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    Drought adaptation of Forests in Europe – Practical Strategies

    Drought adaptation of Forests in Europe

    Extreme droughts are expected to increase in frequency and severity and together with rising temperatures weaken trees, threatening forest ecosystems and the services provided. Extensive tree dieback and mortality related to drought have already been detected across the globe and require more effective strategies to adapt forests to such droughts and increasing forest resistance and resilience.

    With this second FoRISK webinar on the topic of drought, “Drought Adaptation of Forests in Europe – Practical Strategies,” FOREST EUROPE, in collaboration with the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), continues the journey to delve into measures we might take at regional, national or international level to support Europe’s forests to adapt to drought, to unravel the science behind these recommendations and provide practical guidance on the measures.

    Extreme drought event in Central Europe in 2018: Its impact on beech forests and their recovery

    The first presentation by Ansgar Kahmen, Professor of Physiological Plant Ecology and Head of the Department of Environmental Sciences at the University of Basel, Switzerland, focused on the results of his research on the extreme drought in Central Europe in 2018 and its devastating impact on forest trees. In particular, the vulnerability of European beech was discussed, but also whether recovery is possible. First, he explained that 2018 cannot be considered an isolated event, but is part of a trajectory towards a hotter and dryer climate. Six of the nine hottest summers of the past 120 years occurred in the last decade.

    Since the 1980s, an increasing evaporative demand in the atmosphere has been observed, accompanied by a progressive decline in tree health. In beech trees, increasing crown damage became visible, which became even more severe after the extreme years of 2003 and 2018. Measurements of hydraulic conductivity in 2018 showed that vascular transport in beech trees was reduced or interrupted by up to 80%, and had been causing canopy decline. The higher susceptibility of beech compared to oak or maple can be explained by the differences in the rooting depth of the tree species: beech has only a shallow rooting system and is the first to run out of water.

    But what about recovery? Severe symptoms of canopy defoliation were also present in the following year, in spring 2019, and thereafter. It was found that hydraulic conductivity had not been restored in symptomatic branches and that the amount of intact conducting tissue (or xylem area), directly relates with the foliage area of a branch. Drought is causing partial and irreversible loss of hydraulic conductivity, and without a fully developed canopy, the risk for tree mortality is increasing. He concluded his talk with an appeal to reason that, given the unceasing trend towards a new, warmer climate, we better study the acclimation and adaptation potential to future climate, rather than trying to understand a tree’s recovery potential to a past climate.

    How to choose climate-adapted tree species and compositions for future mixed forest stands

    The webinar continued with a presentation by Heidi Döbbeler, forest scientist at the Northwest German Forest Research Station (NW-FVA), working in the department of Forest Growth on the development of a decision support tool for hessian forest owners. The project “Maps for climate risk and forest development types as an improved consulting foundation for new challenges for Hessian forest owners” is part of the integrated climate protection plan and funded in collaboration with the state-owned forest and the German Forest Owners Association by the ministry of Hesse in Germany.

    In Hesse, a temperature rise from 15.4 to 17.3 °C is expected, which will increase evapotranspiration but also extend the growing season. Accompanied by a decrease in precipitation during summer, this will have a negative impact on the climatic water balance and, thus, on the water available for plants during the growing season. In addition to these regional climatic data, information on soil property is a prerequisite for calculating the site-specific water balance, which is an important indicator of drought stress for trees under future climatic conditions at a given forest site in Hesse.

    In a next step, tree species and their potential role in the future were categorized according to their tolerance to drought stress depending on the site water balance. The categories were defined as leading, mixed, accompanying, or excluded tree species. As an example, even if European beech will have problems in Hesse in the future and its distribution will be limited as a result, it will still remain part of the forest. Recommendations for forest development types (FDT) for mixed forests were compiled by intersecting ecograms of leading and mixed tree species. As an example, the oak-beech/hornbeam FDT consists of 80-60 % oak, 20-10 % beech, and 10% accompanying other tree species. 32 FDTs have been defined and include information on management practices, their socio-economic potential and closeness to nature. Every forest owner in Hesse is provided with site-specific recommendations and can freely access the information via a web portal or mobile application. Under the assumption that disturbances have a species or structure-specific effect, facing them by mixed, site-adapted, and structurally rich forests will reduce the susceptibility of forests and instead increase stability and resilience.

