Biomedical subjects

Gene E Likens

Publications and source records attributed to Gene E Likens.

7 recordsLinked to original sources

PubMed2006

Patterns in potassium dynamics in forest ecosystems.

The biotic cycling of potassium (K) in forest systems has been relatively understudied in comparison with nitrogen (N) and phosphorus (P) despite its critical roles in maintaining the nutrition of primary production in forests. We investigated the ecological significance of K in forests from a literature review and data synthesis. We focused on (1) describing patterns of the effects of K availability on aboveground growth and change in foliar tissue of tree species from a variety of forests; and (2) documenting previously unreported relationships between hydrologic losses of K and N in forested watersheds from the Americas. In a review of studies examining tree growth under K manipulations/fertilizations, a high percentage (69% of studies) showed a positive response to increases in K availability in forest soils. In addition, 76% of the tree studies reviewed showed a positive and significant increase in K concentrations in plant tissue after soil K manipulation/fertilization. A meta-analysis on a subset of the reviewed studies was found to provide further evidence that potassium effects tree growth and increased tissue [K] with an effect size of 0.709 for growth and an overall effect size of 0.56. In our review of watershed studies, we observed that concentrations of K typically decreased during growing seasons in streams draining forested areas in the Temperate Zones and were responsive to vegetation disturbance in both temperate and tropical regions. We found a strong relationship (r2 = 0.42-0.99) between concentrations of K and N (another critical plant nutrient) in stream water, suggesting that similar mechanisms of biotic retention may control the flow of these nutrients. Furthermore, K dynamics appear to be unique among the base cations, e.g. calcium, magnesium, and sodium, because the others do not show similar seasonal patterns to K. We suggest that K may be important to the productivity and sustenance of many forests, and its dynamics and ecological significance warrant further study. We suggest that knowledge about the dynamics of this understudied element is imperative for our understanding patterns and processes in forest ecosystems.

Ecosystem↗

PubMed2006

Linking direct and indirect data on dispersal: isolation by slope in a headwater stream salamander.

There is growing recognition of the need to incorporate information on movement behavior in landscape-scale studies of dispersal. One way to do this is by using indirect indices of dispersal (e.g., genetic differentiation) to test predictions derived from direct data on movement behavior. Mark-recapture studies documented upstream-biased movement in the salamander Gyrinophilus porphyriticus (Plethodontidae). Based on this information, we hypothesized that gene flow in G. porphyriticus is affected by the slope of the stream. Specifically, because the energy required for upstream dispersal is positively related to slope, we predicted gene flow to be negatively related to change in elevation between sampling sites. Using amplified DNA fragment length polymorphisms among tissue samples from paired sites in nine streams in the Hubbard Brook Watershed, New Hampshire, USA, we found that genetic distances between downstream and upstream sites were positively related to change in elevation over standardized 1-km distances. This pattern of isolation by slope elucidates controls on population connectivity in stream networks and underscores the potential for specific behaviors to affect the genetic structure of species at the landscape scale. More broadly, our results show the value of combining direct data on movement behavior and indirect indices to assess patterns and consequences of dispersal in spatially complex ecosystems.

Animals↗

PubMed2005

Increased salinization of fresh water in the northeastern United States.

Chloride concentrations are increasing at a rate that threatens the availability of fresh water in the northeastern United States. Increases in roadways and deicer use are now salinizing fresh waters, degrading habitat for aquatic organisms, and impacting large supplies of drinking water for humans throughout the region. We observed chloride concentrations of up to 25% of the concentration of seawater in streams of Maryland, New York, and New Hampshire during winters, and chloride concentrations remaining up to 100 times greater than unimpacted forest streams during summers. Mean annual chloride concentration increased as a function of impervious surface and exceeded tolerance for freshwater life in suburban and urban watersheds. Our analysis shows that if salinity were to continue to increase at its present rate due to changes in impervious surface coverage and current management practices, many surface waters in the northeastern United States would not be potable for human consumption and would become toxic to freshwater life within the next century.

Chlorides↗

PubMed2005

Long-term relationships between SO2 and NOx emissions and SO4(2-) and NO3- concentration in bulk deposition at the Hubbard Brook Experimental Forest, NH.

A highly significant second-order polynomial relation between SO(2) emissions and SO(4)(2-) concentrations during 1970-2000 (r(2)= 0.80, p= <0.001), and a linear relation between NO(x) and NO(3)(-) concentrations during 1991-2000 (r(2)= 0.67, p= 0.004) in bulk precipitation were found for the Hubbard Brook Experimental Forest, NH based on emissions from a 24 h, back-trajectory determined source area. Earlier periods (1965-1980) for SO(2)ratio SO(4)(2-) and longer periods (1965-2000) for NO(x)ratio NO(3)(-) had poorer linear relations, r(2)= 0.03, p= 0.51 and r(2)= 0.22, p= 0.004, respectively. Methodology by the US Environmental Protection Agency for calculating emissions data during this period has changed significantly and frequently, making trend analysis difficult. Given the large potential for errors in estimating emissions and to a lesser extent, deposition, the robust relations between SO(2) emissions and SO(4)(2-) concentrations in bulk precipitation at the Hubbard Brook Experimental Forest show that careful, long-term measurements from a single monitoring site can provide sound and reasonable data on trends in air pollution.

