Fire and herbivory – key drivers of temperate open ecosystems : Effects of consumer control on woody and herbaceous plant communities in temperate wood-pasture

The decline of open and semi-open habitats is an important reason behind the ongoing worldwide loss of biodiversity. Understanding the drivers of habitat openness is an important key for preventing the continued loss of these ancient ecosystems. In other continents, fire and herbivory are well-studied and identified as important processes in preventing open habitats from turning into closed forests. The aim of this thesis is to apply the established concept of consumer control in a new context: the northern areas of temperate Europe. In a large field experiment in a Swedish wood-pasture, we first studied how two consumers, prescribed grass fire and grazing by cattle, alone or in combination, affected the survival and growth of planted tree saplings (I). The tree species demonstrated different strategies to resist fire or herbivory. Either they survived by re-sprouting or they quickly grew out of reach of the consumers. In addition, we studied the effect of the two consumers on the establishment and reproduction of sowed open habitat forbs (II). Herbivory improved the conditions for the establishment of forbs, but fire promoted the production of inflorescences. A release of grazing pressure increased the number of remaining flowers at the end of the growing season, confirming the importance of variation in herbivory intensity over time for the persistence of forbs. In a third study, we studied the interaction between fire and cattle grazing (III). We followed the foraging behaviour of cattle with camera traps and found that when a larger proportion of an area was burned, the cattle preferred to graze in the newly burned areas. This led to a reduction in fuel, altering the future fire pattern. In a final study, we evaluated the long-term effects of fire and herbivory by comparing the biomass of woody and herbaceous vegetation (IV). Without fire and herbivory, trees out-shaded grasses and forbs, leading to conversion from an open grassy ecosystem to a tree-dominated forest. In conclusion, herbivores and potentially also fire can be important drivers of openness, also in temperate Northern Europe, but further research is needed to understand the effect of different fire and grazing regimes

Pyric herbivory in a temperate European wood-pasture system

The term pyric herbivory was first introduced in 2009, describing how fire shapes herbivory as burned areas attract herbivores and, simultaneously, herbivory shapes fuel load and fire behaviour. Pyric herbivory results in a mosaic of patches with varying levels of herbivory and grazing intensity fire intensity and frequency. The importance of pyric herbivory for ecosystem heterogeneity and biodiversity has been described for North American, Australian and African systems, but the concept remains largely untested in a European context. We introduced fire and herbivory in a full-factorial experiment in a temperate European wood-pasture system to test whether pyric herbivory operates in ways comparable to grassy systems elsewhere in the world. Using camera traps, we observed the behaviour of cattle in burned subplots (49 m2) compared with unburned subplots. We measured grass height and the proportion of the subplot that burned as variables affecting cattle preference and to assess how grazing affects fire behaviour. We also examined the effect on plant species and life-form composition after six seasons of treatment. Cattle spent more time grazing in burned than in unburned subplots in the most productive paddock, where a larger proportion of the subplot burned. The proportion of a subplot that burned was positively related to pre-fire grass height. Moreover, both grass height and the proportion of subplot burned declined in the burned subplots during the 6-year study period and fire and cattle grazing altered the relative cover of graminoids and shrubs (Rubus spp.), with more graminoids in grazed and/or burned subplots and more shrubs in ungrazed subplots at the end of the study. Synthesis and applications. In our temperate European wood pasture, fire and (cattle) grazing interacted in ways comparable to pyric herbivory in grassy ecosystems elsewhere in the world, especially in the most productive paddock. Fire attracted grazing, with cattle grazing longer on subplots that burned more fully. Grazing also affected fire, where over the course of our experiment cattle grazing reduced grass height and the proportion of a subplot that burned. We suggest that pyric herbivory is an interesting management method to further explore in the European context to address the loss of biodiversity in open ecosystems, particularly in more productive sites.In our temperate European wood pasture, fire and (cattle) grazing interacted in ways comparable to pyric herbivory in grassy ecosystems elsewhere in the world, especially in the most productive paddock. Fire attracted grazing, with cattle grazing longer on subplots that burned more fully. Grazing also affected fire, where over the course of our experiment cattle grazing reduced grass height and the proportion of a subplot that burned. We suggest that pyric herbivory is an interesting management method to further explore in the European context to address the loss of biodiversity in open ecosystems, particularly in more productive sites.imag

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

1. 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.2. 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.3. 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.4. 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