The effect of the spatial scale of tree harvesting on woody seedling establishment and tree dynamic at Ongoye Forest Reserve.
Louw, Sharon Lilla.
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Subsistence harvesting pressure in most African countries focuses on the small and mostly unreproductive trees found in the understorey stratum and can have potentially insidious ecological effects. Harvest intensities at Ongoye Forest Reserve (OFR) vary significantly across the forest (range = 87 - 567 stumps ha-1), with harvesting focussed exclusively on poles from tree species that grow only in the understorey. Growing evidence indicates that seedling establishment from the pool of species available beneath a closed canopy is greatly influenced by the differential ability of species to take advantage of the short burst of resources in newly-created understorey gaps. Seedling dynamics in these gaps may determine forest tree diversity and dynamics and consequently harvest gaps have the potential to significantly affect natural forest dynamics. This study examined seedling establishment beneath intact understorey and within artificially created understorey gaps of different sizes (single stem gaps, two stem gaps, four stem gaps, eight stem gaps and control ‘gap’, where no stems were removed) that simulated different spatial scales of harvesting intensity of understorey trees. This experiment examined the proposition that successful seedling establishment and natural succession is strongly dependent on the scale of harvesting. Seedling abundance, species richness, irradiance (photosynthetically active radiation and the red to far-red ratio), soil nutrient composition and herbaceous layer cover was measured in each gap size in 2005 before harvesting, and again in 2007. The mean seedling abundance was not significantly different among gap treatments, although there was a trend towards more seedlings in 2005 than 2007. Seedling abundance in all gaps was greater than at control sites beneath the intact understorey. There was a continuous increase in seedling richness in 2007. An average species richness of 4.3 was recorded in the control sites, beneath a shaded understorey. Here, seedling richness increased by 18.24% with the removal of a single understorey tree (Gap 1). Species richness increased with increasing experimental gap size increased so that the greatest mean species richness (6.2 species) was recorded where eight neighbouring trees were removed (Gap 8). Light transmission reaching the seedling stratum was greater in larger gaps and there was a trend towards more seedlings and greater species richness in the higher light environments of such gaps. Soil nutrient levels did not influence seedling abundance and species richness in gaps. The herbaceous layer suppressed seedling establishment. In the largest gaps (115.4m2) created by harvesting, seedling composition was more deterministic than in small gaps where seedling establishment and density was random, accordingly there were more species in larger gaps from a more defined species assemblage. Current harvesting levels of pole-sized understorey trees, where only small gaps are created in the understorey, are unlikely to alter forest dynamics and species composition at OFR. This study demonstrates that harvesting eight adjacent trees crosses the harvest intensity threshold between sustainable natural tree dynamics and a potential successional shift to an alternative state. Clearly, for natural dynamics to be maintained harvesting intensities will have to be regulated.
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