ASSESSMENT OF POTENTIAL BIAS WITH SNOWSHOE HARE FECAL PELLET-PLOT COUNTS

Abstract: The fecal pellet-plot method has been used extensively for snowshoe hare (Lepus americanus) population studies across the species' range, but potential biases associated with the technique have not been addressed adequately. We studied hare pellet-plots in northern Idaho to quantify pellet decomposition rates across environmental gradients, and conducted feeding trials on captive hares to assess the role of diet on pellet production rates. We found that across our study area pellet numbers tended to be higher on plots with high vegetative cover, which likely was a reflection of hare habitat choice rather than lesser pellet decomposition in such habitat. A pellet decomposition experiment indicated that pellet persistence was negatively related to moisture level, and that pellets produced by hares during summer decomposed more quickly than those from winter. We found that only 19% of fecal pellets collected from plots located across northern Idaho were produced by hares during winter. There was a correlation between pellet numbers from plots that were pre-cleared 1 year earlier and estimated numbers of hares on 6 study areas. A similar correlation was lacking for pellet counts from uncleared plots, implying that hare population estimation via pellet-plot counts should involve plot pre-clearing. In captive studies, juvenile hares produced slightly fewer pellets per day per gram of food ingested than adults, but pellet production was similar across diets comprised of 10 different browse species. We conclude that for our study area the fecal pellet-plot method may be subject to notable pellet decomposition bias, and therefore recommend that use of the method elsewhere across the species' range be preceded by assessment of both the pellet-hare density relationship and pellet decomposition rates across habitats.     

Nitrate Assimilation in Higher Plants with Special Reference to the Cocklebur (Xanthium pennsylvanicum Wallr.)

A soluble NADH-dependent nitrate reductase is described forthe shoot system of Xanthium. Young leaves and immature stemtissues contain high levels of the enzyme. They are relativelyrich in free amino acids and amides but store little free nitrate.The specific activity of the enzyme is lower in fully expandedleaves, although these leaves exhibit higher rates of fixationof carbon in photosynthesis than do younger leaves. Neithernitrate nor free amino acids accumulate in the mesophyll ofthe leaf. Older parts of the stem axis accumulate large amountsof soluble nitrogen, almost entirely as free nitrate. Reservesof nitrate in the shoot and root are rapidly depleted if nitrateis removed from the external medium. Nitrate reductase is apparently absent from roots of Xanthium.This finding is supported by analyses of bleeding sap from nitrate-fedplants which show that 95 per cent of the nitrogen exportedfrom roots is present as free nitrate. However, roots are capableof synthesizing and exporting large amounts of amino nitrogenif supplied with reduced nitrogen such as urea or ammonium. A scheme is presented summarizing the main features of the metabolismof nitrate in Xanthium and this is compared with the situationin nitrate-fed plants of the field pea (Pisum arvense L.), aspecies previously shown to be capable of reducing nitrate inits root system.     

Influence of Genotype and Mineral Nutrition on the Distribution of Growth within Plants of Lolium perenne L. grown in Soil

Genotypes of Lolium perenne were grown with two levels of fertilizerapplication. Using exponential growth-rates and parameters derivedfrom them, the rate of growth of each genotype was partitionedinto the rates of initiation of new roots and tillers, and thegrowth of the individual roots and tillers after initiation.The growth of the individual roots was further analysed by measuringchanges in diameter and length of the main root, and the growthin the branches of the main root. The genotypes varied in their response to additional mineralnutrition. The greater the increase in the rate of shoot growthfor a genotype, the greater was its increase in the rate oftiller initiation, the smaller was the increase in the sizeof its tillers, and the greater was the decrease in its rateof root relative to shoot growth. Within a population of genotypes growing with the same levelof mineral nutrition, the greater the rate of shoot growth ofa genotype, the greater was the rate of initiation of new tillersand, at a low level of mineral nutrition, the smaller the sizeof tillers. With a higher level of mineral nutrition, the higherthe rate of tiller production, the smaller or larger was tillersize, depending upon environmental factors other than mineralnutrition. At both levels of mineral nutrition, the greaterthe rate of shoot growth, the smaller was the rate of root growthrelative to shoot growth, due to relatively less growth takingplace in the branches of the main root.