Masters Thesis

The potential effect of timber harvest on understory plants: a 420 year chronosequence study in a temperate coniferous forest

In most temperate coniferous forests, the vast majority of vascular plant species richness can be found in the understory layer. While the large-scale conversion of forests to plantations has made it increasingly important to understand how understory flora responds to timber harvesting, there is a surprising paucity of data concerning this topic. Ideally, long-term studies using permanent plots would be used to directly assess post-logging patterns of recovery, both for whole floras and for individual species of conservation interest. Unfortunately, these studies are lacking, and alternative approaches are critical. I used a 420 year forest chronosequence to assess the relationship between stand age, overstory cover and vascular plant understory richness and composition in one watershed in the Siskiyou Mountains of Oregon. The chronosequence consisted of six young managed (age 7-44) and nine older unmanaged (age 90-427) stands. All stands were similar in underlying geology, slope, elevation, and aspect. I found a non-linear relationship between stand age and richness, in which richness showed a steady decline as young stands entered canopy closure, then increased as stands gained older forest attributes such as light gaps and structural diversity. I also found that percent open canopy was correlated with total percent understory cover, richness, diversity and changes in species composition. In general, young stands were characterized by high shrub and graminoid cover and old stands were characterized by an abundant herb layer. In my study area, a large proportion of young managed stands are currently entering canopy closure, a stage characterized by low vascular plant species richness and abundance. I use my results to discuss the potential effects of past and future forest management on vascular plant understory species.