Subalpine grassland productivity increased with warmer and drier conditions, but not with higher N deposition, in an altitudinal transplantation experiment

Abstract

Multiple global change drivers affect plant productivity of grasslands and thus ecosystem services like forage production and the soil carbon sink. Subalpine grasslands seem particularly affected and may serve as a proxy for the cold, continental grasslands of the Northern Hemisphere. Here, we conducted a 4-year field experiment (AlpGrass) with 216 turf monoliths, subjected to three global change drivers: warming, moisture, and N deposition. Monoliths from six different subalpine pastures were transplanted to a common location with six climate scenario sites (CSs). CSs were located along an altitudinal gradient from 2360 to 1680 m a.s.l., representing an April–October mean temperature change of −1.4 to +3.0 ∘C, compared to CSreference with no temperature change and with climate conditions comparable to the sites of origin. To uncouple temperature effects along the altitudinal gradient from soil moisture and soil fertility effects, an irrigation treatment (+12 %–21 % of ambient precipitation) and an N-deposition treatment (+3 kg and +15 kg N ha−1 a−1) were applied in a factorial design, the latter simulating a fertilizing air pollution effect. Moderate warming led to increased productivity. Across the 4-year experimental period, the mean annual yield peaked at intermediate CSs (+43 % at +0.7 ∘C and +44 % at +1.8 ∘C), coinciding with ca. 50 % of days with less than 40 % soil moisture during the growing season. The yield increase was smaller at the lowest, warmest CS (+3.0 ∘C) but was still 12 % larger than at CSreference. These yield differences among CSs were well explained by differences in soil moisture and received thermal energy. Irrigation had a significant effect on yield (+16 %–19 %) in dry years, whereas atmospheric N deposition did not result in a significant yield response. We conclude that productivity of semi-natural, highly diverse subalpine grassland will increase in the near future. Despite increasingly limiting soil water content, plant growth will respond positively to up to +1.8 ∘C warming during the growing period, corresponding to +1.3 ∘C annual mean warming.