Hydrological niches in terrestrial plant communities: a review

Abstract

Summary1. Despite the fundamental significance of water to plants and the persisting question of how competing species coexist, this is the first review of hydrological niches. We define hydrological niche segregation (HNS) as: (i) partitioning of space on fine-scale soil-moisture gradients, (ii) partitioning of water as a resource and/or (iii) partitioning of recruitment opportunities among years caused by species specializing on particular patterns of temporal variance of water supply (the storage effect). 2. We propose that there are three types of constraint that lead to the trade-offs that underlie HNS. (i) An edaphic constraint creates a trade-off between the supply to roots of O 2 on the one hand vs. water and nutrients on the other. (ii) A biophysical constraint governs gas exchange by leaves, leading to a trade-off between CO 2 acquisition vs. water loss. (iii) A structural constraint arising from the physics of water-conducting tissues leads to a safety vs. efficiency trade-off. 3. Significant HNS was found in 43 of 48 field studies across vegetation types ranging from arid to wet, though its role in coexistence remains to be proven in most cases. Temporal partitioning promotes coexistence through the storage effect in arid plant communities, but has yet to be shown elsewhere. In only a few cases is it possible to unequivocally link HNS to a particular trade-off. 4. Synthesis. The field and experimental evidence make it clear that HNS is widespread, though it is less clear what its precise mechanisms or consequences are. HNS mechanisms should be revealed by further study of the constraints and trade-offs that govern how plants obtain and use water, and HNS can be mechanistically linked to its consequences with appropriate community models. In a changing climate, such an integrated programme would pay dividends for global change research.

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

“…In those locations, it is possible that the root density is larger at depth than near the surface, allowing water to be taken up from depth even during the parts of the year when plentiful water is available in the stream. This idea has led to the view that plants may utilize different niches (Silvertown et al, 2015) by partitioning their roots according to the hydrological conditions of different layers (e.g., Walter's two-layer hypothesis). Specifically, Walter's hypothesis states, in part, that shallow and deeply rooted plants do not compete for the same water resources (Walter, 1939;Weltzin and McPherson, 1997;Schenk and Jackson, 2002a;Schwinning, 2008;Holdo, 2013;Ward et al, 2013).…”

Climate modifies response of non-native and native species richness to nutrient enrichment

“…In addition, trade-offs between water and nitrogen use efficiency have been found across different growth forms (i.e. herbs, shrubs and trees), with the relative position of species on this trade-off affecting their fitness and spatial distribution [30].…”

Prairies Thrive Where Row Crops Drown: A Comparison of Yields in Upland and Lowland Topographies in the Upper Midwest US

“…In contrast, when moisture is excessive, perennial grassland yields are substantially reduced in poorly drained lowlands, at least in monoculture switchgrass (Panicum virgatum L.) systems [12,16]. However, in polyculture grasslands, soil hydrology plays an important role in structuring plant species assemblages, and plant species with adaptations to anaerobic stress are more likely to occur in inundated soils [17,18]. Therefore, it is not clear whether more diverse perennial grasslands would suffer yield losses similar to monoculture grasslands in poorly drained locations.…”