The effects of Phoracantha-induced dieback on photosynthesis, respiration and estimated leaf nitrogen in Eucalyptus pauciflora Photosynthesis, respiration and nitrogen in dieback-affected snow gums

Abstract

Eucalyptus pauciflora stands in Kosciuszko National Park are experiencing severe dieback events induced by Phoracantha beetles, threatening the habitat complexity of subalpine ecosystems. This study investigated the effects of Phoracantha-induced dieback on the maximum photosynthetic rate (A_max), average dark respiration (r_dark) and nitrogen (N) concentration in E. pauciflora leaves, and the relations between A_max, r_dark and estimated leaf N concentration. We collected leaf samples from a healthy stand in Charlotte Pass and a dieback-affected stand in Perisher and measured their rates of photosynthesis and dark respiration using a LI-COR 6400, and estimated leaf nitrogen using a SPAD. We found no difference in A_max and r_dark between leaf categories, but epicormic leaves had significantly higher N content than healthy canopy leaves. In contrast to the literature, we found no correlation between A_max and N. Our findings suggest that E. pauciflora may exhibit a plastic response in photosynthetic rate to drought or dieback stress, but that dieback may damage vascular tissue, limiting dark respiration. The high nitrogen content in epicormic leaves does not have a corresponding increase in photosynthetic rate. These results suggest the effects of Phoracantha attack on the vascular system and source-sink flows within a tree.