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Understanding the effects of climate on the vital rates (e.g., survival, development, reproduction) and dynamics of natural populations is a long-standing quest in ecology, with ever-increasing relevance in the face of climate change. However, linking climate drivers to demographic processes requires identifying the appropriate time windows during which climate influences vital rates. Researchers often do not have access to the long-term data required to test a large number of windows, and are thus forced to make a priori choices. In this study, we first synthesize the literature to assess current a priori choices employed in studies performed on 104 plant species that link climate drivers with demographic responses. Second, we use a sliding-window approach to investigate which combination of climate drivers and temporal window have the best predictive ability for vital rates of four perennial plant species that each have over a decade of demographic data (Helianthella quinquenervis, Frasera speciosa, Cylindriopuntia imbricata, and Cryptantha flava). Our literature review shows that most studies consider time windows in only the year preceding the measurement of the vital rate(s) of interest, and focus on annual or growing season temporal scales. In contrast, our sliding-window analysis shows that in only four out of 13 vital rates the selected climate drivers have time windows that align with, or are similar to, the growing season. For many vital rates, the best window lagged more than 1 year and up to 4 years before the measurement of the vital rate. Our results demonstrate that for the vital rates of these four species, climate drivers that are lagged or outside of the growing season are the norm. Our study suggests that considering climatic predictors that fall outside of the most recent growing season will improve our understanding of how climate affects population dynamics.

Foliar nitrogen (N) and phosphorus (P) concentrations, leaf mass per unit area (LMA), and leaf gas exchange were measured in seven populations of Frasera speciosa along a 1700 m gradient, to assess whether altitudinal variation in net photosynthesis rates was related to foliar N and P concentrations, LMA, and stomatal conductance. A secondary goal was to evaluate whether altitudinal trends in foliar chemistry were independent of phenology of the plants. Leaf N and P concentrations per unit mass changed seasonally in all populations, but increased with altitude throughout the growing season. Leaf mass per unit area decreased with increasing altitude, unlike most other studies which have found LMA to increase with altitude. As a result foliar N per unit area of leaf did not change with increasing altitude. Net photosynthesis rates and stomatal conductance did not change with increasing altitude. Transpiration rates increased, while water-use efficiency and the mole fraction of intercellular CO2 decreased with increasing altitude. The change in LMA was the predominant response to increasing altitude in populations of Frasera, and assuming the decrease in CO2 partial pressure with increasing altitude imposes a constraint on photosynthesis, increased internal conductance and/or increased photosynthetic capacity may have resulted in maintenance of similar field photosynthesis rates among populations.

We investigated whether seedling establishment in two alpine plants in Colorado was enhanced through maternally mediated nurse effects. To benefit from an association with maternal debris, seeds must remain with the debris, resulting in reduced risks to seedlings and enhancement of establishment success. Seedlings of Frasera speciosa (Gentianaceae) and Cirsium scopulorum (Asteraceae) were disproportionately abundant under decaying adult rosettes and infructescences relative to other microsites equidistant from the parent. Beneath decaying infructescences of both species, soil was significantly moister, and evaporation rates from hydrometers that mimicked young seedlings were significantly lower than in exposed (control) microsites. For F. speciosa, mean water potential of seedlings planted under infructescences was also significantly higher than that of seedlings planted into exposed plots. These data suggest that drought stress is reduced for seedlings associated with adult debris. In two populations of F. speciosa, adult cover had significant positive effects on seedling establishment in 1995. These advantages were mediated through enhanced survival of young seedlings, rather than emergence success. Cover enhanced seedling survival by 80% in one population and 145% in the other. In one population of C. scopulorum, no seedlings were established from seeds planted with or without debris in 1994 (an extremely dry year). However, in 1995 (a wetter year), seedlings were seven times more likely to emerge and about four times more likely to survive in association with debris than in exposed plots. These results show that associations with adult debris enhance offspring establishment success in both plant species, and provide one of the first demonstrations of conspecific nursing, a form of parental care, in plants.

