Nov 06

Article published: Impact of defoliation on tree growth for sessile oak and Scots pine following heat-drought events

An article has just been published in “Forest Ecology and Management” on the impact of defoliation on tree growth for sessile oak and Scots pine following heat-drought events:

Javoy, T., Perret, S., Couteau, C., Perot, T., 2025. Impact of defoliation on tree growth for sessile oak and Scots pine following heat-drought events. Forest Ecology and Management 598, 123243. https://doi.org/10.1016/j.foreco.2025.123243.

Highlights:

  • Defoliation strongly reduced 2020–2023 tree growth for sessile oak and Scots pine.
  • The reduction in growth was more important for Scots pine than for sessile oak.
  • Tree size and neighborhood density had the strongest effects on tree growth.
  • Neighborhood composition affected growth but to a lesser extent than did defoliation.
  • Tree size and local competition did not modify the growth-defoliation relationship.

 

Abstract:

European forests have experienced several droughts and heat waves in recent years leading to an increasing level of tree dieback. Quantifying the effects of dieback on tree growth is a crucial issue for forest managers. We monitored tree crown defoliation and tree growth for sessile oak and Scots pine from 2020 to 2023 in the OPTMix experimental network located in central France. We quantified the relationship between tree growth and tree crown defoliation under varying neighborhood conditions. We hypothesized that crown defoliation, tree size and neighborhood competition (local density and composition) would all affect tree growth. We carried out a magnitude analysis to quantify the importance of these different effects on tree growth. We showed that crown defoliation had a strong negative effect for sessile oak (-19.5 % of growth from 42 % to 57 % of defoliation) and a very strong negative effect for Scots pine (-29.5 % of growth from 39 % to 51 % of defoliation). Tree size and neighborhood density had the strongest effects on tree growth for both species, with, respectively, a positive and a negative effect. The proportion of heterospecific species in the neighborhood had the weakest effect, and was positive for sessile oak and negative for Scots pine. We found no interaction between crown defoliation and other predictors, thus indicating that they do not modify the relationship between crown defoliation and radial growth for either species. Forest managers can use defoliation as a rapid indicator of tree health and to help them assess the impact of defoliation on tree growth in various oak-pine forests.

 

Sep 30

Article published: Importance of species identity, species mixing and understory on carbon and nitrogen stocks in sessile oak and Scots pine stands

An article has just been published in “Science of The Total Environment” about the importance of species identity, species mixing and understory on carbon and nitrogen stocks in sessile oak and Scots pine stands:

Korboulewsky, N., Javoy, T., Pérot, T., Ouimet, R., 2025. Importance of species identity, species mixing and understory on carbon and nitrogen stocks in sessile oak and Scots pine stands. Science of The Total Environment 1000, 180410. https://doi.org/10.1016/j.scitotenv.2025.180410

Highlights:

  • Species identity has a greater effect on C and N stocks than does species composition.
  • The effect of mixing species is limited.
  • The understory accounts for up to 29 and 49 % of the vegetation C and N stocks.
  • Root biomass is the main source of uncertainty.
  • The main challenge for foresters is to maintain stocks rather than to increase them.

 

Abstract:

Forests contribute the most to the carbon stocks of the terrestrial biosphere. Estimates are available at different scales, though a large degree of uncertainty remains. In particular, the understory is seldom taken into account. In this study, we measured the carbon and nitrogen stocks of all ecosystem compartments, including trees, understory, deadwood, and soil, in pure sessile oak, pure Scots pine and mixed stands of the two species. We studied the importance of different compartment stocks and the effects of stand composition. We also evaluated different sources of uncertainty related to the calculation parameters of the estimates.
We found that species identity affected ecosystem C and N stocks more than stand composition did. The effects were primarily observed in the forest floor (mainly the organic layer) and the understory. No stand composition effects were observed for either soil C stability or C and N stocks in the mineral horizons, indicating that physicochemical processes dominate. Our results show that the understory can account for more than a quarter of vegetation C stocks and up to one-half of vegetation N stocks in pine low density stands. This importance of the understory was most significant in low-density pure pine stands. The root biomass of the trees and understory vegetation were the primary sources of uncertainty in stock estimations.

 

Graphical abstract:

 

 

Sep 29

Do forest thinnings influence understory vegetation?

Joseph BOULNOIS, supervised by Ludovic Henneron at the Ecodiv Laboratory (University of Rouen), successfully completed his M2 internship. He conducted sampling and measurements on understory species within the OPTMix and Gis-Coop networks during spring-summer 2025.

