California Ecosystems Research Articles

Effects of Fire and Fireline Disturbance on the Plant Community in a Southern California Ecological Reserve

Native plants in most California ecosystems are adapted to fire, but altered fire regimes and disturbance from firefighting activity, such as the construction of firebreaks or firelines, can change plant community composition and the ratio of native to nonnative species. In October 2007, a wildfire burned 710 acres through a chaparral/grassland mosaic on an ecological reserve, providing an opportunity to quantify fire and fireline disturbance impacts on native and nonnative species under differing disturbance conditions. In the spring of 2012 we sampled the plant community in three adjacent sites, focusing on Centaurea melitensis, which is a common nonnative invader after fire in California chaparral. The first site was burned and bulldozed, the second site was burned but not bulldozed, and the third site was not burned or bulldozed. The first site had also been sampled in the spring of 2008. After four years within the burned fireline site, the mean relative cover of C. melitensis decreased from 72% to 28%, but its density increased, and there were increases in the covers of nonnative annual grasses, litter, and native plants. Among the three sites in 2012, both of the burned sites had higher density and cover of C. melitensis and lower relative cover of annual grasses than the unburned site. The only site with notable native perennial presence was the burned fireline. The results of our study suggest that the recruitment of C. melitensis and some native species is promoted by fire. In the absence of additional disturbance by firelines, persistence of these taxa is limited by competition from nonnative annual grasses.

Fate and movement of selenium from drainage sediments disposed onto soil with and without vegetation

Disposal options for salty and selenium-laden agricultural drainage sediments are needed to protect the agricultural ecosystem in Central California. Thus, a 7-year pilot-scale field study evaluated the effects of disposing Se-laden drainage sediment onto soil that was planted with either salado grass (Sporobolus airoides ‘salado’) or cordgrass (Spartina patens ‘Flageo’), or on soil left bare with and without irrigation. Significant decreases in salinity and water-extractable and total soil Se concentrations were observed in all treatments to a depth 30 cm, while water extractable Se and salinity increased most significantly between 30 and 60 cm. Total yields increased over time for both species, while plant Se concentrations were ≈10 and 12 mg kg−1 DM for salado and cordgrass, respectively. The results show that Se and soluble salts disposed of as Se-laden drainage sediment onto light textured soils will significantly migrate to lower depths with or without vegetation.

Exotic species display greater germination plasticity and higher germination rates than native species across multiple cues

Rapid germination or flexible germination cues may be key traits that facilitate the invasion of exotic plant species in new environments. We investigated whether robustness or plasticity in response to environmental cues were more commonly exhibited by exotic than native species during germination, evidenced by (1) exhibiting consistently greater germination rate under a variety of conditions (robustness), or (2) increasing germination rate more strongly than native species in response to favorable conditions (plasticity). We conducted growth chamber germination trials of 12 native and 12 exotic species common to coastal sage scrub, a shrub-dominated Mediterranean-type ecosystem in California. Time to germination and percentage germination were recorded in response to variation in three environmental cues: temperature, day length, and soil moisture. Exotic species, especially annuals, displayed consistently higher germination percentages and more rapid germination than native species. Exotic germination percentages also responded more strongly when conditions were favorable (warm temperatures and high soil moisture), and germinated earlier than natives when conditions were indicative of typical growing season conditions in Mediterranean ecosystems (short day length and cool temperatures). Exotic species had more rapid and prolific germination across a variety of environmental cues and in response to increased resource availability compared with native species, indicating both germination plasticity and robustness. These traits may enable colonization of novel environments, particularly if they allow exotic species to establish earlier in the growing season than native species, setting the stage for seasonal priority effects.

