Experimental manipulation of spatial heterogeneity in Douglas-fir forests:effects on squirrels

Squirrel communities simultaneously composed of abundant populations of Gla ucomys, Tamias, and Tamiasciurus are: (1) a result of high production of se eds and fruiting bodies by forest plants and fungi and complexity of ecosys tem structure, composition, and function; (2) indicative of high carrying c apacity for vertebrate predators and (3) characteristic of old, natural for ests in the Pacific northwest, USA. I hypothesized that silvicultural manip ulation of canopies of second-growth forests could result in spatial hetero geneity that would reproduce the biocomplexity and plant-fungal productivit y associated with high squirrel populations. I predicted that accelerating biocomplexity would require greater than or equal to 20 years, but short-te rm effects of induced heterogeneity would be apparent in 5 years: initial d ecreases followed by increases in Glaucomys populations, nonlinear increase s in Tamias populations, and little change in Tamiasciurus populations. If my predictions proved accurate, confidence in long-term predictions would b e enhanced. I chose 16 13-ha stands with two different management histories for a randomized block experiment and began measuring squirrel populations in 1991. Variable-density thinnings were implemented in spring 1993. Fall and spring populations were measured through fall 1998. Populations respond ed as predicted, except for a treatment-management history interaction. Pre vious conventional thinnings altered ecosystem function such that low Glauc omys populations failed to respond to treatment. Variable-density thinning, in conjunction with retention of biological legacies and management of dec adence, could possibly accelerate biocomplexity in second-growth forest tha t mimics that in old, natural forests. Published by Elsevier Science B.V.