Pattern of forest overstory retention, broad-scale environmental variation and species traits influence spatial patterning of understory vegetation after thinning

Background/Question/Methods

Responses of understory vegetation to forest canopy gaps may depend on broad-scale environmental variation as well as fine-scale processes including plant-plant interactions (e.g., competition, facilitation) and colonization (e.g., dispersal, migration). Therefore, spatial patterns may reflect differences among species in growth form and mode of colonization. For example, clonal reproduction in plants represents an adaptation to persist in stressful low-resource conditions, reducing the capacity of plants to colonize disturbed areas. In contrast, sexual reproduction requires more resources but results in greater dispersal distances. Additionally, interactions among woody and herbaceous species may vary along gradients in canopy cover.

We examined 1) factors affecting the cover of shrubs, forbs, graminoids and ferns in understories at multiple spatial scales, and 2) compared results for species capable of clonal reproduction to results for species that reproduce only by seed. We used an operational-scale manipulative thinning experiment replicated seven times in the Coast and Cascade ranges of western Oregon, USA.  Three thinning treatments and an untreated control, along with a fine-scale spatial network of geo-referenced permanent vegetation sampling plots were delineated at each site. This study focuses on the spatial patterning of understory vegetation observed six years following thinning.

Results/Conclusions

Results of hierarchical linear modeling analysis show that the cover of shrubs and forbs responds similarly to gap creation and reductions in basal area (BA) associated with thinning, and that shrubs reduce the cover of forbs locally. In addition to effects on residual density, thinning treatment affected the cover of some groups suggesting that thinning has other important effects (e.g., physical disturbance of the forest floor and vegetation). Differences in effects of potential direct incident solar radiation, BA and treatment, and differences in patterns of residual spatial correlation among clonal and non-clonal species may reflect differences in the abilities of these groups to persist in stressful environments and colonize disturbed areas, as well as spatial scale over which unmeasured processes operate. This hierarchy of controls results in spatial patterns in forest understories that can have important consequences for future forest development and the maintenance of biodiversity.