In the mesic tallgrass prairies of the Great Plains, US, two C4 grass species, Andropogon gerardii Vitman and Sorghastrum nutans (L.) Nash often coexist with such mutually high abundances (canopy cover percentages)J, population densities, and/or occurrence frequencies that they are considered codominant species (sensu (Gray et al., 2021)(L. Brown, 1985; Duralia & Reader, 1993; Freeman, 1998; Hartnett et al., 1996; Smith & Knapp, 2003). In most years, these abundances result in high aboveground net primary productivity (ANPP) (Grime, 1998; la Pierre et al., 2011; Smith & Knapp, 2003), making these species important contributors to ecosystem functioning and services, such as forage for grazing (Vinton et al., 1993), carbon sequestration and storage (Grime, 1998; Kemp et al., 1994; Mahaney et al., 2008; Omonode & Vyn, 2006), nutrient cycling (Grime, 1998; Mahaney et al., 2008), invasion resistance(Smith et al., 2004), and aesthetic and cultural value. Though the population densities of these two grass species can fluctuate from year to year (Silletti & Knapp, 2002; Towne & Kemp, 2003), their codominant relationship has remained stable in the long term. For example, nearly a century ago, Weaver and Fitzpatrick reported abundances similar to those found today in eastern Kansas (Jones et al., 2016; Weaver & Fitzpatrick, 1932). However, despite their frequent description as codominant species is this region (L. Brown, 1985; Duralia & Reader, 1993; Freeman, 1998; Hartnett et al., 1996; Smith & Knapp, 2003), A. gerardii has been reported to be both more competitive for resources (Silletti et al., 2004; Tilman & Wedin, 1991) and more drought tolerant (Hoffman et al., 2018; Hoffman & Smith, 2018; Silletti & Knapp, 2002; Swemmer et al., 2006).
Given the annually fluctuating population densities of the two species, and the competitive advantages in both wet and dry years attributed to A. gerardii, it is not clear how S. nutans maintains its role in the codominant relationship over the long term. This uncertainty is further shrouded by the many morphological and physiological similarities borne between A. gerardii and S. nutans. These species have multiple life history traits in common, including long-lived genets (Gustafson et al., 2005; Keeler, 2004; Lauenroth & Adler, 2008; USDA, 2021a, 2021b), C4 photosynthetic pathways, reproduction mainly through rhizomatous cloning (i.e., ramets/tillers (Benson & Hartnett, 2006; Lauenroth & Adler, 2008; McKone et al., 1998; USDA, 2021a, 2021b)), and bear similarities in their functional traits (e.g., leaf dimensions, leaf gas exchange rates, ANPP) (Forrestel et al., 2014, 2015) and responses to fire and grazing disturbances (Weaver, 1931; Weaver and Fitzpatrick, 1932; Hadley and Kieckhefer, 1963; Polley et al., 1992; Towne and Kemp, 2003; Bowles et al., 2011; Forrestel, Donoghue and Smith, 2014, 2015). Both are considered strong competitors under nitrogen limiting conditions (Berg, 1995; Silletti and Knapp, 2001; Lett and Knapp, 2003; Mulkey, Owens and Lee, 2008), and are tall-statured when flowering under mesic conditions (Weaver, 1931; Knapp and Hulbert, 1986), but are intolerant of shading (Weaver and Rowland, 1952; Lett and Knapp, 2003) and persistent grazing (Damhoureyeh & Hartnett, 2002; Hartnett et al., 1996; Vinton et al., 1993).
Despite all the similarities between the two grasses, the two grasses differ in a key way – in growth determinacy of tillers (McKendrick..) – which may contribute to maintenance of the codominance relationship in space and time (Gray & Smith, in review). A. gerardii exhibits determinant growth, in which it recruits belowground buds into tillers almost exclusively in the early spring, and these tillers are annual in their lifespan (i.e., senesce in early fall). In contrast, S. nutans exhibits indeterminant growth whereby it can recruit belowground buds into tillers throughout the growing season, and later-recruiting tillers can overwinter as belowground buds and be recruited again the following growing season. This difference in growth determinacy results in contrasting intra-seasonal tiller dynamics, in which A. gerardii tiller numbers consistently decline during the growing season whereas S. nutans tiller numbers often increase or remain stable. These contrasting population dynamics could have important implications for the stability of codominance of the two species, (Gray & Smith, in review), and the stress gradient hypothesis is on possible mechanism that may explain how differing growth determinacy may promote stable codominance, particularly within the context of variation in stressful conditions during the growing season.
