1. In the current biodiversity crisis, one of the crucial questions is how quickly plant communities can acclimate to climate warming and longer growing seasons to buffer the impairment of community functioning. Answering this question is pivotal especially for mountain grasslands that experience harsh conditions but provide important ecosystem services to people.

2. We conducted a reciprocal transplant experiment along an elevation gradient (1920 m vs. 2450 m) in the French Alps to test the ability of plant species and communities to acclimate to warming and cooling. For three years, we measured weekly the timing of phenological events (e.g. start of flowering or greening) and the length of phenological stages linked to demographic performance (e.g. lengths of flowering or greening periods).

3. We found that warming (and cooling) changed the timing of phenological events strongly enough to result in complete acclimation for graminoids, for communities in early and mid-season, but not at all for forbs. For example, warming resulted in later greening of communities and delayed all phenophases of graminoids. Lengths of phenological stages did not respond strongly enough to climate change to acclimate completely, except for graminoids. For example, warming led to an acclimation lag in the community’s yearly productivity and had a strong negative impact on flowering of forbs. Overall, when there was an acclimation failure, responses to cooling were mostly symmetric and confirmed slow acclimation in mountain grasslands.

4. Synthesis. Our study highlights that phenological plasticity cannot prevent impairment of community functioning under climate warming in the short-term. The failures to acclimate after three years of warming signals that species and communities underperform and are probably at high risk of being replaced by locally better-adapted plants.

For the dataset collection and the processing, please refer to the methods section of the manuscript. 

dataFocals_030621.RData or dataFocals_030621.csv

Plot: Treatment groups
    LC: SubalpineControl
    LT: AlpineWarmed
    GC: AlpineControl
    GT: SubalpineCooled

Repetition: Number of plot repetitions

Observation_Date:

Species: Focal species names

Phenophase: Phenological phases as described in the main manuscript

Number_of_individuals: Number of individuals present in the phenophase. For details, please refer to the main manuscript. 

dataNDVI_030621.RData or dataNDVI_030621.csv

Plot: Treatment groups     LC: SubalpineControl
    LT: AlpineWarmed
    GC: AlpineControl
    GT: SubalpineCooled

Subplot: Coding for 1mx1m subplots
    X: Only for Controls
    A,B,C,D: Only for Treatments

Plot_Repetition: Number of 4mx4m plot repetitions

Measurement_Repetition: 3 times measurement repetitions

Measurement_Date:

Measurement_Sm: Gap-filled NDVI measurements. For the methodology, please refer to the main manuscript.