芦芽山针叶林分布上下限土壤温度及含水量的季节差异

Seasonal changes in soil temperature and water content at the upper and lower limits of coniferous forest on Luya Mountain, China

山地小气候特征对解释林线位置、形成机制以及生长⁃气候关系具有重要意义。由于高山气象数据匮乏,尤其是土壤水热数据的缺失,使以往对华北地区山地土壤的温湿度变化特征知之甚少。基于5个整年(2012—2016年)的连续监测,分析了华北芦芽山针叶林分布上下限土壤(10cm)温度和含水量的季节变化特征及差异。结果表明:(1)在芦芽山针叶林分布上限,北坡土壤10月末冻结,5月初解冻,南坡土壤冻结和解冻日均滞后于北坡,生长季内南北坡土壤均温、生长季长度无显著差异(122d,8.1℃和110d,7.6℃);(2)南北坡林线土壤含水量最低值都出现在冬季(1月),最高值则在秋季(10月和9月),并且南坡生长季土壤含水量(0.350 m3/m3)显著大于北坡(0.247 m3/m3);(3)与针叶森林的分布下限(2040 m a.s.l.)相比,林线土壤热量指标(年均温、生长季均温、最热月均温和生长季长度)均明显偏低,而土壤生长季内含水量显著偏大。研究结果揭示了亚高山区土壤冻融过程中温度和含水量的耦合关系,并进一步证实了芦芽山地区针叶树木径向生长在低海拔受干旱胁迫而在高海拔受低温限制。

The characteristics and variations in micro⁃climate across alpine regions play an important role in tree⁃line prediction and the climate⁃growth relationship.Due to a lack of meteorological stations,there is little knowledge about dynamics of soil temperature(ST)and water content(SWC)during freeze⁃thaw cycles in sub⁃alpine regions,especially in north⁃central China.In present study,based on in situ measurements from 2012 to 2016,seasonal changes in ST and SWC at the upper and lower limits for coniferous forest were analysed and compared on Luya Mountain,China.The results showed that(1)soil at treeline thawed in early May and froze in late October on the northern slope,and soil on the southern slope lagged behind about a week.Growing season length and mean STs of growing season have no significant differences between the northern and southern slopes(122d,8.1℃and 110 d,7.6℃,respectively).(2)The lowest SWCs for the northern and southern slopes were in January,and the highest were in September and October.And,in growing season,SWC of treeline at the southern slope was significantly greater than that at the northern slope(0.350m3/m3,0.247m3/m3).(3)The thermal indices for the soil across the treelines were smaller than the values at low elevation(2040 m),whereas SWC during the growing season was significantly greater.These results revealed the coupling relationship between ST and SWC during soil freezing and thawing in sub⁃alpine areas,and confirmed that the radial growth of conifer trees at low⁃elevation on Luya Mountain was affected by soil drought,whereas the growth of high-elevation trees was limited by low temperature.