variation in the demography of a woodland understorey herb (primula vulgaris) along the forest regeneration cycle: projection matrix analysis

Abstract

1 Gaps in the forest canopy allow establishment of many understorey species. As gaps close, their: demographic behaviour is affected by changing environmental conditions. A demographic analysis of Primula vulgaris populations in forest patches with varying canopy openness was carried out to investigate the effect of forest canopy dynamics on the demography of this forest understorey herb. 2 Eight P. vulgaris populations across a range of light conditions were studied in three deciduous temperate forests in Britain. Demographic data were collected in each population during summer 1992, 1993 and 1994, and stage-based population projection matrices were built to describe their demography for two growth periods (1992-93 and 1993-94). 3 Population growth rate (lambda) varied from 0.97 to 1.98 in 1992-93, and from 0.88 to 1.23 in 1993-94. There was a significant positive correlation between lambda and diffuse light for the two periods studied: generally, population growth rate was lower in darker patches, implying that populations decline as the canopy closes. Lower lambda-values in 1993-94 were the result of increased mortality and decreased fecundity compared to 1992-93. In most cases the observed population structures were significantly different from the calculated stable stage distributions. 4 Sensitivity analysis showed that 3,was most sensitive to changes in matrix transitions that corresponded to growth from small to adult stages, especially those that implied fast growth. Elasticity analysis revealed that the contribution of fecundity elements to lambda was small, but populations in brighter patches showed comparatively higher fecundity elasticities than populations under the closed canopy. High elasticities corresponded to the entries that implied growth (especially in brighter patches) or persistence in adult categories (predominantly in darker patches). 5 The observed variation in the demography of P. vulgaris in different patches along the forest regeneration cycle stresses the importance of studying the variation in plant population dynamics across the range of habitat patches in which the plant naturally occurs.