Biology subjects

Ruehr, N. K.

Publications and source records attributed to Ruehr, N. K..

3 recordsLinked to original sources

bioRxiv2025

Soil and atmospheric drought trigger early leaf senescence and increase subsequent canopy mortality risk in temperate forests

Early leaf senescence - an advanced decline in leaf functionality before the typical autumn period - has emerged as recurring drought response in temperate forests. Yet its large-scale patterns, drivers, and impacts on tree vitality remain poorly understood. Using six years (2018-2023) of Sentinel-2 time series across Germany, we estimated the onset of leaf senescence in European beech (Fagus sylvatica) and oak (Quercus robur, Q. petraea) forests. Early leaf senescence was most widespread during exceptional drought years (2018, 2019, and 2022) and showed pronounced spatial variability. Sustained high atmospheric demand (VPD) and critically low soil water potential ({psi}soil) in summer were key drivers identified by logistic regression analysis (F1-score = 0.57), with beech responding more strongly to soil drought and oak to atmospheric drought. Thresholds triggering early leaf senescence (probability >50%) were six weeks of daily maximum VPD [≥]1.9 kPa in beech and [≥]2.1 kPa in oak, or two weeks of root zone {psi}soil [≤]-0.8 MPa and [≤]-0.9 MPa, respectively. Elevated spring VPD further amplified early senescence risk in both species, indicating seasonal legacy effects that may reduce hydraulic function or leaf longevity. Forests with very early and repeated early leaf senescence exhibited elevated canopy mortality in subsequent years, highlighting them as potential warning signals of drought-induced forest decline. K-means clustering of forest stands based on senescence patterns, mortality, and drought sensitivity classified three drought response types - resistant, sensitive, and vulnerable - shaped mainly by local soil water availability and stand structure. Our findings demonstrate that early leaf senescence reflects cumulative drought stress and indicates increased mortality risk, rather than a stress-avoidance strategy. We propose it as a mechanistic indicator that can be incorporated into ecosystem models to improve predictions of forest vulnerability and carbon dynamics under climate extremes, and guide forest management to anticipate climate-induced forest decline.

physiology↗

bioRxiv2024

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=A/gs) and leaf carbon isotope ratio ({delta}13C). We sampled 190 individuals including juvenile and mature trees belonging to 18 temperate broadleaved tree species and 9 genera. We measured guard cell length (GCL), stomatal density (SD), specific leaf area (SLA), gas-exchange, iWUE and leaf {delta}13C as a proxy for long-term WUE. Leaf {delta}13C correlated positively with iWUE for both juvenile and mature trees. Across species, GCL showed a negative and SD a positive effect on iWUE and leaf {delta}13C of both juvenile and mature trees. Within species, however, only GCL was significantly associated with iWUE and leaf {delta}13C. Pioneer species (Populus, Prunus, Betula) showed a significantly lower leaf {delta}13C than climax forest species (Fagus, Quercus, Tilia), but the differentiation was not clear for iWUE. We conclude that GCL and SD can be considered as functional morphological traits impacting the iWUE and leaf {delta}13C of trees, highlighting their potential for rapid phenotyping approaches in ecological studies.

plant biology↗

bioRxiv2021

Unrevealing water and carbon relations during and after heat and hot drought stress in Pinus sylvestris

Forests are increasingly affected by heatwaves, often co-occurring with drought, with consequences for water and carbon (C) cycling. However, our ability to project the resilience of trees to an intensification of hot droughts remains limited. Here, we used single tree cuvettes (n=18) allowing us to investigate transpiration (E), net assimilation (Anet), root respiration (Rroot) and stem diameter change in Scots pine seedlings during gradually intensifying heat or drought-heat stress (max. 42{degrees}C), and post-stress. Further, we assessed indicators of stress impacts and recovery capacities. Under heat stress, well-watered seedlings prevented overheating of leaves effectively via increased E, while under drought-heat leaf temperatures increased to 46{degrees}C. However, leaf electrolyte leakage was negligible, but Fv/Fm declined alongside Anet moderately in heat but strongly in drought-heat seedlings, in which respiration exceeded C uptake. Further, the decrease of needle water potential ({psi}Needle) to -2.7 MPa and relative needle water content (RWCNeedle) under drought-heat reflected a decline of leaf hydraulic conductance (KLeaf) by 90% and stem hydraulic conductivity (KS) by 25%. Alongside, we observed pronounced stem diameter shrinkage. Heat stress alone resulted in low functional impairment and all measured parameters recovered fast. In contrast, larger impacts following combined heat and drought led to the incomplete recovery of KLeaf and KS. Despite Anet tended to be reduced albeit Fv/Fm had recovered, the seedlings net C balance reached control values 2 d after stress release and stem growth rates exceeded control rates in the 2nd week post-stress. This indicates that a new equilibrium of C uptake and release was maintained at the tree level, slowly supporting regaining of stress-induced losses. In summary, we highlight that under moderate heatwaves with low functional impairment, recovery is fast in Scots pine, while in combination with drought hydraulic and thermal stress are intensified, resulting in functional damage and delayed recovery processes. The incomplete recovery of hydraulic conductance indicates limited water transport capacities that could become critical under repeated heat events.

physiology↗