Influence of Small Dams on Downstream Channel Characteristics in Pennsylvania and Maryland: Implications for the Long‐Term Geomorphic Effects of Dam Removal1

Abstract: We evaluate the effects of small dams (11 of 15 sites less than 4 m high) on downstream channels at 15 sites in Maryland and Pennsylvania by using a reach upstream of the reservoir at each site to represent the downstream reach before dam construction. A semi‐quantitative geomorphic characterization demonstrates that upstream reaches occupy similar geomorphic settings as downstream reaches. Survey data indicate that dams have had no measurable influence on the water surface slope, width, and the percentages of exposed bedrock or boulders on the streambed. The median grain diameter (D₅₀) is increased slightly by dam construction, but D₅₀ remains within the pebble size class. The percentage of sand and silt and clay on the bed averages about 35% before dam construction, but typically decreases to around 20% after dam construction. The presence of the dam has therefore only influenced the fraction of finer‐grained sediment on the bed, and has not caused other measurable changes in fluvial morphology. The absence of measurable geomorphic change from dam impacts is explicable given the extent of geologic control at these study sites. We speculate that potential changes that could have been induced by dam construction have been resisted by inerodible bedrock, relatively immobile boulders, well‐vegetated and cohesive banks, and low rates of bed material supply and transport. If the dams of our study are removed, we argue that long‐term changes (those that remain after a period of transient adjustment) will be limited to increases in the percentage of sand and silt and clay on the bed. Thus, dam removal in streams similar to those of our study area should not result in significant long‐term geomorphic changes.     

Perceptions of visibility degradation in Hong Kong

The human dimensions of visibility degradation remain under-researched academically. Based on Zube and Sell's Process Model of the Perception of, and Response to, Environmental Change, a questionnaire survey was conducted in Hong Kong between 5 June 2012 and 26 January 2013 (n = 1,203). The respondents’ perceptions towards visibility degradation were collected. The results show that respondents were moderately knowledgeable to visibility degradation, perceiving the problem as anthropogenic and unfavorable and identifying the fight against the degradation as an important environmental issue. It is further deduced that the respondents’ physical and sociocultural attributes, and the way they gain information about visibility degradation, can affect their perceptions of visibility degradation. The results reveal that improving information channels can positively promote human sensitivity to and understanding of visibility degradation in order to mitigate it. Yet, any attempts to educate the lay public should not assume the existence of a homogenous community of like-minded people.     

Saltcedar control and water salvage on the Pecos river, Texas, 1999–2003

A large scale ecosystem restoration program was initiated in 1997 on the Pecos River in Western Texas. Saltcedar (Tamarix spp.), a non-native invasive tree, had created a near monoculture along the banks of the river by replacing most native vegetation. Local irrigation districts, private landowners, federal and state agencies, and private industry worked together to formulate and implement a restoration plan, with a goal of reducing the effects of saltcedar and restoring the native ecosystem of the river. An initial management phase utilizing state-of-the-art aerial application of herbicide began in 1999 and continued through 2003. Initial mortality of saltcedar averaged about 85–90%. Monitoring efforts were initiated at the onset of the project to include evaluating the effects of saltcedar control on salinity of the river water, efficiency of water delivery down the river as an irrigation water source, and estimates of water salvage. To date, no effect on salinity can be measured and irrigation delivery was suspended in 2002–2003 due to drought conditions. Water salvage estimates show a significant reduction in system water loss after saltcedar treatment. However, a flow increase in the river is not yet evident. Monitoring efforts will continue in subsequent years.     

Production and dispersal of Colletotrichum gloeosporioides spores on Stylosanthes scabra under elevated CO2

This paper reports the effect of twice-ambient (700 ppm) atmospheric CO(2) concentration on infection, disease development, spore production and dispersal of the anthracnose pathogen Colletotrichum gloeosporioides in susceptible (Fitzroy) and partially resistant (Seca) cultivars of the tropical pasture legume Stylosanthes scabra under controlled environment and field conditions. Reduction in plant height due to anthracnose was partially compensated for by growth enhancement at elevated CO(2) in Fitzroy but not in Seca. Anthracnose severity was reduced under elevated CO(2) although the reduction was only significant in Fitzroy. Delayed and reduced germination, germtube growth and appressoria production were partly responsible for the reduced severity. Despite an extended incubation period, C. gloeosporioides developed sporulating lesions faster and produced more spores per day within the same latent period at high CO(2) and ambient CO(2). When Fitzroy seedlings grown at 700 ppm CO(2) were exposed to pathogen inoculum under field conditions, they consistently developed more severe anthracnose with more lesions than seedlings grown at ambient CO(2). The environmental variable, which correlated most strongly with the dispersal and infection of C. gloeosporioides spores in the field, was relative humidity in plant canopy. We have shown that an enlarged Stylosanthes canopy under elevated CO(2) can trap more spores, which can lead to more severe anthracnose under favorable weather. The implications of these findings for perennial Stylosanthes pastures are discussed.     

Environmental psychology or behavioral geography? Either way it was a good start

The paper traces the author's background as a British geographer and the intellectual climate at Clark University in 1968 which led him out of urban social geography into the uncharted domain of children's knowledge of the geographic environment. Following the integration of theory and research on the development of spatial cognition in children came the need to investigate child development in relation to the physical landscape more comprehensively in a Ph.D. dissertation. Subsequent work, built entirely from this early base, continues to investigate children's use, knowledge and experience of physical environment in a variety of settings. Applications of research are made in both the fields of environmental education and environmental planning and design.     

Temperature acclimation and other influences on embryonic duration in the copepod Pseudocalanus sp.

