Gregory S. Okin

Global distribution of atmospheric phosphorus sources, concentrations and deposition rates, and anthropogenic impacts

A worldwide compilation of atmospheric total phosphorus (TP) and phosphate (PO4) concentration and deposition flux observations are combined with transport model simulations to derive the global distribution of concentrations and deposition fluxes of TP and PO4. Our results suggest that mineral aerosols are the dominant source of TP on a global scale (82%), with primary biogenic particles (12%) and combustion sources (5%) important in nondusty regions. Globally averaged anthropogenic inputs are estimated to be similar to 5 and 15% for TP and PO4, respectively, and may contribute as much as 50% to the deposition over the oligotrophic ocean where productivity may be phosphorus-limited. There is a net loss of TP from many (but not all) land ecosystems and a net gain of TP by the oceans (560 Gg P a(-1)). More measurements of atmospheric TP and PO4 will assist in reducing uncertainties in our understanding of the role that atmospheric phosphorus may play in global biogeochemistry.

Practical limits on hyperspectral vegetation discrimination in arid and semiarid environments

Hyperspectral remote sensing is a promising tool for the analysis of vegetation and soils in remote sensing imagery. The purpose of this study is to ascertain how well hyperspectral remote sensing data can retrieve vegetation cover, vegetation type, and soil type in areas of low vegetation cover. We use multiple endmember spectral mixture analysis (MESMA), high-quality field spectra, and AVIRIS data to determine how well full-range spectral mixture analysis (SMA) techniques can retrieve vegetation and soil information. Using simulated AVIRIS-derived reflectance spectra, we find that, in areas of low vegetation cover, MESMA is not able to provide reliable retrievals of vegetation type when covers are less than at least 30%. Overestimations of vegetation are likely, but vegetation cover in many circumstances can be estimated reliably. Soil type retrievals are more than 90% reliable in discriminating dark-armored desert soils from blown sands. This simulation comprises a best-case scenario in which many typical problems with remote sensing in areas of low cover or desert areas are minimized. Our results have broad implications for the applicability of full-range SMA techniques in analysis of data from current and planned hyperspectral sensors. Several phenomena contribute to the unreliability of vegetation retrievals. Spectrally indeterminate vegetation types, characterized by low spectral contrast, are difficult to model correctly even at relatively high covers. Combinations of soil and vegetation spectra have the potential of generating mixtures that resemble an unmixed spectrum from different material, further confounding vegetation cover and soil type retrievals. Intraspecies spectral variability and nonlinear mixing produce uncertainties in spectral endmembers much larger than that only due to instrumental noise modeled here. Having established limits on linear spectral unmixing in areas of low cover through spectral simulations, we evaluate AVIRIS-derived reflectance data from the Mojave Desert, California. We show that MESMA is capable of mapping soil surface types even when vegetation type cannot be reasonable retrieved.

The ecology of dust

Wind erosion and associated dust emissions play a fundamental role in many ecological processes and provide important biogeochemical connectivity at scales ranging from individual plants up to the entire globe. Yet, most ecological studies do not explicitly consider dust-driven processes, perhaps because most relevant research on aeolian (wind-driven) processes has been presented in a geosciences rather than an ecological context. To bridge this disciplinary gap, we provide a general overview of the ecological importance of dust, examine complex interactions between wind erosion and ecosystem dynamics from the scale of plants and surrounding space to regional and global scales, and highlight specific examples of how disturbance affects these interactions and their consequences. It is likely that changes in climate and intensification of land use will lead to increased dust production from many drylands. To address these issues, environmental scientists, land managers, and policy makers need to consider wind erosion and dust emissions more explicitly in resource management decisions.

On soil moisture–vegetation feedbacks and their possible effects on the dynamics of dryland ecosystems

