Drivers of vegetative dormancy across herbaceous perennial plant species.

Vegetative dormancy, that is the temporary absence of aboveground growth for ≥ 1 year, is paradoxical, because plants cannot photosynthesise or flower during dormant periods. We test ecological and evolutionary hypotheses for its widespread persistence. We show that dormancy has evolved numerous times. Most species displaying dormancy exhibit life-history costs of sprouting, and of dormancy. Short-lived and mycoheterotrophic species have higher proportions of dormant plants than long-lived species and species with other nutritional modes. Foliage loss is associated with higher future dormancy levels, suggesting that carbon limitation promotes dormancy. Maximum dormancy duration is shorter under higher precipitation and at higher latitudes, the latter suggesting an important role for competition or herbivory. Study length affects estimates of some demographic parameters. Our results identify life historical and environmental drivers of dormancy. We also highlight the evolutionary importance of the little understood costs of sprouting and growth, latitudinal stress gradients and mixed nutritional modes.

Contributions of day length, temperature and individual variability on the rate and timing of leaf senescence in the common lilac Syringa vulgaris.

Deciduous trees prepare for winter by breaking up chlorophyll and other nitrogen-rich compounds, which are resorbed for storage. Timing is important as senescence too early will waste growing season, while senescence too late risks the loss of the leaf resources to frost. While plants of temperate and boreal regions use decreasing day length as a cue of approaching winter, we show that decreasing temperature may also play a role in the variability of leaf senescence. We investigated the timing of autumnal decrease in photosynthetic efficiency and the concentration of chlorophyll and total carotenoids in nine common lilac (Syringa vulgaris L.) trees over two consecutive years. Day length explained a greater proportion of photosynthetic efficiency, but temperature had a significant additional role, which seems to be related to individual differences. Precipitation and cloudiness did not explain photosynthetic efficiency. Photosynthetic efficiency was higher outside the canopy and at high and middle elevations than inside and low elevations of the canopy. Late onset of senescence led to a steeper decline in photosynthetic efficiency than early senescence. The onset of decline in photosynthetic efficiency differed between years, but there was no difference in the steepest rate of change in photosynthetic efficiency with respect to sampling year or location. Contributions of day-length vs temperature to leaf senescence have important consequences for the adaptability and invasibility of deciduous trees in a changing climate, especially at the edge of species distributions.

Decline in atmospheric sulphur deposition and changes in climate are the major drivers of long-term change in grassland plant communities in Scotland.

The predicted long lag time between a decrease in atmospheric deposition and a measured response in vegetation has generally excluded the investigation of vegetation recovery from the impacts of atmospheric deposition. However, policy-makers require such evidence to assess whether policy decisions to reduce emissions will have a positive impact on habitats. Here we have shown that 40 years after the peak of SOx emissions, decreases in SOx are related to significant changes in species richness and cover in Scottish Calcareous, Mestrophic, Nardus and Wet grasslands. Using a survey of vegetation plots across Scotland, first carried out between 1958 and 1987 and resurveyed between 2012 and 2014, we test whether temporal changes in species richness and cover of bryophytes, Cyperaceae, forbs, Poaceae, and Juncaceae can be explained by changes in sulphur and nitrogen deposition, climate and/or grazing intensity, and whether these patterns differ between six grassland habitats: Acid, Calcareous, Lolium, Nardus, Mesotrophic and Wet grasslands. The results indicate that Calcareous, Mesotrophic, Nardus and Wet grasslands in Scotland are starting to recover from the UK peak of SOx deposition in the 1970's. A decline in the cover of grasses, an increase in cover of bryophytes and forbs and the development of a more diverse sward (a reversal of the impacts of increased SOx) was related to decreased SOx deposition. However there was no evidence of a recovery from SOx deposition in the Acid or Lolium grasslands. Despite a decline in NOx deposition between the two surveys we found no evidence of a reversal of the impacts of increased N deposition. The climate also changed significantly between the two surveys, becoming warmer and wetter. This change in climate was related to significant changes in both the cover and species richness of bryophytes, Cyperaceae, forbs, Poaceae and Juncaceae but the changes differed between habitats.