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

• Published in: Oikos 2008-10, Vol.117 (10), p.1569-1577
• Main Authors: Eränen, Janne K., Kozlov, Mikhail V.
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Betula pubescens; Biological and medical sciences; Competition; Ecology; Fundamental and applied biological sciences. Psychology; General aspects; Plant populations; Stress; Trees
• ISSN: 0030-1299; 1600-0706
• DOI: 10.1111/j.0030-1299.2008.16772.x

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 seedling-seedlings interactions were studied. Positive host-seedling 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 host-seedling competition was detected in a low stress site after two study years. In seedling-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 host-seedling interactions and the difference between host-seedling and seedling-seedling 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.