PLANT DEMOGRAPHIC RESPONSES TO ENVIRONMENTAL VARIATION - DISTINGUISHING BETWEEN EFFECTS ON AGE STRUCTURE AND EFFECTS ON AGE-SPECIFIC VITAL-RATES

1 The vital rates (e.g. fecundity, growth or mortality) of a plant pop ulation can each be expressed as the scalar product of the vectors n(t ), representing the age structure, and m(t), representing the age-spec ific values of the vital rate. We hypothesize that environmental chang e can affect a vital rate of a plant population through one of two mec hanisms: by changing the population age structure (hypothesis 1), or b y altering the age-dependent vital rates (hypothesis 2). 2 We determin ed which of these hypotheses best explained the effects of different g razing treatments, applied in a field experiment, on the population ti ller production of two pasture grasses, Agrostis stolonifera and Loliu m perenne. 3 Monthly censuses of mean per capita tiller production wer e carried out for two years and grazing effects were detected in a num ber of censuses. 4 Tiller production by individual tillers was age-dep endent and increased linearly with tiller age. 5 Grazing treatments si gnificantly changed both the age structures (n(t)) of the tiller popul ations in most censuses and the regression of tiller production on age (m(t)) in a number of censuses. 6 Treatment effects on population til ler production predicted according to hypothesis 1 were calculated usi ng measured values for n(t) specific to each grazing treatment and the mean values for m(t) over all treatments. Conversely, effects predict ed according to hypothesis 2 were calculated using mean values for n(t ) and values for m(t) specific to each grazing treatment. 7 The grazin g effects on tiller production predicted by hypothesis 2 fitted the fi eld data significantly for both species in all grazing treatments wher eas hypothesis 1 never gave a significant fit. Therefore, grazing effe cts on population tiller production arose solely through changes in th e age-specific values for tillering and never through effects on the p opulation age structures. 8 We discuss the reasons for these results a nd consider the mechanisms of the responses to grazing. We also assess the use of these hypotheses in determining the ecological processes g overning plant demographic change.