Doctorant : Estelle Noyer
Université / Institution : M2 EDDEE AgroParis Tech
Durée du contrat : 2013-2016
Titre de la thèse — La réponse individuelle des arbres à l’ouverture de la canopée : caractéristiques de la croissance des arbres de du bois. Conséquences pour la dynamique de recrutement de la canopée.
Équipe d’accueil et encadrant(s) —
LERFoB, Laboratoire d’Etude des Ressources Forêt-Bois, UMR 1092 INRA – AgroParisTech, Institut National de la Recherche Agronomique (INRA) – Centre de Nancy, 54 280 Champenoux, France, http://www7.nancy.inra.fr/foret_bois_lerfob/
EEF, Ecologie & Ecophysiologie Forestières, UMR 1137 INRA – Université de Lorraine, Institut National de la Recherche Agronomique (INRA) – Centre de Nancy, F-54280 Champenoux, France. http://www4.nancy.inra.fr/eef
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Context and state of the art — Natural or anthropic canopy disturbance is a major driver of forest dynamics (Picket and White 1985). However, the approach taken by ecologists to analyse the role of disturbance is mostly descriptive (Frelich 2002) and there have been only few attempts to understand the disturbance processes and their effects on plant community dynamics and individual tree response (Johnson et al 1994).
The process of canopy disturbance involves two successive phases: rapid canopy opening followed by progressive canopy closure, to which the trees must acclimate. Canopy opening has been shown to simultaneously i) increase resource availability (light, water and nutrient) (Gessler et al. 2001, Renneberg et al. 2009), ii) increase air displacements and thus, evaporative demand and mechanical stress (Aussenac 2000), iii) reduce physical contact with adjacent trees, thus reducing support between adjacent crowns and branch collisions, iv) increase air and soil temperature (Aussenac 2000), and v) change light spectral composition.
Individual trees respond to canopy opening with an improved growth that combines an immediate response of radial growth and a delayed response of axial growth. Simultaneously, changes in tree morphology occur, as the stem straightens and becomes more dominant. Growth enhancement of released trees results primarily from the higher resource availability, and requires that trees acclimate to their new environmental conditions. Acclimation to open canopy conditions involves adjustments in hydraulic (stem hydraulic conductivity, vulnerability to cavitation), foliar (leaf mass area, nitrogen content) and stem mechanical properties (Kimura et al. 1998, Caquet et al. 2009, Collet et al. 2011). These adjustments lead to higher photosynthetic performance and, finally, to improved tree growth (Naidu and DeLucia 1997, Aranda et al. 2004).
Tree response to canopy closure has been far less studied. When submitted to increased shade, trees show an immediate decrease in radial growth and a delayed decrease in axial growth, along with reduced leaf gas exchange and reduced leaf mass area (Ziegenhagen and Kausch 1995, Welander and Ottosson 1997, Baudry 2013).
All these studies show that trees growing in changing canopy conditions require at least two to three years after the disturbance to fully acclimate to their new environment. In forests characterised by periodic intermediate disturbance regimes, the young trees are submitted to a succession of open and closed canopy episodes (Harcombe et al., 2002; Kwit and Platt, 2003), and are in a state of constant adjustment to canopy conditions.
In these forests, trees that finally reach the upper canopy have all benefited from gap openings. However, the growth trajectory of the tree is strongly related to the species shade tolerance (Canham 1985, Webster 2005). Intolerant species reach the canopy within a single gap event, whereas shade tolerant species may reach the canopy after multiple gap opening events separated by long closed canopy periods. In these trees, the alternation between open and closed canopy conditions may easily be detected using tree ring analysis, which shows the alternation between periods of high radial growth and periods of low radial growth (Landis et al. 2005 ; Emborg 2007 ; Rozendaal et al. 2010).
Most studies analysing tree response to specific canopy disturbance regimes are based solely on tree ring analysis and, to our knowledge, no studies have examined other possible variables reflecting tree growth such as axial growth or branch development or reflecting tree adjustment to specific environmental factors such as wood anatomical characteristics or stable isotopes. Radial growth is an integrative process that results from the interplay among multiple environmental and ontogenetic factors and tree ring analysis used by itself does not allow disentangling the role of the different factors that may constrain or enhance tree growth during a canopy disturbance event. In order to assess which environmental (light, belowground resource availability, air movements…) or ontogenic (tree size, age, morphology…) factors control tree response to canopy disturbance, tree ring analysis should be combined with the analysis of other variables reflecting the environmental and ontogenic constrains experienced by the tree.
In addition, only very few studies have examined tree response (characterized using tree ring analysis) in relationship to a known canopy disturbance history (but see Rentsch et al. 2003; Hart 2010). In fact, most studies estimate past canopy disturbance, using the established tree ring series and applying criteria to detect past canopy release events (Hart et al. 2012 ; Stan et al. 2010), which strongly limits the analysis of causal relationship between canopy disturbance and tree radial growth.
Aims and specific questions to be addressed — Tree response to canopy disturbance is complex. Several environmental factors known to determine tree growth change simultaneously, and their respective role in tree response have not been analyzed. Growth is an integrative process that results from the interplay among multiple environmental and ontogenetic factors. Understanding tree response to canopy disturbance requires disentangling the role of the different factors on tree growth.
Wood structure and wood properties may be used as markers of tree environment and tree functioning (e.g. Chave et al. 2009). Different wood functions -storage, conduction, support, defense- may be considered, leading to a wide range of potential markers, each related to specific environmental or internal factors (McCaroll and Loader 2004, Rozenberg et al. 2012). Wood offers chronological records of these markers that may be used to assess past tree growth conditions.
The objective of the thesis is to use wood markers and analyze tree response to canopy disturbance, in order to:
- 1. Identify the main environmental constraints that restrict tree growth during canopy disturbance.
