喀斯特峰丛洼地不同类型森林养分循环特征

以中国西南喀斯特峰丛洼地为研究区域用标准木法和收获法对人工林、次生林、原生林3个不同类型森林的6个代表性群落的生物量、营养元素生物循环量及循环特征进行了研究。结果表明:(1)不同类型森林群落乔木各器官的养分含量大小顺序为:叶枝根干,林下植被层和凋落物层的养分含量比较高,其含量普遍高于乔木层各组分,仅次于乔木叶片;各组分中营养元素以K、Ca最高,P、Mg最低;(2)3种类型森林间乔木层的养分积累量总规律表现为原生林(4540.30 kg/hm~2)次生林(2107.09 kg/hm~2)人工林(719.51 kg/hm~2),分别占林分养分积累量的88.30%、79.57%和62.60%;(3)3种类型森林生态系统养分总贮量相差不大,均主要集中在土壤层在各层分配格局有所差异;营养元素的年吸收量和年归还量均为次生林原生林人工林,年吸收量分别为:418.80、271.17和148.79 kg hm~(-2)a~(-1);年归还量分别为:182.98、111.43和43.37 kg hm_(-2)a~(-1);(4)不同类型森林养分利用系数总规律为人工林(0.35)次生林(0.20)原生林(0.10);循环系数则相反,为原生林(0.48)次生林(0.46)人工林(0.30);而周转时间为原生林(37.32)人工林(18.63)次生林(13.93)。喀斯特峰丛洼地土层薄,养分贮存能力差,森林养分循环能力相对较弱,沿着强、中、弱干扰递减梯度,3种类型森林养分利用效率和循环能力呈增长趋势。

Dynamics of nutrient elements in different types of forests in depressions between karst hills

Nutrient availability is a key factor affecting forest plantation growth and forest sustainability. The biological processes of nutrient cycling (including plant uptake, internal cycling of nutrients, litter fall, litter decomposition, and microbial transformations of C, N, and P) help to retain added nutrients and minimize losses due to leaching and erosion. A sound understanding of nutrient cycling in different types of forest types is helpful for forest management, vegetation restoration, and reconstruction and understanding of forest succession. In the present study, three forest types (plantation forest, secondary forest, and primary forest) in depressions between karst hills in southwest China were selected. Along the gradient from plantation forest to secondary forest to primary forest, human disturbance tended to decrease. The biomass and contents of nutrient elements in different forest compartments were investigated to study the nutrient element cycling in these forests. Forest biomass was investigated using the standard-timber and harvesting methods. Nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) in the tree layer, grass layer, shrub layer, and lichen were measured. For the tree layer, the nutrient elements in leaves, boles, branches, and roots were separately measured. Nutrient contents for various organs in the three forest types were in the order leaves > branches > roots > bole. Nutrient contents in the understory layer and litter layer were higher than those in the arbor layer, except for tree leaves. The contents of K and Ca were highest among the five elements, followed by N, while Mg and P were the lowest in various components. The total nutrient contents were 4540.30, 2107.09, and 719.51 kg/hm² in, respectively, primary forest, secondary forest, and plantation forest, which accounted for 88.30%, 79.57%, and 62.60% of total amounts, respectively. The arbor layer''s nutrient contents accounted for most of the total. Among the five elements, the K content and annual accumulation rate in the arbor layer of the plantation forest were greatest, while the Ca content and annual accumulation rate in the arbor layer of the secondary forest and primary forest were greatest. Both annual absorption and annual returns of the five nutrient elements in the three forests changed in the order secondary forest > primary forest > plantation forest. The annual absorption rates were 418.80, 271.17, and 148.79 kg hm^-2 a^-1 for, respectively, secondary forest, primary forest, and plantation forest. The annual return rates were 182.98, 111.43, and 43.37 kg hm^-2 a^-1 for, respectively, secondary forest, primary forest, and plantation forest. The nutrient utilization coefficients in the three forests changed in the order plantation forest > secondary forest > primary forest, while the order was the reverse for the cycling coefficients. The recycling periods changed in the order primary forest > plantation forest > secondary forest. The present study revealed that nutrient cycling was relatively slow in forests of the karst areas. Since the three types of forests covered a gradient of disturbance, our study showed that nutrient contents sharply decreased with disturbance. Therefore, it is crucial to reduce disturbance in order to sustain nutrient elements and facilitate karst vegetation restoration.