Overwintering and growing season dynamics of Trifolium repens L. in mixture with Lolium perenne L.: A model approach to plant-environment interactions

In attempting to increase the reliability of clover contribution in clove/r yegrass systems it is important to understand the roles of (1) specific tra its of the clover genotype (2) climate and (3) their interactions in determ ining clover behaviour in swards. Overwintering and spring growth of white clover (cultivars AberHerald and Huia) grown in binary mixtures with perenn ial ryegrass were measured at 12 European sites ranging in latitude from Re ykjavik, Iceland (64 degrees 30' N) to Pordenone, Italy (46 degrees 30' N). In the overwintering period, tiller density of the grass was assessed and detailed morphological and chemical measurements were made on the clover at each sampling time. During the growing season, the clover contribution to total available biomass was recorded, Detailed climatic data were available at all sites. The annual growth cycle of swards was divided into four func tional periods (spring, summer, autumn and winter). Within each functional period community responses were modelled. The models incorporated independe nt biotic variables characterizing each community within each site at the s tart of the period and independent variables characterizing the climate at each site during the period. The models were linked dynamically by taking, as response variable(s) for a functional period. the independent biotic var iable(s) of the succeeding period. In general, the modelling strategy was s uccessful in producing a series of biologically meaningful linked models. E ssential prerequisites for this were (a) the establishment of a well-devise d common protocol prior to the experiment and (b) the extensive gradients o f climatic and other variables obtained by using numerous sites, AberHerald generally performed as well as. or better than, Hula throughout the annual cycle across the range of climatic conditions encountered, and especially under low temperature conditions in winter and autumn. Clover leaf area ind ex appeared to be a key variable in determining clover performance over win ter and through the following growing season. Grass tiller density had a st rong negative effect on clover content in spring but only at low temperatur es, This emphasizes the importance of a high clover leaf area index in autu mn as the main biotic factor related to spring clover content in milder con ditions. The importance of climatic variables in the models is their use in explaining the reliability of the contribution of clover in clover,: ryegr ass systems. Temperature was the primary climatic determinant of clover res ponse in all periods, having a direct effect on clover content and leaf are a index or mediating the effect of the associate species. Radiation strongl y influenced Clover dynamics during winter and spring but not in the other periods, possibly because it was confounded with the effect of higher tempe rature. Precipitation was positively related to clover growth during spring and autumn and was related to tiller density in a complex manner during au tumn and winter. (C) 2001 Annals of Botany Company.