L-Band SAR Co-Polarized Phase Difference Modeling for Corn Fields

This research aims at modeling the microwave backscatter of corn fields by coupling an incoherent, interaction-based scattering model with a semi-empirical bulk vegetation dielectric model. The scattering model is fitted to co-polarized phase difference measurements over several corn fields imaged with fully polarimetric synthetic aperture radar (SAR) images with incidence angles ranging from 20° to 60°. The dataset comprised two field campaigns, one over Canada with the Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR, 1.258 GHz) and the other one over Argentina with Advanced Land Observing Satellite 2 (ALOS-2) Phased Array type L-band Synthetic Aperture Radar (PALSAR-2) (ALOS-2/PALSAR-2, 1.236 GHz), totaling 60 data measurements over 28 grown corn fields at peak biomass with stalk gravimetric moisture larger than 0.8 g/g. Co-polarized phase differences were computed using a maximum likelihood estimation technique from each field’s measured speckled sample histograms. After minimizing the difference between the model and data measurements for varying incidence angles by a nonlinear least-squares fitting, well agreement was found with a root mean squared error of 24.3° for co-polarized phase difference measurements in the range of −170.3° to −19.13°. Model parameterization by stalk gravimetric moisture instead of its complex dielectric constant is also addressed. Further validation was undertaken for the UAVSAR dataset on earlier corn stages, where overall sensitivity to stalk height, stalk gravimetric moisture, and stalk area density agreed with ground data, with the sensitivity to stalk diameter being the weakest. This study provides a new perspective on the use of co-polarized phase differences in retrieving corn stalk features through inverse modeling techniques from space.

Radiative and microphysical responses of clouds to an anomalous increase in fire particles over the Maritime Continent in 2015

The year of 2015 was an extremely dry year for Southeast Asia where the direct impact of strong El Niño was in play. As a result of this dryness and the relative lack of rainfall, an extraordinary amount of aerosol particles from biomass burning remained in the atmosphere over the Maritime Continent during the fire season. This study uses the Weather Research and Forecasting model coupled with Chemistry to understand the impacts of these fire particles on cloud microphysics and radiation during the peak biomass burning season in September. Our simulations, one with fire particles and the other without them, cover the entire Maritime Continent region at a cloud-resolving resolution (4 km) for the entire month of September in 2015. The comparison of the simulations shows a clear sign of precipitation enhancement by fire particles through microphysical effects; smaller cloud droplets remain longer in the atmosphere to later form ice crystals, and/or they are more easily collected by ice-phase hydrometeors, in comparison to droplets under no fire influences. As a result, mass of ice-phase hydrometeors increases in the simulation with fire particles, so does rainfall. On the other hand, we see no clear sign of temperature differences between the two simulations that could stem from the semi-direct effects of aerosols by absorbing the incoming solar radiation. Clouds are more reflective in the simulation with fire particles as ice mass increases. Combined with the direct scattering of sunlight by aerosols, the simulation with fire particles shows higher albedo over the simulation domain on average. The simulated response of clouds to fire particles in our simulations clearly differs from what was presented by two previous studies that modeled aerosol-cloud interaction in years with different phases of El Niño-Southern Oscillation (ENSO), suggesting a further need for an investigation on the possible modulation of fire-aerosol-convection interaction by ENSO.

Large-scale comparison of biomass and reproductive phenology among native and non-native populations of the seagrass Zostera japonica

Large-scale analysis along latitude or temperature gradients can be an effective method for exploring the potential roles of light and temperature in controlling seagrass phenology. In this study, we investigated effects of latitude and temperature on seagrass biomass and reproductive seasonality. Zostera japonica is an intertidal seagrass with a wide latitudinal distribution expanding from tropical to temperate zones in its native range in Asia, with an additional non-native distribution in North America. We collated available data on phenological traits (timings of peak biomass or reproduction, durations of biomass growth and reproductive season, and maximum biomass or reproductive ratio) from publications and our own observations. Traits were compared among geographic groups: Asia-tropical, Asia-temperate, and North America-temperate. We further examined relationships between traits and latitude and temperature for 3 population groups: Asian, North American, and all populations. Our analysis revealed significant variation among geographic groups in maximum biomass, peak reproductive timing, and maximum reproductive ratio, but not in other traits. Maximum biomass and peak reproductive timing for Asian and all populations were significantly correlated with latitude and temperature. Maximum biomass was highest at mid-latitudes or intermediate temperatures and decreased toward distribution range limits, and peak reproductive timing occurred later in the year at higher latitudes or cooler sites. North American populations showed shorter growth durations and greater reproductive ratios at higher latitude. Different responses observed for North American populations may reflect effects of introduction. Our study demonstrates potential variation among geographic regions and between native and non-native populations.

