Summary We studied water use by Eucalyptus tereticornis Sm. in two plantations, differing in tree density (1800 stems ha−1 at Site I and 1090 stems ha−1 at Site II), in different years. At both sites, stomatal conductance, predawn and midday water potentials and microclimate were measured and used to esti-mate hourly transpiration by the Penman-Monteith equation. Growth in girth was also measured.
Stomatal conductance was closely correlated with atmos-pheric vapor pressure deficit (D); however, stomata did not close completely even at high D (≈ 5.0 kPa). Midday leaf water potentials did not fall below −2.0 MPa during any part of the year at either site. Predawn leaf water potentials were greater than −0.25 MPa during the postmonsoon period, but declined to −0.7 MPa at Site I during the premonsoon period. Transpiration estimates ranged from 0.6 to 1.2 mm h−1 at Site I and from 0.2 to 0.6 mm h−1 at Site II. The extrapolated transpiration values for the rain-free days of the year were 1563 mm and 853 mm for Sites I and II, respectively. Growth in girth was negligible during the premonsoon period. Photo-synthesis was not affected by the minor water stress that developed during the premonsoon period.
Keywords: growth, photosynthesis, stand density, stomatal conductance, transpiration, water relations.
Introduction
The use of eucalypts as plantation trees in the tropics has been criticized on the grounds that they consume water in excessive amounts. During the past decade, therefore, several studies have been undertaken to investigate water use by eucalypts (see reviews by Davidson 1985, Poore and Fries 1985, FAO 1988, Calder 1992, Calder et al. 1992, Calder 1994). In their review, Poore and Fries (1985) concluded that there is no general answer to the question of whether eucalypts consume water in excessive quantities because site conditions, climate and species all influence water use. Thus, there is a need to study the hydrological impact of each eucalypt species under the conditions of interest.
There is little information on water use by eucalypt trees in the humid tropics where annual rainfall is around 3000 mm. There have been several studies on the water relations and water use of eucalypt seedlings in the humid tropics of India
(e.g., Dabral 1970, Rawat et al. 1984, 1985), but the validity of extrapolating the results of these studies to plantation trees has been questioned by Calder et al. (1992).
To gain a better understanding of water use by eucalypt trees in the humid tropics where they have been introduced on a massive scale, we investigated water use by two Eucalyptus tereticornis Sm. stands differing in tree density, which were located in the Kerala State Forest. Eucalyptus tereticornis was chosen for study because it is the major eucalypt species planted in Kerala State, occupying more than 25,000 ha of the total 40,000 ha under eucalypts.
Materials and methods
Site description
Two plantations of E. tereticornis were monitored in different years (1990/91 and 1991/92 for Sites I and II, respectively). Both sites, which are about 250 km apart, are in the Kerala State Forest, in the southwestern part of India. The climate is tropical with annual rainfall ranging from 2500 to 3000 mm, 80% of which is contributed by two monsoons. Site I at Vara-voor in the Trichur Forest Division (10°41′ N, 76°12′ E) had 4-year-old one-stem coppices regenerated from 10-year-old root-stock. Hence the root-stock was 14 years old at the time of the study. Trees at Site II, at Palode in the Trivandrum Forest Division (8°45′ N, 76°59′ E), also had 4-year-old one-stem coppices, however, the coppices had been regenerated after two 10-year rotations. Thus, the root-stocks at Site II were 24 years old. Both sites were situated about 100 m above sea level. Soils at both sites were deep red and lateritic with nearly 1 m of deep loamy soil at the top. At Site I, stand density was 1800 stems ha−1 (original number planted was 2500), whereas at Site II, stand density was 1090 stems ha−1. At both sites, average tree height was 10 m, and average tree diameter varied from 8.5 to 9.0 cm. The canopy in both plantations had not closed at the time of measurement. At both sites, there was a ground cover consisting of several weed species, predomi-nantly Eupatorium sp. and some grasses, most of which dried up during the premonsoon period. This ground cover, along with eucalypt leaf litter, covered the soil in the dry season. There was occasional grazing of the sites by stray cattle.
