Soutenance mémoire
LENGTHENING OF THE DURATION OF XYLOGENESIS ENGENDERS DISPROPORTIONATE INCREASES IN XYLEM PRODUCTION
In the past, the Earth has experienced a number of climate fluctuations. Glaciations and warmings succeeded one another and shaped the world as we know it today (IPCC, 2007). The 21st century is witnessing a new era of climate change and human activities seem to have accelerated the processes (Galloway, 2004; IPCC, 2007). Within the next 100 years, the temperature will increase by about 1.4 to 5.2°C. According to IPCC (2007), warming could be more pronounced at northern latitudes, with dramatic consequences on the life forms of the boreal and arctic ecosystems. The boreal forest covers 25% of the planet’s forest surface and constitutes a huge sink of carbon stored in form of biomass and organic matter, representing one of the most important ecosystems in the world (Burton et al, 2010). It is therefore a priority to understand the effects of climate change on these forests to predict the potential future evolution and the ecological consequences over time.
Like most living organisms, trees follow recurrent phenological activities, such as flowering, fructification, growth resumption and cessation (Forrest & Miller-Rushing, 2010). One of these, xylogenesis, or wood production, occurs annually in trees of temperate and cold climates according to the cycles of summer and winter, producing a distinct pattern on the wood conferred by the tree rings. Plants are unable to escape the unfavourable periods of the year, so the meristems suspend their activity during winter by becoming dormant, a state that is maintained even if environmental requirements of temperature or day length are met. In late winter, a change occurs from dormancy to a new state, quiescence, when growth cannot take place unless the required environmentally-favourable conditions are present (Begum et ai, 2007). At high altitudes or latitudes, the resumption of cambial activity for wood formation is an event mostly driven by temperature, according to either a gradual influence or threshold effect (Rossi et al., 2007; Seo et al, 2008; Swidrak et al, 2011). As a consequence, the warming provided by higher temperatures could allow the threshold or heat sum triggering growth resumption to be reached earlier in spring (Rossi et al, 2011; Boulouf Lugo et al, 2012). It is expected that the consequent longer growing season of cambium will result in increased growth, in terms of wood or forest productivity (Boisvenue & Running 2006; Lupi et al., 2010). However, forecasting the effects of warming on tree growth remains problematic because of the complexity of the abiotic (climate) and biotic (xylogenesis) systems investigated.
The relationship between temperature and growth is the subject of a fervent debate on the mechanisms by which the former influences the latter (Kôrner, 1998). Given the harsh environmental conditions and the short thermally-favourable season, it is not surprising that temperature is one of the key ecological factors controlling growth in boreal ecosystems. However, although numerous hypotheses have been proposed and discussed (Stevens & Fox 1991; Kôrner, 1998; Sveinbjôrnsson, 2000), neither the control mechanisms (e.g. gradual influences versus threshold effects) nor the physiological processes (e.g. carbon assimilation versus allocation) involved have yet been clearly and definitively demonstrated. The growth is the result of cell division during the activity of the meristems that occurs for a precise period of time. Thus, increased growth may be the result of a longer growing season, or a higher growth intensity, or both (Rathgeber et al., 2011; Rossi et al, 2014).
Study area and tree selection
The study was conducted in the boreal forest of Quebec, Canada, where five permanent sites [Simoncouche (abbreviated as SIM), Bernatchez (BER), Mistassibi (MIS), Camp Daniel (DAN) and Mirage (MIR)] were selected in mature even-aged black spruce stands at different altitudes and latitudes . In each site, ten dominant or co-dominant 120-140-year-old trees with upright stems were chosen. Trees with polycormic stems, partially dead crowns, reaction wood or evident damage due to parasites were avoided. The height of the selected trees ranged between 13.1 and 18.3 m and decreased at increasing latitudes. No trend was observed for tree diameter along latitude or altitude .
Xylem sampling and preparation
Tree-ring formation was studied from April to October 2012. Wood microcores were collected weekly following a spiral trajectory on the stem from 30 cm below to 30 cm above breast height (1.3 m) using Trephor (Rossi et al., 2006a). Trephor is chisel shaped tool for a fast recovery of 2 mm diameter microcores. Its cutting tube is hammered into the wood, and the wood sample is separated from the xylem by rotating and extracting the tool like a corkscrew. The very small wounds inflicted by the thin piercing tubes of the tool and the consequently narrow areas of traumatized tissues around the sampling points allowed repeated samplings by microcore extraction (Forster et al., 2000). Samples usually contained the recently formed tree rings and the developing annual layer with the cambial zone and adjacent phloem. Samples were always taken at least 5 cm apart to avoid getting resin ducts on adjacent cores .
