Summary An idealized model was developed to describe leaf CO2 exchange in the leguminous tree Erythrina poep-pigiana (Walpers) O.F. Cook under well-watered field condi-tions. Photosynthetic rate in mature leaves (p) was modeled as a rectangular hyperbolic function of photon flux density (q) and ambient CO2 concentration (ca), relative photosynthetic
capacity (π) was modeled as a logistic s-function of leaf age (la), metabolic dark respiration rate (rm) was modeled as an
exponential function of leaf temperature (Tl), and
photorespi-ration rate (rp) was modeled as a hyperbolic function of ca.
Assimilation rate (ac) was modeled as the difference between
the product of p and π and the sum of rm and rp:
ac=p(q,ca)π(la)−[rm(Tl)+rp(ca)].
The model parameters were estimated separately for five sources of E. poeppigiana (Clones 2660, 2662, 2687 and 2693 and half-sib Family 2431) from field data measured with a portable closed-loop gas exchange system at a humid tropical site in Costa Rica. The between-source differences in leaf CO2 exchange characteristics were small, but statistically signifi-cant. Aboveground biomass production was highest in sources that maintained high relative photosynthetic capacity through-out the leaf life span. Quantum yield varied between 0.046 and 0.067, and light-saturated assimilation rate (q = 2000 µmol m−2 s−1 and Tl = 28 °C) at natural atmospheric ca (350 µmol
mol−1) was 16.8--19.9 µmol m−2 s−1. Increasing c
a to 1000
µmol mol−1 resulted in an approximate doubling of the light-saturated assimilation rate. Foliole nitrogen concentration, which was 45.3--51.2 mg g−1 in mature leaves, was positively correlated with relative photosynthetic capacity. Foliole nitro-gen concentration, quantum yield and maximum assimilation rate of E. poeppigiana are among the highest values observed in tropical woody legumes.
Keywords: assimilation rate, leaf age, leaf temperature, mod-eling, nitrogen concentration, photon flux density, photosyn-thetic capacity, respiration.
Introduction
Erythrina poeppigiana (Walpers) O.F. Cook, which is a nitro-gen fixer (Escalante et al. 1984, Lindblad and Russo 1986), is
a commonly used agroforestry tree species in tropical Amer-ica. It is native to northwestern South America, from Bolivia to Panama and Venezuela, and has been introduced to Central America and several Caribbean islands (Krukoff 1969). Its natural distribution extends from humid tropical lowlands to 1500 m a.s.l. (Holdridge and Poveda 1975). Erythrina poep-pigiana, which reaches a height of over 25 m and a diameter of more than 1 m (Holdridge and Poveda 1975), is the most popular shade tree for coffee plantations in Costa Rica (Russo and Budowski 1986). It is also used for shading cacao (Esca-lante et al. 1984), and more recently for green manuring in alley cropping (Kass et al. 1989, Nygren and Jiménez 1993) and as a protein supplement for dairy cattle (Romero et al. 1993).
Agroforestry trees are generally completely pruned twice or more annually to give a pole with a height of 0.75 to 1.5 m in alley cropping (Kass et al. 1989, Nygren and Jiménez 1993) and 3 m in coffee plantations (Russo and Budowski 1986). The species resprouts easily and the amount of foliage biomass pruned every 6 months varies from 1.52 to 2.70 kg per tree in alley cropping (Nygren and Jiménez 1993) to 6.96 kg per tree (about 2000 kg ha−1) in coffee plantations (Russo and Budowski 1986). Considerable clonal variation in foliage biomass production (ranging from 0.72 to 3.03 kg per tree in 6 months) has been observed in E. poeppigiana (Pérez Castellón 1990).
Dynamic models that describe plant growth on the basis of the response of ecophysiological processes to environmental factors integrate ecological and physiological information about plant productivity in a way that permits prediction of plant productivity in a changing environment. Ecological growth models may be especially useful in tropical agrofore-stry, where resource partitioning between the tree and crop components is the key factor in understanding the functioning of the system and the effect of management practices on it. Construction of an agroforestry system model is a hierarchical process that first requires the formulation of ecological growth models of the component species. The tree component is usually less well understood than the other component species. This is the case for coffee (Russo and Budowski 1986) and cacao (Escalante et al. 1984) plantations with E. poeppigiana shade trees, and for alley cropping of E. poeppigiana with maize and beans (Kass et al. 1989, Nygren and Jiménez 1993).
Leaf CO2 exchange of
Erythrina poeppigiana
(Leguminosae:
Phaseolae) in humid tropical field conditions
PEKKA NYGREN
University of Helsinki, Forestry Field Station, FIN-35500 Korkeakoski, Finland
Received December 3, 1993
Development of a leaf CO2 assimilation model is the first step in the construction of an ecological plant growth model (Thornley and Johnson 1990). To be applicable as part of an ecological agroforestry production model, the leaf CO2 ex-change model should be idealized such that it accurately de-scribes the response of assimilation rate to the most important environmental factors, but does not require detailed plant physiological measurements, which may be difficult to obtain under tropical field conditions. Further, because strong corre-lations between environmental variables make it difficult to separate the effects of too many variables (Berninger and Hari 1993), it is necessary to use simple leaf CO2 exchange models in field research.
I have studied, in the context of the development of an ecological biomass production model, the effects of environ-mental factors, foliole nitrogen concentration and leaf age on the assimilation rate and night respiration of four clones and a half-sib family of E. poeppigiana under humid tropical condi-tions. The results are presented in the form of an idealized leaf CO2 exchange model.
Leaf CO2 exchange model
If we assume that a homogeneous leaf is uniformly irradiated by photon flux density q and the internal CO2 concentration at the photosynthetic sites, ci, is uniform, then photosynthetic rate
p can be described by the semiempirical relationship (Landsberg 1986, cf. Thornley and Johnson 1990):
p = kqqkcci kqq+kcci
, (1)
where kq is the quantum yield for incident photon flux density,
and kc is carboxylation efficiency. The CO2 concentration at the photosynthetic site cannot be measured directly in the field. In the absence of water stress, C3 leaves tend to maintain a constant ratio of ci to the ambient CO2 concentration, ca (Long
1985, 1991):
ci = glca, (2)
where gl is the conductance equivalent to the whole diffusion
pathway from the air through the leaf boundary layer, stomata and mesophyll sap to the chloroplasts. Substitution of ci from
Equation 1 in Equation 2 yields:
p= kqq kcglca kqq + kcglca
. (3)
Because of the strong correlation between gl and kc, they
cannot be estimated separately from field data, but their prod-uct, κc, can be used:
p= kqqκcca kqq+ κcca
, (4)
The assimilation rate, ac, is the difference between
photosyn-thetic rate and daytime respiration rate, rd:
ac=p−rd. (5)
Daytime respiration integrates metabolic respiration inde-pendent of photosynthesis and photorespiration. The metabo-lic respiratory rate, rm, was assumed to depend exponentially
on leaf temperature, Tl (Larcher 1975):
rm=rm(20)exp[kr(Tl−20)] , (6)
where rm(20) is the respiration rate at a leaf temperature of
20 °C and kr is the rate of change of rm.