Wood markers that may be considered are: stem radial and axial growth, ramification and rameal traces, wood density, latewood proportion, reaction and flexure wood, cell types proportion (vessels, fibers, parenchymas), cell wall characteristics (micro fibril angle, pit characteristics), wood hydraulic traits, carbon and oxygen stable isotopes. Some of these wood characteristics have been widely used in tree ecophysiology to analyze tree response to environmental factors in interaction with different internal tree factors (genotype, organ type, cambial age). Others are currently investigated by tree biophysics and wood functional anatomy. Individual characteristics may be related to a specific set of environmental and ontogenic factors. The analysis will distinguish hard traits (i.e. wood characteristics that are explicitly and mechanistically linked to tree functioning) and soft traits (i.e. characteristics that are good proxies of hard traits and easy to measure with high throughput, and potentially are good candidates as wood markers)
The relative importance of the different environmental constraints (light or water limitation, air temperature, mechanical constraint) will be estimated by the combined analysis of the different wood characteristics, measured on trees experiencing canopy disturbances
- 2. Identify wood characteristics that may be used as markers of past canopy disturbance.
Past chronologies of wood characteristics will be established on trees submitted to a known history of canopy disturbance, and the use of each characteristic as a marker of canopy disturbance will be tested.
Novelty of the project — For many years, forest scientists and wood technologists have intensively studied wood variations with environment, genotypes and internal tree variables (as tree architecture or growth velocity) in order to answer practical questions of resource quality (e.g. Zobel 1989). More recently, variations in wood characteristics have been analysed to answer questions on the ecology and functioning of trees. The thesis is based on a large body of literature dealing with existing links between wood properties, tree functioning and tree environment, which will be applied to the understanding of how trees acclimate to canopy disturbances. A multidisciplinary approach that combines wood science, forest ecophysiology, forest ecology and silviculture will be developed.
Tree response to light availability has been a major research question for the past decades. However, the vast majority of the studies focused on contrasted but stable light regimes, and only few studies have analysed trees response to changing light environment. More generally, studies in forest ecology and ecophysiology have largely documented tree response to stable environmental conditions but have much less documented tree response to changing environments, although forest trees typically grow in permanently unstable conditions (resource availability, wind, air temperature…). The thesis aims at moving into this direction, and the analysis of tree response to canopy disturbance will largely be based on existing knowledge on tree response to contrasted and stable environmental factors (light, water, mechanical constraints).
Science and socio-economic issues — Canopy disturbance is a major driver of the dynamics of natural and semi-natural forests.
First, continuous cover forestry is presently developing in Europe and in north America, as a management practice that allows improved soil protection, habitat conservation and flood alleviation while providing regular volumes of timber. Continuous cover silviculture involves the coexistence of several strata of trees and requires that, at some time, young trees are overtopped by adult trees that are progressively removed. Developing silvicultural prescriptions for continuous cover systems requires a better understanding of tree response to canopy disturbance.
Second, strong windstorms are known to occur regularly, creating gaps of various sizes in all forest types (natural, semi natural, or plantation forests). Understanding forest resilience to windstorm and establishing silvicultural prescription to manage forest following storm damages also requires a better understanding of tree response to canopy disturbance.
Finally, relationships between wood properties and canopy opening that will be established during the thesis, may be used as a starting point in a future work focusing on tree response to thinning, a silvicultural intervention used in all semi-natural and plantation forests, largely beyond the scope of continuous cover forestry.
Available equipment, material and methods — The thesis is based on experimental work in natural conditions.
Two experimental networks established in oak and beech stands, will be used to analyse tree response to sudden changes in canopy conditions. A first network includes several experimental sites located in eastern and central France where trees at the pole stage had been submitted to canopy release and measured for 6 years after canopy opening. The second network includes several experimental sites located in the Wallonie region in Belgium where saplings were submitted to a 3-years shading treatment and released again after 3 years.
In addition, a mixed-species stand with a known history of canopy disturbance, will be selected from existing long-term experimental stands in France or in a neighbouring country.
Wood characteristics will be assessed using the facilities of the INRA centre of Nancy, i.e. the platform Xylosciences and the Platform of Functional Ecology.
Main publications of the research team
Caquet B, Montpied P, Dreyer E, Epron D, Collet C 2010 Response to canopy opening does not act as a filter to Fagus sylvatica and Acer sp. advance regeneration in a mixed temperate forest. Ann For Sci 67 :105.
Van Couwenberghe R, Collet C, Lacombe E, Pierrat JC, Gégout JC 2010 Gap partitioning among temperate tree species across a regional soil gradient in windstorm-disturbed forests. For Ecol Manage, 260 :146-154.
Wagner S, Collet C, Madsen P, Nakashizuka T, Nyland RD, Sagheb-Talebi K 2010 Beech regeneration research: From ecological to silvicultural aspects. For Ecol Manage, 259: 2172-2182.
Collet C, Ningre N, Hounzandji API, Constant T, Fournier M 2011 Growth and posture control strategies in Fagus and Acer saplings in response to canopy disturbance. Annals of Botany, 107: 1345-1353.
Roussel M, Dreyer E, Montpied P, Le-Provost G, Guehl JM, Brendel O 2009 The diversity of C-13 isotope discrimination in a Quercus robur full-sib family is associated with differences in intrinsic water use efficiency, transpiration efficiency, and stomatal conductance. J Exp Bot, 60: 2419-2431.
Bonal D, Ponton S, Le Thiec D et al. 2011 Leaf functional response to increasing atmospheric CO2 concentrations over the last century in two northern Amazonian tree species: a historical delta C-13 and delta O-18 approach using herbarium samples. Plant Cell Environment 34 : 1332-1344.
