Commonality and variability in the structural attributes of moist temperate old-growth forests: A global review

Temperate forests have been fundamentally altered by land use and other stressors globally; these have reduced the abundance of primary and old-growth forests in particular. Despite many regional studies, the literature lacks a global synthesis of temperate old-growth structural characteristics. In this study we compare literature derived data on mature and old-growth moist temperate forests with the aim of: (i) exploring global commonalities; (ii) investigating sources of variability among systems; and (iii) highlighting data gaps and research needs. We compiled a dataset of 147 records from 93 papers, and analyzed a set of structural indicators: basal area, stem density, large living trees, live aboveground biomass, quadratic mean diameter, and coarse woody debris volume. These indicators were contrasted between mature and old-growth age classes at a global level and across continents and broad forest types, testing for significance through Monte-Carlo permutation procedure. We also related structural indicators to age, climatic and geographical descriptors. Our results suggest that all structural indicators vary across systems in relation to geographical, compositional, and climatic influences. However old-growth forests showed global commonalities in structure when compared to mature forests: significantly higher densities of large living trees, higher quadratic mean diameter, and higher amounts of live aboveground biomass and coarse woody debris. Furthermore we found inconsistency in the structural variables reported by different papers; lack of studies on temperate forests in Russia, and Western and Central Asia. The findings improve our understanding of old-growth structure and function, and will help inform sustainable forest management and conservation approaches world-wide.