Copyright (c) 2009 University of Nebraska - Lincoln All rights reserved. http://digitalcommons.unl.edu/natrespapers Recent documents in Papers in Natural Resources en-us Sat, 14 Nov 2009 23:17:49 PST 3600 http://digitalcommons.unl.edu/natrespapers/176 http://digitalcommons.unl.edu/natrespapers/176 Fri, 13 Nov 2009 11:20:34 PST Geochemical and geomorphic data from dune fields in southwestern Nebraska provide new evidence that the Nebraska Sand Hills once migrated across the North and South Platte rivers and dammed the largest tributary system to the Missouri River. The Lincoln County and Imperial dune fields, which lie downwind of the South Platte River, have compositions intermediate between the Nebraska Sand Hills (quartz-rich) and northeastern Colorado dunes (K-feldspar-rich). The most likely explanation for the intermediate composition is that the Lincoln County and Imperial dunes are derived in part from the Nebraska Sand Hills and in part from the South Platte River. The only mechanism by which the Nebraska Sand Hills could have migrated this far south is by complete infilling of what were probably perennially dry North Platte and South Platte river valleys. Such a series of events would have required an extended drought, both for activation of eolian sand and decreased discharges in the Platte River system. A nearby major tributary of the North Platte River is postulated to have been blocked by eolian sand about 12,000 14C yr B.P. We propose that an eolian sand dam across the Plattes was constructed at about this same time. Daniel R. Muhs http://digitalcommons.unl.edu/natrespapers/175 http://digitalcommons.unl.edu/natrespapers/175 Fri, 13 Nov 2009 11:15:49 PST The age of sand dunes in the Nebraska Sand Hills has been controversial, with some investigators suggesting a full glacial age and others suggesting that they were last active in the late Holocene. New accelerator mass spectrometry radiocarbon ages of unaltered bison bones and organic rich sediments suggest that eolian sand deposition occurred at least twice in the past 3000 I4C yr B.P. in three widely separated localities and as many as three times in the past 800 14C yr at three other localities. These late Holocene episodes of eolian activity are probably the result of droughts more intense than the 1930s "Dust Bowl" period, based on independent Great Plains climate records from lake sediments and tree rings. However, new geochemical data indicate that the Nebraska Sand Hills are mineralogically mature. Eolian sands in Nebraska have lower K feldspar (and K2O, Rb, and Ba) contents than most possible source sediments and lower K-feldspar contents than dunes of similar age in Colorado. The most likely explanation for mineralogical maturity is reduction of sand sized K feldspar to silt-sized particles via ballistic impacts due to strong winds over many cycles of eolian activity. Therefore, dunes of the Nebraska Sand Hills must have had a long history, probably extending over more than one glacial-interglacial cycle, and the potential for reactivation is high, with or without a future greenhouse warming. Daniel R. Muhs http://digitalcommons.unl.edu/natrespapers/174 http://digitalcommons.unl.edu/natrespapers/174 Tue, 20 Oct 2009 13:49:42 PDT The seasonal cycle of plant community photosynthesis is one of the most important biotic oscillations to mankind. This study built upon previous efforts to develop a comprehensive framework to studying this cycle systematically with eddy covariance flux measurements. We proposed a new function to represent the cycle and generalized a set of phenological indices to quantify its dynamic characteristics. We suggest that the seasonal variation of plant community photosynthesis generally consists of five distinctive phases in sequence each of which results from the interaction between the inherent biological and ecological processes and the progression of climatic conditions and reflects the unique functioning of plant community at different stages of the growing season. We applied the improved methodology to seven vegetation sites ranging from evergreen and deciduous forests to crop to grasslands and covering both cool-season (vegetation active during cool months, e.g. Mediterranean climate grasslands) and warm-season (vegetation active during warm months, e.g. temperate and boreal forests) vegetation types. Our application revealed interesting phenomena that had not been reported before and pointed to new research directions. We found that for the warm-season vegetation type, the recovery of plant community photosynthesis at the beginning of the growing season was faster than the senescence at the end of the growing season while for the coolseason vegetation type, the opposite was true. Furthermore, for the warm-season vegetation type, the recovery was closely correlated with the senescence such that a faster photosynthetic recovery implied a speedier photosynthetic senescence and vice versa. There was evidence that a similar close correlation could also exist for the cool-season vegetation type, and furthermore, the recovery-senescence relationship may be invariant between the warm-season and cool-season vegetation types up to an offset in the intercept. We also found that while the growing season length affected how much carbon dioxide could be potentially assimilated by a plant community over the course of a growing season, other factors that affect canopy photosynthetic capacity (e.g. nutrients, water) could be more