    Potential of thinning to increase drought tolerance

    Following was a presentation by Julia Schwarz, working at the Swiss Federal Research Institute as a researcher for silviculture and climate change and expert in the field of ecology and management of forest ecosystems. She provided insights into whether thinning can offer an adaptation strategy to drought. While long-term adaptation approaches aim at creating structurally and species-rich mixed forests in the future, short- to medium-term adaptation approaches must work with the existing forest stand and thinning might be a way of adaptation to drought stress.

    Thinning reduces the amount of trees per area and thereby promotes the growth and vitality of the remaining trees by reducing the competition for natural resources, such as light, water, and nutrients. Thinning regimes may vary in intensity, frequency, by the age of the trees and the type applied (e.g. crown thinning). The short-term impacts at the stand level have been found to range from positive effects on the water balance due to fewer trees and less leaf area, lower evapotranspiration and higher soil moisture to negative effects due to an increased understory growth, increasing solar radiation and higher soil evaporation. At tree level, trees have more space, increased fine root growth with no change in transpiration or increasing transpiration. In medium terms, trees have been found to increase their crowns, their overall root system, the water extraction capacity and their growth rates. At stand level this means that evapotranspiration is reduced after thinning only for a short period of time before it returns to pre-thinning levels.

    But can the advantages of thinned stands outweigh the disadvantages of un-thinned stands during drought? What are the effects on tree growth during and after drought? Clear answer: thinning can mitigate drought-induced growth decline and mortality due to higher soil water availability and water extraction capacity during moderate droughts, while trees in denser stands experience drought earlier and more extreme. But during an extreme event where soil water is depleted thinning will have no effect. Thinning effects are highly dependent on drought severity. She concluded her presentation with the recommendation for regular, moderate thinnings that will contribute to mitigating growth decline and mortality during moderate droughts.

    Pine and mistletoe– insights from Poland and Switzerland

    Concluding the presentations were Andreas Rigling, Professor of Forest Growth at ETH Zurich and the Swiss Federal Research Institute, with his presentation “The impact of mistletoe on Scots pine performance in a dryer future” together with Wojciech Kędziora, assistant professor at the Institute of Forest Sciences at Warsaw University of Life Sciences and his presentation “Problem with mistletoe in Poland.”

    Mistletoes are hemiparasitic plants, well known for using the host’s water supply for their own photosynthetic capacity. When growing on a thriving host plant, they can get as old as 30 years. In central Europe Scots pine is a known host tree. In the host tree, the infection causes a reduced growth rate of branches, and with increasing infection numbers, branch diameter, and length, but also needle length and age, have been found to be negatively affected until even crown architecture and assimilation capacity are affected.

    But what if a tree’s water balance is put under additional pressure by drought stress? While trees are able to reduce their water loss by closure of their stomata during a drought event, the mistletoes continue to transpire, thereby increasing the tree’s drought stress. With increasing infection numbers, differences in growth are exacerbated, causing defoliation, growth decline and a reduction in carbon assimilation up to 80%, and finally, increasing significantly the risk of tree mortality in combination with drought stress.

    On the positive side, mistletoes not only indirectly contribute to reshaping forests in structure and tree species composition in the long term but also directly improve soil nutrient availability due to their nutrient-rich litter.

    In Poland, mistletoe has only recently gained higher attention, and only after 2015-18 has it been perceived as a major problem. To assess the extent of the forests, ground-based monitoring programs were started, but were soon found to be greatly underestimating the actual infection numbers on trees. Most of the mistletoes are growing in the middle of the tree, which is not visible from the ground. The observation has been that mistletoes prefer older stands, stands with lower soil fertility, taller trees with sparse crowns, and more likely to appear at forest edges. There is great hope in the use of drones for inventory programs, but really, only a few management options are available. While pruning, thinning, or clear-cutting promise only limited success, long-term strategies will rely on rebuilding the forests with more diverse tree species compositions. Wider discussions between forest owners, forest industry, management, and public society, as well as cross-border communication, are needed.

    Cross-border communication and sharing of experiences is going to be the core business of a future FoRISK. This webinar gave an outlook on a future FoRISK in action that promotes cross-border exchange and cooperation. Here, you can find all the presentations shared during the webinar.

    The recording of the event can be found here: