A GLOBAL ANALYSIS OF ROOT DISTRIBUTIONS FOR TERRESTRIAL BIOMES

Understanding and predicting ecosystem functioning (e.g., carbon and w ater fluxes) and the role of soils in carbon storage requires an accur ate assessment of plant rooting distributions. Here, in a comprehensiv e literature synthesis, we analyze rooting patterns for terrestrial bi omes and compare distributions for various plant functional groups. We compiled a database of 250 root studies, subdividing suitable results into 11 biomes, and fitted the depth coefficient beta to the data for each biome (Gale and Grigal 1987). beta is a simple numerical index o f rooting distribution based on the asymptotic equation Y = 1-beta(d), where d = depth and Y = the proportion of roots from the surface to d epth d. High values of beta correspond to a greater proportion of root s with depth. Tundra, boreal fori:st, and temperate grasslands showed the shallowest rooting profiles (beta = 0.913, 0.943, and 0.943, respe ctively), with 80-90% of roots in the top 30 cm of soil; deserts and t emperate coniferous forests showed the deepest profiles (beta = 0.975 and 0.976, respectively) and had only 50% of their roots in the upper 30 cm. Standing root biomass varied by over an order of magnitude acro ss biomes, from approximately 0.2 to 5 kg m(-2). Tropical evergreen fo rests had the highest root biomass (5 kg m(-2)), but other forest biom es and sclerophyllous shrublands were of similar magnitude. Root bioma ss for croplands, deserts, tundra and grasslands was below 1.5 kg m(-2 ) Root/shout (R/S) ratios highest for tundra, grasslands, and cold des erts (ranging from 4 to 7); forest ecosystems and croplands had the lo west R/S ratios (approximately 0.1 to 0.5). Comparing data across biom es for plant functional groups, grasses had 44% of their roots in the top 10 cm of soil (beta = 0.952), while shrubs had only 21% in the sam e depth increment (beta = 0.978). The rooting distribution of all temp erate and tropical trees was beta = 0.970 with 26% of roots in the top 10 cm and 60% in the top 30 cm. Overall, the globally averaged root d istribution for all ecosystems was beta = 0.966 (r(2) = 0.89) with app roximately 30%, 50%, and 75% of roots in the top 10 cm, 20 cm, and 40 cm, respectively. We discuss the merits and possible shortcomings of o ur analysis in the context of root biomass and root functioning.