Chapter 6
Climatic adaptability and yield potentials

Understanding the relationships between the climatic environment and ecophysiological processes of growth, development and yield in trees forms the basis of formulating quantitative descriptions of the climatic adaptability of improved and unimproved provenances and their productivity potentials in land use. Principles of climatic adaptability for plants are described in Kassam, Kowal and Sarraf (1977) and (FAO 1978–81).

Photosynthesis produces the source of a assimilates which plants use for growth, and the rate of photosynthesis is influenced by both temperature and radiation. However, plants are also obliged to undergo sequences of phenological and morphological developments in time and space to allow photosynthetic assimilates to be converted into growth of plant parts and economically useful yields of satisfactory quantity and quality. The development sequence of tree growth in relation to the calendar (i.e. tree phenology) is influenced by climatic factors.

In general, temperature determines the rate of growth and development of plant parts and the tree as a whole. However, in some tree species, temperature may also determine whether a particular development process will begin or not (e.g., chilling requirement for bud formation and floral initiation), the time when bud break will occur, the subsequent rate of development and the time when the process will stop (Cannell and Last 1976).

In the seasonally dry climates of Kenya, ability to survive the dry period is an important adaptability characteristic just as frost hardiness is for survival in the cooler thermal zones at higher altitudes.

Accordingly, in assessments of land suitabilities, consideration has to be given to the specific climatic requirements and adaptability for survival, growth and development.

6.1 PHOTOSYNTHESIS CHARACTERISTICS

All the fuelwood species listed in Table 4.1 have C3 photosynthesis pathway and are classified into two adaptability groups (Table 6.1). Group I species are adapted to operate in cooler conditions (mean temperatures 10–20 °C), whereas Group II species are adapted to operate in warmer conditions (mean temperatures 20–30 °C). Both groups have species with nitrogen fixing capability.

Rates of maximum photosynthesis (Pm) for both adaptability groups are in the range 5–30 kg CH₂O ha^-1 hr^-1 (Landsberg 1986). Species in each adaptability group are therefore further classified into three photosynthesis productivity classes. They are class A, Pm = 5–10 kg CH₂O ha^-1 hr^-1; class B, Pm = 10–20 kg CH₂O ha^-1 hr^-1; and class C, Pm = 20–30 kg CH₂O ha^-1 hr^-1. These photosynthesis rates of productivity class A, B, and C correspond to mean annual total (including foliage, stem and roots) biomass increments of 12.5–25.0, 25.0–40.0 and 40.0–60, 0 t/ha dry weight respectively or annual wood biomass (stem and branch wood) increments of 8.0–15.0, 15.0–25.0 and 25.0–40.0 t/ha dry weight respectively. The relationships between photosynthesis and temperature for these six adaptability classes are presented in Table 6.2.

TABLE 6.1
Adaptability groups for fuelwood species

(N) - Nitrogen fixer.

TABLE 6.2
Relationships between temperature and rate of photosynthesis (kg CH₂O ha^-1 hr^-1) for six adaptability classes of fuelwood species

TABLE 6.3
Rotation length (years) by moisture zones

6.2 ROTATION LENGTH

Rotation length in the model is taken as the age at ‘maximum yield’, and is the point when annual increment is equal to mean annual increment over the total period since establishment (Nilsson 1983).

Rotation length is affected by the photosynthesis productivity class of the species and by length of growing period. Rotation lengths applied in the model are given in Table 6.3.

6.3 CLIMATIC YIELD POTENTIALS

Thermal zone ratings for each of the species are given in Table 6.4. Five suitability classes are employed (i.e., S1, very suitable; S2, suitable; S3, moderately suitable; S4, marginally suitable; and N, not suitable), and the ratings apply to production at all the three levels of inputs.

A rating of S1 indicates that the temperature conditions for tree growth and development are optimal, and that it is possible to achieve the maximum attainable silvicultural yield potential provided there are no moisture or soil-landform limitations. A rating of S2 indicates that there are moderate temperature constraints to growth and development, and that there would be a suppression of yield potential of the order of 25%. A rating of S2 indicates that there are moderate to severe temperature constraints, and that there would be a yield suppression of the order of 50%. A rating of S4 indicates that yield suppression of the order of 75%. A rating of N indicates that temperature conditions are not suitable for production.

Growing period zones which have been considered for yield assessments for each species are shown in Table 6.5 which represents a moisture screen. Table 6.5 is based on the actual research information obtained from local experiments and permanent sample plots.

TABLE 6.4
Thermal zone suitability ratings for fuelwood species

Ecophysiological models have not been widely applied in the estimation of stand and site productivity potentials. A useful description of the state-of-the art is given in Landsberg (1986). Potential attainable yields (total and wood biomass) were derived according to the method developed by the FAO-AEZ Project (Kassam 1977; FAO 1978), and modified to take into account the generally accepted fact that for fuelwood tree species, total biomass yield at 50% of rotation length is 38% of the standing total biomass yield at 100% rotation length (Nilsson 1983).

