The occurrence and magnitude of the tragedy of the commons were affected by soil fertility
In low fertility, old Monkhead showed some tragedy of the commons and modern 92-46 changed the slope of root-total allometic relationships when confronted with an intra-variety neighbour, whilst in high fertility all these patterns disappeared and both varieties showed consistent allocation trajectories between two partition treatments (Table 3). Therefore, our results provided support for the third hypothesis that the occurrence and magnitude of the tragedy of the commons were affected by soil fertility. Different fertilization effects on the old and modern varieties were likely to be driven by different resource strategies and sensitivity to competitive environments (Semchenko and Zobel 2005; Guo et al. 2012; Wang et al. 2014).
Modern varieties which were subjected to human domestication to improve seed yield often possess a smaller root system, shorter root length and smaller diameter of the metaxylem vessels of the seminal roots (Li and Zhang 1999). These characteristics in modern 92-46 make cooperation between individuals possible when confronted with neighbour’s roots. Landraces and old varieties were subjected to long-term natural selection under high-input nutrient conditions (Roucou et al. 2018), and thus resulted in exploitative resource strategies represented by higher growth rates, larger root systems, larger fine-root proportion, and these landraces are likely limited more by light than soil nutrients.
Different resource strategies may result in between-species differences in plant – plant interactions, with resource-exploitative old landraces being more plastic in response to light competition compared with resource-conservative modern varieties. Our results were consistent with this idea; old Monkhead showed significant changes in relative allocation to stems and leaves across fertilization rates, whereas no change was recorded for modern 92-46 (Table 2, Fig. 3c-d). We deduced that under low fertility rates, plant communications were likely mediated by root recognition (Dudley and File 2007; Chen et al. 2012), because of higher relative allocation to root biomass and lower tiller numbers in wheat varieties which minimized extensive between-individual communications at the level of above-ground organs. Under high fertility rates, the lack of differences in allometric relationships between plants in two partition treatments potentially reflected that above-ground sensing had dominated plant communications via the photochrome system (Smith 2000), and that individual plants could develop extensive communications even in the plastic partition treatments due to higher relative allocation to stem&leaf biomass and greater tiller numbers. However, future work is needed to examine whether the occurrence and magnitude of the tragedy of the commons varies with soil fertility by experimentally separating plant communications between above-ground and below-ground components.