Frasera speciosa (Gentianaceae) is a long-lived monocarpic plant with mast seeding, a type of plant whose demography is rarely studied. Using a 35-year dataset from an ongoing study, we described the population dynamics of F. speciosa using matrix projection models, addressed how many years of data are needed to estimate population growth and elasticity values consistently, and examined the effects of mast seeding on demographic parameter estimates. Our results indicated that our population is likely stable, with the annual population growth rate near one (λ = 0.969, λs = 0.967 with 95% CI = 0.923–1.013). Mean generation time was 40 years and the mean estimated lifespan was 6.86 years with a high variance of 181.8 years for individuals starting in the seedling stage, consistent with observations of very long-lived individuals in the field. Elasticity values were highest for the transitions representing stasis and lowest for reproduction. For dataset length, 15 years of data yielded lambda estimates that were within 1% of the long-term estimate, but 20 years of data were needed to yield lambdas with confidence intervals that consistently overlap one. Although mast seeding is generally defined using the inter-annual variation in seed production per unit area, stochastic lambdas were not correlated with this measure of reproduction but instead correlated with per capita dormant seed production and per capita recruit production. Mean survival rate was also positively correlated with stochastic lambda estimates, despite the necessary increase in mortality in years with high reproduction. Our results indicated that the demography of F. speciosa is similar to that of other long-lived monocarps without mast seeding, that long datasets may be needed to capture the variation in demographic rates even for stable populations, and that life history stages with low elasticity values can still be very important to population growth.

Synchronized flowering in populations of Frasera speciosa occurs at 2—4 yr intervals. In the East River Valley, Gunnison County, Colorado, major flowering years were 1969, 1973, 1977, and 1980; flowering occurred in alternate years at the highest site (3750 m). Some flowering in three populations in other years suggests that environmental factors can influence the periodicity of flowering. Frasera flowers are preformed at least 3 yr in advance of flowering, although the causes of floral initiation and subsequent synchronous flowering over broad areas are not clear. Progeny of a single plant produced in the same year may grow at different rates and flower over an extended period at different sizes. For Frasera this provides an alternative to iteroparity or seed dormancy as a means of persisting in an unpredictable environment. Both the increase in seed—set and the lowering of seed predation among Frasera individuals in dense neighborhoods have probably provided the selective basis for the evolution of periodic synchronous flowering.

In Frasera speciosa, a long-lived monocarpic gentian from the Colorado Rocky Mountains, leaves are preformed two to three yr in advance of their appearance above ground. Initiation of a flower stalk may begin as long as three yr before it becomes fully developed during a plant's final year. Whorls of preformed leaves are initiated continuously during the growing season, but they are enlarged only once a yr, when the basal rosette emerges above ground. This pattern of development allows plants to make maximal use of the growing season. The prolonged development may be necessary for the production of an elaborate and massive flower stalk by a slow-growing plant in a habitat with a short growing season. PREFORMATION, "the formation of a large part of a shoot in the primordial state with a period of rest prior to expansion" (Halle, Oldeman and Tomlinson, 1978), is a relatively common phenomenon. Many plants preform leaf buds or floral structures during the season before they appear. For example, a study of springblossoming herbaceous plants in New England found that all twenty-eight species considered had formed the next season's buds to some recognizable stage of development by August or September (Foerste, 1891). Temperate forest trees in the same area also initiate some, if not all of their leaves during the season before they appear (Sacher, 1954; Critchfield, 1970; Marks, 1975; Steingraeber, 1982). Thirteen species of native perennial Alaskan grasses were found to preform inflorescences; these species flower earlier in the season than species introduced from more temperate regions, which do

