The impacts of climate change on forests can be dramatic but can also play out via more subtle interactions between different species. A new study published in Nature Plants demonstrates how the overall effect of climate change on tree reproduction depends on complex interactions between climate, the trees, and the insects that eat their seeds.

Co-author Dr Andrew Hacket-Pain (University of Liverpool) explains, “Many tree species produce large seeds which are packed full of energy and nutrients giving the seedlings the best possible chance to establish as new trees. However, such energy-rich seeds are highly attractive to seed-eating animals, and so trees have evolved various strategies to minimise the loss of seeds to these ‘seed predators’”.

One of these strategies is to suppress the populations of seeding-eating animals by switching between years of ‘bumper’ seed production, and ‘famine’ years with very few seeds. In the famine years, seed-eating animals are starved and so their numbers decrease. Then, during a bumper year, the supressed populations of seed predators are overwhelmed with a surfeit of seeds, and so more seeds can survive to establish the next generation of trees.

In this research, we use data from four decades of long-term monitoring to investigate how climate change has affected the reproduction of one of the UK’s most widespread trees, beech”, explains Dr Peter Thomas at Keele University. “We show that seed production has increased over that time period – but this does not tell the whole story”.

The team found that the long-term increase in seed production has been accompanied by a reduction in the degree of year-to-year variability in seed production, and specifically a reduction in the frequency of the ‘famine years’ which suppress the populations of seed-eating animals. Lead author Dr Michał Bogdziewicz from Adam Mickiewicz University, Poland, explains why this is important: “The main seed predator of beech is a specialist moth (Cydia fagiglandana), whose larvae feed on the developing beech seeds. Frequent ‘famine years’ enable the beech trees to suppress populations of this seed-eating insect. However, the disappearance of the characteristic ‘boom and bust’ seed production patterns in UK beech trees has led to an increase in moth populations. As a result, we have seen a dramatic increase in the percentage of the seeds that are eaten by the larvae – up from around 1% in the 1980s to 40% in recent years”.

Dr Hacket-Pain explains the implications of the study for understanding how beech trees are responding to changes in the climate: “Climate change is leading to an increase in the seed production of beech trees – but we show that any benefit the trees might gain from this increased reproductive effort has been almost entirely offset by higher seed consumption by the moth larvae. The trees are producing more seeds, but gaining almost no return on their increased investment”. It is an excellent example of how the overall response of forests to climate change is dependent on a complex web of interactions between species – it is far from straightforward”.

(photos: Julia Witczuk, Stanisław Pagacz)