Coordination of bark and wood traits underlies forest-to-savanna evolutionary transitions

Aim:

To test the hypothesis that adaptive shifts leading to the assembly of tropical savannas involved coordination between bark and wood traits, and to understand the underlying mechanisms.

Location: Tropical South America.

Taxon: Angiosperms (woody).

Methods:

We compiled data on three bark traits (total, inner, and outer relative bark thickness), wood density, maximum height, five secondary xylem traits, and on species’ habitat information (light environment, climate, soil, and fire history) for Neotropical savanna, forest, and generalist species (biome groups). We tested for pairwise and multivariate associations among traits across species, and if biome group and habitat conditions explained species positions along the resulting strategy axes.

Results:

Traits covaried along four different axes. The first axis was consistent with a trade-off between fire (thick barks) and shade tolerance (low bark to diameter ratio, high vessel density) and contributed to differentiate the three biome groups according to preference for shaded environments. Forest species also differed from savanna and generalist species in a separate axis by being more resource acquisitive. Maximum height and wood density did not strongly trade-off with bark thickness, although maximum height negatively covaried with relative outer bark thickness. Preference for shaded conditions was the main driver of variation in the two principal strategy axes, but temperature, fire and soil sand content also explained differences in plant stature between savanna and generalist species. Main conclusions: Allocation to bark is constrained by trade-offs with wood, opposing shade-tolerant and acquisitive forest species to fire-resistant and conservative savanna species. Rather than a single strategy axis, three axes are necessary to understand the functional differences among savanna, forest, and generalist species. Because two of these axes are controlled by light availability, the associated traits tend to covary in space and time, but not across species.

Methods
This dataset was compiled from 83 published studies conducted in the following tropical South America biomes: moist forest, dry forest and savannas (including flooded savannas). We compiled data on bark thickness divided by stem diameter (i.e., relative bark thickness; relBT), discriminating between the inner (relIBT) and the outer parts (relOBT), vessel diameter (VDi; i.e. vessel lumen diameter, excluding the walls), vessel density (VDe; i.e., quantity of vessels per millimetre square), intervessel pit diameter (IPD; i.e., the size of pits that link the vessels horizontally), vessel element length (VEL), fiber wall thickness (FWT), wood density (WD) and maximum tree height (HMax). We classified species as savanna (s), forest (f), and generalist (g) species (biome groups). The dataset includes both individual and species level information, depending on how the data were reported in the literature studies. Literature sources can be found in the published article.

We also searched the Global Biodiversity Information Facility (GBIF) for occurrence data to the subset of species for which we had a full trait set (doi: 10.15468/dl.tf5yvv) in order to obtain environmental data for these species (the GBIF data is not included in this dryad submission). Mean principal component axes and environmental information per species are presented for the species with available occurrence information (63 species). We excluded occurrence data from outside of Brazil (the main region represented in our dataset). For the remaining occurrences, we obtained data on mean annual precipitation (map), rainfall seasonality (rs), mean annual temperature (mat), fire frequency (fire), soil sand content (sand) and soil cation exchange capacity (cec). We also created an index for the preference of species for shaded environments (shade): a value of 1 was attributed to the least shade tolerant species (i.e., savanna species), 3 to the most shade adapted species (i.e., forest species) and 2 to generalist species. Details can be found in the published article.