Water use by
Eucalyptus tereticornis
stands of differing density in
southern India
JOSE KALLARACKAL and C. K. SOMEN
Plant Physiology Division, Kerala Forest Research Institute, Peechi 680653, Kerala, India
Received July 14, 1995
Weather data
Measurements of maximum and minimum temperatures, rain-fall and relative humidity were recorded at weather stations located nearly 5 km from Site I and about 3 km from Site II. The minimum and maximum temperatures recorded at Site I ranged from 20.8 (January 1991) to 38.2 °C (March 1991). Those recorded at Site II ranged from 18.6 (January 1992) to 33.7 °C (March 1992). Annual rainfall is 3210 and 2465 mm at Sites I and II, respectively. At Site I, the South-West Mon-soon predominates and the wettest months are June to August. The North-East Monsoon occurs during September and Octo-ber (Figure 1a). Rainfall is negligible from NovemOcto-ber to May. Site II is also affected by the monsoons in May to August and in October and November. The year may be divided into three periods----the premonsoon period (January--May), the mon-soon period (June--August), and the postmonmon-soon period (Sep-tember--December). Less water is available to plants during the premonsoon period than during the other periods. The pan evapotranspiration rate (PET) recorded for Sites I and II is shown in Figures 1a and 1b.
Microclimate
To monitor the microclimate 2 m above the canopy, a 12-m-high, steel scaffold tower was installed at each site to mount the meteorological equipment. Air temperature (Ta), and
rela-tive humidity (RH) were measured with a shielded thermistor and a carbon-electrode RH chip (Model 207 Temperature and RH probe, Campbell Scientific Inc., Logan, UT). Wind speed (u) was measured with a cup counter anemometer (Model 014A, Met One, Sunnyvale, CA) equipped with a switch closure mechanism. Net radiation (Rn) was measured with a
Fritschen type net radiometer (REBS Inc., Washington, DC). Shortwave radiation (S) was measured with a pyranometer
sensor (Model LI-200S, Li-Cor, Inc., Lincoln, NE). All sensors were connected to a data logger (Model 21X, Campbell Scien-tific Inc.) that recorded measurements at 5-s intervals and stored the hourly averages.
Stomatal conductance (gs)
A steady-state porometer (Model LI-1600, Li-Cor, Inc.) was used to measure stomatal conductance of leaves at ambient relative humidity. The canopy was treated as a two-layered structure, and the vertical depth of each layer was approxi-mately 2 m. At each site, a total of eight well-exposed sample leaves, one from each canopy layer of four trees accessible from the scaffold tower, were sampled hourly from sunrise to sunset. The porometer measurements were made on both the upper and lower sides of the amphistomatous leaves. Stomatal frequency on both sides of the leaf was determined micro-scopically from epidermal imprints of several sample leaves, which were made with transparent adhesive tape. Daily pat-terns of gs were followed on the sampling days shown in
Table 1.
Leaf water potentials (Ψ)
Predawn and midday leaf Ψ were measured on a total of five fully exposed leaves collected from the upper layers of differ-ent trees on the sampling days shown in Table 1. The sample leaves were enclosed in a polyethylene bag just before detach-ing from the plant. We adopted the precautions for sampldetach-ing outlined by Turner (1988). A Scholander-type pressure cham-ber (Soil Moisture Equipment Corp., Santa Barbara, CA) was used to determine the balance pressure, which was assumed to equal leaf Ψ. Pressure-volume curve analyses were performed according to the method of Turner (1988), to determine the
water potential at the turgor loss point on five days during the premonsoon period.
Canopy conductance (gc)
Canopy conductance was calculated as:
gc=ΣgsL, (1)
where L = leaf area index of the conducting surface, and gs is
the mean stomatal conductance. Mean gs was obtained by
sampling eight leaves distributed in different zones of the canopy each hour. As far as possible, the same leaves were measured every hour.
Leaf area index (L)
Leaf area index was measured with an LAI-2000 canopy analyzer (Li-Cor, Inc.). The values given by the instrument were usually modified by masking one or two horizontal angles scanned by the instrument using the C-2000 software program provided by the manufacturer. Sampling dates are shown in Table 1.
