Stabilization of the pine increment during recent years of low precipitation and high temperatures by removal of the spruce lower storey

In stands dominated by Scots pine in Central European nutrient-poor sandy forest sites, deterioration of pine health due to recent development in climate change has been observed, whereas the vitality of the Norway spruce growing in the lower storey of such stands has remained stable. The study aims to analyse the impact of the spruce lower storey on pine increments and performance in relation to drought stress. Wood core samples were collected using an increment borer from mature pines growing in deep, sandy soils with ground water at a depth that was unreachable by the roots in three plot pairs (blocks) consisting of a pine-spruce plot and a plot with the lower-storey spruce removed. The increments of earlywood and latewood were measured separately. Health status and soil moisture were also evaluated. Spruce removal increased the radial growth (tree ring width - TRW) of the pines, and this manifested earlier in the latewood. Spruce removal significantly changed relationship of TRW and umbrothermic index of released pines. The treatment also increased pine foliation, while increases of topsoil moisture in the transects were insignificant. The study indirectly proves the existence of interspecies competition in the rooting zone for water in soils where the ground water is unreachable by the roots. The flat root system of spruce creates a barrier that reduces the amount of water available for pines with deeper growing roots. To reduce the risk of the negative impact of unfavourable climate change on pine as the target species in commercial mature mixed pine forests with similar site and stand conditions, lower-storey spruce reduction can be recommended, even though this might locally and temporarily lessen the biodiversity of the forest stand.

Automated summary

In Central European nutrient-poor sandy forests dominated by Scots pine, climate change has led to deterioration of pine health while the vitality of the lower-storey Norway spruce remains stable. This study analyzes the impact of lower-storey spruce on pine growth and performance under drought stress. Removing the lower-storey spruce increased the radial growth of pines, particularly in latewood. It also altered the relationship between tree ring width and umbrothermic index. The study suggests that interspecies competition for water in soils with unreachable groundwater occurs, with the spruce's flat root system reducing water availability for the deeper-rooted pines.