芦芽山华北落叶松(Larix principis-rupprechtii)树轮宽度年表对气候因子的响应

在芦芽山地区采集3个不同海拔的华北落叶松(Larix principis-rupprechtii),在传统去趋势的基础上,采用"signal-free"方法对拟合曲线进行修正,避免了中等频率的气候信息引起的拟合偏差,最终建立3个不同海拔树轮宽度标准年表(STD)。同时以10a为界对上述年表进行滤波处理,得到3个低频年表。年表特征值表明,随着海拔升高,年轮平均轮宽变窄,敏感性和高频信息增强,低频信息减弱,这可能与逐渐恶劣的生境有关。中、低海拔年表的低频信息更一致,中、高海拔的高频信息更接近,而高、低海拔无论是标准年表还是高频、低频年表相似性均较差。树轮气候响应分析显示,低海拔STD年表与5月最低温负相关最为显著,STD和低频年表均与5、6月份土壤温度显著负相关,说明生境暖干,树木主要受生长季的干旱胁迫;中海拔STD年表与当年5月最高温正相关最为显著,STD和低频年表与土壤温度相关均不显著,说明生境逐渐变得冷湿,生长季的低温成为树木生长的限制因子;高海拔STD年表与气象要素相关不显著,低频年表与当年4月土壤温度正相关,说明高海拔最为冷湿,并有季节性冻土分布,生长季的土壤低温成为树木生长的限制因子。因此,全球变暖的趋势将更有利于高海拔树木的生长,而低海拔树木的干旱胁迫进一步加剧。

Climate-adial growth relationship of Larix principis-rupprechtii at different altitudes on Luya Mountain

Tree-ring samples were collected from Larix principis-rupprechtii trees at three sites at different altitudes on Luya Mountain, in the Shanxi Province of North Central China. We used a "signal-free" method to alleviate potential biases due to climate medium-frequency disturbances, and generated three standard (STD) chronologies. The mean tree-ring width, first-order auto-correlation (ac1), and standard deviation (SD) of the three chronologies decreased as the altitude increased, and the mean sensitivity (MS), absent rings percentage, and series inter-correlation (r) increased with increasing altitude. The low-frequency and high-frequency chronologies were generated by applying low-pass (f < 0.1037) and high-pass (f > 0.1037) filters. Generally, the climate sensitivity and high-frequency variability of tree-ring chronologies increase with increasing altitude, while mean tree-ring widths and low-frequency variability were lower at high elevations. Low-frequency chronologies from middle and low altitudes exhibit less variation; while high-frequency chronologies from middle and high altitudes are similar to each other. The chronologies of high and low altitudes have few common variations. The correlation function and response function analyses were used to identify the key climate factor limiting the growth of trees at different altitudes. A significant negative correlation was observed between tree rings at low altitudes and the minimum temperature in May, while standard and low-frequency chronologies were significantly negatively correlated with soil temperatures in May and June. This indicates that trees at low altitudes experience drought stress due to the warm, dry climate. The STD chronology is most significantly positively correlated with the maximum May temperature, and significantly negatively correlated with the maximum April temperature at the middle altitude site, which has a relatively cold and wet climate. The relationship between chronology and soil temperature is not significant. High April temperatures intensified drought conditions and suppressed the expansion and absorption of water by cambium cells. Greater moisture and higher temperatures in May were more beneficial for tree growth than high moisture and temperatures in April. The relationships between chronologies (STD and low frequency) and climate factors at the high-altitude site with seasonally frozen soil are not significant, while the low-frequency chronology is positively correlated with April soil temperatures. Low soil temperatures could suppress the activity of cambium cells and delay the beginning of the growing season; thus, leading to the formation of narrow growth rings. Generally, the environmental conditions became increasingly moist and cold as altitude increased, and the factors limiting tree growth changed from growing season drought conditions to low soil temperatures at the beginning of the growing season. It appears that the current warming trend favors coniferous forests at high altitudes. However, drought stress on the mixed broadleaf-conifer forest at the low altitude site may be intensified; this would provide a reasonable advice for the forestry management in arranging the forest pattern in different altitudes.