Transpiration, tracheids and tree rings : linking stem water flow and wood formation in high-elevation conifers

Peters, Richard Louis. Transpiration, tracheids and tree rings : linking stem water flow and wood formation in high-elevation conifers. 2018, Doctoral Thesis, University of Basel, Faculty of Science.


Official URL: http://edoc.unibas.ch/diss/DissB_12926

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Conifers show a biogeographical distribution across a wide range of contrasting environmental conditions, stretching from the Arctic Circle to the equator and Southern Hemisphere. In mountainous ecosystems, conifers can dominate at high elevations with low temperatures severely limiting tree growth and survival. Conifers growing at sites with temperature limiting conditions are highly sensitive to ongoing climatic change, where warmer and drier conditions will impact their growth. Understanding how high-elevation conifers will respond to these changes in climate is critical, as they play a role in regulating terrestrial carbon storage (facilitated by the formation of woody tissue) and water balance (by releasing water to the atmosphere via transpiration). The environmental regulation of wood formation (i.e., tracheid development in conifers), which dictates annual ring-width patterns, is commonly associated with the tree’s photosynthetic activity, while other growth-limiting factors might also be relevant. For example, tree growth requires turgidity in the cambium to exert the pressure necessary for cell expansion, assimilates to lengthen and thicken cell walls, warmth to allow the metabolic reactions to take place, and time for these processes to be completed. Yet, an in-depth study on how important tree hydraulics (i.e., transpiration dynamics) are in regulating “turgor-driven” growth in high elevation forests is lacking.
As part of the LOTFOR project, the general objective of this work is to develop a better mechanistic understanding on how tree hydraulics and environmental factors interact in regulating wood formation and shaping tree rings in high-elevation conifer trees. More specifically, the coupling between stem hydrological cycles and structural carbon dynamics is investigated in the context of increasing temperature and water scarcity. This thesis combines multi-annual records of both intra-annual wood formation data and high-resolution hydraulic measurements within a mechanistic growth model to explain inter- and intra-annual tree growth patterns. To simulate the impact from recent climate change on these mechanisms, a space-for-time experimental setting is applied within the Lötschental, located in the Swiss Alps, where we collected data of two commonly occurring conifer species (Larix decidua Mill. and Picea abies Karst. L.) along an elevation/thermal gradient and contrasting wet and dry sites. Additionally, evaluations are performed on existing methodologies for measuring sap flow and handling large wood anatomical datasets.
Analysing how climate affects tree growth at high elevations requires measurements on inter- and intra-annual growth, frequently obtained from tree rings and wood formation observations, respectively. In CHAPTER 2 of this thesis, more than 150 years of inter-annual growth dynamics along the elevational gradient (derived from tree rings) are assessed in relation to temperature, precipitation and insect activity. An analysis of the recent forest biomass increment increase, derived from the tree-ring width measurements, indicates that the absence of insect outbreaks (since 1981) has caused an equal or even greater impact on carbon sequestration compared to the observed warmer summer temperatures. The presented analysis reveals the relevance of including such biotic drivers and their interactions with climate in models assessing the future productivity and carbon sink capacity of forests. In CHAPTER 4, using the algorithms presented in CHAPTER 3, we monitored intra-annual tracheid development across an 8 °C thermal gradient including two elevational transects (in the Lötschental and Vosges Mountains in France) to investigate cell enlargement and wall thickening dynamics in relation to environmental conditions. Results show that at colder sites, differentiating tracheids compensate for lower rates of cell enlarging and wall thickening by increasing the cell development time, except for the wall-thickening latewood cells. This compensation allows conifer trees to mitigate the influence of temperature on the final tree-ring structure, with important implications for the ring’s size and functioning.
The production of carbohydrates and generation of turgidity in the cambium to initiate growth are tightly linked to the way a tree regulates the flow of water through the soil-plant-atmosphere continuum. For high elevation conifers, the regulation of the stomatal conductance in the leaves is important, as transpiration has to be optimised for minimal water losses during winter and maximum photosynthetic yield during the short vegetative season. Interestingly, sap flow (measured with thermal dissipation probes installed into the water-conducting wood) can be used to derive stomatal conductance, although this application requires proper data processing of raw sap flow measurements to reduce uncertainties. CHAPTER 5 presents a quantification of the uncertainties generated by commonly applied data-processing methods for conifer sap flow measurements. The uncertainty analysis reveals the importance of performing species-specific calibrations of the sap flow probes, determining zero sap flow conditions with environmental measurements, and applying a dampening correction for better estimates of both the variability and absolute values of whole-tree water use. The processed sap flow measurements are used in CHAPTER 6 to address the ability of L. decidua and P. abies in the Lötschental to adjust their conductance response to environmental conditions when growing under persistently colder and drier conditions. The results indicate that the pioneer L. decidua is more plastic in optimizing its conductance response to temperature with increasing elevation, compared to P. abies. Surprisingly, drought sensitive P. abies did not show a stronger downregulation of its stomatal conductance during drought episodes compared to L. decidua. The stronger plasticity of stomatal conductance response to environmental conditions and the higher water-conductance efficiency of L. decidua, compared to P. abies, provides a new insight into how trees differ in water-use strategies and indicates that L. decidua may be well equipped to function under changing future climatic conditions, compared to a climax species such as P. abies.
While mechanistic models can now simulate turgor-driven growth and potentially improve current growth predictions, they lack validation on annual timescales. CHAPTER 7 uses the processed intra-daily sap flow together with site-specific environmental measurements in a mechanistic whole-tree model. The simulated growth dynamics show good agreement with the observed inter- and intra-annual growth in high-elevation conifers (obtained from tree-ring width and xylogenesis observations, respectively). Four years of high-resolution measurements on sap flow and diameter variations were used to apply the mechanistic model for L. decidua or P. abies trees growing along the elevational gradient and in contrasting dry and wet sites in the Lötschental. Good agreement was found between the simulated and observed radial stem growth. Growth was unlikely to occur at temperatures below 2 °C (which is above the photosynthetic minimum) or soil water potentials lower than -0.6 MPa. These results suggest that turgor and its environmental drivers are important for regulating radial growth and should be considered when assessing forest productivity under changing environmental conditions.
If one message becomes clear from this thesis, it is that elevational transect studies provide crucial insights into the effect of persistent changes in growing season temperature (as induced by climate change) on annual tree growth patterns, wood formation dynamics and tree hydraulics. Furthermore, collecting a large variety of tree physiological measurements is vital for testing and validating the mechanisms that regulate tree growth and forest productivity patterns.
Advisors:Kahmen, Ansgar and Fonti, Patrick and Hoch, Günter
Faculties and Departments:05 Faculty of Science > Departement Umweltwissenschaften > Integrative Biologie > Physiological Plant Ecology (Kahmen)
UniBasel Contributors:Peters, Richard Louis and Kahmen, Ansgar and Hoch, Günter
Item Type:Thesis
Thesis Subtype:Doctoral Thesis
Thesis no:12926
Thesis status:Complete
Number of Pages:1 Online-Ressource (240 Seiten)
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Last Modified:27 Jul 2019 04:30
Deposited On:07 May 2019 14:19

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