It is assumed that wood biomass (stem wood and branch wood) is 0.6 of total biomass, foliage biomass 0.2 and root biomass 0.2. Partitioning of total wood biomass into main stem and branch wood biomass is assumed to be in the ratio of 0.8 and 0.2. Leaf area index at maximum annual growth rate is assumed to be 5 or more, and the period of annual growth is equal to the inventoried lengths of growing period. These reference model variables can be modified as appropriate to take into account differences between species and environmental conditions.

The following is an example of biomass calculation for Eucalyptus camaldulensis(adaptability group II, class C). The methodology for the calculation of net biomass and constraint-free yields by suitable thermal zone is derived from Kassam (1977), and is presented in this section.

TABLE 6.5
Moisture screen for fuelwood species

TABLE 6.6
Photosynthetically active radiation on very clear days (ac) (in cal cm^-2 day^-1) and the daily gross photosynthesis rate of standard vegetation canopies on very clear (be) and overcast (bo) days (in kg CH₂, o ha^-1 day^-1) for Pm = 20 kg CH₂O kg ha^-1 hr^-1) (from de Wit 1965)

Net annual total biomass increment at 50% rotation length (Ba) is calculated from the equation:

where;

Assuming that standing biomass at 50 % rotation length is 38 % of the standing biomass at 100% of the rotation length (Br), Br is calculated from the equation:

where:

R = rotation length (years).

Net mean annual total biomass (t/ha) increment (Bm) is calculated from the equation:

Constraint-free mean annual wood biomass yield increment (Bw) is calculated from (Bm) from the equation:

where:

Hi = Harvest index (proportion of the net total biomass of the species that is stem and branch wood).:

The maximum rate of gross biomass production (bgm) is dependent on the maximum rate of photosynthesis (Pm) which is dependent on temperature. Maximum rates of photosynthesis (Pm) for Eucalyptus camaldulensis(group II, class C) by temperature are presented in Table 6.2.

For Pm = 20 CH₂O kg ha^-1 hr^-1 and LAI of 5, bgm is calculated from the equation:

where:

When Pm is greater than 20 kg CH₂O ha^-1 hr^-1, bgm is given by the equation:

When Pm is less than 20 kg ha^-1 hr^-1, bgm is given by the equation:

Net biomass and yield calculations for Eucalyptus camaldulensis for Lamu is presented below.

Climate:

Station: Lamu, Kenya
Location : 2° 16' S and 40° 54' E
Altitude : 30 m
Length of growing period : 140 days
Start growing period : 5 April
End growing period : 25 August
Average radiation (Rg) : 471 cal cm^-2 day
Average day-time temperature : 26.5 °C
Average 24hr mean temperature : 25.3 °C

Fuelwood species:

Species: Eucalyptus camaldulensis
Rotation length: 8.3 years (from Table 6.3)
Leaf area index at maximum growth rate : 3.5
Harvest index (stem/branch wood proportion): 0.6
Species adaptability : Photosynthesis pathway C3, Group II
Productivity class: C

Calculation of daily rate of gross biomass production (bgm):

Photosynthesis rate Pm at 26.5 °C : 25 kg CH₂O ha^-1 hr^-1.

Difference in Pm relative to Pm = 20 kg CH₂O ha^-1 hr^-1: +25%.

Average photosynthetically active radiation on clear days
(Ac) : 351 cal cm^-2 day^-1 (Table 6.6).

Fraction of the day-time when the sky is overcast
(F) : 0.41 (from equation F = (Ac - 0.5Rg)/0.8Ac).

Average rate of gross biomass production for perfectly clear days at
Pm = 20 kg CH₂O ha^-1 hr^-1 (be) : 418 kg CH₂O ha^-1 hr^-1 (Table 6.6).

Average rate of gross biomass production for totally overcast days at
Pm = 20 kg CH₂O ha^-1 hr^-1 (bo) : 222 kg CH₂O ha^-1 hr^-1 (Table 6.6>.

Rate of gross biomass production at Pm = 20 CH₂O kg ha^-1 hr^-1 at LAI of 5 : 338 kg CH₂O ha^-1 day^-1 (from equation 6.5).

Rate of gross biomass production at Pm = 25 kg CH₂O ha^-1 hr^-1 at LAI of 5 (bgm) : 373 kg CH₂O ha^-1 day^-1 (from equations 6.5 and 6.6) or 317 kg CH₂O ha^-1 day^-1 at LAI of 3.5.

Calculation of annual total net biomass production (Ba) and total biomass at rotation length (Br):

Maintenance respiration coefficient at 30 °C : 0.0108 (for non-legume species).

