RAIN-FOREST ON MARACA ISLAND, RORAIMA, BRAZIL - ARTIFICIAL GAPS AND PLANT-RESPONSE TO THEM

Three replicate artificial canopy gaps of four sizes (ca. 40 m(2), ca. 150 m(2), ca, 350 m(2), and ca, 2500 m(2)) were made in rain forest o n Maraca Island. Gaps were defined according to Brokaw (1982) and all vegetation (> 2 m tall) within them was felled and left in place, The nine most numerous dicotyledonous tree (greater than or equal to 10 cm dbh) species of unfelled forest were chosen for studies on size-class distribution, sprouting, and seedling dynamics in the artificial gaps , Seedling dynamics of two pioneer species, one small understory tree species,and a shrub are also presented as well as the response of the herbaceous vegetation. The artificial gaps were ephemeral in that with in a few months of their creation the growth of sprouts and herbaceous vegetation often exceeded 2 m height and hence the strict definition of gap no longer applied. Later, there was much young tree and liana g rowth and the upper branches of the surrounding trees were encroaching into the gaps, reducing their area (after 4 yrs) by as much as 85%. T he gaps caused enhanced seedling recruitment for at least a year. Cecr opia palmata and Jacaranda copia which an well-known pioneer species w ere both recruited in all gaps including the smallest ones. Tetragastr is panamensis behaved similarly to these two species and had many seed lings recruited in all gap sizes. The survivorship and height growth o f all seedlings were recorded in sample quadrats in each gap size. Dat a for nine species, which had reasonably large seedling cohorts, are p resented separately. The gaps reduced the mortality rate and increased the rate of height growth of the seedlings present at the time of gap creation. Of the nine leading species, Brosimum lactescens had the lo west mortality rate and almost all the C. palmata and J. copaia recrui ted after gap creation died by the end of the study. Mean height of se edlings in the forest plots varied little throughout the 4-yr study, w hile small seedlings of B. lactescens, Himatanthus articulatus, and Li ndackeria paludosa had reached heights up to 7 m in the large and very large gaps. The results provide no support for the notion that rain f orest species are adapted to gaps of a certain size. Certainly, there are light-demanding species and shade-bearing species, but the observe d variations within individual species, the heterogeneity of the gap e nvironment, and the ephemeral nature of gaps all make it unlikely that a species is best adapted to one gap size. (C) 1998 Elsevier Science B.V.