A few weeks ago I wrote about a co-authored study led by Ethan Butler in PNAS on mapping global plant trait distributions. This week we have a new paper coming out in Journal of Biogeography, led by Irena Šimova, that focuses on New World trait distributions using different data and approaches, and finds some complementary and thought-provoking patterns.
The new study uses the BIEN database to directly map species occurrences, and integrates this spatial information with species trait data from the BIEN and TRY databases. Critically, this approach uses direct observations of species rather than inferred observations based on plant functional type maps, and provides an important independent source of information for biogeography. Additionally, this study then is able to directly measure the strength of trait-environment relationships, rather than use them to calibrate model predictions, as in Butler et al. Butler succeeds in mapping full distributions of traits within grid cells, while the current study is limited to means and variances because of the data sources. Nevertheless, both find broadly similar spatial patterns, which suggests that we are beginning to move towards a more synthetic understanding of what kinds of organisms live where, and why.
There are two especially exciting findings in the current study.
First, we show that many trait-environment relationships are strong at large spatial grains and extents. Several explained variances exceed 60%, providing some helpful predictive power relative to the very weak patterns reported in earlier studies like Moles et al. 2014. Some of this could be an artifact of the spatial autocorrelation in species distribution data that affected grid-cell trait averages, but it’s hard to know this with certainty. Regardless, there appears to be very strong scale dependence in these trait patterns.
Second, we show that trait patterns and trait-environment relationships are wholly different between woody and herbaceous species. This suggests that these different functional groups play by different rules, and that biogeographic studies that lump these groups together may miss important mechanisms differentiating between them. Based on the maps it looks like herbaceous species are released from many of the broad scale environmental constraints, which might arise because they are more sensitive to finer-scale climate variation that is not predicted by regional climate variation (e.g. within forest understoreys).
The photos and landscapes in this blog piece come from the in the Rocky Mountain subalpine, the Sonoran desert, and the Andean Polylepis forests. It is encouraging to think that this recent informatics work by Šimova and Butler is beginning to build consensus on the drivers of these vast differences in organismal form and function – and also identifying some of the key growth form differences that underlie this diversity.