Photo of green trees seen from above
Photo of green trees seen from above

The diversity of present tree species is shaped by climate change in the last 21,000 years

A new global survey of 1000 forest areas shows how past climate change has had a major impact on the diversity and distribution of tree species we see today. The results can help us predict how ecosystems will react to future changes, thus having an impact on conservation management around the globe.

A research team led by Aarhus University, in collaboration with researchers from more than 50 research institutes around the world, including the Radboud Institute for Biological and Environmental Sciences, has assessed how past climate changes have affected how the composition of tree species in one area differs from the composition of neighbouring areas on six continents.

The most striking finding was that the global pattern of beta diversity in terms of tree species, species characteristics and evolutionary history was closely linked to temperature changes since the peak of the last ice age, which was about 21,000 years ago. Moreover, the researchers show that the effects of historical climate variations on the beta diversity were stronger than the effects of current climatic conditions.

The researchers have studied angiosperm tree species – i.e. species that produce seeds that are covered with a protective layer like a seed shell.  Angiosperms make up about 80 per cent of all plant species, and some of the most common angiosperm tree species are oak, beech, birch, maple, linden, maple, willow, palm, and eucalyptus.

In their research, the researchers combined data from five openly shared databases of tree species and their distributions, with information on the phylogenetic relations between species, and their ecomorphological attributes. They then divided the effects of ancient climate change on different habitats into two components, each with its own technical term:

  • Turnover – i.e. changes due to species replacement. If one species goes extinct in a habitat, another species comes in and fills its ecological role. It turns out that the greater the temperature changes an area has experienced since the Ice Age, the less replacement has occurred in that area.
  • Nestedness. In beta diversity, this term describes a pattern in which the composition of species in a diverse habitat is a subset of the species composition in a different and less diverse one – such that the more diverse habitat contains all the species found in the less diverse one, plus additional species. This is an important concept in understanding the organization of biodiversity because it can help identify areas that are more important for conservation. Habitats with nested species compositions may have lower overall biodiversity, but may contain species not found in other habitats, making them essential for preserving overall biodiversity. And the greater the temperature changes an area has experienced, the more nestedness has occurred. Climate fluctuations have thus wiped out local species that have not been replaced.

The authors found that the influence of the two components shifted from the equator to the poles.  

In tropical areas, turnover – i.e. species replacement – was the most important factor in determining changes in species composition between localities, due to rapid species change.

In temperate regions, nestedness was the primary mechanism for determining changes in species composition, because the species richness declines as we get closer to the poles.

The purpose of the study is to supply the ecology science with a tool to solve the major challenge of understanding how ongoing and near-future climate change reshapes the distribution of biodiversity and ecosystem functioning.

"Because the Earth's climate has changed enormously through geological time, exploring the effects of past climate change on current biodiversity provides an opportunity to understand the risks emerging from ongoing and future human-induced climate change," explains the study's first author, Wubing Xu, who initiated the study at Aarhus University and is now a postdoc at the German Center for Integrative Biodiversity Research (iDiv).

The researchers point out that the study also provides a new understanding of the challenges to ecosystem protection and management of efforts to mitigate the impacts of such changes.

"I hope these findings can aid the development of conservation and management plans that consider the long-term and diverse impacts of climate change on all biodiversity dimensions. Only then is there a realistic chance that we will reach goal A of Kunming-Montreal's Sustainable Development Goals for 2050," says Assistant Professor Alejandro Ordonez from Aarhus University and senior author of the study.

The mentioned goal A for 2050 includes that human induced extinction of known threatened species is halted, and, by 2050, the extinction rate and risk of all species are reduced tenfold.

Beta diversity

Beta diversity is a measure of the variation of species between different habitats or areas. It helps us understand the diversity of life in a given region or ecosystem by comparing the number and types of species present in different locations.

For example, comparing the number and types of birds in a forest versus grassland, beta diversity will help you understand the differences in bird species between the two environments. It can also help identify regions that have unique or rare species, and can be used to monitor changes in biodiversity over time.

Contact information

For further information, please contact one of the scientists involved or team Science communication via +31 24 361 6000 or media [at] (media[at]ru[dot]nl)

Sustainability, Nature