What does the chestnut tree teach us before matriculation? Climate change on tree branches

Just before high school graduation chestnuts bloomed… There are those among us who remember not only the words of this song, but also those looks of trees passed on the way to the exam. For generations we have associated the chestnut tree with high school graduation – its stately white blossoms usually bloomed in late April and early May. Today, this phenomenon is losing its regularity – more and more often we note earlier, delayed or even repeated autumn blooms.

Climate warming, while it may seem like an abstraction with this year’s cool May, is changing nature’s calendar in a well-documented way. There is a lengthening of the growing season across Europe, with the onset of spring in recent decades falling an average of 9-11 days earlier compared to 1951-1990. Chestnuts are blooming, but the data – about which more below – leaves no illusions, as plants are learning the new order, while we are still living according to the old.

Warmer climate, or a word about faster flowering

An increase in average temperatures is a major factor in accelerating plant development in the spring. Long-term observations confirm that chestnut trees clearly respond to warming. In Geisenheim, Germany, for example, observations have been made of chestnut blossoms since 1900, and earlier flower appearances have been observed, especially since the 1990s. In other words, the dates of chestnut blossom onset are getting earlier with each decade.

Recent studies indicate that climate warming affects the entire developmental cycle of the chestnut tree, i.e. all phenological phases (leafing out, flowering, fruiting and leaf shedding) occur earlier, extending the growing season and reproductive season of these trees. With higher spring temperatures, chestnut trees move more quickly into the flowering phase, and stay green longer in autumn – a consequence of the general shift in the life cycle in response to warming.

Similar changes are also being observed in Poland. Scientists from the Institute of Meteorology and Water Management (IMGW-PIB) note that changes in the timing of phenophases are correlated with rising temperatures. Springs come earlier, which entails earlier bursting of leaf buds and flowering in many tree species. Traditionally, chestnut trees bloomed in the first days of May, but warm winters and early spring warming mean that these trees can bloom as early as mid-April.

Reports from naturalists confirm that in some years an earlier start to spring results in the surprisingly early appearance of chestnut flowers. Similar changes apply to many regions of Europe – plants in different countries respond to warming in the same way: an earlier start of vegetation and earlier flowering.

Urban heat island

Climatic conditions in cities can differ significantly from those in rural areas. Dense buildings and concreted surfaces absorb and emit heat. As a result, the surrounding areas are cooler than the heavily urbanized center, especially at night and in early spring.

Research indicates that this local increase in temperatures translates into earlier plant development. For example, observation of chestnut trees in Wroclaw showed that trees growing closer to the city center begin to bloom earlier than those on the periphery. Specimens 1-2 km from the center of Wroclaw bloomed several days earlier than chestnut trees growing about 5 km from the center.

A similar effect has been observed in other cities. In Munich, the trees (e.g., the papillary birch) in the center of the city bloomed noticeably earlier than in suburban areas. Even a small difference in average temperature, e.g. 2-3°C higher in the center, significantly shortens the time needed for plants to start flowering.

It is worth adding that microclimate also plays an important role. Within a city, there may be locally cooler places, such as green areas, parks or waterfront areas. In the aforementioned study, which was conducted in Wroclaw, it was noted that chestnut trees growing near rivers or reservoirs bloomed the latest, as the moisture and lack of heated surfaces somewhat mitigated the impact of the urban environment. Nevertheless, the trend is clear: spring accelerates in cities. This phenomenon has been confirmed in many places in Europe, from Warsaw to Madrid, and is an important aspect of the impact of climate change on nature.

Disconnection with pollinators – troublesome lack of synchronization

Nature is an interconnected system of vessels, which means that plants bloom at specific times of the year, in close conjunction with the activity of pollinating insects. Changing flowering dates can therefore disrupt this synchronization. Unfortunately, observations indicate that climate warming affects flora differently from fauna – the rate of change in plants is on average four times faster than in insects. This means that many plants (including chestnut trees) are shifting their flowering dates faster than insects are modifying their flight and foraging dates.

The consequences can be surprisingly large. Analyses by British and Chinese scientists have shown that already about 60 percent of the plant and insect species studied in Europe are experiencing a disruption of mutual synchrony in the seasonal cycle. What does this mean in practice? If a chestnut tree blooms much earlier than usual, the population of its pollinators (e.g., bees, bumblebees) may not yet be ready to become fully active. The availability of nectar and pollen will not be enough for insects to awaken to life if the air temperature is too low.

When the tree and insects size up, the flowers may remain partially unpollinated. This in turn affects seed (chestnut) setting and the overall reproductive success of the plant. Admittedly, the chestnut tree is often planted as an ornamental avenue tree and its fruits, or chestnuts, are of no use to humans, but it is an important part of the forest or park ecosystem and such changes can have serious consequences.

A food-poor flower attracts fewer insects, which can affect the health of local bee or butterfly populations. This phenomenon is the subject of intense study by ecologists around the world, as pollinators and plants form an interdependent network, the disruption of which can disrupt entire ecosystems.

