Humid tropical rain forest has expanded into eucalypt forest and savanna over the last 50 years

Abstract

Tropical rain forest expansion and savanna woody vegetation thickening appear to be a global trend, but there remains uncertainty about whether there is a common set of global drivers. Using geographic information techniques, we analyzed aerial photography of five areas in the humid tropics of northeastern Queensland, Australia, taken in the 1950s and 2008, to determine if changes in rain forest extent match those reported for the Australian monsoon tropics using similar techniques. Mapping of the 1950s aerial photography showed that of the combined study area (64,430 ha), 63% was classified as eucalypt forests/woodland and 37% as rain forest. Our mapping revealed that although most boundaries remained stable, there was a net increase of 732 ha of the original rain forest area over the study period, and negligible conversion of rain forest to eucalypt forest/woodland. Statistical modeling, controlling for spatial autocorrelation, indicated distance from preexisting rain forest as the strongest determinant of rain forest expansion. Margin extension had a mean rate across the five sites of 0.6 m per decade. Expansion was greater in tall open forest types but also occurred in shorter, more flammable woodland vegetation types. No correlations were detected with other local variables (aspect, elevation, geology, topography, drainage). Using a geographically weighted mean rate of rain forest margin extension across the whole region, we predict that over 25% of tall open forest (a forest type of high conservation significance) would still remain after 2000 years of rain forest expansion. This slow replacement is due to the convoluted nature of the rain forest boundary and the irregular shape of the tall open forest patches. Our analyses point to the increased concentration of atmospheric CO2 as the most likely global driver of indiscriminate rain forest expansion occurring in northeastern Australia, by increasing tree growth and thereby overriding the effects of fire disturbance.

A Broad Approach to Abrupt Boundaries: Looking Beyond the Boundary at Soil Attributes within and Across Tropical Vegetation Types

“…These approaches have undoubtedly increased our understanding of the factors involved in determining boundary locations and dynamics. However, individual boundary dynamics can be highly idiosyncratic [9]–[11] and by focusing on boundaries alone we may overlook or underestimate underlying aspects of landscape heterogeneity and long-term history at a regional scale. By treating boundaries as static vegetation limits we risk overlooking the effects of the vegetation itself on the parameters being measured, the effect of ecosystem-level processes (such as succession and decomposition) occurring in the boundary zone, and the dynamic nature of many boundaries (e.g.…”

“…By treating boundaries as static vegetation limits we risk overlooking the effects of the vegetation itself on the parameters being measured, the effect of ecosystem-level processes (such as succession and decomposition) occurring in the boundary zone, and the dynamic nature of many boundaries (e.g. [1], [11]). In this study we approach abrupt boundaries between rain forest and sclerophyll woodland vegetation at a regional scale in the Australian Wet Tropics, specifically trying to incorporate local diversity of both soils and vegetation types, as well as acknowledging soil-vegetation feedbacks.…”

“…[1], [11]). In a system with ASS, we would expect to find both sharp boundaries, and rapid shifts between vegetation types [23], [25].…”

“…In a system with ASS, we would expect to find both sharp boundaries, and rapid shifts between vegetation types [23], [25]. In North Queensland rainforest margins expand, on average, more than half a metre per year [11], and some boundaries have been shown to shift horizontally more than a metre per year [11], [21]. This means that studies which have compared soil directly across boundaries with the assumption of comparing two types of soil (i.e.…”

Nonresident birds in tropical wet sclerophyll forest, northeast Queensland, Australia

“…Wet sclerophyll forest (also called tall open forest; Tng et al 2012) is unique to Australia. It is dominated by tall trees of the genus Eucalyptus (family Myrtaceae; Beadle 1962).…”