Linking soil hydrologic and tree transpiration dynamics by dendrometer measurements

Trees in forests and urban environments are increasingly under pressure due to extended periods of drought in central Europe. At the same time, their transpiration performance is all the more important to counteract drought and heat. Defining drought itself, as well as trees’ physiological response strategies, can be closely linked to soil hydrological conditions. In a three-year field experiment with young trees of different species planted in different substrates (n = 3 substrates * 9 species * 5 repetitions = 135 trees) in Northern Germany, we found that soil hydraulic properties strongly affected tree vitality, and that the species’ reactions towards unfavorable conditions differed significantly. This might be due to species-specific transpiration regulation strategies under drought stress. In a second step, we now link tree diameter fluctuations, which have been shown to closely correlate with transpiration, to soil water conditions. To this end, subsets of trees were equipped with dendrometers – 15 trees of one species (Quercus cerris) in three different substrates (sand, planting substrate, and loamy silt) in the growing season of 2020, and 21 trees of seven different species (Amelanchier lamarkii, Carpinus betulus ‘Lucas’, Geleditsia traconathos ‘Skyline’, Liquidambar styraciflua, Ostrya carpinifolia, Quercus palustris, Tilia cordata ‘Greenspire’) in sand substrate in the growing season of 2021. The dendrometer measurements were partly combined with soil water tension measurements (2020: n=6, 2021: n=10). By comparing statistical characteristics of the time series, we want to (1) link soil water and transpiration dynamics and (2) differentiate this link for different substrates and species. Such, tree stem diameter fluctuations, coupling soil conditions and physiological properties, might provide insight into the effect of tree strategic responses to soil drought.