Slope aspect, slope length and slope inclination controls of shallow soils vegetated by sclerophyllous heath - links to long-term landscape evolution

On upland Triassic sandstone slopes of the western Blue Mountains, nonswamp, sclerophyllous heath (shrub-dominated vegetation) on shallow soils is commonly found downslope and adjacent to sclerophyllous forest on deeper soils. Some consider heath—and thus shallow soils—as favouring west-facing slopes, which are expected to experience drier microclimates due to insolation, strong and desiccating winds, and severe summer fires. However, our analysis of extensive areas with heath on shallow soils, based on vegetation and topographic maps, and fieldwork of uplands with various degrees of dissection, suggests that aspect is a poor predictor of shallow soils. Rather, shallow soils and heath are found on short slopes and the lower segments of longer slopes with the latter significantly steeper than forested segments. The shallow–deep soil boundary, marked by contrasting modern vegetation structures, does not signify a catchment area threshold, and correspondingly, the vegetation patterns are not in balance with distributary catchment processes, as short slopes are mantled exclusively by shallow soils. Instead, the soil depth boundary represents the propagation of base-level lowering signals, which takes place not only by the headward retreat of knickpoints but also via increased lowering of slope segments adjacent to drainage lines. This leads to steep slopes immediately adjacent to canyons, narrow gorges, and small steep valleys, that are mantled by shallow, discontinuous soils undergoing rapid erosion. These steep slopes persist in the landscape for ≥ 10 My after upland stream rejuvenation until incision of more weatherable Permian sediments, underlying the Triassic cliff-forming sandstones, triggers rapid lateral expansion of gorges. Once shallowly mantled and steeper slopes adjacent to streams are consumed by gorge widening, slopes adjacent to wide gorge clifflines reflect former upland drainage patterns rather than the redirected flow to rapidly widening gorges. Hence, modern vegetation patterns reflect a significant phase of landform development, perhaps combined with enhanced erosion during the Last Glacial Period that is compounded by a humped soil production function on bedrock.16 page(s