Browsing by Person "Hein, Sebastian"

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Radial stem growth response of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) to severe drought depending on soil properties and rooting characteristics
(2024) Spangenberg, Göran; Hein, Sebastian
The increase in severe droughts and heat waves caused by climate change has impacted forests worldwide. One of these effects is the increase in drought-related damage to trees and tree dieback in many regions of the world. This became visible in Central Europe during and after the extreme drought of 2018. Not only severely damaged forests but many still existing forests must also be adapted to the changing climate. In this forest conversion, the adaptation of tree species composition to the changing climatic conditions, e.g., by planting other tree species, is an important measure. Drought tolerance is among the most important criteria for tree species selection. In this context, the planting of non-native tree species that are considered drought tolerant is an increasingly used silvicultural strategy. Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), which is native to North America, is an important substitute tree species with a higher drought tolerance, especially compared to Norway spruce (Picea abies (L.) Karst.), which has experienced severe drought damage in different regions of Europe. However, there has also been an increase in drought-related problems with Douglas-fir in some regions of Europe and North America. There is a need for research on the influence of site and rooting characteristics on the drought tolerance of this tree species. The objective of this dissertation was to investigate the influence of two soil properties (soil texture and plant-available water capacity) and two rooting characteristics (effective rooting depth and fine root density at a 0–40 cm soil depth) on the drought tolerance of Douglas-fir during the extreme drought year 2018. Different indicators derived from measurements of radial stem growth and stem radius changes were used to assess drought tolerance. Seven study sites were selected along a soil texture gradient from sand to clay. To investigate the influence of soil and rooting characteristics, other influencing factors on the drought tolerance of Douglas-fir were kept as consistent as possible. For this purpose, the study sites were selected in close proximity in southern Germany, at an elevation of approximately 500m a.s.l. Since the variety and provenance of Douglas-fir also influence drought tolerance, genetic origin analysis was carried out. As a result, all trees examined were clearly assigned to the coastal variety. They originated from the central area of the natural distribution range of the coastal variety. The soil physical and rooting characteristics were determined on a representative soil profile for each of the seven sites. In the investigations in Chapter II, cores were collected from Douglas-fir trees with an increment borer and tree ring widths were measured. The growth response indices of resistance, recovery, and resilience were derived from this as indicators of drought tolerance. These indices quantify the ability of trees to maintain stem growth during drought stress events and stem growth recovery after drought. In the study in Chapter III, stem radius changes were measured using dendrometers with high temporal and spatial resolution at the same study sites. Based on these data, the water status parameters maximum daily shrinkage and tree water deficit were derived for two pronounced drought periods in the extreme drought year 2018. In addition, growth onset and cessation for 2018 were estimated from dendrometer measurements and compared with the corresponding values for years with more precipitation, 2017 and 2019. Linear mixed-effects models were used to analyze the influence of soil and rooting characteristics on the indicators of drought tolerance and water status in Douglas-fir. The following overall conclusions can be drawn from the results of both studies. In general, Douglas-fir showed a higher drought tolerance on silty and sandy soils. On clay-dominated soils, a higher risk of drought stress is to be expected during very severe drought. A higher plant-available water capacity only reduced drought stress in Douglas-fir at the initial stage of the extreme drought in 2018. In the advanced stage of drought, this positive effect was no longer observed. A greater rooting depth reduced drought stress in the later stage of extreme drought and proved to be an influencing factor that improved drought tolerance in the analysis of the interannual radial growth. Douglas-fir trees only benefited from more intensive rooting in the top 40 cm of soil in the initial stage of extreme drought. In the later drought stage, no significant influence was detected. Based on measurement of the interannual radial stem growth and intra-annual stem radius changes, this drought stress study provides recommendations for Douglas-fir cultivation under the predicted climatic changes with an increased occurrence of severe drought periods. The combined application of the two measurement methods reduced method-specific uncertainties and answered additional research questions. There are comparatively few studies on the drought tolerance of trees that examine the relationships between soil properties, rooting characteristics, and aboveground tree responses to drought. This investigation shows the importance of such studies in understanding the response patterns of trees to very severe drought events.