http://hdl.handle.net/1957/1309 Tue, 21 May 2013 20:33:21 GMT 2013-05-21T20:33:21Z http://hdl.handle.net/1957/38688 Descriptors of natural thermal regimes in streams and their responsiveness to change in the Pacific Northwest of North America Arismendi, Ivan; Johnson, Sherri L.; Dunham, Jason B.; Haggerty, Roy 1. Temperature is a major driver of ecological processes in stream ecosystems, yet the dynamics of thermal regimes remain poorly described. Most work has focused on relatively simple descriptors that fail to capture the full range of conditions that characterise thermal regimes of streams across seasons or throughout the year. 2. To more completely describe thermal regimes, we developed several descriptors of magnitude, variability, frequency, duration and timing of thermal events throughout a year. We evaluated how these descriptors change over time using long-term (1979–2009), continuous temperature data from five relatively undisturbed cold-water streams in western Oregon, U.S.A. In addition to trends for each descriptor, we evaluated similarities among them, as well as patterns of spatial coherence, and temporal synchrony. 3. Using different groups of descriptors, we were able to more fully capture distinct aspects of the full range of variability in thermal regimes across space and time. A subset of descriptors showed both higher coherence and synchrony and, thus, an appropriate level of responsiveness to examine evidence of regional climatic influences on thermal regimes. Most notably, daily minimum values during winter–spring were the most responsive descriptors to potential climatic influences. 4. Overall, thermal regimes in streams we studied showed high frequency and low variability of cold temperatures during the cold-water period in winter and spring, and high frequency and high variability of warm temperatures during the warm-water period in summer and autumn. The cold and warm periods differed in the distribution of events with a higher frequency and longer duration of warm events in summer than cold events in winter. The cold period exhibited lower variability in the duration of events, but showed more variability in timing. 5. In conclusion, our results highlight the importance of a year-round perspective in identifying the most responsive characteristics or descriptors of thermal regimes in streams. The descriptors we provide herein can be applied across hydro-ecological regions to evaluate spatial and temporal patterns in thermal regimes. Evaluation of coherence and synchrony of different components of thermal regimes can facilitate identification of impacts of regional climate variability or local human or natural influences. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. The published article is copyrighted by Wiley-Blackwell and can be found at: http://onlinelibrary.wiley.com/. Wed, 01 May 2013 00:00:00 GMT http://hdl.handle.net/1957/38688 2013-05-01T00:00:00Z http://hdl.handle.net/1957/38575 Post-fire vegetation response to snow in the western United States Blauvelt, Katie The western United States is experiencing significant changes in wildfire and snow regimes as a result of warming temperatures. An amplification of wildfire activity and reduction in snow water equivalent, snow covered area, and earlier spring snowmelt are documented trends that are projected to continue into the future. With an increase in wildfire activity, it is important to understand how a reduction in snow will impact regenerating vegetation in the western United States. The first objective of this study was to assess summer vegetation biomass response to antecedent winter snow on a local scale by determining the physiographic characteristics that influence the relationship between snow and vegetation in the case of the 2002 Biscuit Fire. The second objective was to assess the broad scale regional patterns of regenerating vegetation response to snow, by comparing the correlation between summer vegetation biomass and antecedent winter snow before and after large wildfires across the western United States. Remote sensing data and spatial-temporal statistics were used to analyze the relationship between snow and vegetation. In the local scale analysis, the 2002 Biscuit Fire was analyzed, which burned over 2,000 km² in southwest Oregon and northern California. Nonparametric Multiplicative Regression (NPMR) was used to explore the complex relationships between multiple predictor variables (winter snow frequency, elevation, slope, aspect, and burn severity) and the summer vegetation response variable (enhanced vegetation index, or EVI), before and after the Biscuit Fire burned. The burned area was subset by soil type to determine how soil texture influenced the snow and vegetation relationship. In the regional scale analysis, the Pearson's Correlation Coefficient was calculated to analyze the relationship between winter snow frequency and summer EVI before and after 23 wildfires across the western United States. In the case of the Biscuit Fire, summer EVI responded negatively to snow before the fire, and responded positively to snow after the fire. EVI in coarse-textured skeletal soils exhibited the clearest shift to a positive response to snow after the fire burned, while EVI in fine-textured clay soils did not exhibit this type of shift. The regional analysis proved that wildfire disturbances affect the relationship between snow and vegetation differently across the western United States. Seven fires clustered near the Biscuit Fire in northern California and southwestern Oregon behaved similar to the Biscuit Fire, shifting from a negative pre-fire snow and EVI correlation to a less negative or positive post-fire snow and EVI correlation. The majority of these fires had relatively low average elevations (430 to 1708 m) with greater than 80% forest land cover. Ten fire areas exhibited a significant positive pre and post-fire snow and EVI correlation. The majority of these fires had relatively high average elevations (1612 to 2291 m) and consisted of greater than 50% shrub, scrub, and grass land cover. The local scale analysis suggests that the condition of the vegetation (undisturbed vs. regenerating) and the soil texture in which it