No abstract

The photosynthesis and transpiration rates of winter wheat leaves were measured at different developmental stages and at various levels of mildew infection. Even at low levels of infection, circa 40Jo leaf area being covered, both the assimilation and transpiration rates at light satiation were considerably reduced. Light use efficiency and dark respiration were not significantly affected. The physiological background of these effects was analysed by photosynthesis measurements at different external COz concentrations. It was shown that the carboxylation resistance was increased due to mildew infection and that stomatal resistance, boundary layer resistance and transport resistance in the mesophyll were only indirectly affected.A simulation model was used to compute the consequences of these effects on daily growth rate of a winter wheat canopy at different development stages and with different levels of mildew severity. These computations show that a low infection level may result in a considerable reduction of the crop growth rate. This effect was more pronounced when the sky was clear than overcast.

Recruitment algorithms in forest gap models are examined with particular regard to their suitability for simulating forest ecosystem responses to a changing climate. The traditional formulation of recruitment is found limiting in three areas. First, the aggregation of different regeneration stages (seed production, dispersal, storage, germination and seedling establishment) is likely to result in less accurate predictions of responses as compared to treating each stage separately. Second, the related assumptions that seeds of all species are uniformly available and that environmental conditions are homogeneous, are likely to cause overestimates of future species diversity and forest migration rates. Third, interactions between herbivores (ungulates and insect pests) and forest vegetation are a big unknown with potentially serious impacts in many regions. Possible strategies for developing better gap model representations for the climate-sensitive aspects of each of these key areas are discussed. A working example of a relatively new model that addresses some of these limitations is also presented for each case. We conclude that better models of regeneration processes are desirable for predicting effects of climate change, but that it is presently impossible to determine what improvements can be expected without carrying out rigorous tests for each new formulation.

No abstract

The AFRC model describes the phenology of the wheat plant in relation to its environment in terms of the effect of temp., vernalization and photoperiod on development. In 1976-79 four sowing date experiments were carried out in which the developmental stages were scored at regular time intervals. Two of these experiments, containing 35 sowings of 1 cv. over 2 seasons, were used to evaluate and improve the model. Variation in prediction of anthesis was used as a measure of the 'success' of the model. The unchanged AFRC model predicted anthesis with a coeff. of variation of 5.0%. After several parameter changes this was reduced to 2.8%. When the adapted model was used to predict all developmental stages scored in all 4 experiments, it gave good results for anthesis and maturity. Coeff. of variation were high for stages before stem elongation. Exclusion of extreme sowing dates improved the results of the model. (Abstract retrieved from CAB Abstracts by CABI’s permission)