The influence of competition from herbaceous vegetation and shade on simulated browsing tolerance of coniferous and deciduous saplings

The ability of saplings to tolerate browsing (i.e. the ability to persist with reduced biomass and to compensate for biomass loss) is influenced by the level of stress and their growth strategies. Ultimately, insight into species-specific responses of saplings to browsing, shade and competition from neighbours will help explain diversity, structure and function of grazed ecosystems such as the endangered wood-pasture systems. We measured the survival, whole-sapling biomass and compensatory growth responses of two coniferous (Picea abies and Abies alba) and two deciduous (Acer pseudoplatanus and Fagus sylvatica) tree species to simulated summer browsing (one single clipping event), shade (installation of a shade cloth) and neighbour removal (mowing surrounding vegetation to ground level) treatments and the interactions between them after two-growing seasons. For all species, there were interacting effects on growth of browsing and environmental condition (shade and neighbours). Simulated browsing resulted in relatively smaller growth losses when plants were growing slowly due to competitive conditions related to herbaceous neighbours. Although none of the clipped saplings could fully compensate for their biomass losses, the saplings were closer to compensation under high competitive conditions than under low competitive conditions. Survival of the clipped saplings remained relatively high and was only significantly reduced for Picea and Acer. Picea was least tolerant of competition and was the only species for which growth was not negatively affected by strong irradiance of a mountain pasture. Surprisingly, the tolerance of saplings to herbivory as browsing tolerance was enhanced under conditions that negatively affected sapling performance (i.e. survival and growth). Apparently, the relative impact of browsing at the early sapling stage is linked to tree life history characteristics such as competition and shade tolerance and will be lower in situations with intense competitive interactions and/or strong irradiance.

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

Charlotte Vandenberghe

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.

EPFL2006

Short-term effects of cattle browsing on tree sapling growth in mountain wooded pastures

Alexandre Buttler, François Frelechoux, Charlotte Vandenberghe

Browsing by livestock has been identified as an important factor preventing tree regeneration in wooded pastures. Two field experiments were performed to investigate the effects of cattle browsing on tree sapling growth in a mountainwooded pasture. Two size classes (ca. 12–17 cm and 41–59 cm) of each of 4 species (Picea abies, Abies alba, Acer pseudoplatanus and Fagus sylvatica) were exposed to zero, low and high grazing intensities. We measured the proportion of saplings browsed and the effect of browsing on growth. A mowing treatment within the zero grazing intensity treatment showed no significant effect on sapling growth. One percent of the large saplings but 25% of the small saplings escaped browsing. Saplings were better protected when surrounded by taller vegetation. The proportion of saplings browsed was not significantly different among species although evergreen tree saplings lost a larger proportion of biomass than deciduous species when browsed. Under grazing, average current-year shoot production and total aboveground biomass of all species were significantly reduced. Browsing effects tended to be smaller at the lower grazing intensity. Because the deciduous species were least reduced in aboveground growth, especially under the low grazing intensity, we conclude that at least in short-term, deciduous species are less affected by cattle browsing.

2007

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

Charlotte Vandenberghe

EPFL2006