Developing multisite dynamic models of mixed species plant communities

Data on the development of two white clover cultivars (AberHerald and Huia) in mixed clover/ryegrass swards were available at 12 sites in Europe from experiments conducted for several years under a common protocol. Swards wer e measured up to seven times over winter and up to seven times over the gro wing season. In the overwintering period. detailed morphological measuremen ts were taken for clover at each sampling time and, during the growing seas on. the clover contribution to total available biomass was recorded. Detail ed meteorological data were available at all sites. The development of thes e clover, ryegrass communities over time and sites was modelled. The modell ing strategy had three main elements: (a) division of the annual growth cyc le of the clover/ryegrass community into a number of functional periods: (b ) development of relationships within each functional period using models i ncorporating plot-level biotic variables characterizing each community at t he start of the period and site-level climatic and management variables mea sured during the periods and (c) introduction of a dynamic element by linki ng the models across functional periods. The response variable(s) for a fun ctional period was the biotic independent variable(s) of the succeeding per iod. The object was to produce a dynamic series of models in which communit y development within and across sites was described as a resultant of the i nitial state of the community and climatic and other forces acting on it. T he analysis used a mixed models technique in recognition of the complex err or structure of the data. Various statistical aspects of the modelling are discussed including the models and fitting strategy used, the complexity of the error structure in an experiment that includes sites and years, and th e desirability of transforming certain variables before modelling. The issu es in presenting the results of a series of complex models are discussed an d a graphical tabular approach is outlined. (C) 2001 Annals of Botany Compa ny.