不同树高对荷木液流密度、整树蒸腾量和时滞的效应

人工林面积不断增大,这不仅能解决由于森林砍伐引起的一系列社会问题,而且还对解决水土保持、二氧化碳减排等环境问题起到重要作用。了解人工林的生长特性和蒸腾效率,对植被生长、恢复和管理有着重要意义。为此,该研究连续监测了华南地区12棵不同高度荷木人工林的液流密度,对样树以高度划分等级,采取错位相关法分析不同高度等级胸高处液流与冠层蒸腾的时滞效应。结果表明:气候环境相同时,所有样树胸高处液流日格型相似;荷木林蒸腾量优势木中间木劣势木,所有树木湿季月蒸腾量大于干季月蒸腾量;不同高度等级之间时滞差异显著,劣势木时滞50min,优势木和中间木时滞20min;所有样树干湿季时滞差异不显著,同一高度级两季节时滞差少于10min。这些说明:在干季华南地区土壤水分仍然相对较充足,植物输水阻力没有受到土壤水分降低和长距离水分传导的影响;中间木和优势木时滞短,水力阻力小,蒸腾量大并占据着林段的有利资源;劣势木长势低矮,时滞长,导管阻力大,蒸腾量少,光合作用需要的水热资源少,所以回馈根部的营养物质少,不均衡的营养循环使得林段分化愈明显,劣势木将逐渐从林段中被淘汰。该文指出在荷木人工林生长后期,对于长势低矮,生命力极弱的劣势木应定期砍伐,这样能增加优势木和中间木对光照及水分等有利资源的分配,提高林分质量,增加林地生产力。

Effects of tree height on sap flow, whole-tree transpiration and time lag of Schima superba

Plantation area has been increasing in recent years,which is not the only effective solution for the social consequences caused by forest deforestation but also plays an important role in solving many environmental problems,like erosion control and carbon dioxide reduction. It has great significance for plantation growth,restoration and management by understanding their transpiration efficiency and growth characteristic of plantation. In order to do that,sap flow of 12 trees were measured in a Schima superba plantation in South China. All sample trees were grouped according to height into three ranks:dominant tree,intermediate tree and suppressed tree. Cross-correlation analysis was performed to estimate time lag between sap flux density and PAR measurement being a substitute for canopy transpiration at this site. It showed that in those days with similar environmental conditions,daily courses of sap flow trends at breast height were similar among all the trees. No matter at daily or monthly scale,the sap flux density and whole-tree transpiration of all measured trees were ranked as:dominant tree>intermediate tree>suppressed tree; and at the seasonal scale,the amount of transpiration was higher in wet season than that in dry season. The time lag showed significant variation across different ranges of height,in which time lag of dominant tree and intermediate tree was 20 min,and that of suppressed tree was 50 min. It did not show any significant variation in different seasons,and the differences of time lag in the same height rank were less than 10 min in both wet and dry seasons. This suggested that the soil moisture was relatively sufficient in the dry season in South China,and that the hydraulic resistance of sapwood was not altered by decline of soil moisture and the long distance of conductive pathway. In our site,the time lags of the taller dominant trees and intermediate trees were short with high amount of transpiration and little transport resistance because they took a favorable position in the stand. However,the time lag of the shorter suppressed trees was large with fewer amounts of transpiration due to great transport resistance. They not only could not acquire sufficient resources of water and light for photosynthesis,but also returned limited photosynthate to the root system belowground. The non-uniform nutrient cycle resulted in obvious differentiations within the S. superb,and consequently the suppressed trees would be eliminated from the stand. This paper pointed out that suppressed trees which grow smaller and weaker could be cut regularly in the later growth stage of Schima superb. It could increase the resource distribution of enough water and light for plant photosynthesis,and that improved the stand quality and increases stand productive capacity.