Spatial and temporal patterns of root activity in a species-rich alluvial grassland.

The time and depth of activity of a number of co-existing grassland plants was measured using a technique involving the simultaneous injection to different depths in soil of 3 chemical tracers - Li, Rb and Sr. Root activity at a particular depth was assessed from the concentration of each tracer in leaf tissue.The seven most constant species showed very similar patterns of root activity, which was greater at 5 than at 15 or 25 cm except towards the end of the growth period in late June. Maximum root activity generally occurred earlier than maximum shoot productivity but there was little evidence of differentiation between species. When root activity was assessed as a proportion of total community root activity, by combining tracer concentration and biomass data, seasonal differences between species were more obvious. Using both root activity and productivity data, species were grouped into two main guilds, one active in spring (April-May) and one in summer (June).Correlations of above-ground biomass with root activity at different depths revealed that species of the spring guild were more active in the 5-15 cm horizons and those of the summer guild at 15-25 cm.These patterns suggest that rooting depth and time of activity are strongly linked: early-active species tend to be less productive and shallower-rooted and this combination of characters allows them to escape from competition with more productive species, by being active at a time when deeper soil layers are less hospitable.