Studying the Mediterranean forest functioning under climate change

L’O3HP (Oak Observatory at the OHP) interdisciplinary and experimental approaches to study Mediterranean forest functioning under climate change

The need to improve our understanding of the functioning of Mediterranean forests in the light of their future evolution, has led to the elaboration of a program concerning the Downy Oak forest. It’s one of three major species of importance in the region of the French Mediterranean, and covers more then 250.000 ha in the Provence-Alpes-Côte-d’Azur (PACA) Region.

This program is complementary to pioneer programs at Puechabon for Holm Oak, installed and run by the CEFE in Montpellier, and at Roquefort la Bedoule for Aleppo Pine, installed and run by the INRA Avignon.

The O3HP program is managed and coordinated by a consortium of the IMBE/OSU-PYTHEAS/ECCOREV and is strongly supported by the Institute for Ecology and Environment (INEE) of the CNRS (National Centre for Scientific Research). The installation of the dedicated field site and laboratory has further received substantial financial support by the PACA Region and the Conseil Général des Alpes de Haute Provence.

The central installation consists of an experimental field site, situated at the research center of the Observatory of the Upper-Provence (OHP, UMS CNRS Pythéas) at Saint Michel l’Observatoire, close to Forqualquier in the Alpes of the Upper-Provence. The site is thus named O3HP (« Oak Observatory at the OHP »). The O3HP is part of the national research infrastructure AnaEE-France.

The field site is equipped with a precipitation management system (PMS) to simulate drier climate.
A device is installed above 300 m² of canopy, that dynamically excludes a defined fraction of precipitation (rain & hail) by extending automated covers, and which allows to re-irrigate part of the excluded water. The system manages a reduction in precipitation by 40 % using temperature derived functions based on 50 year records of local meteorological data. This results in a scenario of 500 mm annual precipitation corresponding to about 2°C temperature increase, which is in line with climate predictions for the Mediterranean region (Giorni & Mearns, 2002). The reduction is piloted dynamically between 20% and 60% for year 2100 in relation to year 2000. Therein lays the originality of the system, whilst environmental conditions are hardly affected. During leaf development in spring, exclusion of precipitation events is performed at night-time as not to disturb photomorphogenesis. Intercepted precipitation will be evacuated to a temporary reservoir. An irrigation (sprinkler) system attached to the metal structure will use this water to fine-tune the fraction of excluded precipitation.

The O3HP system is thus organized around 5 elements:

  1. A system of instrumented walkways organized in the form of a cross, each branch of which is 10m long and installed at 2 height levels: 0.80 m and 3.50 m, thus allowing easy access to the canopy and strata lower without disturbing the ground
  2. A rain exclusion system covering approximately half of the plot (300 m2) and designed using a system of drop-down tarpaulins intercepting precipitation.
  3. An irrigation system, currently being installed, on a 300 m2 plot to limit water stress.
  4. A network of sensors (T°, humidity, at different soil and canopy levels, sap flow, etc.), providing real-time information on meso and microclimatic conditions as well as tree activity .
  5. Laboratory parts for simple sample processing, storage and analysis.

Various functional aspects are studied or monitored in particular:

  • biogeochemical cycles and in particular the decomposition of litter
  • gas exchange
  • VOC emissions
  • phenology
  • the water cycle: rainfall, runoff, interception
  • edaphic biodiversity and biodiversity-functioning relationships
  • tree growth (dendroecology, etc.), biomass evolution and carbon storage
CNRS video in French