Northward displacement of optimal climate conditions for ecotypes of Eriophorum vaginatum L. across a latitudinal gradient in Alaska.

Plants are often genetically specialized as ecotypes attuned to local environmental conditions. When conditions change, the optimal environment may be physically displaced from the local population, unless dispersal or in situ evolution keep pace, resulting in a phenomenon called adaptational lag. Using a 30-year-old reciprocal transplant study across a 475 km latitudinal gradient, we tested the adaptational lag hypothesis by measuring both short-term (tiller population growth rates) and long-term (17-year survival) fitness components of Eriophorum vaginatum ecotypes in Alaska, where climate change may have already displaced the optimum. Analyzing the transplant study as a climate transfer experiment, we showed that the climate optimum for plant performance was displaced ca. 140 km north of home sites, although plants were not generally declining in size at home sites. Adaptational lag is expected to be widespread globally for long-lived, ecotypically specialized plants, with disruptive consequences for communities and ecosystems.

Synergistic effects of climate change and harvest on extinction risk of American ginseng.

Over the next century, the conservation of biodiversity will depend not only on our ability to understand the effect of climate change, but also on our capacity to predict how other factors interact with climate change to influence species viability. We used American ginseng (Panax quinquefolius L.), the United States' premier wild-harvested medicinal, as a model system to ask whether the effect of harvest on extinction risk depends on changing climatic conditions. We performed stochastic projections of viability response to an increase in maximum growing-season temperature of 1°C over the next 70 years by sampling matrices from long-term demographic studies of 12 populations (representing 75 population-years of data). In simulations that included harvest and climate change, extinction risk at the median population size (N = 140) was 65%, far exceeding the additive effects of the two factors (extinction risk = 8% and 6% for harvest and climate change, respectively; quasi-extinction threshold = 20). We performed a life table response experiment (LTRE) to determine underlying causes of the effect of warming and harvest on deterministic λ (λd). Together, these factors decreased λd values primarily by reducing growth of juvenile and small adult plants to the large-adult stage, as well as decreasing stasis of the juveniles and large adults. The interaction observed in stochastic model results followed from a nonlinear increase in extinction risk as the combined impact of harvest and warming consistently reduced λ values below the demographic tipping point of λ = 1. While further research is needed to create specific recommendations, these findings indicate that ginseng harvest regulations should be revised to account for changing climate. Given the possibility of nonlinear response like that reported here, pre-emptive adaptation of management strategies may increase efficacy of biodiversity conservation by allowing behavior modification prior to precipitous population decline.

Ecology and conservation of ginseng (Panax quinquefolius) in a changing world.

American ginseng (Panax quinquefolius L.) is an uncommon to rare understory plant of the eastern deciduous forest. Harvesting to supply the Asian traditional medicine market made ginseng North America's most harvested wild plant for two centuries, eventually prompting a listing on CITES Appendix II. The prominence of this representative understory plant has led to its use as a phytometer to better understand how environmental changes are affecting many lesser-known species that constitute the diverse temperate flora of eastern North America. We review recent scientific findings concerning this remarkable phytometer species, identifying factors through its history of direct and indirect interactions with humans that have led to the current condition of the species. Harvest, deer browse, and climate change effects have been studied in detail, and all represent unique interacting threats to ginseng's long-term persistence. Finally, we synthesize our current understanding by portraying ginseng's existence in thousands of small populations, precariously poised to either escape or be drawn further toward extinction by the actions of our own species.

Clinal variation in stomatal characteristics of an Arctic sedge, Eriophorum vaginatum (Cyperaceae).

