Natural Disturbance and Gap‐Phase Regeneration in a Wind‐Exposed Tropical Cloud Forest

Abstract

The patterns and processes of canopy tree death and replacement were studied in the elfin forest of Monteverde, Costa Rica. Natural treefalls and limbfalls in a 5.2—ha study area opened 0.8, 1.4, and 1.0% of the area in three consecutive years with about four gaps°ha—1°yr—1 larger than 4 m2. Forty—one percent of the gaps were formed by uprooted trees, 39% by snapped trees, and the remainder of limbfall, the collapse of epiphyte masses, and dead standing trees killed by lightning. Gaps were found to be spatially aggregated, with more gaps occurring within 17—20 m of one another than expected by chance. Variation among gaps was complex; the first principal component of the variation in eight important gap characteristics among 88 gaps contrasted measures of gap size with the way the gapmaker broke and the position of the gap on the slope, but accounted for only 56% of the total variation. In gaps <8 mo old, the leaf area index was 1.6, and only 8% of the area was not covered by living plants. Leaf area index increased logarithmically with time since gap formation and with gap area; 50% of the mature—forest leaf area index of 5.1 was recovered in 3 yr in gaps of 10 m2 and in 1.5 yr in gaps of 40 m2. Saplings of both shade—tolerant and shade—intolerant canopy tree species were more abundant in gaps than in the understory of mature forest. Sapling density increased with time since gap formation, but, given the effect of time, shade—tolerant sapling density decreased with gap area, while shade—intolerant sapling density increased. Saplings of eight species were concentrated on nurse logs, while those of one other species were concentrated on the mineral soil disturbed by uprooting trees. Experimental investigation of colonization of exposed soil in treefall gaps indicated that buried seeds give rise to many more tree seedlings that seeds dispersed into recent gaps. Many of the saplings in gaps, however, started life as epiphytic seedlings in the crowns of the trees that fell. The dynamics of this lower montane rain forest resemble many lowland forests in regards to the importance of gap—phase regeneration but differ in the sources of canopy gap colonists and in the importance of different substrates for seedling establishment.