Hydraulic traits and their relevance for water use strategies in five broad-leaved tree species of a temperate mixed forest

Without anthropogenic intervention, Central Europe would be covered by about 65% by beech-dominated broad-leaved forests. However, large forest areas have been replaced by coniferous monocultures since the early 19th century resulting in a reduction of tree species diversity and a loss in structural forest diversity, thereby increasing the vulnerability of forest stands to storms, snow, ice, drought and insect damage. Only recently, forestry, in an effort to restore biodiversity, aims at a conversion of pure coniferous forests into mixed stands. If the intensity of soil and atmospheric drought are increasing in the future, the abundance and vitality of tree species will depend on their capability to ensure water uptake. Trees have developed several mechanisms and strategies to overcome water shortage which are closely linked to morphological and physiological traits and thus vary with plant size, species and functional type. This thesis is focused on water relations and hydraulic traits of five co-occurring temperate tree species (Fagus sylvatica L., Acer pseudoplatanus L., Fraxinus excelsior L., Carpinus betulus L., and Tilia cordata Mill.) in the Hainich forest and took place within the framework of the Research Training Group (RTG) 1086 “The role of biodiversity for biogeochemical cycles and biotic interactions in temperate deciduous forests”. The studies were conducted on a research site in the Hainich forest where all five tree species are present with at least five mature tree individuals per species, reaching the upper canopy layer, are of comparable stem size and are accessible with a canopy lift. Starting in 2008, we examined root anatomical and hydraulic traits in the five tree species, analyzed the relation between root anatomy and hydraulic conductivity and investigated the relation of these traits to the species’ drought tolerance. In small-diameter roots (2-6 mm), we measured vessel diameters and vessel density, specific hydraulic conductivity, and the percental loss of conductivity (“native” embolism) during summer. Specific conductivity was positively related to vessel diameter but not to vessel density. Drought-tolerant Fraxinus showed the smallest mean vessel diameters and drought-sensitive Fagus the largest. Specific conductivity was highly variable among different similar-sized roots of the same species with a few roots apparently functioning as “high-conductivity roots”. The results show that coexisting tree species can differ largely in root hydraulic traits with more drought-sensitive trees apparently having larger mean vessel diameters in their roots than tolerant species. However, this difference was not related to the observed root conductivity losses due to embolism. In 2009, we concentrated on sap flux density and diurnal and seasonal stem radial variations of the five tree species. Sap flux density was recorded synchronously at five positions along the root-to-branch flow path of mature trees (roots, three stem positions, branches) with high temporal resolution (2 min) and related to stem radius measurements. The daily amount of stored stem water withdrawn for transpiration was estimated by comparing the integrated flow at stem base and stem top. Our results confirm that internal water stores are playing an important role in the four diffuse-porous species with 5-12 kg d-1 being withdrawn on average in 25-28 m tall trees representing 10-22 % of daily transpiration; in contrast, only 0.5 – 2.0 kg d-1 was withdrawn in ring-porous Fraxinus. Sapwood area size (diffuse- vs. ring-porous) had the largest influence on storage but wood density was also influential. Across the five species, the length of the time lag in flow at stem top and stem base was positively related to the size of stem storage. The stem stores were mostly exhausted when soil matrix potential dropped below -0.1 MPa and daily mean VPD exceeded 3-5 hPa. Different degrees of coupling seem to exist between branch, stem and root flows. We conclude that stem storage is an important factor improving the water balance of diffuse-porous temperate broad-leaved trees in moist periods while it is most likely of low relevance in dry periods and in ring-porous species. Measurements of stem radial variation were also used to generate radial stem growth rates in daily resolution which were analyzed in its response to environmental factors. Daily stem radius change (SRCd) was primarily influenced by the atmospheric demand (RH and VPD) while rainfall, soil matrix potential, temperature and radiation were only secondary factors. SRCd increased linearly with increasing RH and decreasing VPD in all species. The positive effect of a low atmospheric water vapor demand on SRCd was largest in June. We explain the strong positive effect of high RH and low VPD on radial stem increment by lowered transpiration which reduces negative pressure in the conducting system and increases turgor in the stem cambium cells, thereby favoring cell division and expansion. Our results suggest that climate change will impact on the growth performance of plants not only by reductions in soil water availability but also via increases in evaporative demand as it is predicted worldwide with increasing temperatures.