Cyclical Patterns and a Regime Shift in the Character of Phytoplankton Blooms in a Restricted Sub-Tropical Lagoon, Indian River Lagoon, Florida, United States

This paper examines the character of phytoplankton blooms in a restricted sub-tropical lagoon along the Atlantic coast of central Florida. The results of the 23-year study (1997–2020) provide evidence for multiple types of variability in bloom activity, including cyclical patterns, stochastic events, and most prominently a regime shift in composition and intensity. Cyclical patterns (e.g., El Niño/La Niña periods) and stochastic events (e.g., tropical storms) influenced rainfall levels, which in turn impacted nutrient concentrations in the water column and the timing and intensity of blooms. In 2011, a major change occurred in the character of blooms, with a dramatic increase in peak biomass levels of blooms and the appearance of new dominant taxa, including the brown tide species Aureoumbra lagunensis and other nanoplanktonic species. Results of quantitative analyses reveal system behavior indicative of a regime shift. The shift coincided with widespread losses of seagrass community and reduced drift algae biomass. A combination of exceptionally low water temperatures in the winters of 2009/2010 and 2010/2011, hypersaline conditions associated with drought conditions, and high light attenuation caused by blooms appear to have contributed to the widespread and protracted decline in seagrass and drift macroalgal communities in the lagoon, leading to shifts in distribution of internal and external nutrient sources toward phytoplankton.

Variability in periphyton community and biomass over 37 years in Lake Tahoe (CA-NV)

Nuisance periphyton blooms are occurring in oligotrophic lakes worldwide, but few lakes have documented changes in biomass through periphyton monitoring. For decades periphyton has caused concern about oligotrophic Lake Tahoe’s nearshore water quality. To determine whether eulittoral periphyton increased in Lake Tahoe, measures of biomass and dominant communities at 0.5 m below lake level have been monitored regularly at nine shoreline sites starting in 1982, with up to 54 additional sites monitored annually at peak biomass. Lake-wide, this metric of periphyton biomass has not increased since monitoring began. Biomass decreased at many sites and increased at one. Periphyton biomass peaked in March and was low in the summer lake-wide. The northern and western shores had higher biomass than the eastern and southern shores. Biomass varied with lake level. High biomass occurred at sites regardless of urban development levels. As increasing periphyton at Lake Tahoe was first cited in scientific literature in the 1960s, it is possible that periphyton increased prior to our monitoring program. A dearth of published long-term monitoring data from oligotrophic lakes with reported periphyton blooms makes it difficult to determine the extent of this issue worldwide. Long-term nearshore monitoring is crucial for tracking and understanding periphyton blooms.

Assessing Phytoplankton Bloom Phenology in Upwelling-Influenced Regions Using Ocean Color Remote Sensing

Phytoplankton bloom phenology studies are fundamental for the understanding of marine ecosystems. Mismatches between fish spawning and plankton peak biomass will become more frequent with climate change, highlighting the need for thorough phenology studies in coastal areas. This study was the first to assess phytoplankton bloom phenology in the Western Iberian Coast (WIC), a complex coastal region in SW Europe, using a multisensor long-term ocean color remote sensing dataset with daily resolution. Using surface chlorophyll a (chl-a) and biogeophysical datasets, five phenoregions (i.e., areas with coherent phenology patterns) were defined. Oceanic phytoplankton communities were seen to form long, low-biomass spring blooms, mainly influenced by atmospheric phenomena and water column conditions. Blooms in northern waters are more akin to the classical spring bloom, while blooms in southern waters typically initiate in late autumn and terminate in late spring. Coastal phytoplankton are characterized by short, high-biomass, highly heterogeneous blooms, as nutrients, sea surface height, and horizontal water transport are essential in shaping phenology. Wind-driven upwelling and riverine input were major factors influencing bloom phenology in the coastal areas. This work is expected to contribute to the management of the WIC and other upwelling systems, particularly under the threat of climate change.

Effects of defoliation timing on plant nutrient resorption and hay production in a semi-arid steppe

Aims Nutrient resorption is a key plant nutrient conservation strategy, and its response to environmental and management changes is linked to nutrient cycling and production of ecosystems. Defoliation is a major pathway of mowing affecting plant nutrient resorption and production in grasslands, while the effect of defoliation timing has not been unexplored. The aim of this study was to examine the effect of defoliation timing on plant nutrient resorption and production in a steppe ecosystem. Methods We conducted a field experiment in a semi-arid steppe of Inner Mongolia including four treatments: early defoliation, peak defoliation, late defoliation and non-defoliation. We measured plant nitrogen (N) and phosphorus (P) resorption at species and community levels, and quantified plant N and P fluxes in resorption, litter return and hay output. Plant production in the mowing system was assessed by hay production and quality. Important Findings Peak and late defoliation, but not early defoliation, reduced plant community N and P resorption proficiency (RP); and late defoliation reduced N resorption efficiency (RE) but not P resorption efficiency. Peak and late defoliation, but not early defoliation, reduced plant nutrient resorption flux and litter nutrient return flux. Defoliation timing did not alter root nutrient accumulation as nutrient uptake from soil likely compensated the deficit of nutrient resorption. Peak defoliation had the highest hay production and quality, while early defoliation had the lowest. Our results provide new insights into the nutrient cycling in mowing grassland, and imply that the mowing timing can be used as a tool to mediate the balance between conservation and production of steppes, and the early mowing before plant peak biomass period is recommended for conservation of the steppes while keeping sustainable pastoral production.

