Forest filled with gaps : effects of gap size on water and nutrient cycling in tropical rain forest : a study in Guyana

Guyana's forests are selectively logged and a forest management is desired that is economically sustainable and ecologically responsible. Canopy gaps, created by selective logging, induce changes to microclimatic and edaphic conditions. These changes influence the regeneration of the forest in general and of commercial tree species in particular. This study evaluated the influence of gap size on microclimatic conditions, water dynamics and nutrient cycling. The study was carried out in the tropical rain forest of central Guyana in experimental gaps of 50 to 3200m2. Research topics included: microclimate, soil moisture, modelling of solar radiation, evapotranspiration and soil water dynamics, litterfall, decomposition, mineralisation and leaching of nutrients. Emphasis was on differences between gaps of different size for the various parts of the water and nutrient cycle and the spatial variation within gaps and in the gap edge area. Temporal changes were studied by making observations in 0-3, 7 and 13-year-old gaps. The main finding were: 1) soil water availability was rarely limited for growth, 2) the amount of radiation increased with increasing gap size, both inside the gap and in the gap edge area, 3) the amount of litterfall decreased sigmoidal with increasing distance from the gap edge and litter turnover rates were larger in larger sized gaps due to a larger regrowth of the vegetation in these gaps, 4) decomposition rates were not affected by gap size, but decomposition in gaps was slower than in forest, 5) mineralisation rates were affected in gaps larger than 100m2 and mineralisation rates in larger sized gaps had a faster recovery to the level of the forest than in smaller sized gaps, 6) increased leaching was found in gaps larger than 400m2 and increased nutrient losses were still observed in a 7-year-old 3400m2 gap, but not in smaller gaps. This study generated insight into the potential impacts of logging gaps on abiotic and edaphic conditions affecting forest regeneration. The research result suggest that canopy gaps created by logging should preferably not exceed 400m2, if it is the objective of the forest management to maintain the ecological functions of the forest