Growth and Resource Use of the Invasive Grass, Pampasgrass (Cortaderia selloana), in Response to Nitrogen and Water Availability

Exotic invasive species are nonnative species that thrive outside of their native habitat, and while it is difficult to determine which exotic plants will become invasive, successful invaders often share a wide range of traits including high growth rate and reproductive output, vegetative reproduction, high population growth rates, early reproductive age, phenotypic and physiological plasticity, and high resource use efficiency. Here we report on the response of pampasgrass, an important exotic invasive plant of the western United States, to experimental variations in soil nitrogen (N) and water availability. Given its ability to invade a wide variety of ecosystems in southern California, we hypothesized that pampasgrass would have higher water and N use efficiency under conditions of low water and N availability but rapid growth and resource use under conditions of high water and N availability. Our data support this hypothesis and indicate that pampasgrass exhibited large variations in growth, carbon allocation, morphology, and N and phosphorus (P) nutrition to variations in N availability and water table depth. Many of these traits are highly correlated with invasive performance, and the high N and P use efficiency observed under low soil N (control) and water table, coupled with the large increase in physiological performance and resource use under high N and water table, indicate that pampasgrass is highly flexible to soil resource levels that are typical for coastal sage scrub and riparian ecosystems of southern California. Such flexibility in resource use could allow pampasgrass to persist in low-resource environments and expand as resource levels increase.

Interactive effects of anthropogenic nitrogen enrichment and climate change on terrestrial and aquatic biodiversity

Biodiversity has been described as the diversity of life on earth within species, among species, and among ecosystems. The rate of biodiversity loss due to human activity in the last 50 years has been more rapid than at any other time in human history, and many of the drivers of biodiversity loss are increasing, including habitat loss, overexploitation, invasive species, climate change, and pollution, including pollution from reactive nitrogen (Nr). Of these stressors, climate change and Nr from anthropogenic activities are causing some of the most rapid changes. Climate change is causing warming trends that result in poleward and elevational range shifts of flora and fauna, and changes in phenology, particularly the earlier onset of spring events and migration, and lengthening of the growing season. Nitrogen (N) enrichment can enhance plant growth, but has been shown to favor, fast-growing, sometimes invasive, species over native species adapted to low N conditions. Although there have been only a few controlled studies on climate change and N interactions, inferences can be drawn from various field observations. For example, in arid ecosystems of southern California, elevated N deposition and changing precipitation patterns have promoted the conversion of native shrub communities to communities dominated by annual non-native grasses. Both empirical studies and modeling indicate that N and climate change can interact to drive losses in biodiversity greater than those caused by either stressor alone. Reducing inputs of anthropogenic Nr may be an effective mitigation strategy for protecting biodiversity in the face of climate change.

Continuous observation of tree leaf area index at ecosystem scale using upward-pointing digital cameras

Understanding the dynamics of leaf area index (LAI) at ecosystem scale is critical for evaluating and modeling the response of vegetation to environmental variability and change. We present an approach for quantifying daily ecosystem LAI that integrates automatic acquisition of a small number of reference photos, viewed towards the zenith, and a larger number of photos covering a spatially extensive area that were manually acquired during periodic field trips. We present an image archive spanning three years for an oak-savanna ecosystem in California to identify the timing of phenological and disturbance events, and to quantify the seasonal to interannual variability of tree LAI at ecosystem scale. The digital camera-derived LAI, corrected for clumping effects, agreed well (r2=0.94, root mean square error=0.05) with independent estimates of LAI from litterfall traps and the LAI-2000 Plant Canopy Analyzer. Using the Moderate Resolution Imaging Spectroradiometer (MODIS) LAI product, which combined grass and tree LAI, we obtained realistic seasonal patterns of ecosystem LAI that corresponded with tree LAI from the digital cameras for the period when grass was dead. The digital camera method is an easy and inexpensive way to monitor LAI at ecosystem scale. This method can be used for testing and improving phenology models, evaluating remote-sensing-derived LAI products, and quantifying forest structures in rapidly changing environments.

