Lags in phenological acclimation of mountain grasslands after recent warming

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.