Seasonal changes in the hydrogen isotope ratios of xylem waters were measured to determine water sources used for growth in desert plants of southern Utah. While all species used winter-spring recharge precipitation for spring growth, utilization of summer rains was life-form dependent. Annuals and succulent perennials exhibited a complete dependence on summer precipitation. Herbaceous and woody perennial species simultaneously utilized both summer precipitation and remaining winter-spring precipitation, with herbaceous species much more reliant on the summer precipitation component. Several of the woody perennials exhibited no response to summer precipitation. Currently, precipitation in southern Utah is evenly partitioned between winter and summer time periods; however, global circulation models predict that summer precipitation will increase in response to anticipated climate change. Our data indicate that components within the community will differentially responde to the change in precipitation patterns. These results are discussed in relation to impact on competition and possible changes in community structure.
Specific formulations are derived for the correlation between the heterozygosity of a randomly mating parent and its offspring for a diallelic locus, and for the correlation when multiple loci are considered. The expected correlation is maximal, approaching r = 0.50, when allelic frequencies are highly asymmetric, and it is zero when the allelic frequencies are equal to 0.50. Parent-offspring correlations, up to a maximum of 0.50 for diallelic loci, indicate that levels of heterozygosity can respond to selection. Multilocus allozyme data from limber pine, Pinus flexilis, and from horses of standardbred and thoroughbred breeds are used to demonstrate correlations between a parent and its offspring. The Spearman rank correlation between the heterozygosity of a limber pine and the mean heterozygosity of her offspring is r = 0.45. Correlations in the horses range from r =0.16 to 0.32.
Summary1. The responses of night-time dark respiration ( R d ) to temperature and leaf characteristics were measured through the canopies of tree species from two distinct forests -an oak-dominated deciduous forest in north-eastern USA, and a conifer-dominated temperate rainforest in New Zealand. These were chosen to examine the extent to which canopy level changes in dark respiration can be applied across forest biomes, and the appropriateness of scaling rules to calculations of whole-canopy carbon efflux. 2. The response of respiration to temperature differed significantly between species and with height in the canopy. This involved changes in both R d at a reference temperature, and the extent to which R d increased with temperature (described by the energy of activation, E o , or the change in R d over a 10 ° C range, Q 10 ). E o ranged from 25 (lowercanopy leaves) to 53·8 kJ mol − 1 K − 1 (upper-canopy leaves) in the deciduous forest, and from 24-37 kJ mol − 1 K − 1 in the temperate rainforest site. 3.Relationships between respiratory and leaf characteristics indicated that the instantaneous rate of respiration covaries with soluble sugar concentration and leaf nitrogen, but the temperature response of respiration ( E o or Q 10 ) appears to be driven by leaf N. 4. Scaling leaf respiratory carbon loss to the whole-canopy level indicated that simplifying assumptions regarding the variation in respiration and its temperature response with canopy height tend to underestimate carbon loss if the assumptions are based on lower-canopy leaf physiology, but overestimate carbon loss if the assumptions are based on upper-canopy physiology. Thus, canopy-level differences in leaf respiratory characteristics should be considered in modelling efforts attempting to estimate wholecanopy respiration.
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