The combination of premature sprouting with icy weather caprioles becomes a tricky challenge for plants in the course of climate change. An experimental study now makes it clear: deciduous tree species that recover particularly well from damage caused by late frosts could gain decisive developmental advantages under the changed climatic conditions and thus spread.
The trend is clear: mild weather conditions are waking nature up from hibernation unusually early. The deciduous trees often turn green very early in the year. They're just early, you might think. However, studies have already shown that there is a problematic aspect to this development spurt. Because the premature warm phases in the course of climate change can continue to be characterized by freezing cold snaps. Such late frosts can then hit the delicate leaves of the "early awakened" trees hard and thus severely damage young plants in particular.
Frosts in spring have always posed a risk for deciduous trees in our latitudes, to which they had to adapt. But: "Although frost damage is an important topic, little is known about how severely it actually affects our trees," says lead author Frederik Baumgarten from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL). It is also unclear why trees "play with the danger" at all and why they sprout so early despite possible late frosts. Baumgarten and his colleagues have now investigated these questions in an experimental study.
On the trail of early budding and frost damage
The "test subjects" of the research team were biennial plants of four European tree species: wild cherry, English oak, hornbeam and common beech. In order to let them sprout at different times, they were exposed to different temperature conditions in climatic chambers. There was an early and a later group compared to control trees that turned green under "normal" spring conditions. As the buds unfolded, the researchers mimicked a frost event in the climate chambers on a portion of the plants, which resulted in leaf death. They then transplanted all the saplings outdoors and studied how they recovered from the event and developed over the following year and a half.
As the scientists report, the results showed that the timing of budding is a balancing act. "It can be worth taking risks," says Baumgarten. The saplings that sprouted later than average lagged behind their peers in terms of growth. If, on the other hand, they sprouted a few days earlier than the competition, many of the saplings were able to grow a comparatively large amount of biomass. But these advantages are of course associated with the disadvantage of the increased risk of frost damage. In addition, the evaluations showed that the saplings, which sprouted rather early, were increasingly troubled by aphids. The results therefore reflect the fact that the optimal time for leaf bursting has leveled off between certain limits for each tree species over the course of evolution, the scientists explain.
Worthwhile investment in recovery potential
As far as the consequences of frost damage are concerned, there were clear differences between the four tree species examined, the researchers report: All frost-damaged trees grew significantly more slowly than the untreated comparison groups due to the loss of the first green. However, in the case of oak and wild cherry, comparatively effective strategies emerged to deal with the damage. "I was amazed at how well some species recovered from the frost," says Baumgarten.
According to the observations, after the damage, the wild cherries invested their energy in a side shoot that replaced the original main shoot from below. He then advanced to become the new trunk of the young tree. The oaks were also able to recover comparatively well from the damage. As it turns out, this is because they invest in quite a lot of reserve bud. In contrast, 30 percent of the young hornbeam trees did not survive the frost at all. The beeches were also comparatively badly affected by the "shock": They formed smaller leaves and a comparatively sparser crown, the researchers report.
But what do the results mean against the background of climate change? Tree species that have already invested more intensively in strong recovery potential will probably have an advantage in the springs of the future, which will be characterized by more unpredictable weather, say the researchers: This means that species such as wild cherry and English oak could outperform species with characteristics such as hornbeam and prevail red beech. So that would slowly change our forests. "Over time, a new species community that is better adapted to frost could establish itself," says Baumgarten.
Source: Swiss Federal Institute for Forest, Snow and Landscape Research, specialist article: Functional Ecology, doi: 10.1111/1365-2435.14243