The stress-gradient hypothesis (SGH) is a conceptual model that predicts that the frequency of facilitative and competitive interactions will change across abiotic stress gradients, with facilitative interactions being more common in harsh conditions (Bertness and Callaway 1994; Maestre et al. 2009, Malkinson and Tielbörger 2010), and competition being more common under more benign conditions (Maestre et al. 2009; Holmgren and Scheffer, 2010). Experimental studies have shown that a single-factor stress gradient, such as a temperature gradient across latitude, can be important in determining the distribution of plant species (Woodward 1987, De Frenne et al. 2013). Temperature gradients also overlay other dimensions of ecological variation such as precipitation, soil type, and the density of competitors, which may lessen or amplify temperature effects. Therefore, studies that investigate plant performance under field conditions are needed to fully understand how these complex latitudinal gradients determine where plants grow on the landscape.
Eastern redcedar Juniperus virginiana (hereafter ERC) is a widespread woody coniferous tree native to eastern North America and is known for establishing in old fields and limestone glades within the Eastern Deciduous forests (Lawson 1990). However, ERC is also rapidly expanding its geographic range into grasslands across the western United States (Yang et al. 2024). Being widely distributed across latitude from Florida to Nova Scotia (Harper 1912; Anderson 2003), ERC occurs under a broad range of temperature conditions, suggesting that the relative balance of competitive versus facilitative interactions might differ across its range. Understanding how temperatures combined with biotic interactions can affect plant performance and survival will be critical to understand the factors that have limited ERC distributions in the past and those that are currently promoting its expansion. Prolonged exposure to cold temperatures may affect ERC directly by affecting their growth rate, biomass accumulation, physiological activity (e.g., water potential, stomatal conductance, assimilation rate, and transpiration rate), and frost events, which can result in tissue necrosis and death (Theocharis et al. 2012). In addition, subzero temperatures can negatively affect the plant’s water status, through embolisms in the xylem, potentially limiting future gas-exchange capabilities (Huang et al. 2017).
In addition to temperature gradients experienced by ERC across its range, lake-effect snow is an important latitudinal stress gradient shaping the Northeastern deciduous hardwood forests of North America (Niziol et al. 1995, Burnett et al. 2003). This phenomenon is produced when cold dry arctic air passes over the warmer waters of the Laurentian Great Lakes (Monmonier 2012). The cold air absorbs warm water vapor from the lake, increasing its temperature and humidity and causing it to rise upwards, where it freezes, and is subsequently re-deposited, mainly as snow (Niziol et al. 1995, Burnett et al. 2003, Monmonier 2012). Lake-effect snow causes far higher snow production close to the lake than further away (Scott and Huff 1996; Henne et al. 2007), which influences the distribution of vegetation (Henne et al. 2007).
Snow cover can have many benefits (Tomiolo and Ward 2018). For instance, snow cover can protect new growth from extreme temperatures whereas wind and snowpacks insulate soil, thus regulating and preventing soil from freezing even when air temperatures are below zero (Monson et al. 2006; Liptzin et al. 2009; Wilson et al. 2020). Snow also replenishes soil water, which can represent critical resources for deep-rooted woody plants, especially when growing-season moisture is limited (Grippa et al. 2005). However, prolonged snow cover can increase the inactive period for plants, limiting the growing season (Hallinger et al. 2010). This also alters phenological dynamics, which in turn may result in reduced growth and increased plant mortality (Wipf and Rixen 2010; Francon et al. 2017; Carrer et al. 2019). Even for plants that remain physiologically active at lower temperatures, deep snow could be detrimental because ERC is shade-intolerant (Ward 2020, 2021). Lake-effect snow is also associated with other aspects of environmental variation because the highest snowfall occurs in areas that were previously glaciated (Henne et al. 2007). One consequence of historic glaciation is that soils closer to the Great Lakes can be low in nitrogen compared to unglaciated soils farther away (Vitousek and Farrington 1997, Hobbie and Gough 2002, Göransson et al. 2016). ERC occurs throughout northeast Ohio, creating a steep stress gradient along a short latitudinal range (Tomiolo and Ward 2018). This provides a unique opportunity to test the SGH and understand the relative balance of abiotic versus biotic factors in the growth and survival of ERC.
The SGH predicts that moving southward along this gradient (Fig. 1) causes the dynamics between neighboring plants to change, and competitive interactions will begin to outweigh facilitative interactions (Bertness and Callaway 1994; Maestre et al. 2009; Molina-Montenegro et al. 2013). ERC growth rates depend on site quality and competition from other species (Lawson 1990; Hamati et al. 2023). ERC is considered cold-hardy to -43°C, and tolerant of poor-quality soils, suggesting that it is well-suited to endure cold stress. ERC is also considered shade-intolerant (Ward 2020, 2021), which could make deep snow problematic for this species (Anderson 2003). In addition, we still do not know how abiotic stress interacts with competition (biotic stress) to determine the distribution of this species. Previous research by Hamati et al. (2023) showed that ERC is negatively affected by a common invasive grass, Bromus inermis, in the early stages of establishment. However, ERC will outcompete B. inermis once the height of ERC surpasses that of B. inermis (Hamati et al. 2023). Furthermore, previous research has suggested that intraspecific competition negatively affects ERC growth rate because the plants will be competing for the same resources due to their similar niches (Yao et al. 1999, Riddle et al. 2014). Combined with the high tolerance of this species to environmental stress (Lawson 1990; Torquato et al. 2020), such abiotic interactions may have a neutral or facilitative effect on ERC performance and survival at locations closer to Lake Erie, but a negative effect at locations farther from the lake where competition may be more important.
To test this hypothesis, we conducted a field experiment over a 120 km-long range with increasing distance from Lake Erie. ERC saplings were planted at three sites in northeast Ohio (Geauga, Portage, and Tuscarawas; Fig. 1), which differ in their environmental conditions, including snowfall, precipitation, temperature, and soil type. We created a biotic stress gradient component by adding 0, 1, 2, or 4 ERC (replicated) competitors within each site. We gathered data on snowfall and precipitation at each site, and we tested for the effects of site and intraspecific competition on ERC growth and nutrient status (relative growth rate, total biomass, root length, as well as above and below-ground nitrogen concentration), midday water potential, and gas exchange (assimilation rate, transpiration rate, stomatal conductance, and water-use efficiency).
We predicted that ERC sapling growth, nitrogen concentrations, and gas-exchange rates will be lowest for plants growing without competitors and closer to the lake (Song et al. 2016) due to higher precipitation, more shading from snowfall and lower soil nitrogen availability. Similarly, we expected conditions at colder sites to retard the growth of competitors, but that at their highest density competitors will improve ERC growth and performance by reducing wind interception. Furthermore, facilitation will further be supported by the fact that plants closer to the lake will be less water-stressed than ERC at the other sites due to higher precipitation. Finally, we expected the highest growth, gas-exchange rates and tissue nitrogen concentration for ERC plants growing without competitors farthest from the lake, due to warmer temperatures and increased soil nitrogen availability. However, lower precipitation farther from the lake will result in lower soil water availability and more vigorous growth of competitors, which means that they will negatively affect ERC at lower density compared to cold sites.