Understanding the dynamics of treeline ecotones under global change requires long-term ecological and environmental data. The Stillberg ecological treeline research site in the Swiss Alps was established in 1975 by planting 92,000 seedlings of Larix decidua, Pinus cembra and Pinus mugo ssp. uncinata, and has been continuously monitored since then. Here, we present curated long-term data acquired over almost 50 years at the Stillberg site, and we synthesise the major research findings. The long-term datasets comprise 6.5 million ecological and environmental records from the 40-year afforestation experiment, as well as from a 9-year free-air CO2 enrichment experiment crossed with a 6-year soil warming experiment, a 12-year nutrient addition experiment and an 8-year multifactorial tree seedling recruitment experiment. Our datasets further include 38 million records of 25 meteorological parameters measured at an hourly resolution from 1975 to 1996, and at a 10 min resolution since 1997. We provide all datasets and the corresponding metadata as open research data. Almost five decades of research in this treeline ecotone showed high mortality after tree establishment that was closely related to microclimatic variability. The two Pinus species survived at a much lower rate than L. decidua, due to indirect pathogen interactions. Furthermore, CO2 enrichment only increased growth of L. decidua, while warming increased growth of P. mugo ssp. uncinata and two Vaccinium shrub species. Enhanced nutrient availability stimulated growth in tree and understorey shrub species. In addition, soil warming and CO2 enrichment stimulated microbial activity and decreased soil carbon stocks. These findings improve our understanding of ecological processes in the treeline ecotone under global change and confirm the importance of tree growth and establishment limitations. The enhanced availability and quality of these long-term data are expected to foster whole-system approaches and transdisciplinary research syntheses, supporting the development of effective global change adaptation strategies.

What is addressed as growing season in terrestrial ecosystems is one of the main determinants of annual plant biomass production globally. However, there is no well-defined concept behind. Here, we show different facets of what might be termed growing season, each with a distinct meaning: (1) the time period during which a plant or a part of it actually grows and produces new tissue, irrespective of net carbon gain (growing season sensu stricto). (2) The period defined by developmental, that is, phenological markers (phenological season). (3) The period during which vegetation as a whole achieves its annual net primary production (NPP) or a net ecosystem production (NEP), expressed as net carbon gain (productive season) and (4) the period during which plants could potentially grow based on meteorological criteria (meteorological season). We hypothesize that the duration of such a 'window of opportunity' is a strong predictor for NPP at a global scale, especially for forests. These different definitions have implications for the understanding and modelling of plant growth and biomass production. The common view that variation in phenology is a proxy for variation in productivity is misleading, often resulting in unfounded statements on potential consequences of climatic warming such as carbon sequestration.

Earlier snowmelt and more frequent summer drought due to climate warming are considered particularly influential for extratropical alpine plants, which are adapted to a short growing season and high water availability. Here, we explored the combined effects of the two drivers with a field experiment in late-successional alpine grassland in the Swiss Alps (2500 m a.s.l.) over 6-7 years. We advanced and delayed snowmelt by removing and adding snow to experimental plots prior to natural snowmelt for 7 years and combined this treatment with 5 and 10 weeks of summer drought for 6 years. We measured plant biomass formation, community composition and ecosystem respiration, and monitored soil moisture as well as soil temperature. Natural snowmelt dates varied by 42 days across years. Snow manipulations advanced and delayed snowmelt by 4.6 and 8.0 days on average but did not affect annual growth (peak biomass) above- nor below-ground. Interactions between snowmelt and drought were nonsignificant, implying that drought effects were independent of snowmelt. Drought reduced total annual above-ground biomass in the 10-week treatment by 16 & PLUSMN; 7% across years, while the 5-week treatment lowered biomass in the last year only. This decline in biomass was accountable to high drought sensitivity of biomass production in a few forb and graminoid species. In contrast, drought did not affect the biomass production of the dominant sedge Carex curvula, whose proportion of total plant cover increased from 36% in controls to 48% in 10-week drought. Below-ground biomass slightly increased under drought (5-week treatment only), resulting in a higher root mass fraction (both treatments). Despite continued root formation, drought reduced ecosystem respiration by 13%-23% per season, assessed nine times during three growing seasons. Since more than 85% of ecosystem respiration stemmed from below-ground activities and roots continued growing under drought, we assume that soil microorganisms were heavily constrained by the drought treatments. Synthesis. We conclude that snowmelt timing is unrelated to productivity, while recurrent drought will shift biomass allocation from shoots to roots in this typical alpine grassland, with potential implications for grazers but also for nutrient and carbon cycling. Species-specific drought-sensitivity will considerably alter community composition under more frequent drought.

Alpine plants have evolved a tight seasonal cycle of growth and senescence to cope with a short growing season. The potential growing season length (GSL) is increasing because of climate warming, possibly prolonging plant growth above- and belowground. We tested whether growth dynamics in typical alpine grassland are altered when the natural GSL (2–3 months) is experimentally advanced and thus, prolonged by 2–4 months. Additional summer months did not extend the growing period, as canopy browning started 34–41 days after the start of the season, even when GSL was more than doubled. Less than 10% of roots were produced during the added months, suggesting that root growth was as conservative as leaf growth. Few species showed a weak second greening under prolonged GSL, but not the dominant sedge. A longer growing season under future climate may therefore not extend growth in this widespread alpine community, but will foster species that follow a less strict phenology.

Almost all natural terrestrial ecosystems are nutrient limited in terms of growth, and we expect treeline vegetation to be no exception. However, direct constraints of low temperature on tissue formation may superimpose effects of low nutrient availability. We examined growth responses of two tree (Larix decidua and Pinus uncinata) and two dwarf shrub species (Vaccinium myrtillus and Vaccinium gaultherioides) to 12 years of moderate fertilizer addition (NPK applied at a rate of 15 and 30 kg nitrogen ha−1 a−1) along an elevation gradient within the treeline ecotone in the Swiss Alps (2,083 to 2,225 m a.s.l.). We measured annual top‐ and side‐shoot increments as well as stem ring width in trees and shoot increments in dwarf shrubs. Fertilizer addition increased soil nutrient availability, indicated by enhanced soil extractable N, higher concentrations of N, P and K in leaves and higher foliar δ15N. Fertilizer addition stimulated annual growth of all four species: by 11%–20% for L. decidua and 15%–36% for P. uncinata (depending on trait) and by 8%–16% for the two dwarf shrub species. Growth stimulation by the higher fertilizer dose was not significantly stronger than by the lower dose (except for V. gaultherioides), suggesting an overall low nutrient demand for growth and saturation at a rather low nutrient input. Synthesis. Even slightly enhanced nutrient availability can stimulate growth of trees and dwarf shrubs in an alpine treeline ecosystem. Ongoing atmospheric nutrient deposition, in conjunction with global warming, may accelerate plant growth at the treeline.