Greenhouse gas emissions from an arid-zone reservoir and their environmental policy significance: Results from existing global models and an exploratory dataset.

Reservoirs in arid regions often provide critical water storage but little is known about their greenhouse gas (GHG) footprint. While there is growing appreciation of the role reservoirs play as GHG sources, there is a lack of understanding of GHG emission dynamics from reservoirs in arid regions and implications for environmental policy. Here we present initial GHG emission measurements from Lake Powell, a large water storage reservoir in the desert southwest United States. We report CO2-eq emissions from the shallow (< 15 m) littoral regions of the reservoir that are higher than the global average areal emissions from reservoirs (9.4 vs. 5.8 g CO2-eq m-2 d-1) whereas fluxes from the main reservoir were two orders of magnitude lower (0.09 g CO2-eq m-2 d-1). We then compared our measurements to modeled CO2 + CH4 emissions from the reservoir using four global scale models. Factoring these emissions into hydropower production at Lake Powell yielded low GHG emissions per MWh-1 as compared to fossil-fuel based energy sources. With the exception of one model, the estimated hydropower emissions for Lake Powell ranged from 10-32 kg CO2-eq MWh-1, compared to ∼400-1000 kg CO2-eq MWh-1 for natural gas, oil, and coal. We also estimate that reduced littoral habitat under low water levels leads to ∼50% reduction in the CO2 equivalent emissions per MWh. The sensitivity of GHG emissions to reservoir water levels suggests that the interaction will be an important policy consideration in the design and operation of arid region systems.

The biggest bang for the buck: cost-effective vegetation treatment outcomes across drylands of the western United States.

Restoration and rehabilitation are globally implemented to improve ecosystem condition but often without tracking treatment expenditures relative to ecological outcomes. We evaluated the cost-effectiveness of widely conducted woody plant and herbaceous invasive plant removals and seeding treatments in drylands of the western United States from 2004 to 2018 to determine how land managers can optimize efforts. Woody plant cover decreased at a similar rate per dollar spent regardless of vegetation removal type, and the dominant invasive species was reduced by herbicide application. Relatively inexpensive herbicide application also had a large positive effect on seeded perennial grass cover that was enhanced by additional cost; while expensive woody mastication treatments had little effect regardless of additional cost. High seed cost was driven by including a large proportion of native species in seed mixes, and combined with high seeding cost, promoted a short-term (2-3 yr) gain in perennial forb cover and species richness. In contrast, seeding and seed mix cost had no bearing on seeded perennial grass cover, in part, because relatively cheap nonnative seeded species rapidly increased in cover. Our results suggest the differential benefits of commonly implemented treatments aimed at reducing wildfire risk, improving wildlife habitat and forage, and reducing erosion. Given the growing need and cost of restoration and rehabilitation, we raise the importance of specifying treatment budgets and objectives, coupled with effectiveness monitoring, to improve future outcomes.

Forest environmental investments and implications for climate change mitigation.

Forest environmental conditions are affected by climate change, but investments in forest environmental quality can be used as part of the climate change mitigation strategy. A key question involving the potential use of forests to store more carbon as part of climate change mitigation is the impact of forest investments on the timing and quantity of forest volumes that affect carbon storage. Using an economic optimization model, we project levels of U.S. forest volumes as indicators of carbon storage for a wide range of private forest investment scenarios. Results show that economic opportunities exist to further intensify timber management on some hectares and reduce the average timber rotation length such that the national volume of standing timber stocks could be reduced relative to projections reflecting historical trends. The national amount of timber volume is projected to increase over the next 50 yr, but then is projected to decline if private owners follow an economic optimization path, such as with more forest type conversions and shorter timber rotations. With perfect foresight, future forest investments can affect current timber harvest levels, with intertemporal linkages based on adjustments through markets. Forest investments that boost regenerated timber yields per hectare would act to enhance ecosystem services (e.g., forest carbon storage) if they are related to the rate of growth and extent of growing stock inventory.