Values of L obtained with the LAI-2000 were similar to values estimated from stem girth by means of an allometric equation derived from destructive measurements made in a eucalypt plot close to Site I:
lnL=−5.3938 + 2.1444lngbh, (R2 = 0.89) (2)
where gbh is girth at breast height. Aerodynamic conductance (ga)
Aerodynamic conductance (ga) was calculated as the
recipro-cal of aerodynamic resistance (ra) following the equation of
Monteith (1965): height (m), d = zero plane displacement, calculated as 0.64h (where h = crop height (m)), zo = the roughness length (= 0.13h),
and k = von Karman’s constant (= 0.41).
Canopy transpiration (Et)
Canopy transpiration, Et, was estimated hourly using the
Pen-man-Monteith equation (Monteith 1965):
soil heat flux (which can be ignored for daily calculations), ρ = density of air (kg m−3), cp = the specific heat of air at constant
pressure (J kg−2°C−1), ∆ = the slope of the saturation vapor pressure curve for water (kPa °C−1), D = vapor pressure deficit (kPa), ga = aerodynamic conductance (m s−1), γ =
psychromet-ric constant (kPa °C−1), and gc = canopy conductance (m s−1).
Canopy transpiration was calculated hourly assuming a sin-gle-layer model, because the canopy was relatively open and there was little difference in stomatal conductance among leaves within the canopy. We assumed that the leaves were always at ambient temperature, and used atmospheric D in-stead of leaf to air D.
Growth measurements
Diameter growth of 40 eucalypt trees located around each measurement site was followed by monthly measurements of gbh during the study period.
Results Microclimate
The highest temperature at Site I was over 38 °C, which was reached in March 1991. At Site II, the highest temperature was 33.5 °C, which was reached in April 1992. Values of D re-mained relatively high during January to March at both sites in their respective years (> 2.0 kPa) and D reached 5.0 kPa at Site I. From December 1990 to February 1991, u was high at Site I because of a strong easterly wind. Values of ga, calculated
from the wind speed measurements at both sites, during the premonsoon and postmonsoon months are shown in Figures 2a and 2b. Both S and Rn remained relatively high during the
premonsoon period. Stomatal conductance (gs)
The leaves of E. tereticornis are amphistomatous. Stomatal frequency for samples of fully expanded leaves from both sites was 598 ± 18 mm−2 on the abaxial side and 420 ± 18 mm−2 on the adaxial side. Figure 3a shows consecutive gs measurements
made on the adaxial and abaxial surfaces of the same leaves during the course of a day. Values of gs obtained by sampling
two layers of the canopy were not significantly different (Fig-ure 3b). Hence gc was calculated from gs using Equation 1 and
assuming a single-layer model for the canopy.
Diurnal variations in gs followed a consistent pattern,
in-creasing during the morning until noon and then dein-creasing in the afternoon. At both sites, gs was lower during the
premon-soon months compared with the other periods. We found a
Table 1. Measurement dates at the two experimental sites.
Site I Site II
higher range of gs values during the pre- and postmonsoon
months at Site II than at Site I (Figures 4a and 4b).
Multiple regression analysis indicated that Rn and D were
the most important factors controlling gs. We plotted the gs data
obtained at the two sites against the corresponding D values, ignoring the gs values measured when Rn was less than
300 W m−2, and identified the pre- and postmonsoon values (Figures 5a and 5b). In general, the postmonsoon values of gs
were higher than the premonsoon values at Site I (Figure 5a).
The low gs values during the premonsoon period at low
atmos-pheric demand indicate that Ψ could be an important parame-ter controlling gs. Although there is some scatter at low D in
Figure 5a, the general trend is for gs to remain more or less
unchanged (200 ± 50 mmol m−2 s−1) irrespective of D. This trend was less evident at Site II than at Site I (Figure 5b). In contrast, there is a trend for stomatal closure at high values of D (Figure 5). Predawn Ψ did not decline as much at Site II as at Site I, which probably explains why gs did not differ
signifi-Figure 2. Aerodynamic conduc-tance calculated from wind-speed measurements 2 m above the canopy for the premonsoon (closed symbols) and postmon-soon (open symbols) months at Site I (a) and Site II (b).
