EROSION OF TREE DIVERSITY DURING 200 YEARS OF SHIFTING CULTIVATION IN BORNEAN RAIN FOREST

Abstract

The long-term effect of shifting cultivation on tree diversity was examined in West Kalimantan, Indonesia. Ten sites formed a gradient of cultivation history: seven secondary forest fallows (9–12 years old) having experienced from one to 10 prior cycles of long-fallow shifting cultivation plus three adjacent primary forest stands. For trees >10 cm diameter at breast height (dbh), stand-level species density was >70% lower in secondary forest recovering from the first cycle of shifting cultivation than in nearby primary forest. The species density of trees 5–10 cm dbh declined only 30%, and that of stems <5 cm dbh declined by 10%. For stems <10 cm dbh, species density continued to decline with each cycle, but that of large trees remained relatively constant. The evenness of the tree community declined systematically with each cycle. Late successional species were 18% of the species pool <300 m from the border of primary forest but only 2% >1000 m away. The richness of seed rain also declined with distance from primary forest (<100 m vs. 300–500 m). Distance from primary forest was positively correlated with the number of prior cultivation cycles. Thus, the decline in species density and evenness in part reflects long-distance dispersal limitation of late successional species. However, the species density of early successional species (not often found in primary forest) also declined with cultivation history. Thus, factors other than distance to primary forest must play a role. Systematic change in the early successional species pool surrounding a focal patch is a hypothesis for further study. When cultivation history was accounted for, site level species density was not related to soil P, N, Mg, Ca, or K. Soil nutrients also failed to consistently predict species density within a fallow. Dispersal more strongly limits species density than soil nutrients under shifting cultivation in West Kalimantan. More research is necessary on limitations imposed by the regeneration niche and age to reproduction of early successional species. Current trends in regional land use change will greatly accelerate the slow, steady loss of diversity seen during 200 years of long-fallow shifting cultivation.