Theory predicts plants grow roots to compete with only their closest neighbours

Abstract

The combination of individual-based selection with shared access to resources drives individuals to invest more than necessary in taking up their share of resources due to the threat of other individuals doing the same (competitive overinvestments). This evolutionary escalation of investment is common, from deer antlers and peacock feathers to tree height and plant roots. Because plant roots seem to be well intermingled belowground, the simplifying assumption that belowground resources are perfectly well mixed is often made in models—a condition that favours maximal fine-root overinvestments. Here, I develop simple models to investigate the role of space in determining the overlap among individuals belowground and resulting fine-root biomass. Without costs of growing roots through space, evolutionary optimization leads individuals to intermingle their fine roots perfectly and to invest just as much in these roots, whether there are two individuals competing or many. However, if there are any costs of sending roots through soil, investment in fine roots is constrained in amount and spatial extent. Dominant individuals are those that keep their roots in the soil closest to their own stem and the stems of their closest neighbours. These results highlight the importance of space in determining individual strategies as well as competitive networks.

Organizing principles for vegetation dynamics

“…While competition for below-ground resources can be readily modelled by assuming common pools of soil resources for all plants in a stand 86 , the validity of this 'perfect mixing' assumption is not well known. The actual spatial extent of competition for nutrients and water is not well understood despite potentially large implications for key processes such as root growth 99 , water use 47 and whole-plant growth 39 .…”

Plasticity of Root Traits under Competition for a Nutrient-Rich Patch Depends on Tree Species and Possesses a Large Congruency between Intra- and Interspecific Situations

“…Indeed, the importance of C assimilate availability in determining the N uptake capacities of Acer and Fagus was demonstrated by Simon, Li and Rennenberg [53]-i.e., a reduced light availability severely hampering the ammonium and glutamine uptake of Acer but not Fagus under interspecific competition. The strongly reduced fine root biomass of Fagus might thus be interpreted as a 'self-thinning mechanism', reducing the competitive interactions belowground by curtailing the overlap of root/mycorrhizal zones (and leading to increased root zone 'stratification')-as repeatedly shown for mature Fagus sylvatica trees in mixtures (see also theoretical consideration in [38]). While in large parts of Central Europe, the competitive advantage of Fagus sylvatica aboveground is clearly related to its ability to tolerate shade in the juvenile state and pre-empt light as mature trees, evidence is thus increasing that Fagus 'strength' belowground may not be its competitive effect on neighboring roots per se but its highly plastic and C-efficient root system.…”

“…The root biomass production rates, root N concentrations and C:N ratios measured in the competition chambers (CC) were comparable to values from nutrient-rich top soil layers in mature stands dominated by either species [67], suggesting that realistic experimental conditions were established. No signs of fine root over-proliferation were found [38] but individual root biomasses were in general lower in CCs with a competing root compared to isolation (Figure 2a). The stronger decrease in fine root biomass in Fagus as compared to Acer seedlings under competition and the relative competition intensity (RCI) indices illustrate that Fagus seedlings' roots are affected to a greater extent by roots sharing the same soil volume than Acers' (Figure 2b).…”

“…However, the suggested traits (e.g., specific root length (SRL), nitrogen (N) content, specific uptake and respiration rates, and lifespan) are strongly related to resource acquisition or conservation [26]-and thus also central to access mechanisms of exploitative competition. In particular, the fast proliferation of active root surface area into heterogeneously distributed, resource-rich patches might be an important strategy to pre-empt resources ( [8,38,39], but see [40]). Once space has been occupied, traits related to resource mobilization (such as extracellular enzymes exuded by roots or their fungal symbionts [41]) have to be considered.…”

Growth, death, and resource competition in sessile organisms

“…Following Turing’s seminal work on scale-dependent feedback, namely local activation and long-range inhibition, similar principles of pattern formation with local density dependence have been considered ( 10 – 13 ), touching on the more general question of how multiple scales of time and space emerge ( 14 – 16 ). More recent work has connected these principles with mechanisms of biological interaction and environmental feedback ( 17 – 19 ). For spatial patterning, approaches to mechanism range from using perturbations like cascades of tree death to explore self-organized criticality in forests ( 20 – 22 ) to applying Turing-like activation-inhibition concepts to scale-dependent plant processes ( 15 , 16 ), which could be modulated by environmental conditions ( 23 ), to considering how ecosystem engineers modify the local environment to generate bare and densely vegetated patches ( 18 , 24 ).…”