Spectral responses to plant available soil moisture in a Californian grassland

This study established relationships among plant available soil moisture and reflectance and vegetation indices (VIs) derived from AVIRIS and MODIS data in grassland ecosystem in California. Strong correlations were observed between soil moisture and different forms of reflectance in the red-edge, near infrared and shortwave infrared bands. Both greenness-based and canopy-water-based indices were linearly related with soil moisture during the growing season, the wet and the dry season. The relationship was stronger with antecedent soil moisture, particularly in the dry season. Using plant available soil moisture, which is the difference between measured soil moisture and the wilting point, improved the relationship by reducing the soil property effect. Furthermore, results suggested that the difference in sensors had little impact on the relationships in grassland, but the parameters of relationships were influenced by the spatial resolution of sensors.

Patterns of Aboveground Biomass Regeneration in Post-Fire Coastal Scrub Communities

An evaluation of ALOS PALSAR (Phased Array type L-band Synthetic Aperture Radar) data for shrub height and aboveground biomass (AGB) estimation has been performed in scrub-dominated ecosystems of central coastal California. Comparison between AGB field measurements and SAR estimations showed a correlation coefficient of R2 = 0.53 for the HV polarization. Post-fire AGB regeneration was examined two years after the Big Sur Basin Complex fire of 2008. In 2009, the average patch size of AGB density classes between 20-40 Mg ha-1 and 50-70 Mg ha-1 was 0.49 ha and 0.09 ha, respectively. In 2010, patch sizes in these same two AGB density classes increased to averages of 0.8 ha to 0.15 ha, respectively. No strong saturation in the SAR-AGB relationship was found, which indicated the potential for more advanced applications of L-band SAR data for coastal ecosystem AGB mapping.

An integrated ecosystem trophic model for the North and Central Gulf of California: An alternative view for endemic species conservation

Understanding of the intricate interactions of endemic species with anthropogenic impacts of diverse economic interests on ecosystems is of paramount importance to the implementation of effective conservation programs. A trophic mass-balance model was used to analyze the structural properties of the North and Central Gulf of California (NC-GulfCal) ecosystem, the most important fishing area in Mexico and where conservation efforts for protecting the endangered endemic porpoise known as vaquita (Phocoena sinus) have been carried out for the last two decades. Results allowed to discuss the role of fisheries in the ecosystem and its implications for conservation of the endangered endemic porpoise. The trophic model was constructed with 166 functional groups and 3 regional fishing fleets. Abundance and catch from the early 1980s were used for balancing the model. Maturity indices of ascendency and production to respiration ratio indicate this ecosystem had a relatively high resilience and a high potential for development; this is in accordance with the highly variable environment found in the NC-GulfCal ecosystem. Mixed trophic impacts analysis confirms that pelagic and plankton groups have the most generalized impacts in this ecosystem. The model also shows that fisheries in the NC-GulfCal ecosystem, by the early 80s, were already operating in multiple habitats and landing a great variety of species. However, the highest exploitation pressure was focused on a few species. Although vaquita, since then, has also been under a high incidental mortality by gillnet fisheries, results suggest that predation should also be considered as a key factor on vaquita population dynamics and regulations focused exclusively on fisheries probably are not enough to achieve a successful vaquita recovery program.

North Pacific Gyre Oscillation modulates seasonal timing and ecosystem functioning in the California Current upwelling system

On interannual and longer time scales, dynamical and biogeochemical fluctuations in the North Pacific are dominated by two modes of variability, namely the Pacific Decadal Oscillation and the North Pacific Gyre Oscillation (NPGO). In this study the regional expression of the NPGO in the California Current System (CCS) is detailed. The statistical relationship between the NPGO index and nearshore wind variability (mostly upwelling favorable) along the U.S. West coast is strongest in the wintertime (December to March) off Central California. Most importantly, NPGO fluctuations are associated with a seasonal shift of 1–2 months in the onset of the upwelling season. Regional numerical simulations show that an early (late) onset of upwelling during the positive (negative) phase of the NPGO leads to a more (less) productive planktonic ecosystem throughout spring and summer, i.e., several months after the direct NPGO effects on the system have ceased. These results bring new light on the California ecosystem variability as observed in atypical years such as 2005 and 2007.

