IMPACTS OF ROOT COMPETITION IN FORESTS AND WOODLANDS: A THEORETICAL FRAMEWORK AND REVIEW OF EXPERIMENTS

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.

The photosynthetic capacity of leaves is related to the nitrogen content primarily bacause the proteins of the Calvin cycle and thylakoids represent the majority of leaf nitrogen. To a first approximation, thylakoid nitrogen is proportional to the chlorophyll content (50 mol thylakoid N mol-1 Chl). Within species there are strong linear relationships between nitrogen and both RuBP carboxylase and chlorophyll. With increasing nitrogen per unit leaf area, the proportion of total leaf nitrogen in the thylakoids remains the same while the proportion in soluble protein increases. In many species, growth under lower irradiance greatly increases the partitioning of nitrogen into chlorophyll and the thylakoids, while the electron transport capacity per unit of chlorophyll declines. If growth irradiance influences the relationship between photosynthetic capacity and nitrogen content, predicting nitrogen distribution between leaves in a canopy becomes more complicated. When both photosynthetic capacity and leaf nitrogen content are expressed on the basis of leaf area, considerable variation in the photosynthetic capacity for a given leaf nitrogen content is found between species. The variation reflects different strategies of nitrogen partitioning, the electron transport capacity per unit of chlorophyll and the specific activity of RuBP carboxylase. Survival in certain environments clearly does not require maximising photosynthetic capacity for a given leaf nitrogen content. Species that flourish in the shade partition relatively more nitrogen into the thylakoids, although this is associated with lower photosynthetic capacity per unit of nitrogen.