Growth characteristics of Eucalyptus camaldulensis trees differ between adjacent regulated and unregulated rivers in semi-arid temperate woodlands

Abstract

River red gum (Eucalyptus camaldulensis) is the most widely-distributed eucalypt, an important timber resource, and critical to the sustainable function of many Australian waterways. Agricultural development, including decades of river regulation, has resulted in extensive degradation of E. camaldulensis woodlands, and restoration is now required to maintain ecological function in many floodplain areas. To aid restoration planning, we quantified the structural characteristics of E. camaldulensis trees in an intensively farmed region of eastern Australia. We studied tree populations on two rivers with different hydrological regimes: (1) Bogan River - ephemeral and surrounded by dryland agriculture, and (2) Macquarie River - permanent and the focus of irrigated agriculture. The two populations showed clear differences in allometric relationships. Trees on the ephemeral river were shorter with relatively wider canopies than trees with similar stem diameter on the permanent river. Trees on the ephemeral river progressed through stages of senescence at smaller stem sizes than those on the permanent river. Tree growth was episodic and strongly related to tree senescence, but the amount of growth in stem diameter was less on the ephemeral river. Tree senescence and stem diameter had the strongest effects on the probability of trees bearing hollows, with a lesser interactive effect of river hydrology. Variation among individual trees, sites and sub-catchments was large and not fully explained by the site characteristics measured. Our data enable predictions of future vegetation growth and fauna habitat development in semi-arid riparian woodlands that are not managed for silvicultural production. Our findings indicate that differences in the hydrological regimes of inland rivers manifest as differences in structural attributes of E. camaldulensis, even among individuals occupying the same catchment management area. Such differences should be reflected in future management strategies to sustain the function and biodiversity of riparian forests and woodlands.