The ratio of leaf internal (ci) to ambient (ca) partial pressure of CO2, defined here as χ, is an index of adjustments in both leaf stomatal conductance and photosynthetic rate to environmental conditions. Measurements and proxies of this ratio can be used to constrain vegetation model uncertainties for predicting terrestrial carbon uptake and water use.
We test a theory based on the least-cost optimality hypothesis for modelling historical changes in χ over the 1951–2014 period, across different tree species and environmental conditions, as reconstructed from stable carbon isotopic measurements across a global network of 103 absolutely dated tree-ring chronologies. The theory predicts optimal χ as a function of air temperature, vapour pressure deficit, ca and atmospheric pressure.
The theoretical model predicts 39% of the variance in χ values across sites and years, but underestimates the intersite variability in the reconstructed χ trends, resulting in only 8% of the variance in χ trends across years explained by the model.
Overall, our results support theoretical predictions that variations in χ are tightly regulated by the four environmental drivers. They also suggest that explicitly accounting for the effects of plant-available soil water and other site-specific characteristics might improve the predictions.
Stable stratification of the nocturnal lower boundary layer inhibits convective turbulence, such that turbulent vertical transfer of ecosystem carbon dioxide (CO2), water vapor (H2O) and energy is driven by mechanically forced turbulence, either from frictional forces near the ground or top of a plant canopy, or from shear generated aloft. The significance of this last source of turbulence on canopy flow characteristics in a closed and open forest canopy is addressed in this paper. We present micrometeorological observations of the lower boundary layer and canopy air space collected on nearly 200 nights using a combination of atmospheric laser detection and ranging (lidar), eddy covariance (EC), and tower profiling instrumentation. Two AmeriFlux/Fluxnet sites in mountain-valley terrain in the Western U.S. are investigated: Wind River, a tall, dense conifer canopy, and Tonzi Ranch, a short, open oak canopy. On roughly 40% of nights lidar detected down-valley or downslope flows above the canopy at both sites. Nights with intermittent strong bursts of “top-down” forced turbulence were also observed above both canopies. The strongest of these bursts increased sub-canopy turbulence and reduced canopy virtual potential temperature (θv) gradient at Tonzi but did not appear to change the flow characteristics within the dense Wind River canopy. At Tonzi we observed other times when high turbulence (via friction velocity, u∗) was found just above the trees, yet CO2 and θv gradients remained large and suggested flow decoupling. These events were triggered by regional downslope flow. Lastly, a set of turbulence parameters is evaluated for estimating canopy turbulence mixing strength. The relationship between turbulence parameters and canopy θv gradients was found to be complex, although better agreement between the canopy θv gradient and turbulence was found for parameters based on the standard deviation of vertical velocity, or ratios of 3-D turbulence to mean flow, than for u∗. These findings add evidence that the relationship between canopy turbulence, static stability, and canopy mixing is far from straightforward even within an open canopy.
On August 14, 2003, the Seattle Forestry Sciences Laboratory was re-named as its fire research program and partnerships grew. The new name, "Pacific Wildland Fire Sciences Laboratory," highlights the lab's fire science leadership.
Research questions cover the impact of fire on air quality and visibility, wildfire and ecology research, the effects of fire on air, impacts of smoke on human health, and social science (rural and urban wildland interface). We study a wide variety of wildland fire topics: fire behavior, combustion science, biomass assessments, fire ecology, fire management, prescribed fires, fire-climate change interactions, landscape ecology, emissions of greenhouse gases, fire policy, and traditional fire use by indigenous communities.
The Pacific Northwest Research Station's Pacific Wildland Fire Sciences Lab is located in the heart of the Fremont District in Seattle, Washington. Close proximity to the University of Washington facilitates joint research with the University of Washington Wildland Fire Sciences Laboratory.
About 21 station scientists and support staff work at the lab, pursuing a range of scientific inquiry across disciplines including pyrology, hydrology, meteorology, sociology, forestry, and more. The lab frequently hosts visiting scientists and post-graduate fellows through partnership with the University of Washington (UW). A number of UW faculty researchers and graduate students sit at the PNW Research Station's Pacific Wildland Fire Sciences Lab.
