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Wood formation dynamics of Larch (Larix decidua Mill.) along a 1000 m elevation gradient in the French Southern Alps

Abstract : The survival of perennial plants depends on their ability to remain adapted to their ever-changing environment. Vegetation model predictions indicate that climate change will have profound effects on forest production and distribution. Tree response to environmental factors amply relies on cambium ability to produce efficient wood. The main objective of this thesis is to understand which modifications of the wood formation (WF) processes allow trees to grow under contrasted conditions. The xylogenesis of 60 larch trees, in four stands along an elevation gradient of 1000 m in the French Southern Alps was monitored in the 2013. Wood samples were taken every week from tree stem.. Forming wood cells counted according to their differentiation stage (dividing, enlarging, wall thickening and mature). Moreover, the occurrence of wounding tissues were recorded. Meteorological data were obtained from nearby climate stations andrelative extractable water was computed using a water balance model. Thecritical dates of wood formation were computed based on logistic regression, while wood growth dynamics was described using Gompertz models. Dates and rates were then compared along the elevation gradient using bootstrap tests and mixed-effects models. The relative contribution to the total annual production of rates vs. durations was assessed using a sensitivity analysis of a basic physical model. Cambial activity (CA) started around mid-May at the lowest stand and around mid-June at the highest one, which correspond to a delay of 3 days per 100 m. The onsets of the other periods of wood formation followed exactly the same elevation trend, but were lagged in time of a few days up to one or two months. The cessation of CA and WF followed a parabolic trend with trees from the lowest stand finishing first, followed by those from the highest stand, while those from the intermediate elevations finished the last. CA and WF durations also exhibited parabolic trends, but with the shortest season occurring at 2300 m and the longest ones at 1700 m Nevertheless, we discovered that the elevation-related shortening of the growing season, was compensated by an increase in the cell production rate, leading to comparable tree-ring widths in the three upper stands. The lowest stand, on the contrary, exhibited narrow tree rings due to both low production rates and short durations. The sensitivity analysis showed that the final cell number was more related to the rate of production than to its duration. Moreover, our results showed that the number of cambial cell increased with elevation, which compensated the lengthening of the cell cycle, and thus maintained high production rates, even at high elevation. Lastly, we didn't observe any callus tissue in our anatomical sections, but we counted numerous traumatic resin canals. Moreover, a careful examination of Gompertz fitting residuals leaded us to think that microcoring monitoring slightly stimulated cambial activity, disturbing xylogenesis and final tree-ring structure. We believe, however, that the overall impact of cambial stimulation on wood formation phenology and dynamics remains limited. So the delay in the wood formation CA and WF onsets with increasing elevation can be associated with the adiabatic temperature decrease and thus translated into a lengthening of the growing season of about 5 days per °C. On the other hand, our results show that the cessation of cambial activity is more related to photoperiod, even with water shortage being able to hasten growing season termination. However, even if phenology is a key component of forest functioning, our results underlined the pivotal role of the rate of cambial activity in tree growth. Structural investment to sustain cell division rates while elevation increase, thanks to increased number of cambial cells, and despite temperature-related increased of cell cycle length, unveil a strong adaptation mechanism of trees to high elevations.
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Seyedehmasoumeh Saderi. Wood formation dynamics of Larch (Larix decidua Mill.) along a 1000 m elevation gradient in the French Southern Alps. Silviculture, forestry. AgroParisTech, 2017. English. ⟨NNT : 2017AGPT0015⟩. ⟨tel-02895332⟩

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