Doctoral defence: Meeli Alber “Impact of elevated atmospheric humidity on the structure of the water transport pathway in deciduous trees”

On 20 June at 10:00 Meeli Alber will defend her doctoral thesis “Impact of elevated atmospheric humidity on the structure of the water transport pathway in deciduous trees” for obtaining the degree of Doctor of Philosophy (in Plant Ecology and Ecophysiology).

Supervisor:
Professor Arne Sellin, University of Tartu

Opponent:
Associate Professor Vít Gloser, Masaryk University (Czech Republic)

Summary
Plants need water because it is essential resource in order to keep up photosynthesis, grow and survive. Water moves in plants along vascular tissue – xylem, which consists of vessels and tracheids and forms the largest part of plants' water transport pathway. Various environmental factors may affect the structure of water transport system and consequently its water conducting capacity. In northern Europe amount and frequency of precipitation is predicted to rise during this century. This brings along increase in relative air humidity on local or regional scales. In the thesis, I studied how increasing relative air humidity affects structure of xylem and stomata, which influence efficiency of water transport. The data for doctoral thesis was collected from Free Air Humidity Manipulation experimental site situated at Rõka village, eastern Estonia, where two deciduous tree species  silver birch (Betula pendula Roth) and hybrid aspen (Populus tremula L. × P. tremuloides Michx.)  were studied. I found that increased air humidity causes changes in anatomical structure of stems and leaves of broadleaved trees that leads to lower hydraulic efficiency of their water transport system. Trees grown in conditions of elevated air humidity experience reduced transpirational water flux and thus there is no strong environmental pressure for the development of efficient water transport system. The reduced hydraulic efficiency may limit gas exchange and growth rate of trees. Despite of some slight changes in xylem traits, generally wood anatomical structure of studied broadleaves tree species is conservative with respect to climate trends. Xylem susceptibility to cavitation was not affected by humidification treatment in hybrid aspen. Thus, the functional vulnerability of stem-wood does not represent a potential threat to hybrid aspen in the light of long-term regional climate trends. Elevated air humidity caused increase in the share of ray parenchyma in hybrid aspen stems that enhances respiration costs leaving less resource to sustain growth processes. Increased air humidity induced changes in biomass allocation in favour of vascular tissues, which can be treated as a mechanism compensating for reduced hydraulic efficiency of stem-wood.