Could alluvial knickpoint retreat rather than fire drive the loss of alluvial wet monsoon forest, tropical northern Australia?

Abstract

Drainage rejuvenation through headward migration of alluvial knickpoints is common in ephemeral semi-arid streams, but has not yet been described for tropical rivers. In the Australian monsoon tropics (AMT), wet monsoon forests have an important ecological function, and are present along many alluvial valleys and springs within a eucalypt-savanna dominated landscape. Using a combination of LiDAR, remote sensing and field evidence, we observe the ongoing destruction of wet monsoon forest through hydro-geomorphic feedbacks, along with the headward retreat of an alluvial knickpoint at Wangi Creek in Litchfield National Park, Northern Territory. Due to the highly transmissive shallow aquifer along the lower Wangi Creek, this knickpoint retreat leads to a downstream drop in in-channel water level, which in turn drives a decrease in the local groundwater table. The lowered groundwater level causes the shallow anabranches and formerly water saturated peaty floodplain soil to desiccate, which results in a reduction of vegetation density. The resulting dry surface conditions allow annual to bi-annual high frequency low-intensity fires to affect the monsoon forest, while wet rainforest upstream of the knickpoint remains intact. In this paper, we argue that such hydrogeomorphic feedbacks may cause the initial destabilization of the forest, which then provides the necessary conditions for the impact of fire. This scenario thus challenges the prevalent view that fire is a first-order control on the spatial extent of wet monsoonal rainforest in the study area, and provides a new and testable hypothesis for further studies in the AMT.

Forgotten impacts of European land‐use on riparian and savanna vegetation in northwest Australia

“…Some of the changes witnessed in the MP11A record could be explained by other environmental shifts occurring in the Mitchell River catchment. Larsen, May, Moss, and Hacker () demonstrated how the geomorphic process of knick‐point migration caused riparian forest loss in Australia's Northern Territory. Sand slugs released by upstream erosion cause long‐lasting impacts on downstream ecosystems in many Australian rivers (Prosser et al., ).…”

Late Quaternary biotic and abiotic controls on long‐term sediment flux in a northern Australian tropical river system

“…Environmental condition of the study reach. Base map: Canopy height model derived from green airborne LIDAR (reprinted from Larsen et al , ). (a) Intact wet monsoon forest; (b) high floodplain water table with peat (black) and sand deposition on top (light colour); (c) deeply weathered macro‐channel banks; (d) site 2, profile WG‐553: recently drained subsurface showing a Histosol with peat development on fluvial sands (arrow points to location of OSL sampling); (e) site 2, profile WG‐550 shows several levels of trees growing on top of each other; the peat matric between the plants is not yet completely removed; white arrow points to location of radiocarbon sampling; (f) site 2, profile WG‐448 shows almost complete erosion of top soil with three levels of trees growing on top of each other; arrow is pointing to the lowest tree, whose bark was sampled for radiocarbon sampling; (g) spatial extension of site 1, profile WG‐751; white arrow points towards black soil layers alternating with white sands; open woodland (OWL) growing on top of the exposure; (h) site 1, profile WG‐372; below dashed line: deeply weathered macro‐channel boundary; above dashed line cemented and mottled sands.…”

“…Located in a similar hydro‐geomorphic setting to Magela Creek, studies on late Quaternary hydrological variability using the relative height of plunge pool deposits at Wangi Creek (Figure ) point towards a climatically‐driven decrease in flood magnitudes since the Last Glacial Maximum (LGM) (Nott et al , ; Nott and Price, ), however the fluvial dynamics and especially the role of riverine ecosystems in the river corridor evolution over the same timescales remain elusive. In this context, a recent study at Wangi Creek has demonstrated the importance of bio‐hydro‐geomorphic feedbacks in the modern evolution of river–floodplain dynamics (Larsen et al , ). This study found an upstream migrating alluvial knickpoint caused channel incision and hydrological drainage of the previously water‐logged floodplain, leading to increased susceptibility to fire impacts and ultimately to the destruction of the riverine corridor vegetation (Larsen et al , ).…”

Introduction to the themed issue: Wildfire and Geomorphic Systems

“…Brogan et al (2017) examine channel changes through time in response to convective and longer-duration, lower intensity mesoscale rainfall. Larsen et al (2016) examine multi-decadal response of channels to episodic fires in the seasonal tropical environment of northern Australia. A convective storm immediately after the fire created estimated peak unit discharges of 28 m 3 /s/km 2 and substantial channel aggradation.…”