The influence of cattle activity on tree regeneration in wood-pastures

Wood-pastures are semi-natural, highly biodiverse systems maintained by traditional extensive agriculture. They are used for grazing and timber. The shifting mosaics of grassland, shrub thickets and woodland patches in these systems are driven by large herbivores. Current changes in agricultural practices are leading to either intensification or abandonment and they result in a segregation of grassland and forest. This presents a complex conservation problem for these endangered ecosystems. Large herbivores have been identified as an important factor preventing the establishment of trees and the regeneration of woodland. However, successful management is severely hindered by the lack of ecological knowledge about the effects of large herbivores on woody species. This research focussed on the quantification of the impact of cattle activity on the early stages of tree establishment for four tree species (Abies alba, Picea abies, Acer pseudoplatanus and Fagus sylvatica) common in the wood-pastures of the Swiss Jura Mountains. Cattle can have both direct and indirect effects on tree establishment, and the interaction between cattle activity and both competition and facilitation by surrounding vegetation was studied. Experimental field studies (involving seed sowing and sapling planting) were carried out. Both cattle activity and competition from neighbours reduced first-year tree seedling recruitment in wood-pastures. Seedling establishment decreased with increasing cattle grazing intensity. The effects of gaps in the vegetation on seedling emergence were complex, as a result of biotic (i.e. competition, facilitation and predation from rodents, invertebrates and large herbivores) and abiotic (e.g. moisture, temperature and light) interactions, leading to different results in different years. Although overall the chances for tree seeds to make it to seedlings were low, seedling establishment was relatively high for Picea, intermediate for Abies and Acer and low for Fagus. The probability of a sapling of being browsed increased with sapling size and decreased with the height of surrounding vegetation and in the presence of nurse shrubs. It was similar for the four species and under low and high grazing intensities. Saplings had species- and size-specific responses in survival and growth to cattle browsing. Under both low and high grazing intensity, small coniferous saplings (36 %) had a lower survival rate than small deciduous saplings (53 %), which we hypothesize is due to the larger biomass losses stemming from a more vulnerable plant architecture. Under low grazing intensities, large Picea (80 %) and Fagus (67 %) saplings had a higher survival than large Acer (50 %) and Abies (33 %) saplings. This was likely due to differences in sapling tolerances to loss of biomass. Simulated browsing damage resulted in relatively smaller growth losses when small saplings were growing slowly in the presence of neighbours. Shade increased this effect. At least in the short term, the degree of small sapling tolerance was not related to plant performance as saplings that compensated almost for biomass losses still had lower survival and growth rates than less tolerant saplings. Picea was the only species not negatively affected by strong irradiance as found in an open pasture. For all tree species, the facilitative effect of nurse shrubs on small saplings was highest at intermediate levels of grazing intensity. Shrubs were more heavily damaged at high intensity than low intensity. Consequently, escaping browsing, sapling survival and growth was significantly increased by shrubs under low grazing intensity but not under high grazing intensity. Moreover, the positive effects of shrubs tended to be higher for coniferous species, and in particular Picea which was more sensitive to both competition from neighbours and browsing. In conclusion, the resistance of trees to cattle activity varies among the early stages of tree establishment and is affected by tree species, grazing intensities and environmental conditions. Both competitive and facilitative interactions found between young trees and surrounding vegetation illustrate the complex nature of plant-plant and plant-animal interactions during the tree establishment phase in wood-pastures. The insight provided into the interactions between cattle grazing and tree establishment in wood-pastures will contribute to improve ecological theory and models, in addition to informing the management and conservation of wood-pasture systems.