Early stages of secondary succession on abandoned cropland in north-east Borneo Island

A study on community structure and floristic change in the early stages of the tropical old-field succession in an intensive shifting cultivation area, Sabah, north-east Borneo Island, was conducted in March and September 1997. Dense therophytic communities were established soon after abandonment, and rapidly changed: namely, from Galinsoga parviflora (in approximately 1-2 months) to Crassocephalum crepidioides (in approximately 3-5 months) and further to Conyza sumatrensis (in approximately 1 year). These therophytic communities were replaced by perennial grass and shrub species, such as Imperata cylindrica and Eupatorium odoratum, within 3 years after abandonment. At this stage, a few pioneer tree species such as Trema orientalis made a sparse canopy layer approximately 3-4 m high. The three pioneer annuals have wind-dispersed small seeds. The plant densities of the three annuals did not appear to change between areas fallow for 1 month and 4 months. Almost all of the seedlings of the three annuals might simultaneously have invaded the fallowed field soon after abandonment. The three pioneer annuals take a different amount of time to reach their respective mature sizes. This resulted in the rapid successional changes in the tropical old-field succession. The seed-bank annuals, whose seeds germinate after the winter pre-chilling, occurred in the temperate old-fields as the first-year pioneers under seasonal condition. Intermittent shifting cultivation and tropical conditions without seasonality favour the occurrence of wind-dispersed annuals with a spatially fugitive strategy.