Interactions between developmental phenology, carbon movement, and storage constrain demography in the understory clonal herb Podophyllum peltatum L.

Little is known about how carbon integration and storage dynamics affect and are affected by demography in field populations. We sought to elucidate this link by examining dynamic patterns of carbon integration relative to the timing of demographically significant developmental decisions regarding shoot type determination in mayapple, Podophyllum peltatum, a clonal plant with large and persistent rhizomes. Individual rhizome systems growing in natural populations were fed 14CO2 either in late-April, early-May, or mid-June, then harvested at intervals throughout the current season and into the next. When distribution of label was examined we found that carbon fixed at different times in the growing season is used differently: April-fixed assimilate remained in the labeled shoot or was moved into the old rhizome, May-fixed assimilate was found predominantly in the old rhizome, while early-June fixed assimilate moved into the old rhizome and the extending new ramet. Movement of assimilate into the old rhizome appeared to have precedence over formation of additional new ramets. Despite significant within season changes in location of dominant sinks within rhizome systems, there was little redistribution of labeled assimilate: early fixed assimilate was not used to fuel later within season growth, however, assimilate was redistributed between seasons. Vegetative and sexual systems differed in the distribution only of April-fixed assimilate. This was observed even though early labeling occurred prior to anthesis. Sexual systems retained a greater proportion of assimilate in the stem than did vegetative ones, which exported more to the old rhizome. 14C-distribution patterns did not vary between systems differing in future demographic status suggesting that the developmental decision regarding shoot type is based on resources acquired in prior years. We explore the hypothesis that preformation and storage are functionally linked traits that permit plants to coordinate the developmental determination of structures differing in cost and demographic function with known resource status. We conclude that demography influences and is influenced by integrative physiology and that physiological restrictions on within season redistribution of assimilates constrain plants' capacities to respond to short-term environmental variation. Such constraints may affect plants' abilities to respond to rapid environmental change in the Anthropocene.

Live substrate positively affects root growth and stolon direction in the woodland strawberry, Fragaria vesca.

Studies of clonal plant foraging generally focus on growth responses to patch quality once rooted. Here we explore the possibility of true plant foraging; the ability to detect and respond to patch resource status prior to rooting. Two greenhouse experiments were conducted to investigate the morphological changes that occur when individual daughter ramets of Fragaria vesca (woodland strawberry) were exposed to air above live (non-sterilized) or dead (sterilized) substrates. Contact between daughter ramets and substrate was prohibited. Daughter ramet root biomass was significantly larger over live versus dead substrate. Root:shoot ratio also increased over live substrate, a morphological response we interpret as indicative of active nutrient foraging. Daughter ramet root biomass was positively correlated with mother ramet size over live but not dead substrate. Given the choice between a live versus a dead substrate, primary stolons extended preferentially toward live substrates. We conclude that exposure to live substrate drives positive nutrient foraging responses in F. vesca. We propose that volatiles emitted from the substrates might be effecting the morphological changes that occur during true nutrient foraging.

Preferential allocation to beneficial symbiont with spatial structure maintains mycorrhizal mutualism.

Mutualisms, beneficial interactions between species, are expected to be unstable because delivery of benefit likely involves fitness costs and selection should favour partners that deliver less benefit. Yet, mutualisms are common and persistent, even in the largely promiscuous associations between plants and soil microorganisms such as arbuscular mycorrhizal fungi. In two different systems, we demonstrate preferential allocation of photosynthate by host plants to the more beneficial of two AM fungal symbionts. This preferential allocation could allow the persistence of the mutualism if it confers sufficient advantage to the beneficial symbiont that it overcomes the cost of mutualism. We find that the beneficial fungus does increase in biomass when the fungi are spatially separated within the root system. However, in well-mixed fungal communities, non-beneficial fungi proliferate as expected from their reduced cost of mutualism. Our findings suggest that preferential allocation within spatially structured microbial communities can stabilize mutualisms between plants and root symbionts.

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.

Shifts in patterns of microhabitat occupation by six closely related species of mosses along a complex altitudinal gradient.

Changes in patterns of microhabitat occupation were examined for six closely related moss species (family Polytrichaceae) found growing together along a complex altitudinal gradient on the northeast face of Mount Washington, New Hampshire. Little evidence could be found to support the hypothesis that the relative distributions of these six moss species were determined by competitive interactions occurring among them. Instead, the data support the hypothesis that changing patterns in the relative distributions of these six moss species result from differences in microhabitat availability among sites. The moss species appear to behave in an opportunistic manner, occupying a wide array of microhabitats as these microhabitats become available to them.