Effectiveness of thinning in mitigating post-drought legacy effects on Chinese fir in subtropical plantation forests, Southeastern China

Abstract

Legacy effects following drought are widely detected across worldwide forests, significantly affecting the growth recovery and susceptibility of trees after droughts. Thinning is a common forest management practice used to alter tree growth and growth-climate relationships. Although the effects of thinning on tree response during drought have been investigated, how thinning modulates post-drought legacy effects remains largely unknown. In this study, based on tree-ring data of 140 trees, we examined the effects of thinning on post-drought legacy effects using the quantile mixed effect model for Chinese fir (Cunninghamia lanceolata) in Southeastern China. The tree-ring data were stratified sampling from a thinning experiment applied 10 years ago in 8-year-old plantations and included four thinning intensities (20%, 25%, 33%, and 50% reduction of tree number) and an unthinned control treatment. Drought legacy effects of tree growth positively depended on tree social status with the magnitudes and variations larger in higher status classes. Dominant large trees without thinning management had the greatest drought legacy effects. Although the effects of thinning varied slightly at different quantiles, they all indicated that thinning mitigated the growth legacies after drought and the reduction effect was more pronounced with increasing thinning intensity. Thinning could also reduce post-drought climate sensitivities, but only after moderate thinning (20% and 25% thinning intensity). Heavier thinning (33% and 50% thinning intensity) instead enhanced tree growth responses to climate changes following drought. Thinning intensity needs to be carefully considered to really reap the post-drought benefits of forest thinning management. Our findings suggested that mild thinning offered an alleviation of climate dependency following drought in addition to reducing drought legacy effects on growth, benefiting tree recovery from drought. The results of this study are useful to inform management adaptive strategies for drought-vulnerable plantations under increasingly frequent droughts.