林线树轮生长与净初级生产力的夏至日最大化耦合机制及其气候变化响应意义

How to understand the limitation to alpine treeline growth is a long-debated issue, and it is still unclear if tree-ring growth could be served as an indicator of the change in net primary productivity and how they are closely linked over the growing season. We aim to test the hypothesis that in response to the short growing-season length in alpine environments, woody plant growth would be tightly coupled with canopy carbon gain through systematically regulating their phenological characteristics and physiological metabolisms and by synchronizing their maxima to the longest day length (around the summer solstice), which allows plants to safely complete secondary cell wall lignification before winter. In this way, the low temperature in the early growing season is the primary factor limiting both tree-ring growth and photosynthetic productivity. Based on long-term observation data for seasonal measurements of tree-ring radial increment, sap flow, litter-fall, leaf/tree-ring carbon isotope ratios and leaf nitrogen concentration etc. across two timberline forests in southeast Tibet, our tasks are to: 1) model the dynamics of photosynthetic productivity, 2) examine if seasonal tree-ring growth rate and photosynthetic productivity are more sensitive to the air and soil temperature changes in the early growing season (before the summer solstice); 3) if seasonal tree-ring growth rate and photosynthetic productivity generally peak around the summer solstice, and if both are well correlated. When the hypothesis is confirmed, we can further establish a model relating net primary productivity (NPP) with tree-ring records (time series of ring width and carbon isotope ratio) in recent 50 years. Such knowledge would be important to predicting the trend of timberline NPP changes in response to global warming, and understanding why recent climatic warming causes contrasting growth responses of trees in cold environments, the long-debated issue in global change science.

目前仍不清楚林线树轮生长变化是否能指示净初级生产力的长期变化，因为有关其动态耦合机制仍缺乏了解，且高海拔树木生长受限及其对气候变化的响应机制一直存在争议。申请项目旨在验证最新提出的夏至日最大化耦合机制假说(适应高寒环境的木本植物围绕夏至日系统调节其物候和生理代谢，使树轮生长和净初级生产力的最大值同时出现在夏至日附近，以有利于在严冬来临前完成其新组织的木质化)，拟依托藏东南两典型树种林线的长期定位观测研究平台，检测并确认：1）树木径向生长速率和净初级生产力的最大值是否同步出现在夏至日附近，两者是否普遍存在显著正相关关系；2）树木径向生长速率和光合生产力是否普遍对生长季早期（夏至日之前）温度变化更敏感。据此阐明高寒树轮生长与净初级生产力的季节动态耦合机制，建立基于树轮记录估算亚高山林线净初级生产力长期变化的模型方法，并为长期争论的高海拔树木生长受限及其对气候变化的响应机制问题提供新的机理解释。

本项目旨在验证研究假设：由于高海拔地区植物生长季短，适应高寒环境的木本植物围绕夏至日（一年中最大日长）系统调节其物候和生理代谢过程，使树轮生长、光合能力和氮素利用效率在夏至日附近出现最大化，以确保新形成组织有足够时间和资源在严冬来临之前完成木质化，这时生长季早期低温环境是树轮生长的主要限制因子。我们依托中科院藏东南站色季拉山林线长期定位观测研究平台（阴坡急尖长苞冷杉林线与阳坡方枝柏林线），开展了两树种林线树木径向生长量、树干液流速率、林冠凋落物量和林内相关气象要素的长期定位观测（2007-2017），采样测定了叶和树轮碳稳定同位素值及氮含量的季节和年际变化，获得了如下新认识：. 1、不同树种、年龄和观测年份的树木径向生长速率均在夏至日附近出现最大值，基于当年叶氮和碳同位素模拟的潜在NPP也在夏至日附近出现峰值，两者存在密切正相关。. 2、在生长季早期（夏至日之前），土壤温度升高显著促进树木径向生长和树干液流，但对氮素利用效率无显著影响；夏至日以后，氮素利用效率随土壤温度升高而下降，但树木径向生长和树干液流与土壤温度无关。. 3、树木径向生长量与凋落物量动态普遍存在一种滞后关系。在年际变化上，树轮宽度指数与前年5/6月-当年5/6月凋落物量存在密切正相关；在季节变化上，每月（每两月）的径向生长量与前一个月（前两个月）的凋落物量存在密切正相关。. 观测数据支持项目研究假设，表明指向夏至日的光周期钟在很大程度上调控着高寒树木生长和养分利用过程及其温度敏感性，树木生长的相关生理代谢速率在夏至日附近发生“共振”能确保新形成木质部组织有足够时间在严冬来临之前完成木质化。观测数据进一步显示，树轮生长的碳源主要来自于去年和当年的光合产物，树木径向生长围绕夏至日的最大化过程与树冠叶凋落动态紧密耦合，并通过养分回流再利用提高了植物在生长季早期的光合生产力和氮素利用效率，避免了土壤低温环境的不利影响。研究发现为基于树轮记录检测亚高山森林生态系统生产力和养分循环变化提供了新的理论和方法，拓展了树轮记录的生理生态学意义。