Last May 17 and 18, the Forest Ecosystems research unit of Nogent-sur-Vernisson (Irstea, EFNO) welcomed the 9th Irstea Measurement Network meeting. On this occasion, a group of forty people from all IRSTEA centers went to visit OPTMix. After an introduction about the Orleans’s forest and OPTMix network the visit focus on four main points:
– Impact of herbivores measurements;
– Biodiversity measurements: an exemple with vascular flora and bryophytes;
– Tree growth measurements: from inter-annual to intra-annual growth;
– Water balance measurements.

The functioning of perched water tables and their impacts on tree growth are still poorly documented while soils with temporary waterlogging are very frequent in France. How does perched water table change during winter? Are these changes influenced by the forest stand features? Is this perched water table a disadvantage or an advantage for the growth of mature trees? In order to brighten these questions, an internship supervised by Nathalie Korboulewsky (DR, Irstea Nogent-sur-Vernisson) and Jordan Bello (Phd Student, Irstea Nogent-sur-Vernisson) began the 3rd of april 2017 for five month (Anna-Karine Jean, Université de Jean Monnet Saint-Etienne). This internship will attempt:

• to characterize the perched water table regime during winter for the different silvicultural treatments of the OPTMix experiment;
• to link oak (Quercus petraea) and pine (Pinus sylvestris) growth with different variables of the perched water table (minimum depth, time of presence, date/speed of apparition, withdrawal)

To reach these objectives, data recorded since 2014 on each plot with soil moisture sensors, automatic and non-automatic piezometers will be used (see plot below).

Clay layer depths are mean values of the measured clay layer depths of each plot having the same species composition (pure Oak, pure Pine or Mixture). Groundwater table depth values are the mean values of the manually collected piezometric measurements in each plot having the same species composition (pure Oak, pure Pine or Mixture) and at the same date.

The application forms to perform a new experiment in the OPTMix plots are available here (french version only). You will find a form for external demands (for researchers belonging to another research unit than the Irstea-EFNO research unit at Nogent-sur-Vernisson) and a form for internal demands.

To help you in this process please contact Nathalie Korboulewsky.

This week began the installation of aluminum tubes at a depth of 125cm for measuring soil moisture content with a neutron probe. These tubes are installed near the pits already equipped with probes recording water content (CS616 – Campbell Scientific). The measurements of the neutron probe, considered as reference measurements, will be used to validate the calibration equations of the CS616 probes. This work will be carried out during an internship in spring 2017 in collaboration with Cyril Dejean of the Irstea G-EAU research unit in Montpellier. This internship is supported by Irstea’s measurement network.

An article on predicting solar radiation transmittance in pure and mixed stands has just been published in Forest Ecology and Management. This work was carried out on the experimental plots of the OPTMix device. Below are the key points of the article.

Perot, T., A. Mårell, N. Korboulewsky, V. Seigner and P. Balandier (2017). « Modeling and predicting solar radiation transmittance in mixed forests at a within-stand scale from tree species basal area. » Forest Ecology and Management 390: 127-136. doi: 10.1016/j.foreco.2017.01.023

Highlights:

• Radiation under forest canopy was modelled in mono-specific and mixed stands.
• The Beer-Lambert law can be used in mixed stands with the basal area of each species.
• Extinction coefficients were not different between mixed and mono-specific stands.
• Transmittance can be predicted at local scale within a stand with local basal area.
• Predictions of the model are unbiased with a relative mean absolute error of 20%.

The annual meeting of the project took place on 10, 12 and 13 January, 2017 at Irstea Nogent-sur-Vernisson. The participants presented recent results and ongoing works concerning the different topics of the project. They also presented the program of future works.

Data form T-Lidar of the ScanComp project (see corresponding article) were analyzed by a Master1 student (Enrick Potiron, UMR Lerfob, supervisors Ignacio Barbeito and Mathieu Dassot). Dendrometric variables (diameter, height, crown surface, crown volume …) were extracted using the Computree platform of the French National Forest Office (http://computree.onf.fr/). Below are some pictures obtained from the T-Lidar data.

Two classes of the Chesnoy High school (45) participated in the collection of understorey litter and soil sampling the 14 of November and the 1st of December.

Following the meeting with the Forest Health Department in June (see related article), evaluation of tree health was performed on OPTMix plots in collaboration with the University of Orléans (Cécile Barbaroux, Aurélien Salle, LBLGC laboratory), the National Forest Office (Michel Sottejeau) and the Forest Health Department (DSF). This operation took place from September 5 to September 14. The DSF scoring protocols were applied to about 400 trees. These scores will help interpret the results on the intra-annual growth and storage of glucidic reserves (see related article).

After a very wet spring, a severe drought has occured from July to August with very low rainfall (20 mm for the two months) and high temperatures (days above 35°C). These weather conditions added to the harsh soil conditions of the Orleans forest led to significant stress for the trees. To better understand the effects of drought on tree functionning, experiments were conducted during the summer as part of the thesis of Jordan Bello (PhD student, Irstea). Two methods were carried out: the first method is based on measuring the base water potential of trees and the second is based on the measurement of oxygen isotope ratio in soil and wood. These methods make it possible to estimate the stress suffered by trees and the depth of water uptake by each species. The sampling was designed in order to see whether stress and depth of water uptake during drought depend on stand composition (monospecific or mixed).