Comparison of clear-cut and old-growth forest understory community structure 33 years after a volcanic eruption

Background/Question/Methods

Ecosystem response to large disturbances, such as volcanic eruptions, is likely to depend on pre-disturbance conditions and pre-existing vegetation. Here, we compare the response of clear-cut and old-growth forest understory plant communities to disturbance produced by tephra deposits from the eruption of Mount St. Helens in 1980. Previous research focused on the understory plant community response over 30 years in old-growth forests; ours is the first study to compare communities in paired old-growth and clear-cut forests. We use two sites, one with high and one with low diversity of herbaceous understory plants prior to the eruption, in order to clarify the potential range of responses. At each site we paired a clear-cut that predated the eruption with an adjacent old-growth forest, of either high or low diversity. All other topo-edaphic factors were controlled for, so as to allow comparison of responses to tephra in the different forest types. Accordingly, our data compare the relative understory community trajectories in clear cut and old-growth forests 33 years following massive tephra deposition disturbance.

Results/Conclusions

Mosses, herbs, and shrubs on clear-cut sites remain sparse 33 years after the eruption, and percent vegetative cover was higher in the old-growth. Clear-cut sites had soil surfaces with a high percent cover of barren tephra and relatively open canopy structure, whereas old-growth sites had thickly developed soil O-horizons overlaying 1980 tephra deposits. Clear-cuts sites remained dominated by ruderal, shade-intolerant species generally, but also by Vaccinium shrub species common in the old-growth sites. Community analysis (NMS ordination and MRPP) demonstrated clear separation between high diversity sites and low diversity sites, and between clear-cut and old-growth sites. Communities in old-growth forest were much more divergent than those in clear-cuts, suggesting that old-growth conditions may enhance niche differences governing plant distribution. Nevertheless, clear-cuts were more diverse than old-growth sites, especially the low diversity site comparison for evenness and Shannon's Diversity Index (H'). Simpson's (D) diversity was significantly higher in clear-cut sites for all site comparisons. Overall, these data suggest that intact old-growth forests buffered the impact of volcanic tephra disturbance on understory communities. These differences are likely explainable by greater old-growth canopy cover and soil O-layer development over 33 years of recovery.