Joseph D. White

Remote Sensing of Forest Fire Severity and Vegetation Recovery

Burned forested areas have patterns of varying burn severity as a consequence of various topographic, vegetation, and meteorological factors. These patterns are detected and mapped using satellite data. Other ecological information can be abstracted from satellite data regarding rates of recovery of vegetation foliage and variation of burn severity on different vegetation types. Middle infrared wavelengths are useful for burn severity mapping because the land cover changes associated with burning increase reflectance in this part of the electromagnetic spectrum. Simple stratification of Landsat Thematic Mapper data define varying classes of burn severity because of changes in canopy cover, biomass removal, and soil chemical composition. Reasonable maps of burn severity are produced when the class limits of burn severity reflectance are applied to the entire satellite data. Changes in satellite reflectance over multiple years reveal the dynamics of vegetation and fire severity as low burn areas have lower changes in reflectance relative to high burn areas. This results as a consequence of how much the site was altered due to the burn and how much space is available for vegetation recovery. Analysis of change in reflectance across steppe, riparian, and forested vegetation types indicate that fires potentially increase biomass in steppe areas, while riparian and forested areas are slower to regrow to pre-fire conditions. This satellite-based technology is useful for mapping severely burned areas by exploring the ecological manifestations before and after fire.

Testing scale dependent assumptions in regional ecosystem simulations

. We present a Regional Ecosystem Simulation System (RESSys) which uses satellite data to define vegetation properties, topographic and soil data to define site characteristics, and a climate generator program to build a topographically sensitive microclimate map. We use a 150-km2 mountainous forested watershed in Glacier National Park to test the consequences of modeling various ecosystems processes using different versions of RESSys with increasing simplification of the landscape: (1) spatial scaling generated using 30 m x 30 m Landsat Thematic Mapper data versus 1 km x 1 km Advanced Very High Resolution Radiometer data for vegetation definition; (2) modeling hydrologic dynamics produced by using a topographic routing model versus a simple soil ‘bucket’ model; (3) variable landscape partitioning based on patterns of topographic complexity; and (4) representation of annual net primary productivity (ANPP) using an absorbed photosynthetic active radiation (APAR) model. We evaluate results of these simulations by comparison with average values and areal distributions of photosynthesis, evapotranspiration, hydrologic outflow, and ANPP. Our primary goal is to test whether areal average flux of carbon and water can be scaled linearly over a complex landscape. We found that daily photosynthesis could be predictably estimated between modeling scales with correlation coefficients ranging between 0.89 to 0.99. ANPP was highly correlated among the modeling scales with maximum differences between ANPP prediction of ca. 0.5Mg C ha-1 yr-1. Evapotranspiration was similarly predictable between scales but was influenced by differences associated with hydrologic modeling. Hydrologic outflow was not highly correlated between different modeling scales as a function of the different hydrologic models used at different scales.

Assessing simulated ecosystem processes for climate variability research at Glacier National Park, USA

Glacier National Park served as a test site for ecosystem analyses that involved a suite of integrated models embedded within a geographic information system. The goal of the exercise was to provide managers with maps that could illustrate probable shifts in vegetation, net primary production (NPP), and hydrologic responses associated with two selected climatic scenarios. The climatic scenarios were (a) a recent 12-yr record of weather data, and (b) a reconstituted set that sequentially introduced in repeated 3-yr intervals wetter–cooler, drier–warmer, and typical conditions. To extrapolate the implications of changes in ecosystem processes and resulting growth and distribution of vegetation and snowpack, the model incorporated geographic data. With underlying digital elevation maps, soil depth and texture, extrapolated climate, and current information on vegetation types and satellite-derived estimates of leaf area indices, simulations were extended to envision how the park might look after 120 yr. The p...

Estimates of New Zealand forest and scrub biomass from the 3-PG model

Abstract We present the application of a simple physiological model (3-PG) for estimating biomass accumulation in New Zealand vegetation. The simulation was performed utilising monthly surfaces of temperature, precipitation, and radiation coupled with digital soil maps classified into fertility classes at a 1-km 2 resolution. From 3-PG simulations, we investigated trends in long-term biomass accumulation by simulating vegetation growth over the entire country for 100 years. The predictions were compared with data collected at a number of spatial scales including: (1) individual plot measurements of forest and scrub stem biomass; (2) average stem biomass for South Island forest types; and (3) total vegetation biomass for forest and scrub vegetation types for North and South Islands. The model was calibrated by comparing simulated stem biomass data to literature values for individual plots. Simulated and plot based estimates of stem biomass were highly correlated ( r 2 =0.98) once key parameters were calibrated for New Zealand vegetation. 3-PG predictions correlated well with regional estimates of aboveground stem biomass for the South Island of New Zealand ( r 2 =0.82) and also with total vegetation biomass for the entire country ( r 2 =0.72). However, both were slightly underestimated due to factors associated with assumptions about allocation of biomass to roots, soil characteristics, and stand age. The results indicate that climate and soil fertility exert considerable control on biomass accumulation at the broad scale. Application of the 3-PG model can provide accurate national estimates of biomass that supplement field-based programs by improving large-scale field sampling efficiency and highlight research related to physiology, biomass allocation patterns, and environmental mapping.

