Soil nitrogen availability, plant luxury consumption, and herbivory by white-tailed deer.

N is often cited as a limiting factor for sapling growth in northeastern USA forests. However, under conditions of elevated soil N, seedlings and saplings of some tree species exhibit luxury consumption of N, leading to elevated tissue N concentration. While this pool of plant N may have benefits for saplings if light levels change, it may also increase the risk of herbivory by white-tailed deer (Odocoileus virginianus L.). We conducted a field fertilization experiment to test the hypothesis that saplings increase stem tissue N when soil N availability is elevated. We fertilized saplings of nine tree species under closed and open canopies. Two of the nine species, yellow birch (Betula alleghaniensis) and black cherry (Prunus serotina), had significant increases in radial growth when fertilized under high light. Six of the nine species showed increased stem N concentration under either low light or high light conditions. Under low light conditions, black cherry and sugar maple (Acer saccharum) had significantly higher concentrations of stem N when fertilized. Regardless of fertilization treatment, yellow birch had significantly greater stem N in saplings under low light conditions when compared with saplings found in high light conditions. Under high light conditions, fertilization resulted in increased stem N in saplings of white ash (Fraxinus americana) and eastern hemlock (Tsuga canadensis). There was a trend for saplings of red maple (Acer rubrum) and white ash to show elevated stem N concentration when fertilized under low light. Red oak (Quercus rubra), American beech (Fagus grandifolia) and white pine (Pinus strobus) did not show any evidence of luxury consumption. We compared browse frequency of fertilized and unfertilized saplings within similar forest types. Browse frequency was consistently higher on fertilized saplings. When averaged across all species, however, the difference between treatments was not significant. In contrast, we found significantly higher browse rates for fertilized saplings that had been previously identified as luxury N consumers. Our results indicate that increased soil N availability has positive effects on at least some of the species under high light, but, has potentially negative indirect effects on a larger group of species under both high and low light due to a higher risk of herbivory.

Effects of acorn production and mouse abundance on abundance and Borrelia burgdorferi infection prevalence of nymphal Ixodes scapularis ticks.

Risk of exposure to Lyme disease is a function of the local abundance of nymphal Ixodes ticks that are infected with the etiological agent, the spirochete Borrelia burgdorferi. We monitored abundance of white-footed mice (the principal B. burgdorferi reservoir in the eastern and central United States) and acorns (a critical food resource for mice), and Ixodes scapularis ticks, as well as ambient temperature (cumulative growing degree days) and growing season precipitation, in a forested landscape of southeastern New York State from 1994 to 2000. We found that acorn production in autumn strongly influenced abundance of white-footed mice the following summer and that abundance of mice in summer, when larval ticks are active, influenced the abundance of infected nymphs the following year. Consequently, the abundance of infected nymphal ticks can be predicted from acorn production 1.75 years earlier. Monitoring of natural fluctuations in acorn production thus supports results of prior acorn addition experiments that were conducted at small spatial scales. Growing degree days and precipitation either had no significant effect on density of nymphs or marginally increased the explanatory power of models that included acorns or mouse density as independent variables. We conclude that, at our study site in New York, the risk of human exposure to Lyme disease is affected by mouse density in the prior year and by acorn production 2 years previously.

Intrinsic density-dependent regulation of vole populations.

Considerable controversy exists over the role of density-dependent processes in controlling animal population size. In populations that fluctuate cyclically or erratically, for example many voles and insects, theory predicts that either density-dependence is weak, or that density-dependent responses lag behind density. One key mechanism for lagged density-dependence is a delay in regeneration of food resources following heavy exploitation. Here we show that meadow vole (Microtus pennsylvanicus) populations respond immediately to high density by reducing breeding effort and hence population growth, disproving the hypothesis that density-dependence is weak. In addition, vole populations do not show a delay in growth following marked reduction in plant biomass (their source of food and cover). We conclude that intrinsic density-dependence processes tend to stabilize vole populations, and that cyclic dynamics are not caused by lagged effects of resource exploitation.

A method for stabilization of monoamine oxidases in homogenates of rat intestine epithelium.

In a homogenate of epithelium isolated from the small intestine of male Wistar rats, the amine oxidase activity with 10(-3)M tyramine was 9200 +/- 200 nmol (g tissue)-1 h-1 of which 91% was due to the A form of monoamine oxidase (MAO) and 9% to the B form. Semicarbazide-sensitive amine oxidase activity was not detected with either 10(-3)M tyramine or 10(-4)M benzylamine as substrate. However, it was detectable in the homogenate of the gut residue where the activity with 10(-4)M benzylamine was 3600 +/- 200 nmol (g tissue)-1 h-1. The MAO activity, in homogenates of epithelium prepared with 0.1 M sodium phosphate pH 7.4, was stable at 4 degrees C for at least 6 h whilst at minus 20 degrees C it decreased by 70% within 24 h. Incorporation of 10% (v/v) glycerol into the homogenization medium stabilized the enzymes. The total activity and proportions due to MAO-A and MAO-B and kinetic constants for tyramine and 5-hydroxytryptamine, did not alter during 5 weeks storage at -20 degrees C. The ability to store tissue homogenates should facilitate studies of intestinal amine oxidases.