Figure 3. Stomatal conduc-tance at Site I on October 9, 1990. (a) Diurnal variations in stomatal conductance on the adaxial (closed symbols) and abaxial (open symbols) sides of the same leaf. Data are shown for two leaves ( , h
cantly between the pre- and postmonsoon periods. At Site II, D rarely went above 3.0 kPa.
Leaf water potentials (Ψ)
At Site I, predawn Ψ reached its lowest value of −0.71 MPa in May 1991, whereas the lowest midday Ψ values (−1.68 MPa) were reached in March and May 1991 (Figures 6a and 6b). The high predawn and midday Ψ values noted in April 1991 were
probably the result of a few summer showers (see Figure 1). At Site II, minimum predawn and midday Ψ values occurred in January 1992 and were −0.34 MPa and −1.40 MPa, respec-tively. Thus, trees at Site II experienced less water stress than trees at Site I. Examination of the seasonal variation in Ψ showed that values were highest during the monsoon and the postmonsoon periods. Analysis of the pressure-volume curves (data not presented) showed that Ψ at turgor loss point was
Figure 4. Diurnal variation in stomatal conductance at Site I (a) and Site II (b). Postmon-soon (open symbols) and pre-monsoon (closed symbols) values are shown. Each point is the mean of individual measurements on a total of eight leaves, four from each layer of the canopy. Lines represent measurements on the dates shown in Table 1.
Figure 5. Relationship be-tween stomatal conductance (gs) and vapor pressure
defi-cit (D) at Site I (a) and Site II (b). Only gs values taken at a
−1.75 MPa, indicating that, at both sites, the trees probably do not reach the turgor loss point at any time during the year. The water tables at both sites, which were monitored at nearby wells, remained between 10 and 15 m deep during the dry season. During the monsoon and postmonsoon periods, the water tables decreased to 2--3 m deep as a result of soil recharge and saturation by the prolonged and heavy monsoon rains.
Leaf area index (L)
Leaf area indices at Sites I and II were 2.17 and 0.60, respec-tively. Seasonal variation in L (data not shown) was negligible at both sites during the study period.
Transpiration
Maximum hourly transpiration, which was observed between 1000 and 1600 h, ranged between 0.4 and 1.2 mm h−1 at Site I, and 0.2 and 0.6 mm h−1 at Site II (Figure 7). At Site I, transpi-ration rates during the pre- and postmonsoon periods were similar, whereas at Site II transpiration rates were generally higher during the premonsoon period than during the postmon-soon period. When the hourly values were accumulated to give daily rates, we obtained values of 3.5 to 7.7 mm day−1 at Site I compared with values of 2.0 to 4.9 mm day−1 at Site II. We extrapolated the daily values to obtain seasonal transpiration values of 729 and 834 mm for the pre- and postmonsoon
Figure 6. Seasonal variations in pre-dawn (open symbols) and midday (closed symbols) leaf water poten-tials at Site I (a) and Site II (b). Each data point is the mean of five meas-urements. Bars indicate ± SE.
periods, respectively, giving a total of 1563 mm at Site I. The corresponding values at Site II were 248 mm and 605 mm, giving a total of 853 mm.
Growth
At both sites, water stress during the premonsoon period re-sulted in a reduction in diameter growth (Figures 8a and 8b). The reduction was less pronounced at Site II than at Site I, probably because of the relatively higher water potentials. We observed the production of new leaves during the premonsoon months, indicating that not all growth was inhibited.
Discussion
Both study sites are located in the foothills of the Western Ghats range of mountains. Rainfall was about 30 percent greater at Site I than at Site II during the periods studied. We also found microclimatic differences in temperature, RH, and vapor pressure deficit between the sites. In addition to different sampling years between sites, these differences are the result of the effects on Site I of the Palghat Gap: a gap in the Western Ghats range of mountains through which the dry winds of the Deccan blow during certain seasons.
Diurnal gs values showed seasonal variation. At both sites,
gs was lower during the premonsoon season than during the
other periods. The lack of significant variations in gs between leaves of the upper and lower canopy layers, which was also observed by Roberts et al. (1992), could be the result of the open nature of the canopy as well as the pendulous leaves of this species. In E. grandis W. Hill ex Maiden., gs varies among
leaves depending on their position in the canopy (Kallarackal 1997).
