Water consumption of beech, spruce and Douglas fir in pure and mixed stands in a wet and a dry year - Testing predictions of the iso/anisohydry concept.

A rising atmospheric vapour pressure deficit (VPD) increases forest transpiration and depletes soil moisture reserves, exposing trees to stress and reducing groundwater recharge. How stand water consumption varies with the species composition, is not well known, but is crucial for managing water resources. We measured stand-level transpiration of nearby pure European beech, Norway spruce and Douglas fir stands and a beech-Douglas fir mixture on deep sandy soil with sap flux systems during a wet and a dry year to compare the species' water use patterns under varying water availability and examine species mixing effects.

Linking stomatal size and density to water use efficiency and leaf carbon isotope ratio in juvenile and mature trees.

Water-use efficiency (WUE) is affected by multiple leaf traits, including stomatal morphology. However, the impact of stomatal morphology on WUE across different ontogenetic stages of tree species is not well-documented. Here, we investigated the relationship between stomatal morphology, intrinsic water-use efficiency (iWUE) and leaf carbon isotope ratio (δC).

Coordinated hydraulic traits influence the two phases of time to hydraulic failure in five temperate tree species differing in stomatal stringency.

Worldwide, forests are increasingly exposed to extreme droughts causing tree mortality. Because of the complex nature of the mechanisms involved, various traits have been linked to tree drought responses with contrasting results. This may be due to species-specific strategies in regulating water potential, a process that unfolds in two distinct phases: a first phase until stomatal closure, and a second phase until reaching lethal xylem hydraulic thresholds.

Addressing controversies in the xylem embolism resistance-vessel diameter relationship.

Although xylem embolism is a key process during drought-induced tree mortality, its relationship to wood anatomy remains debated. While the functional link between bordered pits and embolism resistance is known, there is no direct, mechanistic explanation for the traditional assumption that wider vessels are more vulnerable than narrow ones. We used data from 20 temperate broad-leaved tree species to study the inter- and intraspecific relationship of water potential at 50% loss of conductivity (P ) with hydraulically weighted vessel diameter (D ) and tested its link to pit membrane thickness (T ) and specific conductivity (K ) on species level.

Anatomical adjustments of the tree hydraulic pathway decrease canopy conductance under long-term elevated CO2.

The cause of reduced leaf-level transpiration under elevated CO2 remains largely elusive. Here, we assessed stomatal, hydraulic, and morphological adjustments in a long-term experiment on Aleppo pine (Pinus halepensis) seedlings germinated and grown for 22-40 months under elevated (eCO2; c. 860 ppm) or ambient (aCO2; c.

Mutually inclusive mechanisms of drought-induced tree mortality.

Unprecedented tree dieback across Central Europe caused by recent global change-type drought events highlights the need for a better mechanistic understanding of drought-induced tree mortality. Although numerous physiological risk factors have been identified, the importance of two principal mechanisms, hydraulic failure and carbon starvation, is still debated. It further remains largely unresolved how the local neighborhood composition affects individual mortality risk.

Soil water availability and branch age explain variability in xylem safety of European beech in Central Europe.

Xylem embolism resistance has been identified as a key trait with a causal relation to drought-induced tree mortality, but not much is known about its intra-specific trait variability (ITV) in dependence on environmental variation. We measured xylem safety and efficiency in 300 European beech (Fagus sylvatica L.) trees across 30 sites in Central Europe, covering a precipitation reduction from 886 to 522 mm year.

Rapid hydraulic collapse as cause of drought-induced mortality in conifers.

Understanding the vulnerability of trees to drought-induced mortality is key to predicting the fate of forests in a future climate with more frequent and intense droughts, although the underlying mechanisms are difficult to study in adult trees. Here, we explored the dynamic changes of water relations and limits of hydraulic function in dying adults of Norway spruce ( L.) during the progression of the record-breaking 2018 Central European drought.

Maximum-likelihood estimation of xylem vessel length distributions.

Vessel length is an important functional trait for plant hydraulics, because it determines the ratio of flow resistances posed by lumen and pit membranes and hence controls xylem hydraulic efficiency. The most commonly applied methods to estimate vessel lengths are based on the injection of silicon or paint into cut-off stem segments. The number of stained vessels in a series of cross-sections in increasing distance from the injection point is then counted.