The 10th International Conference on Agricultural and Biological Sciences
(ABS 2024)

Abstract: Forest ecosystems have a key role in climate change mitigation, and they provide a wide spectrum of ecosystem services. Meanwhile, climate change will have significant consequences for growth of trees and functioning of forest ecosystems with a possible impact on the economic value of forestland. Scenarios for future climate predict a rise in precipitation and humidity at high latitudes including Northern Europe and the Baltic region. To predict the responses of forest ecosystems to climate change, we investigated the effect of increased air relative humidity (RH) on the growth and functioning of two economically important deciduous tree species – Betula pendula and Populus × wettsteinii – at the Free Air Humidity Manipulation (FAHM) facility in Estonia. The FAHM system enables RH to be increased over the ambient level during mist fumigation, which alters water flux through the canopy of deciduous forest stand. The results are obtained during 10 years of humidity manipulation. Several structural and functional responses occurred as a consequence of the humidification manipulation in both above- and belowground tree parts. Transpirational flux through the trees declined, resulting in reduced nutrient uptake, photosynthetic capacity and growth rate. Competitively dominant B. pendula trees showed faster post-stress growth recovery, and size-growth relations in P × wettsteinii stands indicated more intensive root competition in humidified stands. Also, changes in tree biomass allocation occurred: the ratio of non-photosynthetic to photosynthetic tissues increased leading to larger maintenance costs, leaving less resources for growth. The humidified birches changed the biomass and morphology of the physiologically most active tree parts - increased fine-root biomass (per tree biomass and per 1m2), as well as specific areas of fine roots (SRA) and leaves (SLA). After birches had adapted their biomass allocation, their initially decreased biomass production recovered. At the stand level, biomass allocation to the belowground part of the understory increased at first (after two years of humidification treatment) more than two times, resulting in altered forest carbon balance – young stands in the humidification treatment became carbon sinks compared to the control stands that were carbon neutral. However, after four years of humidification, the reaction disappeared, while plant production did not differ anymore between treatments. Diverse changes in the hydraulic architecture of trees occurred, whereas plant hydraulic efficiency decreased, resulting in a greater risk of hydraulic dysfunction under drought conditions. Changes in leaf secondary metabolism and chemical composition of wood occurred, indicating a typical stress response of plants. Since 2020, FAHM experiment includes Picea abies as a new test species, planted in mono- and mixed-species (with B. pendula) sub-plots. Further studies on the effect of elevated air humidity on forest ecosystems are needed during a longer period in order to ascertain whether these results show a long-term acclimation trend or a short-duration temporary response.

Biography: Dr. Eele Õunapuu-Pikas was granted a PhD degree in the field of plant ecology and ecophysiology at the University of Tartu, Estonia, in 2014. Her doctoral thesis addressed the ecophysiological consequences of spatial and temporal variability of leaf hydraulic conductance in woody plants. Currently, she is working as a research fellow at the Department of Botany, Institute of Ecology and Earth Sciences, University of Tartu. Her major is Plant Science and Climate Change with specific emphasis on plant ecophysiology and water relations. She belongs to a research group investigating forest ecosystem acclimation to increasing atmospheric humidity and interacting environmental drivers. In addition, she has expertise in other fields of plant ecology, for example experimental work with vesicular-arbuscular mycorrhiza and using the method of fluctuating asymmetry in assessment of tree vitality in relation to environmental stress factors.