Plant phenological responses to a long‐term experimental extension of growing season and soil warming in the tussock tundra of Alaska

Abstract

Climate warming is strongly altering the timing of season initiation and season length in the Arctic. Phenological activities are among the most sensitive plant responses to climate change and have important effects at all levels within the ecosystem. We tested the effects of two experimental treatments, extended growing season via snow removal and extended growing season combined with soil warming, on plant phenology in tussock tundra in Alaska from 1995 through 2003. We specifically monitored the responses of eight species, representing four growth forms: (i) graminoids (Carex bigellowii and Eriophorum vaginatum); (ii) evergreen shrubs (Ledum palustre, Cassiope tetragona, and Vaccinium vitis-idaea); (iii) deciduous shrubs (Betula nana and Salix pulchra); and (iv) forbs (Polygonum bistorta). Our study answered three questions: (i) Do experimental treatments affect the timing of leaf bud break, flowering, and leaf senescence? (ii) Are responses to treatments species-specific and growth form-specific? and (iii) Which environmental factors best predict timing of phenophases? Treatment significantly affected the timing of all three phenophases, although the two experimental treatments did not differ from each other. While phenological events began earlier in the experimental plots relative to the controls, duration of phenophases did not increase. The evergreen shrub, Cassiope tetragona, did not respond to either experimental treatment. While the other species did respond to experimental treatments, the total active period for these species did not increase relative to the control. Air temperature was consistently the best predictor of phenology. Our results imply that some evergreen shrubs (i.e., C. tetragona) will not capitalize on earlier favorable growing conditions, putting them at a competitive disadvantage relative to phenotypically plastic deciduous shrubs. Our findings also suggest that an early onset of the growing season as a result of decreased snow cover will not necessarily result in greater tundra productivity.

Ecotypic differences in the phenology of the tundra species Eriophorum vaginatum reflect sites of origin

“…(2015) showed that if the growing season is artificially brought forward by removing snow in May, the buds of seven of eight dominant tundra plant species, including E. vaginatum , will break earlier, but also senesce earlier. As a result, there was no increase in growing period, suggesting that annual growth time is under genetic control shaped by selection pressure (Rosa et al., 2015). Another experimental early‐season increase in snow in the high Arctic (Svalbard) produced a similar effect whereby late onset of growth results in late senescence for all species studied (Semenchuk et al., 2016).…”

“…As with Rosa et al. (2015), the change in the timing of phenology had no effect on the period of active growth, suggesting a genetic control over the window for growth in arctic plants (Semenchuk et al., 2016). Consequently, the present study and previous work may have important implications for future vegetation dynamics under climate change.…”

“…While there were differences in the timing of senescence between ecotypes, there was no significant effect of warming on timing of senescence, a finding that concurs with other warming studies (Bjorkman, Vellend, Frei, & Henry, 2017; Rosa et al., 2015) as well as long‐term datasets which show that phenology of arctic plants is relatively unresponsive to recent climate change (Oberbauer et al., 2013). A long‐term warming experiment also found that phenology of high arctic plants was not responsive to warming, but year‐to‐year variation in phenology could be explained by snowmelt date (Bjorkman, Elmendorf, Beamish, Vellend, & Henry, 2015).…”

“…Over recent decades, warmer temperatures and smaller winter snowpacks have caused a significant lengthening of the potential growing season in arctic ecosystems, with both earlier thaw and later freeze‐up (Euskirchen et al., 2006; Park et al., 2016). However, snow removal and addition experiments to manipulate the start of the growing season have shown that phenology can be shifted, resulting in earlier green‐up followed by earlier senescence (Rosa et al., 2015; Semenchuk et al., 2016). In both of the cited experiments, there was no net increase in growth period following the earlier onset of growth, suggesting that there may be a genetic basis for duration of seasonal growth (Rosa et al., 2015).…”

“…However, snow removal and addition experiments to manipulate the start of the growing season have shown that phenology can be shifted, resulting in earlier green‐up followed by earlier senescence (Rosa et al., 2015; Semenchuk et al., 2016). In both of the cited experiments, there was no net increase in growth period following the earlier onset of growth, suggesting that there may be a genetic basis for duration of seasonal growth (Rosa et al., 2015). These results raise the question whether locally adapted arctic plants have the capacity to respond to longer growing seasons or whether other factors associated with climate change (e.g., community change and stimulation of growth [Elmendorf, Henry, Hollister, Bjork, Bjorkman, et al., 2012]) are driving observed (Epstein et al., 2012) “greening” patterns.…”