Dendrochronological methods can be used to analyze growth reductions in annual rings and changes in morphology that are indicative of defoliation, and thus to reconstruct the history of insect outbreaks. However, yet the application has been scarce and there are very few records about insects outbreaks in the Hengduan Mountains.
Dr. FAN Zexin of Xishuangbanna Tropical Botanical Garden (XTBG) and Dr. Achim Bräuning of University of Erlangen-Nürnberg compiled tree-ring data from 4 host (Larix potaninii) and 4 non-host (Abies and Picea) sites in the central Hengduan Mountains, southwestern China. They analyzed growth patterns in larch (Larix potentii) and non-host spruce (Picea brachytyla, P. likiangensis) and fir (A. georgei) tree-ring chronologies and detected growth depressions in larch tree populations.
The researchers applied two methods (i.e. host and non-host comparison approach and running minima approach) to identify growth depressions induced by larch insect outbreaks from those caused by climatic and other abiotic factors.
They found that larch trees show higher inter-series and cross-sites correlations than co-occurring fir and spruce, which indicate that the larch chronologies contain a higher common signal related to biotic and/or abiotic factors. Radial growth depressions of larch trees, accompanying with reduced latewood density, were found synchronously within and across sties in the central Hengduan Mountains.
The tree-ring data provide evidences that insect defoliators have infected larch trees growing in the central Hengduan Mountains repeatedly. Eight major outbreaks coincided at regional scale were identified over the past 250 years. Spectral analyses indicate that insect outbreaks occurred regularly every 11–13 years, with no such periodicity exist in non-host tree-ring chronologies.