Tree stomatal regulation and water use in a changing climate - From tropical to boreal ecosystems
Abstract
Rising levels of atmospheric carbon dioxide concentration ([CO2]) and temperature have the
potential to alter stomatal behavior and tree water use, which has implications for forest hydrology
and climate. Many models assume decreases in stomatal conductance (gs) and plant water use under
rising [CO2], which has been invoked as the causes for the positive global trend in river runoff over
the past century. Plant water use is, however, also affected by changes in temperature, precipitation,
land use and management and climate change-induced alterations in ecosystem structure. Still, there
is no consensus about the contribution of different drivers to temporal trends of evapotranspiration
(ET) and river runoff. There is great variation in stomatal and photosynthetic responses to [CO2] and
temperature among plant species, and the factors controlling it are still poorly understood, in
particular for boreal and tropical tree species.
This thesis investigated the effects of elevated [CO2] and temperature on the stomatal functioning,
tree hydraulics, canopy leaf area and whole-tree water use of mature Picea abies and young
Eucalyptus globulus trees grown in whole-tree chambers in boreal and temperate areas, respectively.
In the boreal study, the tree-level experiment was complemented with data on historical trends and
patterns in ET of large-scale boreal landscapes, using climate and runoff data from the past 50 years,
in order to assess water-use responses to past climate change in Swedish boreal forests. The thesis
also explored the temperature responses of photosynthesis as well as the taxonomic and functional
controls of the large interspecific variation in stomatal CO2 responsiveness and photosynthetic
capacity in a broad range of tropical woody species.
Results demonstrated that neither mature P. abies nor young E. globulus saved water under
elevated [CO2], and that warming did not increase their transpiration as decreased gs cancelled the
effect of higher vapour pressure deficit in warmed air. Also, Swedish boreal ET increased over the
past 50 years while runoff did not significantly change, with the increase in ET being related to
increasing precipitation and forest standing biomass over time. In E. globulus, neither elevated [CO2]
nor warming treatment affected gs, stomatal density or length, or leaf area-specific plant hydraulic
conductance. Furthermore, elevated [CO2] increased both total canopy leaf area and tree water use,
while warming did not have any significant influence on either of these variables. In the tropical
studies, the optimum temperature for the maximum rate of photosynthetic electron transport (Jmax)
was lower in the native than in the exotic species. The daytime peak leaf temperatures greatly
exceeded (by up to 10 °C) the photosynthetic optimum temperatures, in particular in the native
montane rainforest species. Lastly, all studied plant taxonomic groups exhibited stomatal closure
responses to increased [CO2], but none of the functional characteristics investigated could explain the
variation in stomatal CO2 responses among tropical woody species. The interspecific variation in
photosynthetic capacity was related to within leaf nitrogen allocation rather than to area-based total
leaf nutrient content.
The findings of this thesis have important implications for the projections of future water use of
forests, showing that changes in tree structural responses (e.g. size, canopy leaf area and hydraulics)
are more important than the effects of elevated [CO2] or warming on leaf transpiration rates. The lack
of reductions in gs under elevated [CO2] in P. abies and E. globulus conflicts with the present
expectation and model assumption of substantial leaf-level water savings under rising CO2. In the
tropical biome, the evidence of pronounced negative effects of high temperature on the
photosynthesis of native montane tree species indicates high susceptibility of these ecosystems to
global warming. Furthermore, the results on stomatal and photosynthetic responses in a broad range
of tropical species contribute with important data for this comparatively poorly researched biome.
Parts of work
Hasper TB, Wallin G, Lamba S, Hall M, Jaramillo F, Laudon H, Linder S, Medhurst J,
Sigurdsson B, Räntfors M, Uddling J (2015) Water use by Swedish boreal forests in a
changing climate. Functional Ecology, ::doi:: 10.1111/1365-2435.12546 Hasper TB, Barton CVM, Crous KY, Quentin AG, Ellsworth DS, Uddling J. Stomatal
and water-use responses of Eucalyptus globulus to elevated CO2 and warming.
Manuscript. Vårhammar A, Wallin G, McLean CM, Dusenge EM, Medlyn BE, Hasper TB,
Nsabimana D, Uddling J (2015) Photosynthetic temperature responses of tree species in
Rwanda: evidence of pronounced negative effects of high temperature in montane
rainforest climax species. New Phytologist, 206, 1000-1012. ::doi::10.1111/nph.13291 Hasper TB, Dusenge EM, Breuer F, Uwizeye FK, Wallin G, Uddling J. Stomatal CO2
responsiveness and photosynthetic capacity of tropical woody species in relation to
phylogeny and functional traits. Submitted to Oecologia.
Degree
Doctor of Philosophy
University
University of Gothenburg. Faculty of Science
Institution
Department of Biological and Environmental Sciences ; Institutionen för biologi och miljövetenskap
Disputation
Fredagen den 4 december 2015, kl. 10.00, Hörsalen, Botan, Carl Skottsbergs gata 22B, Göteborg.
Date of defence
2015-12-04
thomas.hasper@bioenv.gu.se
thomas.hasper@gmail.com
Date
2015-11-12Author
Berg Hasper, Thomas
Keywords
Climate change
Carbon dioxide
Temperature
Transpiration
Water use
Stomata
Tropical
Temperate
Boreal
Tree
Publication type
Doctoral thesis
ISBN
978-91-85529-88-9
978-91-85529-84-1
Language
eng