Human societies depend on an Earth system that operates within a constrained range of nutrient availability, yet the recent trajectory of terrestrial nitrogen (N) availability is uncertain. Examining patterns of foliar N concentrations and isotope ratios (delta N-15) from more than 43,000 samples acquired over 37 years, here we show that foliar N concentration declined by 9% and foliar delta N-15 declined by 0.6-1.6 parts per thousand. Examining patterns across different climate spaces, foliar delta N-15 declined across the entire range of mean annual temperature and mean annual precipitation tested. These results suggest declines in N supply relative to plant demand at the global scale. In all, there are now multiple lines of evidence of declining N availability in many unfertilized terrestrial ecosystems, including declines in delta N-15 of tree rings and leaves from herbarium samples over the past 75-150 years. These patterns are consistent with the proposed consequences of elevated atmospheric carbon dioxide and longer growing seasons. These declines will limit future terrestrial carbon uptake and increase nutritional stress for herbivores.

/pdf/isotopic-evidence-for-oligotrophication-of-terrestrial-2x1k1z69if.pdf

Isotopic evidence for oligotrophication of terrestrial ecosystems

Should we see urban trees as effective solutions to reduce increasing ozone levels in cities

Risks associated with exposure of individual plant species to ozone (O3) are well documented, but implications for terrestrial biodiversity and ecosystem processes have received insufficient attention. This is an important gap because feedbacks to the atmosphere may change as future O3 levels increase or decrease, depending on air quality and climate policies. Global simulation of O3 using the Community Earth System Model (CESM) revealed that in 2000, about 40% of the Global 200 terrestrial ecoregions (ER) were exposed to O3 above thresholds for ecological risks, with highest exposures in North America and Southern Europe, where there is field evidence of adverse effects of O3, and in central Asia. Experimental studies show that O3 can adversely affect the growth and flowering of plants and alter species composition and richness, although some communities can be resilient. Additional effects include changes in water flux regulation, pollination efficiency, and plant pathogen development. Recent research is unraveling a range of effects belowground, including changes in soil invertebrates, plant litter quantity and quality, decomposition, and nutrient cycling and carbon pools. Changes are likely slow and may take decades to become detectable. CESM simulations for 2050 show that O3 exposure under emission scenario RCP8.5 increases in all major biomes and that policies represented in scenario RCP4.5 do not lead to a general reduction in O3 risks; rather, 50% of ERs still show an increase in exposure. Although a conceptual model is lacking to extrapolate documented effects to ERs with limited or no local information, and there is uncertainty about interactions with nitrogen input and climate change, the analysis suggests that in many ERs, O3 risks will persist for biodiversity at different trophic levels, and for a range of ecosystem processes and feedbacks, which deserves more attention when assessing ecological implications of future atmospheric pollution and climate change.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/ece3.2568

Current and future ozone risks to global terrestrial biodiversity and ecosystem processes

Canopy transpiration and its cooling effect of three urban tree species in a subtropical city- Guangzhou, China

[Extract] Understanding the drought physiology of woody plants has become an increasingly important focus of plant science research in recent years, spurred on by concern about the potential for widespread tree mortality in response to drought stress caused by global warming (McDowell et al., 2008; Allen et al., 2010). However, there is one aspect of how woody plants cope with water deficits that has received very little attention: the role of carbon fixation by photosynthetic bark. This process may become especially important for maintaining physiological activity in woody tissues of drought-stressed trees as the supply of photosynthate from leaves dwindles, due to stomatal closure and impaired phloem translocation. In this issue of New Phytologist, Vandegehuchte et al. (pp. 998–1002) bring this process into the spotlight, and provide a compelling argument for why we should take more notice of that often-hidden, green layer of tissue that is ubiquitous beneath the smooth-bark surfaces of woody plants (Scott, 1907; Pfanz et al., 2002; Dima et al., 2006; Rosell et al., 2015).

