Effects of forest canopy density and epixylic vegetation on nutrient concentrations in decaying logs of a subalpine fir forest

Background:Deadwood and the associated epixylic vegetation influence nutrient cycles in forest ecosystems.Open canopies strongly regulate deadwood decomposition and disrupt epixylic vegetation on logs.However,it is unclear how the forest canopy density and epixylic vegetation growth affect the nutrient concentrations in deadwood.Methods:We measured the concentrations of nitrogen(N),phosphorus(P),potassium(K),calcium(Ca),sodium(Na),magnesium(Mg),and manganese(Mn)in experimentally exposed decaying logs placed in gaps,at the edge of gaps,and under the closed canopy during a four-year decomposition experiment in a Subalpine Faxon fir forest(Abies fargesii var.faxoniana)on the eastern Qinghai-Tibetan Plateau,China.To assess the effect of the epixylic vegetation,we experimentally removed it from half of the logs used in the study.Results:Under open canopy conditions in the gap and at the edge,the concentrations for most of the nutrients in the bark and the highly decayed wood were lower than under the closed canopy.The effect of the epixylic treatment on nutrient concentrations for all but K and Na in barks varied with the decay classes.Significantly lower concentrations of N,P,Ca,and Mn following the removal of epixylic vegetation were observed in the wood of decay class IV.Epixylic vegetation significantly increased most nutrient concentrations for decaying barks and wood under open canopy conditions.In contrast,epixylic vegetation had no or minimal effects under the closed canopy.Conclusions:Forest canopy density and epixylic vegetation significantly alter the nutrient concentrations in decaying logs.Open canopies likely accelerate the rate of nutrient cycling between the epixylic vegetation and decaying logs in subalpine forests.

亚高山森林倒木碳氮磷化学计量特征随林窗位置的变化

森林林窗对太阳辐射和降水(主要是降雨和降雪)的再分配可造成林窗内外微环境条件的异质性。本研究旨在通过改变微环境,测试林窗对倒木碳(C)、氮(N)和磷(P)化学计量比的影响。在青藏高原东部的亚高山森林中,将I-V腐解等级的岷江冷杉(Abies faxoniana)倒木分别置于林窗中心(GC)、林窗边缘(GE)和郁闭林下(CC)进行原位培养,分别于2013-2016年8月测定了腐烂树皮、边材和心材的C、N、P含量。研究结果表明,树皮N浓度从CC到GC呈下降趋势,而边材和心材则表现出相反的趋势。此外,边材和心材IV、V腐解等级的C/N从CC到GC呈下降趋势,心材V腐解等级的N/P从CC到GC呈增加趋势,这表明森林林窗对高度腐解等级倒木的C/N/P化学计量比有强烈影响。简而言之,高度腐烂的倒木更容易受到微环境的影响,必要时应适当调整郁闭林下或林窗中心的高度腐烂的倒木比例。