Long-term impact of prescribed burning on water use efficiency, biological nitrogen fixation, and tree growth of understory acacia species in a suburban forest ecosystem of subtropical Australia

Prescribed burning is a useful tool in a suburban forest of subtropical Australia for bushfire risk management. Toohey Forest has been managed by controlled burns and was used in this study. This study examined the long-term impacts of prescribed burning on biological nitrogen fixation (BNF) of understory acacia species estimated by 15 N natural abundance method, water use efficiency (WUE) as reflected in foliar carbon isotope composition (δ13C), and plant growth 6–14 years after prescribed burning in a suburban Toohey forest ecosystem of subtropical Australia. Four sites (S1, S2, S3, and S4) were established in Toohey Forest, Queensland, Australia. Foliar samples were collected from two understory species of acacia (A. leiocalyx and A. disparimma) and reference plants (E. psammitica) at each study site for three seasons (growing seasons of Spring October 2018 and Autumn May 2019, and non-growing season of Winter August 2019). Foliar total nitrogen (N) concentrations of A. leiocalyx were higher at sites of S1, S3, and S4 than those of A. disparimma after 6–14 years of prescribed burning. Both species still depended upon BNF for their N supply, with a higher dependence maybe in winter than in summer at S3. The highest BNF rates for A. leiocalyx at all sites were also found in the winter of August 2019, while the lowest BNF rates were detected during the autumn of May 2019. The BNF was estimated as 56.5% for A. leiocalyx and 52.8% for A. disparimma at S1, with the corresponding values of 74.6% at S2, 63.4% at S3, and 85.0% at S4, respectively. This study demonstrates that A. disparimma for S1, S2, S3, and S4 had relative higher WUE than A. leiocalyx in the winter of August 2019 after 6–14 years of prescribed burning. There was a significant higher plant growth rate (tree height) of A. leiocalyx at sites of S1, S2, and S4, compared with that of A. disparimma. Both young acacia plants were more active in BNF than older plants, with the BNF peaked between 6 and 8 years after prescribed burning. Conversely, BNF was lower after 9–14 years of prescribed burning. There were significant and positive relationship between foliar δ13C and δ15N, highlighting the role of BNF in improving plant WUE and subsequently tree growth in the suburban native forest.