2 RESOURCE ALLOCATION PARAMETERS
Chapter 5: Validation of the Structural Behaviour of the Single Ramet Model
1.4 Growth response to defoliation
the plant over the life of the plant. The seasonal accumulation of carbohydrate production and carbohydrate demand generated by the model indicates that although total photosynthate production exceeds total demands, photosynthate production in the second season is less than carbohydrate demand during that season (indicated by the dip in accumulated carbohydrate in Figure 5.8). This means that model growth in the second spring relies on a substantial amount of carbohydrate generated in the first season, which is unrealistic given the small amount of storage in over-wintering tillers. This indicates that the present fixed ideal root-to-shoot ratio, awllbi,t, of 3.2 cannot be supported by the single- tiller ramet. The next chapter concentrates on resource allocation when storage is limited.
Photosynthetic production during the second spring is restricted by the limited photosynthetic surface constructed during the second season and the reduction of photosynthetic production due to light inhibition by the surrounding canopy (Figure 5.9).
Presently leaf area index (LAI) is set equal to the sum of BAI and SAI only. This is because internodes of Themeda triandra are brown and as such are assumed to be non- photosynthetic. Therefore they remove an insignificant amount of the photosynthetically active radiation. The peak leaf area index is achieved by a tiller during its vegetative growth phase due to the large surface area of leaf blades.
The effect of LAI above each layer on the photosynthetic rate within that layer, Fp(LAI), shows the clear inverse relationship with LAI (Figure 5.10). The net effect on photosynthetic efficiency of photosynthetic tissues is indicated (Figure 5.11) showing that photosynthesis is strongly impacted during the second spring, both during regrowth after winter and during senescence after flowering. The restriction in early spring arises because all blades and sheathes die back over winter so new phytomers have to expand from the apical bud in the second spring. The apical meristem remains close to the ground through the early part of the second growing season so new blades and sheaths start growing below most of the aerial plant matter and experience strong light restriction.
101 201 301 401 501 601 701 801 Iteration (days)
0-100
> 1 0 0 - 2 0 0
>200 - 300
> 3 0 0 - 4 0 0
> 4 0 0 - 5 0 0
> 5 0 0 - 6 0 0
> 6 0 0 - 7 0 0
> 7 0 0 - 8 0 0
> 8 0 0 - 9 0 0
>900
Figure 5.9 Leaf area index (LAI) in different height layers as they change over time for a single-tiller ramet. (Environmental conditions: Plant activity during the growing and non-growing season as defined in Section 4.3 of Chapter 4. Growing season proceeds from Yearday 1 to Yearday 242 of each year. Soil nitrogen is not limiting to growth.)
1.2
0.4 0.2
1 1 1 ; 1 1 1 1 —
1 101 201 301 401 501 601 701 801
Iteration (days)
0-100
>100-200
>200 - 300
>300 - 400
>400-500
>500-600
>600-700
•>700-800
>800-900
>900
Figure 5.10 Photosynthetic leaf area index multiplier (FP(LAI)) in each height layer on a single-tiller ramet. The FP(LAI) represents the level to which light is dissipated by intervening plant material in the shoot canopy, and therefore is inversely proportional to the LAI at any given height in the canopy. (Environmental conditions: Plant activity during the growing and non- growing season as defined in Section 4.3 of Chapter 4. Growing season proceeds from Yearday 1 to Yearday 242 of each year. Soil nitrogen is not limiting to growth.)
09
~0> 0.45
^ • ^ -
CO CO
the
c
5* CO O O
CO CO CO
b
CD 3 CO CO*-*
o
• 4 - ^
JC CD
Net 4—» syn
o
o4
0.4 0.35
0.3 0.25
0.2 0.15
0.1 0.05
0
101 201 301 401 Iteration (days)
501
Figure 5.11 Efficiency of photosynthetic tissues (blades and sheaths) on a single-tiller ramet.
(Environmental conditions: Plant activity during the growing and non-growing season as defined in Section 4.3 of Chapter 4. Growing season proceeds from Yearday 1 to Yearday 242 of each year. Soil nitrogen is not limiting to growth.)
and blade material below the burn height. Live stem material below the burn height is not destroyed. Cutting is used to demonstrate the response of the model to defoliation.
For the simulation two cuts were applied annually on day 86 and day 184 at a height of 60 mm for both events. Total masses of individual organs (Figure 5.12), summed live masses of organ types (Figure 5.13) and shoot mass (Figure 5.14) were reduced abruptly by each defoliation event. The imbalance in the actual root-to-shoot ratio induces root dieback, which releases additional carbohydrate for allocation to shoot growth. The defoliation event also removes overlying dead material, which allows more light to penetrate to the new phytomers. This increases the photosynthetic efficiency of the remaining leaf surface after each defoliation event. The first defoliation event in the second year of growth (iteration 451) removed all live blade and sheath material and removed the stem to a height of 60 mm above the ground as specified for this simulation. This decapitated the apical meristem and forced the tiller into Stasis (Phenophase 3). Death followed rapidly and the simulation was completed more quickly (less iterations) because the amount of material available to decay was much reduced by the defoliation events.
2 COMPARISON OF SIMULATED DATA WITH EMPIRICAL DATA UNDER NATURAL ENVIRONMENTAL CONDITIONS
In order to demonstrate that the model predicts the growth of Themeda triandra tillers with sufficient accuracy, it is necessary to compare the behaviour of tillers generated by the TILLERTREE model to T. triandra tillers growing under natural conditions. The data of an ontogenetic study of individual tiller growth of T. triandra in response to different defoliation treatments by Tainton & Booysen (1965) is suitable for such an analysis. This study was conducted at Ukulinga, so the same T. triandra ecotype has been used to parameterise the model (Chapter 4) as was measured by Tainton & Booysen (1965), making the data directly comparable.
