Influence of light, soil fertility, and deer browsing on the spatial structure, relative growth rates, and population demography of Trillium grandiflorum

Trillium grandiflorum (trillium) is a widespread and locally abundant but slow-growing perennial temperate forest herb that is eaten by white-tailed deer. Trillium plants growing on plots with high light and high soil nitrogen were less likely to be browsed by deer. In 1998 and 1999, deer preferentially consumed larger plants, but this pattern fell short of statistical significance. Deer may preferentially select high density trillium patches over low density patches. The proportion of plants consumed within a patch was unaffected by the number of individuals in the patch, regardless of the spatial scale. Spatial aggregation of plants did not affect an individual plant's risk of being eaten.; I examined how RGR (relative growth rate) responds to defoliation, past reproduction and current reproduction at 4 sites in northern Wisconsin. ANCOVA indicated that plant size affected RGR, and not past or current reproduction. Plants that flowered in 1998 were likely to do so again in 1999, whereas nonflowering plants in 1998 were unlikely to flower in 1999. Defoliation in 1998 had no effect on reproductive status in 1999. ANCOVA also revealed significant differences in RGR due to defoliation, study site, site x defoliation interaction, and plant size.; I investigated trillium demography using a stage-based matrix population model using 7 discrete life cycle stages. Individual plants were classified by stage both in 1998 and 1999. A summary matrix based on data pooled over sites predicted the long-term growth rate of the population to be -3.53% per year. Without any browsing, the long-term growth rate of the population would remain negative, although the rate of population decline be less rapid. Rates of population decline increase if foraging deer focus more selectively on flowering plants. Deer browsing has a negative effect on the proportion of plants remaining in the nonflowering stage, the vital rate with the highest elasticity value. If conditions remain the same, trillium populations will decline by 40% over the next 10 years, and 90% over the next 50 years. Deer density estimates did not predict local damage to trillium.