What happens to oak growth and survival when there is both competition and browsing?

Abstract

Competition from neighboring vegetation and browsing by large herbivores are two of the most important factors affecting the structure and dynamics of temperate forests. While the previous literature has been able to identify individual negative effects from competition or browsing, no one has yet identified and quantified both the individual and the joint effects. Still, when plants face both competition and browsing, it is possible that the combined effect is not simply a sum of the individual negative effects, but perhaps a more complicated situation, where plants perform either better (in case there is also a facilitative effect from the neighboring vegetation) or worse (if the effects amplify each other) than they would if experiencing only one of the two factors. In this paper, we focus on regeneration of oak (Quercus robur L) to study these questions. We analyze a rich data set from a large long-term field experiment conducted at multiple sites in mixed temperate forests in southern Sweden over almost a decade. By the use of four separate treatments on each site - (i) neither competition, nor browsing, (ii) only competition, (iii) only browsing, and (iv) both competition and browsing - we can identify and quantify both the individual and combined effects of competition and browsing on oak growth and survival. We find that both competition and browsing individually affect growth and survival negatively. For growth, browsing has the largest effect, while competition is the larger problem from a survival point of view. When the plant experiences both competition and browsing, the combined, negative, effect is larger than either individual effect for survival, but for growth, the relationship is more complicated, and the surrounding woody vegetation offers at least some protection for the oak, reducing the negative effects from browsing.