[1] Soil moisture is the environmental variable synthesizing the effect of climate, soil, and vegetation on the dynamics of water-limited ecosystems. Unlike abiotic factors (e.g., soil texture and rainfall regime), the control exerted by vegetation composition and structure on soil moisture variability remains poorly understood. A number of field studies in dryland landscapes have found higher soil water contents in vegetated soil patches than in adjacent bare soil, providing a convincing explanation for the observed preferential establishment of grasses and seedlings beneath tree canopies. Thus, because water is the limiting factor for vegetation in arid and semiarid ecosystems, a positive feedback could exist between soil moisture and woody vegetation dynamics. It is still unclear how the strength of such a feedback would change under different long-term rainfall regimes. To this end, we report some field observations from savanna ecosystems located along the south-north rainfall gradient in the Kalahari, where the presence of relatively uniform sandy soils limits the effects of covarying factors. The data available from our field study suggest that the contrast between the soil moisture in the canopy and intercanopy space increases (with wetter soils under the canopy) with increasing levels of aridity. We hypothesize that this contrast may lead to a positive feedback and explore the implications of such a feedback in a minimalistic model. We found that when the feedback is relatively strong, the system may exhibit two stable states corresponding to conditions with and without tree canopy cover. In this case, even small changes in environmental variables may lead to rapid and largely irreversible shifts to a state with no tree canopy cover. Our data suggest that the tendency of the system to exhibit two (alternative) stable states becomes stronger in the more arid regions. Thus, at the desert margins, vegetation is more likely to be prone to discontinuous and abrupt state changes.

Do Changes in Connectivity Explain Desertification

Arid and semiarid regions cover more than 40% of Earths land surface. Desertification, or broadscale land degradation in drylands, is a major environmental hazard facing inhabitants of the worlds deserts as well as an important component of global change. There is no unifying framework that simply and effectively explains different forms of desertification. In this article, we argue for the unifying concept that diverse forms of desertification, and its remediation, are driven by changes in the length of connected pathways for the movement of fire, water, and soil resources. Biophysical feedbacks increase the length of connected pathways, explaining the persistence of desertified landscapes around the globe. Management of connectivity in the context of environmental and socioeconomic change is essential to understanding, and potentially reversing, the harmful effects of desertification.

Distribution of vegetation in wind‐dominated landscapes: Implications for wind erosion modeling and landscape processes

Dust emission and wind erosion from arid and semiarid environments provide a major source of global atmospheric aerosols. Well-known relations between wind stress and saltation sand flux for sand sheets and relations between sand flux and dust emission by sandblasting have enabled construction of dust models that have only been partly successful in predicting atmospheric mineral dust concentrations. Most models of wind erosion assume that vegetation is evenly distributed. Through the use of field, Fourier transform, and semivariogram analysis, we show that mesquite dunelands in the Chihuahuan Desert of southern New Mexico, United States, have anisotropic shrub distributions. Elongated areas of bare soil, “streets,” which are aligned with the prevailing winds may partially explain discrepancies between observed and predicted atmospheric dust concentrations. Soils in the streets are not protected from winds blowing down the streets and may therefore produce more dust than if vegetation were more evenly distributed. Currently, few desert landscape evolution models take the role of wind explicitly into account. The existence of streets implies that wind plays a major role in the evolution of vegetated arid and semiarid landscapes with wind-erodible soils. Here wind acts in tandem with water to enforce islands of fertility centered around individual shrubs and may provide an explanation for reduced soil fertility observed in shrublands. Furthermore, in order for mathematical models of dust flux to be successful in these landscapes, new landscape models are required which incorporate the existence and orientation of streets.

Responses of wind erosion to climate-induced vegetation changes on the Colorado Plateau

Projected increases in aridity throughout the southwestern United States due to anthropogenic climate change will likely cause reductions in perennial vegetation cover, which leaves soil surfaces exposed to erosion. Accelerated rates of dust emission from wind erosion have large implications for ecosystems and human well-being, yet there is poor understanding of the sources and magnitude of dust emission in a hotter and drier climate. Here we use a two-stage approach to compare the susceptibility of grasslands and three different shrublands to wind erosion on the Colorado Plateau and demonstrate how climate can indirectly moderate the magnitude of aeolian sediment flux through different responses of dominant plants in these communities. First, using results from 20 y of vegetation monitoring, we found perennial grass cover in grasslands declined with increasing mean annual temperature in the previous year, whereas shrub cover in shrublands either showed no change or declined as temperature increased, depending on the species. Second, we used these vegetation monitoring results and measurements of soil stability as inputs into a field-validated wind erosion model and found that declines in perennial vegetation cover coupled with disturbance to biological soil crust resulted in an exponential increase in modeled aeolian sediment flux. Thus the effects of increased temperature on perennial plant cover and the correlation of declining plant cover with increased aeolian flux strongly suggest that sustained drought conditions across the southwest will accelerate the likelihood of dust production in the future on disturbed soil surfaces.