The poor correlation between gs and D indicates that some
factor(s) beside Rn interacted with D. The importance of plant
water status was demonstrated when the pre- and postmonsoon
values were separated. If leaf water potential was not involved in regulating gs, the gs values observed during the premonsoon
period would have been as high as those observed during the postmonsoon period when atmospheric demand was low. Al-though we did not observe predawn leaf water potentials in-dicative of severe water stress, the sensitivity of gs to Ψ was demonstrated. For example, a predawn Ψ of less than −0.25 MPa altered gs considerably. Davies and Zhang (1991) reported that drying out of part of the root system can affect gs, even though
predawn Ψ remains unchanged. The role of Ψ at Site II was less clear than at Site I, probably because of the milder water stress experienced by trees at Site II (cf. Figure 6b). At both sites, stomatal closure in response to increasing D was appar-ent. In several plant species, gs is well correlated with D (Lange
et al. 1971, Schulze et al. 1972, Calder 1978, Roberts 1978, Colquhoun et al. 1984, Kallarackal 1993). Pereira et al. (1986, 1987) found a close correlation between gs and D in E. globu-lus Labill.; however, the correlation also depended on leaf water potential. Based on our own results and those of other studies, we conclude that, in eucalypts, stomatal conductance is regulated by atmospheric vapor pressure deficit as well as by leaf water potential.
Predawn leaf water potential is a good indicator of the water potential of soil accessed by the roots (Crombie et al. 1988). At both sites, trees had predawn Ψ values greater than −0.25 MPa during the postmonsoon period. From the midday Ψ values, it seems that the turgor loss point was never reached at either site. The Ψ values also showed that trees at Site I were more water stressed than trees at Site II. The relatively high Ψ values of trees at Site II may be associated with the rootstocks being ten years older at Site II than at Site I. It is also possible that the low transpiration rates at Site II, as a result of low L, could have led to higher water potentials. Compared with eucalypts grown in other parts of the world, the Ψ values reported in our study are high (Pereira et al. 1986,
mann and Milburn 1982). Relatively high leaf water potentials are characteristic of several tropical trees (Robichaux et al. 1984). However, Roberts et al. (1992) reported midday water potentials as low as --3.6 MPa in E. tereticornis growing in the neighboring state of Karnataka where the annual rainfall is approximately 800 mm. The relatively high water potentials that we observed during the premonsoon suggest that the tree roots have access to the water table. The water table during the premonsoon period was between 10 and 15 m deep. Several studies in Australia have shown that eucalypt roots sometimes access deep phreatic aquifers (Carbon et al. 1982, Greenwood et al. 1985).
Hourly transpiration at Site I was twice that at Site II. The increase in transpiration rates during the premonsoon period compared to the postmonsoon period at Site II reflects the increased atmospheric demand during this period. Although a much higher atmospheric demand was also observed at Site I during the premonsoon period than during the postmonsoon period, it was not accompanied by an increase in transpiration rate, perhaps because the low leaf water potentials enhanced stomatal closure at Site I, resulting in a reduction in stomatal conductance (Figures 4a and 4b). However, the diurnal pattern of transpiration rate at Site I (Figure 7a) did not follow the diurnal pattern of stomatal conductance (Figure 4a). Likewise, at Site II, transpiration rates were greater during the premon-soon period than during the postmonpremon-soon period, whereas gs
was higher during the postmonsoon period.
Transpiration at Site I was 95% of PET, whereas at Site II it was nearly 60% of PET. If we assume that transpiration during the monsoon months is negligible because of low vapor pres-sure deficit and overcast conditions, the transpiration figures reported here represent nearly 50% of the annual rainfall at both sites. Whitehead and Kelliher (1991) have reported a similar loss in a Pinus radiata D. Don catchment in New Zealand. In Karnataka, India, Calder et al. (1992) and Harding et al. (1992) have reported transpiration losses equal to or slightly less than the annual rainfall. However, annual rainfall reported at these sites was much less than at our study sites. Greenwood et al. (1985) reported that eucalypt evapotranspi-ration can exceed annual rainfall by four times.