At this point it is necessary to point out that the extension growth of individual organs responds to the incident R:FR ratio of the local environment. Low R:FR stimulates organs to extend to greater potential lengths, while high R:FR stimulates tillers to grow to minimal potential lengths. Therefore individual tiller size is partly determined by the local light environment. In this section the model's performance is tested against the average estimates recorded by Tainton & Booysen (1965) for tillers recruited at the start of spring
101 201 301 401 Iteration (days)
501 601
D)
S E°
Dtal
c m rn
0.016 - 0.014 - 0.012 - 0.01 - 0.008- 0.006- 0.004- 0.002-
101 201 301 401 Iteration (days)
0.035
101 201 301 401
Iteration (days)
501
§ 8 S8
E
I
s
lower
U -
0.03 0.025 0.02 0.015 0.01 0.005 0
101 201 301 401 Iteration (days)
501
Figure 5.12 Total organ masses on a single-tiller ramet subjected to cutting defoliation. Treatment: cut height = 60 mm, applied annually on day 86 and day 184. The R:S ratio is fixed at 3.2 at all times. (Environmental conditions: Plant activity during the growing and non-growing season as defined in Section 4.3 of Chapter 4. Growing season proceeds from Yearday 1 to Yearday 242 of each year. Soil nitrogen is not limiting to growth.)
0.5
V) CO CO
E
ive
c
CO
org
1
EE 3
CO
0.4 0.3 0.2 0.1 0
101 201 301 401 Iteration (days)
501
Figure 5.13 Summed live mass of organ ( blade mass; sheath mass; — • — internode mass; flower mass) of a single-tiller ramet subjected to a cutting treatment.
Treatment: cut height = 60 mm, applied annually on day 86 and day 184. Carbohydrate storage is restricted while N is not restricted. The R:S ratio is fixed at 3.2 at all times. (Environmental conditions: Plant activity during the growing and non-growing season as defined in Section 4.3 of Chapter 4. Growing season proceeds from Yearday 1 to Yearday 242 of each year. Soil nitrogen is not limiting to growth.)
101 201 301 401
Iteration (days)
501
Figure 5.14 Change in live root mass ( ) and live shoot mass ( ) of a single- tiller ramet subjected to a cutting treatment. Treatment: cut height = 60 mm, applied annually on day 86 and day 184. Carbohydrate storage is restricted while N is not restricted. The R:S ratio is fixed at 3.2 at all times. (Environmental conditions: Plant activity during the growing and non- growing season as defined in Section 4.3 of Chapter 4. Growing season proceeds from Yearday 1 to Yearday 242 of each year. Soil nitrogen is not limiting to growth.)
that grew in a sward that was only defoliated just before the tillers were initiated and never again during the development of the tillers. Data collected at Ukulinga for the present study included an identical treatment, called the No Defoliation treatment (Appendix Al).
Therefore organ extension data obtained from this treatment, presented in Table 4.1 in Chapter 4 is used for the simulations that follow to improve the comparison of the simulated data and the empirical data of Tainton & Booysen (1965).
In order to compare the simulated data to the empirical data, it is necessary to include the environmental variables that restrict growth. Therefore the functions that restrict growth in response to average air temperature, fg(TaVe), and soil water pressure, fg(4/s) as presented in Chapter 4, are included in the model for this simulation. Data for Ts and Tave is supplied for the years 1961 to 1963, to match the field data collected by Tainton & Booysen (1965). Environmental data (average temperature, minimum temperature, soil water potential) for the Ukulinga site for the years when Tainton & Booysen (1965) conducted their study was obtained using the ACRU Agrohydrological Model (Schulze 1995).
The data set collected by Tainton & Booysen (1965) contains a limited number of parameters. Importantly, all organ mass measurements were taken as tiller total (live plus dead) mass. Also the data set did not divide leaves into sheaths and blades, but rather treated them as one component. Summed leaf parameters are easily calculated from the model.
Stem material is live throughout the growing phase of the tiller, so this parameter can be compared directly. Most important in the validation process is comparison of the temporal distribution of growth processes and how closely the model follows the empirical data. It is anticipated that there will be some discrepancy between masses simulated during the reproductive phase and the empirical data because the data set used for the organ parameters was calculated from tillers that were larger than those measured by Tainton & Booysen (1965) (see Appendix Al). Reference will be made to the data set in Appendix Al when differences arise in order to validate some results predicted by the model that appear to be different to the data collected by Tainton & Booysen (1965).
The decay rate parameter, RDecay, was an unknown parameter. Therefore a number of simulations were conducted with different values for RDecay in order to establish a value that generated similar data to that of Tainton & Booysen (1965). It was concluded that a value of 0.012 g"1 g"1 d"1 gave a similar representation to the data presented by Tainton &
Booysen (1965) and this was used as a measure for the decay of Themeda triandra in the model. The growth of a single tiller ramet was then simulated again for comparison with the
empirical data of Tainton & Booysen (1965). No other parameter or growth pattern was tweaked for this simulation.
The simulated data is compared to the empirical data in two ways. Firstly the simulated data and empirical data are compared non-rigorously, using maximum values for certain seasonal parameters. Secondly the simulated data and empirical data are compared rigorously by graphical comparison of the seasonal growth behaviour visually and by using a formal statistical analysis of goodness-of-fit.