How biotic, abiotic, and functional variables drive belowground soil carbon stocks along stress gradient in the Sundarbans Mangrove Forest?

Mangrove forests, some of the most carbon-dense ecosystems on Earth, play an important role in climate change mitigation through storing carbon in the soil. However, increasing anthropogenic pressures and sea level rise are likely to alter mangrove forest structure and functions, including the major source of carbon in mangrove ecosystems - below-ground soil carbon stocks (BSCS). Although estimating soil carbon stocks has been a popular practice in the mangroves, but poorly understood the (I) the linkage between BSCS and key ecosystem drivers (i.e., biotic, abiotic, and functional) and in (II) determining the pathways of how BSCS and multiple forest variables interact along stress gradients. This lack of understanding limits our ability to predict ecosystem carbon dynamics under future changes in climate. Here, we aimed to understand how abiotic factors (such as salinity, canopy gap fraction, nutrients, and soil pH), biotic factors (e.g., structural parameters, canopy packing, and leaf area index, LAI), and forest functional variables (e.g., growth and aboveground biomass stocks, AGB) affect BSCS (i.e., soil organic carbon, SOC, and root carbon, RC) using spatiotemporal data collected from the Sundarbans Mangrove Forest (SMF) in Bangladesh. We observed that BSCS decreased significantly with increasing salinity (e.g., from 70.6 Mg C ha-1 in the low-saline zone to 44.6 Mg C ha-1 in the high-saline zone). In contrast, the availability of several macronutrients (such as nitrogen, phosphorous, and potassium), LAI, species diversity, AGB, and growth showed a significant positive effect on SOC and RC. Stand properties, including tree height, basal area, density, canopy packing, and structural diversity, had a non-significant but positive impact on RC, while tree height and basal area significantly influenced SOC. Pathway analysis showed that salinity affects BSCS variability directly and indirectly by regulating stand structure and restricting nutrients and forest functions, although basal area, nutrients, and LAI directly enhance RC stocks. Our results indicate that an increase in nutrient content, canopy density, species diversity, and leaf area index can enhance BSCS, as they improve forest functions and contribute to a better understanding of the underlying mechanisms.

Drivers of nocturnal stomatal conductance in C3 and C4 plants.

Nocturnal water losses were for long considered negligible, but it is now known that incomplete stomatal closure during the night leads to significant water losses at leaf, plant and ecosystem scales. However, only daytime transpiration is currently accounted for in evapotranspiration studies. Important uncertainties on the drivers of nocturnal water fluxes hinder its incorporation within modelling frameworks because some studies indicate that night-time stomatal drivers may differ from day-time responses. Here, we synthesise the studies on nocturnal stomatal conductance (gn) to determine underlying drivers through a systematic literature review and, whenever possible, meta-analytical techniques. Similar to daytime responses, we found negative effects of vapour pressure deficit, predawn water potential, air temperature, and salinity on gn across the plant species. However, the most apparent trend was an increase of gn from the beginning until the end of the night, indicating significant and widespread endogenous regulation by the circadian clock. We further observed how neither elevated CO2 nor nutrient status affected gn significantly across species. We also did not find any significant associations between gn and elevated ozone or increasing plant age. There was a paucity of studies on climatic extremes such heat waves and also few studies connected gn with anatomical features such as leaf specific area or stomatal density. Further studies are also needed to address the effects of plant sex, abscisic acid concentrations and genotypic variations on gn. Our findings solve the long-term conundrum on whether stomatal responses to daytime drivers are the same as those that during the nighttime.

Radiation and Drought Impact Residual Leaf Conductance in Two Oak Species With Implications for Water Use Models.

Stomatal closure is one of the earliest responses to water stress but residual water losses may continue through the cuticle and incomplete stomatal closure. Residual conductance (g res ) plays a large role in determining time to mortality but we currently do not understand how do drought and shade interact to alter g res because the underlying drivers are largely unknown. Furthermore, g res may play an important role in models of water use, but the exact form in which g res should be incorporated into modeling schemes is currently being discussed. Here we report the results of a study where two different oak species were experimentally subjected to highly contrasting levels of drought (resulting in 0, 50 and 80% losses of hydraulic conductivity) and radiation (photosynthetic photon flux density at 1,500 µmol m-2 s-1 or 35-45 µmol m-2 s-1). We observed that the effects of radiation and drought were interactive and species-specific and g res correlated positively with concentrations of leaf non-structural carbohydrates and negatively with leaf nitrogen. We observed that different forms of measuring g res , based on either nocturnal conductance under high atmospheric water demand or on the water mass loss of detached leaves, exerted only a small influence on a model of stomatal conductance and also on a coupled leaf gas exchange model. Our results indicate that, while understanding the drivers of g res and the effects of different stressors may be important to better understand mortality, small differences in g res across treatments and measurements exert only a minor impact on stomatal models in two closely related species.