important at this time scale. These results and insights demonstrate that the proposed method of analysis and system of terminology can serve as a foundation for studying the dynamics of plant community photosynthesis and such studies can be fruitful. Lianhong Gu http://digitalcommons.unl.edu/natrespapers/173 http://digitalcommons.unl.edu/natrespapers/173 Tue, 20 Oct 2009 13:31:25 PDT Vegetation albedo is a critical component of the Earth's climate system, yet efforts to evaluate and improve albedo parameterizations in climate models have lagged relative to other aspects of model development. Here, we calculated growing season albedos for deciduous and evergreen forests, crops, and grasslands based on over 40 site-years of data from the AmeriFlux network and compared them with estimates presently used in the land surface formulations of a variety of climate models. Generally, the albedo estimates used in land surface models agreed well with this data compilation. However, a variety of models using fixed seasonal estimates of albedo overestimated the growing season albedo of northerly evergreen trees. In contrast, climatemodels that rely on a common two-stream albedo submodel provided accurate predictions of boreal needle-leaf evergreen albedo but overestimated grassland albedos. Inverse analysis showed that parameters of the two-stream model were highly correlated. Consistent with recent observations based on remotely sensed albedo, the AmeriFlux dataset demonstrated a tight linear relationship between canopy albedo and foliage nitrogen concentration (for forest vegetation: albedo = 0.01 + 0.071%N, r2 = 0.91; forests, grassland, and maize: albedo = 0.02 + 0.067%N, r2 = 0.80). However, this relationship saturated at the higher nitrogen concentrations displayed by soybean foliage. We developed similar relationships between a foliar parameter used in the two-stream albedo model and foliage nitrogen concentration. These nitrogen-based relationships can serve as the basis for a new approach to land surface albedo modeling that simplifies albedo estimation while providing a link to other important ecosystem processes. David Y. Hollinger http://digitalcommons.unl.edu/natrespapers/172 http://digitalcommons.unl.edu/natrespapers/172 Tue, 20 Oct 2009 13:13:17 PDT We developed a stoichiometrically explicit computer model to examine how heterotrophic uptake of nutrients and microbial mineralization occurring during the decay of leaves in streams may be important in modifying nutrient concentrations. The simulations showed that microbial uptake can substantially decrease stream nutrient concentrations during the initial phases of decomposition, while mineralization may produce increases in concentrations during later stages of decomposition. The simulations also showed that initial nutrient content of the leaves can affect the stream nutrient concentration dynamics and determine whether nitrogen or phosphorus is the limiting nutrient. Finally, the simulations suggest a net retention (uptake > mineralization) of nutrients in headwater streams, which is balanced by export of particulate organic nutrients to downstream reaches. Published studies support the conclusion that uptake can substantially change stream nutrient concentrations. On the other hand, there is little published evidence that mineralization also affects nutrient concentrations. Also, there is little information on direct microbial utilization of nutrients contained in the decaying leaves themselves. Our results suggest several directions for research that will improve our understanding of the complex relationship between leaf decay and nutrient dynamics in streams. J. R. Webster http://digitalcommons.unl.edu/natrespapers/171 http://digitalcommons.unl.edu/natrespapers/171 Tue, 20 Oct 2009 13:04:20 PDT Movements of deer can affect population dynamics, spatial redistribution, and transmission and spread of diseases. Our goal was to model the movement of deer in Nebraska in an attempt to predict the potential for spread of chronic wasting disease (CWD) into eastern Nebraska. We collared and radio-tracked >600 white-tailed deer (Odocoileus virginianus) and mule deer (Odocoileus hemionus) in Nebraska during 1990-2006.We observed large displacements (>10 km) for both species and sexes of deer, including migrations up to 100 km and dispersals up to 50 km. Average distance traveled between successive daily locations was 166m for male and 173 for female deer in eastern Nebraska, and 427m for male and 459 for female deer in western Nebraska. Average daily displacement from initial capture point was 10m for male and 14m for female deer in eastern Nebraska, and 27m for male and 28m for female deer in western Nebraska.We used these data on naturally occurring movements to create and test 6 individual-based models of movement for white-tailed deer and mule deer in Nebraska, including models that incorporated sampling from empirical distributions of movement lengths and turn angles (DIST), correlated random walks (CRW), home point fidelity (FOCUS), shifting home point (SHIFT), probabilistic movement acceptance (MOVE), and probabilistic movement with emigration (MOVEwEMI). We created models in sequence in an attempt to account for the shortcomings of the previous model(s). We used the Kolmogrov-Smirnov goodness-of-fit test to verify improvement of simulated annual displacement distributions to empirical displacement distributions. The best-fit model (D = 0.07 and 0.08 for eastern