Effects of Climate change on Mediterranean forest functioning

PhD student                  Mathieu SANTONJA
Duration 2011-2014
Supervisor Virginie BALDY

1. Introduction

During the past decades it became increasingly obvious that ecosystems and the goods and services they generate for the human society are critical to the Earth’s life-support system, and that they are increasingly threatened by global environmental change (Millenium ecosystem assessment, Watson, 2005). Besides direct economic impacts, e.g. in terms of fiber production, flood mitigation, water purification, pollinisation and soil fertility, the human society is affected in different, sometimes more subtle ways by the loss of biological diversity and ecosystems functions associated. Applied ecologists, environmental managers and even economists are in urgent need of more ecological knowledge on the consequences of global change for the long-term functioning and viability of ecosystems. Without an understanding of the role of organisms, functional groups and ecological communities that provide services to the society, it is not possible to develop wise management plans or conservation policies (Luck et al., 2009).

Mediterranean ecosystems are under particular strong human pressure, both directly through one of the highest human population growth rates in Europe and enormous touristic activities, and indirectly through climate change with substantially decreasing precipitation and increasing summer temperatures.

Indeed, the Mediterranean climate is characterized by strong summer water stress, linked to high temperatures combined with low rainfall (Le Houérou, 2005). In the French Mediterranean region, the 2003 summer is however considered as exceptional over the last five centuries, as in most of Europe (Luterbacher et al., 2004). The monthly maximum and minimum temperatures of May, June and August exceeded the normal values (1961-2002) by 4 to 6°C, those of July by 2°C, with very little rainfall during 4 months. The drought continued in 2004 and 2005, with a rainfall deficit reaching respectively 50 and 40 % for the first 6 months (Vennetier et al., in press).

Because of the rapid climate warming and the decreasing rainfall in spring and summer forecast for the 21st century in the Mediterranean basin (Hesselbjerg-Christiansen and Hewitson, 2007) and the related increase in climate variability (Schar et al., 2004), such extreme events and successions of dry years should become more frequent in the coming decades (Beniston and Diaz, 2004). These periods of low water availability and high temperatures that limit physiological processes will be of major importance, limiting trees' photosynthetic activity and growth. As most Mediterranean soils are eroded (Butzer, 2005), their water holding capacity is often low which can extend periods of water limitation and increase the impact of drought events. Tree survival may be at stake in the case of extreme soil water depletion (Landmann et al., 2003; Bréda et al., 2006).

In the french Mediterranean area and particularly in the PACA Region, the pubescent oak Quercus pubescens (Willd.) forest dominates with more than 250,000 ha, contributing to the structure and the identity of natural landscapes in french Mediterranean hinterland.

 Q. pubescens, through its ecology, physiology, history, its relationship with human societies over time, is a model of choice for studying the evolution and functioning of Mediterranean ecosystems and more generally the evolution of communities subjected to stresses and disturbances. Often occurring in the transition of several climatic influences, and thus particularly sensitive and responsive to climate change, Q. pubescens is an interesting model to monitor changes affecting the Mediterranean area. Moreover, the importance of this oak (marcescent malacophyllous species) in Mediterranean ecosystems is subject to debate. The precise study of its performance, particularly in terms of photosynthetic efficiency, and this, in comparison with the green oak (evergreen sclerophyllous species), is a particularly interesting, especially if one puts this study in the context of the expected global warming. The question is what will be the importance of this tree species in the decades to come if we consider that climate change will result primarily from a drying and warming affecting summer. In other words, does climate change may alter:

  • · Biodiversity in forest ecosystems?
  • · Repartition and Functioning of Mediterranean Oak forests?
  • · Culturally and historically important landscapes of the Mediterranean region and their touristic and recreational value?

 The need to better understand the functioning of the oak forest in order to provide input for the debate regarding its future led to the establishment of an ambitious project including long-term ecological studies of the Q. pubescens strongly supported by the Centre National de la Recherche Scientifique and the PACA Region.