grows affects its response to winter snow. The low water holding capacity of coarse-textured soils and the short root-lengths of regenerating vegetation may result in greater dependence on snow as a water resource. Regionally, vegetation type and elevation may affect the vegetation's response to snow; short-rooted shrubs at higher elevations above the transient snow zone may be more dependent on snow as a water resource. These results suggest that the relationship between snow and vegetation is not constant, depending on the condition of the vegetation. Increases in wildfire activity and a reduction of snow in the future may impact successional trajectories in certain regions where vegetation may have historically relied on snowmelt to regenerate. Graduation date: 2013 Thu, 02 May 2013 00:00:00 GMT http://hdl.handle.net/1957/38575 2013-05-02T00:00:00Z http://hdl.handle.net/1957/38567 Impacts of aerosol particles on the microphysical and radiative properties of stratocumulus clouds over the southeast Pacific Ocean Twohy, C. H.; Anderson, J. R.; Toohey, D. W.; Andrejczuk, M.; Adams, A.; Lytle, M.; George, R. C.; Wood, R.; Saide, P.; Spak, S.; Zuidema, P.; Leon, D. The southeast Pacific Ocean is covered by the world's largest stratocumulus cloud layer, which has a strong impact on ocean temperatures and climate in the region. The effect of anthropogenic sources of aerosol particles on the stratocumulus deck was investigated during the VOCALS field experiment. Aerosol measurements below and above cloud were made with a ultra-high sensitivity aerosol spectrometer and analytical electron microscopy. In addition to more standard in-cloud measurements, droplets were collected and evaporated using a counterflow virtual impactor (CVI), and the non-volatile residual particles were analyzed. Many flights focused on the gradient in cloud properties on an E-W track along 20 degrees S from near the Chilean coast to remote areas offshore. Mean statistics, including their significance, from eight flights and many individual legs were compiled. Consistent with a continental source of cloud condensation nuclei, below-cloud accumulation-mode aerosol and droplet number concentration generally decreased from near shore to offshore. Single particle analysis was used to reveal types and sources of the enhanced particle number that influence droplet concentration. While a variety of particle types were found throughout the region, the dominant particles near shore were partially neutralized sulfates. Modeling and chemical analysis indicated that the predominant source of these particles in the marine boundary layer along 20 degrees S was anthropogenic pollution from central Chilean sources, with copper smelters a relatively small contribution. Cloud droplets were smaller in regions of enhanced particles near shore. However, physically thinner clouds, and not just higher droplet number concentrations from pollution, both contributed to the smaller droplets. Satellite measurements were used to show that cloud albedo was highest 500-1000 km offshore, and actually slightly lower closer to shore due to the generally thinner clouds and lower liquid water paths there. Thus, larger scale forcings that impact cloud macrophysical properties, as well as enhanced aerosol particles, are important in determining cloud droplet size and cloud albedo. Differences in the size distribution of droplet residual particles and ambient aerosol particles were observed. By progressively excluding small droplets from the CVI sample, we were able to show that the larger drops, some of which may initiate drizzle, contain the largest aerosol particles. Geometric mean diameters of droplet residual particles were larger than those of the below-cloud and above cloud distributions. However, a wide range of particle sizes can act as droplet nuclei in these stratocumulus clouds. A detailed LES microphysical model was used to show that this can occur without invoking differences in chemical composition of cloud-nucleating particles. This is the publisher’s final pdf. The published article is copyrighted by Copernicus Publications and can be found at: http://publications.copernicus.org/. Tue, 05 Mar 2013 00:00:00 GMT http://hdl.handle.net/1957/38567 2013-03-05T00:00:00Z http://hdl.handle.net/1957/38558 Assessing the stability of tree ranges and influence of disturbance in eastern US forests Woodall, C.W.; Zhu, K.; Westfall, J.A.; Oswalt, C.M.; D’Amato, A.W.; Walters, B.F.; Lintz, H.E. Shifts in tree species ranges may occur due to global climate change, which in turn may be exacerbated by natural disturbance events. Within the context of global climate change, developing techniques to monitor tree range dynamics as affected by natural disturbances may enable mitigation/adaptation of projected impacts. Using a forest inventory across the eastern U.S., the northern range margins of tree distributions were examined by comparing differences in the 95th percentile locations of seedlings to adults (i.e., trees) by 0.5° longitudinal bands over 5-years and by levels of disturbance (i.e., canopy gap formation). Our results suggest that the monitoring of tree range dynamics is complicated by the limits of forest inventory data across varying spatial/temporal scales and the diversity of tree species/environments in the eastern U.S. The vast majority of tree and seedling latitudinal comparisons across measurement periods and levels of disturbance in the study were not statistically different from zero (53 out of 60 comparisons). A potential skewing of ranges towards a northern limit was suggested by the stability of northern margins of tree ranges found in this study and shifts in mean locations identified in previous work. Only a partial influence of disturbances on tree range dynamics during the course of the 5-years was found in this study. The results of this study underscore the importance of continued examination of the role of disturbance in tree range dynamics and refined range monitoring techniques given future forest extent and biodiversity implications. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.elsevier.com/. Mon, 07 Jan 2013 00:00:00 GMT http://hdl.handle.net/1957/38558 2013-01-07T00:00:00Z