密度对缺苞箭竹凋落物生物元素动态及其潜在转移能力的影响

研究了一个生长季节内,缺苞箭竹(Fargesia denudata)-紫果云杉(Picea purpurea)原始林下不同密度缺苞箭竹凋落物及其生物元素含量的动态,比较了凋落物与新鲜叶中生物元素含量的差异,探讨了生物元素在缺苞箭竹体内的潜在内转移能力。研究结果表明:在生长季节内,缺苞箭竹凋落物量随着缺苞箭竹密度增加而增大。凋落物中C、N、P、K含量随着缺苞箭竹密度增加而减小,但Ca、Mg含量随着缺苞箭竹密度增加而增大。凋落物和新鲜叶中的C含量无显著差异,且二者均无明显的季节变化规律;凋落物的N、P、K含量表现为在5、6、7月依次升高,7月以后逐渐下降的格局,且凋落物中的含量明显低于新鲜叶;凋落叶的Ca含量明显高于新鲜叶,但无明显的季节变化规律;凋落叶的Mg含量在缺苞箭竹指数生长期最低,而新鲜叶中Mg含量在缺苞箭竹指数生长期最高。缺苞箭竹密度对生物元素的动态变化规律无显著影响。内转移率表现为K>N>P,且P的内转移率随着缺苞箭竹密度的增加而升高,但缺苞箭竹密度对K、N的内转移能力影响较小;C在缺苞箭竹植株体内的内转移现象不明显;Ca在凋落物中的积累率随缺苞箭竹密度增加而增大;Mg元素的积累率随着缺苞箭竹密度增加越来越高,而内转移率越来越低。

EFFECTS OF FARGESIA DENUDATA DENSITY ON SEASONAL CHANGES IN LITTER NUTRIENT CONCENTRATIONS AND THEIR POTENTIAL RETRANSLOCATION

Dwarf bamboo (Fargesia denudata), a staple food of the giant panda, is the dominant synusia of the forest ecosystem in Wanglang National Nature Reserve, which plays an important role in the maintenance of forest ecosystem structure and function. The seasonal changes of nutrients in bamboo litter and nutrient retranslocation is one of the most profound ecological processes in the forest system. The seasonal change of nutrients in bamboo litter and nutrient retranslocation is regulated by the density of the bamboo stand as well as by biotic and abiotic factors. Although there have been a number of studies on nutrient cycling in bamboo forests, the effects of the bamboo density has not been investigated. The influence of bamboo density on the dynamics on litterfall, nutrient concentrations in bamboo litter, the differences in nutrient concentrations between litter and fresh leaves, and potential nutrient retranslocation were studied in three bamboo communities with different densities (D1 with （220±11） stems·m-2, D2 with （140±7） stems·m-2, and D3 with （80±4） stems·m-2) beneath a bamboo-fir (Picea purpurea) forest. Plots were chosen that had similar slope, aspect, soil parent material and texture, tree canopy, etc. Bamboo litter was collected in each stand and fresh leaves collected over a growing period from May to October, 2003, and nutrient concentrations (C, N, P, K, Ca and Mg) analyzed. The results indicated that litterfall production of bamboo increased with stem density during the growing period. The concentrations of C, N, P and K in litter decreased with stem density, but Ca and Mg showed the opposite pattern. It was found that N, P and K concentrations were lower in litter than in fresh leaves, increased from May to July, and decreased after July. Carbon concentrations in both litter and fresh leaves did not vary with the stem densities and month. Calcium concentrations were higher in litter than in fresh leaves over the growing season, but there was no significant monthly variation. Litter had the lowest Mg concentrations in July (during the exponential growth period) whereas fresh leaves showed the opposite pattern. No significant monthly differences in nutrients were found among the three communities. The potential retranslocation capacity of P was strongly regulated by stem density, but N and K retranslocation was not influenced by stem density. The potential retranslocation capacity of N, P and K showed the ranked order as K>N>P. The retranslocation of C in bamboo was not significantly different among the three bamboo communities. The accumulation of Ca in litter increased with stem density. The accumulation of Mg in litter increased with stem density, but the retranslocation capacity decreased with stem density. Density, therefore, greatly influenced the seasonal changes of nutrient concentrations in litter and their potential retranslocation capacity.