Biochemically, photorespiration is based on the dual affinity of Rubisco for O2 and CO2: in light, photorespiration competes with photosynthesis for Rubisco. If we assume that, at a con-stant ambient CO2/O2 concentration ratio, the photorespiration rate is proportional to the photosynthetic rate, then the absolute photorespiration rate cannot be estimated from field data. Changes in ambient CO2 concentration alter the CO2/O2 con-centration ratio and, subsequently, the photorespiration rate; low ambient CO2 concentrations favor photorespiration, whereas high ambient CO2 concentrations suppress it (Ed-wards and Walker 1983). This dependency can be modeled as a hyperbolic function:
rd = rm+ αr/ca, (7)
where αr is a parameter. The complete function for the
assimi-lation rate is:
Physiological changes that affect photosynthetic capacity occur during leaf development. Subsequently, photosynthetic capacity is affected by leaf age (Larcher 1975, Farquhar and von Caemmerer 1982). Nitrogen is needed for Rubisco regen-eration (von Caemmerer and Farquhar 1981), and leaf nitrogen concentration is also an important determinant of photosyn-thetic capacity (Mooney et al. 1984). These effects can be taken into account by introducing the relative photosynthetic capacity π into Equation 5:
ac=π p−rd. (9)
Leaf age, la, was used as an indicator of physiological leaf
development. A modification of the logistic s-curve was ap-plied to describe the relationship between la and π:
π= απ− βπla 1+ exp(γπ−kπla)
, (10)
where απ and βπ are parameters that define the maximum value of π (the asymptote), kπ is the initial rate of change of π, and γπ
An asymptotic function was applied to describe the relation-ship between foliole nitrogen concentration, nl, and relative
photosynthetic capacity (cf. Cromer et al. 1993):
π=απ(1 − exp[−kn(nl−nmin)]),
(11)
where nmin is the minimum foliole nitrogen concentration
re-quired, kn is the rate of change of the relative photosynthetic
capacity, and απ is the maximum photosynthetic capacity (asymptote).
Field data
Experimental site
The experiment, which was designed to collect data to model the growth of E. poeppigiana, was carried out at the experi-mental farm of the Centro Agronómico Tropical de Investi-gación y Enseñanza (CATIE), situated in Turrialba, Costa Rica (9°53′ N, 83°39′ W, 600 m a.s.l.). The experiment consisted of two groups of trees. In one group of 30 trees, the root systems were separated by a plastic barrier down to a depth of 1 m and the ground was kept completely weed-free by manual weeding to facilitate root and nodule sampling. Another group of 30 trees was grown without root enclosures and weeding, but the ground vegetation was cut every 2 months.
The experiment was established in March 1991 at a planting density of 4 × 4 m. The tree material consisted of 1.5-m long rooted cuttings of four clones selected by the Nitrogen Fixing Tree Project of CATIE (2660, 2662, 2687 and 2693), and 4-month-old greenhouse-grown seedlings of a half-sib family (2431 of the Latin American Forest Seed Bank, CATIE). Six trees of each source were planted according to a completely randomized design in each group.
The trees were pruned and pollarded to 1.5 m on December 12, 1991. A second pruning was carried out on June 12, 1992 in accordance with normal management practices for E. poep-pigiana on Costa Rican coffee farms and CATIE’s alley crop-ping experiments.
Climatic data for the study site, collected by an automatic weather station (Delta-T Devices, Cambridge, U.K.) are pre-sented in Figure 1. Average daytime relative humidity varied from 77.1% in March to 82.3% in October; nighttime humidity was saturating throughout the year. The maximum daytime temperature was 28--30 °C throughout the year. Nighttime minimum temperatures dropped to 13--15 °C from December through March, and were above 17 °C in June and July. Comparison with long-term climatic data (since 1943) from the CATIE weather station indicated that rainfall from Decem-ber 1991 until the end of February 1992 was considerably lower than the long-term mean (long-term means of 328, 171 and 141 mm, respectively). In May, more than half (119 mm) of the monthly precipitation was received during a rain storm. The atmospheric pressure in Turrialba is stable throughout the year (94.2 ± 0.6 kPa).
Gas exchange measurements
The gas exchange measurements were made between March 3 and June 9, 1992, with a battery-operated Li-Cor LI-6200 portable photosynthesis system (Li-Cor Inc., Lincoln, NE, USA), which contains an infrared gas analyzer for determina-tion of assimiladetermina-tion rate and funcdetermina-tions as a steady-state porometer for the determination of transpiration rate. The system was used in closed mode in all measurements. The measuring apparatus included a 0.25-l Plexiglas assimilation chamber lined with Teflon to reduce water adsorption, a quan-tum sensor (calibration accuracy ± 5%) attached outside the chamber to monitor photon flux density during the gas ex-change measurements, a linearized thermistor (accuracy ± 0.5 °C) to record chamber temperature, a chromel-constantan thermocouple (accuracy ± 1 °C) to measure leaf temperature, and a Vaisala Humicap thin film capacitance sensor (Vaisala Ltd., Helsinki, Finland) to record the relative humidity inside the chamber.
The measuring system was controlled by a datalogger, which also calculated assimilation and transpiration rates and stomatal conductance for H2O according to the formulae pre-sented by von Caemmerer and Farquhar (1981). For every sample, the sensor and gas analyzer output were read approxi-mately every second, and the mean and range of the instanta-neous values were stored. The calculation of CO2 exchange rate was based on the mean slope of CO2 depletion in the chamber (Li-Cor Inc. 1990a, 1990b).
Most of the gas exchange measurements were carried out on the 30 trees with root enclosures, because more information was available about these ramets and half-sib family individu-als than the trees without enclosures. Because the trifoliate leaves of E. poeppigiana are large (surface area of a mature foliole is about 1.9--2.7 dm2), only part of a foliole was en-closed in the assimilation chamber. The chamber temperature was about 4--5 °C higher than the ambient air temperature in full sunlight, and about 2 °C higher during the nighttime respiration measurements. The warm air flowing from the infrared gas analyzer was presumably responsible for the tem-perature increase at night, and the greater warming in sunny conditions was due to radiation absorption by the chamber walls. However, with a normally transpiring leaf in the
ber the temperature remained constant (range ± 1 °C) for more than 20 min even in full sun.
Chamber temperature and the water vapor pressure deficit were correlated with photon flux density (r = 0.87 and r = 0.88, respectively). Leaf temperature was strongly correlated with both chamber temperature and photon flux density according to the linear regression (R2 = 0.99):
Tl= 0.647 + 0.942Ta+0.00137q, (12)
where Ta is chamber temperature.
The responses of assimilation and transpiration to changes in photon flux density were measured on the terminal foliole of a selected 5th to 8th fully open trifoliate (2--3 weeks old) leaf from early morning to midday on a sunny day. Only sun leaves were selected, because the leaves of E. poeppigiana emerge in full sun and the older leaves are shaded by the growing foliage. Preliminary tests indicated that there were no differences in assimilation rates among the folioles of the same leaf. A total of five leaves could be monitored during one day, and the measurements were repeated on one leaf from each of the trees growing in the root enclosure section. Records in which the range of instantaneous values of photon flux density was more than 10% of the average or 100 µmol m−2 s−1 during the recording were eliminated from the analysis.
