Tree lines influence soil temperatures, sward growth dynamics and litter decomposition in permanent grassland

Climate change is likely to have a negative impact on soil functions in agro-ecosystems in the future. Alley cropping combines trees and agricultural crops or grassland on the same agricultural area. It has recently been discussed as more sustainable and resilient than comparable systems without trees for the temperate zone. So far, the influence of tree lines on biomass growth and decomposition of aboveground litter in the adjacent grassland is not fully clear. We measured the effect of tree lines on soil temperatures of an adjacent grass sward at five positions differing in distance and orientation (facing the tree line in the East or West) in a temperate alley-cropping system. Growth dynamics (sward height and soil cover) as well as litter half-life times for two cutting frequencies (four vs. two annual cuts) were monitored during two experimental years. The botanical composition at the transect positions was evaluated using the dry weight rank method. We found lower soil temperatures close to the trees, even though the proportion of soil cover was slightly lower close to the trees. The sward heights did not show a consistent pattern with respect to distance to the trees. Annual biomass accumulation was greatest at 6 m distance to the trees. An intermediate distance to the trees had a beneficial effect on productivity compared to the position with the lowest tree effect (25 m). Litter half-life times decreased with increasing sward height without a consistently positive/negative influence of the tree line over the examined growth periods. Lower litter half-life times in taller grass swards indicate that the sward height is the main factor driving litter decomposition while the influence of the trees is of minor importance.