A 34-year study has found that viable seed production in five major European forest tree species has fallen by more than 30%.
Researchers said warmer summers were consistently linked to lower seed production, while moisture and spring temperatures appeared to have less influence.
Oaks and Scots pine recorded the steepest declines, at about 65% and 64% respectively, according to the study.
The authors said forests may continue to look intact even as their ability to reproduce and recover is already weakening.
The findings raise concerns for forest regeneration, biodiversity, carbon storage and long-term climate resilience.
Climate change is reducing the ability of major European forest trees to reproduce, with viable seed production falling by more than 30 per cent over three decades, according to a new study. The findings suggested that forests may be affected by warming at a deeper biological level than previously understood.
Trees may continue to stand for decades while gradually losing their ability to reproduce effectively, the authors noted. “If reproduction is under chronic strain, the forest may still look intact, while its capacity to replace itself is already weakening,” said Jessie Foest, lead author of the study and a researcher at the Adam Mickiewicz University Forest Biology Centre in Poland.
The study, published in the journal Nature Climate Change on May 12, 2026, analysed 34 years of seed-harvest data from Poland between 1988 and 2021. Researchers examined 40,530 observations across five major European tree species: English oak (Quercus robur), sessile oak (Quercus petraea), Scots pine (Pinus sylvestris), silver fir (Abies alba) and European beech (Fagus sylvatica).
The study found widespread declines in viable seed production across the species analysed. Both oaks recorded a decline of about 65 per cent, while Scots pine saw a fall of about 64 per cent, the researchers found. Silver fir and European beech recorded declines of 44 per cent and 32 per cent respectively, according to the study.
The records were based on post-sorting seed lots, meaning the seeds had passed quality control and largely excluded empty, damaged or infested seeds. The decline therefore points to reduced effective reproductive output, rather than only a change in total litter, flowers or cones, the study said.
“Our results suggest that climate change is already reducing the reproductive capacity of major European trees,” Foest said. The next question is whether these seed declines are now translating into weaker regeneration in forests. That is the link we urgently need to test, she added.
Rising summer temperatures consistently reduced seed production across species, the researchers found. Moisture and spring temperatures appeared to matter much less, the study said. In European beech, the decline was consistent with disrupted masting, the researchers said.
Masting refers to the pattern in which trees produce large seed crops in some years and far fewer seeds in others. Disrupted masting in beech was associated with a reduction in viable seed production because of increased seed predation and reduced pollination, the research said.
The findings extend earlier species-level reports of reduced fecundity in temperate and boreal forests to the community scale, the authors said. Fecundity refers to the capacity of a tree or population to produce offspring.
In Scots pine, marked declines in fecundity combined with increasing climate-driven mortality suggest that reproductive output may become insufficient to offset population losses, the study said. However, the researchers said further work was needed to quantify the downstream demographic consequences.
Partial buffering in beech and fir probably reflects differences between species in reproductive biology and sensitivity to climate during flower and fruit development, the study said.
The results support evidence of a widespread decline in forest fecundity, the researchers concluded. The direction and scale of the declines align with independent evidence of large losses in viable seeds and fruit production elsewhere, the study said.
The researchers cited a 50 per cent decline in viable seeds in European beech in the UK, a 40 per cent decline in cone production in pinyon pine in New Mexico and an 80 per cent decline in fruit production in Gabon.
“The scale of change matches or exceeds contemporary declines in growth and increases in mortality,” the study concluded. The authors said fecundity appears to be a strongly climate-sensitive demographic rate and an early signal of population stress, which could drive forest restructuring under ongoing environmental change.
Trees may survive temporarily while reproducing less successfully, the authors noted.
Increased seed supply can partially buffer negative climate effects on regeneration, but this may not be enough if fecundity declines at the same time as climatic conditions for seedling establishment worsen, the study said.
Those combined effects could interact and potentially accelerate population decline, the researchers warned. “Trees are not only vulnerable to the negative effects of climate change when seedlings or adult trees die,” Foest said.