Correlation of the abundance of bacteria catalyzing phosphorus and nitrogen turnover in biological soil crusts of temperate forests of Germany

Soil P pools are strongly driven by microbial activities, and vice versa, P pools shape bacterial communities and their functional potential. Biological soil crusts (biocrusts) represent a microbial hotspot for nutrient turnover. We compared biocrusts and bulk soil samples from different temperate beech (Fagus sylvatica L.) forests representing a gradient in soil texture, nutrient concentrations, and pH values at biocrust peak biomass. We measured the total and plant-available P and N concentrations and assessed the bacterial potential to mineralize (phoD, phnX), solubilize (gcd), and take up P (pstS and pitA) and mineralize (chiA, apr) and fix N (nifH) by quantifying the respective marker genes (qPCR). We found an increase of absolute and relative bacterial abundance involved in P turnover in biocrusts, but the strategy to acquire P differed between the regions as bacteria harboring the starvation-induced pstS gene were most abundant where labile P was lowest. In contrast, the region with lowest total P concentrations has a higher potential to utilize more stable phosphonates. N mineralization was strongly correlated to P turnover at regions with increased labile N and P concentrations. Interestingly, the potential to fix N was highest in the bulk soil where total P concentrations were highest. Even though the correlation of N and P turnover is strongest if their ratio is low, the acquisition strategy strongly depends on soil properties.

Phenology and plant functional type dominance drive CO2 exchange in seminatural grasslands in the Pyrenees

Understanding the mechanisms underlying net ecosystem CO2 exchange (NEE) in mountain grasslands is important to quantify their relevance in the global carbon budget. However, complex interactions between environmental variables and vegetation on NEE remain unclear; and there is a lack of empirical data, especially from the high elevations and the Mediterranean region. A chamber-based survey of CO2 exchange measurements was carried out in two climatically contrasted grasslands (montane v. subalpine) of the Pyrenees; assessing the relative contribution of phenology and environmental variables on CO2 exchange at the seasonal scale, and the influence of plant functional type dominance (grasses, forbs and legumes) on the NEE light response. Results show that phenology plays a crucial role as a CO2 exchange driver, suggesting a differential behaviour of the vegetation community depending on the environment. The subalpine grassland had a more delayed phenology compared to the montane, being more temperature than water constrained. However, temperature increased net CO2 uptake at a higher rate in the subalpine than in the montane grassland. During the peak biomass, productivity (+74%) and net CO2 uptake (NEE +48%) were higher in the subalpine grassland than in the montane grassland. The delayed phenology at the subalpine grassland reduced vegetation's sensitivity to summer dryness, and CO2 exchange fluxes were less constrained by low soil water content. The NEE light response suggested that legume dominated plots had higher net CO2 uptake per unit of biomass than grasses. Detailed information on phenology and vegetation composition is essential to understand elevation and climatic differences in CO2 exchange.

Studies of NH4+ and NO3- uptake ability of subalpine plants and resource-use strategy identified by their functional traits

The leaf economics spectrum (LES) is based on a suite of leaf traits related to plant functioning and ranges from resource-conservative to resource-acquisitive strategies. However, the relationships with root traits, and the associated belowground plant functioning such as N uptake, including nitrate (NO3-) and ammonium (NH4+), is still poorly known. Additionally, environmental variations occurring both in time and in space could uncouple LES from root traits. We explored, in subalpine grasslands, the relationships between leaf and root morphological traits for 3 dominant perennial grass species, and to what extent they contribute to the whole-plant economics spectrum. We also investigated the link between this spectrum and NO3- and NH4+ uptake rates, as well as the variations of uptake across four grasslands differing by the land-use history at peak biomass and in autumn. Although poorly correlated with leaf traits, root traits contributed to an economic spectrum at the whole plant level. Higher NH4+ and NO3- uptake abilities were associated with the resource-acquisitive strategy.Nonetheless, NH4+ and NO3- uptake within species varied between land-uses and with sampling time, suggesting that LES and plant traits are good, but still incomplete, descriptors of plant functioning. Although the NH4+: NO3- uptake ratio was different between plant species in our study, they all showed a preference for NH4+, and particularly the most conservative species. Soil environmental variations between grasslands and sampling times may also drive to some extent the NH4+ and NO3- uptake ability of species. Our results support the current efforts to build a more general framework including above- and below-ground processes when studying plant community functioning.