Age-related changes in tree structure and function: mechanisms and consequences to forest ecosystems processes

幼树和老树在冠层结构、叶片气体交换和化学成分以及暴露于寄生虫和病原体等生物因素方面通常存在很大差异。这种“与年龄有关”的变化可能是生态系统过程的重要驱动因素；它们在管理方面也至关重要，因为采伐通常涉及砍伐较大的树木。提出的研究建立在我的研究小组最近的一系列发现的基础上，这些发现表明，一些温带树木在冠内叶面积指数上表现出长时间的下降，植物寄生虫负荷对叶片生理有很大的影响，这在一定程度上解释了某些物种“与年龄相关”的光合作用下降。(1)利用生态生理学研究方法，在热带和温带两种不同地点，对成虫对林冠乔木叶片的影响程度、年龄依赖性和影响程度进行了研究。在影响非常大的情况下，将利用对照接种研究，结合诊断性气体交换和荧光测量，详细分析生理机制。(2)研究了树木形态和功能的年龄依赖性变化对土壤过程的影响，特别是验证了老树自养呼吸减少的假设，以及与叶片C:N比增加相关的凋落物分解率降低的假设。(3)我们将研究不同树龄和大小的树冠下森林更新模式的趋势，验证老树由于光穿透增加而增加对光照要求更高的树种的吸收的假设。(4)利用机载激光雷达、多光谱/高光谱反射率、叶级光谱反射率和透射率测量数据，探索树木年龄和衰老遥感的潜力——特别是测试与冠层内LAI与年龄相关的减少相关的一致信号，以及与年龄相关的叶片结构和化学变化相关的叶片光谱信号的变化。

Young and old trees commonly differ greatly in terms of canopy structure, leaf gas-exchange and chemistry, and exposure to biotic factors such as parasites and pathogens. Such "age-related" changes may act as important drivers of ecosystem processes; they are also critical in a management context, since harvesting generally involves the removal of larger trees. The proposed research builds on a set of recent discoveries from my research group showing that some temperate trees show a long period of decline in intra-crown leaf area index, and that plant parasite loads can have large impacts on leaf physiology that partially explain "age-related" photosynthetic decline in some species. Four inter-related studies are proposed: (1) We will utilize ecophysiological techniques to evaluate the prevalence, age-dependence, and magnitude of impacts of gall-forming arthropods on foliage of adult canopy trees at one tropical and one temperate site. In cases of very large effects, the physiological mechanisms will be analyzed in detail making use of controlled inoculation studies coupled with diagnostic gas-exchange and fluorescence measurements. (2) We will examine effects of age-dependent changes in tree form and function on soil processes, specifically testing the hypotheses that older trees will show reduced autotrophic respiration, and reduced litter decomposition rates related to increasing C:N ratio of foliage. (3) We will examine trends in forest regeneration patterns beneath crowns of trees varying in age and size, testing the hypothesis that older trees will show enhanced recruitment of more light-demanding tree species due to increased light penetration. (4) The potential for remote sensing of tree age and senescence will be explored using data from airborne LiDAR, multi- / hyper-spectral reflectance, and leaf-level spectral reflectance and transmittance measurements -- specifically testing for consistent signals related to age-related reductions in within-canopy LAI, and for changes in leaf spectral signals related to age-related changes in leaf structure and chemistry.