History and biological consequences of forest fragmentation: A study of Trillium ovatum in southwestern Oregon.

Abstract

Many land-use practices result in the conversion of continuous ecosystems into archipelagos of habitat surrounded by a sea of agriculture and urban development. The result of this process of habitat fragmentation is thought to be an increase in population-level extinctions and a reduction in regional biodiversity. I studied fragmentation in the Sucker Creek watershed, a 250 km$\sp2$ area in southwestern Oregon. Beginning in the 1850s, forested habitat was rapidly depleted throughout the surrounding region, though large remnants of forest existed in the watershed until the 1950s. Between the 1950s and 1980s, however, 287 km of road was built in the watershed and 34% of the older forest was logged. I studied the influence of fragmentation on an understory herb, Trillium ovatum. This plant is useful for demographic studies because individuals can be aged. Based on data collected in 1949 and a survey I conducted, I estimated that clearcutting results in the mortality of almost all trillium ($\sim$97.6%), and remaining plants are not recruiting new individuals. I also determined the age of individuals in eight populations of trillium within separate fragments. Populations within $\sim$65 m of forest edges have had almost no recruitment of plants since the time of the adjacent clearcutting; these populations are expected to decline, although I could not quantify extinction probabilities. It is clear, however, that these declining populations have higher probabilities of reaching extinction than interior populations which had significantly higher levels of recruitment. Forest remnants provide much less suitable habitat for trillium than their actual area might indicate. I used the same eight populations to explore potential mechanisms of reduced recruitment. This work suggests that several factors can be eliminated as possible mechanisms: flowering phenology, seed dispersal by ants and yellow jackets, germination, herbivory, and survivorship of established plants. Two factors measured potentially contribute to observed reductions in recruitment near edges: increased seed predation by rodents and decreased seed production via changes in pollination. As well, a separate study suggests that inbreeding and outbreeding depression occur in T. ovatum, and thus changes in mating patterns due to fragmentation can potentially influence population growth.