Transient facilitative effects of heather on Scots pine along a grazing disturbance gradient in Scottish moorland

Abstract

Summary1 Facilitation between neighbouring plants can promote species survival and regulate community composition. However, the role of facilitation varies along environmental severity gradients. It is important to understand the shape of this relationship to improve our ability to predict the impact of a changing environment on biodiversity. 2 We used Scots pine saplings growing within heather to examine the shape of the relationship between facilitative interactions (protection from browsing) and the severity of the environment (deer browsing intensity). We also investigated whether protection from browsing translated into a biomass response of saplings. 3 In the first winter following planting heather had a facilitative effect on saplings by reducing the probability of browsing. This effect was strongest at intermediate deer browsing intensities, thus producing a hump-backed relationship between facilitative effects and the severity of the environment. 4 Protection from browsing did not lead to longer-term biomass gains for the saplings. The competitive effects of heather on sapling growth therefore outweighed the beneficial effects of protection from browsing. 5 These results provide much-needed information on the shape of the severity-interactions relationship with respect to a key natural disturbance phenomenon (herbivory), and demonstrate that an observable interaction relationship does not necessarily translate into a biomass response. 6 This illustrates the complex and potentially transient nature of plant-plant interactions, and the potential difficulty that would be associated with using shelter effects of heather as a management tool to promote Scots pine regeneration.

Effect of nitrogen availability on forest understorey cover and its consequences for tree regeneration in the Austrian limestone Alps

“…A facilitative effect of ground vegetation on young trees by protection against herbivores has repeatedly been reported (e.g. Callaway 1992;Gill 1992;Hester et al 2000;Kupferschmid and Bugmann 2005;Baraza et al 2006;Brooker et al 2006;Gómez-Aparicio et al 2008). Although many of these studies were conducted in non-forest systems and with shrub species as benefactors, which may have more pronounced protective effects than herbs and grasses (Harmer 2001), our results indicate that dense graminoid understorey vegetation may indeed reduce browsing on juvenile trees.…”

Does the strength of facilitation by nurse shrubs depend on grazing resistance of tree saplings?

“…grazer mediated) effects on palatable herbs, shrubs or trees (Bakker et al, 2004;Callaway et al, 2005;Smit, Be´guin, Buttler, & Mueller-Schaerer, 2005;Smit, den Ouden, & Mu¨ller-Scha¨rer, 2006). Brooker, Scott, Palmer, and Swaine (2006) and Smit, Vandenberghe, den Ouden, and & Mu¨ller-Scha¨rer (2007) found a hump-backed relationship between facilitation of saplings by a nurse plant and the level of grazing disturbance. The reduced facilitation for saplings under high grazing intensities was related to high net search effort by the herbivores and increased damage to nurse plants.…”

Season- And Herbivore-Dependent Competition and Facilitation in a Semiarid Savanna

“…There is growing evidence pointing to the intensity (not simply presence or absence) of grazing as an important driver of facilitation (Rebollo et al 2002, Graff et al 2007, and the prevalence of facilitation is predicted to increase with increasing grazing stress (Bertness and Callaway 1994, but see Brooker et al 2006, Smit et al 2007). There are two major reasons to believe higher grazing intensity may have coincided with the dry season in my study: 1) dry seasons in general are associated with increases in grazing pressure on limited forage and/or with higher herbivore selectivity (Demment and Van Soest 1985, Scoones 1995, Belovsky 1997 Facilitation of Cynodon appears to be a case of "associational resistance" in which some plants gain an advantage by "hiding" among others of higher or lower palatability (Root 1973, Attsat and O'Dowd 1976, Hjältén et al 1993 A major assumption of associational resistance is that herbivores are deterred by the less palatable neighbor species.…”

“…In savanna ecosystems, where the duration and intensity of herbivory and rainfall vary tremendously over time and space, herbivores-and their interactions with wet/dry cycles-have the potential to be major drivers of long-term ecosystem change. We know that herbivores can influence succession (e.g., Dormann et al 2000, Fagan and Bishop 2000, Bokdam 2001, Skarpe 2001, Seabloom and Richards 2003, that intensity of herbivory matters (e.g., Rebollo et al 2002, Brooker et al 2006, Graff et al 2007, and that different guilds of herbivores have different and sometimes interactive effects on the plant community (McNaughton 1978, Young et al 2005). But we must explicitly test how herbivory interacts with temporal and spatial variation in plant community dynamics in order to elucidate and make meaningful predictions about the relationships between short-term and long-term dynamics in savanna ecosystems.…”

“…Herbivory also affects the net direction of plant-plant interactions by mediating both positive and negative interactions (Gurevitch et al 2000, Hambäck and Beckerman 2003, Veblen-4 Callaway et al 2005, Brooker et al 2006, Graff et al 2007. If a plant species is selectively grazed, its fitness may be decreased, thereby altering competitive dynamics within the plant community (Gurevitch et al 2000, Fowler 2002).…”