Nutrient-poor grasslands cover a substantial portion of the terrestrial surface and provide important ecosystem services ranging from forage and livestock production to stabilization of erosion-prone soils. The future of these grasslands under increased drought frequency and severity is difficult to predict, especially given the complex ecological interactions and feedbacks among plants, other organisms, and physical environmental factors (Sardans and Peñuelas 2012).
Dominant and subordinate plants demonstrate variable responses not just to precipitation inputs but to nutrient availability. In turn, nutrient dynamics are affected not only by water availability but also by plant uptake and by the mediation of availability by soil biota, especially symbiotic mycorrhizae. While much has been learned about the role played by mycorrhizae in plant water and nutrient uptake (Johnson 2010), there is still much more to understand about species-level variation in fungal colonization and in the resulting influence on plant resource uptake.
As drought stress increases in grasslands, how might we expect these complex dynamics to vary? There is value in understanding what tradeoffs might underlie the different behaviours of dominant and subordinate species in responding to resource availability or episodes of stress (Mariotte 2014). In particular, we lack information on the relative merits of opportunistic responses to nutrient availability versus maintaining balanced tissue composition (stoichiometric homeostasis), which might be important to overall plant performance.
Our Editor’s Choice article, Stoichiometric N:P flexibility and mycorrhizal symbiosis favour plant resistance against drought, explores differences among grassland species in these responses.
Pierre Mariotte, Alberto Canarini, and Feike Dijkstra established rain-out shelters to impose drought (50 % reduction of precipitation) in plots within a managed Australian grassland, and followed the biomass production of both dominant and subordinate grasses in drought and control plots. After a year they examined mycorrhizal colonization, water use efficiency, and tissue nitrogen (N) and phosphorus (P) contents in detail for one dominant (Paspalum dilatatum) and one subordinate (Cynodon dactylon) species.
Dominant grasses displayed substantial reductions in biomass in the drought treatment plots while subordinate species showed no such reduction in growth. Both the species studied in detail had higher water use efficiency (reflected in carbon isotope ratios) under drought treatment, but the effect was greater for the subordinate species. The subordinate also showed much higher levels of arbuscular mycorrhizal development under drought than the dominant grass.
Most intriguing were striking differences between the two species in patterns of nutrient uptake. The dominant P. dilatatum showed very little variation in tissue N concentrations, even as soil moisture and mycorrhizal arbuscule development varied within and between treatments. Plant N:P ratio was similarly invariable for the dominant, suggesting that this species closely regulated N:P stoichiometry. The subordinate C. dactylon, however, demonstrated great stoichiometric flexibility, with N:P ratios increasing with the abundance of mycorrhizal arbuscules and as soils became drier.
The authors speculate that the dominant species’ tight regulation of nutrient stoichiometry comes at some cost to efficiency of resource uptake and use, thereby reducing its ability to maintain above-ground growth under drought conditions. The impressive differences in nutrient stoichiometry shown by two co-occurring grasses suggest that we have much more to learn about how plant abilities to regulate nutrient uptake affect competitive interactions and the relative resilience of dominant and subordinate species.
Laura Foster Huenneke, Associate Editor for Journal of Ecology