Effects of forest age and forest structure on epiphytic lichen biomass and diversity in a Douglas-fir forest
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Epiphytic lichens contribute to the biological diversity of old forests, but little is known about the changes that occur in lichen communities as forests age and become structurally more complex. We compared forest age and forest structure as indicators of lichen biomass, richness and community in a Douglas fir (Pseudotsuga menziesii) stand on the Gifford Pinchot National Forest in Washington, USA. Epiphytic lichens were sampled in 1995 and 1996 on 21, 13-ha forested units, ranging from 75 to 165 years old. Canopy lichen litterfall was sampled in 15, 12.5 m2 plots per unit. Mean lichen litterfall biomass differed significantly between years, but species composition did not. Forest structure variables were refined using principal component analysis resulting in the first two principal components accounting for 75.5% of the variation in the samples-by-structural-variables matrix. Principal components 1 and 2 represented structure and were compared to forest age by correlating each with lichen biomass, richness and community composition. Multiple regression was also used to test the relationship of age and eight structural variables against lichen biomass, richness and community composition. In both years, and in both analyses, forest structure explained more variance in lichen biomass and richness than forest age, whereas age explained more variance in lichen community composition. The structural variables that were important for predicting lichen biomass differed from those predicting lichen richness. In mature forests, structure may be a better predictor of lichen biomass and diversity than forest age. Techniques that emphasize structure have the potential to help forest managers assess mature stands for their biological value.