Age-related decline of stand biomass accumulation is primarily due to mortality and not to reduction in NPP associated with individual tree physiology, tree growth or stand structure in a Quercus-dominated forest

Age-related reductions in stand biomass accumulation are frequently observed in old-growth forests. The phenomenon may be caused by reduced production, increased mortality or both. The relative importance of production and mortality is not well studied, so the mechanisms controlling age-related decline of stand biomass accumulation remain unclear. In this study, conducted in a Quercus-dominated deciduous forest in the Northeastern USA, we examined whether age-related decline in stand above-ground biomass (AGB) accumulation could be explained by reduction of above-ground net primary production (NPP) (growth of surviving trees) that may be associated with (i) physiological constraints within individual trees or (ii) changes in stand structure, or by (iii) age-related, increasing tree mortality in stands up to 135 years old. Few previous studies have tested these hypotheses simultaneously within the same forest. We did not find evidence for a reduction in individual tree growth associated with age-related physiological constraints, in terms of foliar carbon assimilation capacity, photosynthesis ⁄ respiration balance, nitrogen availability or hydraulic constraints on carbon gain. Over the period of 1937–2006, we did not observe alterations in stand structure, and the above-ground NPP of the Quercus forest was generally stable. However, we did find that the primary mechanism driving age-related decline of stand AGB accumulation was biomass loss due to the death of large, dominant trees. Our results indicate that shifts in mortality from the loss of small trees to the loss of large trees, rather than changes in above-ground NPP, drives age-related decline in stand AGB accumulation in this forest. Synthesis. We found that within the range of stand development stages analysed, the age-related decline of stand AGB accumulation in a Quercus-dominated forest was primarily due to mortality of large, dominant trees and not due to changes in above-ground NPP associated with tree physiology, individual tree growth or stand structure. This result indicates that tree demography and the influence of climate change on disturbances may need to be integrated into models to predict the change of above-ground carbon stock of some old-growth forests

Stoichiometry patterns in the androdioecious Acer tegmentosum

CITATION: Zhang, X., et al. 2016. Stoichiometry patterns in the androdioecious Acer tegmentosum. Scientific Reports, 6:35022, doi:10.1038/srep35022.The original publication is available at http://www.nature.com/srepENGLISH ABSTRACT: This study evaluates stoichiometry patterns in the androdioecious Acer tegmentosum, a species characterized by a rare reproductive system where males and hermaphrodites coexist. Altogether 31 hermaphrodites and 29 male plants were harvested and samples of leaves, current-year shoots, branches and coarse roots were analyzed to explore gender differences in biomass, C, N and P concentrations of these four components. The nitrogen to phosphorus relationship of each component was examined using SMA estimates. Males had significantly greater amounts of leaf and coarse root dry matter content than hermaphrodites. C, N and P stoichiometry differed significantly between genders, especially in the newly emerging vegetative components (leaves and shoots). Males had higher C/N and C/P ratios in current-year shoots and lower C/P ratios in leaves and branches. Hermaphrodites had higher N/P ratios in the leaves and branches. Males had higher rates of increase in leaf P content than hermaphrodites. This study suggests that stoichiometry patterns may be significantly affected by gender.https://www.nature.com/articles/srep35022Publisher's versio