Amplifying effects of recurrent drought on the dynamics of tree growth and water use in a subalpine forest
We conducted an in situ rain-exclusion experiment at a subalpine tree line forest in the Austrian Alps where we exposed the two widespread European tree species European larch (Larix decidua) and Norway spruce (Picea abies) to three years of recurrent summer drought. We studied tree growth and tree water relations using high-resolution measurements of stem diameter variations and xylem sap flow as an estimate of whole tree transpiration. We combined these measurements analyses of xylem anatomy, non-structural carbohydrates and their carbon-isotopic composition.
We found that pronounced negative effects of drought on tree growth and stem radius variations from the beginning of the experiment that were related to a sustained diurnal and periodic depletion of internal stem water storage. Most importantly, while drought effects on stem radius variations did not increase, the effects on growth strongly amplified under repeated drought. Furthermore, xylem sap flow and its response to its main driver vapor pressure deficit was not limited during the first two years of the experiment, but abruptly decreased from the beginning of the third drought. While both tree species strongly responded to drought, spruce showed more a pronounced increase in the negative effects of recurrent drought than larch, indicating a higher vulnerability of this species to repeated droughts.
Altogether, our study shows that recurrent droughts progressively increase the impacts of drought. A future climate with an increased drought frequency may therefore impose strong legacies on tree and forest functioning.
Stem radius variations for Larix decidua (a–c) and Picea abies (d–f) in the growing periods of the 3 study years 2016, 2017 and 2018. Lines and background shadings indicate mean ± 1 SE for stem radius variations (n = 6 trees per species and treatment). Colours denote treatment (blue = control, orange = drought). Background ribbons indicate periods when rainout shelters were closed.
Oberleitner, F., Hartmann, H., Hasibeder, R., Huang, J., Losso, A., Mayr, S., Oberhuber, W., Wieser, G., Bahn, M. (2022). Amplifying effects of recurrent drought on the dynamics of tree growth and water use in a subalpine forest. Plant, Cell and Environment 45: 2617-2635. DOI: https://doi.org/10.1111/pce.14369
Widespread greening suggests increased dry-season plant water availability in the Rio Santa valley, Peruvian Andes
Changing water availability is making climate change tangible and is often a great concern for affected societies. In rural settings of the global south where the livelihood of many people relies on subsistence-based agriculture, changes in seasonality of climate variables or increasing weather extremes can have far-reaching consequences including rural exodus and increase of poverty and vulnerability. The slopes of the Rio Santa basin, located in the Peruvian Andes, are home to many farmers whose livelihoods have already been increasingly challenged by economical and societal developments in the past decades. The local agriculture is mostly rain-fed and plant-growth is determined by the appearance of seasonal rainfalls which are occurring during an annual cycle of a distinct wet and dry season.
Recently, farmers reported that they are facing changed timing and intensities of seasonal rainfalls with detrimental effects on crop yields. These reports were not supported by analysis of historical meteorological records in previous research, but there remains considerable uncertainty regarding the quality of the data. To overcome this, we exploited 20 years of satellite-derived vegetation greenness data as a proxy for water availability. In comparison to the analysed rainfall products, these data are available in unpreceded spatio-temporal resolution and allowed us to gain new insights into recent trends and changes in the variability of plant available water.
We confirmed that no clear conclusion can be drawn from several rainfall datasets but we find a significant increase of plant greenness in the Rio Santa basin, particularly pronounced during the dry season. This indicates an overall increase of plant available water over the past two decades. In agreement to these greening patterns, we found a delayed end of the growing season which either implies a later retreat of the seasonal rainfalls or larger amounts of rainfall during the wet season feeding storages of the hydrological system. The start of the growing season, however, fluctuates highly from year to year with variation of up to two months, governing the overall growing season length. This variability is likely linked to the perception of local farmers, as it hampers the planning of sowing dates and overall complicates successful farming.
The ridgelines display the variability in timing of the growing season for each of the 20 growing seasons between 2000 and 2020. A smaller width of the distribution can be interpreted as plant growth in the Rio Santa basin being temporally more uniformly, while a larger width shows larger deviations of the timing. Additionally, the plot displays the large interannual variability of the start of the growing season and the increasingly delayed end of the growing season.
Hänchen, L., Klein, C., Maussion, F., Gurgiser, W., Calanca, P., & Wohlfahrt, G. (2022). Widespread greening suggests increased dry-season plant water availability in the Rio Santa valley, Peruvian Andes. Earth System Dynamics 13(1): 595-611. DOI: https://doi.org/10.5194/esd-13-595-2022
Photophysiological investigations of the temperature stress responses of Zygnema spp (Zygnematophyceae) from subpolar and polar habitats (Iceland, Svalbard)
Climate change in the form of elevated temperatures is predicted to affect areas in high latitudes especially. The Arctic even bears the unfortunate title of the fastest-warming area worldwide. An increase of only a few degrees can severely affect the flora in polar and subpolar regions. The already harsh environmental conditions of these habitats, result in a flora containing only a low amount of vascular plants. Zygnematophycea, a class of freshwater green algae, which are well adapted to these conditions are one of the main primary producers in polar hydro-terrestrial habitats. Studying their adaptation strategies and response to increased temperatures is vital in predicting the future of these ecosystems.
In the present study, we exposed two field sampled strains of Zygnema, originating from a subpolar (Iceland) and a polar (Svalbard) island, to experimentally elevated temperatures of up to 25°C. To quantify their responses, we measured a variety of different photo-physiological parameters and their phenolic content. Our results show, that both Zygnema strains were not able to adapt to the elevated temperatures, as for example, their photosynthetic performance decreased with increased temperatures. This suggests a low-temperature adaptation and a high sensitivity of the photosynthetic apparatus to higher temperatures for (sub-) polar Zygnematophhycae.
Overall, our study illustrates the negative effect of temperature increase on Zygnematophyceae and highlights the possible severity of global warming. As global change proceeds at an alarming rate, such studies are highly relevant for predicting the future development of plant communities in these ecosystems.
Zygnema strains were sampled from Iceland and Svalbard (with picture from the sampling site in Iceland). The algal strains were not able to adapt to elevated temperatures and showed a decrease in photosynthetic activity, suggesting a low-temperature adaptation.
Permann, C., Pierangelini, M., Remias, D., Lewis, L.A., Holzinger, A. (2022). Photophysiological investigations of the temperature stress responses of Zygnema spp (Zygnematophyceae) from subpolar and polar habitats (Iceland, Svalbard). PHYCOLOGIA, 61(3): 299–311. DOI: https://doi.org/10.1080/00318884.2022.2043089