Maxine A. Watson

Is Evolution Necessary for Range Expansion? Manipulating Reproductive Timing of a Weedy Annual Transplanted beyond Its Range

Ecologists often consider how environmental factors limit a species’ geographic range. However, recent models suggest that geographic distribution also may be determined by a species’ ability to adapt to novel environmental conditions. In this study, we empirically tested whether further evolution would be necessary for northern expansion of the weedy annual cocklebur (Xanthium strumarium) in its native North American range. We transplanted seedlings beyond the northern border and photoperiodically manipulated reproductive timing, a trait important for adaptation to shorter growing seasons at higher latitudes within the range, to determine whether further evolution of this trait would result in a phenotype viable beyond the range. Earlier reproductive induction enabled plants to produce mature seeds beyond the range and to achieve a reproductive output similar to those grown within the range. Therefore, evolution of earlier reproduction in marginal populations would be necessary for northward range expansion. This study is the first to empirically show that evolution in an ecologically important trait would enable a species to survive and reproduce beyond its current range. These results suggest that relatively few traits may limit a species’ range and that identifying evolutionary constraints on such traits could be important for predicting geographic distribution.

Timing of shoot senescence and demographic expression in the clonal perennial Podophyllum peltatum (Berberidaceae)

The timing of leaf senescence may be imposed by the environment or controlled internally by the plant: the latter form of senescence we term endogenous senescence. Controls on the timing of endogenous senescence may reflect an evolutionarily derived compromise between the plants carbon and mineral nutrient requirements. To examine the validity of this hypothesis, we examined the relationship between variation in mean shoot senescence time and demographic status in the long-lived perennial understory herb, the mayapple, Podophyllum peltatum. We found that mean shoot senescence time extends over a 30-d period, with the timing related to the recent history of the rhizome system, the demographic status of the current shoot, and the demographic status of the terminal bud on the extending rhizome axis (i.e., whether it will form a sexual or vegetative shoot in the following year). We found that (1) sexual shoots senesce later than vegetative shoots; (2) fruiting shoots senesce later than sexual shoots without fruits; and (3) shoots on rhizome systems where last years rhizome segment was larger senesce later than those with a shorter ultimate rhizome segment. Of particular interest are our observations that (4) current shoots that give rise to larger new rhizome segments senesce later than those that give rise to smaller ones and (5) current shoots on rhizome systems that give rise to sexual new shoot buds senesce later than those that give rise to vegetative ones. We conclude that, in mayapple, the timing of endogenous shoot senescence is influenced by current and future reproductive status as well as the past and current vigor of the rhizome system. The patterns of relationship that we identify are consistent with the hypothesis that carbon rather than mineral nutrients is the resource most limiting mayapple growth. Significant differences in mean shoot senescence time also were detected among colonies and among years. These differences suggest the existence of genotype and environmental effects on the expression of endogenous leaf senescence time. It remains unclear whether variation in mean shoot senescence time, in and of itself, significantly affects the future demographic fate of rhizome systems.

Resource storage and the expression of clonal plant life histories

Life history—the relative timing of growth, reproduction and mortality—is a fundamental aspect of organisms’ population biology and influences how evolutionary pressures act upon them. Plants manifest a great diversity of life histories, showing significant variation in the extent of growth of vegetative organs vis á vis the timing of sexual reproduction (Harper 1977). It is possible to view life history evolution as a means of seeking solutions to different sets of environmental challenges; thus research has been directed at understanding the relationship between patterns of environmental variation and plant life histories (Stearns 1992). Life history also can be viewed as a solution to a resource-based problem: what pattern of resource use maximizes fitness under some set of environmental conditions? Much work has been done on annual plants, where bigger tends to be better. This may be due to increases in the size or efficiency of the resource gathering apparatus, or to the size of the meristem population available to form flowers or inflorescences (Watson 1984; Bonser and Aarssen 2003). King and Roughgarden (1982) predicted that annual plant species should switch from vegetative growth to sexual reproduction when the biomass of vegetative organs equals the biomass that the sexual organs ultimately will attain. Their interpretation is a developmental one, in that it examines the basis for the developmental shift between vegetative growth and reproduction. But it is also a resource-based hypothesis in that it implies that plants (1) assess their resource status and alter their development based on that assessment and (2) that a virtually complete transfer of resources occurs from vegetative tissues, where they are stored, into reproductive organs as they mature. Little attention has been paid to the effect of storage on the timing, within the life history, of the commitment of meristems to vegetative versus sexual function. If accrual of resources is important for annuals, it should be equally so for perennials that can forgo sexual reproduction in a given year while still increasing lifetime fitness through growth, clonality or both. Storage is a predominant feature of plant biology—serving as a bridge between periods of resource abundance and scarcity—over time scales from minutes to years (Chapin III et al. 1990). Perhaps because storage is so ubiquitous, it has been easy to

Volatile organic compounds (VOCs) drive nutrient foraging in the clonal woodland strawberry, Fragaria vesca

Background and AimsIt was previously demonstrated that stolons of Fragaria vesca respond to patches of varying nutrient quality; however, the mechanism of patch-detection remained unknown. Here we provide support for a process by which F. vesca perceives nutrient-rich patches, consistent with nutrient foraging prior to rooting.MethodsVolatile organic compounds (VOCs) emitted from unsterilized and sterilized field substrates were collected and analyzed by stir-bar headspace extraction gas chromatography-mass spectrometry using a method modified for soil and litter systems. Selected compounds were chosen to represent unsterilized and sterilized field substrates. These synthetic volatile compound mixtures were then applied to neutral substrate to test the ability of F. vesca to choose between unsterilized versus sterilized substrates.ResultsPrimary stolons exhibited chemotropism towards unsterilized (natural) substrates and grew away from the sterilized volatile substrates when the alternate choice was a negative control. We conclude that the presence of carboxylic acids tends to stimulate stolon elongation and chemotropism while aldehydes, ketones and monoterpenes tend to suppress it.ConclusionsWe provide evidence that developing stolons of F. vesca forage for nutrient-rich patches via volatile cues similar to those emitted from the soil through microflora activity.