The response of assimilation rate to ambient CO2 concentra-tion was determined by injecting CO2-enriched air into the closed system and following the decrease in CO2 concentration from about 1000 µmol mol−1 to the CO2 compensation point of the assimilation rate. This took about 20 min. The results of McDermitt et al. (1989) indicate that the CO2 response curves measured by this method are comparable with the results from steady-state laboratory measurements. Because the leaves were transpiring at a high rate, the increase in leaf temperature did not cause any problems. The CO2 response curve measure-ments were repeated on each tree within the root enclosure section at saturating photon flux density (1600--2000 µmol m−2 s−1) and on an overcast day (photon flux density of 300--1000 µmol m−2 s−1).
The metabolic dark respiration rate was determined from nighttime measurements carried out shortly after sunset and just before dawn on a selected leaf on each tree with a root enclosure. Dew formation disturbed the measurements before dawn and so the transpiration records were not analyzed. Stabilization of the measuring apparatus with the ambient humidity before dawn took 30 to 90 min compared with 10 to 15 min during the daytime.
The effect of leaf age on relative photosynthetic capacity was determined from two sets of measurements. Rapid changes in young leaves were followed on one marked leaf per tree in three trees per source within the root enclosure section. The measurements were carried out every 2 days between 1 and 14 days after leaf emergence, and a control measurement was made 22 days after leaf emergence. The measurements were performed at ambient CO2 concentration and at saturat-ing photon flux density (1600--2000 µmol m−2 s−1) whenever possible. Long-term changes in relative photosynthetic
capac-ity during the life span of the leaves were monitored every 2 weeks at saturating photon flux density on one leaf in every tree. If the selected leaf became shaded at an older age, the shading foliage was carefully bent to expose the leaf to full sun. If this was not possible the leaf was rejected and no measurements were made.
Measurement of aboveground biomass production
Leaf biomass and area were determined at the pruning on June 12, 1992 by means of the relationship between branch cross-sectional area below the first leaf or bifurcation and leaf biomass or area supported by the branch as described pre-viously (Nygren et al. 1993). Leaf litter was collected weekly from the root enclosure section. The litter lying inside the plastic barrier around each tree was assumed to have fallen from the same tree. The biomass of pruned green twigs and woody branches was determined by weighing all the pruned material for each tree separately, and drying the subsamples for 24 h at 105 °C to determine the water content.
On December 12, 1991 and June 12, 1992, stem volume was determined by measuring the pole diameter at 20 cm from the base and 20 cm from the top. The volume of the pole was estimated as a cylinder. The mean of the pole cross-sectional areas at the measuring points was used as the basal area of the cylinder (Kilkki 1982). Core samples of known volume were taken from the pole and dried for 24 h at 105 °C to determine the wood bulk density. The stem volume increment between December 12, 1991 and June 12, 1992 was multiplied by the average bulk density (0.25 kg dm−3) to calculate stem biomass increment.
Determination of leaf life span and nitrogen concentration
Average leaf life span was determined by marking 10 newly emerged leaves on each tree in the root enclosure section. Initially, the leaves were checked twice a week and severe insect or pathogen damage was noted. After the first appear-ance of senescent leaves, the leaves were checked daily and the age at shedding due to senescence was noted. The number of leaves shed as a result of herbivory or pathogen damage before senescence was used to compute an average herbivory/patho-gen loss percentage by source.
To determine changes in foliole nitrogen concentration dur-ing the leaf life span, 25 newly emerged leaves were marked after pruning in each tree in the root enclosure section. Folioles of five leaves were sampled 2 and 4 weeks after marking, and thereafter at monthly intervals for the determination of total nitrogen content by the micro-Kjeldahl method (Müller 1961). An additional sample of yellow leaves, removed just before shedding, was also analyzed.
Specific leaf mass was determined from the foliole nitrogen concentration samples. The area of each foliole was deter-mined by means of Chacón’s equation (Chacón Espinoza 1990):
Af=lmwmff, (13)
maximum width of the foliole, ff is the form factor and Af is the
area of the foliole. The folioles were measured in the field to the nearest millimeter with a ruler. The form factor applied was 0.58 (Chacón Espinoza 1990). Because the samples for nitro-gen determination were dried for 48 h at 70 °C to avoid volatilization losses of N, their mass was corrected to give a value equivalent to the samples dried for 24 h at 105 °C by multiplying the sample mass by 0.95 (Nygren et al. 1993).
Parameter estimation and model evaluation
Parameter values for Equations 8 and 10 were estimated from the data by applying the multivariate secant method for least squares curve fitting (SAS Institute Inc., Cary, NC, USA). The overall fit of the models was evaluated by means of the mod-eling efficiency, EF (Mayer and Butler 1993):
EF = 1 −Σ(yi−y
^i)2 Σ(yi−y
_
)2 , (14)
where yi is the value of the ith observation of the dependent
variable, y^i its estimated value for the ith observation, and y _
is the mean of the observed values of the dependent variable. In the case of linear regression with intercept, EF is equal to the determination coefficient, R2. In the case of nonlinear regres-sion, EF equally measures the proportion of variance explained by the model, but its lower limit is negative infinity because the data are compared against a fixed line, not against the best fit line. The upper limit of EF is unity also in the case of nonlinear regression (Mayer and Butler 1993).
The parameter estimation was carried out separately for each source. To detect the significance of the between-source differences, analyses of residual variance of the fitted models for each source and for the combined data set were carried out (Mead and Curnow 1983).
The parameters of the metabolic respiration model were estimated from the night respiration measurements. The value of rm(20) was determined as the average night respiration rate
measured at 20 °C (measurements before dawn). The measure-ments after sunset were mainly carried out at a leaf temperature of 24 °C. Their average was computed and kr determined
analytically from Equation 6.
Results
Metabolic respiration
The night respiration rate at 20 °C was 0.60--0.70 µmol m−2 s−1, except in Clone 2693 (Table 1). More variability among sources was observed at 24 °C than at 20 °C. Accordingly, the response of the metabolic respiration rate to leaf temperature varied among sources and was strongest in Clone 2662 and weakest in Clone 2693. The metabolic respiration rate approxi-mately doubled for every 10 °C increase in leaf temperature in Clones 2660, 2662 and 2687, but increased only 1.17-fold in Clone 2693 and about 1.75-fold in the half-sib Family 2431.
Effects of photon flux density and ambient CO2 concentration on assimilation rate
A typical response of assimilation rate of mature (2- to 3-week-old) leaves of E. poeppigiana to changes in photon flux density is presented in Figure 2 for Clone 2660. Most of the observed variation was caused by depletion of the ambient CO2 concen-tration in the closed-loop system during the measurements, but a minor part of the variation was due to variations in leaf temperature among measurements. Differences in assimilation rates between leaves were small (Figure 2). Maximum assimi-lation rate was about 20 µmol m−2 s−1 in all sources studied.
