Effects of forest fragmentation on seed production of the understory herb Trillium camschatcense

We investigated the effects of forest fragmentation on reproduction of a common understory perennial, Trillium camschatcense , in Hokkaido, Japan. We measured seed production of 12 (1998) and 14 (1999) populations, and we explored the relationships of seed production to the number of flowering plants (population size) and to the surrounding landscape condition (landscape type). We discriminated between two landscape types, isolated and continuous, to describe the quantity and distribution of forests around T. camschatcense populations. Population size may be correlated with attractiveness to pollinators and the availability of compatible mates, whereas landscape condition can affect the abundance of pollinators. The number of seeds per flower was related to population size in 1999, but not in 1998. In both years, small populations of <50 flowering plants produced few seeds. Although the effect of landscape type was not significant, continuous populations with abundant forest series in their neighborhood tended to produce a higher number of seeds than isolated populations located in small, isolated forest. We attribute the decrease in seed production to pollen limitation because the stigmatic pollen load had a positive relationship to seed production and addition of pollen by hand increased seed production. Variation in population structure (four stage classes) among populations was better explained by population size than number of seeds produced, suggesting the relative importance of population size for the demography of T. camschatcense . Larger populations had a higher proportion of seedlings, possibly because large populations had fewer edge effects, which reduce seedling recruitment and survival. Small populations with 220 flowering plants or less showed almost no seedling recruitment. The data on seed production and population structure suggest that continuous populations with at least 1000 flowering plants may be required to avoid the effects of fragmentation.