The Editor’s Choice for our February issue is “Spatial mapping of root systems reveals diverse strategies of soil exploration and resource contest in grassland plants“ by Lepik et al. This article combines fine‐scale measurements of plant root distributions with spatial statistics, yielding new insights into plant behavioural strategies.
Here the handling Editor, JC Cahill, provides an overview of the research and highlights the novel methodology used in this study.
Upon turning over a greenhouse pot filled with plants, we find an entangled mat of roots, seeming quite different from Darwin’s poetic entangled bank. A persistent question among plant ecologists has been whether there is order within this mess. The ecological literature is replete with expectations, or perhaps hope, that plants partition the soil habitat through differential root placement, helping explain how so many species can coexist on so few limiting resources. In both classical and modern community assembly conceptualisations, minimising spatial overlap among neighbours should reduce the tyranny of competitive exclusion and promote coexistence.
Underlying the concept of habitat partitioning is the field of behavioural ecology, with an abundance of studies demonstrating contingencies in habitat use and home range overlap. For example, the warblers in MacArthur’s trees adjust their spatio-temporal use patterns depending upon which other species were present, rather than expressing fixed occupancy patterns. Similarly, root occupancy in the soil can be contingent on the distributions of nutrients and neighbours, lending plausibility to the idea of fine-scale habitat differentiation in the soil. Lacking has been a holistic analysis of how plant home ranges, territoriality, and foraging strategies may change as a function of neighbourhood properties. Here, the new study of Lepik and colleagues plays a critical role in filling the knowledge gap.
The research team grew mixtures of eight species in pots at different densities and combinations to understand how neighbourhood structure modified root home ranges and habitat use. A novelty of the work was the reinvigoration of a laborious but effective methodology to map fine root distributions, developed by Martyn Caldwell nearly 30 years prior. Due to careful species selection, Lepik and colleagues ensured their focal species had colour variation, and thus they could map the distributions of fine roots by species. At harvest, the team froze the soil, cut slices at two depths, and mapped the populations or roots. Equally novel is what the research team did next, which was to run the data through a series of analyses widely used in behavioural ecology to estimate home ranges and aggregation, but for no good reason, not historically applied to plant roots.
Habitat occupancy patterns by plant roots was highly contingent on species identities, the scale of observation, and neighbourhood structure. There was no evidence for an overall tendency of reduced core foraging area as density increased, nor was there strong evidence for root segregation among species. Instead, at the larger (2mm) scale, there was evidence that plant roots aggregated with interspecific neighbours more than expected by chance. There is some intriguing evidence of multiple behavioural strategies within these data, with a greater overlap in plant home ranges as size disparity among neighbouring plants increased.
This study helps open doors into a more nuanced understanding of plant (social) interactions, drawing upon the deep literature and evolutionary understanding developed by animal biologists. The evidence provided does not support widespread habitat differentiation in the soil, but the results support aspects of some game-theoretic expectations of plant social encounters. It is unclear what generalisations will emerge as behavioural understanding is applied to plant social interactions, but Lepik and colleagues have provided a critical tool set to allow us to find out.
JC Cahill Associate Editor, Journal of Ecology
You can read the full article by Lepik, Abakumova, Davison, Zobel & Semchenko here: Spatial mapping of root systems reveals diverse strategies of soil exploration and resource contest in grassland plants.