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SUMMARYNon-random collecting practices may bias conclusions drawn from analyses of herbarium records. Recent efforts to fully digitize and mobilize regional floras offer a timely opportunity to assess commonalities and differences in herbarium sampling biases.We determined spatial, temporal, trait, phylogenetic, and collector biases in ∼5 million herbarium records, representing three of the most complete digitized floras of the world: Australia (AU), South Africa (SA), and New England (NE).We identified numerous shared and unique biases among these regions. Shared biases included specimens i) collected close to roads and herbaria; ii) collected more frequently during spring; iii) of threatened species collected less frequently; and iv) of close relatives collected in similar numbers. Regional differences included i) over-representation of graminoids in SA and AU and of annuals in AU; and ii) peak collection during the 1910s in NE, 1980s in SA, and 1990s in AU. Finally, in all regions, a disproportionately large percentage of specimens were collected by a few individuals. These mega-collectors, and their associated preferences and idiosyncrasies, may have shaped patterns of collection bias via ‘founder effects’.Studies using herbarium collections should account for sampling biases and future collecting efforts should avoid compounding these biases.

Summary 1.Variation in functional community composition is expected to influence the extent of exotic species invasions. Yet, whether resident functional groups control invasion through their relative biomass (mass ratio hypothesis) or by traits other than biomass (identity hypothesis) remains poorly understood. 2. We performed a 6-year experiment to determine the effects of removing different functional groups on exotic species biomass in a Flooding Pampa grassland, Argentina. Functional groups were defined by life-form (grasses or forbs), phenology (winter or summer) and origin (native or exotic). Removal of each functional group was compared against the removal of an equivalent amount of random biomass. Exotic group responses were monitored over 4 years of continuous removals, and after 2 years of recovery without manipulations. 3. Removal of dominant native summer grasses caused the greatest impact on exotic species and overall community composition. Native summer-grass removal significantly increased exotic grass (120%) and forb (730%) biomass beyond the level (46% and 180%, respectively) expected from deleting a similar amount of biomass at random. Exotic annual grasses showed only a transient increase, whereas exotic forb invasion persisted even after 2 years without removals. 4. Removing subordinate, native or exotic winter grasses, and rare native forbs significantly promoted exotic forbs, but to the same level (300%) as random biomass removals. Total grass removal increased exotic forbs to half the extent expected from adding the effects of single grass group removals. Dispersal limitation and harsh abiotic conditions may constrain exotic forb spread into such heavily grass-depleted patches. 5. Synthesis. The impact of losing a functional group on the magnitude and persistence of invasion reflected its relative contribution to community biomass. Identity attributes other than biomass (e.g. phenological niche) further enhanced the biotic control that dominant native grasses exerted on established exotic species. Our findings highlight the community legacies of past disturbances to dominant functional groups.

Abstract. The expansion of plant species into new sites is limited by a combination of environmental conditions and the capacity for adaptive variability in trait expression. Here, we investigated whether and how adaptation to forest edge conditions might be limiting the spread of the invasive plant Alliaria petiolata (garlic mustard) into the forest interior in eastern Massachusetts. We conducted a common garden experiment to test whether plants from forest edge vs. forest interior microhabitats differ in their plasticity and physiological responses to experimental shading. All plants in the experiment responded to shading with reductions in growth, photosynthetic activity, and reproduction regardless of the source environment, indicating a high degree of plasticity and a strong likelihood that most seeds in our study populations are produced at the forest edge. Effects of the source habitat on physiological function were detectable, but small compared to the magnitude in growth and reproductive responses to light. Plants from the forest edge physiologically outperformed those from the interior when grown in the shade, but had equally low reproductive success. Plants originating in the forest interior, in contrast, demonstrated greater allocation to growth in relation to photosynthesis and reproduction compared to plants originating at the forest edge. We compare our findings to earlier work on the importance of plasticity for invasive spread in this species, and conclude that failure by garlic mustard to invade some forest interior sites is due in part to overwhelming reproductive and physiological disadvantages in low light. We further suggest that in some cases shade tolerance in this species is constrained in favor of plastic responses that optimize fitness in high light conditions. The implications for geographic variation in the spread and management of this species are discussed.

Significant changes in plant phenology and flower production are predicted over the next century, but we know relatively little about geographic patterns of this response in many species, even those that potentially impact human wellbeing. We tested for variation in flowering responses of the allergenic plant, Ambrosia artemisiifolia (common ragweed). We grew plants originating from three latitudes in the Northeastern USA at experimental levels of CO2 (400, 600, and 800 µL L−1). We hypothesized that northern ecotypes adapted to shorter growing seasons would flower earlier than their southern counterparts, and thus disproportionately allocate carbon gains from CO2 to reproduction. As predicted, latitude of origin and carbon dioxide level significantly influenced the timing and magnitude of flowering. Reproductive onset occurred earlier with increasing latitude, with concurrent increases in the number of flowers produced. Elevated carbon dioxide resulted in earlier reproductive onset in all ecotypes, which was significantly more pronounced in the northern populations. We interpret our findings as evidence for ecotypic variation in ragweed flowering time, as well in responses to CO2. Thus, the ecological and human health implications of common ragweed’s response to global change are likely to depend on latitude. We conclude that increased flower production, duration, and possibly pollen output, can be expected in Northeastern United States with rising levels of CO2. The effects are likely, however, to be most significant in northern parts of the region.