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Georg-August University Göttingen (2014)

Drought Adaptation of the Fine Root System and Hydraulic Architecture of Larix sibirica at its Southern Distribution Limit General introduction

Chenlemuge Tselmeg

Titre : Drought Adaptation of the Fine Root System and Hydraulic Architecture of Larix sibirica at its Southern Distribution Limit General introduction

Auteur : Chenlemuge Tselmeg

Université de soutenance : Georg-August University Göttingen

Grade : Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultäten 2014

Résumé
Taiga-steppe ecotone is a natural grassland habitat with isolated forest islands and low precipitation (c. 100-250 mm yr-1), and is an important part of the boreal biome. The southernmost distribution limit of Siberian taiga occurs in the mountainous areas of Northern and Western Mongolia. In these areas, larch (Larix sibirica) forest patches are normally restricted to the upper part of humid north-facing slopes, whereas steppe covers south-facing slopes and most valley bottoms. Temperatures have been rising faster here than the the global average for decades, but precipitation has not increased concomitantly. Induced by the increasing aridity, a decades-long decline in the growth and regeneration of the larch forests, and differentiation in the growth performance of L. sibirica between within-stand microhabitats (forest edge vs. forest interior) was evidenced by earlier work. However, the physiological mechanisms underlying these ecological responses to drought stress and climate warming are not fully understood. As a contribution to answer these questions, bio- and necromass and morphological properties of fine roots, xylem hydraulic conductivity and wood anatomical properties along the flow pass (root, stem, and branch) together with tree productivity in monospecific L. sibirica stands were studied in this thesis. The study was conducted in two dry sites (the Altai and the Khangai Mountains) in the Mongolian forest-steppe ecotone and a moist site in Central Europe (the Ore Mts. in Germany). The dry sites contain natural L. sibirica stands while the moist site is a L. sibirica plantation outside its natural range. Field work was carried out during three continuous growing seasons (July/August 2010, 2011 and 2012) which started with two dry years (2010–2011) and a subsequent wet year (2012). Variations in fine root mass and morphological properties between the dry and moist years and between the dry and moist site were examined. Spatial distribution of fine roots was also investigated in the Mongolian Altai. The hydraulic architecture of L. sibirica between the dry and moist sites was compared in relation to macroclimate (precipitation, temperature) and tree productivity. We also studied how branch and coarse root hydraulic architecture and xylem conductivity, fine root biomass and necromass, and fine root morphology of L. sibirica respond to different microhabitat conditions in terms of water availability. Fluctuations in fine root biomass (FRB) between years and among study areas revealed high plasticity in fine root system of L. sibirica to different precipitation regimes. In general, increasing FRB was associated with increasing precipitation. Comparatively deep fine root distribution was found in the larch stand in the Altai Mt., which might be an adaptation to top soil drying. However, no obvious plastic change in fine root morphology was observed. Mean annual precipitation was directly related to wood anatomical and hydraulic traits across the three sites, and a higher sapwood area-specific hydraulic conductivity in both roots and branches is correlated with higher productivity in terms of annual radial stem increment. We found a significant reduction of branch hydraulic conductivity in the putatively more drought-affected forest interior in the driest Mongolian Altai site, while no branch xylem modification occurred in the moister Khangai Mountain site. Hydraulic conductivity was several times larger in roots than in branches, but root hydraulics was not influenced by stand density or mean annual precipitation. Very low fine root biomass : necromass ratios at all sites, and in the forest interior in particular, suggest that L. sibirica maintains a relatively high root conductivity by producing large conduits, which results in high root mortality due to embolism during drought. In summary, our results suggest that L. sibirica is adapted to the semi-arid climate at its southernmost distribution limit by considerable plasticity of the branch hydraulic system and a small but apparently dynamic fine root system.

Mots clés  : Drought ; forest-steppe ecotone ; root dieback ; Hydraulic conductivity ; Wood anatomy ; Drought adaptation ; Larix sibirica ; Mongolia

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