Experimental evidence for weakened tree nutrient use and resorption efficiencies under severe drought in a subtropical monsoon forest

Aims The functions of global forests are threatened by the increasing frequency of severe drought.Due to drought inducing reductions in soil nutrient availability,efficiencies of nutrient use and resorption of trees become crucial for forest functions and biogeochemical cycles.However,understanding the dynamics of responses of foliar nutrient use and resorption efficiencies to drought,especially in tropical or subtropical forests,is still limited.Our goal was to detect whether and how the importance of leaf nutrient use and resorption changes across different species in the hot and wet forests when suffering drought stress in different months.Methods Based on a 70%throughfall exclusion experiment in a subtropical forest,we collected green and senesced leaves of Schima superba and Lithocarpus glaber in different months from October 2016 to May 2019,to estimate the effects of drought on leaf nitrogen(N)and phosphorus(P)use and resorption efficiencies(i.e.NUE and PUE,NRE and PRE).Important Findings The effects of drought on nutrient use and resorption efficiencies varied between species and months.Based on a 2-year observation,drought had no effect on S.superba,but significantly decreased NUE,NRE and PRE of L.glaber by 3.4%,20.2%and 7.1%,respectively.Furthermore,the negative drought effects were aggravated by the natural summer drying in 2017.As a result,NUE and PUE of L.glaber were significantly depressed by 17.2%and 58.1%,while NRE and PRE were significantly reduced by 56.5%and 53.8%in August 2017.Moreover,the responses of NRE,PRE and NUE to drought were related with soil moisture(SM)for L.glaber,and when SM decreased to a threshold near 9 v/v%,drought effects were shifted from unresponsive to negative.Our results highlight a species-specific threshold response of nutrient use under drought in a subtropical forest.

日益频发的干旱严重威胁着全球森林生态系统的功能。由于干旱胁迫抑制了土壤养分可利用性,因此植物的养分利用和重吸收效率对森林生态系统的功能以及生物地球化学循环至关重要。然而,目前对于植物养分利用和重吸收效率在干旱条件下的动态响应规律的理解十分有限,且在(亚)热带区域尤为明显。因此,本研究主要探讨了在湿热森林中不同植物物种在不同月份遭受干旱胁迫时,其养分利用和重吸收效率的重要性是否会发生变化以及如何变化。本研究在2016年10月-2019年5月期间,依托一处位于亚热带常绿阔叶林的隔离70%自然穿透雨的干旱实验平台,采集了木荷(Schima superba)和石栎(Lithocarpus glaber)两个物种在不同月份的鲜叶及凋落叶,用于分析干旱对两种植物叶片氮磷利用和重吸收效率的影响(NUE和PUE,NRE和PRE)。研究结果表明,干旱对氮磷利用和重吸收效率的作用在不同植物物种和月份之间有差异。基于两年的观测结果显示,干旱对木荷的养分利用和重吸收效率无显著影响,却使石栎的NUE、NRE和PRE分别降低了3.4%、20.2%和7.1%。另外,2017年夏季发生的自然干旱进一步加剧了干旱对石栎养分重吸收的负作用。在2017年8月,石栎的NUE和PUE在干旱处理下分别降低了17.2%和58.1%,而NRE和PRE分别下降了56.5%和53.8%。此外,石栎的NRE,PRE和NUE对干旱的响应显著依赖于土壤水分条件,即当土壤湿度降至约9 v/v%时,存在一个阈值使干旱处理的效果从无影响转变为负作用。我们的结果表明,在干旱条件下亚热带常绿阔叶林树木的养分利用呈现出了物种特定的阈值响应。