Does input of rich litter facilitate tree growth? : growth and climate growth response of oak (Quercus robur) growing in the neighborhood of black cherry (Prunus serotina)

Atmospheric nitrogen deposition and former land use have led to widespread acidification of forest soils and disturbed nutrient balances, which has been linked to reduced forest vitality and tree growth. The admixture of rich-litter tree species in forests dominated by poor-litter species that further accelerate soil acidification may alleviate the impact of soil acidification on poor sandy soils. Admixture of litter with high base cation content has a positive impact on topsoil pH, as well as base saturation and may potentially affect growth and vitality of trees. To date, the effect of introducing rich-litter species on the long-term growth of co-occurring tree species has not been systematically studied. In this study, we aimed to evaluate the effect of the rich-litter species Prunus serotina on the growth of the poor-litter tree species Quercus robur by means of dendrochronology. We hypothesized that the presence of Prunus will – through soil amelioration - positively affect both the growth and the drought susceptibility of Quercus. We investigated the growth of co-occurring Quercus and Prunus trees in six forest sites with sandy, poor soils in the Netherlands and Germany. We compared tree-ring patterns of oaks growing next to cherry trees (influenced), oaks growing next to other oaks (uninfluenced) and the cherry trees. Tree-ring widths were analyzed on (a) average annual growth variation and basal area increment; (b) the climate sensitivity of the annual growth; (c) the growth response to selected drought years; and (d) the tree-individual growth variation. For most analyses, influenced and uninfluenced oaks showed little differences. Individual growth variation between oaks did show a division between influenced and uninfluenced trees in some sites. However, this division seemed to be linked to spatial separation rather than rich-litter effects. Overall, this study indicated that there is no clear evidence for a facilitating effect of prunus on the growth of oak. It is likely that the studied sites were too poor to capture an effect of the improved soil conditions on the growth of oak, or that competition for water may have overruled any direct soil effect by prunus. We conclude that this study did not provide evidence the rich litter species Prunus serotina does positively affect the growth of Quercus robur on the studied acidic poor sandy soils.