The response curves to ambient CO2 concentration showed little variation (Figure 2). The curves at different photon flux densities had the same form, but differed in maximum assimi-lation rate. The curves measured at photon flux densities of 1560 and 2060 µmol m−2 s−1 had similar maximum rates, about 40 µmol m−2 s−1. The observed maximum was slightly above 40 µmol m−2 s−1 in all sources, indicating a strong response to the elevated CO2 concentration. The apparent ‘‘lack of fit’’ of the estimated assimilation rate at 900 µmol m−2 s−1 was due to differences between leaves; the curve was fitted to all observations on Clone 2687, not to a specific leaf.
No apparent systematic deviation was observed in the plots of model residuals against photon flux density, ambient CO2 concentration and leaf temperature, except for a slight under-estimation at low ambient CO2 concentration in Clone 2662 (Figure 3). The range of residuals was large, but they were systematically distributed on both sides of the zero (perfect fit) line. The same was also true for the plot of the residuals against leaf-to-air water vapor pressure difference (Figure 3), indicat-ing that the exclusion of this environmental factor from the
assimilation rate model was justified. The modeling efficiency varied between 0.88 and 0.96 (Table 2).
The quantum yield for incident radiation, kq, was similar in
the half-sib Family 2431 and Clones 2660 and 2687, but was significantly lower (significance determined according to the overlap of 95% confidence intervals) in Clone 2662 and higher in Clone 2693 (Table 2). Slightly more variation was observed in the value of κc, which combines the conductance of the CO2 pathway from the air to the chloroplasts and the carboxylation efficiency. It was highest in Clones 2660, 2662 and 2687. The 95% confidence intervals for the quantum yield and κc
esti-mates were relatively narrow, indicating a reliable fit of these parameters.
The value of the photorespiration parameter, αr, was highest
in Clone 2662, followed by Clone 2687, and was lowest in Clone 2693; however, only the difference between Clones 2662 and 2693 was significant (Table 2). The confidence intervals of αr were wider than those of kq and κc, indicating
that αr parameter estimation was less accurate than the
pa-rameter estimation of photosynthetic rate. Because significant differences were observed in the values of the parameters of
photosynthetic rate, analysis of residual variance of the com-plete assimilation rate models (Equation 8) fitted for each source and for the combined data set was carried out. The results indicated highly significant differences (F12,1034 = 14.05, P < 0.001).
Model estimates of the assimilation rates at a photon flux density of 2000 µmol m−2 s−1, a leaf temperature of 28 °C and an ambient CO2 concentration of 350 µmol mol−1 were 17.0, 18.9, 18.1, 19.9 and 16.8 µmol m−2 s−1 in the half-sib Family 2431 and Clones 2660, 2662, 2687 and 2693, respectively. At 1000 µmol mol−1, the respective assimilation rates were esti-mated to be 40.1, 43.5, 40.6, 44.7 and 41.5 µmol m−2 s−1.
Effect of leaf development on relative photosynthetic capacity
Leaf age satisfactorily explained changes in the relative photo-synthetic capacity of the leaves. The modeling efficiency var-ied between 0.81 and 0.92 (Table 3), and the residuals plotted against leaf age, although scattered, did not show any system-atic deviation (Figure 4). The slope of the model was signifi-cantly steeper in the half-sib Family 2431 and Clone 2693 than in Clones 2660 and 2662. The 95% confidence interval of the slope parameter βπ of Clone 2660 included zero, indicating that the parameter was not important for the model. The con-fidence intervals of the values of the parameters γπ and kp,
which measure initial rate of change of the relative photosyn-thetic capacity, overlapped between all sources. According to the analysis of residual variance, the differences between the relative photosynthetic capacity models fitted by source were significant (F16,829 = 13.27, P < 0.001).
Equation 10 did not hold for senescent leaves from about 6 days before shedding, but the relative photosynthetic capac-ity declined with a steep slope (Figure 5). This senescence slope was determined analytically as a straight line, which intercepted the curve of Equation 10 at a leaf age correspond-ing to 6 days before the average sheddcorrespond-ing age, and was zero at the average shedding age of each source (Table 4).
The average leaf life span was short and varied among sources; leaves of the half-sib Family 2431 and Clone 2662 had a life span almost 2 weeks longer than leaves of Clones 2660 and 2687 (Table 4). The premature loss of leaves as a result of herbivory or pathogen damage was lowest in Clone 2662 at 8.3%, and highest in Clone 2687 at 50.0%. Herbivory or pathogen loss was 28.3, 35.0 and 48.3% for sources 2660, 2431 and 2693, respectively.
Three patterns of leaf development were detected (Figure 5): long life span with a considerable decrease in photosynthetic capacity as a function of leaf age (half-sib Family 2431), short life span with almost constant photosynthetic capacity (Clone 2660) and short life span with a considerable decrease in photosynthetic capacity (Clone 2687). Development of leaves in Clones 2662 and 2693 followed intermediate patterns (Ta-bles 3 and 4): long life span with a moderate decrease in photosynthetic capacity (Clone 2662) and intermediate life span with a considerable decrease in photosynthetic capacity (Clone 2693).
Figure 2. Top: Assimilation rate as a function of photon flux density in mature leaves of Clone 2660 of Erythrina poeppigiana. Symbols represent observations on different leaves (n = 6) and the curve represents the estimate by Equation 8 at ambient CO2 concentration
(300 µmol mol−1) and a leaf temperature of 28 °C. Bottom: Assimila-tion rate as a funcAssimila-tion of ambient CO2 concentration at photon flux
Foliole nitrogen concentration and leaf mass/leaf area ratio
The foliole nitrogen concentration was high and declined with leaf age (Figure 6). However, even senescent leaves had a nitrogen concentration of 23.0--26.4 mg g−1, which was about
half the value observed in vigorous 28- to 56-day-old leaves. The leaf mass/leaf area ratio was low in young leaves (38--46 g m−2) and increased with leaf age stabilizing at 60--70 g m−2 (Figure 7).
Equation 11 provided a satisfactory description of the
rela-Table 2. Parameter values with 95% confidence interval for the rectangular hyperbolae describing the dependence of assimilation rate on ambient CO2 concentration and incident photon flux density in mature leaves of Erythrina poeppigiana; Equation 8 adjusted to data on the effects of photon
flux density and CO2concentration.
