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Wed, 26 Apr, 2023 - 17:42

Anja Murray

Each wild species has its own approach to keeping in tune with the seasons. Trees have ways to measure the intensity of sunlight and the ratio of daylight to darkness, registering environmental cues in order to know when the time is right to sprout new leaves in spring. It's not just as simple as measuring temperature and the span of daylight hours though. After an especially cold winter, for example, some tree leaves unfurl earlier the following spring. Conversely, after a mild winter, leaves can take longer to appear. Trees even have ways to calculate the number of milder days — what we might call an ability to count. By biding their time carefully, trees decipher the optimal timing to focus their energy on producing a whole new canopy of leaves for the growing season ahead.

In nature, as in life, timing is everything

Phenology is the study of the timing of seasonal life-cycle events in plants and animals: when plants produce leaves and flowers; when insects and mammals awake from hibernation; when reproduction begins; and migrations occur. Every single wild creature, from moths to shrews to blackbirds, synchronises its life cycle with the seasonal events of the plants upon which they depend. Concern is now growing about a phenomenon known as 'phenological mismatch' also called 'phenological asynchrony'. This is when plants and animals that have evolved together, finely tuned to the life cycles of all they depend on in their ecological community, are suddenly out of sync due to once dependable seasonal variations thrown all askew by a rapidly warming climate system.

One often-cited example of phenological asynchrony is the relationship between oak leaves, caterpillars, and great tits. Certain butterflies lay their eggs on oaks just when the leaves are beginning to emerge, providing fresh sustenance for the caterpillars to chomp through when they hatch. Great tits have evolved to time the hatching of their eggs with the first flush of oak leaves, generally emerging now as April merges into May, ensuring an abundance of caterpillars to feed the chicks with. As oak leaves develop they become rich in tannin — one of the compounds that traditionally made oak useful for tanning leather — as a defence mechanism against its leaves all being eaten. Tannin has a bitter taste, and while the caterpillars can cope with this, some baby birds cannot. This is why many song birds time the hatching of their brood to coincide with the first flush of oak leaves. If the leaves emerge early, or the nestlings hatch late, the caterpillars that eat the tannin-rich leaves are themselves rich in tannin, and baby great tits can suffer double the natural mortality rate from consuming indigestible tannin-rich caterpillars.

Swifts arriving in Ireland in May to feed on the abundance of insects here, can suffer malnutrition when spring is late in coming and insect abundance is delayed. After the long migration from Africa, they need to eat a lot of insects to revive their depleted energy stores.

A baffling quantity of plant–animal interactions have been harmonised over thousands of years, providing the everyday mechanics of how ecosystems remain functional and resilient. When a few things go out of kilter, systems can begin to unravel. Some of these relationships are now beginning to fall askew as a result of rapidly shifting seasonal patterns, and while some species are learning to adapt quickly, many species won’t be able to adapt in time to survive.

Long-term phenological records in trees show that a rise in global temperature generally leads to earlier timing of spring events, such as when leaves on trees unfold. This is leading to a plethora of as yet unknown mismatches in timing of vital life cycle events between interdependent species. In Ireland, phenological studies have been ongoing since the establishment in 1966 of four gardens in the ‘International Phenological Garden’ network. Through this data, we know that the leafing of some trees species is now occurring 30 days earlier than 30 years ago. As yet unknown consequences could already be cascading through ecosystems.

Adaptations and consequences

Earlier leaf emergence can result in earlier peak abundance of other insects. In the Netherlands, Blue tit populations are responding to this change, laying eggs earlier. The effect of changing tree phenology is being transmitted up the food web, ultimately changing nesting behaviour in blue tits. Blue tits are an adaptable species, unlike many others.

The ultimate effects of so many unfolding mismatches are unknown for most plant animal interactions. For many tree species, warmer winters cause delayed bud burst in spring, which adversely affects flowering and seed germination, reducing overall productivity. In forest ecosystems around the world, as well as with tree crops such as fruit and nuts, such chain reactions of knock on impacts will have potentially far–reaching consequences.

Other phenological studies are revealing that temperature is not the only factor impacting the seasonal timing of phenomena such as bud burst. In experiments, tree seedlings have been grown in a controlled atmosphere with elevated carbon dioxide levels. In some tree species, bud burst is delayed by 2–3 weeks at the beginning of the spring growing season where concentrations of carbon dioxide are higher than at present levels. In other species, higher concentrations of carbon dioxide cause leaves to fall prematurely in autumn.

Around the Mediterranean, the key factor determining the timing of life cycles of wild plants and animals is not temperature but aridity. An exceptionally dry winter in the eastern Mediterranean is already being felt among olive and almond farmers, who are in turn are drawing more water from groundwater reserves for irrigation, and springs across the region are running dry for the first time in decades.

Here in Ireland, some agriculturalists speak eagerly about the longer growing season for grass that will result from warming climate. But there are many downsides too, too many to even predict, such as the doubling likelihood of fodder crises in the coming decades as a result of climate change.

As we enjoy the leaves emerging on deciduous trees, we must also bear in mind not to take anything for granted. Reducing emissions and planning for ecosystems adaptation is a task we cannot continue to shirk.