Global-scale climatic controls of leaf dry mass per area, density, and thickness in trees and shrubs

Leaf dry mass per unit area (LMA) is a product of leaf thickness (T) and of density (D). Greater T is associated with greater foliar photosynthetic ra tes per unit area because of accumulation of photosynthetic compounds; grea ter D results in decreased foliage photosynthetic potentials per unit dry m ass because of lower concentrations of assimilative leaf compounds and decr eases in intercellular transfer conductance to CO2. To understand the consi derable variation in T and D at the global scale, literature data were anal yzed for 558 broad-leaved and 39 needle-leaved shrubs and trees from 182 ge ographical locations distributed over all major earth biomes with woody veg etation. Site climatic data were interpolated from long-term world climatol ogies (monthly precipitation, surface temperature) or modeled using the Can adian Climate Center Model (monthly global solar radiation). Influences of total annual precipitation (W-T), precipitation of the driest month (W-min) , monthly mean precipitation of the three driest months in the year (W-3min ), highest monthly precipitation (W-max), precipitation index ([W-max - W-m in]/W-T), mean, minimum, and maximum annual monthly temperatures, and daily annual mean global solar radiation (R) on LMA, D, and T were tested by sim ple and multiple linear and log-linear regression analyses. In broad-leaved species, LMA and T increased with increasing R and mean temperature and sc aled weakly and negatively with precipitation variables, but D was negative ly related only to precipitation. Similar relationships were also detected in needle-leaved species, except that, in multiple regression analysis, pre cipitation did not significantly influence leaf thickness, and R was positi vely related to D. Given that increases in LMA and T are compatible with en hanced photosynthetic capacities per unit leaf area, but also with greater costs for construction of unit surface area, positive effects of solar irra diance and surface temperature on these variables are indicative of shorter leaf pay-back times in conditions of higher irradiance and temperature all owing construction of leaves with higher photosynthetic potential. To gain insight into the scaling of leaf density with site aridity, correlations of D with the leaf elastic modulus close to full turgor (epsilon) and with th e leaf osmotic potentials (pi) at full and zero turgor were analyzed. Both low pi, which is compatible with low leaf water potential, and high epsilon , which permits large adjustment of leaf water potential with small changes in leaf water content, may facilitate water uptake from drying soil. Leaf elastic modulus was independent of T and was weakly related to LMA; but the re were close positive associations of epsilon with D and leaf dry to fresh mass ratio, which is an estimate of apoplastic leaf fraction. Consequently , changes in D bring about modifications in leaf elasticity and allow toler ance of water limitations. Across all the data, epsilon and the estimates o f pi were negatively related. However, given that pi varied only fourfold, but epsilon 10-fold, I conclude that osmotic adjustment of leaf water relat ions is inherently limited, and that elastic adjustment resulting from chan ges in leaf structure may be a more important and general way for plants to adapt to water-limited environments.