Report
Teaser, summary, work performed and final results
Periodic Reporting for period 1 - PHLOEMAP (Hydraulic functional traits as determinants of forest function and drought responses. Putting xylem and phloem attributes into the functional trait map.)
Teaser
Forests provide key ecological services such as the regulation of energy, carbon and water balances from local to global scale. These are however under severe pressure due to climate and land use changes, especially in the Mediterranean basin where limited water availability...
Summary
Forests provide key ecological services such as the regulation of energy, carbon and water balances from local to global scale. These are however under severe pressure due to climate and land use changes, especially in the Mediterranean basin where limited water availability is prominent. It is thus crucial to identify the forest species and areas that are likely to be more vulnerable to reduced water availability and establish how changes in forest composition and structure will impact key ecosystem services.
In this project, we used the functional trait concept to bridge between tree, population, species and forest level to enlarge our capacity to predict where and when dramatic changes in forest ecosystem services are likely to occur. We focussed on the role of the xylem and phloem tissue and cells in tree and forest functioning in relation to drought.
Our objectives were to
(1) compile an extensive dataset of xylem and phloem trait values representing the trait variability existing along gradients in water availability at regional scale addressing both within-species and between-species variability;
(2) integrate this set of xylem and phloem traits with a comprehensive set of other hydraulic and functional traits;
(3) use the integrated dataset to study trait relationships and describe tree responses to drought so to determine the combination of traits that is the best predictor of drought resistance at intra- and interspecific level; and
(4) test how and to which extent variability in xylem and phloem traits influences forest performance at different ecological scales and forest vulnerability under climate change scenarios.
The expected final conclusions of the project are:
- About one fourth of trait variability can be attributed to the within-species level and should thus not be neglected when forecasting responses of trees and forests to climate change.
- Trait variation is larger between species than between populations of the same species with water availability not having a large direct effect on branch tissue proportions and conduit sizes lowering forest adaptation possibilities to environmental changes.
- Strong conclusions on whole-tree performance under changing environmental conditions should at least incorporate several hydraulic traits.
Work performed
We started with finalising a large sampling campaign in Catalonia (ES) and with sample preparation for phloem and xylem trait assessment. In total, 450 trees from 6 species and 90 populations were selected for the study. For each species (three Fagaceae and three Pinaceae, the dominant forest vegetation of the region), the populations were chosen along a gradient in water availability.
Of each individual tree, a branch section was taken into the lab to make a micrometre thin section with a sliding microtome. With a camera attached to a microscope, digital images from all 450 stained micro-sections were obtained on which xylem and phloem traits could be quantified. This generated a phloem and xylem anatomy dataset of 16 traits (Obj 1) that will be contributed to the TRY Plant Trait Database.
Tree-specific hydraulic traits and population-specific measurements were also taken. In total, 20 leaf and xylem hydraulic traits were measured for each individual tree. Population specific variables were divided in 3 groups: 8 soil variables, 4 forest structure variables and 6 climate variables. Integration of the anatomy and physiology traits and the environmental variables lead to a comprehensive dataset allowing to study trait relationships and describe tree responses to drought (Obj 2).
Through linear modelling, we concluded that variation in tissue balances were larger between species than between populations of the same species and that water availability did not have a large direct effect on tissue proportions or conduit sizes (Obj 3), opposite to what was expected. Joint effort of the PHLOEMAP and FUN2FUN projects showed that: about one fourth of trait variability can be attributed to the within-species level, trait coordination differed between inter- and intra-specific level and hydraulic traits are crucial to predict whole-tree performance under changing environmental conditions (Obj 4). All scientific results will be published in the course of 2018 in four journal papers.
During dataset build-up, 3 dissemination activities were done: (i) the publication of 2 articles and 2 videos for a general public showing the beauty, the complexity and the ecological importance of wood structure, (ii) the publication of a book chapter on the anatomy and xylem functioning of oaks and (iii) the coordination of the outreach activities of the EU COST Action STReESS including two outreaching exhibitions in which several aspect of the PHLOEMAP project were integrated. The first scientific results of the project were presented at the 3rd International Xylem Meeting in Bordeaux (FR) in 2017 and further results will be presented at the TRACE conference (DE, 04/18), the Gordon Research Conference on vascular biology (US, 06/18) and the 42nd New Phytologist Symposium (US, 07/18). An exhibition activity on the future of forests in a changing climate closely related to this project is planned in March 2018 in Barcelona. On my personal website “Let’s treeâ€, I have included an explanation that is easily understandable by a broader public on the importance of trees and forests and the role the water transporting cells herein.
Final results
The results of this project and their impact on forest resource management are:
- With about 1/4th of trait variability being attributed to the within-species level, our study reinforces the fact that this source of variation should not be neglected in studies addressing the forecast of how trees and forests are likely to change under different climate scenarios. This result pleas for an update on the current methods used to build forest projections for the future by integrating this source of variation.
- Trait variation was found to be larger between species than between populations of the same species. Also, trait coordination differed between inter- and intra-specific level and no clear pattern was found when trait co-variation networks were compared among species. More research should be dedicated to find the exact driving forces of trait variation at intra-specific level under different ecological conditions in forests worldwide, herewith unravelling the exact causal relationships of multi-dimensional trait adjustments. These insights are crucial to design effective plans of mitigation and adaptation against climate change.
- The branch tissue functional balance analyses as well as the (unfinished) conducting cell analyses show that water availability does not have large direct influence on tissue proportions or conduit sizes. An indirect effect of water availability through growth rate rather than a direct effect could be observed. An important implication of this result is that forest adaptation to changing environmental conditions might be less easy than when a direct climate response would exist for these traits.
- The results of our work also pleas for an integrative approach when addressing whole-tree performance under changing environmental conditions, i.e. strong conclusions should not be based on a selected group of traits only (for example leaf economic traits only) but instead at least incorporate a sufficient number of hydraulic traits.