Land surface skin temperature captures thermal environments of C3 and C4 grasses
Numerous studies have documented ecological sorting of C3 and C4 grasses along air temperature gradients. However, phylogenetically structured analyses suggest that closely related C3 and C4 grasses (in the same PACMAD clade: Panicoideae, Arundinoideae, Chloridoideae, Micrairoideae, Aristidoideae and Danthonioideae) occur in environments with similar air temperature, challenging our understanding of how the photosynthetic pathway influences grass biogeography. To better understand thermal differences between C3 and C4 grass lineages, we analysed the surface radiative temperature as an alternative measure that is more closely aligned with plant microclimate.
Hawaiian Islands, USA.
We used the MODIS land surface temperature (LST) product, a satellite-based measurement of radiative temperature. We compared LST with mean annual air temperature (MAT) for locations where C3 and C4 grass species were collected. We also utilized other satellite products, like MODIS tree cover, as a proxy for relative habitat openness where these grasses occur.
Comparisons of C3 lineages [in BEP (Bambusoideae, Ehrhartoideae and Pooideae) and PACMAD clades] and C4 lineages (PACMAD clade only) illustrate the differing thermal environments for each group. C4 taxa are found in the environments with the highest MAT, followed by C3 PACMAD species. By contrast, C3 PACMAD species are found in the environments with the coolest LST, and the LST values for C3 BEP species are substantially higher than their MAT values. The difference in LST between C3 and C4 PACMADs is larger than the difference in MAT between these groups.
Though LST has been used infrequently in ecology and biogeography, it is intimately related to water and energy balance and ecosystem structure, and should more accurately capture plant temperatures and microclimates than MAT. Our results provide support for a pronounced thermal difference in the environments of closely related C3 and C4 grass taxa, and show that C3 PACMADs occur in the coolest and least variable thermal environments, probably due to the greater tree cover of these habitats.