Dynamic chlorophyll fluorescence parameters and photosynthetic capacity of <em>Quercus mongolica</em> and <em>Tilia amurensis</em> seedlings were measured immediately after suddenly exposing to 10%, 30% and 100% daylight from greenhouse (5% relative light intensity). The aim of the study was to investigate the light acclimation and photoprotective mechanisms to the change of light environment on the two species. The results showed that: maximal photochemical efficiency of PSⅡ(<em>F</em>_v/<em>F</em>_m) decreased immediately on sudden exposure to 100% daylight, with the maximal decrease occurring 3 days (0.52) after transfer on <em>Q. mongolica</em> and 1 day (0.67) on <em>T. amurensis</em>. The decrease on <em>Q. mongolica</em> was obviously higher than that on <em>T. amurensis</em>. During the light acclimation, the <em>F</em>_v/<em>F</em>_m was followed by a subsequent recovery to the initial value, indicating that photodamage in the two species did not occur in the short term; the value of the photosynthetic capacity (<em>P</em>_max) and actual efficiency of PSⅡ (<em>Ф</em>PSⅡ) under 30% daylight was higher than 10% and 100% daylight, indicating that relative to high and low light, moderate light was better to the growth of seedlings; the variation of <em>P</em>_max, <em>F</em>_v/<em>F</em>_m, <em>Ф</em>PSⅡ, NPQ between 100% and 30% daylight in <em>Q. mongolica</em> was greater than that in <em>T. amurensis</em>, indicating that <em>Q. mongolica</em> was more susceptible to high light than <em>T. amurensis</em>; and the two species dissipated their excess light energy both depending on the increase of non-photochemical quenching of chlorophyll fluorescence (NPQ) and the Car/Chl ratio, and on the decrease of the Chl content.

Light is a key resource for tree performance and hence, tree species partition spatial and temporal gradients in light availability. Although light distribution drives tree performance and species replacement during secondary forest succession, we yet lack understanding how light distribution changes with tropical forest development.This study aims to evaluate how changes in forest structure lead to changes in vertical and horizontal light heterogeneity during tropical forest succession.We described successional patterns in light using a chronosequence approach in which we compared 14 Mexican secondary forest stands that differ in age (8-32 years) since agricultural abandonment. For each stand, we measured vertical light profiles in 16 grid cells, and structural parameters (diameter at breast height, height and crown dimensions) for each tree.During succession, we found a rapid increase in stand size (basal area, crown area and length) and stand differentiation (i.e. a gradual leaf distribution along the forest profile), which leads to fast changes in light conditions and more light heterogeneity. The inflection points of the vertical light gradient (i.e. the absolute height at which 50% relative light intensity is attained) rapidly moved towards higher heights in the first 20 years, indicating that larger amounts of light are intercepted by canopy trees. Light attenuation rate (i.e. the rate of light extinction) decreased during succession due to slower accumulation of the crown area with height. Understorey light intensity and heterogeneity slightly decreased during succession because of an increase in crown size and a decrease in lateral gap frequency. Understorey relative light intensity was 1.56% at 32 years after abandonment.. During succession, light conditions changed linearly, which should lead to a continuous and constant replacement of species. Especially in later successional stages, stronger vertical light gradients can limit the regeneration of light-demanding pioneer species and increase the proportion of shade-tolerant late-successional species under the canopy. These changes in light conditions were largely driven by the successional changes in forest structure, as basal area strongly determined the height where most light is absorbed, whereas crown area, and to a lesser extent crown length, determined light distribution.© 2021 The Authors. Journal of Ecology published by John Wiley & Sons Ltd on behalf of British Ecological Society.

叶面积指数(LAI)的空间异质性对研究植物的生长状况、分布格局及其对气候变化的响应机制至关重要, 然而关于不同因素对解释LAI空间变异相对贡献率的报道尚少。该研究依托小兴安岭9.12 hm ² (380 m × 240 m)谷地云冷杉林固定样地, 采用LAI-2200植物冠层分析仪测定了228个小样方(20 m × 20 m)的LAI, 基于地统计学方法分析了LAI的空间异质性; 测定了每个小样方的28个林分因子和10个土壤因子, 利用主轴邻距法(PCNM)量化了空间因子, 并采用方差分解的方法解析了林分、土壤、空间因子及其相互作用对LAI空间变异的相对贡献率。结果表明: LAI在37 m尺度内具有强烈的空间自相关, 且在不同方向上LAI呈现相异的空间格局; 3种因子及其相互作用共同解释了LAI空间变异的50.4%, 其中空间因子的贡献率最大, 单独解释了LAI空间变异的25.5%; 中等树(5 cm &lt;胸径≤ 10 cm)的密度和主要树种(冷杉(Abies nephrolepis)和云杉(Picea spp.))的胸高断面积均与LAI显著正相关, 质量含水率与LAI显著负相关。总体来看, 空间自相关对小兴安岭谷地云冷杉林LAI空间异质性的决定作用明显强于林分因子和土壤因子。