Seasonal timing of first rain storms affects rare plant population dynamics

A major challenge in forecasting the ecological consequences of climate change is understanding the relative importance of changes to mean conditions vs. changes to discrete climatic events, such as storms, frosts, or droughts. Here we show that the first major storm of the growing season strongly influences the population dynamics of three rare and endangered annual plant species in a coastal California (USA) ecosystem. In a field experiment we used moisture barriers and water addition to manipulate the timing and temperature associated with first major rains of the season. The three focal species showed two- to fivefold variation in per capita population growth rates between the different storm treatments, comparable to variation found in a prior experiment imposing eightfold differences in season-long precipitation. Variation in germination was a major demographic driver of how two of three species responded to the first rains. For one of these species, the timing of the storm was the most critical determinant of its germination, while the other showed enhanced germination with colder storm temperatures. The role of temperature was further supported by laboratory trials showing enhanced germination in cooler treatments. Our work suggests that, because of species-specific cues for demographic transitions such as germination, changes to discrete climate events may be as, if not more, important than changes to season-long variables.

A Summary of Fire Frequency Estimates for California Vegetation before Euro-American Settlement

California fire regimes have been altered from those that occurred prior to Euro-American settlement, and are predicted to continue to change as global climates warm. Inclusion of fire as a landscape-level process is considered essential to successful ecological restoration in many ecosystems, and presettlement fire regimes provide foundational information for restoration or “realignment” of ecosystems as climate change and land use changes progress. The objective of our study was to provide an up-to-date, comprehensive summary of presettlement fire frequency estimates for California ecosystems dominated by woody plants, and to supply the basis for fire return interval departure (FRID) mapping and analysis in California. Using the LANDFIRE Biophysical Settings (BpS) vegetation-fire regime types as a framework, we used literature review and the outcomes of regional expert workshops to develop twenty-eight presettlement fire regime (PFR) groups based on similarity of their relationships with fire. We then conducted an exhaustive review of the published and unpublished literature pertaining to fire return intervals (FRIs) observed prior to significant Euro-American settlement in the twenty-eight PFRs, and summarized the values to provide a single estimate of the mean, median, mean minimum, and mean maximum FRI for each PFR.Much variability was evident among PFRs, with mean FRIs ranging from 11 yr to 610 yr, and median FRIs ranging from 7 yr to 610 yr; mean minimum FRIs ranged from 5 yr to 190 yr, and mean maximum FRIs ranged from 40 yr to 1440 yr. There was also high variability within many PFRs, and differences between minimum and maximum FRIs ranged from 32 yr to 1324 yr. Generally, median FRIs were lowest for productive drier forests such as yellow pine, dry and moist mixed conifer, and oak woodland (7 yr, 9 yr, 12 yr, and 12 yr, respectively). Median FRIs were highest for less productive woodlands such as pinyon-juniper (94 yr), high elevation types such as subalpine forest (132 yr), very dry types such as desert mixed shrub (610 yr), and productive moist forests such as spruce-hemlock (275 yr mean FRI). Our summary of California’s presettlement fire regimes should be a useful reference for scientists and resource managers, whether they are seeking a general estimate of the central tendency and variability of FRIs in a broadly defined vegetation type, background information for a planned restoration project or a mechanistic model of vegetation-fire interactions, or a list of literature pertaining to a specific vegetation type or geographic location.

Mapping groundwater dependent ecosystems in California.

