华北落叶松液流速率的优势度差异及其对林分蒸腾估计的影响

Variation of Sap Flow Density of Larix principis-rupprechtii with Dominances And Its Impact on Stand Transpiration Estimation

将实测的杨树液流速率和林木边材厚度(或胸径)经尺度扩展得到林分蒸腾,已成为常用的野外测定方法,但此法没有考虑其他树形因子的影响,当林分密度大、光竞争激烈时,会导致蒸腾估计误差偏大。为认识主要树形因子对液流速率的影响,并为改进样树液流向林分蒸腾的尺度扩展方法提供依据,在六盘山北侧半干旱区华北落叶松人工林内,利用热扩散探针对5株不同优势度样树的液流速率进行了连续观测,研究了生长季中期(叶面积指数达到峰值并保持稳定)不同土壤水分条件下不同优势度树木的液流速率差异。结果表明:优势度越大的树木,其液流在日内的启动越早,结束越晚,到达峰值越早,峰值也越大;日均液流速率明显比优势度小的树木高;液流速率对太阳辐射和饱和水汽压差瞬时变化的响应敏感性比优势度小的树木高,而对土壤水分条件的响应敏感性则弱于后者,但整体上对环境条件的响应趋势一致,不同优势度树木间液流速率的相对差异比较稳定。相关分析表明:液流速率与优势度(或相对树高)、树高呈极显著正相关,与冠长、胸径显著正相关,而与冠幅、边材面积正相关但不显著。利用拟合的优势度与液流速率之间的线性关系(R2=0.95)计算了样地内所有树木的液流速率及其平均值,即林分平均液流速率,该值比常用方法计算结果低16%。建议今后在利用样树液流速率测定结果进行尺度扩展计算林分液流速率和蒸腾时,增加考虑优势度等主要树形因子的影响。

It is a basic approach to estimate the stand transpiration through computing the average of sap flow density （ Js ） from a number of sample trees and then up-scaling. However, without considering the effect of tree size or spatial position in community on Js, the approach is likely to either under- or over-estimate the stand transpiration, especially in a high-density stand. In order to understand the difference in Js among trees of different size or degree of dominance, and further improve the accuracy of the up-scaling approach, a field experiment was conducted in a Larix principis-rupprechtii stand located at the semi-arid region of Liupan Mountains. In the experiment, the J, of five sample trees with different dominances （relative height） was continuously monitored by using thermal diffusion probes. Then the differences in Js among individual trees under three kinds of soil water conditions were analyzed with the data at mid-growing season when the tree leaf area reached to the maximum and remained relatively stable. It is found that the J, in trees with higher dominance （ ＂higher trees＂ ） started earlier in the morning but ended later at night than in trees with lower dominance （ ＂lower trees＂ ）. In addition, the maximum of J, during the daytime appeared earlier and was higher in higher trees. Thus the daily average of Js in higher trees was apparently higher as well. Furthermore, the Js in higher trees was more sensitive to the solar radiation （ R, ） and the vapor pressure defi- cit （VPD） on a 5-min time scale, while on daily scale to the soil drought, higher trees showed less sensitive than the lower trees, implying a stronger ability for obtaining light and water. In general, however, no significant differ- ence was found in the pattern of J, response to environmental conditions among sample trees, and the relative differ- ence in J, was relatively stable. Correlation analysis indicated that the most important factors positively affecting the J, were the degree of dominance （relative height） and the tree height （P 〈0.01 ）, followed by the canopy length, the diameter at breast height （DBH） （P 〈 0.05 ）, the canopy width and the sapwood area （ P 〉 0.05 ）. In an im- proved approach, the ,Is of the forest stand was taken as the average of Js for all individual trees in the stand which was calculated by using the linear relation between J, and degree of dominance. The value calculated by the average of J, for the five sample trees was 16% lower than that calculated by the widely used approach. Therefore it is proposed that the degree of dominance should be taken into account in the up-scaling approach for the stand＇ s Js or transpiration estimate in future.