Data from: How tree species identity and diversity affect light transmittance to the understory in mature temperate forests

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. 2. 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. 3. 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. 4. Synthesis 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.

光是植物生长的核心资源，在森林生态系统中尤为关键，因其显著的垂直结构会使得光从林冠（canopy）到林地地表（forest floor）依次被截获。不同乔木树种的透光量与透光异质性存在差异。基于互补效应（complementarity effects）与生态位填充（niche filling）原理，我们推测相较于纯林（monocultures），混交林（mixed stands）的透光量与透光空间异质性均会下降。
2. 本研究通过乔木与灌木盖度的差异，探究了树种组成与多样性对展叶期（leaf expansion）前、展叶期及展叶后光照可获得性时空变异（spatiotemporal variation）的影响程度。我们选取了三种透光特性迥异的乔木树种的不同组合样地，构建了从纯林到三种树种混交林的多样性梯度。采用半球摄影法（hemispherical photography）测定了抵达林地地表的透光率（light transmittance）。
3. 在所有观测时段内，乔木多样性提升均会导致冠层填充（canopy packing）增加，且林地地表的空间光照异质性降低。在展叶期，不同乔木树种的透光率存在差异，因此展叶物候可能是林下植物（understory plant species）光照环境变异的重要来源。尽管本研究未直接测定展叶后冠层尺度的透光率，但该透光率大概率存在树种间差异，且因冠层填充度提升而在混交林中有所下降。然而，互补性的灌木层使得本研究各样地中所有树种组合的林地地表光照水平趋于一致。
4. 综合分析 我们发现，互补性的灌木层会在特定的乔木树种组合中利用更高的光照可获得性。因此，林地地表的资源最终由乔木层与灌木层的共同作用决定。混交树种配置会降低光照量的异质性，从而减少非生物生态位的变异。