BackgroundMost groundwater conservation and management efforts focus on protecting groundwater for drinking water and for other human uses with little understanding or focus on the ecosystems that depend on groundwater. However, groundwater plays an integral role in sustaining certain types of aquatic, terrestrial and coastal ecosystems, and their associated landscapes. Our aim was to illuminate the connection between groundwater and surface ecosystems by identifying and mapping the distribution of groundwater dependent ecosystems (GDEs) in California.Methodology/Principal FindingsTo locate where groundwater flow sustains ecosystems we identified and mapped groundwater dependent ecosystems using a GIS. We developed an index of groundwater dependency by analyzing geospatial data for three ecosystem types that depend on groundwater: (1) springs and seeps; (2) wetlands and associated vegetation alliances; and (3) stream discharge from groundwater sources (baseflow index). Each variable was summarized at the scale of a small watershed (Hydrologic Unit Code-12; mean size = 9,570 ha; n = 4,621), and then stratified and summarized to 10 regions of relative homogeneity in terms of hydrologic, ecologic and climatic conditions. We found that groundwater dependent ecosystems are widely, although unevenly, distributed across California. Although different types of GDEs are clustered more densely in certain areas of the state, watersheds with multiple types of GDEs are found in both humid (e.g. coastal) and more arid regions. Springs are most densely concentrated in the North Coast and North Lahontan, whereas groundwater dependent wetlands and associated vegetation alliances are concentrated in the North and South Lahontan and Sacramento River hydrologic regions. The percentage of land area where stream discharge is most dependent on groundwater is found in the North Coast, Sacramento River and Tulare Lake regions. GDE clusters are located at the highest percentage in the North Coast (an area of the highest annual rainfall totals), North Lahontan (an arid, high desert climate with low annual rainfall), and Sacramento River hydrologic regions. That GDEs occur in such distinct climatic and hydrologic settings reveals the widespread distribution of these ecosystems.Conclusions/SignificanceProtection and management of groundwater-dependent ecosystems are hindered by lack of information on their diversity, abundance and location. By developing a methodology that uses existing datasets to locate GDEs, this assessment addresses that knowledge gap. We report here on the application of this method across California, but believe the method can be expanded to regions where spatial data exist.

The carbon budget of California

Abstract The carbon budget of a region can be defined as the sum of annual fluxes of carbon dioxide (CO2) and methane (CH4) greenhouse gases (GHGs) into and out of the regional surface coverage area. According to the state government's recent inventory, California's carbon budget is presently dominated by 115 MMTCE per year in fossil fuel emissions of CO2 (>85% of total annual GHG emissions) to meet energy and transportation requirements. Other notable (non-ecosystem) sources of carbon GHG emissions in 2004 were from cement- and lime-making industries (7%), livestock-based agriculture (5%), and waste treatment activities (2%). The NASA-CASA (Carnegie Ames Stanford Approach) simulation model based on satellite observations of monthly vegetation cover (including those from the Moderate Resolution Imaging Spectroradiometer, MODIS) was used to estimate net ecosystem fluxes and vegetation biomass production over the period 1990–2004. California's annual NPP for all ecosystems in the early 2000s (estimated by CASA at 120 MMTCE per year) was roughly equivalent to its annual fossil fuel emission rates for carbon. However, since natural ecosystems can accumulate only a small fraction of this annual NPP total in long-term storage pools, the net ecosystem sink flux for atmospheric carbon across the state was estimated at a maximum rate of about 24 MMTCE per year under favorable precipitation conditions. Under less favorable precipitation conditions, such as those experienced during the early 1990s, ecosystems statewide were estimated to have lost nearly 15 MMTCE per year to the atmosphere. Considering the large amounts of carbon estimated by CASA to be stored in forests, shrublands, and rangelands across the state, the importance of protection of the natural NPP capacity of California ecosystems cannot be overemphasized.

