Climate control of wood formation measured at high precision in a southern boreal pioneer and a climax tree species (Larix sibirica and Betula platyphylla)
Final Report Abstract
Tree-ring analyses have repeatedly shown that the annual radial stem increment in boreal forest trees in Inner Asia and southern Siberia is dominantly limited by drought. While these forests are drought-limited due to their geographic position in the ecotone to the steppe biome, global climate change has intensified drought limitation, which is increasingly becoming relevant even in more northern forests formerly limited by low temperatures. In the project, mechanisms behind the control of stemwood formation were studied with a combination of tree-ring analysis, weakly anatomical analysis of cambium activity and xylem differentiation, continuous electronic dendrometer records, monthly analysis of tree water relations, and weekly analysis of non-structural carbohydrate (NSC) concentrations. These analyses were combined with continuous measurements of air temperature, relative humidity, soil temperature at 10 depths, as well as volumetric soil water content and soil matric potential at 3 depths and supplemented by analyses of C stock density and δ13C and δ15N signatures. The study was conducted in monospecific larch (Larix sibirica) or birch (Betula platyphylla) and mixed larch-birch forests of Mongolia, thus including two species occupying contrasting ecological niches in the boreal forest as latesuccessional (larch) and early-successional (birch) species. The study was originally planned for 2 years of field work, but since the first year (2017) included one of Mongolia’s strongest droughts on record, field work was extended to a total of 3 years with 2018/19 being moist years, because trees were supposed to show aftereffects of the drought in the first subsequent year and average behavior in the second moist year. Annual stem increment was drought-limited as expected, but anatomical study showed that the onset of cell division and enlargement was controlled by air temperature. During drought, both processes ceased due to falling soil matric potential, causing the drought limitation of annual stem increment. Dendrometry further revealed a control of stem radius changes by soil temperature, while only diurnal variability was related to VPD. Birch resumed growth immediately after the drought, whereas larch showed aftereffects, investing parts of its assimilates in growth, but much in soluble sugars for constituting drought and cold resistance. Larch showed strong osmotic adjustment during the drought and had larger hydraulic safety margins based on the gap between minimum water potential (Ψ) and embolism resistance and turgor loss point. NSC concentrations in the fast-growing birch were much lower and most NSC were allocated to starch. Investment in wall thickening in larch generated a bulk elastic modulus (ε) that increased with decreasing relative water content (RWC) and thus allowed rapid lowering of Ψ with progressive soil drying, which is combined with the larches’ high potential for osmoregulation. In birch, ε decreased with decreasing RWC, enhancing drought vulnerability. However, reduced cell wall thickening and lignification reduced ε at low RWC close to turgor loss point even in larch in the year following the drought. This reduction of ε explains the aftereffect of the drought on larch in the year following the drought, because with reduced capability to downregulate Ψ at soil drying larch has to close its stomata earlier, which lowers the photosynthetic carbon gain. The high C allocation priority of larch to soluble sugars is likely to partly compensate for reduced ε, but can only be achieved by less investment in growth, which explains why relationships of annual stem increment are often found with previous year’s droughts in tree-ring analysis. Overall, the study provides new insights in mechanisms (involving C allocation and tree hydraulics) and in climatic factors controlling the phenology of wood formation in drought-limited boreal forests.
Publications
- (2018) Drought effects on hydraulic traits and relations to tree-ring width in Larix sibirica growing in forest stands of varying size in the Mongolian forest-steppe. Annals of Forest Science 75: 30
Khansaritoreh E, Schuldt B, Dulamsuren Ch
(See online at https://doi.org/10.1007/s13595-018-0701-2) - (2019) Effects of forest fragmentation on organic carbon pool densities in the Mongolian foreststeppe. Forest Ecology and Management 433: 780-788
Dulamsuren Ch, Klinge M, Bat-Enerel B, Ariunbaatar T, Tuya D
(See online at https://doi.org/10.1016/j.foreco.2018.10.054) - (2019) Hydraulic architecture and vulnerability to drought-induced embolism in southern boreal tree species of Inner Asia. Tree Physiology 39: 463-473
Dulamsuren Ch, Abilova Sh, Bektayeva M, Eldarov M, Schuldt B, Leuschner C, Hauck M
(See online at https://doi.org/10.1093/treephys/tpy116) - (2021) Drought stress mitigation by nitrogen in boreal forests inferred from stable isotopes. Global Change Biology 27: 5211-5224
Dulamsuren Ch, Hauck M
(See online at https://doi.org/10.1111/gcb.15813) - (2021) Organic carbon stock losses by disturbance: comparing broadleaved pioneer and late-successional conifer forests in Mongolia's boreal forest. Forest Ecology and Management 499: 119636
Dulamsuren Ch
(See online at https://doi.org/10.1016/j.foreco.2021.119636)
