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

Abstract

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. 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.

Light and warming drive forest understorey community development in different environments

“…Our findings thus support the notion that adaptive management to global change can restore small statured spring‐flowering geophytes with high conservation priority in postagricultural forest, but only when light is a limiting resource to enable coexistence with other species (Vandekerkhove, Verheyen, & De Keersmaeker, ; Verheyen et al, ). A structurally rich forest with a dense shrub layer can easily develop on former agricultural land over the course of a few decades to provide such a suitable forest microclimate (De Schrijver et al, ; Fridley & Wright, ; Sercu et al, ). The only remaining condition for settlement of slow colonizing spring‐flowering geophytes is then to alleviate dispersal limitation (Baeten et al, ; Barker et al, ).…”

Plant functional trait response to environmental drivers across European temperate forest understorey communities

“…The lack of effects of light availability on the investigated traits is surprising, as light is considered a key driver of the functional composition and dynamics of understorey communities (Lieffers et al ; Bartemucci et al ; Bartels & Chen, ; Bernhardt‐Römermann et al, ; De Frenne et al, ; Sercu et al, ). We sampled in quite closed mature forests with a mean canopy openness of 18.3% (min.…”

Hiding from the climate: Characterizing microrefugia for boreal forest understory species

“…As forest productivity and stand density have generally increased in recent decades (Felton et al, 2017), a microclimate buffering of macroclimate warming effects on understory vegetation is conceivable, and has been suggested for temperate forests (De Frenne et al, 2013). However, a too dense canopy might be detrimental for understory species for other reasons such as low light levels, interception of rain and high litter fall (Barbier, Gosselin, & Balandier, 2008;Sercu et al, 2017). More research is needed to understand the combined historical effects of forest management (e.g.…”