PREMISE OF THE STUDY: In a large reciprocal transplant experiment, Eriophorum vaginatum tussocks transplanted along a latitudinal gradient in Alaska's interior exhibited genetic differentiation and phenotypic plasticity for vegetative traits. Using the same tussocks 30 yr later, we used estimates of growing season temperature at each site to ask whether there was a climatic cline for stomatal density, size, and conductance. METHODS: We created impressions of the abaxial leaf surfaces of the transplanted individuals for viewing under a microscope and measured stomatal density (SD) and length (SL) for 224 individuals. We used SD and SL to estimate stomatal conductance (C). Separate one-way analyses of variance were performed to quantify the effect of population genetic differences and latitudinal environmental variation on stomatal characteristics. KEY RESULTS: Our data suggest that stomatal size was influenced by both genetics and environment and that plasticity for stomatal density produced highest densities at the coolest sites. Stomatal conductance increased with decreasing temperature of site from which the populations originated. CONCLUSIONS: Our results demonstrate a cline in stomatal conductance in E. vaginatum, with some ability of populations to plastically produce an appropriate phenotypic response in a new environment. Because the species is a dominant species in many arctic plant communities, its ability to produce an appropriate stomatal phenotype and to optimize water use efficiency by decreasing stomatal conductance in warmer environments could affect both community composition and total primary productivity in future, warmer climates.

Experimental test for adaptive differentiation of ginseng populations reveals complex response to temperature.

BACKGROUND AND AIMS: Local climatic adaptation can influence species' response to climate change. If populations within a species are adapted to local climate, directional change away from mean climatic conditions may negatively affect fitness of populations throughout the species' range. METHODS: Adaptive differentiation to temperature was tested for in American ginseng (Panax quinquefolius) by reciprocally transplanting individuals from two populations, originating at different elevations, among temperature treatments in a controlled growth chamber environment. Fitness-related traits were measured in order to test for a population × temperature treatment interaction, and key physiological and phenological traits were measured to explain population differences in response to temperature. KEY RESULTS: Response to temperature treatments differed between populations, suggesting genetic differentiation of populations. However, the pattern of response of fitness-related variables generally did not suggest 'home temperature' advantage, as would be expected if populations were locally adapted to temperature alone. CONCLUSIONS: Failure consistently to detect a 'home temperature' advantage response suggests that adaptation to temperature is complex, and environmental and biotic factors that naturally covary with temperature in the field may be critical to understanding the nature of adaptation to temperature.

Evidence of local adaptation in the demographic response of American ginseng to interannual temperature variation.

Bioclimatic envelope models of species' responses to climate change are used to predict how species will respond to increasing temperatures. These models are frequently based on the assumption that the northern and southern boundaries of a species' range define its thermal niche. However, this assumption may be violated if populations are adapted to local temperature regimes and have evolved population-specific thermal optima. Considering the prevalence of local adaptation, the assumption of a species-wide thermal optimum may be violated for many species. We used spatially and temporally extensive demographic data for American ginseng (Panax quinquefolius L.) to examine range-wide variation in response of population growth rate (λ) to climatic factors. Our results suggest adaptation to local temperature, but not precipitation. For each population, λ was maximized when annual temperatures were similar to site-specific, long-term mean temperatures. Populations from disparate climatic zones responded differently to temperature variation, and there was a linear relation between population-level thermal optima and the 30-year mean temperature at each site. For species that are locally adapted to temperature, bioclimatic envelope models may underestimate the extent to which increasing temperatures will decrease population growth rate. Because any directional change from long-term mean temperatures will decrease population growth rates, all populations throughout a species' range will be adversely affected by temperature increase, not just populations at southern and low-elevation boundaries. Additionally, when a species' local thermal niche is narrower than its range-wide thermal niche, a smaller temperature increase than would be predicted by bioclimatic envelope approaches may be sufficient to decrease population growth.

Effects of self-pollination and outcrossing with cultivated plants in small natural populations of American ginseng, Panax quinquefolius (Araliaceae).

For rare plants, self-pollination and inbreeding can increase in small populations, while unusual levels of outcrossing can occur through restoration efforts. To study both inbreeding and outcrossing, we performed experimental pollinations using Panax quinquefolius (American ginseng), a wild-harvested plant with a mixed mating system. For inbreeding, plants were either cross-pollinated within the population or self-pollinated, which resulted in a higher proportion of seeds from self-pollinated flowers. For outcrossing, wild plants were either cross-pollinated within the population or with cultivated plants from West Virginia or Wisconsin. Offspring of all crosses were followed for 4 yr. Two-yr-old seedlings from self-pollination had 45% smaller leaf areas and 33% smaller heights relative to those from cross-pollination. Leaf area is a positive predictor of longer-term survival in wild populations. Our results suggest inbreeding depression, which is unexpected in this self-fertile species. Seedlings from crosses with cultivated plants had 127% greater leaf area and 165% greater root biomass relative to outcrosses within the population. The accelerated growth suggests genetic differences between wild and cultivated populations, but outbreeding depression may not appear until later generations. Assessment of the ultimate fitness consequences of introducing cultivated genotypes requires monitoring over longer time periods.

