Daniel Montesinos

Genders in Juniperus thurifera have different functional responses to variations in nutrient availability

• Differences in reproductive investment can trigger asymmetric, context-dependent, functional strategies between genders in dioecious species. However, little is known about the gender responses of dioecious species to nutrient availability. • We experimentally fertirrigated a set of male and female Juniperus thurifera trees monthly for 2 yr. Water potential, photosynthesis rate and stomatal conductance were measured monthly for 2 yr, while shoot nitrogen (N) concentration, carbon isotopic composition (δ(13) C), branch growth, trunk radial growth and reproductive investment per branch were measured yearly. • Control males had lower gas exchange rates and radial growth but greater reproductive investment and higher water use efficiency (WUE; as inferred from more positive δ(13) C values) than females. Fertirrigation did not affect water potential or WUE but genders responded differently to increased nutrient availability. The two genders similarly increased shoot N concentration when fertilized. The increase in shoot N was associated with increased photosynthesis in males but not in females, which presented consistently high photosynthetic rates across treatments. • Our results suggest that genders invest N surplus in different functions, with females presenting a long-term strategy by increasing N storage to compensate for massive reproductive masting events, while males seem to be more reactive to current nutrient availability, promoting gas-exchange capacity.

Conflicting Selective Forces Underlying Seed Dispersal in the Endangered Plant Silene diclinis

Selective pressures underlying the distance of seed dispersal in Silene diclinis were evaluated in natural populations. We expected that high seed predation and high seedling density around mother plants would favor relatively long‐distance dispersal and that microhabitat conditions might favor short‐distance dispersal. We experimentally altered seed density and seed dispersal distance and found that seed predation by ants increased with density but not with distance. Observations of naturally occurring seedlings showed that their survival probability was higher at lower density and when closer to female plants. Overall, calculated cumulative survival probabilities (including both seed predation and seedling survival) favored limited dispersal. Observations of naturally occurring seedlings and adults showed a strongly clumped spatial population structure. Selection for reduced dispersal and the resulting reduction of colonization ability make S. diclinis populations extremely sensitive to environmental stochasticity, and in the long run, this could contribute to the extinction of this endangered species as a consequence of changing land‐use patterns.

Native congeners provide biotic resistance to invasive Potentilla through soil biota

Soil biota can facilitate exotic plant invasions and these effects can be influenced by specific phylogenetic relationships among plant taxa. We measured the effects of sterilizing soils from different native plant monocultures on the growth of Potentilla recta, an exotic invasive forb in North America, and conducted plant-soil feedback experiments with P. recta, two native congeners, a close confamilial, and Festuca idahoensis, a native grass species. We also reanalyzed data comparing the ability of P. recta to invade experimentally constructed congeneric monocultures vs. monocultures of a broad suite of non-congeners. We found that monocultures as a group, other than those of the native P. arguta, were highly invasible by P. recta. In contrast, this was not the case for monocultures of P. arguta. In our first experiment, the biomass of P. recta was 50% greater when grown in soil from F. idahoensis monocultures compared to when it was grown in soils from P. arguta or P. recta monocultures. Sterilizing soil from F. idahoensis rhizospheres had no effect on the biomass of P. recta, but sterilizing soil from P. arguta and P. recta rhizospheres increased the biomass of P. recta by 108% and 90%, respectively. In a second experiment, soil trained by F. idahoensis resulted in a positive feedback for P. recta. In contrast, soils trained independently by each of the two native Potentilla species, or the closely related Dasiphora (formerly Potentilla) resulted in decreases in the total biomass of the invasive P. recta indicating strong negative feedbacks. Soil trained by P. recta also resulted in intraspecific negative feedbacks. Our results demonstrate substantial negative feedbacks for an invader in its nonnative range under certain conditions, and that native congeners can mount strong biotic resistance to an invader through the accumulation of deleterious soil biota.

Neo-Allopatry and Rapid Reproductive Isolation

Over the past 3 centuries, many species have been dispersed beyond their natural geographic limits by humans, but to our knowledge, reproductive isolation has not been demonstrated for such neo-allopatric species. We grew seeds from three species of Centaurea (Centaurea solstitialis, Centaurea calcitrapa, and Centaurea sulphurea) that are native to Spain and have been introduced into California, and we tested to what extent seed production was affected by pollen source. Compared with within-population crosses, seed production decreased by 52% and 44%, respectively, when C. solstitialis and C. sulphurea from California were pollinated with conspecific pollen from native populations in Spain. This implies rapid evolution of reproductive isolation between populations in their native and nonnative ranges. Whether reproductive isolation has evolved following the introduction of other species is unknown, but additional cases are likely, considering the large number of neo-allopatric species.

Dispersal pathways and genetic differentiation among worldwide populations of the invasive weed Centaurea solstitialis L. (Asteraceae).

