Overstorey and juvenile response to thinning and drought in a jarrah (Eucalyptus marginata Donn ex Sm.) forest of southwestern Australia

Abstract

Aims: Forest thinning is expected to affect tree water use and carbon assimilation, but the related influence from climate variability is little known. Recent forest thinning in the Wungong catchment coincided with a record dry year following the thinning, which provides a rare opportunity to understand the climate influence on the thinning effect. Methods: A field experiment was conducted to examine changes before and after thinning, especially the rainfall, soil moisture, leaf water status, tissue isotope signature (13 C and 15 N) and N concentration of overstorey and understorey juvenile trees of Eucalyptus marginata (Donn ex Sm.). Results: Despite the post-thinning drought, surface soil was moister and juvenile jarrah plants were less water stressed, attributable to reduced rain interception and transpiration as a result of less canopy cover. The overstorey was under stress but mainly due to drought rather than by thinning. The concentration of N declined in both tree stems and juvenile leaves along with available N in soil, suggesting a soil N limitation. No treatment effects were detected from leaf relative water content and tissue isotope signature (13 C and 15 N). Conclusions: The drought effects were superimposed over the thinning effects on overstorey growth, with stemwood δ13C being a major indicator of water stress. The water relations and carbon assimilation of understorey juveniles were however dependent more on topsoil moisture, and the wetter soil during the year following thinning enhanced growth activity and hence the depletion of 13 C (more negative δ13C) in juvenile leaves.