Forest stand density influences understory biodiversity and functioning, but the relationships between silvicultural practices and plant functional traits remain little explored. This study, conducted on an experimental network of 24 plots, analyzes the effect of different thinning intensities on the composition, biomass, and leaf traits of understory herbaceous species. Trait measurements (SLA, LDMC, nitrogen content, and average plant height) were performed on several dominant species, including seedlings of Quercus petraea. Results show that canopy opening favors heliophilous species and increases total biomass, while shade-tolerant species decline. Functionally, thinned stands display higher SLA and nitrogen content, indicating a resource-acquisitive strategy that supports growth. However, these patterns vary among species, reflecting high interspecific variability. Stand density thus emerges as a key factor structuring understory communities and their functioning. Silvicultural practices, particularly thinning, therefore represent a potential lever to guide biodiversity dynamics and the resilience of managed forests.

Sep 29

X-Risks: Mixed Forest Stands Facing Water Stress and Foliage Herbivory

As part of the PEPR FORESTT X-Risks project, oak leaf samples were collected from resprouts in early September. Out of a total of 1,200 leaves (from low-density plots), measurements of pigments and nitrogen (using the Dualex device), as well as traits such as specific leaf area (SLA), and observations of damage caused by herbivorous insects (see photo) were conducted. The goal is to assess the combined effect of water stress risk and herbivory (including chewers, sap feeders, gall makers, skeletonizers, and the presence of stink bugs) depending on the stand composition.

This protocol—excluding Dualex measurements—is also being implemented in two other networks: ORPHEE (led by Hervé Jactel) and GMAP (led by Xavier Morin), focusing on other oak-based mixtures. These measurements will be repeated over a period of four years. The individuals being monitored are marked with tape (see photo).

Jul 01

Article published: Capacity of a forest to buffer temperature: Does canopy tree species matter?

An article has just been published in “Agricultural and Forest Meteorology” on the importance of canopy tree species on the capacity of forest cover to buffer temperatures during summer:

Perot, Thomas, Marc Saudreau, Nathalie Korboulewsky, Anders Mårell, et Philippe Balandier. Capacity of a forest to buffer temperature: Does canopy tree species matter? Agricultural and Forest Meteorology 371 (2025): 110646. https://doi.org/10.1016/j.agrformet.2025.110646

Highlights:
  • Air temperature was monitored in sessile oak and Scots pine forests from 2018 to 2020;
  • Tree species and tree density effects on the summer buffering capacity were tested;
  • Incident and intercepted irradiance were accounted for in the analyses;
  • Tree species had a significant effect on the temperature buffering capacity;
  • Oak provided a better temperature buffer than pine when light interception was equal;

 

Abstract:

We studied the effects of tree species and tree density on the capacity of a forest to buffer understory temperatures during the summer period. We dissociated tree-species and light effects by integrating incident solar irradiance and its proportion intercepted by the canopy into our analyses. We measured solar radiation and air temperature over three consecutive years (2018, 2019 and 2020) in 16 plots in Central France composed of mature stands of sessile oak and Scots pine with three types of composition: monospecific oak, monospecific pine and oak-pine mixture, and two levels of tree density. Air temperature and solar radiation were recorded simultaneously in the experimental plots and in a reference plot without forest cover.
Our results show that the higher the incident irradiance the greater the difference in minimum temperature, and that the higher the intercepted irradiance the greater the difference in maximum temperature between below-canopy and open conditions. We found that tree species had a significant effect on the buffering capacity even when the light factor was taken into account. For a given incident irradiance and a given proportion of intercepted irradiance, the pine plots buffered understory temperatures less than the oak plots. Our results also show that higher maximum temperatures occurred in the understory than in open conditions for low values of intercepted irradiance and high values of incident irradiance, especially in the plots where Scots pine was present. The two species differ in leaf albedo and in their ability to regulate transpiration during droughts and these two characteristics may explain our results. Our study shows that Scots pine is less able to buffer summer temperatures than sessile oak. These results are of interest to forest managers since reducing stand density and mixing tree species are considered to be silvicultural strategies that can help cope with climate change.

Jul 01

Article published: How do mixing tree species and stand density affect crown defoliation after heat-drought events?

An article has just been published in “Forest Ecology and Management” on the effects of stand composition and stand density on tree crown defoliation of sessile oak and Scots pine during the five years following a severe heat-drought events:

Javoy, T., Perret, S., Perot, T., (2025). How do mixing tree species and stand density affect crown defoliation after heat-drought events? FOREST ECOLOGY AND MANAGEMENT 586. https://doi.org/10.1016/j.foreco.2025.122684

Highlights:
  • Oak and pine tree crown defoliation was monitored from 2020 to 2024 in 18 plots;
  • Oak defoliation increased steadily with a steeper slope in mixed than in pure stands;
  • Pine defoliation increased more abruptly in pure stands and decreased in 2024;
  • For both species larger trees had less crown defoliation than smaller ones;
  • We found no stand-density effect on tree crown defoliation for either species.

