Allen, R. B. | Search bioRxiv

bioRxiv2023

Forest demography and biomass accumulation rates are associated with transient mean tree size vs density scaling relations

Linking individual and stand-level dynamics during forest development reveals a scaling relationship between mean tree size and tree density in forest stands, which integrates forest structure and function. However, the nature of this so-called scaling law and its variation across broad spatial scales remains unquantified and its linkage with forest demographic processes and carbon dynamics remains elusive. Here we develop a theoretical framework and compile a broad-scale dataset of long-term sample forest stands (n = 1433) from largely undisturbed forests to examine the association of temporal mean tree size vs density scaling trajectories (slopes) with biomass accumulation rates and the sensitivity of scaling slopes to environmental and demographic drivers. The results empirically demonstrate a large variation of scaling slopes, ranging from -4 to -0.2, across forest stands in tropical, temperate and boreal forest biomes. Steeper scaling slopes are associated with higher rates of biomass accumulation, resulting from a lower offset of forest growth by biomass loss from mortality. In North America, scaling slopes are positively correlated with forest stand age and rainfall seasonality, thus suggesting a higher rate of biomass accumulation in younger forests with lower rainfall seasonality. These results demonstrate the strong association of the transient mean tree size vs density scaling trajectories with forest demography and biomass accumulation rates, thus highlighting the promise of leveraging forest structure properties to predict forest demography, carbon fluxes and dynamics at broad spatial scales. Significance StatementMean tree size vs density scaling relationships are thought to predict forest function at broad spatial scales. Here we develop a theoretical framework based upon demographic processes and empirical evidence from forest inventory data to demonstrate a strong association of the transient mean tree size and density scaling trajectories (slopes) with forest demography and biomass accumulation rates. This strong association is pervasive across forest biomes and suggests a negative relationship between scaling slope and biomass accumulation rate (resource availability). Our results highlight the promise of leveraging forest structure (i.e., inferred from high resolution remote sensing data or fused into size-structured demographic models) to evaluate forest demography, carbon fluxes and dynamics at broad spatial scales.

ecology↗

bioRxiv2022

Robustness of field studies evaluating biodiversity responses to invasive species management in New Zealand

Benefits of invasive species management for terrestrial biodiversity are widely expected and promoted in New Zealand. Evidence for this is presented in policy and scientific reviews of the literature, but the robustness and repeatability of the underpinning evidence-base remains poorly understood. We evaluated the design of field-based studies assessing biodiversity responses to invasive species management in 155 peer-reviewed articles published across 46 journals from 2010 - 2019. Each study was assessed against nine principles of experimental design, covering robustness of sampling and avoidance of bias. These principles are important in New Zealand to detect treatment effects from environmental variability driven by underlying gradients such as soil fertility, climate and disturbance. Fifty two percent of studies defined a sampling universe and 68% of studies specified the treatment. Whereas, 54%, 74%, and 50% of studies did not utilise replication, representatively sample the universe, or quantify invasive species, respectively. Ninety five percent of studies quantified biodiversity responses, although a high proportion of these did not representatively sample replicates. Initial conditions and accounting for effects of experimental implementation were not utilised in 57% and 84% of studies respectively. No studies avoided observer/analyst bias using blinding methods, despite this being widely adopted in other fields. Ordinal logistic regression showed these principles varied in how robustly they were applied among categories of biodiversity responses and invasive species. Our findings suggest that greater attention to experimental design principles is desirable: supported by researchers, funding agencies, reviewers, and journal editors. Greater resources is not necessarily a solution to these design issues. Undertaking fewer studies, that are individually more expensive because they better adhere to experimental design principles, is one alternative. Our intent in this article is to improve the robustness of future field studies for at least some principles. Robust designs have enduring value, reduce uncertainty and increase our understanding of when, where and how often the impacts of invasive species on biodiversity are indeed reversible.

ecology↗