Frasera speciosa is a conspicuous perennial in the Rocky Mountains where it forms discrete colonies whose inflorescences are characterized by almost total absence in some years and greater abundance in others. This sporadic yet synchronous flowering was the most conspicuous feature of the reproductive biology of the species and its adaptive value was worth investigation. In a three—mile stretch of the East River Valley, Colorado, the following data were gathered over a period of three years: the distribution and abundance of colonies, the floral biology, the dispersal of insect visitors among the flowers of Frasera and its floral associates, the frequency of floral predation and seed—set. Although the percentage of plants with inflorescences was always very low and despite the occasional wide spatial separation of individuals, Frasera invariably attracted the greatest number and diversity of floral visitors and never yielded less than 52% seed—set. It was also shown that pollination was affected by a wide variety of inset visitors that maintained cross—pollination at a frequency of approximately 15%. In no colonies was floral predation nearly as heavy as in its sympatric associate Lupinus and it was concluded that seed—set Frasera was not significantly affected by predispersal herbivory. The local synchrony revealed by the occurrence of discrete colonies and the overall synchrony revealed in the almost total absence of floral colonies in some years and their abundance in others is viewed as a strategy for predator avoidance. These mechanisms, together with other aspects of the reproductive biology, also reduce pressure from sympatric species competing for pollinators. It is out that they confer reproductive advantage to a minority species which may otherwise rapidly decline to extinction in the presence of intense competition. The flowering regime of Frasera combines a predator avoidance system which yields widely dispersed colonies in space and in time with a pollination system which successfully exploits the maximum diversity of floral visitors and maintains excellent seed—set whenever and wherever the colonies appear. The systems are clearly complementary in preventing the build—up of predator populations while maintaining an attractive forage source for potential pollen vectors. The combined effect is to maintain the abundance of the species in a variety of stress environments, in turn resulting in a remarkably wide geographic success. It is considered likely that similar systems will turn out to be very common among entomophilous plants in both temperate and tropical regions.

The temperature and water relations of the largleafed, high-elevation species Frasera speciosa, Balsamorhiza sagittata, and Rumex densiflorus were evaluated in the Medicine Bow Mountains of southeast Wyoming (USA) to determine the influence of leaf size, orientation, and arrangement on transpiration. These species characteristically have low minimum stomatal resistances ( 260 mg m-2s-1 for F. speciosa). Field measurements of leaf and microclimatic parameters were incorporated into a computer simulation using standard energy balance equations which predicted leaf temperature (Tleaf) and transpiration for various leaf sizes. Whole-plant transpiration during a day was simulated using field measurements for plants with natural leaf sizes and compared to transpiration rates simulated for plants having identical, but hypothetically smaller (0.5 cm) leaves during a clear day and a typically cloudy day. Although clear-day transpiration for F. speciosa plants with natural size leaves was only 2.0% less per unit leaf area than that predicted for plants with much smaller leaves, daily transpiration of B. sagittata and R. densiflorus plants with natural leaf sizes was 16.1% and 21.1% less, respectively. The predicted influence of a larger leaf size on transpiration for the cloudy day was similar to clear-day results except that F. speciosa had much greater decreases in transpiration (12.7%). The different influences of leaf size on transpiration between the three species was primarily due to major differences in leaf absorptance to solar radiation, orientation, and arrangement which caused large differences in Tleaf. Also, simulated increases in leaf size above natural sizes measured in the field resulted in only small additional decreases in predicted transpiration, indicating a leaf size that was nearly optimal for reducing transpiration. These results are discussed in terms of the possible evolution of a larger leaf size in combination with specific leaf absorptances, orientations and arrangements which could act to reduce transpiration for species growing in short-season habitats where the requirement for rapid carbon fixation might necessitate low stomatal resistances.

Different response patterns in net photosynthesis (A) leaf conductance (g) and water use efficiency (WUE= a/transpiration) in three subalpine plants occurred during experimental sun/shade transitions that simulated natural cloudcover. In Frasera speciosa Dougl., a large-leaved herb characteristic of open sites, g was relatively insensitive to transitions in irradiance and variations in A. However, large decreases in leaf temperature resulted in reduced transpiration during shade intervals and relatively constant WUE throughout the experimental sun/shade regime. In the understory herb, Arnica cordifolia Hook., patterns of A and g were similar during sun/shade transitions, but WUE was substantially reduced compared to steady-state levels. A third, somewhat intermediate pattern of A, g, and WUE was found in Artemisia tridentata L., an open site shrub. Higher intercellular CO2 values in A. tridentata suggested that internal, cellular limitations to A were high relative to stomatal limitations in this shrub when compared to the herbaceous species.

Permission to post granted by: The American Midland Naturalist Department of Biological Sciences University of Notre Dame