Maintenance respiration coefficient at 25.3 (Ct) °C :
0.0070 (from equation C_t = C₃₀ (0.0044+0.0019T+0.0010T²).

Ba = 25.3 t/ha (from equation 6.1).

Br = 170.2 t/ha (from equation 6.2).

Calculation of mean annual total biomass increment (Bm) and mean annual wood biomass yield increment (Bw):

Bm = 170.2/8.3 = 20.5 t/ha (from equation 6.3).

Bw = 0.6×20.5 = 12.3 t/ha (from equation 6.4).

Total biomass productivity estimates (Bm) in terms of mean annual increments (t/ha dry weight) are given in Table 6.7 for high level of inputs by length of growing period for species with and without nitrogen fixing ability for lie three photosynthesis productivity classes. For the low level of inputs circumstance, site yield potentials are assumed to be 50% of those at the high level of inputs. At intermediate level of inputs, yield potentials are assumed to be half-way between the low and the high levels of inputs. Total biomass productivity for intermediate and low levels of inputs are given in Tables 6.8 and 6.9 respectively. Wood biomass yield estimates (Bw) in terms of mean annual increments (t/ha dry weight) are given in Tables 6.10, 6.11 and 6.12 respectively for high, intermediate and low levels inputs circumstances. Wood biomass estimates in the growing period zones allowable by the moisture screen in Table 6.5 are presented in the Appendix in Tables A6.1, A6.2 and A6.3 for high, intermediate and low levels of inputs respectively.

All tree species are matched to total lengths of L1, L2, L3 and L4. Yields in Tables 6.7 to 6.12 apply to years with normal length of growing period, i.e. growing period with a humid period during which precipitation is greater than potential evapotranspiration. For years with intermediate growing periods, i.e. growing periods with no humid period, full water requirements cannot be met and yield reductions are assumed to be of the order of 50% on all soils except Fluvisols and Gleysols. The percentage of occurrence of intermediate lengths of growing periods in all LGP-Pattern zones is 100% in LGP zone 1–29 days; 65% in LGP zone 30–59 days; 25% in LGP zone 60–89 days; 10% in LGP zone 90–119 days; and 5% in LGP zone 120–149 days.

At this stage in the model development, it has not been possible to take into account in the climatic suitability assessment other climatically driven constraints such as pests and diseases and workability, which may reduce yield. It should be possible to take such constraints into account in the future as the information and research base for fuelwood production improves.

An exception to the general methodology for climatic suitability assessment applies to areas occupied by Fluvisols because the length of growing period does not fully reflect their particular circumstance with regards to moisture regime.

Land use on Fluvisols is generally governed by the depth, intensity and duration of flooding which occurs in the low lying areas of these soils. These flooding attributes are generally controlled not by the amount of ‘on site’ rainfall but by external factors such as river flood regime, hydrological features of catchment area and catchment-site relationships.

Fluvisols ratings are presented in Table 6.13 for the three levels of inputs using seven suitability ratings, S1, S2, S3, S4, S5, S6 and N, corresponding to potential biomass yield suppressions of zero, 25%, 50%, 75%, 90%, 95% and 100% respectively.

TABLE 6.7
Total biomass yield potential (Bm) without constraints (mean annual increment in t/ha dry weight) at high level of inputs

Pm - maximum photosynthesis rate in kg CH₂O ha^-1 hr^-1

TABLE 6.8
Total biomass yield potential (Bm) without constraints (mean annual increment in t/ha dry weight) at intermediate level of inputs

Pm - maximum photosynthesis rate in kg CH₂O ha^-1 hr ^-1

TABLE 6.9
Total biomass yield potential (Bm) without constraints (mean annual increment in t/ha dry weight) at low level of inputs

Pm - maximum photosynthesis rate in kg CH₂O ha^-1 hr^-1

TABLE 6.10
Wood biomass yield potential (Bm) without constraints (mean annual increment in t/ha dry weight) at high level of inputs

Pm - maximum photosynthesis rate in kg CH₂O ha^-1 hr ^-1

TABLE 6.11
Wood biomass yield potential (Bm) without constraints (mean annual increment in t/ha dry weight) at intermediate level of inputs

Pm - maximum photosynthesis rate in kg CH₂O ha^-1 hr^-1

TABLE 6.12
Wood biomass yield potential (Bm) without constraints (mean annual increment in t/ha dry weight) at low level of inputs

Pm - maximum photosynthesis rate in kg CH₂O ha^-1hr ^-1

TABLE 6.13
Fluvisols ratings

¹ Yields (t/ha) refer to high level inputs circumstances for the intermediate and low level inputs circumstances reference maximum yields are 75% and 50% respectively of yields at the high level.

TABLE 7.1
Soil units