Monsoon forests in north-western Australia. II. Forest-savanna transitions

Presence-absence data for tree species in over 1000 quadrats, 10 ° 20m, on 144 transects were analysed by the divisive classificatory program TWINSPAN in order to define wet or dry monsoon forest, ecotone and savanna assemblages. A sorted table revealed that there was continuous floristic variation among these six vegetation types, although the abundance of tree species varied highly significantly between assemblages. Both wet and dry monsoon forests have higher stem densities, greater basal area, more tree species, higher litter cover and lower grass cover than surrounding savannas. Wet monsoon forest soils have significantly more moisture than surrounding savanna, but on the dry monsoon forest-savanna boundary there is no significant difference in soil moisture. Both wet and dry monsoon forest soils are more fertile than those in surrounding savannas, although there is considerable variation in the concentration of nutrients within and between the two forest formations. The greater fertility of the forest probably reflects superior nutrient accretion compared with the savanna, rather than indicating that monsoon forests are restricted to inherently fertile sites. Half the wet monsoon forest boundaries sampled had ecotonal quadrats separating forest from savanna, while only 18% of the dry monsoon forest boundaries had such structurally and floristically intermediate quadrats. A limited number of tree species was found to dominate both wet and dry forest ecotones. Wet forest ecotones are overall environmentally more similar to the surrounding savanna than the adjoining forests. Dry forest ecotones are environmentally intermediate between the dry forest and savanna. Both wet and dry forests have less fire damage than surrounding savannas. In contrast to the edaphic control of most wet monsoon forests, dry forests are typically restricted to fire protected niches. The distribution of both wet and dry forest, ecotone and savanna on boundaries with no edaphic or topographic discontinuities is thought to reflect a dynamic relationship between these assemblages.