Acid Rain↗

PubMed2005

Trends in atmospheric ammonium concentrations in relation to atmospheric sulfate and local agriculture.

Ammonium (NH(4)(+)) concentrations in air and precipitation at the Institute of Ecosystem Studies (IES) in southeastern New York, USA declined over an 11-year period from 1988 to 1999, but increased from 1999 to 2001. These trends in particulate NH(4)(+) correlated well with trends in particulate SO(4)(2-) over the 1988-2001 period. The NH(4)(+) trends were not as well correlated with local cattle and milk production, which declined continuously throughout the period. This suggests that regional transport of SO(4)(2-) may have a greater impact on concentrations of NH(4)(+) and subsequent deposition than local agricultural emissions of NH(3). Ammonium concentrations in precipitation correlated significantly with precipitation SO(4)(2-) concentrations for the 1984-2001 period although NH(4)(+) in precipitation increased after 1999 and SO(4)(2-) in precipitation continued to decline after 1999. The correlation between NH(4)(+) and SO(4)(2-) was stronger for particulates than for precipitation. Particulate NH(4)(+) concentrations were also correlated with particulate SO(4)(2-) concentrations at 31 of 35 eastern U.S. CASTNet sites that had at least 10 years of data. Air concentrations of NH(4)(+) and SO(4)(2-) were more strongly correlated at the sites that were located within an agricultural landscape than in forested sites. At most of the sites there was either no trend or a decrease in NH(4)(+) dry deposition during the 1988-2001 period. The sites that showed an increasing trend in NH(4)(+) dry deposition were generally located in the southeastern U.S. The results of this study suggest that, in the northeastern U.S., air concentrations of NH(4)(+) and subsequent deposition may be more closely linked to SO(4)(2-) and thus SO(2) emissions than with NH(3) emissions. These results also suggest that reductions in S emissions have reduced NH(4)(+) transport to and NH(4)(+)-N deposition in the Northeast.

Agriculture↗

PubMed2003

Role of soil freezing events in interannual patterns of stream chemistry at the Hubbard Brook Experimental Forest, New Hampshire.

Soil freezing is a disturbance of the below ground environment, potentially resulting in increased losses of NO3- and surface water acidification. Here, we report the effects of soil freezing on interannual variation in stream chemistry at the Hubbard Brook Experimental Forest, New Hampshire. Data from 1970 to 1997 of soil frost depth, snow cover, precipitation, air temperature, and stream discharge and chemistry were used in a stepwise linear regression model to select the variables that best predicted deviations of annual stream concentrations from 4-year running averages. Variables quantifying soil freezing severity were selected as significant predictors of short-term fluctuations in stream K+, NO3-, Ca2+, and Mg2+ concentrations from 1970 to 1989, explaining 59 and 47% of the short-term variability in K+ and NO3-, respectively. Fine-root mortality and disturbance of root-soil-microbe interactions, with subsequent effects on decomposition and nutrient uptake, likely contributed to the mobilization of K+ and NO3- to streamwater following severe soil freezing events. The relationship between soil freezing and stream chemistry, however, weakened during the period 1990-1997. Because soil freezing has had inconsistent effects on stream chemistry during the period 1970-1997, it is unclear whether future changes in the frequency, duration, and depth of soil freezing events as the result of changes in the snow cover regime under a warmer climate will have significant impacts on the losses of NO3- and nutrient-base cations from temperate northern ecosystems.

Climate↗

PubMed2002

Mycorrhizal weathering of apatite as an important calcium source in base-poor forest ecosystems.

The depletion of calcium in forest ecosystems of the northeastern USA is thought to be a consequence of acidic deposition and to be at present restricting the recovery of forest and aquatic systems now that acidic deposition itself is declining. This depletion of calcium has been inferred from studies showing that sources of calcium in forest ecosystems namely, atmospheric deposition and mineral weathering of silicate rocks such as plagioclase, a calcium-sodium silicate do not match calcium outputs observed in forest streams. It is therefore thought that calcium is being lost from exchangeable and organically bound calcium in forest soils. Here we investigate the sources of calcium in the Hubbard Brook experimental forest, through analysis of calcium and strontium abundances and strontium isotope ratios within various soil, vegetation and hydrological pools. We show that the dissolution of apatite (calcium phosphate) represents a source of calcium that is comparable in size to known inputs from atmospheric sources and silicate weathering. Moreover, apatite-derived calcium was utilized largely by ectomycorrhizal tree species, suggesting that mycorrhizae may weather apatite and absorb the released ions directly, without the ions entering the exchangeable soil pool. Therefore, it seems that apatite weathering can compensate for some of the calcium lost from base-poor ecosystems, and should be considered when estimating soil acidification impacts and calcium cycling.

Apatites↗