Leuschner, C. | Search bioRxiv

bioRxiv2025

Isohydric species show earlier drought-induced declines in stem water content, rehydration, sap flow, and growth than anisohydric species

O_LIIsohydric species reduce water potential fluctuations through more stringent stomatal regulation. It is unclear whether isohydric behavior leads to smaller reversible diurnal stem shrinkage (i.e. greater nocturnal stem rehydration) during drought and a lower drought sensitivity of radial growth compared to anisohydric behavior. C_LIO_LIWith synchronous high-resolution sap flow and dendrometer measurements in mature anisohydric European beech and isohydric Douglas fir trees in pure and mixed stands, we quantified declines in stem water content (SWC), stem rehydration, sap flow and radial growth during soil dry-down and determined the critical soil moisture levels (expressed as Relative Extractable Water, REW) and elapsed desiccation time until 30-90% reductions in these traits. C_LIO_LISap flow and growth started to decline in both species at REW [~]0.6, preceding declines in stem rehydration. Water-spending Douglas fir approached 50% drops in SWC, sap flow, stem rehydration and growth during soil dry-down faster than beech, indicating higher drought sensitivity. In mixture, both species reached these reduction levels later than in monoculture, suggesting positive mixing effects on the species drought resistance. C_LIO_LIOur findings demonstrate that SWC, sap flow and radial growth decrease earlier than nocturnal stem rehydration, with isohydric and anisohydric species exhibiting different timelines of physiological downregulation during soil dry-down. C_LI

physiology↗

bioRxiv2017

The Kobresia pygmaea ecosystem of the Tibetan highlands – origin, functioning and degradation of the world’s largest pastoral alpine ecosystem

Kobresia pastures in the eastern Tibetan highlands occupy 450000 km2 and form the worlds largest pastoral alpine ecosystem. The main constituent is an endemic dwarf sedge, Kobresia pygmaea, which forms a lawn with a durable turf cover anchored by a felty root mat, and occurs from 3000 m to nearly 6000 m a.s.l. The existence and functioning of this unique ecosystem and its turf cover have not yet been explained against a backdrop of natural and anthropogenic factors, and thus its origin, drivers, vulnerability or resilience remain largely unknown. Here we present a review on ecosystem diversity, reproduction and ecology of the key species, pasture health, cycles of carbon (C), water and nutrients, and on the paleo-environment. The methods employed include molecular analysis, grazing exclusion, measurements with micro-lysimeters and gas exchange chambers, 13C and 15N labelling, eddy-covariance flux measurements, remote sensing and atmospheric modelling.\n\nThe following combination of traits makes Kobresia pygmaea resilient and highly competitive: dwarf habit, predominantly below-ground allocation of photo assimilates, mixed reproduction strategy with both seed production and clonal growth, and high genetic diversity. Growth of Kobresia pastures is co-limited by low rainfall during the short growing season and livestock-mediated nutrient withdrawal. Overstocking has caused pasture degradation and soil deterioration, yet the extent remains debated. In addition, we newly describe natural autocyclic processes of turf erosion initiated through polygonal cracking of the turf cover, and accelerated by soil-dwelling endemic small mammals. The major consequences of the deterioration of the vegetation cover and its turf include: (1) the release of large amounts of C and nutrients and (2) earlier diurnal formation of clouds resulting in (3) decreased surface temperatures with (4) likely consequences for atmospheric circulation on large regional and, possibly global, scales.\n\nPaleo-environmental reconstruction, in conjunction with grazing experiments, suggests that the present grazing lawns of Kobresia pygmaea are synanthropic and may have existed since the onset of pastoralism. The traditional migratory rangeland management was sustainable over millennia and possibly still offers the best strategy to conserve, and possibly increase, the C stocks in the Kobresia turf, as well as its importance for climate regulation.

ecology↗