The replacement of native C₄‐dominated grassland by C₃‐dominated shrubland is considered an ecological state transition where different ecological communities can exist under similar environmental ...conditions. These state transitions are occurring globally, and may be exacerbated by climate change. One consequence of the global increase in woody vegetation may be enhanced ecosystem carbon sequestration, although the responses of arid and semiarid ecosystems may be highly variable. During a drier than average period from 2007 to 2011 in the northern Chihuahuan Desert, we found established shrubland to sequester 49 g C m⁻² yr⁻¹on average, while nearby native C₄grassland was a net source of 31 g C m⁻² yr⁻¹over this same period. Differences in C exchange between these ecosystems were pronounced – grassland had similar productivity compared to shrubland but experienced higher C efflux via ecosystem respiration, while shrubland was a consistent C sink because of a longer growing season and lower ecosystem respiration. At daily timescales, rates of carbon exchange were more sensitive to soil moisture variation in grassland than shrubland, such that grassland had a net uptake of C when wet but lost C when dry. Thus, even under unfavorable, drier than average climate conditions, the state transition from grassland to shrubland resulted in a substantial increase in terrestrial C sequestration. These results illustrate the inherent tradeoffs in quantifying ecosystem services that result from ecological state transitions, such as shrub encroachment. In this case, the deleterious changes to ecosystem services often linked to grassland to shrubland state transitions may at least be partially offset by increased ecosystem carbon sequestration.
The impacts of climate extremes on terrestrial ecosystems are poorly understood but important for predicting carbon cycle feedbacks to climate change. Coupled climate–carbon cycle models typically ...assume that vegetation recovery from extreme drought is immediate and complete, which conflicts with the understanding of basic plant physiology. We examined the recovery of stem growth in trees after severe drought at 1338 forest sites across the globe, comprising 49,339 site-years, and compared the results with simulated recovery in climate-vegetation models. We found pervasive and substantial "legacy effects" of reduced growth and incomplete recovery for 1 to 4 years after severe drought. Legacy effects were most prevalent in dry ecosystems, among Pinaceae, and among species with low hydraulic safety margins. In contrast, limited or no legacy effects after drought were simulated by current climate-vegetation models. Our results highlight hysteresis in ecosystem-level carbon cycling and delayed recovery from climate extremes.
Hydrogen has been inferred to occur in enhanced concentrations within permanently shadowed regions and, hence, the coldest areas of the lunar poles. The Lunar Crater Observation and Sensing Satellite ...(LCROSS) mission was designed to detect hydrogen-bearing volatiles directly. Neutron flux measurements of the Moon's south polar region from the Lunar Exploration Neutron Detector (LEND) on the Lunar Reconnaissance Orbiter (LRO) spacecraft were used to select the optimal impact site for LCROSS. LEND data show several regions where the epithermal neutron flux from the surface is suppressed, which is indicative of enhanced hydrogen content. These regions are not spatially coincident with permanently shadowed regions of the Moon. The LCROSS impact site inside the Cabeus crater demonstrates the highest hydrogen concentration in the lunar south polar region, corresponding to an estimated content of 0.5 to 4.0% water ice by weight, depending on the thickness of any overlying dry regolith layer. The distribution of hydrogen across the region is consistent with buried water ice from cometary impacts, hydrogen implantation from the solar wind, and/or other as yet unknown sources.
Satellite remote sensing provides unmatched spatiotemporal information on vegetation gross primary productivity (GPP). Yet understanding of the relationship between GPP and remote sensing ...observations and how it changes with factors such as scale, biophysical constraint, and vegetation type remains limited. This knowledge gap is especially apparent for dryland ecosystems, which have characteristic high spatiotemporal variability and are under‐represented by long‐term field measurements. Here we utilize an eddy covariance (EC) data synthesis for southwestern North America in an assessment of how accurately satellite‐derived vegetation proxies capture seasonal to interannual GPP dynamics across dryland gradients. We evaluate the enhanced vegetation index, solar‐induced fluorescence (SIF), and the photochemical reflectivity index. We find evidence that SIF is more accurately capturing seasonal GPP dynamics particularly for evergreen‐dominated EC sites and more accurately estimating the full magnitude of interannual GPP dynamics for all dryland EC sites. These results suggest that incorporation of SIF could significantly improve satellite‐based GPP estimates.
Key Points
SIF captures seasonal GPP dynamics better than EVI and PRI, especially for evergreen forest sites
SIF is more sensitive than EVI and PRI to site‐level interannual GPP variability across all southwestern North America ecoregions
Incorporation of SIF could significantly improve satellite‐based GPP estimates for dryland ecosystems
Aims
(1) To develop a 3D root distribution model for piñon-juniper woodland using only tree species, sizes and locations as input. (2) To interpret a two-year time series of soil moisture relative to ...root distributions.
