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  1. The phylogenetic distance between species often predicts differences in ecologically important traits. The phylogenetic diversity and structure of biological communities can inform our understanding of the processes that shape those communities, and there is a well-developed framework for comparing phylogenetic structures of communities. However, particularly in studies of phylogenetic distances from one focal species to other members of its assemblage (a one-to-many framework), the standard metrics of community-wide studies encounter significant limitations due to the left-skewed distribution of pairwise phylogenetic distances in most biological communities. For studies that require estimating the degree of phylogenetic isolation of a focal taxon, the mean phylogenetic distance (MPD) usually provides little power to distinguish among taxa because it is heavily weighted by the many ways to be distantly related, whereas the nearest taxon distance (NTD) is highly idiosyncratic and ignores cases where multiple close relatives may contribute equally strongly to influence the focal species. Here we highlight the value of examining the cumulative distribution of phylogenetic distances in studies that take a focal-species approach. We describe and discuss the benefits of two new metrics. An integrated metric of phylogenetic distances (AUPhyDC) uses information from the whole cumulative distribution, whereas the tenth quantile (PD10) ismore »an extremely simple metric that improves on NTD by capturing the influence of multiple close relatives on ecological interactions. Several recent examples found that PD10 did a better job of revealing ecological patterns than NTD or MPD. We provide R code to facilitate the use of these approaches and advocate for the inclusion of PD10 along with NTD and MPD in statistical packages for phylogenetic ecology.« less
    Free, publicly-accessible full text available July 1, 2023
  2. Abstract The relationships that control seed production in trees are fundamental to understanding the evolution of forest species and their capacity to recover from increasing losses to drought, fire, and harvest. A synthesis of fecundity data from 714 species worldwide allowed us to examine hypotheses that are central to quantifying reproduction, a foundation for assessing fitness in forest trees. Four major findings emerged. First, seed production is not constrained by a strict trade-off between seed size and numbers. Instead, seed numbers vary over ten orders of magnitude, with species that invest in large seeds producing more seeds than expected from the 1:1 trade-off. Second, gymnosperms have lower seed production than angiosperms, potentially due to their extra investments in protective woody cones. Third, nutrient-demanding species, indicated by high foliar phosphorus concentrations, have low seed production. Finally, sensitivity of individual species to soil fertility varies widely, limiting the response of community seed production to fertility gradients. In combination, these findings can inform models of forest response that need to incorporate reproductive potential.
    Free, publicly-accessible full text available December 1, 2023
  3. Tree fecundity and recruitment have not yet been quantified at scales needed to anticipate biogeographic shifts in response to climate change. By separating their responses, this study shows coherence across species and communities, offering the strongest support to date that migration is in progress with regional limitations on rates. The southeastern continent emerges as a fecundity hotspot, but it is situated south of population centers where high seed production could contribute to poleward population spread. By contrast, seedling success is highest in the West and North, serving to partially offset limited seed production near poleward frontiers. The evidence of fecundity and recruitment control on tree migration can inform conservation planning for the expected long-term disequilibrium between climate and forest distribution.
  4. McGlinn, Daniel (Ed.)
    Free, publicly-accessible full text available June 1, 2023
  5. Abstract Arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) associations are critical for host-tree performance. However, how mycorrhizal associations correlate with the latitudinal tree beta-diversity remains untested. Using a global dataset of 45 forest plots representing 2,804,270 trees across 3840 species, we test how AM and EcM trees contribute to total beta-diversity and its components (turnover and nestedness) of all trees. We find AM rather than EcM trees predominantly contribute to decreasing total beta-diversity and turnover and increasing nestedness with increasing latitude, probably because wide distributions of EcM trees do not generate strong compositional differences among localities. Environmental variables, especially temperature and precipitation, are strongly correlated with beta-diversity patterns for both AM trees and all trees rather than EcM trees. Results support our hypotheses that latitudinal beta-diversity patterns and environmental effects on these patterns are highly dependent on mycorrhizal types. Our findings highlight the importance of AM-dominated forests for conserving global forest biodiversity.
  6. Despite its importance for forest regeneration, food webs, and human economies, changes in tree fecundity with tree size and age remain largely unknown. The allometric increase with tree diameter assumed in ecological models would substantially overestimate seed contributions from large trees if fecundity eventually declines with size. Current estimates are dominated by overrepresentation of small trees in regression models. We combined global fecundity data, including a substantial representation of large trees. We compared size–fecundity relationships against traditional allometric scaling with diameter and two models based on crown architecture. All allometric models fail to describe the declining rate of increase in fecundity with diameter found for 80% of 597 species in our analysis. The strong evidence of declining fecundity, beyond what can be explained by crown architectural change, is consistent with physiological decline. A downward revision of projected fecundity of large trees can improve the next generation of forest dynamic models.