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An increasing number of studies have indicated that positive interactions among plant species may drive community function and structure in various ecosystems. Recent examples come from ecosystems such as high mountain vegetations (Nunez, Aizen & Ezcurra 1999; Choler, Michalet & Callaway 2001), deserts (Munzbergova & Ward 2002), forests (Arevalo & Fernandez-Palacios 2003) and pastures (Callaway, Kikvidze & Kikvidze 2000). Such spatial associations suggest facilitative effects, as one species directly or indirectly ameliorates the environment of the other. Understanding such interactions between plants helps to clarify the structure, diversity and dynamics of plant communities and ecosystems (Castro, Zamora & Hodar 2002). So far, most evidence for these facilitative interactions between plants comes from descriptive studies, and empirical evidence is rare but needed, especially for grazed ecosystems (Callaway, Kikvidze & Kikvidze 2000).
Wooded pastures are traditional semi-natural ecosystems, extensively grazed and logged, with natural regeneration of both grassland and woodland. Today, they are among the most endangered ecosystems in Europe, threatened by both intensification and abandonment of management. Wooded pastures have probably been developed by humans since Neolithic times (Pott & Hüppe 1991) but others suggest a natural origin via now extinct large herbivores as Bos primigenius and Equus ferus (Vera 2000). Remnants of this once-abundant system are nowadays well known for their high conservation values, and serve as examples for nature development projects in western Europe, where large herbivores are increasingly reintroduced in former agricultural areas in order to increase biodiversity (Olff et al. 1999). While an understanding of the mechanisms regulating biodiversity maintenance is a prerequisite for sustainable management in these systems, it is often lacking.
Plants have evolved various traits against herbivory, such as chemical, morphological and phenological defence mechanisms, and these vary in their effectiveness (Crawley 1983). As a consequence, the survival probability of spatially associated tree saplings may depend on the effectiveness of the facilitator's defence strategy and vary among facilitator plants with different defence mechanisms.
Sapling size affects the probability of being grazed by herbivores, with small tree saplings less likely to be discovered by herbivores and therefore less likely to be damaged (Rao et al. 2003). Consequently, small tree saplings may need less protection against browsing than larger tree saplings. Alternatively, large tree saplings may be easier to detect and hence avoided (when not palatable to herbivores), in contrast with small tree saplings which may be grazed accidentally together with surrounding, preferred vegetation. Thus, associational resistance may depend on the size of the tree sapling.
Tree saplings growing near unpalatable plants may be protected effectively against cattle grazing but could suffer more from competition with their facilitators (for light, nutrients, water, etc.) than tree saplings growing in open swards or grassland (Berkowitz, Canham & Kelly 1995; Rousset & Lepart 2000). Accordingly, growth of protected tree saplings may be reduced as a result of competition with their facilitators, leading to a trade-off between protection against grazers and being limited in growth by unpalatable plants.
In this field study we tested the proposed facilitative effects of a physically and chemically defended unpalatable species on tree sapling performance. We transplanted Picea abies (L.) Karst. saplings of two size classes in plots with and without unpalatable plants, and in plots with unpalatable plants removed to ground level. Tree sapling performance was followed throughout two grazing seasons. In accordance with the associational resistance theory, we hypothesized that tree saplings planted in plots with unpalatable plants would have higher survival but reduced growth rates. Furthermore, we expected to find differences in protective effects of the two unpalatable species and different performances of the two size classes of tree saplings.