 

Abstract:

Severe droughts and heat waves have increased over the last decades, leading to forest dieback worldwide. In this context, reducing stand density and mixing tree species are two silvicultural options being investigated by forest managers to limit the impact of climate change on forest health and productivity. We studied tree size effect and stand composition and density effects on crown defoliation in adult trees belonging to two species: sessile oak (Quercus petraea, Matt. (Liebl.)) and Scots pine (Pinus sylvestris, L.). We monitored tree crown defoliation at 18 experimental plots located in central France for five consecutive years following the extreme heat wave of 2019. Our results highlight two different dynamics for crown defoliation in the two tree species. For Scots pine, we observed an abrupt severe increase in defoliation followed by a significant decrease in 2024, while sessile oak, crown defoliation continued to increase steadily after 2020. For both species, larger trees had significantly less crown defoliation than small trees. We found higher defoliation for sessile oak in mixtures and conversely, higher defoliation for Scots pine in pure stands. Finally, we found no difference in crown defoliation between low-density and medium-density stands for either species.

Jun 19

Save the date! symposium 2026 to celebrate 10 years of OPTMix, June 8 to June 12, 2026

The OPTMix (Oak Pine Tree Mixture) French field experiment is celebrating its 10 anniversary with an international symposium inviting us to question management options in the face of climate change:

“Adapt forest management to climate change: modulating stand composition, stand density and browsing pressure”

The symposium will take place in Nogent-sur-Vernisson, France, from June 8 to June 12, 2026.

Click on the link (pdf file) or the picture below for full details:

SAVE THE DATE symposium 10th anniversary of OPTMix

Jun 16

Measurements and sampling as part of Erwan Le Roux’s thesis on the effect of ungulates on soil biodiversity

As part of Erwan Le Roux’s thesis, co-supervised by Stéphane Bazot (ESE Univ Orsay) and Nathalie Korboulewsky, a measurement campaign and soil sampling on a selection of OPTMix plots (O12 low density open and closed) and Renecofor (inside and outside the enclosure) is carried out until July. The measurements on O12 were made at the end of May with soil compaction measurements (with the “Clegg hammer”) and soil respiration on the same soil samples transects as Claire Populus thesis’s last year. Soil samples will be used to determine soil microbial diversity and mineral nitrogen. The compaction results are immediate and we observed a more compact soil in the open plot vs. the closed one (47 vs. 36 Gm units, n=41/plot, significant difference). We will never know if this is due to the 4 legs or 2 legs animals, but given the simplicity and ease of this device, it makes you want to make measurements in the other plots.

Jun 16

Soil and vegetation sampling for the ANR project “PLASTRAIT”

A team from the University of Rouen came last week to sample soil and vegetation as part of the PLASTRAIT project:

“Plasticity in leaf litter quality of tree species: consequences for soil nitrogen cycling and plant-plant interactions.”

Last Thursday and Friday (June 12-13), Ludovic Henneron, from the Ecodiv unit, accompanied by three colleagues, sampled soil and vegetation on pure oak stands on OPTMix (O12-O214-O593, medium and low density). Other plots from the GIS Coop are also sampled. The aim is to study the nitrogen cycle and leaf traits of the species (tree and undergrowth). They installed five measurement points per plot, marked with a yellow stick. PVC tubes are inserted into the soil and will be retrieved at the end of July. The green oak leaf samples planned by EFNO will be used by Ecodiv for additional measurements.

Jan 30

ECOLOGGING: A new connected, reliable and scalable instrumented station tested on OPTMix

As part of an AnaEE 2024 project, Pierre Bordenave (Instrumentation Assistant Engineer, Pierroton Forest Experimental Unit – UEFP) develop and test a low-cost microclimate measurement station in various situations. The goal is to maintain high sensor quality for research while keeping the whole station low-cost and ensuring scalability, as additional sensors (such as soil moisture sensors, water table height sensors, or dendrometers) can be connected.

OPTMix is a forest site where the durability of the devices, solar panel power supply, and 4G data transmission will be closely observed due to its location under tree cover.

Pierre and his colleague Bernard traveled from Bordeaux to deploy this station in just one hour. It is fully operational, and we are already receiving hourly data (average or total sum of measurements every 20 seconds) on our computers. A simple graphical visualization will soon be available for everyone.