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Stable isotopes and Mediterranean ARidification: Tree Rings in Ecological and Environmental Studies

Final Report Summary - SMARTREES (Stable isotopes and Mediterranean ARidification: Tree Rings in Ecological and Environmental Studies)

Background and project objectives

In the Mediterranean region, the combined impact of current aridity with climate change will particularly threaten forest ecosystems. In addition, global change is likely to impact carbon sequestration in the forest sector, and trade-offs between mitigation and adaptation measures may arise in the future. In this regard, there is a need to understand long-term plant responses to climate changes in order to define the most appropriate adaptation measures.

Stable isotopes in tree-rings as ecophysiological tools at different time scales

Tree rings provide an easily datable archive that can be explored for physiologically relevant signals. In particular, the analysis of carbon and oxygen stable isotope composition (d13C and d18O, respectively) in tree-rings combines the ability to integrate physiological information over different time scales, which allows validating the results over a great number of sites. However, although the basic principles for d13C and d18O in plants are well known, there are still some unsolved questions. Thus, there is a need to assess how genetic and environmental variations affect physiological processes at the leaf level, as well as post-photosynthetic fractionation steps at the tree level, in order to apply d13C and d18O in studies aiming at understanding plant physiological response to environmental factors.

In this context, the objectives of the project SMARTREES could be divided in two main blocks:

1) Basic research to better understand the underlying mechanisms determining stable isotope composition in trees and their relationship with physiological processes: to determine the effect of mesophyll parameters on d13C and d18O, to characterise fractionation steps in d13C and d18O from leaves to tree rings and to assess the role of hydraulic and stomatal limitations in response to drought.
2) Applied research to develop the use of stable isotopes as tools to characterise genetic material for reforestation (assessing the sources of genetic variability in d13C and d18O, and uncoupling adaptation and acclimation effects on the d13C of tree rings) and to monitor long-term response to global changes in Mediterranean pines (linking point measurements with measures at the ecosystem level and comparing the environmental response of co-occurring species).

Project activities and main results

1) Assessing genetic and plastic variability for d13C and d18O

The sources of genetic variability in d13C and d18O were assessed on 3-year old seedlings of Pinus pinaster Ait. from 25 geographic origins, grown in pots in a common garden trial maintained by the holding TRAGSA in Orense (North Western Spain). In late spring, 8 trees from each of the 25 locations were monitored for physiological parameters and needles were sampled to determine d13C and d18O in leaf organic matter. At the end of the growing season, plants were harvested and d13C and d18O were measured in the last tree ring. As a potential indirect measure of xylem hydraulic properties, the seasonal pattern of wood density in the same tree ring was assessed using a resistograph (F-300-S, IML-GmbH, Heidelberg). We found relatively small genetic variability for gs, but a clear differentiation between 'maritime' and 'continental' populations in terms for gm. In addition, although this data is still being processed, we found substantial differences in wood density among provenances, suggesting a significant role of hydraulic limitations in population differences.

2) Linking d18O with leaf hydraulic conductance and mesophyll conductance for CO2

Leaf water gets isotopically enriched through transpiration and diffusion of enriched water through the leaf depends on transpiration flow and the effective path length (L). Effective pathlength (L) is a parameter of leaf water enrichment model, theoretically linked to water pathways from veins to evapourative sites. In a recent paper derived from this project, Ferrio et al. (2012, Plant Cell and Enviroment) related for the first time L with measurable mesophyll variables, such as leaf lamina hydraulic conductance (Klam) or mesophyll conductance for CO2 (gm). The working hypothesis was that restrictions in water pathways would cause a reduction in Klam and an increase in L. As a secondary hypothesis, we proposed that, given the common pathways for water and CO2 involved, a similar response should be found in gm. The strong correlation found between L, Klam and gm supports the idea that water and CO2 share an important part of their diffusion pathways through the mesophyll, and has been further discussed in a recent review paper about mesophyll conductance (Flexas et al., 2012, Plant Science).

3) Retrospective study of tree dieback

The aim of this study was to compare the historical record of healthy and declined trees in a Pinus sylvestris L. stand located in the Gúdar Range (Iberian System, Spain), before and after a dieback episode occurring just after a strong thermal contrast in autumn 2001. In this area, a progressive increase in annual temperature has been recorded during the last decades. We hypothesised that the adjustment to these new conditions should be detected in tree-ring archives (radial growth, carbon -d13C- and oxygen -d18O- isotopes), and that this adjustment would differ in healthy and declined trees. We observed a decline in radial growth and an increase in water-use-efficiency from 1975 to 2004, being faster in declined trees. Overall, our results indicate that a differential history in terms of water-use determined the fate of individuals during the dieback event, and point to a combination of water stress and low storage capacity as main drivers for dieback (Voltas et al., submitted).

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