The impact of water-soluble environmental aerosols on the drought tolerance of forest trees

Forest trees in Germany and around the world are showing increasing drought damage symptoms and higher mortality rates due to climate change. Trees suffer direct damage from drought periods and heat waves, and forest ecosystems are becoming less resilient, which increases the damage caused by biotic and abiotic factors. Hygroscopic aerosols could be another largely overlooked factor increasing drought stress in plants. Since the Industrial Revolution, aerosol pollution has increased significantly. Some of these aerosols are hygroscopic salts, that can draw moisture from the air and liquefy as a result (deliquescence). The HAS hypothesis (HAS: hydraulically activated stomata; Burkhardt, 2010) is a mechanistic model explaining the connection between hygroscopic salts and drought stress. According to the hypothesis, hygroscopic salts can activate stomata by forming a connection between the substomatal cavity and the leaf surface. Through this connection, liquid water can be transported in a bidirectional flow. This can lead to increased, uncontrollable water loss via the stomata. <br> The present study investigated the influence of deposited hygroscopic aerosol particles on the drought tolerance of forest trees. In this German-wide field study, leaves and needles of <em>Fagus sylvatica</em> (European beech), <em>Quercus petraea</em> (sessile oak), <em>Pinus sylvestris</em> (Scots pine), and <em>Picea abies</em> (Norway spruce) were sampled over a period of three years. Minimum epidermal conductance (g_min) and the water potential at turgor loss point (π_TLP) were measured as indicators of drought tolerance. To determine hygroscopic aerosol deposition, the leaves and needles were washed, and the chemical composition of the wash solution was analyzed. The nutritional status of the leaves and needles and their nitrogen and carbon isotopic ratios were analyzed. The hydrophobicity of the surfaces was determined using contact angles. A greenhouse experiment compared needle and leaf surfaces with and without aerosols using electron microscopy and wax analysis. <br> The main novelty of this study is that significant correlations were found between the concentration of the washable ions and the drought tolerance parameters for each tree species, despite highly variable conditions. The existence of these significant correlations makes causal relationships likely, as they were established based on a mechanistic hypothesis and a targeted experimental approach. <br> In <em>P. sylvestris</em>, g_min and sodium were positively and significantly correlated. This supports the study's main hypothesis that aerosol deposition hydraulically activates stomata, thereby increasing g_min and reducing drought tolerance. This result corresponds with the filter approach to canopy interaction by Ulrich (1983), in which sodium does not interact with the leaf surfaces, allowing dry deposition to be inferred. For <em>P. sylvestris</em>, Ulrich's approach can also be used to explain the buffering of H₃O⁺ by the base cations. <br> For the other tree species, a large number significant correlations between washable ions and g_min as well as π_TLP were also recorded. These results are largely consistent with the HAS hypothesis, but in some cases new perspectives or assumptions regarding canopy interaction are necessary. Over the course of the year, the correlations demonstrate that HAS develops faster in <em>Q. petraea</em> than in <em>P. sylvestris</em>. Increased HAS and thus higher g_min then leads to more effective ion uptake, which also explains the significant negative correlations between the g_min and the ions. A rapid build-up of HAS can be advantageous for trees, as it enables them to absorb both nutrients and water through their stomata. The potentially functionally adapted leaf surfaces of tree species, combined with increasing atmospheric drought due to climate change and insufficiently strong reductions in air pollution, pose a drought risk for forests. The entire field of aerosol-plant interactions, should be investigated further, as it is important for both the nutrient and the water balance of trees.