Although direct comparison between our experimental sites is not possible because observations were made in different years, it is likely that the lower density of trees at Site II, which resulted in lower L, was mainly responsible for the lower transpiration rates at this site compared with those at Site I. Nevertheless, it is difficult to conclude that spacing is the sole reason for the difference in transpiration rates between the sites, because rainfall and atmospheric demand were much higher at Site I than at Site II. However, some recent studies in eucalypt catchments have shown that increased spacing and thinning can increase water yield by reducing transpiration (ICAR 1987, Stoneman and Schofield 1989, Cornish 1993).
In conclusion, we observed differences in water use charac-teristics of E. tereticornis at two sites with different climatic and silvicultural conditions. Although direct comparisons be-tween the sites was not possible, we obtained circumstantial evidence of lower water use in the more widely spaced
planta-tion. Stomatal conductance was regulated by atmopheric vapor pressure deficit as well as by leaf water potential. Even when roots were subjected to only minor water stress, stomatal conductance was considerably affected. Although transpira-tion by the more widely spaced plantatranspira-tion was only half that of the more closely spaced plantation, growth was affected during the premonsoon period depending on the magnitude of water stress. These results have implications for forest man-agement programs in the tropics where water conservation is an important issue during afforestation.
Acknowledgments
We are grateful to Dr. Ian Calder, Institute of Hydrology, Wallingford, UK who advised us on methodology during the early stages of this work. We thank the Ministry of Environment and Forests, Government of India for financial assistance.
References
Calder, I.R. 1978. Transpiration observations from a spruce forest and comparisons with predictions from an evaporation model. J. Hy-drol. 18:33--47.
Calder, I.R. 1992. Water use of eucalypts----a review. In Growth and Water Use of Forest Plantations. Eds. I.R. Calder, R.L. Hall and P.G. Adlard. John Wiley and Sons, Chichester, England, pp 167--175. Calder, I.R., R.L. Hall and P.G. Adlard. 1992. Growth and water use
of forest plantations. John Wiley and Sons, Chichester, England, 381 p.
Calder, I.R., M.H. Swaminath, G.S. Kariyappa, N.V. Srinivasalu, K.V. Srinivasa Murthy and J. Mumtaz. 1992. Deuterium tracing for the estimation of transpiration from trees. Part 3. Meaurement of tran-spiration from Eucalyptus plantation. India. J. Hydrol. 130:37--48. Calder, I.R. 1994. Eucalyptus, water and sustainability. ODA Forestry
Series No. 6. Instititute of Hydrology, Wallingford, Oxon, 14 p. Carbon, B.A., F.J. Roberts, P.F. Farringron and J.D. Beresford. 1982.
Deep drainage and water use of forests and pastures grown on deep sands in a Mediterranean environment. J. Hydrol. 55:53--64. Colquhoun, I.J., R.W. Ridge, D.T. Bell, W.A. Loneragan and J. Kuo.
1984. Comparative studies in selected species of Eucalyptus used in rehabilitation of the northern jarrah forest, Western Australia. I. Patterns of xylem pressure potential and diffusive resistance of leaves. Aust. J. Bot. 32:367--373.
Cornish, P.M. 1993. The effects of logging and forest regeneration on water yields in a moist eucalypt forest in New South Wales, Aust. J. Hydrol. 150:301--322.
Crombie, D.S., J.T. Tipett and T.C. Hill. 1988. Dawn water potential and root depth of trees and understory species in south western Australia. Aust. J. Bot. 36:621--631.
Dabral, B.G. 1970. Preliminary observation in potential water require-ment in Pinus roxburghii, Eucalyptus citridora, Populus casala and
Dalbergia latifolia. Indian For. 96:775--780.
Davidson, J. 1985. Setting aside the idea that eucalypts are always bad. FAO Working Paper No. 10, FAO, Rome.
Davies, W.J. and J. Zhang. 1991. Root signals and the regulation of growth and development of plants in drying soil. Annu. Rev. Plant Physiol. 42:55--76.
Greenwood, E.A.N., L. Klein, J.D. Beresford and G.D. Watson. 1985. Differences in annual evaporation between graded pasture and Eucalyptus species in plantations on a saline farm catchment. J. Hydrol. 78:261--278.
