Level and pattern of overstory retention interact to shape long-term responses of understories to timber harvest

Background/Question/Methods

In many regions of the world, variable retention has replaced clearcut logging as the principal method of regeneration harvest. A fundamental goal of variable retention is to enhance the ecological values of managed forests, including maintenance or recovery of late-seral species. Two basic elements of forest structure can be manipulated to achieve specific ecological objectives—the level (amount) and spatial pattern of retained trees. Greater retention should favor closed-canopy species that depend on shaded understories and should inhibit early-seral species that benefit from disturbance. Although dispersed retention reduces microclimatic extremes relative to open areas, it may be ineffective for species sensitive to disturbance or to changing environmental conditions—these may require undisturbed forest patches. Here we present decade-scale responses of forest understories to variable-retention harvests as part of a regional-scale experiment in mature coniferous forests of the Pacific Northwest, the DEMO Study. Replicated at each of five sites, experimental treatments contrast both the level of retention (40 vs. 15% of original basal area) and its spatial distribution (dispersed vs. aggregated in 1-ha patches). We explore changes in community composition and in the abundance and diversity of species with differing seral roles for which we anticipated contrasting responses to treatments.

Results/Conclusions

For most vascular plant groups (early seral, forest generalist, and late seral), changes in cover and richness from the original forest were reduced at higher levels of retention and in dispersed treatments. Although retained forest patches (1-ha aggregates) were stable, changes in adjacent harvested (cleared) areas were significantly greater than in dispersed treatments. Late-seral herbs were highly sensitive to level and pattern of retention, with extirpations most frequent in the cleared areas of aggregated treatments and at low levels of dispersed retention. In contrast, these environments supported the greatest abundance and diversity of early-seral species. Forest-floor bryophytes showed large and persistent declines regardless of treatment, suggesting that threshold levels of disturbance or stress were exceeded. Although aggregates provided refugia for disturbance-sensitive species, they appear susceptible to edge effects, particularly when isolated. The ability to achieve multiple ecological or silvicultural objectives with variable retention may require the spatial partitioning of habitats to include dispersed retention, larger undisturbed aggregates, and cleared areas.