Growth and Development of Forest Stands on High pH Soils and Marl

Marl is a high-pH subsoil layer that occurs frequently in the peatlands in the midlands of Ireland. Shallow marl is thought to prevent forestry plantations growing successfully and current policy in Ireland does not support planting where marl is found within 70 cm of the surface. However there is conflicting evidence about whether this depth limit is appropriate. The hypothesis that plantations can be successfully established on sites where marl or other high-pH soils (identified using dilute hydrochloric acid ) prevail was investigated in this study. Existing forest plantation sites were identified where marl or other high-pH soils were present and these sites were sampled using gradient directed transects, following the gradients of tree growth, or depth to high-pH soil if no gradient in tree growth was observed in preliminary screening. At each sample point along the transect the tree growth, crown condition, and soil profile were recorded. At subsets of these sample points soil samples were taken and foliar samples were taken from the crown of the trees for spectroscopy analysis. Examples of trees from all species surveyed were found to be growing at commercially viables rates in the presence of high-pH soil material shallower than 70 cm but growth rates varied. There was no clear evidence of soil or tree properties measured in the study interacting with the depth to high-pH soil material to determine tree growth rates. Tree foliar health did not relate to the depth to high-pH soil material under individual trees. Results indicated that the soil profiles were very heterogeneous within sites, even over small spatial scales. These results have implications for expanding the area available for afforestation into areas with shallow high-pH subsoils, although thorough sampling grids are recommended to exploit the full potential of forestry on sites with high-pH soil present. Further research should aim to identify further factors that act alongside the depth to high-pH material to drive tree growth on high-pH soil sites.