芦芽山林线华北落叶松径向变化季节特征

Abstract

利用点状树木径向变化记录仪对芦芽山林线树种华北落叶松树木茎干的径向变化进行了一年的连续观测,分析了华北落叶松茎干径向日变化规律及茎干累积变化的季节动态。结果表明:华北落叶松茎干日变化在温暖季和寒冷季存在着相反的变化模式。在温暖季,茎干径向日变化归因于空气温度导致树木蒸腾作用强度的日变化而使茎干组织水分发生变化;在寒冷季,茎干径向日变化主要是因为空气温度通过热力学原理导致的"茎干冻融作用"。华北落叶松年内茎干径向变化存在4个不同阶段:1)春季茎干水分恢复期,2)夏季茎干快速增长期,3)秋季茎干脱水收缩期,4)冬季茎干相对稳定期。在不同阶段,影响华北落叶松茎干径向变化的环境因子并不一致。土壤温度为生长季中控制华北落叶松茎干径向生长的主导因子。;The treeline, which is defined as the upper limit of altitudinal tree distribution, is one of the most conspicuous vegetation boundaries worldwide. Treeline environments are generally characterized by harsh climatic conditions, which restrict tree growth and metabolic functions. Tree growth at the treeline is very sensitive to environmental changes, and it may be immediately influenced by small changes to limiting factors. Therefore, tree growth at the treeline can be used as an amplifier of external environmental changes, which has important implications for global climate change research. Fine resolution studies of tree radial variation on short temporal scales can be useful for exploring the interplay of the main physical variables that trigger the radial variation. From 19 Oct. 2008 to 17 Oct. 2009, we continuously monitored the stem radial variation of <em>Larix principis-rupprechtii</em> trees using point dendrometers, at the treeline of the Luya Mountains, in Shanxi, northern China. Our results showed that there were two reverse daily variation patterns in the warm and cold seasons, which were related to the daily transpiration rates and daily stem freeze-thaw cycles, respectively. In the warm seasons, the diurnal stem variation pattern was resolved into three phases: (1) radius contraction, (2) radius expansion, and (3) radius increment. Generally, during daytime, when tree transpiration exceeded water absorption, stem diameters usually decreased, and stem expansion occurred at night because of greater water absorption by the roots compared with water loss due to transpiration. In our study, a wide range of stem radius variations occurred during the stem freeze-thaw cycles during the cold seasons when the air temperature fluctuated around -5℃. The course of the cumulative radial variation during the year showed similarities among individuals representing the characteristic seasonal patterns. The average net stem radial increment of <em>Larix principis-rupprechtii</em> during the growing season was (2014.1±240.5) μm. Annual stem radial variation of <em>Larix principis-rupprechtii</em> was divided into four distinct periods: (1) spring stem rehydration, (2) summer stem rapid growth, (3) autumn stem dehydration contraction, and (4) winter stem stagnation. During different growth periods, the major environmental factors controlling stem radial variation were different. During the first period, the large radial increments were due to stem tissue rehydration by root pressure in early spring, and soil water content became the crucial environmental factor influencing stem radius variation. During the period of rapid stem growth in the summer, soil temperature was the major determining factor. Low soil temperature can inhibit net photosynthesis of <em>Larix principis-rupprechtii</em>, distribution of non-structural carbohydrates in the stem, and root activity and water uptake. The third period was characterized by the cessation of increments accompanied by dehydration of stem cells. These changes accompanied the declines in temperature and soil water content. Changes occurring in the cambia likely allow for over-wintering, which may be a survival strategy to avoid frost damage during cold winter conditions. During the fourth period, <em>Larix principis-rupprechtii </em>entered the dormancy stage and the stem remained relatively stable. It was observed that fluctuations in the radius of<em> Larix principis-rupprechtii</em> were mainly related to the changes in air temperature.