Tree source kq κc αr SSE df EF1
2431 0.055 0.067 455 1438 209 0.88
0.049--0.061 0.062--0.072 323--586
2660 0.056 0.076 449 555 190 0.96
0.052--0.060 0.072--0.079 360--538
2662 0.046 0.079 677 568 202 0.96
0.043--0.048 0.076--0.083 582--771
2687 0.054 0.081 541 1565 224 0.91
0.049--0.058 0.076--0.086 418--663
2693 0.067 0.063 391 969 209 0.95
0.059--0.075 0.059--0.066 296--485
Combined 0.053 0.073 497 5926 1046 0.92
0.051--0.056 0.071--0.075 445--549
1 Abbreviations: k
q = quantum yield (mol CO2 assimilated/mol photons incident on leaf), κc = product of total conductance of the CO2 pathway
from air to chloroplast and carboxylation efficiency (mol m−2 s−1), αr = parameter of photorespiration rate, SSE = sum of squared errors, df =
degrees of freedom for error, and EF = modeling efficiency.
tionship between foliole nitrogen concentration and relative photosynthetic capacity; the modeling efficiency was 0.98--0.99 in all sources (Table 5). The value of parameter απ (maxi-mum photosynthetic capacity) was fixed beforehand to the value estimated from the fit of Equation 10 to the data on the effect of leaf age on relative photosynthetic capacity (Table 3),
because there were too few foliole nitrogen concentration data to estimate the three parameter values reliably. According to the analysis of residual variance, the models fitted by source differed significantly (F8,11 = 6.72, P < 0.001).
Table 3. Parameter values with 95% confidence interval for the logistic s-curves describing the relationship between leaf age and relative photosynthetic capacity in Erythrina poeppigiana; Equation 10.
Tree source απ βπ γπ kπ SSE df EF1
2431 1.26 0.0067 4.78 0.53 1860 246 0.81
1.20--1.32 0.0057--0.0077 3.77--5.79 0.41--0.65
2660 1.10 0.0021 6.25 0.81 1170 153 0.88
1.03--1.17 −0.00065--0.0048 4.67--7.83 0.60--1.03
2662 1.05 0.0039 5.71 0.79 708 162 0.92
1.01--1.10 0.0026--0.0052 4.63--6.78 0.63--0.94
2687 1.12 0.0064 4.10 0.68 1024 140 0.85
1.06--1.18 0.0043--0.0086 3.30--4.90 0.53--0.83
2693 1.21 0.0072 5.66 0.66 510 128 0.90
1.15--1.28 0.0056--0.0088 4.58--6.74 0.52--0.80
Combined 1.16 0.0059 5.12 0.67 6622 845 0.84 1.13--1.18 0.0053--0.0065 4.57--5.67 0.59--0.74
1 Abbreviations: α
π, βπ = parameters of the maximum photosynthetic capacity (asymptote), kπ = initial rate of change of the relative photosynthetic capacity (day−1), γπ = parameter, SSE = sum of squared errors, df = degrees of freedom for error, and EF = modeling efficiency.
Figure 4. Residuals of the relative photosynthetic capacity model for
Erythrina poeppigiana (Equation 10) as a function of leaf age. Figure 5. Relative photosynthetic capacity as a function of leaf age inthe half-sib Family 2431 and Clones 2660 and 2687 of Erythrina
poeppigiana. Calculated according to Equation 10. Table 4. Average leaf shedding age ± SD and the equations describing
the ‘‘senescence slope’’ of relative photosynthetic capacity as a func-tion of leaf age in Erythrina poeppigiana.
Tree source Leaf shedding Senescence slope Application age (days) equation range (days) 2431 87 ± 16 π = 10.4 − 0.120la1 82--87
2660 74 ± 10 π = 11.8 − 0.160la 69--74
2662 86 ± 15 π = 10.6 − 0.123la 81--86
2687 73 ± 9 π = 8.41 − 0.115la 68--73
2693 80 ± 17 π = 9.03 − 0.113la 75--80
Combined 81 ± 15 π = 9.57 − 0.120la 76--81
1 Abbreviations: π = relative photosynthetic capacity (unitless), and l
a
= leaf age (days).
Stomatal conductance
Stomatal conductance for CO2 varied between 158 and 238 mmol m−2 s−1 at photon flux densities below 1000 µmol m−2 s−1 and between 190 and 366 mmol m−2 s−1 at higher photon flux densities (Table 6). A typical response of assimilation rate to stomatal conductance is presented in Figure 8. Stomatal conductance was strongly affected by photon flux density and the dependence of assimilation rate on stomatal conductance followed the same pattern as its dependence on photon flux density (cf. Figure 2).
Aboveground biomass production
Aboveground biomass production between December 12, 1991 and June 12, 1992 was highest in Clone 2660, followed by Clone 2662, and was lowest in Clone 2693 (Table 7). The proportion of harvestable biomass (foliage, green twigs and woody branches) was highest in sources with high total above-ground biomass production (76 and 70% in Clones 2662 and 2660, respectively). The within-source variation in biomass production was lowest in Clone 2660 and highest in the half-sib Family 2431 and Clone 2662. Litterfall consisted of foliage
litter only and presented the highest within-source variation of the biomass compartments, probably because its measurement was less accurate than that of the other compartments.
Correlations were calculated between the parameters of the CO2 exchange model (Tables 1--3) and the production of total aboveground and harvestable biomass. The only significant correlations were found between both total and harvestable biomass production and the parameter βπ (r = −0.99, P = 0.0019 and r = −0.99, P = 0.0003, respectively), which meas-ures the rate of decline of the relative photosynthetic capacity as a function of leaf age. This indicates that sources that maintain a high photosynthetic capacity during the entire leaf life span produce more biomass than sources that only main-tain a high photosynthetic capacity for part of the leaf life span. Also the leaf area on June 12, 1992 was correlated with βπ
(r = −0.88, P = 0.0494). Weaker correlations were detected between rate of change of metabolic respiration rate, kr, and the
total aboveground (r = 0.73, P = 0.1667) and harvestable biomass production (r = 0.73, P = 0.1596) and leaf area on June 12, 1992 (r = 0.76, P = 0.1327).
Discussion
Field data were used to determine the parameter values for an idealized model describing the effect of environmental factors and leaf development on leaf CO2 exchange in E. poeppigiana. The assimilation model was divided into four components, the photosynthetic rate in mature leaves, relative photosynthetic capacity, metabolic respiration and photorespiration. The pho-tosynthetic rate in mature leaves was described as a function of the photon flux density and ambient CO2 concentration, and the relative photosynthetic capacity as a function of leaf age. It was introduced into the model as the coefficient of the basic photosynthesis model described by Equation 4. This is consis-tent with the observations of Long et al. (1993) that the quan-tum yields of 11 taxa were not affected by the age of the photosynthetic organs, but that the maximum assimilation
Figure 7. Leaf mass/leaf area ratio as a function of leaf age in four clones and a half-sib family (2431) of Erythrina poeppigiana.
Table 5. Parameter values with 95% confidence interval for the asymptotic curves describing the relationship between foliole nitrogen concentration and relative photosynthetic capacity in Erythrina poeppigiana; Equation 11.
Tree source απ kn nmin SSE df EF1
2431 1.26 0.085 26.5 0.0090 3 0.99
0.064--0.106 25.0--28.0
2660 1.10 0.109 23.3 0.0028 2 0.99
0.057--0.160 22.0--24.6
2662 1.05 0.091 22.9 0.0038 2 0.99
0.054--0.129 20.9--25.0
2687 1.12 0.072 26.2 0.0142 2 0.98
0.026--0.118 21.6--30.8
2693 1.21 0.059 26.3 0.0051 2 0.99
0.039--0.078 23.3--29.3
Combined 1.16 0.073 24.6 0.2057 19 0.99
0.061--0.084 23.5--25.8
1 Abbreviations: α
π= maximum photosynthetic capacity (asymptote), kn = rate of change of relative photosynthetic capacity (g mg−1), nmin =
rates tended to be lower in young and old organs than in mature photosynthetic organs. A temperature dependence of quantum yield (Farquhar and von Caemmerer 1982) was not observed; it may have been obscured by the strong correlation between photon flux density and leaf temperature.
