暖温带和亚热带树木初级和次级生长的协调关系及机制研究

Primary (budburst, shoot and foliage) and secondary (cambium and xylem) growth of trees play a critical role in fixing carbon in terrestrial ecosystems. Under climate warming, it is essential to precisely investigate growth dynamics and processes of primary and secondary growth, as well as their coordination relations and the mechanisms behind. This may allow better understanding and quantification of the role of climate change playing in tree growth, forest structure and composition as well as succession, which may greatly improve the prediction capability of ecosystem energy exchange and carbon cycle models. Our study will be conducted in warm temperate forests in Jigongshan Natural Reserve, Henan Province and subtropical evergreen rain forests in Shimentai Natural Reserve (Yingde), Guangdong Province. The purposes of the study are to 1) weekly monitor budburst, shoot and foliage growth, variation in auxin concentration, and cambium activity and xylem formation of dominant tree species in these ecosystems during three growing seasons (2016-2018) using the recently developed micro-sampling approach, 2) determine the onset, termination, and duration of the primary and secondary growth and their growth rates by nonlinear fitting, 3) explore and compare their potential relations with the changes in auxin concentration and environmental factors among sites and across species, and 4) quantify the potential relationships between primary and secondary growth as well as auxin concentration and environmental factors using the Mixed-effects model and further elucidate the physiological mechanisms underlying them. We expect this study can greatly improve our understanding in carbon allocation within a tree during the growing season and tree physiology in subtropical and temperate forest ecosystems.

树木的初级（枝、叶和芽）和次级生长（形成层和木质部）对生态系统固碳起着至关重要的作用。探讨气候变暖下初级和次级生长的动态过程及它们之间的协调关系和内在生理机制，有助于深层次理解和量化气候变化在树木生长、森林生态系统结构和组分变化以及群落演替中的作用，从而提高生态系统能量循环和碳循环模型的预报能力。本研究拟在河南鸡公山和广东石门台国家自然保护区，用最新的微采样技术在3 个生长季（2016-2018）内每周连续监测主要树种树干形成层活动和木质部的形成过程，并同时监测枝、叶和芽的生长及生长素含量的变化，通过非线性函数拟合来定量初级和次级生长的开始、结束、生长期及生长率等；用线性和非线性拟合定量其与环境胁迫的关系并对比样点间和树种间的差异；用混合效应模型来定量初级和次级生长及生长素和主要环境胁迫之间潜在数量关系并阐明其生理机制。本研究对暖温带和亚热带树木生长季内碳的分配机制将有突破性的认识。

随着全球气候变暖，树木的初级生长和次级生长都面临着挑战。而且近年来研究发现，树木的初级生长和次级生长对气候变化的响应存在明显的空间和时间差异，且因种而异。自我国亚热带到温带过渡带这一广大地区，过去对树木的研究多集中在较长时间尺度上，在年内动态的研究上，由于缺乏必要的手段导致我们对该地区树木生长动态了解甚少。我们利用微树心技术，连续3年每周监测温带地区的河南鸡公山和亚热带地区的广东省石门台优势树种径向生长动态，并结合气候、氮沉降、生长素等方面，利用线性和非线性混合模型、决策树等分析手段，定量化研究影响树木生长的主要因素。并结合其他地区已得的数据，综合分析树木生长如何应对气候变化。.目前得到以下主要结果：.1）不同树种随气候变暖春季展叶时间和秋季落叶时间存在差异，春季展叶时间在1951-1980年间提前，之后降低；而秋季落叶时间则依种而异。春季展叶时间对海拔梯度的响应呈非线性趋势，表明不同海拔的气候变暖差异较大，但不同海拔梯度的物候越来越趋于一致。此外春季霜冻对物种的危害也存在种间差异：对温度敏感的树种霜冻危害更大。.2）亚热带地区树木全年都在生长，温度是决定木质部细胞增大的主要因子。该地区的马尾松在1月左右存在一个较短的半休眠期，且在干季（10-次年3月）的生长情况优于湿季（4-9月份）。生长素和氮沉降都可以促进森林生长初期木质部的生长速度，从而增加亚热带地区的森林碳汇。.3）温带地区树木生长存在明显的休眠期（11-来年2月），生长高峰期总体在春夏季。氮沉降对树木的作用因种而异，对栓皮栎的作用不明显，而对麻栎有较为显著的促进作用。此外林冠氮添加的促进作用强于林下氮添加，表明叶片的截流作用也是氮元素促进植物生长的重要途径。此外北半球，主要是温带和寒带地区温度升高导致的干旱是森林衰退的主要原因。