Plant Demographic Responses to Environmental Variation: Distinguishing between Effects on Age Structure and Effects on Age-Specific Vital Rates

Abstract

1 The vital rates (e.g. fecundity, growth or mortality) of a plant population can each be expressed as the scalar product of the vectors nt, representing the age structure, and mt, representing the age-specific values of the vital rate. We hypothesize that environmental change can affect a vital rate of a plant population through one of two mechanisms: by changing the population age structure (hypothesis 1), or by altering the age-dependent vital rates (hypothesis 2). 2 We determined which of these hypotheses best explained the effects of different grazing treatments, applied in a field experiment, on the population tiller production of two pasture grasses, Agrostis stolonifera and Lolium perenne. 3 Monthly censuses of mean per capita tiller production were carried out for two years and grazing effects were detected in a number of censuses. 4 Tiller production by individual tillers was age-dependent and increased linearly with tiller age. 5 Grazing treatments significantly changed both the age structures (n,) of the tiller populations in most censuses and the regression of tiller production on age (m,) in a number of censuses. 6 Treatment effects on population tiller production predicted according to hypothesis 1 were calculated using measured values for n, specific to each grazing treatment and the mean values for m, over all treatments. Conversely, effects predicted according to hypothesis 2 were calculated using mean values for n, and values for m, specific to each grazing treatment. 7 The grazing effects on tiller production predicted by hypothesis 2 fitted the field data significantly for both species in all grazing treatments whereas hypothesis 1 never gave a significant fit. Therefore, grazing effects on population tiller production arose solely through changes in the age-specific values for tillering and never through effects on the population age structures. 8 We discuss the reasons for these results and consider the mechanisms of the responses to grazing. We also assess the use of these hypotheses in determining the ecological processes governing plant demographic change.