Differentiation among populations of Sedum wrightii (Crassulaceae) in response to limited water availability: water relations,CO2 assimilation,growth and survivorship

Summary Sedum wrightii is one of only a few species in the Crassulaceae for which there is evidence for a high degree of variability in the ratio of daytime to nighttime CO₂ assimilation. There are both environmental and genetic components to this variability. S. wrightii grows over a wide altitudinal gradient. The purpose of this study was to compare low, intermediate, and high altitude populations with respect to the degree of CAM expression and the capability to tolerate limited water availability. We utilized clonallyreplicated genotypes of plants from each population in common environment greenhouse experiments. Genetic differences among the populations were found in long-term water use efficiency, in 24 hour CO₂ exchange patterns, in biomass ¹³C values, in carbon allocation, and in water status and ultimately survival during prolonged drought. The differences among the populations appear to be closely related to differences in the native habitats. The low altitude, desert plants had the greatest ability to grow and survive under conditions of limited water availability and appear to have the greatest shift to nighttime CO₂ uptake during periods without water, while the high altitude plants had the poorest performance under these conditions and appear to shut down net carbon uptake when severely water limited.     

The distance dependence prediction of the Janzen-Connell hypothesis: a meta-analysis

The Janzen-Connell hypothesis explains the maintenance of tropical diversity through the interacting effects of parent-centered dispersal patterns and distance- and density-dependent propagule survival. These effects were thought to support regular spacing of species within tropical forest, enhancing diversity. One of the predictions of the hypothesis is that seed and seedling survival should improve with increased parental distance. Although there are many independent tests of this hypothesis for individual species, there are few synthetic studies that have brought these data together to test its validity across species. This paper reports the results of a meta-analysis of the effect of distance on enhancing propagule survival, employing an odds-ratio effect size metric. We found no general support for the distance-dependent prediction of the hypothesis, and conclude that further testing to explore this hypothesis as a diversity-maintaining mechanism is unnecessary. However, we did find that distance from parent slightly reduces survivorship in the temperate zone, as contrasted with the tropics, and we saw stronger evidence in support of the hypothesis for seedlings than for seeds. The phenomenon of enhanced propagule survival with distance from the parent may be important for the population biology of particular species, but it is not a general phenomenon across communities, life history stages or life forms.     

Sources of variation in growth, form, and survival in dwarf and normal-stature pitch pines (Pinus rigida, Pinaceae) in long-term transplant experiments

Determining the relative contributions of genetic and environmental factors to phenotypic variation is critical for understanding the evolutionary ecology of plant species, but few studies have examined the sources of phenotypic differentiation between nearby populations of woody plants. We conducted reciprocal transplant experiments to examine sources of variation in growth rate, form, survival, and maturation in a globally rare dwarf population of pitch pine (Pinus rigida) and in surrounding populations of normal-stature pitch pines on Long Island, New York. Transplants were monitored over a 6-yr period. The influence of seedling origin on height, growth rate, survival, and form (single-stemmed vs. multi-stemmed growth habit) was much smaller than the effect of transplanting location. Both planting site and seed origin were important factors in determining time to reproduction; seedlings originating from dwarf populations and seedlings planted at the normal-stature site reproduced earliest. These results suggest that many of the differences between dwarf and normal-stature pitch pines may be due more to plastic responses to environmental factors than to genetic differentiation among populations. Therefore, preservation of the dwarf pine habitat is essential for preserving dwarf pine communities; the dwarf pines cannot be preserved ex situ.     

Integrating the statistical analysis of spatial data in ecology

In many areas of ecology there is an increasing emphasis on spatial relationships. Often ecologists are interested in new ways of analyzing data with the objective of quantifying spatial patterns, and in designing surveys and experiments in light of the recognition that there may be underlying spatial pattern in biotic responses. In doing so, ecologists have adopted a number of widely different techniques and approaches derived from different schools of thought, and from other scientific disciplines. While the adaptation of a diverse array of statistical approaches and methodologies for the analysis of spatial data has yielded considerable insight into various ecological problems, this diversity of approaches has sometimes impeded communication and retarded more rapid progress in this emergent area. Many of these different statistical methods provide similar information about spatial characteristics, but the differences among these methods make it difficult to compare the results of studies that employ contrasting approaches. The papers in this mini-series explore possible areas of agreement and synthesis between a diversity of approaches to spatial analysis in ecology.     

Sources of Variation in Leaf Shape among Two Populations of Achillea Lanulosa

Achillea lanulosa has complex, highly dissected leaves that vary in shape and size along an altitudinal gradient. Plants from a high and an intermediate altitude population were clonally replicated and grown in a controlled environment at warm and cool conditions under bright light. There were genetic differences among populations and among individuals within populations in leaf size and shape. Heritabilities for leaf size and shape characters were moderate. Leaves of the lower altitude population were larger and differed from the higher altitude plants in both coarse and fine shape. Plastic response to temperature of the growth environment paralleled the genetic differentiation between low and high altitude populations. There was no apparent trade-off between genetic control over morphology and the capacity for directional plastic response to the environment. Differences in leaf dissection and size at contrasting altitudes in this species are the result of both genetic divergence among populations and of acclimative responses to local environments.