Impacts of biomass burning emissions and land use change on Amazonian atmospheric phosphorus cycling and deposition

Phosphorus (P) availability constrains both carbon uptake and loss in some of the worlds most productive ecosystems. In some of these regions, atmospheric aerosols appear to be an important, if not dominant, source of new P inputs. For example, previous work suggests that mineral aerosols from North Africa bring significant amounts of new phosphorus to the P-impoverished soils of the Amazon Basin. Here we use recent observations and atmospheric transport modeling to show that the Amazon Basin itself appears to be losing atmospheric phosphorus to neighboring regions as a consequence of biomass burning emissions, anthropogenic sources of mineral aerosols and primary biogenic particles. Observations suggest that biomass burning emissions and human disturbance are responsible for ∼23% of the phosphorus flux in the Amazon. Although biomass burning and disturbance may bring new phosphorus into nondisturbed regions, as a whole the Amazon appears to be losing phosphorus through the atmosphere. Phosphorus lost via atmospheric transport from the Amazon is deposited in the adjacent oceans and in other regions downwind. These results suggest that land use change within the Amazon may substantially increase phosphorus availability to the remaining undisturbed forests, and that this fertilization mechanism could potentially contribute to recent changes in carbon uptake measured in undisturbed stands, as well as fertilizing downwind ocean regions.

Impacts of atmospheric nutrient deposition on marine productivity: Roles of nitrogen, phosphorus, and iron

Nutrients are supplied to the mixed layer of the open ocean by either atmospheric deposition or mixing from deeper waters, and these nutrients drive nitrogen and carbon fixation. To evaluate the importance of atmospheric deposition, we estimate marine nitrogen and carbon fixation from present-day simulations of atmospheric deposition of nitrogen, phosphorus, and iron. These are compared with observed rates of marine nitrogen and carbon fixation. We find that Fe deposition is more important than P deposition in supporting N fixation. Estimated rates of atmospherically supported carbon fixation are considerably lower than rates of marine carbon fixation derived from remote sensing, indicating the subsidiary role atmospheric deposition plays in total C uptake by the oceans. Nonetheless, in high-nutrient, low-chlorophyll areas, the contribution of atmospheric deposition of Fe to the surface ocean could account for about 50% of C fixation. In marine areas typically thought to be N limited, potential C fixation supported by atmospheric deposition of N is only ~1%-2% of observed rates. Although these systems are N-limited, the amount of N supplied from below appears to be much larger than that deposited from above. Atmospheric deposition of Fe has the potential to augment atmospherically supported rates of C fixation in N-limited areas. In these areas, atmospheric Fe relieves the Fe limitation of diazotrophic organisms, thus contributing to the rate of N fixation. The most important uncertainties in understanding the relative importance of different atmospheric nutrients are poorly understood speciation and solubility of Fe as well as the N:Fe ratio of diazotrophic organisms.

Atmospheric fluxes of organic N and P to the global ocean

The global tropospheric budget of gaseous and particulate non-methane organic matter (OM) is re-examined to provide a holistic view of the role that OM plays in transporting the essential nutrients nitrogen and phosphorus to the ocean. A global 3-dimensional chemistry-transport model was used to construct the first global picture of atmospheric transport and deposition of the organic nitrogen (ON) and organic phosphorus (OP) that are associated with OM, focusing on the soluble fractions of these nutrients. Model simulations agree with observations within an order of magnitude. Depending on location, the observed water soluble ON fraction ranges from similar to 3% to 90% (median of similar to 35%) of total soluble N in rainwater; soluble OP ranges from similar to 20-83% (median of similar to 35%) of total soluble phosphorus. The simulations suggest that the global ON cycle has a strong anthropogenic component with similar to 45% of the overall atmospheric source (primary and secondary) associated with anthropogenic activities. In contrast, only 10% of atmospheric OP is emitted from human activities. The model-derived present-day soluble ON and OP deposition to the global ocean is estimated to be similar to 16 Tg-N/yr and similar to 0.35 Tg-P/yr respectively with an order of magnitude uncertainty. Of these amounts similar to 40% and similar to 6%, respectively, are associated with anthropogenic activities, and 33% and 90% are recycled oceanic materials. Therefore, anthropogenic emissions are having a greater impact on the ON cycle than the OP cycle; consequently increasing emissions may increase P-limitation in the oligotrophic regions of the worlds ocean that rely on atmospheric deposition as an important nutrient source.