Hobi, M. L. | bioRxiv Science

bioRxiv2022

Tree growth enhancement drives a persistent biomass gain in unmanaged temperate forests

While enhanced tree growth over the last decades has been reported in forests across the globe, it remains unclear whether it drives persistent biomass increases of the stands, particularly in mature forests. Enhanced tree growth and stand-level biomass are often linked with a simultaneous increase in density-driven mortality and a reduction in tree longevity. Identifying empirical evidence regarding the balance between these processes is challenging due to the confounding effects of stand history, management, and environmental changes. Here, we investigate the link between growth and biomass via the shift in the negative relationship between average tree size and stand density (tree number). We find increasing stand density for a given tree size in unmanaged closed-canopy forests in Switzerland over the past six decades and a positive relationship between growth and stand density - qualitatively consistent with simulations by a mechanistic, cohort-resolving ecosystem model (LM3-PPA). Model simulations show that, in the absence of other disturbances, enhanced growth persistently increases biomass stocks despite simultaneous decreases in carbon residence time and tree longevity, independent of assumptions about the drivers of tree mortality. However, the magnitude of simulated changes critically depends on the shape of the mortality parameterizations. Our analyses reconcile reports of growth-induced reductions of tree longevity with model predictions of persistent biomass increases, and with our finding of a trend towards denser forests in response to growth - also in mature stands.

ecology↗

bioRxiv2022

Rising novelty and homogenization of breeding bird communities in the U.S.

AimHuman modification has profound effects on the diversity of ecological communities. Yet, surprisingly little is known about how abiotic novelty due to human modification relates to biological novelty as measured by shifts in species composition from historical baselines. Using space-for-time substitution, we ask a) whether high human modification results in biotic homogenization or heterogenization across different spatial scales; b) if high modification results in the formation of novel, "no-analog" communities; and c) whether changes in bird community composition varies in response to proxies of historical land-use and duration-of-exposure to anthropogenic disturbances. LocationConterminous United States. Time Period2012 - 2016. Major taxa studiedPasseriformes. MethodsWe analyzed continent-wide avian biodiversity data from an online checklist program, eBird, to examine how shifts in breeding bird species composition have been impacted by human modification at regional and continental scales and tested four hypotheses related to how abiotic novelty resulting from human modification generates biological novelty. ResultsAt regional scales, bird communities in highly human-modified areas exhibited similar levels of {beta}-diversity as those in the least modified areas. However, at the continental scale, spatial turnover in community composition was lower in human-modified areas, suggesting that anthropogenic disturbance has a strong homogenizing effect on bird communities at that scale. Lastly, human modification contributed more to community composition in regions where natural disturbance was infrequent and Euro-American settlement occurred later, consistent with the hypothesis that exposure to historical disturbances can shape how contemporary bird communities respond to human modification. Main conclusionsThe observed patterns of increased biotic novelty and homogenization in regions with less frequent disturbances and a longer history of human modification suggests that future extensive human modification could result in further homogenization of bird communities, particularly in the western U.S. We argue that current human-modified environments hold great potential for biodiversity conservation.

ecology↗