Ecosystem legacy of the introduced N2-fixing tree Robinia pseudoacacia in a coastal forest.

Habitat invasibility has been found to increase dramatically following the alteration of ecosystem properties by a nonnative species. Robinia pseudoacacia, black locust, is a nitrogen-fixing, clonal tree species that aggressively invades open habitats and expands outside of plantations worldwide. Robinia pseudoacacia stands in Cape Cod National Seashore were particularly susceptible to a hurricane in 1991 that caused widespread blowdown and a dramatic reduction in Robinia in some stands. We used this change to investigate the lasting ecological effects of this nonnative species on this upland coastal ecosystem. We established replicate clusters of 20 × 20 m field plots within 50 m of each other that contained native pitch pine (Pinus rigida) and oak (Quercus velutina, Q. alba) forest, living Robinia stands, and stands in which Robinia was eliminated or reduced to less than 5% cover by the hurricane. Net nitrification and extractable soil nitrate concentration differed significantly between stand types, in the order Robinia > former Robinia > pine-oak. Nonnative species cover differed significantly between each stand type, in the order Robinia > former Robinia > pine-oak. Invasion of Robinia pseudoacacia increased soil net nitrification and nitrogen availability and precipitated a change in forest species composition that favored nonnative species. The presence of elevated soil nitrogen and nonnative species persisted at least 14 years after the removal of the original invading tree species, suggesting that the invasion of a tree species left a legacy of altered soil biogeochemistry, a higher number of nonnative species, and greater nonnative species cover.

Effect of recurrent selfing on inbreeding depression and mating system evolution in an autopolyploid plant.

Genome duplication resulting in polyploidy can have significant consequences for the evolution of mating systems. Most theory predicts that self-fertilization will be selectively favored in polyploids; however, many autopolyploids are outcrossing or mixed-mating. Here, we examine the hypothesis that the evolution of selfing is restricted in autopolyploids because the genetic cost of selfing (i.e., inbreeding depression) increases monotonically with successive generations of inbreeding. Using the herbaceous, autotetraploid plant Chamerion angustifolium, we generated populations with different inbreeding coefficients (F= 0, 0.17 and 0.36) through three consecutive generations of selfing and compared their magnitudes of inbreeding depression in a common environment. Mating system estimates for four natural populations confirmed that tetraploid selfing rates (s(m) = 0.25, SE = 0.02) are similar to those of diploids (s(m) = 0.12, SE = 0.12; F1,2 = 1.34, P= 0.37) indicating that both cytotypes are predominantly outcrossing. Compared to an outbred control line, mean inbreeding depression for seed production, survival, and height (vegetative and total) in the inbred line differed among generations (inbreeding coefficients). Across all stages, inbreeding depression (relative to control) was positively related to generation (inbreeding coefficient). Although the initial costs of inbreeding in extant and newly synthesized polyploids may be low compared to diploids, the monotonic increase in inbreeding depression with repeated inbreeding may limit the extent to which selfing variants are favored.

Floral morphology mediates temporal variation in the mating system of a self-compatible plant.

The mating system of self-compatible plants may fluctuate between years in response to ecological factors that cause variation in the deposition of self pollen vs. outcross pollen on stigmas. Such temporal variation may have significant ecological and evolutionary consequences, but it has rarely been studied, and the mechanisms that mediate temporal variation have almost never been investigated. We tested for variation in the proportion of seeds self-fertilized (s) between two years within 19 populations of the short-lived herb Aquilegia canadensis. Selfing varied widely among populations (range in s = 0.17-1.00, mean s = 0.82) but was inconsistent across years, indicating significant temporal variation. Three populations exhibited especially wide swings in the mating system between years. Mean s did not decrease with increasing population size (N), nor was the fluctuation in s associated with mean N or the change in N. As expected, s declined with increasing separation between anthers and stigmas within flowers (herkogamy), and s fluctuated to a greater extent in populations with more herkogamous flowers. Self-compatible plants can experience wide temporal variation in self-fertilization, and floral traits such as herkogamy may mediate temporal variation by forestalling self-pollination and thus allowing outcrossing during periods when pollinators are frequent.