A MODEL OF DRY-MATTER PARTITIONING IN TREES

In mechanistic growth models, the description of assimilate allocation or dry matter partitioning plays a key role. Although theoretical con cepts of allocation exist, they include many parameters that cannot be quantified. Therefore, many growth models use descriptive keys that r epresent the proportions of dry matter or carbohydrates assigned to ea ch plant component. I have developed a model to describe the dynamic p artitioning of dry matter in individual trees, and used it to investig ate the effects of growth conditions on the partitioning pattern in Do uglas-fir (Pseudotsuga menziesii (Mirb.) France) and beech (Fagus sylv atica L.). The model estimates the fractions of total available dry ma tter that should go to certain plant parts, based on the concept of st ructural balances. Both mechanistic and allometric relationships betwe en tree components are used to model conditions for the dynamic distri bution of dry matter. The model was to used to estimate the effects of dominance position, site conditions, and thinning on growth partition ing. The fractions of the annual current increment of total dry matter gradually changed with tree age, but the changes were relatively smal l, especially after age 20. Compared with beech, Douglas-fir invested more dry matter in foliage, especially at the cost of the branch and s tem components. Trees of average basal area invested more dry matter i n branches and less in stem than suppressed trees, and their estimated increase in stem diameter over time generally fitted the yield table data well. Stem diameter development was underestimated at higher ages only in the case of a Douglas-fir tree of average basal area on a poo r site. Over time, the proportion of standing biomass in foliage and f ine root fractions showed a gradual decline, whereas there was a gradu al increase in the proportion of standing biomass in the stem fraction . These age-related changes were attributed to different loss rates am ong components. Analysis of the effects of thinning revealed that a di scontinuous reduction in stem number results in a slow decrease in par titioning to the stem. The most obvious response to thinning consisted in a sharp decrease in partitioning to fine roots and foliage, and an increased investment in branches. Stem diameter growth appeared relat ively constant in response to thinning, indicating that it will increa se almost linearly with time. I conclude that the model is able to rep roduce the development of an individual tree over time, both in terms of stem diameter and biomass. The model is thus suitable for simulatin g the effects of competition for resources on growth and development o f forest stands.