Understanding and predicting ecosystem functioning (e.g., carbon and water fluxes)
and the role of soils in carbon storage requires an accurate assessment of plant rooting
distributions. Here, in a comprehensive literature synthesis, we analyze rooting patterns
for terrestrial biomes 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 β to the data for each biome (Gale and Grigal 1987).
β is a simple numerical index of rooting distribution based on the asymptotic equation
Y=1-βd, where d = depth and Y = the proportion of roots from the surface to depth
d. High values of β correspond to a greater proportion of roots with depth. Tundra,
boreal forest, and temperate grasslands showed the shallowest rooting profiles (β=0.913,
0.943, and 0.943, respectively), with 80-90% of roots in the top 30 cm of soil; deserts
and temperate coniferous forests showed the deepest profiles (β=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 across biomes, from approximately 0.2 to 5 kg m-2. Tropical
evergreen forests had the highest root biomass (5 kg m-2), but other forest biomes
and sclerophyllous shrublands were of similar magnitude. Root biomass for croplands,
deserts, tundra and grasslands was below 1.5 kg m-2. Root/shoot (R/S) ratios were
highest for tundra, grasslands, and cold deserts (ranging from 4 to 7); forest ecosystems
and croplands had the lowest R/S ratios (approximately 0.1 to 0.5). Comparing data
across biomes for plant functional groups, grasses had 44% of their roots in the top
10 cm of soil. (β=0.952), while shrubs had only 21% in the same depth increment (β=0.978).
The rooting distribution of all temperate and tropical trees was β=0.970 with 26%
of roots in the top 10 cm and 60% in the top 30 cm. Overall, the globally averaged
root distribution for all ecosystems was β=0.966 (r 2=0.89) with approximately 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 our analysis in the context of root biomass
and root functioning.