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

Abstract

Squirrel communities simultaneously composed of abundant populations of Glaucomys, Tamias, and Tamiasciurus are: (1) a result of high production of seeds and fruiting bodies by forest plants and fungi and complexity of ecosystem structure, composition, and function; (2) indicative of high carrying capacity for vertebrate predators and (3) characteristic of old, natural forests in the Pacific northwest, USA. I hypothesized that silvicultural manipulation of canopies of second-growth forests could result in spatial heterogeneity that would reproduce the biocomplexity and plant-fungal productivity associated with high squirrel populations. I predicted that accelerating biocomplexity would require ≥20 years, but short-term effects of induced heterogeneity would be apparent in 5 years: initial decreases followed by increases in Glaucomys populations, nonlinear increases in Tamias populations, and little change in Tamiasciurus populations. If my predictions proved accurate, confidence in long-term predictions would be 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 responded as predicted, except for a treatment–management history interaction. Previous conventional thinnings altered ecosystem function such that low Glaucomys populations failed to respond to treatment. Variable-density thinning, in conjunction with retention of biological legacies and management of decadence, could possibly accelerate biocomplexity in second-growth forest that mimics that in old, natural forests.