Not all organisms respond equally to climate change. Some insects adapt more easily to new conditions. Also, some plants are able to use a wide range of pollinators or are independent of them, relying on wind-pollination. These buffering mechanisms reduce the risk of disturbance. Nonetheless, the trend of systematic climate warming observed in recent decades raises legitimate concerns – differences between plant development phases and pollinator activity are becoming more numerous and are not always without impact on ecosystems.

The price of a premature spring

The warmer winter and early spring also bring with them some danger to the plants – this is the risk of damage from late frosts. When a chestnut tree puts out young leaves and flower buds earlier than usual, it becomes vulnerable to any wave of cold that hits it. In Polish conditions, late frosts happen in April and even in May, as happened this year. Plants that bloom too early can be damaged by the sudden return of winter temperatures, inhibiting their further development and reducing future seed yield. Such a false spring (false spring), when a period of warmth is interrupted by frigid weather, is particularly dangerous for species that begin vegetation early.

Scientists analyzing data on trees from several thousand sites in Europe have found that as the climate warms, the risk of frost damage increases precisely in species that leaf out earliest. Such harmed pioneers of spring include the chestnut tree, as well as the black alder or the white birch.

In contrast, late-leafed trees (e.g., beech, ash, oak) have not yet experienced a significant increase in risk. A warm, early spring usually won’t manage to stimulate them to bloom before the end of the frost period anyway. Chestnut trees are therefore at a disadvantage: while warming extends their growing season, it also exposes them to a greater likelihood of frost damage. In extreme cases, one severe frost during flowering can destroy almost all of the tree’s flowers, affecting its reproduction and condition in a given year.

Gardeners and foresters try to counteract the effects of frost (e.g., by fumigating orchards or shielding young trees), but this is virtually impossible for large trees such as chestnut trees. Spring weather remains unpredictable. As the weather warms, the pre-spring development of plants will become more frequent, and thus the risk of young organs coming into contact with frost – greater.

Cooked chestnuts

Climate change affects chestnut trees not only by shifting their flowering dates, but also by the overall condition of the trees, especially in regions with warm climates. Spain provides an interesting example here. The chestnut tree, although native to the mountainous region of the Balkans, has long been planted in Spanish cities (such as Madrid and Segovia) for shade and ornamentation.

Experts from the Spanish Arborist Society and the Madrid Botanical Garden warn that at the current rate of climate warming, chestnut trees could disappear from many Spanish cities by the middle of the 21st century. They are already showing signs of weakening. In hot, dry summers, their leaves turn yellow and wither prematurely (sometimes as early as August), and the trees become more susceptible to disease.

Cities such as Segovia have begun replacing chestnut trees with other tree species more resistant to heat and water shortages. Madrid’s Parque del Retiro – a city park famous for its avenue of chestnut trees – has seen a noticeable thinning of its tree stand, and The Times is warning that climate change is threatening chestnut trees across Spain. The problem is not only extreme heat, with air temperatures reaching 40 degrees Celsius, but also prolonged periods of drought and warm winters and nights that disrupt the trees’ resting cycle.

Chestnut trees are best able to tolerate temperate climate conditions – with cool winters and moderate hydration during the growing season. In the warmer, increasingly dry climate of the Iberian Peninsula, they function at the limit of their capabilities. Too long a growing season, combined with repeated episodes of drought, leads to a weakening of the trees’ condition. When it’s hot almost all year round, the plant doesn’t have a chance to have a natural resting rhythm, consumes more resources and depletes them faster. This makes it more susceptible to diseases and pest invasions. Climate forecasts for southern Europe leave no illusions – conditions will become increasingly difficult, and chestnut trees may gradually disappear from the urban landscape.

The situation of chestnut trees in Spain has demonstrated a larger problem – not all tree species traditionally used in urban greening will be able to cope with the new climatic realities. Work is already underway in many countries to identify plants that better tolerate heat and water deficits. While the chestnut tree may give way to more hardy trees over time, it still plays an important role in the biological and aesthetic structure of urban ecosystems – and needs conscious and targeted protection in the transition.

A few words to conclude

The example of the chestnut tree shows how global warming is shaping specific phenomena in the natural world that we are witnessing. The acceleration of flowering dates is one of the best documented effects of rising temperatures. These changes appear at first glance to be positive (it’s greener for longer, flowers are sooner to please the eyes after winter), but they actually bring enormous challenges to ecosystems. Disrupted synchronization between plants and pollinators can disrupt pollination, and the risk of late frosts threatens prematurely flowering trees. In metropolitan areas, where the urban heat island effect is at work, plants wake up faster, but face drought and heat stress in the summer heat.

The chestnut tree, previously associated with matriculation exam dates, has become a living indicator of ongoing climate change. Scientists stress the importance of continuing to monitor the situation. Polish researchers, in cooperation with international scientific centers, track these trends, and the results of their work help forecast nature’s response to the future climate. This knowledge is essential to adequately protect ecosystems – from urban avenues to forests – in the face of upcoming challenges. The story of chestnut trees learning the new calendar is a reminder that climate change is happening here and now, affecting everything, even the appearance of the tree outside our window.

In the article, I used, among others:

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