4.3 Species-specific responses of leaf senescence and last
flower dates
Warming-induced changes in leaf senescence and last flower dates were
highly variable among species. First, unlike leaf out and first flower
dates, leaf senescence and last flower dates are more likely jointly
controlled by the complex interactions among multiple factors, such as
soil temperature, soil moisture, soil nutrient availability, and
photoperiod (Ernakovich et al.2014; Estiarte & PeƱuelas 2015;
Gill et al. 2015). Changes in
these factors may advance or delay the last dates of plant phenology,
depending on their balanced effects. Second, the underlying driving
mechanisms for plant growth and maintenance of plant metabolic
activities are also highly variable among species
(Myers-Smith et al. 2015). For
example, some legumes are reported to be sensitive to reductions in soil
moisture, while forbs are expected to be more responsive to
warming-induced changes in soil N availability
(White et al. 2000;
Engelbrecht et al. 2007). Third,
even species within the same plant functional group were also highly
varied in their responses of leaf senescence and last flower dates to
experimental warming. For example, warming advanced leaf senescence and
last flower dates for Gentiana squarrosa but delayed them forHeteropappus altaicus and Artemisia scoparia , despite the
fact that they are all forbs. The advancement of leaf senescence and
last flower dates of Gentiana squarrosa in the early-growing
could confer a competitive advantage by allowing this species to avoid
the shading effects induced by other taller species later in the growing
season. In addition, plant functional traits (e.g., leaf morphology and
plant height) are also reported to have critical impacts on plant
phenology even within the same plant function group
(Guerin et al. 2012;
Dorji et al. 2013). These results
call for caution when using plant functional group as a predictor of the
effects of warming on last dates of plant phenology.
Our results from the cold Tibetan Plateau stress that warming-induced
shifts in phenological firsts are inadequate to predict the ecological
consequences of climatic warming on plant phenology and plant community
turnover. Rather, we find considerable variability among species in how
warming impacts the first and last dates of their phenological patterns.
As a consequence, it is warming-induced changes in full phenological
periods and phenological lasts, and how this varies among species that
drive how warming reshapes species dominance. Our results provide novel
insights for understanding the effects of climate warming on plant
phenology and plant community turnover in a climatically sensitive
ecosystem, and underscore the need to assess how climatic warming will
impact the phenological lasts and the full phenological periods.