Temperature and nutrient availability influence radial growth of Picea abies at opposite slopes in a treeline ecotone

Abstract

Treeline ecotones in complex mountain landscapes are exposed to pronounced differences in irradiation and soil nutrient availability. Different amounts of nutrients and direct solar energy can influence tree stem growth, especially in lower parts of a treeline ecotone, where trees are still temperature limited, though located below the upper margin of tree life. We hypothesized that, at two sites located on north- and south-facing slopes, differences in nutrient availability outperform temperature differences in modulating stem growth rates, while growth phenology is driven by temperature seasonality. To test this hypothesis, we compared the growth phenology and kinetics of Picea abies in the lower part of a treeline ecotone between a north-facing slope with relatively nutrient-rich soils and a south-facing slope with nutrient-poor soils. We analysed intra-annual wood formation, soil and air microclimate, and soil and needle nutrient contents. Our results showed that thermal differences between south- and north-facing slopes are small but nontrivial, involving higher daytime temperature at the south-facing slope and longer irradiation at the north-facing slope during the middle part of the growing season. The timings of growth onset and maximum growth rate were almost synchronized between both slopes. Accordingly, annual stem growth at both sites was most sensitive to the meteorological conditions at the start of the growing season and around the summer solstice. However, the absolute growth rate was higher on the north-facing slope, consistent with a higher availability and content of base cations in the soil and needles. Our results suggest that temperature governs growth phenology at the lower part of the treeline ecotone, but nutrient availability modulates the growth rate in the peak season when temperature no longer limits cambial activity. We demonstrated that the effect of nutrient availability can be superior to the effect of slope aspect for stem growth rates of Picea abies located in the lower part of a treeline ecotone in a temperate mountain range.