Digging pits in high tree density plots: soil description, soil sampling and sensor installation

From December 10, 2018, pits will be dug in high tree density plots. These pits will allow a fine soil description, soil sampling in the different horizons and the installation of sensors for the measurement of the soil moisture, the level of the water table and the soil temperature.

Pictures coming soon…

OPTMix annual meeting, January 8, 10 and 11, 2019

The OPTMix annual meeting will take place on January 8, 10 and 11, 2019 at Irstea Nogent-sur-Vernisson.

Visits on OPTMix

On Thursday, October 18th, two groups visited the OPTMix device:

  • in the morning: 36 students from the Mesnières Forest High School in Normandy. For the occasion, Richard Chevalier (Irstea) who was their contact, was present to welcome them and present Irstea.
  • in the afternoon: about fifteen people from the regional CNPF, mainly the Presidents and representatives of the various CNPF progress groups. The group took the opportunity to thank Eric Sevrin for his work and to wish him good luck in his new position at the IDF.

These groups exchanged with:

  • Sandrine Perret for the silviculture topics,
  • Camille Couteau for the metrology topics,
  • Yann Dumas for the biodiversity topics,
  • Nathalie Korboulewsky for the topics about water and mineral resources use.



Article published: How do mixing tree species and stand density affect seasonal radial growth during drought events?

An article has just been published in “Forest Ecology and Management” . This article deals with the effects of species interactions and stand density on radial growth during a drought event. To perform this study, the intra-annual growth of oaks and pines was measured for 3 years on 216 trees distributed over 18 plots of the OPTMix device. This work is part of Jordan Bello’s thesis (Irstea Nogent-sur-Vernisson).

Bello, J., P. Vallet, T. Perot, P. Balandier, V. Seigner, S. Perret, C. Couteau and N. Korboulewsky (2019). « How do mixing tree species and stand density affect seasonal radial growth during drought events? » Forest Ecology and Management 432: 436-445. doi: 10.1016/j.foreco.2018.09.044


  • Lower stem density improved growth for oak but not for pine.
  • Mixing tree species had an effect on growth only at medium stand density.
  • Stand density did not influence resistance to drought.
  • Oak and pine showed opposite mixture effects on resistance to drought.
  • No effect of tree size on resistance to drought was found.



Forecasted climate change impacts on temperate forest ecosystems include increased summer drought. Forest managers can increase the resistance of forest stands against summer drought by reducing stand density and favoring tree species mixtures. These strategies have been studied separately, but their combined effect on increasing forest stand resistance to summer drought is unknown.

The main objective of our study was to quantify tree species interaction effects on radial growth during a water stress period and to determine whether these effects changed with different levels of competition reflected by stand density.

The study was based in the Orleans state forest (Central France) at a long-term triplet experimental site (OPTMix) with pure and mixed stands of mature Quercus petraea and Pinus sylvestris. The experimental design comprised three repetitions of two densities (low and medium) in each composition (pure oak, mixed stands, pure pine). We monitored tree radial growth with 216 manual dendrometers placed throughout 18 plots, on small, medium and large trees. We analyzed three consecutive years with contrasted water stress: no water stress, a summer stress period, and a late summer stress period.

We found that mixture did not improve tree growth of the either species during the summer water stress period. On the other hand, there was a mixture effect during the late summer water stress period but only in medium-density stands inversely for the two species studied. More growth occurred for oaks in mixtures while, inversely, more growth occurred for pines in monocultures. A density effect occurred only for oaks, which grew more in lower-density stands than in medium-density stands. Finally, tree size did not influence seasonal resistance to drought.

Maintenance of data loggers and sensors

A total of 22 dataloggers and 494 sensors are continuously operating on OPTMix plots. A maintenance operation of the dataloggers and sensors was carried out from 2018-09-11 to 2018-09-14. The purpose of this operation was to review all the sensors returning abnormal data in order to identify the origin of these anomalies (defective sensors, connection problems, datalogger or multiplexer problems) and to make the necessary operations to correct them.

Of the 27 anomalies resolved, 17 corresponded to a malfunction at the multiplexer level (2 multiplexers replaced), 6 corresponded to a wiring problem and 4 corresponded to an error in the database of the dataloggers and sensors (no sensor).

Of the 9 unresolved anomalies, 8 correspond to a malfunction of the sensor (2 thermocouples are permanently out of order) and one corresponds to a multiplexer malfunction.

Of the 8 sensors that malfunctioned, 4 were removed (2 automatic piezometers and 2 automatic dendrometers) to repair them.

Defective sensors will be replaced soon.


Calibration of the 66 light sensors

The 66 light sensors of the OPTMix device were disassembled from the data loggers during a short period in order to carry out a calibration using reference sensors in an open ground. This calibration will update the correction coefficients used to convert the millivolts returned by the sensors into a unit of measurement of solar radiation. These operations (disconnection, calibration and reconnection of sensors) was carried out between 2018-08-03 and 2018-08-24.


New measurements of soil water content with a neutron probe in drought conditions

This summer, new measurements of soil water content were carried out by Cyril Dejean (Irstea G-EAU unit in Montpellier) on the OPTMix plots using a neutron probe. These new measurements carried out during a period of severe drought (14/08/2018) will complete the range of measurements required to calibrate the soil water content sensors (CS616 – Campbell Scientific) installed in the OPTMix plots (see also here) .