Embryos of Pseudocalanus sp. captured near Halifax, Nova Scotia, Canada, in spring take longer to develop than do those from the warmer waters of summer and autumn. This is related to the larger size of females and their eggs in the colder seasons. An experiment revealed size-assortative mating and the influence of male size as well on embryonic duration. The seasonal change is contrary to the slight reduction of embryonic duration after short-term cold acclimation of females in the laboratory. There is no evidence that embryos in the cold season develop more rapidly than expected from the overall relationship between embryonic duration and female size. However, they do, like embryos after short-term cold acclimation, develop relatively more rapidly at low temperatures; i.e., there is clockwise rotation of the rate-temperature response, equivalent to the temperature adaptation that has been demonstrated among species of copepods. This seasonal rotation may also be related to seasonal variations in size, since larger females from a given season produce embryos that show such rotation. Thus, there is no evidence for seasonal compensation as a consequence of temperature acclimation underlying changes that are attributable to differences in size. Evidently there is also a strong genetical component in duration of development of embryos produced by individual females.     

Behavioural switching in a central place forager: patterns of diving behaviour in the macaroni penguin (Eudyptes chrysolophus)

Recording the activity of animals as they migrate or forage has proven hugely advantageous to understanding how animals use their environment. Where animals cannot be directly observed, the problem remains of how to identify distinct behaviours that represent an animal’s decision-making process. An excellent example of this problem is that of foraging penguins, which travel to sea to find prey to provision their young. Without direct sampling of the prey field, we cannot calibrate patterns of movement with prey capture, and therefore we cannot determine how different activities link to decision-making. To overcome this, we use a hidden markov model (HMM), which is a machine-learning technique that seeks to identify the underlying states of a system from observable outputs. We apply HMM to determine classes of behaviour from repetitive dives. We take dive data from 103 breeding macaroni penguins at Bird Island, South Georgia, for which we have measures of weight gain over a trip. We identify two classes of behaviour; those of short-shallow and long-deep dives. Using these two behaviours, we calculate the transition probabilities between these states and analyse these data to determine what predicts variation in the transition probabilities. We found that the stage of reproduction during a season, the sex and year of an individual influenced the probability of transition between long-deep and short-shallow sequential dives. We also found differences in the hourly transition rates between the four reproductive stages (incubation, broodguard, crèche and premoult) over a daily cycle. We conclude that this application of HMMs for behavioural switching is potentially useful for other species and other types of recorded behaviour.     

Plant-soil microorganism interactions: heritable relationship between plant genotype and associated soil microorganisms

Although soil microbial communities are known to play crucial roles in the cycling of nutrients in forest ecosystems and can vary by plant species, how microorganisms respond to the subtle gradients of plant genetic variation is just beginning to be appreciated. Using a model Populus system in a common garden with replicated clones of known genotypes, we evaluated microbial biomass and community composition as quantitative traits. Two main patterns emerged. (1) Plant genotype influenced microbial biomass nitrogen in soils under replicated genotypes of Populus angustifolia, F1, and backcross hybrids, but not P. fremontii. Genotype explained up to 78% of the variation in microbial biomass as indicated by broad-sense heritability estimates (i.e., clonal repeatability). A second estimate of microbial biomass (total phospholipid fatty acid) was more conservative and showed significant genotype effects in P. angustifolia and backcross hybrids. (2) Plant genotype significantly influenced microbial community composition, explaining up to 70% of the variation in community composition within P. angustifolia genotypes alone. These findings suggest that variation in above- and belowground traits of individual plant genotypes can alter soil microbial dynamics, and suggests that further investigations of the evolutionary implications of genetic feedbacks are warranted.     

Application of tree-ring isotopic analyses to reconstruct historical water use of riparian trees.

Historical patterns of water source use by trees inferred from long-term records of tree-ring stable isotopic content could assist in evaluating the impact of human alterations to natural stream flow regimes (e.g., water impoundments, stream flow diversions, and groundwater extraction). Our objective was to assess the utility of the hydrogen stable isotopic composition (SD) of tree rings as an index of historical water source use by riparian trees. We investigated the influence of site conditions that varied in climate and hydrology on the relationship between deltaD of Populus xylem water (deltaD(xyl)) and tree-ring cellulose (deltaD(cell)). deltaD(xyl) and deltaD(cell) were strongly correlated across sites (r2 = 0.89). However, the slope of this relationship was less than 1, indicating that factors other than deltaD(xyl) influenced deltaD(cell). Inverse modeling with an isotopic fractionation model for tree-ring cellulose suggested that the lack of one-to-one correspondence between deltaD(xyl) and deltaD(cell) was due to the influence of the hydrogen isotopic content of the atmospheric water vapor (deltaD(atm)). Empirically measured values of deltaD(cell) were typically within the seasonal range of deltaD(cell) predicted from the fractionation model. Sensitivity analyses showed that changes in deltaD(xyl) generally had a greater influence at high-elevation montane sites, whereas deltaD(xyl) and deltaD(atm) had about equal influence on deltaD(cell) at low-elevation desert sites. The intrasite relationship between deltaD(cell) and deltaD(xyl) among individual trees was poor, perhaps because of the within-site spatial variation in hydrologic conditions and associated tree physiological responses. Our study suggests that historical variation in deltaD(cell) of Populus provides information on historical variation in both time-integrated water source use and atmospheric conditions; and that the influence of atmospheric conditions is not consistent over sites with large differences in temperature and humidity. Reconstruction of xylem water sources of Populus in riparian ecosystems from deltaD(cell) will be more direct at higher elevation mountain sites than at low-elevation desert sites.