Radial growth of Picea abies is controlled by joint effects of temperature and nutrient availability at the lower part of 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 resulting in variation of growth rates and growth phenology across lower parts of treeline ecotone. We hypothesized that at two contrasting 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 the course of seasonal temperature. To test this hypothesis, we compared the growth phenology and kinetics of Picea abies in the lower part of 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 and its response to ambient climate, as well as soil and air microclimate and soil and needle nutrient content. Our results showed that thermal differences in treeline ecotones between south and north-facing slopes in temperate mountains are small but nontrivial involving higher daytime temperatures at south-facing slopes and longer irradiation of north-facing slopes during the middle part of growing season. The timing of growth onset and maximum growth rate were almost identical on 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 the 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 conclude 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 treeline ecotone in temperate mountain range.