Article published: Better predicting sessile oak and Scots pine growth in a changing climate

An article has just been published in “Agricultural and Forest Meteorology” on the growth of sessile oak and Scots pine. The goal of this work was to integrate climatic variables into models to better predict growth in the context of climate change. This work was performed using IGN data and ring-width data collected in the OPTMix plots.

Vallet, P. and T. Perot (2018). “Coupling transversal and longitudinal models to better predict Quercus petraea and Pinus sylvestris stand growth under climate change.” Agricultural and Forest Meteorology 263: 258-266. doi: 10.1016/j.agrformet.2018.08.021


  • Large-scale NFI data provide growth models including silvicultural effects.
  • Tree rings data provide models for annual modulation of growth by climate.
  • Coupling both models allowed to develop climate-dependent stand growth models.



Climate change has swept away the former general principles of long-term stability in forest productivity. New types of models are needed to predict growth and to plan forest management under future climate conditions. These models must remain robust for silvicultural practices and variations in climate. In this study, we present a new type of model development to achieve these goals. Our study focused on pure and mixed stands of Quercus petraea and Pinus sylvestris in central France. We used National Forest Inventory (NFI) data: respectively, 525 and 548 pure plots of Quercus petraea and Pinus sylvestris, and 68 plots of mixed species. We also used 108 tree cores from an experimental site of the same species. The cores cover the period from 1971 to 2013, making a total of 4572 individual annual increments. We coupled two types of models. One was developed with NFI data (transversal data). This model takes into account mean diameter and stand density effects on stand growth. It includes a set of biophysical factors accounting for stand fertility. The other one was developed with the data from tree cores (longitudinal data), and provides a climate modulation thanks to the correlation between ring width and yearly climate. The model with tree core data reveals the influence of December to July rainfalls on yearly variability in stand growth for Quercus petraea and of May to August rainfalls for Pinus sylvestris. We obtained a coupled model that allowed us to project growth up to 2100 for all the different IPCC scenarios but one; the model was outside its area of validity beyond 2060 for the RCP 8.5 scenario.

Article published: The effect of deer browsing and understory light availability on stump mortality and sprout growth capacity in sessile oak

An article presenting results on the effect of deer browsing and understory light availability on stump mortality and sprout growth capacity in sessile oak has just been published in Forest Ecology and Management. The work was carried out on parts of the experimental facility OPTMix. The article is the result of work carried out by two Master student projects: Jessica Maurize’s Master 1 (University of Perpignan Via Domitia) in 2015 and Master 2 by Kamel Zouaydi (University of Lorraine) in 2016.

Mårell, A., Hamard, J.-P., Pérot, T., Perret, S., Korboulewsky, N. 2018. The effect of deer browsing and understory light availability on stump mortality and sprout growth capacity in sessile oak. Forest Ecology and Management 430:134-142. doi: 10.1016/j.foreco.2018.08.015

Red deer (left) and roe deer (right) browsing resprouts on the OPTMix experimental study plots.

Graphical abstract


  • Deer browsing increased stump mortality of sessile oak even at low deer densities.
  • Deer browsing inhibited sprout growth of sessile oak even at low deer densities.
  • Sprout growth increased linearly with increasing understory light availability.
  • Stump survival declined linearly with greater parent tree diameter.
  • Increased understory light availability did not compensate for deer impacts.



Coppice forestry is a conventional silvicultural practice that takes advantage of a tree’s capacity to respond to disturbances by sprouting. Sprouting capacity is determined by many intrinsic and extrinsic factors such as parent tree age/size, understory light availability and deer browsing, which, under closed canopy conditions are important limiting factors for stump survival or sprout growth. However, the combined effect of potentially confounding abiotic and biotic factors on stump survival and sprout growth remains elusive, even more so under closed canopy conditions. This study aims to quantify the effect of deer browsing on stump mortality and sprout growth under closed-canopy conditions and to compare this effect with other known determinants. Here we show that stump survival and sprout growth in sessile oak (Quercus petraea Matt.) depend on deer browsing, understory light availability and the diameter of the parent tree. By studying paired fenced-unfenced plots, we confirmed that deer browsing decreased stump survival and inhibited sprout growth. Furthermore, by taking advantage of a gradient in understory light availability in monospecific and mixed stands of sessile oak and Scots pine (Pinus sylvestris L.), we showed a clear positive linear relationship between sprout growth and light availability. This relationship explained the observed differences among stand composition types. Finally, we found that increased understory light availability did not compensate for losses due to deer browsing. In the absence of deer browsing, our results demonstrate that sessile oak stumps regenerate well under closed-canopy conditions and maintain a moderate sprouting capacity at least until the age of 70–80 years old. Partial thinning could therefore be a potential tool to renew light-demanding tree species such as sessile oak in mixed high-forest stands. Nevertheless, we do not recommend coppicing sessile oak under closed canopies unless the oak stumps are protected from deer browsing and understory light availability is optimized as much as possible despite closed-canopy-management objectives.

Monitoring soil microbial activity

Promoted by Stéphane Bazot from UMR 8079, Laboratory Ecology, Systematics and Evolution of Paris-sud University, a team led by Gaëlle Vincent came to OPTMix in March and mid-June for two campaigns to monitor soil microbial activity. Soil samples were taken for structural analyzes of the communities of bacteria, archaea and fungi. To complete, soil respiration measurements were coupled for the campaign in June to characterize this part of the carbon cycle as a function of stand density.

At the same time, the Irstea team associated with Sébastien Gogo, from ISTO Orléans, measured soil respiration with large chamber in mixed stands like last year. In addition to the inter-annual variation, the measurements will allow inter-device comparison.