Herbarium collection-based phylogenetics of the ragweeds (Ambrosia, Asteraceae).

Ambrosia (Asteraceae) is a taxonomically difficult genus of weedy, wind-pollinated plants with an apparent center of diversity in the Sonoran Desert of North America. Determining Ambrosia's evolutionary relationships has been the subject of much interest, with numerous studies using morphological characters, cytology, comparative phytochemistry, and chloroplast restriction site variation to produce conflicting accounts the relationships between Ambrosia species, as well as the classification of their close relatives in Franseria and Hymenoclea. To resolve undetermined intra-generic relationships within Ambrosia, we used DNA extracted from tissues obtained from seed banks and herbarium collections to generate multi-locus genetic data representing nearly all putative species, including four from South America. We performed Bayesian and Maximum-Likelihood phylogenetic analyses of six chloroplast-genome and two nuclear-genome markers, enabling us to infer monophyly for the genus, resolve major infra-generic species clusters, as well as to resolve open questions about the evolutionary relationships of several Ambrosia species and former members of Franseria. We also provide molecular data supporting the hypothesis that A. sandersonii formed through the hybridization of A. eriocentra and A. salsola. The topology of our chloroplast DNA phylogeny is almost entirely congruent with the most recent molecular work based on chloroplast restriction site variation of a much more limited sampling of 14 North American species of Ambrosia, although our improved sampling of global Ambrosia diversity enables us to draw additional conclusions. As our study is the first direct DNA sequence-based phylogenetic analyses of Ambrosia, we analyze the data in relation to previous taxonomic studies and discuss several instances of chloroplast/nuclear incongruence that leave the precise geographic center of origin of Ambrosia in question.

Herbarium specimens reveal a historical shift in phylogeographic structure of common ragweed during native range disturbance.

Invasive plants provide ample opportunity to study evolutionary shifts that occur after introduction to novel environments. However, although genetic characters pre-dating introduction can be important determinants of later success, large-scale investigations of historical genetic structure have not been feasible. Common ragweed (Ambrosia artemisiifolia L.) is an invasive weed native to North America that is known for its allergenic pollen. Palynological records from sediment cores indicate that this species was uncommon before European colonization of North America, and ragweed populations expanded rapidly as settlers deforested the landscape on a massive scale, later becoming an aggressive invasive with populations established globally. Towards a direct comparison of genetic structure now and during intense anthropogenic disturbance of the late 19th century, we sampled 45 natural populations of common ragweed across its native range as well as historical herbarium specimens collected up to 140 years ago. Bayesian clustering analyses of 453 modern and 473 historical samples genotyped at three chloroplast spacer regions and six nuclear microsatellite loci reveal that historical ragweed's spatial genetic structure mirrors both the palaeo-record of Ambrosia pollen deposition and the historical pattern of agricultural density across the landscape. Furthermore, for unknown reasons, this spatial genetic pattern has changed substantially in the intervening years. Following on previous work relating morphology and genetic expression between plants collected from eastern North America and Western Europe, we speculate that the cluster associated with humans' rapid transformation of the landscape is a likely source of these aggressive invasive populations.

Pollen clumping and wind dispersal in an invasive angiosperm.

Pollen dispersal is a fundamental aspect of plant reproductive biology that maintains connectivity between spatially separated populations. Pollen clumping, a characteristic feature of insect-pollinated plants, is generally assumed to be a detriment to wind pollination because clumps disperse shorter distances than do solitary pollen grains. Yet pollen clumps have been observed in dispersion studies of some widely distributed wind-pollinated species. We used Ambrosia artemisiifolia (common ragweed; Asteraceae), a successful invasive angiosperm, to investigate the effect of clumping on wind dispersal of pollen under natural conditions in a large field. Results of simultaneous measurements of clump size both in pollen shedding from male flowers and airborne pollen being dispersed in the atmosphere are combined with a transport model to show that rather than being detrimental, clumps may actually be advantageous for wind pollination. Initial clumps can pollinate the parent population, while smaller clumps that arise from breakup of larger clumps can cross-pollinate distant populations.

Sedimented organic nitrogen isotopes in freshwater wetlands record long-term changes in watershed nitrogen source and land use.

Although historic land use is widely recognized as an important determinant of watershed N cycling, efforts to examine land use legacy effects are limited by incomplete historical data. This research evaluates N isotopes of sedimented organic matter (delta15N(org)), in a palynological context, as a long-term proxy of changes in N source to wetland biota. N and S isotope measurements of organic sediments, fossil plant fragments, and living plants are used to explore isotope stratigraphies of wetland sediment cores. Processes potentially contributing to isotope stratigraphies are investigated including the following: a change in N source, diagenesis, and denitrification. We document the delta15N(org) stratigraphy of a core from the Smithsonian Environmental Research Center, MD, U.S.A. spans approximately 350 years, during which time delta15N(org) increases from +2 per thousand to +7 per thousand. Reconstructed population density and wastewater inputs to the watershed suggest that the increase in delta15N reflects changing land use from forested conditions to increasing nutrient inputs from human waste. Our results illustrate the importance of hydrologic connectivity in delivering waste-derived N in a watershed characterized by relatively low human population density. These results also demonstrate how this approach can expand the temporal horizon over which we can assess human impacts to watershed N dynamics.