Short‐term propagation of rainfall perturbations on terrestrial ecosystems in central California

AbstractQuestion: Does vegetation buffer or amplify rainfall perturbations, and is it possible to forecast rainfall using mesoscale climatic signals?Location: Central California (USA).Methods: The risk of dry or wet rainfall events was evaluated using conditional probabilities of rainfall depending on El Niño Southern Oscillation (ENSO) events. The propagation of rainfall perturbations on vegetation was calculated using cross‐correlations between monthly seasonally adjusted (SA) normalized difference vegetation index (NDVI) from the Advanced Very High Resolution Radiometer (AVHRR), and SA antecedent rainfall at different time‐scales.Results: In this region, El Niño events are associated with higher than normal winter precipitation (probability of 73%). Opposite but more predictable effects are found for La Niña events (89% probability of dry events). Chaparral and evergreen forests showed the longest persistence of rainfall effects (0‐8 months). Grasslands and wetlands showed low persistence (0‐2 months), with wetlands dominated by non‐stationary patterns. Within the region, the NDVI spatial patterns associated with higher (lower) rainfall are homogeneous (heterogeneous), with the exception of evergreen forests.Conclusions: Knowledge of the time‐scale of lagged effects of the non‐seasonal component of rainfall on vegetation greenness, and the risk of winter rainfall anomalies lays the foundation for developing a forecasting model for vegetation greenness. Our results also suggest greater competitive advantage for perennial vegetation in response to potential rainfall increases in the region associated with climate change predictions, provided that the soil allows storing extra rainfall.

Archaeology Meets Marine Ecology: The Antiquity of Maritime Cultures and Human Impacts on Marine Fisheries and Ecosystems

Interdisciplinary study of coastal archaeological sites provides a wealth of information on the ecology and evolution of ancient marine animal populations, the structure of past marine ecosystems, and the history of human impacts on coastal fisheries. In this paper, we review recent methodological developments in the archaeology and historical ecology of coastal regions around the world. Using two case studies, we examine (a) a deep history of anthropogenic effects on the marine ecosystems of California's Channel Islands through the past 12,000 years and (b) geographic variation in the effects of human fishing on Pacific Island peoples who spread through Oceania during the late Holocene. These case studies--the first focused on hunter-gatherers, the second on maritime horticulturalists-provide evidence for shifting baselines and timelines, documenting a much deeper anthropogenic influence on many coastal ecosystems and fisheries than considered by most ecologists, conservation biologists, and fisheries managers.

How to quantify tree leaf area index in an open savanna ecosystem: A multi-instrument and multi-model approach

Savannas are spatially heterogeneous, open ecosystems, thus efforts to quantify canopy structure with methods developed for homogeneous, closed canopies are prone to failure. We examine the applicability of two direct (litterfall, allometry) and five indirect (LAI-2000, TRAC, digital hemispheric photography, digital cover photography, traversing radiometer system) methods to determine leaf area index across a 9 ha domain in an oak-savanna ecosystem in California, USA. Interpretation of the leaf area index measurements is supported by two gap-fraction models. We recommend that leaf inclination angle distribution should be characterized first. For this purpose, we propose a simple, reliable and reproducible method using a digital camera. We show that the combination of digital cover photography and LAI-2000 could provide spatially representative leaf area index, gap fraction and element clumping index. Based on these two indirect methods, we quantify spatially representative element clumping index and leaf area index at ecosystem scale as 0.49 ± 0.10 (mean ± 95% confidence interval) and 0.77 ± 0.27, respectively. In contrast to previous studies in northern ecosystems, measurement and modeling results suggest that element clumping index decreases with view zenith angle, most likely due to apparent changes of tree distribution pattern with the view zenith angle. Our results highlight the importance of ecosystem-scale clumping effects for the adequate quantification of tree leaf area index in savannas. Finally, we suggest a protocol to quantify leaf area index and its associated canopy structure variables in open canopy ecosystems.

