Photosynthetic tolerance to non‐resource stress influences competition importance and intensity in an invaded estuary

Abstract

In an attempt to clarify the role of environmental and biotic interactions on plant growth, there has been a long-running ecological debate over whether the intensity and importance of competition stabilizes, increases or decreases across environmental gradients. We conducted an experiment in a Chinese estuary to investigate the effects of a non-resource stress gradient, soil salinity (from 1.4‰ to 19.0‰ salinity), on the competitive interactions between native Phragmites australis and invasive Spartina alterniflora. We linked these effects to measurements of photosynthetic activities to further elucidate the underlying physiological mechanism behind the competitive interactions and the driver of invasion. The experiments revealed that while biomass of both species decreased in the presence of the other, competition did not alter photosynthetic activity of either species over time. P. australis exhibited high photosynthetic activity, including low chlorophyllase activity, high chlorophyll content, high stomatal conductance and high net photosynthetic rate, at low salinity. Under these conditions, P. australis experienced low competitive intensity, leading to high biomass production and competitive exclusion of S. alterniflora. The opposite was observed for S. alterniflora: while competitive intensity experienced by P. australis increased with increasing salinity, and photosynthetic activity, biomass, competitive dominance and the importance of competition for P. australis growth decreased, those of S. alterniflora were stable. These findings demonstrate that S. alterniflora invasion driven by competitive exclusion are likely to occur and expand in high salinity zones. The change in the nature of competition along a non-resource stress gradient differs between competitors likely due to differences in photosynthetic tolerance to salinity. The driver of growth of the less-tolerant species changes from competition to non-resource stress factors with increasing stress levels, whereas competition is constantly important for growth of the more-tolerant species. Incorporating metrics of both competition intensity and importance, as well as linking these competitive outcomes with physiological mechanisms, is crucial to understanding, predicting, and mediating the effects of invasive species in the future.

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

“…This is not necessarily in contradiction with studies that find a net positive effect of neighbours in stressful environments [14,32]. Facilitation among plants may be driven by physiological interactions with the environment and may be independent of the competitive interactions considered here [33][34][35]. Tang et al [33] show that the interconnection of competition for resources and tolerance of a non-resource stress complicate studies comparing competition and facilitation.…”

“…Facilitation among plants may be driven by physiological interactions with the environment and may be independent of the competitive interactions considered here [33][34][35]. Tang et al [33] show that the interconnection of competition for resources and tolerance of a non-resource stress complicate studies comparing competition and facilitation. Qi et al [34] outline a clear stress gradient hypothesis that takes into account this complication.…”

Plants maximize competition while minimizing competitors belowground: a theoretical analysis of incentives for root competition in space

“…In order to focus the investigation here, we have ignored many of the fascinating aspects of belowground plant ecology – interactions with microbial communities (Bever et al 2009), depth distributions (Schenk and Jackson 2002), nutrient hot spots (Hodge 2004; Chen et al 2018), hydraulic lift (Yu and D’Odorico 2015), facilitation among individuals (Tang et al 2018; De Parseval et al 2017), and even size variation (Goldberg et al 2017). But, the simplified framework has allowed us to clearly understand surprising and potentially fundamental feedbacks.…”

Plant species’ range type determines local responses to biotic interactions and land use

“…For stress-tolerant species, competitive ability is expected to decrease toward less stressful conditions (Tang et al 2018). For stress-tolerant species, competitive ability is expected to decrease toward less stressful conditions (Tang et al 2018).…”

“…The competitive ability of a species is always relative to others. For stress-tolerant species, competitive ability is expected to decrease toward less stressful conditions (Tang et al 2018). This might ultimately result in range restrictions under more benign conditions, where interspecific competition should be stronger, because of the greater competitive ability of the resident less stress-tolerant species (Grime 1988, Liancourt et al 2005.…”