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.13723

The benefits of recycling: how photosynthetic bark can increase drought tolerance

Canopy stomatal uptake of NOX, SO2 and O3 by mature urban plantations based on sap flow measurement

It is widely accepted that substantial nighttime sap flux (J s,n) or transpiration (E) occurs in most plants, but the physiological implications are poorly known. It has been hypothesized that J s,n or E serves to enhance nitrogen uptake or deliver oxygen; however, no clear evidence is currently available. In this study, sap flux (J s) in Eucalyptus grandis × urophylla with apparent stem photosynthesis was measured, including control trees which were covered by aluminum foil (approximately 1/3 of tree height) to block stem photosynthesis. We hypothesized that the nighttime water flux would be suppressed in trees with lower stem photosynthesis. The results showed that the green tissue degraded after 3 months, demonstrating a decrease in stem photosynthesis. The daytime J s decreased by 21.47%, while J s,n decreased by 12.03% in covered trees as compared to that of control, and the difference was statistically significant (P < 0.01). The linear quantile regression model showed that J s,n decreased for a given daytime transpiration water loss, indicating that J s,n was suppressed by lower stem photosynthesis in covered trees. Predawn (ψ pd) of covered trees was marginally higher than that of control while lower at predawn stomatal conductance (g s), indicating a suppressed water flux in covered trees. There was no difference in leaf carbon content and δ(13)C between the two groups, while leaf nitrogen content and δ(15)N were significantly higher in covered trees than that of the control (P < 0.05), indicating that J s,n was not used for nitrogen uptake. These results suggest that J s,n may act as an oxygen pathway since green tissue has a higher respiration or oxygen demand than non-green tissue. Thus, this study demonstrated the physiological implications of J s,n and the possible benefits of nighttime water use or E by the tree.

Suppression of nighttime sap flux with lower stem photosynthesis in Eucalyptus trees.

Stomatal uptake of O3 in a Schima superba plantation in subtropical China derived from sap flow measurements.

In plantations of three different re-vegetation tree species (Schima superba, Acacia auriculaeformis and Eucalyptus citriodora) in southern China, the stem sap flow of individuals at different DBH classes were monitored using Granier's thermal dissipation probes. With synchronously-measured meteorological data, the canopy stomatal conductance (g(c)) was determined and the responses of g(c) to environmental variables were analyzed. We found that daytime mean g(c) in S. superba forest on average was significantly higher than those of A. auriculaeformis and E. citriodora plantations during a year (except in March). In the three plantations, g(c) was positively logarithmically correlated with photosynthetically active radiation (PAR) (P < 0.001), with a higher sensitivity of g(c) to PAR during wet season than that of dry season. By contrast, a negative logarithmical correlation between g(c) and vapor pressure deficit (VPD) was observed, with a higher sensitivity of g(c) to VPD during the wet season. Additionally, a higher partial correlation coefficient between g(c) and VPD was observed during wet season, indicating that VPD played a more important role in regulating the behavior of stomata during wet season. In general, the sensitivity of g(c) to VPD decreased with the decreases of soil water content, but more manifest decreases were found in S. superba and E. citriodora forests than in A. auriculaeformis plantation, while the descend degree in S. superba and E. citriodora forests were equal. Overall, our results demonstrated that the native species S. superba is more suitable for re-vegetation in southern China than the exotic species A. auriculaeformis and E. citriodora.

[Characteristics of canopy stomatal conuctance in plantations of three re-vegetation tree species and its sensitivity to environmental factors].

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Canopy stomatal uptake of NOX, SO2 and O3 by mature urban plantations based on sap flow measurement

Suppression of nighttime sap flux with lower stem photosynthesis in Eucalyptus trees.

Stomatal uptake of O3 in a Schima superba plantation in subtropical China derived from sap flow measurements.

[Characteristics of canopy stomatal conuctance in plantations of three re-vegetation tree species and its sensitivity to environmental factors].