Biodiversity facets, canopy structure and surface temperature of grassland communities

Canopy structure is an important driver of the energy budget of the
grassland ecosystem and is, at the same time, altered by plant diversity.
Diverse plant communities typically have taller and more densely packed
canopies than less diverse communities. With this, they absorb more
radiation, have a higher transpiring leaf surface, and are better coupled
to the atmosphere which leads to cooler canopy surfaces. However, whether
plant diversity generally translates into a cooling potential remains
unclear and lacks empirical evidence. Here, we assessed how functional
identity, functional diversity, and species richness of grassland
communities in the Jena Experiment predict the mean and variation of plant
surface temperature mediated via effects of canopy structure. Using
terrestrial laser scanning, we estimated canopy structure describing
metrics of vertical structure (mean height, LAI), the distribution
(evenness), and the highest allocation (center of gravity) of biomass
along height strata. As metrics of horizontal structure, we considered
community stands gaps, canopy surface variation, and emergent flowers. We
measured surface temperature with a thermal camera. We used SEM models to
predict biodiversity effects on the surface temperature during two
seasonal peaks of biomass. Before the first cut in May, herb-dominated
communities directly promoted lower leaf surface temperatures. However,
communities with a lower center of gravity (mostly herb-dominated) also
increased canopy surface temperatures compared with grass-dominated
communities with higher biomass stored in the top canopy. Grass-dominated
communities showed a smaller variation of surface temperatures, which was
also positively affected by species richness via an increase in mean
height. In August, mean surface temperature decreased with increasing
community clumpiness and LAI. The variation of surface temperature was
greater in herb-dominated than in grass-dominated communities and
increased with plant species richness (direct effects). Synthesis: The
mean and variation of canopy surface temperature were driven by
differences in functional group composition (herbs- vs. grass dominance)
and to a lesser extent by plant diversity. These effects were partly
mediated by the metrics of canopy structure but also by direct effects
unrelated to the structural metrics considered.