Fire‐ and herbivory‐driven consumer control in a savanna‐like temperate wood‐pasture: An experimental approach

Abstract

Abstract Fire and herbivory are fundamental top‐down processes, structuring grass–tree ratios in ecosystems across a diversity of climates. Both are plant consumers that can strongly control the recruitment of woody seedlings and saplings to taller height classes. Without consumer control, many grass‐dominated ecosystems would convert into woodlands or forests. While extensively studied in savannas, few have explored the effects of these disturbance regimes on woody recruitment under temperate conditions. We exposed saplings of five common European tree species to fire and herbivory in a full factorial experiment in a savanna‐like wood‐pasture. After 3 years, we evaluated the effects of fire and herbivory on tree sapling survival and height increment. The tree species used, varying in traits and in expected response to fire and herbivory, were Scots pine Pinus sylvestris, Norway spruce Picea abies, European oak Quercus robur, Silver birch Betula pendula and Small‐leaved lime Tilia cordata. Fire and herbivory had a negative effect on sapling survival for all species except for Q. robur, which was not affected by fire. Both processes reduced height increment of B. pendula, while only herbivory reduced the height increment of P. sylvestris and Q. robur. At the same time, B. pendula and P. sylvestris had some of the highest increments, together with P. abies, which had unaffected height increment in all treatments. T. cordata, on the other hand, had a negative height increment across all treatments. Overall, the combined effect of fire and herbivory was similar to the effect of herbivory alone on both survival and height increment, indicating no additional effect of fire when herbivores were present. Synthesis. Our experiment showed how fire and herbivory can strongly affect the recruitment of European temperate tree saplings on a wood‐pasture, potentially leading to comparable consumer control described for ecosystems elsewhere (e.g. savannas). Two strategies to deal with fire and herbivory were identified: tolerance (Q. robur) and avoidance (P. sylvestris and B. pendula). We conclude that both fire and herbivory may have been important drivers of structure and species composition in open ecosystems in temperate Europe in the past.