Charcoal Distribution Affects Carbon and Nitrogen Contents in Forest Soils of California

Fire is the dominant natural disturbance regime in most ecosystems of California. The long‐term relict of fire is charcoal, which has been shown to increase N mineralization and also represents a pool of chemically stabilized C whose quantity and spatial distribution have not been well characterized in forest soils. We examined the charcoal content in three different ecosystems of the Sierra Nevada, including oak woodland (low elevation), mixed conifer (middle elevation), and red fir (high elevation). Using a fine‐scale (2.5‐m minimum resolution) spatially explicit sampling protocol applied to plots ranging between 0.25 to 0.5 ha, we examined the autocorrelation of forest floor and mineral soil properties including charcoal C. Charcoal C content ranged from 1000 to 5000 kg ha−1 in the surface 6 cm of soil and increased with increasing elevation and latitude. Spatial patterns of forest floor and mineral soil properties were generally patchy at a scale of 5 to 20 m, except in the mixed conifer ecosystem (no pattern). The patchiness that existed at the other sites was largely a result of the distribution of total C and total N in the mineral soil. A spatial mixed‐effect ANOVA indicated that charcoal had a 10 to 20% effect on C and N contents in both forest floor and mineral soil surface horizons, independent of other parameters including ecosystem type and total C or N. These results provide evidence that charcoal has a relationship with soil C and N content, which may influence soil biogeochemistry.

Molecular hydrogen uptake by soils in forest, desert, and marsh ecosystems in California

The mechanism and environmental controls on soil hydrogen (H2) uptake are not well understood but are essential for understanding the atmospheric H2 budget. Field observations of soil H2 uptake are limited, and here we present the results from a series of measurements in forest, desert, and marsh ecosystems in southern California. We measured soil H2 fluxes using flux chambers from September 2004 to July 2005. Mean H2 flux rates and standard deviations were −7.9 + −4.2, −7.6 + −5.3 and −7.5 + −3.4 nmol m−2 s−1 for the forest, desert, and marsh, respectively (corresponding to deposition velocities of 0.063 + −0.029, 0.051 + −0.036, 0.035 + −0.013 cm s−1). Soil profile measurements showed that H2 mixing ratios were between 3% and 51% of atmospheric levels at 10 cm and that the penetration of H2 into deeper soil layers increased with soil drying. Soil removal experiments in the forest demonstrated that the litter layer did not actively consume H2, the removal of this layer increased uptake by deeper soil layers, and the exposure of subsurface soil layers to ambient atmospheric H2 levels substantially increased their rate of uptake. Similar soil removal experiments at the desert site showed that extremely dry surface soils did not consume H2 and that fluxes at the surface increased when these inactive layers were removed. We present a model of soil H2 fluxes and show that the diffusivity of soils, along with the vertical distribution of layers that actively consume H2 regulate surface fluxes. We found that soil organic matter, CO2 fluxes, and ecosystem type were not strong controllers of H2 uptake. Our experiments highlight H2 diffusion into soils as an important limit on fluxes and that minimum moisture level is needed to initiate microbial uptake.

Chloroform, carbon tetrachloride and methyl chloroform fluxes in southern California ecosystems

Chloroform (CHCl 3), carbon tetrachloride (CCl 4), and methyl chloroform (CH 3CCl 3) are important carriers of chlorine to the stratosphere and account for an estimated 15% of the total organic chlorine in the troposphere, roughly equivalent to chlorine load due to methyl chloride (CH 3Cl). The tropospheric burden of chlorine has declined since 1994, largely due to the restriction of CH 3CCl 3 and CCl 4 use as specified by the Montreal Protocol. However, few field studies have been conducted on the terrestrial-atmosphere exchange of these chlorinated hydrocarbons, leading to uncertainties about the natural cycling of these trace gases. This work shows the results of 75 flux measurements conducted in a variety of southern California ecosystems, including coast sagebrush, chamise chaparral, creosote bush scrub, shoreline, and coastal salt marsh. We find no evidence of a significant soil sink in these ecosystems but rather a small net source of CHCl 3 and possibly CCl 4.