Increasing intraspecific facilitation in exposed environments: consistent results from mountain birch populations in two subarctic stress gradients

Abstract

The stress-gradient hypothesis predicts that the importance of facilitation relative to competition should increase with increasing stress. The hypothesis has received support from several environments, but multi-gradient studies on the generality of the hypothesis are exceptionally rare. A within-species experiment with mountain birch Betula pubescens subsp. czerepanovii was conducted to test the hypothesis in the extreme ends of two subarctic stress gradients (elevation and seashore) in the Kola Peninsula, northwestern Russia. The high stress sites were characterized by strong winds, temperature extremes and potentially drought. The negative effects of abiotic stress on the study seedlings were verified from performance characteristics. Effects of adult hosts as well as seedlingÁseedlings interactions were studied. Positive hostÁseedling interactions dominated in each study site, and three out of four performance variables indicated stronger positive net effects in the high stress sites. In the seashore gradient also seedling survival gave similar interpretations. Also a temporal shift towards hostÁseedling competition was detected in a low stress site after two study years. In seedlingÁ seedling interactions competition dominated, but the effects were weak, likely due to the 'noise' caused by genetic and environmental factors. Our results support the stress-gradient hypothesis and its generality in subarctic environments, as the interpretations were similar for both stress gradients and several fitness-related variables. The temporal variation in hostÁseedling interactions and the difference between hostÁseedling and seedlingÁseedling effects hint on size-dependency of plant interactions: facilitation might dominate when the benefactor is substantially larger than the beneficiary, while competition may be stronger when the plants are of similar size and developmental status.

Unexpected mechanisms sustain the stress gradient hypothesis in a tropical alpine environment

“…(), Kikvidze et al 2005; (2) Dullinger et al. (); (3) Eränen & Kozlov (); (4) le Roux & McGeoch (, ); (5) Venn et al. (); (6) Cavieres & Badano () (see also Chilean references cited in our paper); (7) Onipchenko et al.…”

Beyond competition: the stress‐gradient hypothesis tested in plant–herbivore interactions

“…The SGH predicts that the importance of negative competitive effects is higher under benign environmental conditions, whereas positive facilitative effects increase in importance as environmental stress (either biotic or abiotic) increases (Bertness andCallaway 1994, Callaway andWalker 1997). The SGH is supported by various studies conducted in a wide variety of plant systems and with different environmental gradients (e.g., Pugnaire and Luque 2001, Callaway et al 2002, Wright et al 2006, Era¨nen and Kozlov 2008. Most of those studies involved some kind of environmentally mediated positive interaction performed by a potential competitor, as habitat amelioration, physical protection from enemies, nutrient enrichment, or chemical protection vs. light, nutrient, or space competition (see Callaway 1995, Stachowicz andHay 1999).…”

Treeline form - a potential key to understanding treeline dynamics

“…Shelter from direct sky exposure is a critical facilitative neighbour interaction enhancing seedling survival and controlling the distribution of new recruits (Wardle, 1985b; Germino & Smith, 1999). At lower elevations, competitive interactions tend to dominate and recruitment is negatively associated with proximity to neighbours (Olofsson, 2004), whereas this pattern is reversed near the treeline (Eränen & Kozlov, 2008). By creating its own benign microclimate, the treeline forest becomes relatively insensitive to climatic deterioration.…”