Vegetation dynamics in Amazonian treefall gaps

In a 1-ha plot of old-growth tierra firme forest near San Carlos de Rio Negro, Venezuela, 88 trees ≥10cm dbh out of 786 died over a 10yr period. Most deaths resulted in the formation of small (5-100m²) canopy openings. Occasionally, large gaps are formed in this region when strong winds topple many trees together. Soil nutrient levels in single-treefall gaps did not differ in a predictable fashion in response to microhabitat within gap or 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 played a dominant role in treefall gap succession. Under closed forest, advance regeneration has mean annual survivorship of c80%; height growth is only a few centimetres a year, and leaf retention times frequently exceed 4yr. Four years after gap formation, advance regeneration accounted for 97% of all trees ≥1m 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. Plant growth within treefall gaps was influenced by gap size, gap microhabitat type, gap age, and plant size. Tree seedlings did 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. -from Authors