Temperature dependence, spatial scale, and tree species diversity in eastern Asia and North America

Abstract

The increase of biodiversity from poles to equator is one of the most pervasive features of nature. For 2 centuries since von Humboldt, Wallace, and Darwin, biogeographers and ecologists have investigated the environmental and historical factors that determine the latitudinal gradient of species diversity, but the underlying mechanisms remain poorly understood. The recently proposed metabolic theory of ecology (MTE) aims to explain ecological patterns and processes, including geographical patterns of species richness, in terms of the effects of temperature and body size on the metabolism of organisms. Here we use 2 comparable databases of tree distributions in eastern Asia and North America to investigate the roles of environmental temperature and spatial scale in shaping geographical patterns of species diversity. We find that number of species increases exponentially with environmental temperature as predicted by the MTE, and so does the rate of spatial turnover in species composition (slope of the species-area relationship). The magnitude of temperature dependence of species richness increases with spatial scale. Moreover, the relationship between species richness and temperature is much steeper in eastern Asia than in North America: in cold climates at high latitudes there are more tree species in North America, but the reverse is true in warmer climates at lower latitudes. These patterns provide evidence that the kinetics of ecological and evolutionary processes play a major role in the latitudinal pattern of biodiversity.

Fig. 1.

Relationships between log-transformed species richness and reciprocal ambient temperature (1/kT) at different spatial scales in eastern Asia (A–D) and North America (E–H). Following the previous studies, these figures (log(S) ∼ 1/kT) are usually termed as the Arrhenius plots, and −E is the slope of the relationships, which represents the activation energy of metabolism (9). To save space, we show data for only 4 sample grids: 50 × 50 km (A and E), 100 × 100 km (B and F), 250 × 250 km (C and G), and 400 × 400 km (D and H).

Fig. 2.

Changes in slopes, E-values (A) and goodness of fit, R² values (B) of regression models for log-transformed species richness as a function of temperature (1/kT) with grid size for eastern Asia (points and solid line) and North America (circles and dashed line). The shaded region represents the slopes (−0.7 ∼ −0.6) predicted by Allen et al. (10) and Brown et al. (9). Both slopes and R² values increase with spatial scales on the 2 continents. For details, see Table S1.

Fig. 3.

Relationship between log-transformed tree species richness and temperature (1/kT) in forests of the mountainous regions in eastern Asia. The data, collected by the Department of Ecology of Peking University (23), come from 318 forest plots, each 600 m² in area, from 19 research sites across eastern China (inset map shows the site locations). This Arrhenius plot has a slope of −0.81 (R² = 0.65). For details of data collection, see ref. .

Fig. 4.

Residuals of the Arrhenius plots (i.e., models of log(species richness) vs. 1/kT shown in Fig. 1) as functions of log-transformed annual precipitation in eastern Asia (A–D) and North America (E–H). The R² in the figure represents the additional proportions of variance in species richness explained by annual precipitation after the effects of temperature are eliminated. To save space, we show plots at 4 spatial scales: 50 × 50 km (A and E), 100 × 100 km (B and F), 250 × 250 km (C and G), and 400 × 400 km (D and H).

Fig. 5.

Relationships between species richness, area, and temperature along a gradient of increasing average annual environmental temperature in eastern Asia and North America. (A–F) Species-area relationships, plotting average species richness as a function of sample area on logarithmic axes, for sample sites with different temperatures in eastern Asia (points and solid line) and North America (circles and dashed line); to save space, we show relationships for only 6 temperatures: −12 °C, −4 °C, 0 °C, 4 °C, 12 °C, and 20 °C. (G) Relationship between the slopes of species-area relationships (as shown in A–F) and environmental temperature in eastern Asia (points and solid line) and North America (circles and dashed line). For the derivation of the temperature dependence of z, see SI Appendix.

Fig. 6.

Comparisons of species richness at high latitudes (>42°N) in eastern Asia (white bars) and North America (dark bars) using data of 1,370 forest plots in the 2 continents (398 plots for eastern Asia, see ref. ; and 972 plots for North America, see http://www.vegbank.org/). Species richness of canopy trees (DBH >10 cm) is compared using a t test in every latitudinal band of 2° (i.e., 42–44°N, 44–46°N, 46–48°N, and 48–50°N) from 42°N to 50°N. The result indicates that North America has significantly more tree species than eastern Asia at high latitudes. **P < 0.01; ***P < 0.001; N.S., P > 0.05.