溪頭孟宗竹林之不同竹桿年齡水分利用特性

Abstract

孟宗竹 (Phyllostachys pubescens) 在台灣為一廣泛分布竹類, 其林分擴張可能影響當地生態系的水分循環與利用，其蒸散、水分潛勢、樹液流與水分導度會是當地森林生態系水分利用循環的重要因子。為了往後竹林經營，孟宗竹的水分利用特性值得探討研究。在前人研究中，已發現孟宗竹林分相比鄰近常見針葉造林地，有較高的樹液流流速和年蒸散量。然而竹桿年齡對孟宗竹水分利用特性卻尚未有進一步探討，這對生長壽命相對較短的孟宗竹來說相當重要。因此本實驗目的在於探討孟宗竹年齡對 (1) 葉片水分潛勢及 (2) 竹桿水分導度之影響 (3) 將前述兩點結果與前人研究之孟宗竹樹液流特性整合，並大致闡述竹桿年齡對各水分利用特性相互作用之影響。本研究以一、二、三、四、五、大於五年生孟宗竹為研究對象。葉片水分潛勢於2017年四月至 2018 四月以植物水分潛勢計量測。竹桿染色試驗將竹桿內維管束染色，以計算染色維管束面積與數量分別占整體維管束面積與數量之比例，並用於探討竹桿水分導度。 研究結果顯示五年生與大於五年生的孟宗竹在正午時其葉片水分潛勢顯著較小，但在黎明時，葉片水分潛勢於各年齡間並無顯著差異。竹桿染試驗中，四、五和大於五年生的孟宗竹相比於其他年生孟宗竹，有較小的染色面積比例和染色個數比例。根據上述觀察和前人研究推論，五年生和大於五年生的孟宗竹有著過低的葉片水分潛勢，此現象可能是源於五年生和大於五年生的孟宗竹中較低的竹桿水分導度，而此推測也在染色試驗中得證。造成染色比例較低的原因可能為孟宗竹維管束中的栓塞和木質素物質的填充。綜上所述，此研究推測孟宗竹在年齡大於四年後有較高可能性發生水分利用衰退。而在水分利用特性的相互關係中，此研究證實了老齡竹桿中較低的水分導度可能造成過低的葉片水分潛勢，而此水分利用特性可能抑制氣孔導度，進而降低樹液流流速以避免維管束氣穴現象和其他水分逆境。

Moso Bamboo (Phyllostachys pubescens) is a widely spread bamboo in Taiwan. The expansion of Moso bamboo in Taiwan may alter the water cycle in local ecosystem. Water use characteristics, such as transpiration, sap flux, plant water status and culm conductivity in Moso bamboo forest, can determine the water cycle in a forest ecosystem. Thus, in order to deal with bamboo stand management, it is necessary to study the water use characteristics of Moso bamboo. Previous studies revealed that Moso bamboo had a higher sap flux density; hence, higher annual transpiration than coniferous forests in the surrounding around Taiwan. However, the relationship between the aging of culm and water use characteristics is rarely investigated for Moso bamboo, which has a relatively shorter life span. This study aimed to clarify the age effects on (1) leaf water potential and (2) culm conductivity. Moreover, data on (3) sap flux density investigated in a previous study were integrated with the results of leaf water potential and culm conductivity in this study to comprehensively elaborate the age effects on water use characteristics of Moso bamboo. The 1, 2, 3, 4, 5 and >5-year-old bamboos were selected for observation. Leaf water potential from July 2017 to April 2018 was measure by a pressure chamber. Culm stain experiment was carried out on bamboo culms to estimate the culm conductivity using the ratio of the stained vascular area or number to the total vascular area or number, respectively. The results showed that 5 and >5-year-old groups had significantly lower midday leaf water potential than the other four age groups. However, no distinctive difference was found in predawn leaf water potential among six age groups. The 4, 5 and >5-year-old groups had less ratio of stained area or number to the total vascular area or number than the other groups. Based on the observation in the present study and the findings in previous studies, excessively low midday leaf water potential for 5 and >5-year-old-groups might result from poor culm hydraulic conductivity. The less stained area ratios of 4, 5 and >5-year-old groups might be the evidence of poor culm hydraulic conductivity. The less stained area ratio was probably due to embolism and tyloses blockage in vascular bundles in older culms. Therefore, Moso bamboo probably starts to show senescence in water use after 4years in age. In terms of relationship to water use, this study confirmed that low culm hydraulic conductivity in the older culms might result in excessively low midday water potential. Such water use characteristics in the older culms may inhibit the stomatal conductance and lead to low sap flux density to avoid cavitation and water stress.