and western Nebraska, respectively) included a probabilistic-movement chance with emigration (MOVEwEMI) and resulted in an optimal daily movement length of 350m (maximum daily movement length of 2800m for emigrators) for eastern Nebraska and 370m (maximum of 2960m) for western Nebraska. The proportion of deer that moved as emigrators was 0.10 and 0.13 for eastern and western Nebraska, respectively. We propose that the observed spread of CWD may be driven by large movements of a small proportion of deer that help to establish a low prevalence of the disease in areas east of the current endemic area. Our movement models will be used in a larger individual-based simulation of movement, survival, and transmission of CWD to help determine future surveillance and management actions. Charles J. Frost http://digitalcommons.unl.edu/natrespapers/170 http://digitalcommons.unl.edu/natrespapers/170 Tue, 20 Oct 2009 12:50:48 PDT Coyote (Canis latrans) food habits were detennined from 490 scats collected from October 1994 to October 1995 at DeSoto National Wildlife Refuge (DNWR), along the Nebraska/Iowa border. Mammals occurred most frequently, as measured by percent-of-scats (POS) , followed by vegetation, birds, and invertebrates. Mammals also constituted the largest portion of coyote diet, as determined by fresh weight correction factors. Within the mammalian category, white-tailed deer (Odocoileus virginianus) occurred most frequently and constituted the largest portion of diet by fresh weight correction factors. White-tailed deer occurrence and importance in diet peaked in June, which corresponds to the fawning period of white-tailed deer at DNWR. Mammals occurred in greater than 75 POS in all months except July and August, when mammals occurred in 38 and 30 POS, respectively. In July and August, vegetation in coyote scats, primarily mulberries (Morus spp.), was highest at 88 and 83 POS, respectively. Invertebrate occurrence peaked in May and in September. Bird occurrence peaked in December, which corresponded with the snow goose (Chen caerulescens) migration, and May, which corresponded with the nesting period for several species of ground nesting birds. Jeffrey J. Huebschman http://digitalcommons.unl.edu/natrespapers/169 http://digitalcommons.unl.edu/natrespapers/169 Tue, 20 Oct 2009 12:45:08 PDT Using an index based on observations of breeding behaviors, we estimated reproductive success of 31 territorial grasshopper sparrows (Ammodramus savannarum) on Conservation Reserve Program fields in southeast Nebraska. Reproductive success was 52%, and no difference was detected between birds holding interior (>100 m from the edge) vs. edge territories. However, grasshopper sparrows appeared to avoid nesting within 50 m of edge habitats. Territories ranged from 0.36-1.24 ha, and territory size did not differ between successful and unsuccessful males. Jennifer M. DeLisle http://digitalcommons.unl.edu/natrespapers/168 http://digitalcommons.unl.edu/natrespapers/168 Tue, 20 Oct 2009 12:40:37 PDT A study to determine the feasibility of producing forage for grazing livestock under trees was conducted as a step toward evaluating the potential for silvopasture systems in the northern and central Great Plains. The effects of overstory leaf area index (LAI), percentage understory light transmittance (LT), and soil moisture (SM) on yield and crude protein (CP) of big bluestem [Andropogon gerardii Vitman; (BB)], smooth bromegrass [ Bromus inermis Leyss.; (SB)], and mixtures with birdsfoot trefoil [ Lotus corniculatus L.; (BFT)] were examined. The study was conducted in both Scotch pine (Pinus sylvestris L.) and green ash (Fraxinus pennsylvancia Marsh.) tree plantations, at the University of Nebraska Agriculture Research and Development Center near Mead, Nebraska. Thirty-six plots representing a wide range of canopy cover were selected at each location and seeded in April 2000 to BB, SB, or mixtures with BFT. Measurements of LAI, LT, and SM were taken throughout the 2001-growing season and plots were harvested in June and September 2001. Soil moisture generally did not explain much of the variability in yield or CP for BB, SB, or BFT. Cumulative LAI or LT averaged over the growing season was the best predictor of yield or CP, particularly under the pine. Yields of BB and SB increased as LAI decreased or LT increased. Conversely, the CP of BB and SB increased as LT decreased for both the June and September harvests. Both BB and SB maintain relatively high productivity under partial shading; however, BFT yields were low at LT levels below75%. Michael El. L. Perry http://digitalcommons.unl.edu/natrespapers/167 http://digitalcommons.unl.edu/natrespapers/167 Tue, 22 Sep 2009 13:27:57 PDT Vertebrate pest control deals with wild animals that create health hazards, damage resources, or become a general nuisance. Some people prefer the terms "wildlife damage control" or "animal damage control" because they focus on controlling damage rather than controlling animals. This is an important principle. Vertebrate animals that cause damage often have many positive values. In dealing with these situations our objective should always be to reduce or prevent damage; this does not necessarily require the killing of animals. I use the term "vertebrate pest control" because I believe it is less ambiguous than "damage control." When I call an animal a "pest," I do so realizing this is a subjective term, and another person may have a different opinion. Robert M. Timm