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.