This program, coordinated by the CNRS and the University of Provence, focuses on achieving an experimental platform developed on the site of the Haute Provence Astronomical Observatory (at Saint Michel l’Observatoire): the O3HP (« Oak Observatory at OHP »). In particular, using a system of instrumented gateways installed permanently at the canopy, we will monitor performance and functioning of this forest ecosystem, by subjecting it to water stress variables simulated potential climate change.

With experimental rain exclusion, we will simulate aridification predicted for years to come, by excluding 30% of incident precipitation, and to reproduce the rainfall conditions that prevailed on average between 1950 and 1980, with intake of artificial rainfall.

scaffolding

Two levels of gateways: at the canopy (3.5 m, left)  and at the soil (80 cm, right) levels.

The changes can make a reduction and/or increased rainfall and the frequency on the phenology of Q. pubescens (e.g. bud burst, growth phases, flowering and fruiting, leaf fall) will be analyzed on all canopy over several annual cycles through the facilities put in place.

Moreover, climate changes effects on Mediterranean forested ecosystem functioning will be investigated with an emphasis on processes related to the carbon cycle related to primary productivity and secondary metabolism, soil fertility, allowing to contribute to develop predictive tools for climate change effects on ecosystem carbon sequestration, a major ecosystem service. This experimental test lays the ground for data based risk assessments for Mediterranean ecosystems under climate change.

This research project then attempts to contribute filling the gap of knowledge on how climate change affects ecosystem processes and associated ecosystem services for the understudied Mediterranean region.

 
2. Research design, milestones and methods

Based on experimental site described above, we will test the climate change on different forested ecosystems processes related to carbon cycle (see the Research design scheme below) with an original and innovative approach, as it includes green and brown forested ecosystem foodchains:

  1. We will monitor phenology of Q. pubescens over several annual cycles by determining bud burst, growth phases, flowering and fruiting, leaf fall by litter traps already put in place.
  2. Assessing the effect of drought induced by an experimental climate change on Biogenic Volatile Organic Compounds (BVOC) emissions in order to quantify the carbon emitted by minor compounds (e.g. BVOC) vs major form, e.g. CO2. BVOC emissions will be sampled using dynamic head-space chambers and analysis will be performed by standard gas chromatography-mass spectrometry. On-line PTR-MS analysis will be test for global quantification of BVOC at ecosystem scale.
  3. The production of leaves determines the amount of litter reaching the forested soil, and available for decomposers. Leaf litter breakdown is a vital process in ecosystem functioning. It is mainly a biological process involving three types of organisms: fungi, bacteria and invertebrates (macroinvertebrates and soil mesofauna). As environmental factors are of greatest importance in breakdown efficiency, this process has been recently considered to assess stream functional integrity. Two impacts could be expected: a direct negative effect of drought on decomposers and a positive or negative indirect effect by the change induced by drought on leaf chemistry and particularly secondary compounds leaf content. Leaf litter breakdown will be studied by the “litter bags” method, but we will replace bags by mesoscosms in order to avoid containment effect.
  4. We will determine soil diversity, particularly that of decomposers with a focus on microorganisms and organisms belonging to mesofauna. Indeed, collembolan communities are influenced by soil conditions and perturbation. As collembolan have difficulties to re-colonize a disturbed site owing to their low dispersal rates, they can be considered as long-term witnesses of environmental disturbance affecting bioclimatic conditions and the physical characteristics of the soil, particularly endemism vulnerability. The arthropods will be extracted from the soil samples and litter bags samples using Berlese-Tullgren funnels at a temperature of 18-23°C. Collembolan will be counted and identified to species level. Microbial communities are highly sensitive to (i) the quantity and quality of organic matter entering the soil and to (ii) the water availability, because it directly governs the intensity of soil microbial processes such as respiration and nutrient cycling. Microbial communities will be extracted following litter-crushing/saline suspension and subsamples will then be used to determine (i) the capacity to degrade predefined metabolic substrates and (ii) bacterial and fungal diversity of using ARISA and T-RFLP molecular fingerprints respectively. Moreover, we will characterize litter and soil microbial processes by potential microbial activities related to the C cycle (substrate induced respiration and decomposition of cellulose paper).
  5. From leaf litter breakdown efficiency follow different dynamics of soil carbon storage. As part of global change, it would be interesting to test if soils with rain exclusion would react as a source or sink of carbon. Most of the data available on soil as a source vs sink of carbon concern tropical forests. These forests affect atmospheric carbon dioxide concentrations, and hence are modulate the rate of climate change, by being both a source of carbon, from land-use change (deforestation), and a sink or source of carbon in remaining undisturbed forest. These fluxes are the least understood and most uncertain major fluxes within the global carbon cycle. Data on the carbon balance and ecology of tropical forests, showing concerted changes consistent with undisturbed tropical forests presently functioning as a carbon sink of ~1 Pg C y-1. However, predictions suggest that this sink is unlikely to continue. Global Circulation Models including dynamic vegetation and an interactive carbon cycle show tropical forests may become a mega-source of carbon: under "business as usual" atmospheric carbon dioxide concentrations could reach 980 ppmv by 2100.  Soil water status is critical for C efflux and storage, especially in water-limited ecosystems such as many Mediterranean ecosystems during the summer drought period. Carbon content will be determined on different organic horizons (OL, OF, OH) and mineral soil by CHN analyzer.

