Relationships of remnant trees to vascular undergrowth communities in the western Cascades: a retrospective approach

Managers of federal forests in the U.S. Pacific Northwest are charged with managing timber harvest while maintaining ecosystem functions and biological diversity associated with old, unmanaged forests. Thus, timber-oriented management is being replaced by ecosystem management, which includes leaving live trees (green trees), snags, and coarse woody debris after harvest. We used a retrospective approach to assess potential effects of green-tree retention on vascular undergrowth in the western Cascades of Oregon. Fourteen natural two-storied stands of remnant trees (> 300 yr) over 65-125 yr old regeneration were paired with neighboring stands of the same age but lacking remnants. Species richness and cover of shrubs, all herbs, and late-successional shrubs and herbs did not differ systematically between stands with and without remnant trees. However, communities of herbs and shrubs varied with the percentages (% BA/ha) of Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) in the regeneration, which varied with remnant densities. Regeneration densities and percentages of western hemlock in there generation were positively correlated with remnant tree densities. In contrast, percentages of Douglas-fir in the regeneration and overall conifer volume were negatively correlated with remnant densities. Undergrowth cover and species richness was generally greater in relatively open, Douglas-fir dominated forests, which were associated with physiographically drier sites and fewer remnant trees. Results for these natural, two-storied stands suggest that variable levels of green-tree retention in managed forests may provide for a diversity of undergrowth communities