Recruitment, mortality and growth in semi-arid conifer-eucalypt forest: Small trees insure against fire and drought

Mathias Neumann; Chris S Eastaugh; Mark A Adams

doi:10.1111/jbi.14522

Recruitment, mortality and growth in semi-arid conifer-eucalypt forest: Small trees insure against fire and drought

J Biogeogr. 2023 Feb;50(2):291-301. doi: 10.1111/jbi.14522. Epub 2022 Dec 13.

Authors

Mathias Neumann^{1

2}, Chris S Eastaugh³, Mark A Adams¹

Affiliations

¹ Faculty of Science, Engineering and Technology Swinburne University of Technology Melbourne Victoria Australia.
² Institute of Silviculture University of Natural Resources and Life Sciences Vienna Austria.
³ Forestry Corporation New South Wales Dubbo New South Wales Australia.

Abstract

Aim: Well-managed semi-arid forests help offset global change by storing significant amounts of carbon above- and belowground and maintaining hydrological cycles. Larger trees have been the focus of many studies due to their carbon storage and habitat quality, yet recruitment and small trees are important components of ecosystem resilience and recovery. Here, we study the impacts of disturbances (including harvesting) on recruitment, mortality and growth for a mixed conifer-broadleaf semi-arid forest type using long-term data.

Location: Pilliga Forest in New South Wales, inland eastern Australia.

Taxon: Callitris-Eucalyptus forests.

Methods: We used data from permanent sample plots (PSPs) spanning 55 years, calculated stand structure, gains and losses and determined reasons for tree death (harvesting, fire, wind, drought and other effects). We extracted climate and fire data for the PSP locations using spatial analysis.

Results: Stocking of studied forests remained stable (modest increase in basal area and stem density), despite harvesting and wildfires over 6 decades. Compared to stands in the 1940s and prior to European settlement, current forests are composed of more trees per unit area, and these trees have smaller diameters. Recruitment and sustained presence of small trees have buffered impacts of recurring drought, fire and harvesting. Fires are a common feature of the studied ecosystems and fire impacts have increased in the past 20 years, especially in unmanaged stands, where fires have reduced tree carbon by >50%.

Main conclusions: Recruitment and growth of small trees are critical to offset carbon losses due to fire, drought and harvesting. All size classes have important ecological values in semi-arid forests and must be included in long-term monitoring programmes. Long-term data offer unique insights into combined effects of climate change, management and disturbances, especially for fire-prone ecosystems, where small trees are often susceptible to fire.

Keywords: Australia; Callitris glaucophylla; Eucalyptus crebra; ecosystem restoration; forest dieback; management; monitoring; pre‐European settlement; species mixture effects; wildfire impacts.