Harding, R.J., R.L. Hall, M.H. Swaminath and K.V. Srinivasa Mur-thy. 1992. The soil moisture regimes beneath forest and an agricul-tural crop in southern India----measurements and modelling. In
Growth and Water Use of Forest Plantations. Eds. I.R. Calder, R.L. Hall and P.G. Adlard. John Wiley and Sons, Chichester, England, pp 244--269.
ICAR (Indian Council of Agricultural Research). 1987. Effect of bluegum plantation on water yield in Nilgiri hills. Bulletin No. T-18.0-3. Central Soil and Water Conservation Research and Train-ing Inst. Research Centre, Udhagamandalam, 67 p.
Kallarackal, J. 1993. Studies on water use, photosynthesis and growth of eucalypts. KFRI Research Report No. 88, KFRI, Peechi, Kerala, 85 p.
Kallarackal, J. and C.K. Somen. 1997. An ecophysiological evaluation of the suitability of Eucalyptus grandis for planting in the tropics. For. Ecol. Manage. In press.
Lange, O.L., R. Losch, E.-D. Schulze and L. Kappen. 1971. Response of stomata to changes in humidity. Planta 100:76--86.
Monteith, J.L. 1965. Evaporation and environment. In The State and Movement of Water in Living Organisms. Ed. G.A. Fogg. Symp. Soc. Exp. Biol. No. 19. Academic Press, London, pp 205--234. Pereira, J.S., J.D. Tenhunen, O.L. Lange, W. Beyschlag, A. Meyes and
M.M. David. 1986. Seasonal and diurnal patterns in leaf gas ex-change of E. globulus trees growing in Portugal. Can. J. For. Res. 16:177--184.
Pereira, J.S., J.D. Tenhunen and O.L. Lange. 1987. Stomatal control of photosynthesis of Eucalyptus globulus Labill. trees under field conditions in Portugal. J. Exp. Bot. 38:1678--1688.
Poore, M.E.D. and C. Fries. 1985. The ecological effects of eucalylp-tus. FAO Forestry Paper No. 59. FAO, Rome, 87 p.
Rawat, P.S., B.B. Gupta and J.S. Rawat. 1984. Transpiration as af-fected by soil moisture in Eucalyptus tereticornis seedlings. Indian For. 110:35--39.
Rawat, P.S., B.B Gupta and J.S. Rawat. 1985. Transpiration, stomatal behaviour and growth of Eucalyptus hybrid seedlings under differ-ent soil moisture levels. Indian For. 111:1095--1110.
Roberts, J.M. 1978. The use of the ‘‘tree cutting’’ techniques in the study of the water relations of Norway spruce (Picea abies (L.) Karst). J. Expt. Bot. 29:465--471.
Roberts, J.M., P.T.W. Rosier and K.V. Srinivasa Murthy. 1992. Physi-ological studies in young Eucalyptus stands in southern India and their use in estimating forest transpiration. In Growth and Water Use of Forest Plantations. Eds. I.R. Calder, R.L. Hall and P.G. Adlard. John Wiley and Sons, Chichester, England, pp 226--243. Robichaux, R.H., P.W. Rundel, L. Stemmermann, J.E. Candfield, S.R.
Morse and W.E. Friedman. 1984. Tissue water deficits and plant growth in wet tropical environments. In Physiological Ecology of Plants of the Wet Tropics. Eds. E. Medina, H.A. Mooney and C. Vàzquez-Yanes. Dr. W. Junk Publishers, The Hague, pp 99--112. Schulze, E.-D., O.L. Lange, U. Buschbom, L. Kappen and M. Evenari. 1972. Stomatal responses to changes in humidity in plants growing in the desert. Planta 108:259--270.
Stoneman, G.L. and N.J. Schofield. 1989. Silviculture for water pro-duction in jarrah forest of Western Australia: an evaluation. For. Ecol. Manage. 27:273--293.
Turner, N.C. 1988. Measurement of plant water status by the pressure chamber technique. Irrig. Sci. 9:289--308.
Whitehead, D. and F.M. Kelliher. 1991. Modeling the water balance of a small Pinus radiata catchment. Tree Physiol. 9:17--33.