Chaparral shrublands are the dominant wildland vegetation type in Southern California and the most extensive ecosystem in the state. Disturbance by wildfire and climate change have created a dynamic landscape in which biomass mapping is key in tracking the ability of chaparral shrublands to sequester carbon. Despite this importance, most national and regional scale estimates do not account for shrubland biomass. Employing plot data from several sources, we built a random forest model to predict aboveground live biomass in Southern California using remote sensing data (Landsat Normalized Difference Vegetation Index (NDVI)) and a suite of geophysical variables. By substituting the NDVI and precipitation predictors for any given year, we were able to apply the model to each year from 2000 to 2019. Using a total of 980 field plots, our model had a k-fold cross-validation R2 of 0.51 and an RMSE of 3.9. Validation by vegetation type ranged from R2 = 0.17 (RMSE = 9.7) for Sierran mixed-conifer to R2 = 0.91 (RMSE = 2.3) for sagebrush. Our estimates showed an improvement in accuracy over two other biomass estimates that included shrublands, with an R2 = 0.82 (RMSE = 4.7) compared to R2 = 0.068 (RMSE = 6.7) for a global biomass estimate and R2 = 0.29 (RMSE = 5.9) for a regional biomass estimate. Given the importance of accurate biomass estimates for resource managers, we calculated the mean year 2010 shrubland biomasses for the four national forests that ranged from 3.5 kg/m2 (Los Padres) to 2.3 kg/m2 (Angeles and Cleveland). Finally, we compared our estimates to field-measured biomasses from the literature summarized by shrubland vegetation type and age class. Our model provides a transparent and repeatable method to generate biomass measurements in any year, thereby providing data to track biomass recovery after management actions or disturbances such as fire.
The coastal zone of southeast Alaska contains thousands of streams and rivers that drain one of the wettest, carbon‐rich, and most topographically varied regions in North America. Watersheds draining temperate rainforests, peatlands, glaciers, and three large rivers that flow from the drier interior of the Yukon Territory and British Columbia discharge water and dissolved organic carbon (DOC) into southeast Alaskan coastal waters. This area, which we have designated the southeast Alaska drainage basin (SEAKDB), discharges about twice as much water as the Columbia or Yukon Rivers. An understanding of the timing, location, and source of water and DOC guides research to better understand the influence of terrestrial outputs on the adjacent marine systems. Additionally, a spatially extensive understanding of riverine DOC flux will improve our understanding of lateral losses related to terrestrial carbon cycling. We estimate 1.17 Tg C yr−1 of DOC enters the adjacent marine system along with 430 km2 of freshwater that influences estuary, shelf, and Gulf of Alaska hydrology. We estimate that 23% to 66% of the DOC entering coastal waters is bioavailable and may influence metabolism and productivity within the marine system. The combination of the large and spatially distributed water and DOC input, long and complex shoreline, large enclosed estuarine volume, and bounded nearshore coastal currents suggests that the physiographic structure of southeast Alaska may have a significant impact on the metabolism of riverine DOC in coastal marine ecosystems.
The 741 million acres of forestland in the United States play a role in mitigating the effects of climate change by sequestering nearly 16 percent of the atmospheric carbon dioxide emissions produced annually in our country. Reducing the conversion of forestland to other uses and planting even more trees, whether through afforestation or reforestation, would increase the nation’s carbon storage capacity. The U.S. Department of Agriculture (USDA) has several incentive programs to accomplish these goals.
Researchers with the USDA Forest Service and Portland State University modeled various scenarios to determine how carbon sequestration would increase if the agency increased its financial investment in these tree planting and forest conservation programs. They also modeled how a 10-percent reduction in the area burned by stand-replacing wildfires could affect carbon sequestration. Because increasing levels of atmospheric carbon has a social cost, they calculated the monetary value of the carbon sequestrated.
The research team found that afforestation and reforestation policies yielded the greatest return in carbon sequestration. By 2050, 469 teragrams (Tg) of carbon dioxide equivalent per year (CO2eq/yr) could be sequestered compared to a baseline scenario of 323 TgCO2eq/yr. They estimated the cost of expanding afforestation and reforestation programs at $6.5 billion, far less than the estimated $93.6 billion in monetary benefits that the increased carbon sequestration from expanding these programs was projected to yield.