Wind drives nocturnal, but not diurnal, transpiration in Leucospermum conocarpodendron trees: implications for stilling on the Cape Peninsula

Comparison of environmental controls on daily actual evapotranspiration dynamics among different terrestrial ecosystems in China

Terrestrial actual evapotranspiration (ET_a) is a key state variable modulating land surface processes; however, it remains challenging to recognize how daily ET_a responses to environmental variables vary with ecosystems, which is relevant for understanding various ecosystem processes. To this end, comprehensive datasets were synthesized for 22 field sites equipped with eddy covariance towers from the ChinaFLUX network and the National Tibetan Plateau Data Center, which included four different ecosystems across China (e.g., barren or sparsely vegetated-BAR, grassland, forest, and cropland ecosystems). The impacts of environmental variables on daily ET_a variations at each field site were assessed by the boosted regression tree (BRT) method. Overall, the BRT results showed that invariant of ecosystem types, net radiation (Rn) was the primary control on daily ET_a dynamics with an average contribution of 62.9 % for all the sites, followed by leaf area index-LAI (18.4 %), vapor pressure deficit (8.2 %), soil water content-SWC (7.4 %), and wind speed (3.1 %). More importantly, it was revealed that the interactions of daily ET_a with energy (Rn) and water (SWC) supplies could be significantly modified by vegetation through various regulation mechanisms. Specifically, for the BAR and grassland ecosystems, the influences of vegetation on daily ET_a highly depended on local land cover and hydrological conditions (e.g., groundwater). For the forest ecosystem, the contrasting impacts of LAI between evergreen and temperate forests were largely attributed to the differences in LAI dynamics over growing seasons, while physiological differences in crops affected the daily ET_a responses to LAI variations at cropland sites. This study underscored the varying interactions of daily ET_a with Rn and SWC, which could be further shaped by vegetation through a variety of regulation mechanisms across different ecosystems.

Anthropogenic changes to the nighttime environment

How the relative impacts of anthropogenic pressures on the natural environment vary between different taxonomic groups, habitats, and geographic regions is increasingly well established. By contrast, the times of day at which those pressures are most forcefully exerted or have greatest influence are not well understood. The impact on the nighttime environment bears particular scrutiny, given that for practical reasons (e.g., researchers themselves belong to a diurnal species), most studies on the impacts of anthropogenic pressures are conducted during the daytime on organisms that are predominantly day active or in ways that do not differentiate between daytime and nighttime. In the present article, we synthesize the current state of knowledge of impacts of anthropogenic pressures on the nighttime environment, highlighting key findings and examples. The evidence available suggests that the nighttime environment is under intense stress across increasing areas of the world, especially from nighttime pollution, climate change, and overexploitation of resources.

Consistent responses to moisture stress despite diverse growth forms within mountain fynbos communities

Understanding climate change impacts on the Cape Floristic Region requires improved knowledge of plant physiological responses to the environment. Studies examining physiological responses of mountain fynbos have consisted of campaign-based measurements, capturing snapshots of plant water relations and photosynthesis. We examine conclusions drawn from prior studies by tracking in situ physiological responses of three species, representing dominant growth forms (proteoid, ericoid, restioid), over 2 years using miniature continuous sap flow technology, long-term observations of leaf/culm water potential and gas exchange, and xylem vulnerability to embolism. We observed considerable inter-specific variation in the timing and extent of seasonal declines in productivity. Shallow-rooted Erica monsoniana exhibited steep within-season declines in sap flow and water potentials, and pronounced inter-annual variability in total daily sap flux (J_s). Protea repens showed steady reductions in J_s across both years, despite deeper roots and less negative water potentials. Cannomois congesta-a shallow-rooted restioid-was least negatively impacted. Following rehydrating rain at the end of summer, gas exchange recovery was lower in the drier year compared with the normal year, but did not differ between species. Loss of function in the drier year was partially accounted for by loss of xylem transport capacity in Erica and Cannomois, but not Protea. Hitherto unseen water use patterns, including inter-annual variability of gas exchange associated with contrasting water uptake properties, reveal that species use different mechanisms to cope with summer dry periods. Revealing physiological responses of key growth forms enhances predictions of plant function within mountain fynbos under future conditions.