内蒙古东部天然白桦林的凋落物性质和储量及其随温度和降水梯度的变化格局

Litter stocks and chemical quality of natural birch forests along temperature and precipitation gradients in eastern Inner Mongolia,China

作为碳素和养分循环的重要组分,地表凋落物如何响应全球气候变化日益受到重视。中国北方半干旱地区森林草原过渡带斑块状分布的森林对气候变化引起的水热变化的响应较为敏感,但是对这些森林地表处于不同分解阶段的凋落物的化学性质和储量的格局及其如何响应温度、降水变化的报道较少。该研究分析了内蒙古东部地区处于年平均气温和年降水量梯度上的12个天然白桦(Betula platyphylla)林不同凋落物层次(最上层的初步分解层L1,中间的半分解层L2,最下层的腐殖质层L3)的化学性质及现存量后发现:1)随着分解的进行(即从L1到L3层),氮、磷浓度显著增加,可提取物浓度基本不变,酸溶性组分(acid soluble fraction,AS)浓度下降,酸不溶性组分(acid insoluble fraction,AIF)浓度增加。2)各元素现存量均在L3层最高,表明凋落物分解缓慢,养分积累。3)年平均气温和年降水量对凋落物有机组分(AS组分和AIF组分)的性质无显著影响,但L3层元素储量随年平均气温升高而增加,可能由于年平均气温较高的地点森林生产力更高,从而导致叶凋落物量增加,但由于受水分限制(尤其是在夏天),这些地点的凋落物分解速率不变或更低,使凋落物积累更为明显。上述结果表明:腐殖质层是这些白桦林的一个重要的碳及养分库,未来在降水没有明显变化的情况下,这一区域的升温可能会增加白桦林地表凋落物储量。

Aims Forest litter is both a large source of CO2 released from terrestrial ecosystems to the atmosphere and a critical sink of carbon and nutrients for plant growth. Studying dynamics of forest litter and its response to temperature and precipitation changes can improve our understanding of forest carbon and nutrient cycles and their interactions with projected climate change. Our objective was to examine the stocks and chemical quality of forest litter in natural birch （Betula platyphylla） forests that vary in both annual mean temperature （AMT） and annual mean precipitation （AMP）. Methods During July and August 2008,we measured the standing stocks and concentrations of carbon,nitrogen and phosphorus and organic fractions （extractives,acid soluble fraction （AS） and acid insoluble fraction （AIF）） of three litter layers （L1：slightly decomposed layer,L2：half-decomposed layer and L3：humus layer） in the forest floor of 12 birch forests in Inner Mongolia,China. Important findings Along the decomposition gradient （i.e.,from L1 to L3）,nitrogen and phosphorus concentra-tions increased,AS concentration decreased,AIF concentration increased but extractives did not show significant change. Temperature and precipitation did not have significant effects on carbon fractions but at sites where AMT was higher,carbon stocks in L3 layer were higher,probably because of greater accumulation at higher-temperature sites as a result of higher litter production but similar decomposition rate compared to lower-temperature sites. These findings indicate that the litter layer （particularly the L3 layer） is an important carbon and nutrient pool at the ecosystem scale and future increases in temperature without concurrent increases in precipitation may enhance litter accumulation in these natural birch forests.