Abstract
The ‘stress-gradient hypothesis’ predicts increasing facilitative interactions with increasing environmental stress, but it remains unclear if the prevailing type of interaction (i.e. facilitative or competitive) between dominant and subordinate plant species occurring in harsh environments is dependent on the plant functional type. In addition, most plant-species removal experiments in grasslands are short-term (1–2 years), which may imprecisely reflect transient effects arising from methodological limitations. We conducted a dominant species removal experiment in a subalpine ecosystem, containing a mosaic of grass-dominated and shrub-dominated community patches, both of which are common in the subalpine zone of the Qinghai-Tibetan Plateau. We examined the direction and magnitude of the effects of three co-dominant grass and a dominant shrub species on subordinate species richness and biomass over a 6-year period. Removal of the dominant grass species alleviated their competitive pressure on subdominant grasses, which resulted in similar total and grass biomass detected in the final year of the study. By contrast, shrub removal showed no effects on its subordinate species biomass. Furthermore, neither the removal of the dominant shrubs nor the grasses altered their respective subordinate species richness. Thus, in subalpine ecosystems that experience harsh environmental conditions, our results showed that the direction of interactive effects of dominant plant species on subordinate species may be dependent on the plant functional type and are not necessarily facilitative. Furthermore, we showed that longer-term plant-removal experiment observations may be required to better determine the effects of species removal for this subalpine and other montane ecosystem(s).
Highlights
Environmental changes such as drought, warming, nitrogen deposition or land management practices are increasingly common in grassland ecosystems globally (e.g. Shaw et al, 2002; Menge and Field, 2007; Wardle et al, 2011)
Forbs comprised between 67 and 83% of the total species richness (6e26 total species per plot), and this functional group represented the largest proportion of plant species in the grass plots of this subalpine ecosystem (Fig. 2A; Table 1)
Our results showed that the plant functional types of the dominant plant species as well as transient effects are important to consider to some extent
Summary
Environmental changes such as drought, warming, nitrogen deposition or land management practices (e.g. firesuppression and grazing intensity changes) are increasingly common in grassland ecosystems globally (e.g. Shaw et al, 2002; Menge and Field, 2007; Wardle et al, 2011). Bertness and Callaway, 1994; Grime, 1998; Smith and Knapp, 2003; Smith et al, 2020) It is unclear if competitive or facilitative interactions prevail in driving vegetation dynamics in ecosystems that contain dominant plants from different plant functional groups and typically experience stressful environmental conditions. This is because contrasting plant functional/ physiological trait(s) may confer different types of plant-plant interaction with respect to the environmental stress factor(s) the community experiences, and field experiments examining interactions in stressful environments have produced inconsistent findings (Olofsson et al, 1999; Pugnaire et al, 2015; Michalet et al, 2015). It is valuable to accurately determine the predominant type of plant-plant interaction occurring in grasslands experiencing stressful abiotic environments over a longer period of time, which can in turn, inform their management and restoration
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