The microcores were placed in Eppendorf microtubes with an ethanol solution (10% in water) and stored at 5 °C to avoid tissue deterioration. Microcores were dehydrated with successive immersions in ethanol and D-limonene and embedded in paraffin (Rossi et ah, 2006a). Transverse sections of 6-10 um thickness were cut from the samples with a rotary microtome. The sections were stained with cresyl violet acetate (0.16% in water) and examined within 10-25 minutes under visible and polarized light at magnifications of 400-500x to differentiate the developing and mature xylem cells.
Microscopic observations
In each sample, the radial number of cells in the cambial zone, radial enlargement phase, cell wall thickening phase, and mature cells were counted along three radial rows. In cross section, cambial cells were characterized by thin cell walls and small radial diameters (Rossi et ah, 2006b). The dormant cambium was composed of 4-5 closelyspaced cells. At the onset of cambial activity, the cambial zone began to widen rapidly (within a week) as the number of cells increased, revealing that cell division had started. During cell enlargement, the tracheids were composed of a protoplast still enclosed in the thin primary wall but with radial diameter at least twice that of a cambial cell. Observations under polarized light discriminated the zones of enlarging and cell wall thickening of tracheids. Because of the arrangement of cellulose microfibrils, the developing secondary walls shone when observed under polarized light. Instead, no glistening was observed in enlargement zones where the cells were still composed of primary wall (Abe et al., 1997). The progress of cell wall lignification was detected with cresyl violet acetate reacting with the lignin (Rossi et al, 2006b). Lignification was shown by a colour change from violet to blue. The colour change over the whole cell wall revealed the end of lignification and the tracheid reaching maturity (GriCar et al, 2005) .
The cell number in the 3 rows was averaged for each tree and used to assess onset and ending of xylogenesis. In spring, when at least one horizontal row of cells was observed in enlargement, xylem formation was considered to have begun. In late summer, when no further cell was observed in wall thickening and lignification, xylem formation was considered complete. Cambium phenology was computed in day of the year (DOY) corresponding to the dates of (1) first enlarging cell, (2) first wall-thickening cell, (3) first mature cell, (4) ending of cell enlargement, and (5) ending of cell wall lignification. The duration of xylogenesis was assessed as the number of days occurring between the onset of cell enlargement and the ending of cell wall lignification. The duration of cell production was assessed according to the phase of cell enlargement rather than on cell division in the cambial zone because (i) cambium can be active without necessarily increasing its cell number, (ii) cambium produces indistinct xylem and phloem cells, which are identical before undergoing differentiation; (iii) onset and ending of cell division and enlargement occur at approximately the same time, generally within one week (Griëar et al, 2009; Rathgeber et al, 2011).
Weather stations
At each site, a standard weather station was installed in a forest gap to measure air temperature, precipitation and snow depth. Snow depth was measured with an acoustic distance sensor that quantifies the elapsed time between emission and return of an ultrasonic pulse and automatically corrects for variations of the speed of sound during the year using the measurements of air temperature. Data were collected every 15 minutes and recorded as averages every hour by means of CR10X dataloggers (Campbell Scientific Corporation). Daily mean values were later calculated with the time series obtained from the 24 measurements per day. Annual statistics were calculated from October 2011 to September 2012, the period with available data for all weather stations.
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Table des matières
INTRODUCTION GÉNÉRALE
Lengthening of the duration of xylogenesis engenders disproportionate increases in xylem production
Abstract
Introduction
Material and methods
Study area and tree selection
Xylem sampling and preparation
Microscopic observations
Weatherstations
Statistical analyses
Results
Climatic characteristics of the sites
Dynamics of xylem formation
Duration vs. rate of cell production
Phenology vs. temperature
Discussion
Linear and nonlinear growth patterns
Growth patterns and temperature
Growth patterns and climate change
Acknowledgements
References
Caption list
Supporting information
Modem portrayal of a boreal relationship: the black spruce and its phenology
Abstract
Introduction
Material and methods
Study area and tree selection
Xylem sampling and preparation
Microscopic observations
Model definition and application
Results
Dynamics of xylem formation
Model definition
Model application
Discussion
Patterns of change in phenology and growth
Model application
Temperature vs photoperiod
Making prediction
Conclusion
Acknowledgements
Reference
Caption list
CONCLUSION GÉNÉRALE
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