自然演替过程中红松幼苗生存策略与物候镶嵌驱动机制

Korean pine (Pinus koraiensis) as a dominant species of the climax in northeast China can hardly recruit from under its own canopy, rather it needs shade provided by deciduous species in secondary forest dominated by birch (Betula platyphylla) and aspen (Populus davidiana), a successional stage during the recovery to the climax status. This implies that the species is shade tolerant. On the other hand, the species shows very good performance for full light in nursery or regeneration on bare land, revealing its light demanding feature. There is no way to explain this contradictory simply by the theory of light response. Although it is generally thought that the successful regeneration under secondary deciduous forest is just because the high level of light availability, the applicants found that irradiance under the canopy of densely closed secondary forest is even weaker than under the old growth. It is thus believed that there exists a particular mechanism driving the persistence or extinction of the seedling population; one factor that can not be ignored is the light regime under canopy and the development rhythm of the seedlings. This project, based on the theory of phenological mosaicing, aims to explain the alternation or overlap of phenological processes between functional groups and the shade avoidance strategy of Korean pine seedlings by clarifying the balance of energy, in terms of carbon budget and physiological activity, with the closure dynamics of different functional groups that lead to declining light availability which is considered as the critical factor affecting seedling growth. Through the response patterns for seedlings during light-favorable or light-unfavorable period, the persistence of Korean pine population during long-term succession will be explained, and it is expected to enrich the population strategy theory and to provide scientific basis for managing the precious forest ecosystem.

红松在以自身为优势种的原始林下更新困难，需要白桦山杨次生林为其创造一定的荫蔽条件，说明红松是耐荫的。另一方面，红松全光下育苗和裸地造林后表现良好，又说明红松具有喜光的特点。这是两种相反的极端现象，无法用简单的光响应理论解释。一般认为，次生林中红松更新良好是因为光照条件优越。然而申请者发现，高度郁闭的次生林内光照强度比原始林还弱。这说明红松幼苗种群的维持或消亡有另外的驱动机制，其中微生境中光能供给模式和红松本身的发育节律的耦合是不可忽略的因素。本项目以林冠不同功能群郁闭过程及其导致的光供给水平变化作为影响幼苗生长的关键驱动因子，以幼苗体内能量收支平衡为关键生理过程，探索功能群之间物候镶嵌-重叠关系以及红松幼苗的避荫策略。通过能量收支平衡分析物候进程中光照利好期和不利期幼苗的响应特征，揭示顶极种红松种群在演替过程中的维持机理，丰富种群对策理论，也为珍贵的阔叶红松林生态系统管理提供科学依据。

红松耐阴性（也称耐荫性）与幼苗更新机理是学术界长期争论的问题。红松在以其自身为优势种的原始林中不能更新，只见幼苗不见幼树。该现象说明红松不耐阴，同时红松可以在全光条件下造林也说明了这一点。但是红松在林冠高度郁闭的阔叶林中能够顺利更新，而且在生长旺盛的夏季，次生林中的光照并不比原始林优越，这又说明红松是耐阴的。这两种似乎相互矛盾的现象一直困扰着生态学家。为了回答这个问题，本项目首先提出了物候镶嵌假说，即红松幼苗利用次生林春秋季节光照优越的条件进行光合作用，积累糖类物质以度过夏季漫长的阴暗期，而原始林由于有常绿树种红松的遮阴影响，红松幼苗不能进行充足的光合作用。为此本项目以红松为中心开展了综合研究，包括群落中全部树种的光合作用及其对光照强度的响应、幼苗在不同光照条件下的存活与生长、幼苗非结构碳不同季节收支平衡、各树种展叶过程观测与模拟、林内光照强度的连续观测等，重点研究红松幼苗在林内和场圃控制条件下对光照的生理生态响应特征。研究结果和假说一致，即红松是阳性树种，在林内，春秋季节光能水平是决定红松幼苗存活的关键因素。和阔叶树种不同，红松适应冷凉条件，在阔叶树种处于休眠状态时，红松仍然能进行光合作用。研究过程中对红松所处植物群落中的所有乔灌木树种以及主要草本植物的光合作用进行了对比测定，连续观测了木本植物的展叶物候并建立了不同尺度的展叶模型、模拟积温水平对展叶的驱动作用。本项目的研究不仅回答了红松种群的维持机理问题，也解释了植物群落中不同种群共存的生理生态与环境因子联合驱动机制，同时为森林经营实践中通过改善林内光照条件以促进阳性树种更新提供理论依据。