Size structure of populations within populations: leaf number and size in crowded and uncrowded Impatiens pallida individuals

Summary We compared the size distributions of leaves on naturally-occurring crowded and experimentally thinned uncrowded individuals of Impatiens pallida in southeastern Pennsylvania. Crowding decreased the number of leaves on individual plants and altered the distribution of leaf size. Crowded individuals had smaller leaves, but the size (length) inequality of the leaf population did not change. The relationships between the height of a plant and the mean and maximum length of its leaves were significantly different for crowded and uncrowded plants. There were weak positive relationships between height and total leaf area, and height and total number of leaves for uncrowded plants, whereas crowded plants showed tighter but curvilinear relationships between these variables. Our results point out the strengths and the limitations of viewing canopies as populations of modules.     

Prolonged intermittent footschock stress decreases Met and Leu enkephalin levels in brain with concomitant decreases in pain threshold

The influence of a 21 day intermittent footshock regimen upon enkephalin levels in brain and adrenals was examined in the rat. Changes in pain sensitivity as well as analgesic and hyperthermic responsiveness to morphine (7.5 mg/kg) were also monitored. Following the stress regimen, Met and Leu enkephalin levels were decreased by 40 to 50% in brain, but were unchanged in adrenals. Post-stress pain thresholds were markedly decreased in stressed animals while the analgesic properties of morphine were enhanced. Core body temperature of stressed animals was significantly raised, but the hyperthermic response to morphine was unchanged.     

Limitations to CO2-induced growth enhancement in pot studies

Recently, it has been suggested that small pots may reduce or eliminate plant responses to enriched CO₂ atmospheres due to root restriction. While smaller pot volumes provide less physical space available for root growth, they also provide less nutrients. Reduced nutrient availability alone may reduce growth enhancement under elevated CO₂. To investigate the relative importance of limited physical rooting space separate from and in conjunction with soil nutrients, we grew plants at ambient and double-ambient CO₂ levels in growth containers of varied volume, shape, nutrient concentration, and total nutrient content. Two species (Abutilon theophrasti, a C₃ dicot with a deep tap root andSetaria faberii, a C₄ monocot with a shallow diffuse root system) were selected for their contrasting physiology and root architecture. Shoot demography was determined weekly and biomass was determined after eight and ten weeks of growth. Increasing total nutrients, either by increasing nutrient concentration or by increasing pot size, increased plant growth. Further, increasing pot size while maintaining equal total nutrients per pot resulted in increased total biomass for both species. CO₂-induced growth and reproductive yield enhancements were greatest in pots with high nutrient concentrations, regardless of total nutrient content or pot size, and were also mediated by the shape of the pot. CO₂-induced growth and reproductive yield enhancements were unaffected by pot size (growth) or were greater in small pots (reproductive yield), regardless of total nutrient content, contrary to predictions based on earlier studies. These results suggest that several aspects of growth conditions within pots may influence the CO₂ responses of plants; pot size, pot shape, the concentration and total amount of nutrient additions to pots may lead to over-or underestimates of the CO₂ responses of real-world plants.     

A computer program for characterizing root system branching patterns

A computer program is presented which measures the length, branching patterns and distribution of link length within a root system. The program skeletonizes digitized images of root systems, loads these images into a binary tree data structure and uses this data structure to characterize the root systems. Measurements of the root length and topological parameters of root systems of Senecio vulgaris made by hand and by computer program were linearly related, with r² values greater than 0.99 in all cases.     

Variation in the oxytocin receptor gene influences neurocardiac reactivity to social stress and HPA function: a population based study

Oxytocin (OT) is a nonapeptide neurohormone that is involved in a broad array of physiological and behavioral processes related to health including hypothalamic-pituitary-adrenal (HPA) axis functioning, autonomic nervous system (ANS) activity and social behaviors. The present study sought to explore the influence of genetic variation in the oxytocin receptor (SNP; rs53576) on autonomic and neurohormonal functioning across both resting and psychological stress conditions in a population based sample of older adults. Results revealed that A carrier males showed higher levels of resting sympathetic cardiac control as compared to their G/G counter parts. However, G/G participants displayed significantly higher levels of sympathetic reactivity to psychological stress with G/G males showing the highest levels of sympathetic response to stress. Although no significant effects were detected for heart rate or parasympathetic cardiac control across resting and stress conditions, results revealed that G/G participants generally displayed heightened stroke volume and cardiac output reactivity to the psychological stressor. Furthermore, analysis of diurnal fluctuations in salivary cortisol revealed that G/G participants displayed lower awakening cortisol levels and less variation in salivary cortisol across the day as compared to A carrier individuals.     

Effect of naloxone on analgesia induced by food deprivation

Naloxone (4 mg/kg) or saline was administered to animals under food deprived and non-deprived conditions prior to testing pain sensitivity in the tail flick test. Food deprived animals exhibited significantly elevated latencies in comparison to latencies observed under non-deprived conditions. This analgesia was diminished by treatment with the opiate receptor antagonist, naloxone. These findings suggest that analgesia induced by food deprivation is mediated in part by opiate receptor systems.     