The stress gradient hypothesis posits that as the intensity of environmental stress increases, the functional sum of the effects of the multiple, simultaneously occurring interactions between competing species becomes less negative as some negative effects are mitigated, and/or the effects of some positive interactions are enhanced, making the presence of certain interspecific and otherwise deleterious neighbors beneficial for survival, growth, and/or reproduction relative to their absence (Bertness & Callaway, 1994; Brooker & Callaghan, 1998; Callaway & Walker, 1997; Olofsson et al., 1999; Ploughe et al., 2018). Alternatively, the two-phase resource dynamics hypothesis (Goldberg & Novoplansky, 1997) proposes that possibly similar outcomes can occur because resources are typically available in pulses, and interactions between plants and their abiotic environment become more important relative to resource competition as resources become less frequent (i.e., become more resource stressed). Such shifts in environmental conditions are common in mesic tallgrass prairie where A. gerardii and S. nutans codominate. Here, there is a high probability of drought (or dry conditions) occurring during each growing season, despite on average relatively high annual rainfall (Craine et al., 2012; Hayden, 1998; Knight et al., 1994). Drought-associated shifts in net interactions between neighboring species may occur if, for example, a competing species translocates water from deeper to shallower soils through tap roots (i.e., hydraulic lift (Dohn et al., 2013; Joffre & Rambal, 1988; Weltzin & Coughenour, 1990)), if a species has physical defense mechanisms that extend protection to neighbors against the exacerbating effects of herbivores (Callaway, 1992; García et al., 2003; McAuliffe, 1986; Vinton et al., 1993), if heavy canopy cover of a species that reduces subcanopy light availability also reduces soil evaporation rates (Escudero et al., 2005; Kikvidze et al., 2006; Pugnaire et al., 2004) or increases humidity within the canopy(Aguirre et al., 2021; Cowles et al., 2016; A. Wright et al., 2014; A. J. Wright et al., 2021), or even if drought-induced mortality in one species increases the availability of resources for the surviving species that may otherwise have been depleted through intra-specific competition (de Dios et al., 2014; Lloret et al., 2004; Ploughe et al., 2018).
Evidence for intra-annual shifts between net negative vs. positive interactions has been observed in codominant species under stressful conditions induced by water limitation later in the growing season. For example, two codominant plant species in a subalpine environment were reported to shift between overall competitive to facilitative relationships as water availability regularly declined during the latter weeks of growing seasons of each year (Kikvidze et al., 2006). In their report, the authors attributed negative interactions in the early season to competition for light. This negative effect was reduced as precipitation declined and leaf cover decreased, and it was speculated that soil moisture may also have been conserved in the mixed communities compared to monocultures. Similarly, such shift in the sum of interaction effects between A. gerardii and S. nutans, may be a mechanism responsible the stability of their codominant relationship. That is, if S. nutans, purportedly the less competitive and drought tolerant of the two species (Hoffman et al., 2018; Hoffman & Smith, 2018; Silletti et al., 2004; Silletti & Knapp, 2002; Swemmer et al., 2006), benefits from the presence neighboring A. gerardii individuals (relative to intra-specific ones) in the drier months of the growing season, the presence of A. gerardii individuals may increase the fitness of S. nutans during that time and reduce the probability of its competitive exclusion. Moreover, because S. nutans is able to recruit new tillers throughout the growing season while A. gerardii is not (McKendrick et al., 1975), drought-driven senescence of A. gerardii may facilitate the emergence and growth of young S. nutans tillers by opening gaps in the canopy for light to reach the understory, allowing S. nutans populations to increase in density and recover from the asymmetry of competition suffered during the early season.
To test whether stressful conditions induced by late-season drought can shift the overall relationship between A. gerardii and S. nutans to one that is more facilitative, we performed a controlled greenhouse experiment using artificial communities. We compared the performance of these species in communities with interspecific mixes to those with only intraspecific neighbors. Using a simple response surface design, we tested the following hypotheses: 1) Water limitation (stress) would diminish the per capita performance of both species at both low and high community densities; 2) Increased community density would reduce per capita performance of each species in monoculture at both high and low water availability levels; 3) In accordance with the stress-gradient hypothesis, interspecific neighbors would alleviate a portion of the negative effects of water limitation relative to monocultures at a given total community genet density.