Root functional traits and growth rates in savanna trees and grasses

Root-based functional traits are relatively overlooked as drivers of
savanna plant community dynamics, an important gap in water-limited
ecosystems. Recent work has shed light on patterns of trait coordination
in roots, but less is known about the relationship between root functional
traits, water acquisition, and plant demographic rates. Here, we
investigated how fine-root vascular and morphological traits are related
in two dominant PFTs (C3 trees and C4 grasses from the savanna biome),
whether root traits can predict plant relative growth rate (RGR), and
whether root trait relationships differ in trees and grasses. We used root
data from 21 tree and 18 grass species grown under greenhouse conditions,
and quantified a suite of vascular and morphological root traits. We used
a principal components analysis (PCA) to identify common axes of trait
variation, compared trait correlation matrices between the two PFTs, and
investigated the relationship between PCA axes and individual traits and
RGR. We found that there was no clear single axis integrating vascular and
morphological traits, but found that vascular anatomy predicted RGR in
both trees and grasses. Trait correlation matrices differed in trees and
grasses, suggesting potentially divergent patterns of trait coordination
between the two functional types. Our results suggested that, despite
differences in trait relationships between trees and grasses, root
conductivity may constrain maximum growth rate in both PFTs, highlighting
the critical role that water relations play in savanna vegetation dynamics
and suggesting that root water transport capacity is an important
predictor of plant performance in the savanna biome.