Methods
The study was conducted in a piñon (
Pinus edulis
(Englem.)) -juniper (
Juniperus monosperma
(Englem.) Sarg.) woodland in New Mexico. We extracted roots from 720 soil blocks (30 cm × 10 cm × 10 cm) cut from the walls of three 10-m long and 1.5-m deep trenches. Roots were sorted by species and diameter class. Distribution models were developed for the dry weight of roots ≤5 mm in diameter. Soil water content and water potentials were measured in soil profiles under tree cluster and canopy gaps for 2 years, including a protracted dry-down period.
Results
Piñon had twice the root dry mass of juniper, similar to the ratio of canopy projection areas. Root densities were ca. 50% lower in soils under canopy gaps compared to tree clusters and the species ratio did not significantly differ between clusters and gaps. Piñon root density declined faster with soil depth and distance from the stem compared to juniper. A hard caliche layer at 60–80 cm soil depth had no apparent effect on the already low root density at that depth. Overall, the models explained 66% (piñon) and 54% (juniper) of the spatial variation in root density at the scale of sampled soil blocks. During an 8-month dry period, soil moisture declined faster in regions of higher root density: in shallow soil and under tree clusters. This left a reserve of plant-available soil water in the deep soil under canopy gaps.
Conclusions
Horizontal variation in root density in these open woodlands is predictable and an important component of the dynamic interactions between plant and soil. Under wet and dry conditions, soil water content is substantially different under tree clusters and canopy gaps.
Current steering and current focusing are stimulation techniques designed to increase the number of distinct perceptual channels available to cochlear implant (CI) users by adjusting currents applied ...simultaneously to multiple CI electrodes. Previous studies exploring current steering and current focusing stimulation strategies are reviewed, including results of research using computational models, animal neurophysiology, and human psychophysics. Preliminary results of additional neurophysiological and human psychophysical studies are presented that demonstrate the success of current steering strategies in stimulating auditory nerve regions lying between physical CI electrodes, as well as current focusing strategies that excite regions narrower than those stimulated using monopolar configurations. These results are interpreted in the context of perception and speech reception by CI users. Disparities between results of physiological and psychophysical studies are discussed. The differences in stimulation used for physiological and psychophysical studies are hypothesized to contribute to these disparities. Finally, application of current steering and focusing strategies to other types of auditory prostheses is also discussed.
Extremes in climate may severely impact ecosystem structure and function, with both the magnitude and rate of response differing among ecosystem types and processes. We conducted a modeling analysis ...of the effects of extreme drought on two key ecosystem processes, production and respiration, and, to provide a broader context, we complemented this with a synthesis of published results that cover a wide variety of ecosystems. The synthesis indicated that across a broad range of biomes, gross primary production (GPP) was generally more sensitive to extreme drought (defined as proportional reduction relative to average rainfall periods) than was ecosystem respiration (ER). Furthermore, this differential sensitivity between production and respiration increased as drought severity increased; it occurred only in grassland ecosystems, and not in evergreen needle-leaf and broad-leaf forests or woody savannahs. The modeling analysis was designed to enable a better understanding of the mechanisms underlying this pattern, and focused on four grassland sites arrayed across the Great Plains, USA. Model results consistently showed that net primary productivity (NPP) was reduced more than heterotrophic respiration (Rh) by extreme drought (i.e., 67% reduction in annual ambient rainfall) at all four study sites. The sensitivity of NPP to drought was directly attributable to rainfall amount, whereas the sensitivity of Rh to drought was driven by soil drying, reduced carbon (C) input and a drought-induced reduction in soil C content – a much slower process. However, differences in reductions in NPP and Rh diminished as extreme drought continued, due to a gradual decline in the soil C pool leading to further reductions in Rh. We also varied the way in which drought was imposed in the modeling analysis; it was either imposed by simulating reductions in rainfall event size (ESR) or by reducing rainfall event number (REN). Modeled NPP and Rh decreased more by ESR than REN at the two relatively mesic sites but less so at the two xeric sites. Our findings suggest that responses of production and respiration differ in magnitude, occur on different timescales, and are affected by different mechanisms under extreme, prolonged drought.
ExoMars is a two-launch mission undertaken by Roscosmos and European Space Agency. Trace Gas Orbiter, a satellite part of the 2016 launch carries the Fine Resolution Neutron Detector instrument as ...part of its payload. The instrument aims at mapping hydrogen content in the upper meter of Martian soil with spatial resolution between 60 and 200 km diameter spot. This resolution is achieved by a collimation module that limits the field of view of the instruments detectors. A dosimetry module that surveys the radiation environment in cruise to Mars and on orbit around it is another part of the instrument.
This paper describes the mission and the instrument, its measurement principles and technical characteristics. We perform an initial assessment of our sensitivity and time required to achieve the mission goal. The Martian atmosphere is a parameter that needs to be considered in data analysis of a collimated neutron instrument. This factor is described in a section of this paper. Finally, the first data accumulated during cruise to Mars is presented.
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