Vegetation Dynamics in Amazonian Treefall Gaps

Abstract

In a 1—ha plot of old—growth tierra firme forest near San Carlos de Rio Negro, Venezuela (northwest Amazon Basin), 88 trees ≥ 10 cm dbh out of 786 die over a 10—yr period. Most deaths resulted in the formation of small (5—100 m²) canopy openings (gaps). Occasionally, large gaps are formed in this region when strong winds topple many trees together. In five small (single—treefall) gaps and one large (multiple—treefall) gap we studied changes in soil fertility and nutrient leaching, and also plant establishment, mortality, and growth during the first 4 yr following gap formation. Gaps were divided into four zones, or microhabitats: a trunk zone, an open zone (between bole and forest edge), a crown zone, and a root—pitzone. Sampling was conducted in each microhabitat. Soil nutrient levels in single—treefall gaps did not differ in a predictable fashion in response to microhabitat within gap of gap age. Moreover, except for a small, short—term increase in NO₃—N, leaching losses from single—treefall gaps did not differ from forest levels. Advance regeneration plays a dominant role n treefall gap succession at San Carlos. Under closed forest, advance regeneration has a mean annual survivorship of °80%; height growth is only a few centimetres a year, and leaf retention times frequently exceed 4 yr. Four years after gap formation, advance regeneration accounted for 97% of all trees ≥ 1 m tall in the single—treefall gaps and 83% of all trees in the multiple—treefall gap. Almost all trees in both gap types were of primary forest species; pioneer trees comprised only a small fraction of the regrowth. In general, microhabitat within gaps did not influence plant density, or plant establishment and mortality patterns. However, mortality was much higher for individuals that germinated after gap formation than for individuals present as advance regeneration. Plant growth within treefall gaps was influenced by gap size, gap microhabitat type, gap age, and plant size. Seedlings and saplings of forest trees in the multiple—treefall gap grew three times as fast as those in the single—treefall gaps. Within single—treefall gaps, height growth was greater in the trunk and open zones than in the crown zone, and trees generally grew more slowly as gaps aged. Furthermore, tree growth was positively correlated with tree size, causing the size differential between trees of different heights to expand as gaps aged. As San Carlos, tree seedlings do not grow to the canopy during a single gap event; rather, canopy closure occurs by growth of larger pole—sized trees that survived treefalls or by lateral expansion of bordering subcanopy trees. Our results indicate that neither gap size, microhabitat with gaps, nor gap age have measurable effects on nutrient loss, nor do they appear to affect plant density, plant establishment, or plant mortality. Size, microhabitat, and temporal affects are minimized, in large part, because of the great importance of advance regeneration in gap succession.