Deer browsing and population viability of a forest understory plant.

American ginseng is the premier medicinal plant harvested from the wild in the United States. In this study, seven populations of ginseng plants were censused every 3 weeks during the growing season over 5 years to monitor deer browse and harvest and to project population growth and viability. The minimum viable population size was approximately 800 plants, a value greater than that of all populations currently being monitored. When simulated deer browsing rates were reduced 50% or more, population viability rose sharply. Without more effective deer population control, ginseng and many other valuable understory herbs are likely to become extinct in the coming century.

ENVIRONMENT-DEPENDENCE OF QUANTITATIVE GENETIC PARAMETERS IN IMPATIENS PALLIDA.

Population response to selection depends on the presence of additive genetic variance for traits under selection. When a population enters an alien environment, environment-induced changes in the expression of genetic variance may occur. These could have large effects on the response to selection. To investigate the environment-dependence of genetic variance, we conducted a reciprocal transplant experiment between two ecotypically differentiated populations of Impatiens pallida using the progeny of a standard mating design. The floodplain site was characterized by high water availability, moderate temperatures, and continuous dense stands of Impatiens. The hillside site was drier, with larger temperature extremes and supported only scattered patches of Impatiens with significantly lower seed production and earlier mortality. Estimates of heritability were low for each of the 13 traits measured in each population and site (range from 0-28%). Additive genetic variance for life-history traits tended to be larger than for morphological traits, but genetic variance in fitness was estimated to be not significantly different from zero in all cases. Significant heritability was detected in both populations for one trait (date of first cleistogamous flower) known to be closely related to fitness on the hillside. In general, heritability was reduced for populations when grown in the hillside site relative to the floodplain site, suggesting that stress acts to reduce the expression of genetic variance and the potential to respond to selection there. Consistent reductions in heritability associated with more stressful environments suggest that populations invading such sites may undergo little adaptive differentiation and be more prone to local extinction.

PHENOTYPIC SELECTION IN AN ARTIFICIAL POPULATION OF IMPATIENS PALLIDA: THE IMPORTANCE OF THE INVISIBLE FRACTION.

Multiple-regression techniques for measuring phenotypic selection have been used in a large number of recent field studies. One benefit of this technique is its ability to discern the direct action of selection on traits by removing effects of correlated traits. However, covariation among traits expressed at different stages in an organism's life history is often poorly estimated because individuals that die before reaching adulthood cannot be measured as adults. Accurate estimates of trait covariances are necessary for the correct interpretation of the direct action of selection on a trait. If phenotypic characters expressed at different life-history stages are of interest, and mortality occurs between stages, the components of the selection model will be biased by not including those individuals that died (the "invisible fraction").

Interactive effects of resource availabilities and defoliation on photosynthesis, growth, and mortality of red oak seedlings.

Responses of forest trees to defoliation by insects such as gypsy moth vary greatly from site to site and from individual to individual. To determine whether some of this variation could be explained by variation in other stress factors, red oak (Quercus rubra L.) seedlings were exposed to low and high light, water, mineral nutrient, and defoliation treatments, in a complete factorial design in a greenhouse. Significant interactions were observed among factors for photosynthesis, growth, and mortality, indicating that the response to defoliation was influenced by other stresses. Defoliation increased the photosynthetic capacity per unit leaf area of seedlings grown in the low-water, but not in the high-water, regime. In response to defoliation, growth of seedlings in a low-mineral-nutrient, or low-light, regime was depressed less than that of seedlings grown in a high-mineral-nutrient, or high-light, regime. However, defoliation resulted in a similar percent reduction in biomass in all seedlings in both the high and the low light, water, and mineral nutrient treatments. Defoliation-induced mortality of shaded plants was twice that of plants grown in full sun.