Direct and indirect effects of climate on richness drive the latitudinal diversity gradient in forest trees

Abstract

Climate is widely recognised as an important determinant of the latitudinal diversity gradient. However, most existing studies make no distinction between direct and indirect effects of climate, which substantially hinders our understanding of how climate constrains biodiversity globally. Using data from 35 large forest plots, we test hypothesised relationships amongst climate, topography, forest structural attributes (stem abundance, tree size variation and stand basal area) and tree species richness to better understand drivers of latitudinal tree diversity patterns. Climate influences tree richness both directly, with more species in warm, moist, aseasonal climates and indirectly, with more species at higher stem abundance. These results imply direct limitation of species diversity by climatic stress and more rapid (co-)evolution and narrower niche partitioning in warm climates. They also support the idea that increased numbers of individuals associated with high primary productivity are partitioned to support a greater number of species. LetterClimate and the latitudinal tree diversity gradient 247 Figure 4 The effects of forest structural attributes on tree diversity derived from the within-forest plot structural equation modelling analyses. Panels a, b and c at the scale of 20 m 9 20 m, and panels d, e and f at the scale of 50 m 9 50 m. The effect of stem abundance on tree species richness showed a significant latitudinal trend at the scale of 20 m 9 20 m (panel b; P < 0.01, R 2 = 0.27). Standardised path coefficients AE 1 SE are shown; SE's are smaller than the size of the symbol for some forest plots. Colours indicate increasing absolute latitude from pink to turquoise.

Testing the diversity–biomass relationship in riverine fish communities

“…However, indirect effects of temperature on biomass exceeded those of direct effects and suggested that biomass increased with increasing temperature, as found for marine fish communities (Duffy et al., 2016). This suggests that available energy limits fish species and functional diversity in stream systems, as for many other taxonomic groups and ecological systems (Chu et al., 2018; Clarke & Gaston, 2006; Field et al., 2009; Hawkins et al., 2003; Swenson et al., 2012). One potential mechanism that might explain the contrasting direct and indirect effects of temperature on biomass might be the observed inverse relationship between temperature and body size (Blanchet et al., 2010).…”

Direct and indirect effects of environmental factors, spatial constraints, and functional traits on shaping the plant diversity of montane forests

“…Our results indicate that environmental factors have direct effects related to increasing plant community species richness and abundance. Precipitation has been reported to promote the richness of trees and shrubs, and increasing MAT elevated the richness of shrubs in subalpine mountains as well as grasslands (Chu et al, ; Lin, Xia, & Wan, ; Xiong et al, ). The results from the SEMs showed that MAT may decrease the tree canopy and indirectly inhibit tree diversity (Figure ).…”

“…Overall responses to climate change are primarily driven by direct effects, suggesting that the response of dominant/single species to climate change may be adequate for forecasting the impacts of climate change within specific communities. The size of the indirect effects also depends on the size of the direct effects experienced by species in the community (Chu et al, ). For the differences in the direct and indirect responses of trees and shrubs to environmental factors, ecological niche differences may influence the magnitude of indirect effects.…”

“…Precipitation has been reported to promote the richness of trees and shrubs, and increasing MAT elevated the richness of shrubs in subalpine mountains as well as grasslands (Chu et al, 2019;Lin, Xia, & Wan, 2010;Xiong et al, 2019). The results from the SEMs showed that MAT may decrease the tree canopy and indirectly inhibit tree diversity (Figure 2) (Chu et al, 2019). For the differences in the direct and indirect responses of trees and shrubs to environmental factors, ecological niche differences may influence the magnitude of indirect effects.…”

Niche width of above‐ and below‐ground organisms varied in predicting biodiversity profiling along a latitudinal gradient

“…Metabolic theory proposes that higher temperature results in higher metabolic rates and biochemical processes (Brown, Gillooly, Allen, Savage, & West, 2004), exponentially increasing species mutation and speciation, and finally elevating species numbers (Brown, 2014). This relationship between temperature and biodiversity has been intensively explored in forests for below‐ground microbial communities, showing better predictions in microbial biodiversity and spatial scaling rates than other environmental variables (e.g., pH; Deng et al., 2018;Wu et al., 2018;Zhou et al., 2016), and temperature was also shown to be the important predictor in shaping plant distribution and diversity (Chu et al., 2019;Peters et al., 2016;Rice et al., 2019). In addition, as circumneutral pH (close to neutral pH) and adequate water content provide suitable conditions for a majority of organisms to colonize and reproduce, edaphic pH and water availability (precipitation) are also important variables in shaping community structures and components for both above‐ and below‐ground organisms (Ratcliffe et al., 2017;Rice et al., 2019;Toledo et al., 2011;Zeppel, Wilks, & Lewis, 2014;Zhang et al., 2016).…”