 

 
3. Scheme

overview

Scheme of the research approach, including different forested ecosystems processes related to carbon cycle and risks associated to climate change.

 
4. References

Beniston M, Diaz HF (2004) The 2003 heat wave as an example of summers in a greenhouse climate ? Observations and climate model simulations for Basel Switzerland. Glob planet change 44:73-81.

Bréda N, Huc R, Granier A, Dreyer E (2006) Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences. Annals of Forest Science, 63(6):625-644.

Butzer KW (2005) Environmental history in the Mediterranean world: cross-disciplinary investigation of cause-and-effect for degradation and soil erosion. Journal of Archaeological Science, vol. 32(12):1773-1800

Hesselbjerg-Christiansen J, Hewitson B (2007) Regional Climate Projection. In: IPCC (2007) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon S Qin D, Manning M,  Chen Z,  Marquis M, Averyt KB Tignor M Miller HL (eds), Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA pp 872-887.

Landmann G, Bréda N, Houllier F, Dreyer E, Flot JL (2003) Sécheresse et canicule de l’été 2003 : quelles conséquences pour les forêts françaises ? Rev. For. LV-4:299-308.

Le Houérou HN (2005) The Isoclimatic Mediterranean Biomes: Bioclimatology, Diversity and Phytogeography. Le Houérou (eds) Montpellier.

Luck GW, Harrington R, Harrison PA, Kremen C, Berry PM, Bugter R, Dawson TP, De Bello F, Diaz S, Feld CK, Haslett JR, Hering D, Kontogianni A, Lavorel S, Rounsevell M, Samways MJ, Sandin L, Settele J, Sykes MT, Van den Hove S, Vandewalle M, Zobel M (2009). Quantifying the contribution of organisms to the provision of ecosystem services. Bioscience 59, 223-235.

Luterbacher J, Dietrich D, Xoplaki E, Grosjean M, Wanner H (2004) European Seasonal and Annual Temperature Variability, Trends, and Extremes Since 1500. Science 303:1499-1503.

Schar C, Vidale PL, Lûthi D, Frei C, Hâberli C, Liniger MA, Appenzeller C (2004) The role of increasing temperature variability for European summer heat waves. Nature 427:332-336.

Vennetier M, Thabeet A, Gadbin-Henry C, Ripert C, Prevosto B, Borgniet L, Vila B, Guibal F, Ray R, Buron , Zanetti C (in press). Conséquences de la canicule et de la sécheresse 2003 sur les pins méditerranéens. In: Landman G Landeau S (eds) "Sécheresse et canicule 2003. Contribution des dispositifs de suivi et d’observation des forêts à la quantification des effets immédiats et à court terme", QUAE, Paris.

Watson RT, Zakri AH, Zedan H (2005) Millenium Ecosystem Assessment. Ecosystems and HumanWell-being: biodiversity synthesis, DC, Washington.