Paleoclimate and current climate collectively shape the phylogenetic and functional diversity of trees worldwide

Abstract

Trees are of vital importance for ecosystem functioning and services at local to global scales, yet we still lack a detailed overview of the global patterns of tree diversity and the underlying drivers, particularly the imprint of paleoclimate. Here, we present the high-resolution (110 km) worldwide mapping of tree species richness, functional and phylogenetic diversities based on ∼7 million quality-assessed occurrences for 46,752 tree species (80.5% of the estimated total number of tree species), and subsequent assessments of the influence of paleo-climate legacies on these patterns. All three tree diversity dimensions exhibited the expected latitudinal decline. Contemporary climate emerged as the strongest driver of all diversity patterns, with Pleistocene and deeper-time (>107 years) paleoclimate as important co-determinants, and, notably, with past cold and drought stress being linked to reduced current diversity. These findings demonstrate that tree diversity is affected by paleoclimate millions of years back in time and highlight the potential for tree diversity losses from future climate change.

Differential responses of forest strata species richness to paleoclimate and forest structure

“…Our study points to a dominant role of paleoclimate in shaping geographic patterns of plant species in temperate forests. Since the LGM, climate change oscillation has promoted environmental heterogeneity, potentially influencing species diversity (Svenning and Skov, 2007b;Stewart et al, 2016;Guo et al, 2020).…”

“…For example, growing season temperature is the most important factor shaping plant species distributions in northeast Asia (Wang et al, 2009), whereas precipitation plays a major role in Inner Asia (Liu et al, 2018) and North America (Harrison et al, 2006;Zhang et al, 2014). Besides current climate, paleoclimate can also have long-lasting influences on the patterns of species diversity (Bruelheide et al, 2018) by determining refugia, colonization, migration, and extinction (Svenning and Skov, 2007b;Jimenez-Alfaro et al, 2018;Guo et al, 2020). For instance, in temperate forests, the Last Glacial Maximum (LGM) had a measurable imprint for understanding current tree species richness (Svenning and Skov, 2007b), whereas both the LGM and current climate influenced understory herb species richness (Jiménez-Alfaro et al 2018).…”

Biodiversity mediates ecosystem sensitivity to climate variability

“…As many species lack trait information, we imputed missing values via a machine learning gap-filling algorithm with Bayesian Hierarchical Probabilistic Matrix Factorization (BHPMF 90 ), which is a robust technic that has been used in other global trait-related studies 67,[91][92][93] . The facts that many traits are strongly correlated and evolutionarily closely related species tend to occupy similar functional space suggest that imputation approaches must benefit from including allometric relationships among traits and evolutionary relationships among species.…”

“…We repeated the imputation approach for each one of the 100 phylogenies and averaged the resulting specieslevel imputed traits. We further used the minimum and maximum values per trait of the observed data as thresholds, replacing the gap-filled data with observed minimum or maximum when outside of the thresholds 92,93 . Finally, we selected eight ecologically relevant and commonly used traits 67,92,93 for further functional diversity analyses: leaf nitrogen content, leaf phosphorus content, leaf dry matter content, specific leaf area, leaf area, wood density, plant max height and seed dry mass.…”

Quaternary climate change explains global patterns of tree beta-diversity

“…Despite many efforts to understand spatial distributions of tree diversity, only a few recent studies have mapped global patterns of species richness, phylogenetic and functional diversity of trees [49][50][51] . To better understand the drivers of spatial variation of tree diversity and determine the potential impacts of ongoing climate change, it is essential to evaluate the influence of past climate change on global present-day patterns of tree beta-diversity.…”