Von Caemmerer and Farquhar (1981) observed a decrease in both the maximum assimilation rate and carboxylation effi-ciency in Phaseolus vulgaris grown at low nitrogen availabil-ity and concluded that nitrogen is needed for Rubisco regeneration. In the present study, the observed decrease in photosynthetic capacity as a function of leaf age may have been caused by the simultaneous decrease in foliole nitrogen concentration. This possibility is supported by the close
rela-tionship between foliole nitrogen concentration and relative photosynthetic capacity, and suggests that foliole nitrogen concentration could be used instead of leaf age to model relative photosynthetic capacity. The relationship between fo-liole nitrogen concentration and relative photosynthetic capac-ity was curvilinear as has been observed in Gmelina arborea Roxb. leaves (Cromer et al. 1993).
A constant ratio was assumed between ambient and chloro-plast CO2 concentration. The conventional method of estimat-ing internal leaf CO2 concentration by applying stomatal conductance or resistance estimated from transpiration data (von Caemmerer and Farquhar 1981, Landsberg 1986, Thorn-ley and Johnson 1990, Long and Hällgren 1993) was avoided for three reasons. First, a constant ratio between ambient and intercellular CO2 concentration has been observed in leaves of well-watered C3 plants (reviewed by Long 1985). Second, according to the optimization theory (Cowan 1977), gas ex-change is optimal when the maximal amount of carbohydrates is produced per unit of water transpired. In non-water-limited leaves this implies that stomatal opening is determined by the capacity of mesophyll tissue to fix carbon (Wong et al. 1979), or that stomatal conductance is affected by the same factors as assimilation rate (cf. Wong et al. 1985). Third, the determina-tion of stomatal conductance is based on a series of measure-ments, the accuracy of which is often poor under humid tropical conditions. For example, an error of 1 °C in leaf temperature between 30 and 31 °C means a 0.25 kPa error in the estimate of water vapor pressure in the substomatal cavity. Further, a 2% error in the humidity sensor output causes a 15% error in conductance computed according to von Caemmerer and Farquhar (1981), when the air humidity is about 80%. However, the error increases to 70% at an air humidity of 90% (Field and Mooney 1984); 80--90% was a common humidity range at the experimental site.
Daytime respiration was divided into metabolic respiration, which was modeled as a function of leaf temperature, and photorespiration, which was modeled as a function of ambient CO2 concentration. The metabolic respiration rate, which was determined from the night respiration rate measurements and was assumed to continue unaltered during the daytime (cf. Edwards and Walker 1983), approximately doubled with a 10 °C increase in leaf temperature. At typical leaf temperature
Table 7. Average aboveground biomass production ± SD (kg per tree) of Erythrina poeppigiana from December 12, 1991 to June 12, 1992. Means followed by the same letter within a row are not significantly different (Duncan’s multiple range test at 5%).
Biomass compartment Tree source
2431 2660 2662 2687 2693
Foliage 1.48 ± 0.52 b 2.44 ± 0.25 a 2.54 ± 0.74 a 1.75 ± 0.44 b 1.47 ± 0.42 b Green twigs 0.74 ± 0.39 b 1.55 ± 0.24 a 0.99 ± 0.53 b 0.73 ± 0.25 b 0.60 ± 0.19 b Woody branches 0.90 ± 0.82 c 2.21 ± 0.25 a 1.63 ± 0.81 ab 1.13 ± 0.40 bc 0.72 ± 0.35 c Total harvestable 3.12 ± 1.70 c 6.19 ± 0.58 a 5.16 ± 2.00 ab 3.61 ± 1.00 bc 2.79 ± 0.96 c Litterfall 0.66 ± 0.42 a 1.34 ± 0.80 a 0.93 ± 0.43 a 1.21 ± 0.35 a 1.19 ± 0.56 a Stem increment 1.28 ± 1.02 a 1.37 ± 0.42 a 0.75 ± 0.41 ab 0.72 ± 0.38 ab 0.38 ± 0.30 b Total aboveground 5.06 ± 2.91 b 8.90 ± 0.92 a 6.86 ± 2.66 ab 5.55 ± 1.28 b 4.36 ± 1.35 b Leaf area (m2) 26.3 ± 9.2 bc 43.9 ± 4.4 a 32.5 ± 9.4 bc 34.0 ± 8.4 b 22.8 ± 6.5 c Figure 8. Assimilation rate as a function of stomatal conductance for
CO2 in mature leaves of Clone 2687 of Erythrina poeppigiana at
different photon flux densities.
Table 6. Average stomatal conductance for CO2± SD (mmol m−2 s−1)
in mature leaves of Erythrina poeppigiana. Tree source Photon flux density
values at the experimental site, metabolic respiration rates were 1--2 µmol m−2 s−1.
At ambient CO2 concentration, the photorespiration rate was comparable to the metabolic respiration rate. In the model, it was assumed that, at a constant atmospheric CO2/O2 concen-tration ratio, the photorespiration rate was proportional to the photosynthetic rate, and that the photorespiration term in the model only measures the deviation of photorespiration rate from photosynthetic rate when the CO2/O2 concentration ratio is altered as a result of variations in atmospheric CO2 concen-tration. Inclusion of the photorespiration term in the model was valid because a model with only a temperature-dependent respiration term overestimated the assimilation rate at low ambient CO2 concentrations. At high atmospheric CO2 con-centration, however, the photorespiration term approached zero and could be omitted from the model.
Both the solubility of CO2 in mesophyll cell sap relative to O2 solubility and the specificity of Rubisco for CO2 relative to O2 change in favor of O2 as a function of increasing leaf temperature, which should result in an increased photorespira-tion rate relative to the photosynthetic rate (Long 1991). How-ever, such dependence of photorespiration rate on temperature was not observed in this study. Attempts to model the pho-torespiration term by applying the changes in CO2/O2 solubil-ity ratio and CO2/O2 specificity factor suggested by Long (1991), or by a simple ‘‘black box’’ type dependence of pho-torespiration rate on leaf temperature and ambient CO2 con-centration, resulted in a poorer fit of the assimilation rate model to the data than was achieved with Equation 8.
The failure to detect any dependence of the photorespiration term in Equation 8 on temperature may be caused by several interacting factors. Both the solubility ratio and Rubisco speci-ficity factor are less affected by temperature at the high tem-peratures typical of tropical environments. According to the formulae presented by Long (1991), the average rate of change of the internal CO2/O2 concentration ratio is 1% per 1 °C between 25 and 35 °C. Most of the assimilation rate measure-ments were carried out within this temperature range. The
average rate of change of the Rubisco specificity factor is somewhat higher, 4% per 1 °C within the same temperature range. These changes are small enough to be obscured by the high correlation between photon flux density and leaf tempera-ture inside the assimilation chamber. They are also small compared to the doubling of the internal CO2/O2 concentration ratio, calculated according to Long (1991), in favor of CO2 when ambient CO2 concentration increases from 350 to 700 µmol mol−1.
