The annual meeting of the project took place on 8, 10 and 11 January, 2019 at Irstea Nogent-sur-Vernisson. These days were an opportunity to review the past year in terms of management and maintenance and also to discuss the scientific aspects: the scientific studies done and those planned and to discuss new experiments or new collaborations to develop.

The following scientific works were presented:

Growth and productivity

• “Determining the link between density and productivity of forest stands in a context of climate change. Application to two tree species: Quercus petraea and Pseudotsuga menziesii ” by Maxime Brière. Thesis project 2019-2021 supervised by Eric Dufrêne (UMR ESE)
• “Transmitted light as a tool to monitor tree leaf phenology and development applied to Quercus petraea” by Thomas Pérot (Irstea)

Resources use

• “First approach of water balance for soil with temporary water table on the OPTMix plots” by Philippe Balandier (Irstea). master 2 traineeship of Jérôme Tricaud (university of Aix-Marseille).
• “Responses of sessile oak and Scots pine to water stress according to stand composition and stand density” by Jordan Bello (Irstea). Thesis project 2016-2018 supervised by Nathalie Korboulewsky (Irstea) and Patrick Vallet (Irstea)
• “Soil carbon and cycle” by Nathalie Korboulewsky and Lucie Vincenot (ECODIV, Irstea university of Rouen Normandie)
• “Impact of stand density on soil organic matter dynamics” by Gaëlle Vincent (UMR ESE)

Biodiversity

• “CLIMATICK Project” by Isabelle Lebert (INRA)
• “Ecological characteristics of forest stands promoting
  an invasive alien species, Campylopus introflexus (Bryophytes)” by Yann Dumas (Irstea)
• “Dung beetle communities and dung fungi in forest ecosystems grazed by large herbivores” by Christophe Baltzinger (Irstea)

Forest understory dynamics

• “Role of bryoflora in the forest interaction network: implications for the coexistence of woody plant species” by Laura Chevaux. Thesis project 2019-2021 supervised by Anders Marell (Irstea) and Philippe Balandier

From 10 to 18th of December, 2018, pits have been dug in high tree density plots. The following operations were performed:

• a fine soil description,
• soil sampling in the different horizons,
• installation of sensors for the measurement of soil moisture, level of the water table and soil temperature.

An article has just been published in the “Plant science” journal on the water uptake depths of sessile oak and Scots pine growing in mixed stands during a severe drought. The data were collected in 2016 on the OPTMix plots. To determine the water uptake depths, isotopic signatures (oxygen) of wood and soil during the water stress period were compared. The results show that during a severe drought Sessile Oak and Scots Pine growing in mixed stands have different water uptake depths.

Bello, J., N. J. Hasselquist, P. Vallet, A. Kahmen, T. Perot and N. Korboulewsky (2019). « Complementary water uptake depth of Quercus petraea and Pinus sylvestris in mixed stands during an extreme drought. » Plant and Soil. doi: 10.1007/s11104-019-03951-z

Abstract

Aims: The growing demand from forest managers is to identify silvicultural practices to overcome projected water scarcity during the next decades. One solution is to mix tree species in the same stand, thereby increasing resource partitioning and minimizing competition for limited soil water. This study investigates the mixture approach for Quercus petraea (Matt.) Liebl. and Pinus sylvestris L. during an extreme summer drought event.

Methods: During the summer drought event in 2016, we analyzed the isotopic signatures of large- and small-tree xylem and soil water throughout the soil profile to assess the depth of water uptake for both tree species. We also measured predawn leaf water potentials (PLWP) to assess water availability for individual tree species.

Results: When grown in pure stands, both species primarily utilized soil water near the surface. In contrast, partial niche complementarity for limited water resources

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…

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

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.

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

Highlights

• 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.

Abstract

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.

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.

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.

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) .