Abstract
Climate-induced increases in tree mortality are reported for many forests worldwide. Understanding the potential effects on carbon pools requires long-term monitoring of changes in forest biomass. We measured above-ground biomass of living trees (AGB) over a 34 year period (1982-2016) in permanent plots spanning a range of forest stand ages, species compositions, and topographic settings in a subalpine forest in the Colorado Front Range. Stand-level and species-level AGB varied spatially and temporally in relation to stand age, successional processes and site moisture classification. Young (c. 122 years) postfire stands composed of lodgepole pine had lower average AGB than older (> 250 years) mixed-species stands. Mesic stands had higher AGB than xeric or hydric stands of similar age. At the level of individual species, significant shifts in AGB among species were primarily explained by successional replacement of shade-intolerant pines by shade-tolerant Engelmann spruce and subalpine fir. The permanent plot network recorded significant shifts in species dominance and tree densities from 1982-2016 reflecting successional patterns developing over several centuries as well as the effects of recent localized windthrow, insects, and pathogens. Despite increases in tree mortality, there was a general pattern of increasing AGB across the forest.
