As above, so below? Comparing above- and below-ground functional traits

While above-ground trait ecology using such measures as specific leaf area is well-studied and important in determining adaptive strategies in differing conditions, much less is known about below-ground traits and whether they vary in analogous ways. Unlike leaves, below-ground organs have functions, such as physical support of the plant, that go beyond resource acquisition. The inaccessible and very plastic nature of roots has hindered efforts at understanding their particular economics.

In a new article just published in Annals of Botany, lead author Guy M. Taseski and colleagues studied four root functional traits – specific root length, branching intensity, root tissue density, and root dry matter – both within and between species, along a fine-scale hydrological gradient. Key above-ground traits were sampled as well. The researchers sought to determine the degree to which these traits vary along the axes studied and whether below-ground traits conform to the same economic pressures as above-ground traits.

The authors found that root traits varied both within and among species as soil water levels changed. Within species differences were smaller than between species differences, as expected based on above-ground traits. In particular, root tissue density decreased in wetter environments, with root dry-matter content tracking the same trend. Species with low root tissue density and dry-matter content – that is, soft, spongey roots – were excluded from drier microsites. These findings “suggest that species are unable to achieve a very high root tissue density in dry sites and a very low root tissue density in wet sites, and thus are unable to maintain a broad niche breadth throughout the site,” write the authors.

The data did not support a significant positive correlation between key above-ground and below-ground measures which had been considered roughly analogous, such as specific leaf area and specific root length. The authors give three reasons why leaf trait and root trait analogies may not work. First, the differences in lifespans of roots and leaves changes the economic calculations for the two. Second, resource availability and depletion above-ground and below-ground are fundamentally different. And third, opposing supply gradients, such as the case in which increasing soil moisture equates to decreasing oxygen, do not occur in the same way above-ground.

This new understanding is valuable for predicting how plants will react to changes in their soil environment. “As below-ground trait research develops towards the level of understanding that we have for above-ground traits two steps forward are necessary: first, refining and establishing the root traits which warrant more attention is crucial, and second, understanding the structure of within-species variation across multiple scales,” write the authors.