The effects of broad-leaved trees on the associated biodiversity of improved grassland within silvopastoral systems.

Abstract

Agroforestry systems are highly complex biological systems, which involve the growth of woody plants in combination with crops, pasture or a combination of the two. They are multi functional systems which offer a sustainable means of fulfilling a number of economic and conservation objectives simultaneously. Given the low tree cover and large proportion of the many small landholdings (35ha) in Northern Ireland, farmers are reluctant to convert large areas of farmland into woodland even with an advantageous grant support system. Within the current reform of the CAP there is a need to develop sustainable management systems which can deliver positive environmental benefits, increase biodiversity, and enhance the farmed landscape. The integration of trees and pasture in a silvopastoral system, whereby livestock graze between protected, broad-leaved trees, is therefore a viable land use alternative.

The aim of this study was to examine the effect of trees on the associated biodiversity of improved grassland within silvopastoral systems. Birds, invertebrates and flora were monitored in ash and sycamore silvopastoral systems and two forms of pasture management were compared. It is proposed that the developing root systems of ash and sycamore trees in silvopastoral systems have improved soil physical characteristics, such as porosity and drainage. Soil improvements in ash and sycamore silvopasture led to increased soil drainage and penetrability, increasing the activity and diversity of burrow constructing fauna, which will have beneficial effects in terms of nutrient cycling and decomposition.

The presence of maturing trees in pasture increased spider and ground beetle diversity through changes in the structure and botanical diversity of the sward. Silvopastoral swards contained a greater diversity of grass and herb species than tree-less pasture, and structural heterogeneity was significantly enhanced by the presence of ash and sycamore trees. Vegetation stratification and increased prey populations associated with structurally and botanically diverse vegetation, are important determinants of habitat preference for ground beetles and spiders, which form an integral part of the ecosystem.

Enhancements in invertebrate diversity may increase bird numbers and species richness. Woodland birds appear to be attracted to silvopastoral systems and are often found in conjunction with farmland species. As the trees mature and the canopy closes, there is likely not only to be a shift in bird species composition, but also a change in how bird communities utilize the silvopasture. It is proposed that seasonal differences in feeding behaviour, in ash and sycamore silvopasture, will be influenced by bird breeding behaviour.

It is likely that postponement of canopy closure through shoot prunmg of silvopastoral trees will maintain a diverse balance between birds and sward and soil invertebrates associated with both grassland and forest habitats. This study shows that although pasture productivity is unaffected by the presence of trees in silvopastoral systems, trees bring additional physiological benefits to grazing livestock whilst enhancing floristic, invertebrate and bird diversity above that of typical agricultural land.

Date of Award	Dec 2000
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	James McAdam (Supervisor) & Alan Bell (Supervisor)