Long-term climate forcing by atmospheric oxygen concentrations

Change was in the air The atmospheric fraction of molecular oxygen gas, O2, currently at 21%, is thought to have varied between around 35 and 15% over the past 500 million years. Because O2 is not a greenhouse gas, often this variability has not been considered in studies of climate change. Poulson and Wright show that indirect effects of oxygen abundance, caused by contributions to atmospheric pressure and mean molecular weight, can affect precipitation and atmospheric humidity (see the Perspective by Peppe and Royer). These effects may thus have produced significant changes in the strength of greenhouse forcing by water vapor, surface air temperatures, and the hydrological cycle in the geological past. Science, this issue p. 1238; see also p. 1210 Atmospheric oxygen concentrations may have had an important indirect effect on climate in the distant past. [Also see Perspective by Peppe and Royer] The percentage of oxygen in Earth’s atmosphere varied between 10% and 35% throughout the Phanerozoic. These changes have been linked to the evolution, radiation, and size of animals but have not been considered to affect climate. We conducted simulations showing that modulation of the partial pressure of oxygen (pO2), as a result of its contribution to atmospheric mass and density, influences the optical depth of the atmosphere. Under low pO2 and a reduced-density atmosphere, shortwave scattering by air molecules and clouds is less frequent, leading to a substantial increase in surface shortwave forcing. Through feedbacks involving latent heat fluxes to the atmosphere and marine stratus clouds, surface shortwave forcing drives increases in atmospheric water vapor and global precipitation, enhances greenhouse forcing, and raises global surface temperature. Our results implicate pO2 as an important factor in climate forcing throughout geologic time.

ARE WATERSHED AND LACUSTRINE CONTROLS ON PLANKTONIC N2 FIXATION HIERARCHICALLY STRUCTURED

N2 fixation can be an important source of N to limnetic ecosystems and can influence the structure of phytoplankton communities. However, watershed-scale conditions that favor N2 fixation in lakes and reservoirs have not been well studied. We measured N2 fixation and lacustrine variables monthly over a 19-month period in Waco Reservoir, Texas, USA, and linked these data with nutrient-loading estimates from a physically based watershed model. Readily available topographic, soil, land cover, effluent discharge, and climate data were used in the Soil and Water Assessment Tool (SWAT) to derive watershed nutrient-loading estimates. Categorical and regression tree (CART) analysis revealed that lacustrine and watershed correlates of N2 fixation were hierarchically structured. Lacustrine conditions showed greater predictive capability temporally. For instance, low NO3(-) concentration (<25 microg N/L) and high water temperatures (>27 degrees C) in the reservoir were correlated with the initiation of N2 fixation seasonally. When lacustrine conditions were favorable for N2 fixation, watershed conditions appeared to influence spatial patterns of N2 fixation within the reservoir. For example, spatially explicit patterns of N2 fixation were correlated with the ratio of N:P in nutrient loadings and the N loading rate, which were driven by anthropogenic activity in the watershed and periods of low stream flow, respectively. Although N2 fixation contributed <5% of the annual N load to the reservoir, 37% of the N load was derived from atmospheric N2 fixation during summertime when stream flow in the watershed was low. This study provides evidence that watershed anthropogenic activity can exert control on planktonic N2 fixation, but that temporality is controlled by lacustrine conditions. Furthermore, this study also supports suggestions that reduced inflows may increase the propensity of N2-fixing cyanobacterial blooms in receiving waters of anthropogenically modified landscapes.