Long-Term Nitrogen Additions and Nitrogen Saturation in Two Temperate Forests

This article reports responses of two different forest ecosystems to 9 years (1988–96) of chronic nitrogen (N) additions at the Harvard Forest, Petersham, Massachusetts. Ammonium nitrate (NH₄NO₃) was applied to a pine plantation and a native deciduous broad-leaved (hardwood) forest in six equal monthly doses (May–September) at four rates: control (no fertilizer addition), low N (5 g N m^-2 y^-1), high N (15 g N m^-2 y^-1), and low N + sulfur (5 g N m^-2 y^-1 plus 7.4 g S m^-2 y^-1). Measurements were made of net N mineralization, net nitrification, N retention, wood production, foliar N content and litter production, soil C and N content, and concentrations of dissolved organic carbon (DOC) and nitrogen (DON) in soil water. In the pine stand, nitrate losses were measured after the first year of additions (1989) in the high N plot and increased again in 1995 and 1996. The hardwood stand showed no significant increases in nitrate leaching until 1995 (high N only), with further increases in 1996. Overall N retention efficiency (percentage of added N retained) over the 9-year period was 97–100% in the control and low N plots of both stands, 96% in the hardwood high N plot, and 85% in the pine high N plot. Storage in aboveground biomass, fine roots, and soil extractable pools accounted for only 16–32% of the added N retained in the amended plots, suggesting that the one major unmeasured pool, soil organic matter, contains the remaining 68–84%. Short-term redistribution of ¹⁵N tracer at natural abundance levels showed similar division between plant and soil pools. Direct measurements of changes in total soil C and N pools were inconclusive due to high variation in both stands. Woody biomass production increased in the hardwood high N plot but was significantly reduced in the pine high N plot, relative to controls. A drought-induced increase in foliar litterfall in the pine stand in 1995 is one possible factor leading to a measured increase in N mineralization, nitrification, and nitrate loss in the pine high N plot in 1996. Received 2 April 1999; Accepted 29 October 1999.     

The allometry of root production and loss in seedlings of Acer rubrum (Aceraceae) and Betula papyrifera (Betulaceae): implications for root dynamics in elevated CO2

Total root production (∑P), total root loss (∑L), net root production. (NP), and biomass production were determined for seedlings of Betula papyrifera and Acer rubrum in ambient and elevated CO₂ environments. ∑P, ∑L, and NP were calculated from sequential, independent observations of root length production through plexiglass windows. Elevated CO₂ increased ∑P, ∑L, and NP in seedlings of Betula papyrifera but not Acer rubrum. Root production and loss were qualitatively similar to whole-plant growth responses to elevated CO₂. Betula showed enhanced ∑P, ∑L, and biomass with elevated CO₂ but Acer did not. However, the observed effects of CO₂ on root production and loss did not alter the allometric relationship between root production and root loss for either Acer or Betula. Thus, in this experiment, elevated CO₂ did not affect the relationship between root production and root loss. The results of this study have important implications for the potential effects of elevated CO₂ on root dynamics. Elevated CO₂ may lead to increases in root production and in root loss (turnover) where the changes in root turnover are largely a function of the magnitude of root production increases.     

Elevated CO2 and the magnitude and seasonal dynamics of root production and loss in t Betula papyrifera

The impact of elevated atmospheric CO₂ on belowground plant growth is poorly understood relative to its effects on aboveground growth. We carried out a study of the seasonal dynamics of gross root production and death to determine how elevated CO₂ affected the dynamics of net and gross root production through a full growing season. We quantified gross root production and root loss from sequential, in situ images of fine roots of t Betula papyrifera in ambient (375 ppm.) and elevated (700 ppm) CO₂ atmospheres from 2 weeks following germination through leaf senescence. We found that elevated CO₂ led to increases in the magnitude of cumulative gross production (P) and cumulative gross loss (L) of roots. However, the effect of elevated CO₂ on these processes was seasonally dependent. Elevated CO₂ led to greater levels of enhancement in P early in the growing season, prior to maximum standing root length (NP). In contrast, elevated CO₂ led to greater levels of enhancement in L in the last half of the growing season, after maximum NP had been reached. This difference in the timing of when elevated CO₂ affects P and L led to a transitory, early enhancement in NP. By the end of the growing season, there was no significant effect of elevated CO₂ on NP, and P was 87% greater than NP for ambient CO₂ and 117% greater in elevated CO₂. We conclude that static assessments of belowground productivity may greatly underestimate gross fine root productivity and turnover and this bias can be exaggerated with elevated CO₂.     

Elevated CO2 alters deployment of roots in “small” growth containers

Since European badgers (Meles meles L.) form non-cooperative groups in parts of their geographic range, but are solitary elsewhere, their social systems have been at the centre of a debate about the evolution of group living in the Carnivora. In a recent review of models of non-cooperative sociality, Woodroffe and Macdonald (1993) presented evidence in favour of two hypotheses, which suggested that badger groups might form because either the distribution of blocks of foodrich habitat, or the economics of excavating new setts, prevented the division of group territories into individual territories. We present data upon the response of badger spatial organisation to a reduction in food-patch dispersion, brought about by the conversion of carthwormpoor arable land to earthworm-rich pasture over a 15-year period. This change in the distribution of earthworm-rich habitats was accompanied by territory fission, facilitated by the excavation of new setts. This indicates that the availability of sett sites had not constrained territory size at the start of the study. However, sett distribution did define the size and configuration of the daughter territories. We also show that variation among territories in the availability of food-rich habitats was reflected in the reproductive rates and body weights of the groups that inhabited them, although there was no detectable effect upon group size.