Effect of Hillslope Gradient on Vegetation Recovery on Abandoned Land of Shifting Cultivation in Hainan Island, South China

In the present study, we investigated the effect of hillslope gradient on vegetation recovery on abandoned land of shifting cultivation in Hainan Island, south China, by measuring community composition and structure of 25-year-old secondary forest fallows along a hillslope gradient (up-, middle-, and down-slope position).A total of 49 733 free-standing woody plant stems higher than 10 cm and belonging to 170 species, 112 genera, and 57 families was found in the three 1-hm2 investigation plots. Stem density was highest in the down-slope stand and lowest in the up-slope stand. Species richness and the Shannon-Wiener index were both highest in the middle-slope stand, and lower in the down-slope and up-slope stands. The recovery forest fallows on different hillslope positions were all dominated by a few species. The five most abundant species accounted for 70.1%, 58.8%, and 72.9% of total stem densities in stands in the up-, middle-, and down-slope positions, respectively. The five species with the greatest basal areas accounted for 74.5%,84.3%, and 74.7% of total stem basal area for the up-, middle-, and down-slope positions, respectively. The number of low-density species (stem abundance less than five) increased from the up-slope position downward. Of the nine local common species among three different functional groups, the short-lived pioneer species dominated the up-slope position, but long-lived pioneer species dominated the middle- and down-slope positions. The climax species of primary tropical lowland rain forest was found in the downslope position. Both the mean diameter at breast height (DBH) and mean height of the trees increased with decreasing hillslope gradient. The stem density and basal area in different size classes were significantly different in stands in different slope positions. Our results indicated that the rate of secondary succession varies, even over small spatial scales caused by the hillslope gradient, in early vegetation recovery.