A combined watershed–water quality modeling analysis of the Lake Waco reservoir: I. Calibration and confirmation of predicted water quality

A coupled watershed–reservoir modeling system was applied to the Lake Waco reservoir and watershed to test possible sources of seasonal excess nutrient concentrations in Lake Waco. The Soil Water Assessment Tool (SWAT) was used for modeling watershed production of water and nutrients. This model included small lakes and dairies in the watershed, located spatially using geographic data. For the reservoir, the 2-dimensional, hydrodynamic model CE-Qual-W2 was used with inputs derived from the SWAT simulations. Calibration of the models was based on observed hydrographic information and stream and reservoir nutrient data. The relationship between predicted and observed stream flow values for the North Bosque River, the major tributary to the reservoir, were highly correlated (r 2= 0.86) for the calibration period 1997–1998. Predicted daily nutrient values near the inflow of the North Bosque into Lake Waco reservoir were variable but similar to previously estimated annual loading values. Comparison of predicted water quality characteristics from the CE-Qual-W2 model with observed values showed acceptably reliable correspondence seasonally. Chlorophyll-a was used as the main measure for calibration accuracy due to its importance for reservoir management and was predicted to within 6.6% based on the mean percent error or an absolute root mean square error of 9.76 μg/L of observed values. The prediction of stream values from the SWAT model and reservoir nutrient concentrations were sensitive and robust for the next phase of the Comprehensive Lake Waco Study, which includes evaluation of multiple watershed and reservoir management options.

3-PG Productivity Modeling of Regenerating Amazon Forests: Climate Sensitivity and Comparison with MODIS-Derived NPP

Abstract Potential forest growth predicted by the Physiological Principles in Predicting Growth (3-PG) model was compared for forest and deforested areas in the Legal Amazon to assess potential differing regeneration associated with climate. Historical deforestation and regeneration have occurred in environmentally marginal areas that influence regional carbon sequestration estimates. Effects of El Nino–induced drought further reduce simulated production by decreasing soil water availability in areas with shallow soils and high transpiration potential. The model was calibrated through comparison of literature biomass and with satellite-based estimates. Net primary productivity (NPP) for mature Amazonian forests from the 3-PG model was positively correlated (r 2 = 0.77) with a Moderate Resolution Imaging Spectroradiometer (MODIS)-derived algorithm, though with some bias. Annual total NPP for the study area using a 1961–90 average climatology was 4.6 Pg C yr−1, which decreased to 4.2 Pg C yr−1 when simulate...

A combined watershed–water quality modeling analysis of the Lake Waco reservoir: II. Watershed and reservoir management options and outcomes

In this study, calibrated watershed and reservoir models are used to explore a range of possible watershed conditions and potential management options to reduce available nutrients and algal growth in the Lake Waco reservoir. The management options are divided between watershed and reservoir options. The watershed management options include wetland construction, manure haul-off, agriculture conversion to pasture, absolute nutrient retention in the watershed and control of urban nutrient run-off. For the reservoir, management options of phosphorus inactivation and increased algal consumption by grazers were evaluated. For all individual management scenarios, only complete conversion of agricultural lands into rangeland decreased nutrient levels and algae growth significantly and achieved target levels for chlorophyll-a and total phosphorus. Combined management scenarios including wetland construction, manure haul-off from dairy operations and increased in-reservoir herbivory could further reduce chlorophyll-a and nutrient values, but with less efficiency than agricultural conversion alone. The management option study showed that decreasing nutrient inputs and water clarity were important factors for controlling algal growth in Lake Waco, and that substantial reduction in total phosphorus is needed to achieve target conditions.

Loss of Neighbors, Fire, and Climate Effects on Texas Red Oak Growth in a Juniper-dominated Woodland Ecosystem

Abstract In this study, we assessed growth response of Quercus buckleyi (Texas red oak), a current codominant deciduous oak species in central Texas woodlands, to changes in competition, fire, and climate over time to evaluate factors related to documented regional decline of this species. For this analysis, we collected 372 tree slabs of Texas red oaks from the woodlands of the Balcones Canyonlands National Wildlife Refuge near Austin, Texas, from which we aged fire scars and measured tree-ring widths to calculate basal area increment and ring-width indices. To determine canopy conditions of these trees over time, we used historical aerial photos from 1937 to 2004 acquired approximately every 15 y to evaluate changes in woody vegetation cover for the locations of the trees sampled. Our results showed that trees affected by loss of local woody vegetation cover, as evaluated by the aerial photographs, and fire had higher average basal area increment than trees without fire evidence and loss of cover. These differences were significant when aspect and slope were added to the analysis. For climate, we found significant correlation between annual Palmer Drought Severity Indices and ring-width indices for the time interval of 1937–1978, but not after, indicative of potential recent decoupling between tree-ring changes and climate. We found drought to potentially be a major driver of community change in this system as it affects tree-ring response, fire, and mortality assessed from sampled trees.