State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing, China; University of Chinese Academy of Sciences, Beijing, China.
State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing, China.
Sci Total Environ. 2021 Aug 15;782:146924. doi: 10.1016/j.scitotenv.2021.146924. Epub 2021 Apr 6.
Recent warming over the Tibetan Plateau (TP) is approximately twice the global-mean surface temperature increase and poses a threat to the healthy growth of forests. Although many studies have focused on whether recent climate warming has caused forest growth decline on the TP, it remains unclear how asymmetric warming, that is faster increasing nighttime temperature than daytime, impacts forest growth decline. We explored this question by using a ring-width index series from 1489 juniper trees (Juniperus prezwalskii and J. tibetica) at 50 sites on the TP. We calculated the percentage of trees with growth decline (PTD) to reconstruct historical forest growth decline and employed a piecewise structural equation meta-model (pSEM) and linear mixed model (LMM) to explore influencing factors. We found that the PTD has decreased since the late 19th century, with an abrupt decreasing trend since the 1980s. Results of the pSEM show that winter minimum temperature has a stronger indirect negative effect on the variation in PTD (β = -0.24, p < 0.05) compared to that of the weak indirect positive effect of summer maximum temperature (β = 0.16, p < 0.05). The results of LMM show that the variation in PTD is directly negatively (p < 0.001) affected by both winter minimum temperature and summer total precipitation, but the former has a greater independent contribution than the latter (with 17.7% vs 2.5% of variances independently explained, respectively). These results suggest that increased winter minimum temperature substantially mitigates the growth decline in juniper forests on the TP. As the minimum temperature generally occurs at night, we conclude that the asymmetric increase in nighttime temperature has decreased the incidence of juniper forest growth decline on the TP under climate warming. This alleviating effect of nighttime warming is likely due to reduced low-temperature constraints and reduced damage to tree growth.
青藏高原（TP）最近的变暖幅度大约是全球平均地表温度上升的两倍，对森林的健康生长构成了威胁。尽管许多研究都集中在最近的气候变暖是否导致了 TP 上的森林生长下降，但夜间温度比白天上升更快的非对称变暖如何影响森林生长下降仍不清楚。我们通过使用 1489 棵（包括青海云杉和川西云杉）树木的年轮宽度指数系列，在 TP 上的 50 个地点进行了研究。我们计算了具有生长下降的树木百分比（PTD），以重建历史森林生长下降，并采用分段结构方程元模型（pSEM）和线性混合模型（LMM）来探讨影响因素。我们发现，自 19 世纪末以来，PTD 呈下降趋势，自 20 世纪 80 年代以来呈急剧下降趋势。pSEM 的结果表明，冬季最低温度对 PTD 的变化具有更强的间接负效应（β=-0.24，p<0.05），而夏季最高温度的间接正效应较弱（β=0.16，p<0.05）。LMM 的结果表明，PTD 的变化直接受到冬季最低温度和夏季总降水量的负向影响（p<0.001），但前者的独立贡献大于后者（分别解释方差的 17.7%和 2.5%）。这些结果表明，冬季最低温度的升高极大地减轻了 TP 上青海云杉和川西云杉林的生长下降。由于最低温度通常发生在夜间，我们得出结论，在气候变暖的情况下，夜间温度的非对称增加减少了 TP 上青海云杉和川西云杉林生长下降的发生。夜间变暖的这种缓解作用可能是由于低温限制减少和对树木生长的损害减少所致。
