Impact of drought on European Beech (Fagus sylvatica) trees at dry temperate forest

Abstract: Background and aim:
Drought will become severe and more frequent in the temperate forests owing to increase in temperature, decrease or shift in the precipitation. This phenomenon obliged researchers to assess the health and ecology of forest trees in central Europe; particularly at their distribution limit where stress could be more severe due to the shortage of soil water or edaphic drought. This study aims to look at the ecological behaviour and survival strategies of European beech (Fagus sylvatica L.) trees at their natural distribution limit in dry forest stands located in Germany and Switzerland. The following questions were addressed to achieve the aim: 1) Is the above ground biomass differing at dry sites compared to the productive sites? 2) How is ‘vitality’ affected by differing combinations of environmental factors under edaphic drought? 3) Is there any survivability threshold for the trees? 4) How is growth sensitivity differing in respect to edaphic and climatic drought? 5) What are the physiological mechanisms opted by the trees in the dry forests under climatic drought?
Methods:
The morphological and physiological responses of the trees to drought stress in their natural growing environment were measured. Together with that, the influence of extreme climatic drought on the trees was studied. Different environmental factors like available soil water storage capacity (AWC), soil pH and bulk density, availability of canopy light, intra- and inter-specific neighbourhood interactions, tree species and structural diversity were measured for the beech trees, to find out the influences of those factors on the crown die-back of the trees.
Trees’ morphological reaction to stress was measured by the above ground biomass, looking particularly on living and dead biomass of a standing tree. Next, the amount of crown die-back was quantified for each tree by preparing an allometric model incorporating the data of the living and dead parts of the crown that was collected during the field surveys. Furthermore, this tree specific crown die-back was analysed by using regression function of multiple abiotic and biotic variables as mentioned above.
The annual growth was measured in the tree rings of the collected discs at 1.3m stem height from the felled trees. The chronologies of last 50 years were prepared for each group of the trees based on the AWC values of the inventory plots and categorised into “dry” and “less-dry” plots. The trend in climate was modelled for the last 65 years, based on the climatic data gathered from the nearest climate stations, and stand specific drought indexes for last 65 years were calculated. Temperature, precipitation, and the drought index were used as the climatic variables in the analysis. The growth sensitivity was assessed based on the rate of annual growth and the variation in growth. The climate to annual growth relationship and the resilience of the trees to the events of recurrent droughts were assessed and compared between dry and less-dry plots. The resilience was measured by quantifying the values of resistance and recovery derived from the empirical index (i.e. the Lloret’s index), for the six recurrent drought years.
Finally, the physiological mechanism was measured for the trees by using the values of stable carbon (δ13C) and oxygen (δ18O) isotope ratios in the wood materials of the tree rings. The intrinsic water use efficiency (WUEi) was calculated based on the δ13C values. The δ13C, δ18O, and WUEi values were again compared for the trees from the dry and the less-dry plots. The climatic dependency was assessed looking on to the relation between the climatic parameters and the δ13C, δ18O and WUEi values. The physiological mechanisms opted by the trees under the drought condition was assessed based on a dual isotope model (i.e. Scheidegger’s model), by combining the δ13C and δ18O values, and compared between the dry and less-dry plots.
Results:
The living above ground biomass of the beech trees from the dry sites was lower compared to the biomass of the trees from the productive and well-drained sites. The plants are growing on dry forest stands are stunted due to getting less soil available water.
The overall crown die-back of beech trees is decreased with the increase of AWC, and light availability. Tree species diversity and intra-specific interaction had a negative influence on the overall crown die-back. An assessment of upper crown die-back showed the high negative influence of AWC, light, intra-, and inter-specific interaction, and tree species diversity. This means with the increase of AWC and light availability; the crown die-back is decreasing. Tree species diversity and neighbouring plant interactions had played a positive or facilitating role to reduce crown die-back. As the upper crown die-back is directly linked with the drought stressed mortality or branch cavitation, these factors could be functionally related to the drought stress submitted on the trees. It means that the trees are growing on low AWC and low light availability, have a higher risk of mortality. However, this could be mitigated by the higher species diversity and neighbouring interactions. Interestingly, a survivability threshold based on the crown die-back (58% crown die-back) was found for the beech trees growing in the dry sites. The beech trees die after crossing this threshold in the studied forest stands.
The growth sensitivity was found higher in the trees from dry plots. Those trees had lower growth rate and higher growth variation compared to the trees from less-dry plots. This explain the higher magnitude of submitted stress on the trees from dry plots. The climate–growth dependency was found greater in the trees from dry plots. The recovery and resilience for last three drought years were higher in the trees from less-dry plots. Whereas the resistance was found greater in the dry plots, but in only during the extreme drought year of 2003.
The isotopic data assessment shows that the beech trees of the same population growing in the drought-stressed forests have a variation for their physiological behaviour. Trees in dry plots had higher δ13C, WUEi and δ18O values compared to the less-dry plots, which shows that the trees in dry plots are facing higher amount of water stress during the growing period. Similar to the growth analysis by tree rings, the isotopic signals also showed higher climatic dependency in the trees on dry plots than in the less-dry plots, which physiologically support the drought-driven mortality risk of the trees grown in the dry plots. Interestingly, different stomatal regulations were found for the trees of dry and less-dry plots. Trees growing on drier plots had shown isohydric behaviour with stomatal resistance and decreasing photosynthetic capacity. The opposite anisohydric trend was found in the trees growing on less-dry plots with increasing stomatal conductance and photosynthetic capacity. It means that the trees from dry plots are more sensitive to drought, and the trees from less-dry plots are more tolerant to drought. However, during the extreme drought year like 2003, trees from both dry and less-dry plots are showing stomatal resistance by opting isohydric behaviour. It signifies that the beech trees could alter their stomatal regulation depending on the drought stress submitted at their natural environmental growing condition.
Conclusions:
The beech trees growing on dry temperate forests have lower biomass due to stunted growth compared to the well-drained and productive forest stands. Soil available water and the light availability are the important stress factors in the dry forests influencing the overall health of the trees. The diverse mixed forests in dry areas could mitigate the impact of drought by reducing the crown die-back. Beech trees have a survivability threshold regarding the magnitude of the crown die-back, when grown under the stress of water. The growth sensitivity of the trees differs based on the available soil water. The trees growing in areas with contrasting AWC have alternative physiological mechanisms and stomatal behaviour to survive from the growth stress. They even could alter the stomatal behaviour within their lifetime depending on the magnitude of the stress. This Ph.D. study will enrich the knowledge on the ecology and the survival strategies of the beech trees in the dry forest sites under the climatic drought. The population of naturally growing beech trees in dry forest areas should be conserved as these stands could act as a source of seeds that might be more resistant to drought

Object Details

• Location: Deutsche Nationalbibliothek Frankfurt am Main
• Extent: Online-Ressource
• Language: Englisch
• Notes: Universität Freiburg, Dissertation, 2016

Classification and Topics

• Keyword: Rotbuche ; Waldökosystem ; Trockenheit ; Buche ; Baum ; Europa

Contributors, Places and Time

• Event: Veröffentlichung
• (where): Freiburg
• (who): Universität
• (when): 2024
• Creator: Chakraborty, Tamalika
• Contributor: Reif, Albert ; Matzarakis, Andreas ; Werner, Christiane ; Albert-Ludwigs-Universität Freiburg. Fakultät für Umwelt und Natürliche Ressourcen

Further information

• DOI: 10.6094/UNIFR/246597
• URN: urn:nbn:de:bsz:25-freidok-2465978
• Rights: Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.
• Last update: 25.03.2025, 1:52 PM CET