When the parameters of the leaf CO2 exchange model were compared with the aboveground biomass production, only the parameter measuring changes in photosynthetic capacity as a function of leaf age, βπ, showed a significant correlation; the photosynthetic capacity of the most productive tree sources did not decrease much during the leaf life span. In the idealized assimilation model, βπ was the only parameter that was indica-tive of the plant’s ability to integrate photosynthetic production over time. The correlation between βπ and biomass production emphasizes the importance of following leaf CO2 exchange characteristics for a long time period, if the aim is to relate these characteristics to biomass production. Comparisons be-tween leaf assimilation and biomass production have usually been made based on the instantaneous assimilation rate meas-ured at light-saturated conditions, and the results vary from positive correlation in 10 tropical tree species (Muthuchelian 1992) to no correlation in provenances of Acacia mangium Willd. (Atipanumpai 1989) and negative correlation in half-sib families of Robinia pseudoacacia L. (Mebrahtu et al. 1991).
The maximum assimilation rate of E. poeppigiana is among the highest observed in tropical woody legumes (Table 8), and is comparable with the values observed in Prosopis chilensis (Mol.) Stuntz. (Pinto 1989). The high photosynthetic capacity of E. poeppigiana may be a result of high foliole nitrogen concentrations (Table 8 and cf. Mooney et al. 1984).
The quantum yield of E. poeppigiana was comparable with the quantum yield observed in the neotropical rain forest Caesalpinoids, Copaifera venezuelana Pittier and Harms (0.065) and Hymenaea courbaril L. (0.056), in laboratory
Table 8. Maximum assimilation rate and nitrogen concentration in mature leaves of some tropical leguminous trees.
Species Maximum assimilation rate Leaf nitrogen concentration Reference (µmol m−2 s−1) (mg g−1)
Erythrina poeppigiana 16.8--19.9 45.9--55.6
Acacia mangium 6 20.0--25.6 Atipanumpai 1989
Acacia auriculiformis 10--12 Trivedi et al. 1992
Acacia koa 7.1 Hansen and Steig 1993
Cassia siamea 24.4 Yamoah et al. 1986
Copaifera venezuelana 8 28.1 Langenheim et al. 1984
Erythrina berteroana 36.7--45.1 Pérez Castellón 1990
Erythrina fusca 33.6--36.3 Pérez Castellón 1990
Erythrina variegata 6 22.8 Mutchuchelian 1992
Flemingia congesta 33.3 Yamoah et al. 1986
Gliricidia sepium 44.6 Yamoah et al. 1986
Hymenaea courbaril 5 21.1 Langenheim et al. 1984
Leucaena leucocephala 10 29.7 Mutchuchelian 1992
Pentaclethra macroloba 7 Oberbauer and Strain 1985
measurements (Langenheim et al. 1984), but higher than that in the Mimosaceae Pentaclethra macroloba (Willd.) Kuntze (0.027, Oberbauer and Strain 1985) and most temperate tree species (data compiled by Landsberg 1986). Long et al. (1993) reported higher quantum yields (mean 0.074 ± 0.008), but their measurements were carried out in a light integrating sphere.
Few data are available on the response of tropical legumi-nous trees to elevated CO2 concentrations. Pinto (1989) re-ported an assimilation rate of about 28 µmol m−2 s−1 at 900 µmol mol−1 in P. chilensis, and the response curve saturated at a higher rate than in E. poeppigiana. Long (1991) calculated that the maximum assimilation rate of C3 plants should theo-retically increase by 105% at 35 °C when the atmospheric CO2 concentration increases from 350 to 650 µmol mol−1. Accord-ing to the model presented here, the maximum assimilation rate of E. poeppigiana under the same conditions would in-crease from 16.5--19.2 to 28.9--31.4 µmol m−2 s−1, i.e., 71--78%, depending on the source.
The model satisfactorily described the effects of photon flux density and leaf temperature on CO2 assimilation and night respiration rate within the naturally occurring range of these factors. It also functioned over a wide range of ambient CO2 concentrations, which improves its applicability in a changing environment. If the assumption of the absence of drought does not hold, then the inclusion of stomatal conductance may improve the model performance, and if the leaf temperatures are lower than those in the present study, the effect of tempera-ture on photorespiration rate may be more important than observed in this study.
Acknowledgments
I thank the Nitrogen Fixing Tree Project of CATIE for providing me with the selected tree material, study site and other research facilities during my stay in Turrialba, and the Department of Forest Ecology, University of Helsinki, for kindly lending me the LI-6200 portable photosynthesis system. The useful comments of Prof. Pertti Hari and Mr. Frank Berninger on earlier versions of the manuscript are grate-fully acknowledged. Mr. John Derome reviewed the English of the manuscript. The study was financed by the Academy of Finland.
References
Atipanumpai, L. 1989. Acacia mangium: studies on the genetic vari-ation in ecological and physiological characteristics of a fast-grow-ing plantation tree species. Acta For. Fenn. 206, 92 p.
Berninger, F. and P. Hari. 1993. Optimal regulation of gas exchange: evidence from field data. Ann. Bot. 71:135--140.
von Caemmerer, S. and G.D. Farquhar. 1981. Some relationships between the biochemistry of photosynthesis and the gas exchange of leaves. Planta 153:376--387.
Chacón Espinoza, J.C. 1990. Análisis del crecimiento del follaje en tres especies de Erythrina en Costa Rica. M.Sc. Thesis. CATIE, Turrialba, Costa Rica, 77 p.
Cowan, I.R. 1977. Stomatal behavior and environment. Adv. Bot. Res. 4:117--227.
Cromer, R.N., P.E. Kriedemann, P.J. Sands and L.G. Stewart. 1993. Leaf growth and photosynthetic response to nitrogen and phospho-rus in seedling trees of Gmelina arborea. Aust. J. Plant Physiol. 20:83--98.
Edwards, G. and D.A. Walker. 1983. C3, C4: Mechanisms, and cellular
and environmental regulation of photosynthesis. Blackwell Scien-tific Publications, Oxford, U.K., 542 p.
Escalante, G., R. Herrera and J. Aranguren. 1984. Fijación de ni-trógeno en árboles de sombra (Erythrina poeppigiana) en cacao-tales del norte de Venezuela. Pesq. Agropec. Bras. 19:223--230. Farquhar, G.D. and S. von Caemmerer. 1982. Modeling of
photosyn-thetic response to environmental conditions. In Physiological Plant Ecology II. Water Relations and Carbon Assimilation. Encyclope-dia of Plant Physiology, New Series 12B. Eds. O.L. Lange, P.S. Nobel, C.B. Osmond and H. Ziegler. Springer-Verlag, Berlin, Ger-many, pp 549--587.
