HABITAT FRAGMENTATION AND DEMOGRAPHIC-CHANGE FOR A COMMON-PLANT - TRILLIUM IN OLD-GROWTH FOREST

I studied the influence of forest fragmentation on an understory herb, Trillium ovatum, in the Siskiyou Mountains of Oregon, where logging p ractices over the past 35 yr have created a mosaic of fragments surrou nded by clearcuts and tree plantations. The age of trillium plants can be estimated by counting the annual constrictions on their rhizomes. Based on data collected by Whittaker in 1949 (i.e., pre-fragmentation) and a survey I conducted in 1995, I estimated that the process of cle arcutting and subsequent conifer planting results in the mortality of almost all trillium (similar to 97.6%). In general, the remaining plan ts are not recruiting new individuals, even in sites clearcut 30 yr ag o. Thus, trillium is restricted to smaller amounts of remnant, uncut f orest. My study also demonstrated that populations in forest remnants that were within similar to 65 m of forest-clearcut edges have had alm ost no recruitment of young plants since the time of the adjacent clea rcutting, while forest interior populations contained higher recruitme nt levels. Projections based on these recruitment estimates indicated that edge populations will decline in size and interior populations wi ll not decline. This study provides the first evidence of demographic changes in plant populations resulting from habitat fragmentation, and it offers evidence for the mechanisms responsible for such demographi c changes.