Sercu Bram K, Baeten Lander, van Coillie Frieke, Martel An, Lens Luc, Verheyen Kris, Bonte Dries
Department of Biology Terrestrial Ecology Unit (TEREC) Ghent University Gent Belgium.
Department Forest and Water Management Forest & Nature Lab Ghent University Gontrode Belgium.
Ecol Evol. 2017 Nov 11;7(24):10861-10870. doi: 10.1002/ece3.3528. eCollection 2017 Dec.
Light is a key resource for plant growth and is of particular importance in forest ecosystems, because of the strong vertical structure leading to successive light interception from canopy to forest floor. Tree species differ in the quantity and heterogeneity of light they transmit. We expect decreases in both the quantity and spatial heterogeneity of light transmittance in mixed stands relative to monocultures, due to complementarity effects and niche filling. We tested the degree to which tree species identity and diversity affected, via differences in tree and shrub cover, the spatiotemporal variation in light availability before, during, and after leaf expansion. Plots with different combinations of three tree species with contrasting light transmittance were selected to obtain a diversity gradient from monocultures to three species mixtures. Light transmittance to the forest floor was measured with hemispherical photography. Increased tree diversity led to increased canopy packing and decreased spatial light heterogeneity at the forest floor in all of the time periods. During leaf expansion, light transmittance did differ between the different tree species and timing of leaf expansion might thus be an important source of variation in light regimes for understory plant species. Although light transmittance at the canopy level after leaf expansion was not measured directly, it most likely differed between tree species and decreased in mixtures due to canopy packing. A complementary shrub layer led, however, to similar light levels at the forest floor in all species combinations in our plots. . We find that a complementary shrub layer exploits the higher light availability in particular tree species combinations. Resources at the forest floor are thus ultimately determined by the combined effect of the tree and shrub layer. Mixing species led to less heterogeneity in the amount of light, reducing abiotic niche variability.
光对于植物生长至关重要，在森林生态系统中尤为重要，因为其垂直结构显著，导致从树冠到林地的光被连续拦截。不同树种在透光量和透光异质性方面存在差异。由于互补效应和生态位填充，我们预计混交林相对于纯林，光透射率在数量和空间异质性方面都会降低。我们通过树木和灌木覆盖度的差异，测试了树种身份和多样性对叶展开前、展开期间和展开后光可利用性时空变化的影响程度。选择了具有不同透光率的三种树种的不同组合样地，以获得从纯林到三种树种混交的多样性梯度。用半球形摄影测量到达林地的光透射率。在所有时间段内，树木多样性增加导致树冠郁闭度增加，林地的空间光异质性降低。在叶展开期间，不同树种的光透射率确实存在差异，因此叶展开时间可能是林下植物物种光环境变化的一个重要来源。虽然叶展开后树冠层的光透射率没有直接测量，但很可能树种之间存在差异，并且由于树冠郁闭，混交林中的光透射率会降低。然而，互补的灌木层使得我们样地中所有物种组合在林地的光水平相似。我们发现，互补的灌木层利用了特定树种组合中更高的光可利用性。因此，林地的资源最终由树木层和灌木层的综合作用决定。物种混交导致光量的异质性降低，减少了非生物生态位的变异性。
