RESPONSES OF JUVENILE TREES TO ABOVE-GROUND AND BELOW-GROUND COMPETITION IN NUTRIENT-STARVED AMAZONIAN RAIN-FOREST

In several tropical lowland rain forests, removal of root competition by trenching around seedlings and saplings has been shown to have litt le or no impact on growth rate. However, we found that trenching incre ased the aboveground growth rate of saplings and seedlings of a wide v ariety of species in an Amazonian caatinga. This low-statured forest d evelops on waterlogged, humus-rich, white-sand soil that is extremely low in available nitrogen. Analysis of foliar nutrient concentrations and a fertilizer experiment confirmed previous findings that nitrogen is limiting, while measures of soil moisture potential and stomatal re sistance gave no indication of water shortage. We suggest that belowgr ound competition has an impact on growth rate because (1) established trees allocate a large proportion of biomass to fine-root production, further reducing the availability of nutrients in an inherently poor s oil, and (2) penetration of daylight to the sapling layer is greater t han under other tropical lowland rain forests (1-3% vs. 0.5-1%). Altho ugh trenching had no effect on percentage mortality (12% over 18 mo), it reduced the percentage of naturally regenerating seedlings exhibiti ng net leaf loss from 28 to 8%. Surprisingly, the magnitude of respons e was similar in gaps and understory and was similar among species, su ggesting that trenching had a larger effect on allocation than on the rate of photosynthesis or respiration.