Field, C. and H.A. Mooney. 1984. Measuring gas exchange of plants in the wet tropics. In Physiological Ecology of Plants of the Wet Tropics. Eds. E. Medina, H.A. Mooney and C. Vázquez-Yánes. Dr. W. Junk Publishers, The Hague, The Netherlands, pp 129--138. Hansen, D. and E. Steig. 1993. Comparison of water-use efficiency
and internal leaf carbon dioxide concentration in juvenile leaves and phyllodes of Acacia koa (Leguminosae) from Hawaii, estimated by two methods. Am. J. Bot. 80:1121--1125.
Holdridge, L.R. and L.J. Poveda. 1975. Arboles de Costa Rica. Tropi-cal Science Center, San José, Costa Rica, 546 p.
Kass, D.L., A. Barrantes, W. Bermudez, W. Campos, M. Jiménez and J.F. Sánchez. 1989. Resultados de seis años de investigación de cultivo en callejones (alley cropping) en La Montaña, Turrialba. El Chasqui 19:4--15.
Kilkki, P. 1982. Metsänmittausoppi. University of Helsinki, Depart-ment of Forest Mensuration and ManageDepart-ment, Research Notes 7, 189 p.
Krukoff, B.A. 1969. Supplementary notes on the American species of
Erythrina III. Phytologia 19:113--175.
Landsberg, J.J. 1986. Physiological ecology of forest production. Academic Press, London, U.K., 198 p.
Langenheim, J.H., C.B. Osmond, A. Brooks and P.J. Ferrar. 1984. Photosynthetic responses to light in seedlings of selected Ama-zonian and Australian rain forest tree species. Oecologia 63:215--224.
Larcher, W. 1983. Physiological plant ecology. 2nd Edn. Springer-Ver-lag, Berlin, Germany.
Li-Cor Inc. 1990a. The LI-6200 primer----an introduction to operating the LI-6200 portable photosynthesis system. Li-Cor Inc., Lincoln, NE, USA.
Li-Cor Inc. 1990b. Technical reference. Li-Cor Inc., Lincoln, NE, USA.
Lindblad, P. and R. Russo. 1986. C2H2-Reduction by Erythrina poep-pigiana in a Costa Rican coffee plantation. Agroforestry Syst. 4:33--37.
Long, S.P. 1985. Leaf gas exchange. In Photosynthetic Mechanisms and the Environment. Eds. J. Barber and N.R. Baker. Elsevier, Amsterdam, The Netherlands, pp 453--500.
Long, S.P. 1991. Modification of the response of photosynthetic pro-ductivity to rising temperature by atmospheric CO2 concentrations:
has its importance been underestimated? Plant Cell Environ. 14:729--739.
Long, S.P. and J.E. Hällgren. 1993. Measurement of CO2 assimilation
by plants in the field and the laboratory. In Photosynthesis and Production in a Changing Environment: a Field and Laboratory Manual. Eds. D.O. Hall, J.M.O. Scurlock, H.R. Bolhàr-Nor-denkampf, R.C. Leegood and S.P. Long. Chapman and Hall, Lon-don, U.K., pp 129--167.
McDermitt, D.K., J.M. Norman, J.T. Davis, T.M. Ball, T.J. Arkebauer, J.M. Welles and S.R. Roemer. 1989. CO2 response curves can be
measured with a field-portable closed-loop photosynthesis system. Ann. Sci. For. 46(suppl.):416--420.
Mayer, D.G. and D.G. Butler. 1993. Statistical validation. Ecol. Model. 68:21--32.
Mead, R. and R.N. Curnow. 1983. Statistical methods in agriculture and experimental biology. Chapman and Hall, London, U.K. Mebrahtu, T., J.W. Hanover, D.R. Layne and J.A. Flore. 1991. Leaf
temperature effects on net photosynthesis, dark respiration and photorespiration of seedlings of black locust families with contrast-ing growth rates. Can. J. For. Res. 21:1616--1621.
Mooney, H.A., C. Field and C. Vázquez-Yánes. 1984. Photosynthetic characteristics of wet tropical forest plants. In Physiological Ecol-ogy of Plants of the Wet Tropics. Eds. E. Medina, H.A. Mooney and C. Vázquez-Yánes. Dr. W. Junk Publishers, The Hague, The Neth-erlands, pp 113--128.
Muthuchelian, K. 1992. Biomass productivity relative to net photosyn-thetic rate, ribulose-1,5-bisphosphate carboxylase activity, soluble protein and nitrogen content in ten tree species. Photosynthetica 26:333--339.
Müller, L. 1961. Un aparato micro-Kjelldahl simple para análisis rutinarios rápidos de materia vegetal. Turrialba 11:17--25. Nygren, P. and J.M. Jiménez. 1993. Radiation regime and nitrogen
supply in modeled alley cropping systems of Erythrina poep-pigiana with sequential maize--bean cultivation. Agroforestry Syst. 21:271--285.
Nygren, P., S. Rebottaro and R. Chavarría. 1993. Application of pipe model theory to non-destructive estimation of leaf biomass and leaf area of pruned agroforestry trees. Agroforestry Syst. 23:63--77.
Oberbauer, S.F. and B.R. Strain. 1985. Effects of light regime on growth and physiology of Pentaclethra macroloba (Mimosaceae) in Costa Rica. J. Trop. Ecol. 1:303--320.
Pérez Castellón, E.E. 1990. Evaluación del ensayo clonal de Erythrina
spp. en San Juan Sur, Turrialba, Costa Rica. M.Sc. Thesis. CATIE, Turrialba, Costa Rica, 111 p.
Pinto, M. 1989. CO2 assimilation in young Prosopis plants. Ann. Sci.
For. 46(suppl.):433--438.
Romero, F., S. Abarca, L. Corado, J. Tobón, M. Kass and D. Pezo. 1993. Producción de leche de vacas en pastoreo suplementadas con poró (Erythrina poeppigiana) en el trópico húmedo de Costa Rica.
InErythrina in the New and Old Worlds. Eds. S.B. Westley and M.H. Powell. Nitrogen Fixing Tree Research Reports, Special Is-sue, pp 223--230.
Russo, R. and G. Budowski. 1986. Effect of pollarding frequency on biomass of Erythrina poeppigiana as a coffee shade tree. Agrofor-estry Syst. 4:145--162.
Thornley, J.H.M. and I.R. Johnson. 1990. Plant and crop modeling: a mathematical approach to plant and crop physiology. Clarendon Press, Oxford, U.K., 669 p.
Trivedi, P.K., U.V. Pathre and P.V. Sane. 1992. Photosynthetic charac-teristics of Acacia auriculiformis. Photosynthetica 26:617--623. Wong, S.C., I.R. Cowan and G.D. Farquhar. 1979. Stomatal
conduc-tance correlates with photosynthetic capacity. Nature 282:424--426. Wong, S.C., I.R. Cowan and G.D. Farquhar. 1985. Leaf conductance in relation to rate of CO2 assimilation. II. Effects of short-term exposures to different photon flux densities. Plant Physiol. 78:826--829.
