Extreme events receive increased attention in ecology as they can have substantial ecological effects. However, generalizing across extremes and their effects on various ecosystems remains challenging. One factor which hampers generalizations is the fact that the susceptibility of ecosystems changes over time. Here, I examples of late spring frost sensitivity to illustrate this phenomenon.
Late spring frost events after early bud break of deciduous forest trees can result in strong leaf damage. Here, the effects of an extreme late frost event in May 2011 was quantified for the dominant native forest tree European beech (Fagus sylvatica L.), within 7 forest stands in southern Germany, based on tree-ring widths of 135 trees.
Furthermore, sensitivity to the same frost event (air temperature - 4.3 °C; after leaf unfolding of all species) in 170 woody species from the entire Northern hemisphere which are grown in the Ecological-Botanical Gardens Bayreuth, Germany, was observed. For all taxa, distribution range characteristics, climatic parameters of origin and phenological strategy were linked to late frost sensitivity.
Results/Conclusions
Both data sets showed severe damage by a frost event which, in comparison to regional absolute minimum temperatures, was not extreme at all but which was the most extreme minimum temperature ever recorded in this region that late in the year.
Compared to the previous 5 years, tree-ring width was on average reduced by 46%. This reduction was correlated with May minimum temperature at the sites (R² = 0.59, p = 0.026), indicating a strong relationship between growth reduction and the magnitude of the late spring frost.
The observations across 170 species in the Ecological-Botanical Gardens showed that range characteristics and the prevalent climatic parameters across the species' native ranges were strongly related to their susceptibility to late spring frost damage (BRT: ROC = 0.737). Further, sensitive species unfolded their leaves two weeks later than tolerant species.
Taken together, late frost damage is an example which clearly demonstrates the crucial role of timing of extreme events. Given the right timing, even events which are in absolute terms not extreme (compared to winter minimum temperatures) can have strong effects. Phenology, on the other hand, is synchronized with species’ sensitivity in an attempt to optimize safety margins against extreme events. Efforts which aim at generalizing across extreme events clearly need to take such timing issues into account.