OPTMix, a forest laboratory to study the effects of climate change

© G. Maisonneuve/Irstea
© G. Maisonneuve/Irstea

As with all other ecosystems, forests will have to adapt to climate change. Researchers from the Irstea Nogent-sur-Vernisson center are using the experimental OPTMix site, located deep within a forest, to study pressures affecting the ecosystem and test management solutions that could alleviate their impact.

Managers are currently considering several strategies to make forests more resistant and resilient to climate change and to maintain the ecosystem, its biodiversity and the services it provides. These include encouraging mixed forests that combine several species and reducing the density of trees within stands.

To check whether these options were really effective, in 2015, scientists from the Irstea's Forest Ecosystems unit set up an experimental site unique in Europe: the OPTMix site (Oak Pine Tree Mixture experiment, OPTMix Project). Located in the Orleans national forest and made up of a network of 30 instrumented plots (devices measuring microclimate and water balance, dendrometers, etc.), the site0052569(1).jpg is used to study the operation of mixed forests in temperate areas and the various pressures they face over the medium and long term.

The site is populated by two tree species that are particularly common in France, sessile oak (hardwood) and Scots pine (softwood), representing one of the most common combinations in the country (thus ensuring results can be generalized). The site is equipped to monitor the impact of climate change as well as that of wild ungulate populations (deer, roe deer, boar). These herbivore species feed on young tree sprouts, and as their numbers have increased in France over the last 30 years, they can disturb a forest's regeneration phase and create significant issues for forest managers.

"The challenge is to understand how these factors impact and interact with growth and productivity in these forest species as well as forest regeneration and biodiversity. The plots we have set up can be used to compare various scenarios: some are planted with a single species (only oak, or only pine) while others have a mixture of both species. Each type of stand (pure or mixed) is created in two different densities: a medium density of about 400 trees per hectare (which corresponds to current practices) and a low density of approximately 200 trees per hectare. Finally, some plots are enclosed while others are not, in order to provide or restrict ungulate access," clarifies Anders Mårell, researcher from the Forest Ecosystems unit.

Initial results on the regeneration of mixed forests

As part of one of the first studies that have been carried out using the OPTMix1 site, scientists have been looking at a long-standing way of growing wood for heating and poles: the regeneration of shoots (or sprouts) from a stump once the tree has been cut down completely. "We wanted to see if this process, known as vegetative regeneration (in contrast to sexual regeneration which is based on seed growth) could be a relevant method of renewing sessile oaks in mixed forests, and whether it would be compatible with a herbivore presence," explains Mårell.

 

Oak sprouts grazed by deer.
Oak sprouts grazed by deer. . © J-P. Hamard/Irstea
Doe grazing on sprouts
Doe grazing on sprouts © Y. Boscardin/Irstea

By comparing the development of oak sprouts in enclosed and open plots that have more or less dense plant cover, scientists were able to study the effect of deer grazing on sprouts 2 and highlight several results:

  • sprout mortality is higher in plots to which deer have access than in those that are protected;
  • where deer have access, there is almost no sprout growth; shoots are maintained at a height of 20 cm, while they grow to 50 cm per year in enclosed plots;
  • although oak is a pioneer species that requires a lot of light to grow, it is capable of producing sprouts under cover, even with low levels of light;
  • finally, regardless of the light gradient tested, growth remains poor in the presence of herbivores, which indicates that bright light does not compensate for the effects of grazing.

"These results show that the practice of cutting/sprouting oaks to regenerate a mixed forest is only possible under certain conditions, either by maintaining a low herbivore population or by implementing protective measures against these animals," notes the researcher. Nevertheless, the practice does present one benefit: young oaks grow faster from vegetative regeneration than sexual regeneration, reaching a height that allows them to escape herbivores in only 5 years compared to 15 years; installing protective measures would therefore only be necessary for a very limited period.

This project is one of the first in a long series that are only possible due to the OPTMix site. Over time, these projects will be used to provide recommendations to help managers implement relevant solutions to preserve forests from global changes.

For more information

1- The effect of deer browsing and understory light availability on stump mortality and sprout growth capacity in sessile oak. Forest Ecology and Management 255(12), 3973-3979.

2- Growth from sprouts are more nutritious and therefore more appetizing for herbivores than young sprouts grown from sexual regeneration.