The natural history of introduced species is often unclear due to a lack of historical records. Even when historical information is readily available, important factors of the invasions such as genetic bottlenecks, hybridization, historical relationships among populations and adaptive changes are left unknown. In this study, we developed a set of nuclear, simple sequence repeat markers and used these to characterize the genetic diversity and population structure among native (Eurasian) and non-native (North and South American) populations of Centaurea solstitialis L., (yellow starthistle). We used these data to test hypotheses about the invasion pathways of the species that were based on historical and geographical records, and we make inferences about historical relationships among populations and demographic processes following invasion. We confirm that the center of diversity and the native range of the species is likely the eastern Mediterranean region in the vicinity of Turkey. From this region, the species likely proceeded to colonize other parts of Europe and Asia via a slow, stepwise range expansion. Spanish populations were the primary source of seed to invade South America via human-mediated events, as was evident from historical records, but populations from the eastern Mediterranean region were also important. North American populations were largely derived from South America, but had secondary contributors. We suggest that the introduction history of non-native populations from disparate parts of the native range have allowed not just one, but multiple opportunities first in South America then again in North America for the creation of novel genotypes via intraspecific hybridization. We propose that multiple intraspecific hybridization events may have created especially potent conditions for the selection of a noxious invader, and may explain differences in genetic patterns among North and South America populations, inferred differences in demographic processes, as well as morphological differences previously reported from common garden experiments.

Invasive and non-invasive congeners show similar trait shifts between their same native and non-native ranges

Differences in morphological or ecological traits expressed by exotic species between their native and non-native ranges are often interpreted as evidence for adaptation to new conditions in the non-native ranges. In turn this adaptation is often hypothesized to contribute to the successful invasion of these species. There is good evidence for rapid evolution by many exotic invasives, but the extent to which these evolutionary changes actually drive invasiveness is unclear. One approach to resolving the relationship between adaptive responses and successful invasion is to compare traits between populations from the native and non-native ranges for both exotic invaders and congeners that are exotic but not invasive. We compared a suite of morphological traits that are commonly tested in the literature in the context of invasion for three very closely related species of Centaurea, all of which are sympatric in the same native and non-native ranges in Europe and North America. Of these, C. solstitialis is highly invasive whereas C. calcitrapa and C. sulphurea are not. For all three species, plants from non-native populations showed similar shifts in key traits that have been identified in other studies as important putative adaptive responses to post-introduction invasion. For example, for all three species plants from populations in non-native ranges were (i) larger and (ii) produced seeds that germinated at higher rates. In fact, the non-invasive C. calcitrapa showed the strongest trait shift between ranges. Centaurea solstitialis was the only species for which plants from the non-native range increased allocation to defensive spines, and allocated proportionally less resources to reproduction, patterns contrary to what would be predicted by theory and other empirical studies to enhance invasion. Our results suggest caution when interpreting the commonly observed increase in size and reproductive capacity as factors that cause exotics to become invaders.

Growth and competitive effects of Centaurea stoebe populations in response to simulated nitrogen deposition.

Increased resource availability can promote invasion by exotic plants, raising concerns over the potential effects of global increases in the deposition of nitrogen (N). It is poorly understood why increased N favors exotics over natives. Fast growth may be a general trait of good invaders and these species may have exceptional abilities to increase growth rates in response to N deposition. Additionally, invaders commonly displace locals, and thus may have inherently greater competitive abilities. The mean growth response of Centaurea stoebe to two N levels was significantly greater than that of North American (NA) species. Growth responses to N did not vary among C. stoebe populations or NA species. Without supplemental N, NA species were better competitors than C. stoebe, and C. stoebe populations varied in competitive effects. The competitive effects of C. stoebe populations increased with N whereas the competitive effects of NA species decreased, eliminating the overall competitive advantage demonstrated by NA species in soil without N added. These results suggest that simulated N deposition may enhance C. stoebe invasion through increasing its growth and relative competitive advantage, and also indicate the possibility of local adaptation in competitive effects across the introduced range of an invader.

Type I error hinders recycling: a response to Rohr and Martin

Rohr and Martin [1] propose that scientific reviews should be recycled in order to reduce the burden of repeated peer-review processes. Many other interesting initiatives are already reducing such burdens in different ways. For instance, and to cite only two examples, PLoS ONE is publishing papers based strictly on scientific quality and not on competition for limited space in a printed journal. This policy naturally reduces the number of reviews that a paper needs before publication, since reviews are only aimed to select good (and not better) manuscripts.

Inter-regional hybrids of native and non-native Centaurea sulphurea inherit increased competitive ability from the non-natives

Background: Exotic species can rapidly develop adaptations to their non-native regions, such as increased size and competitive ability. Although these traits are believed to be responsible for invasive success, some non-invasive exotic species display them too. This suggests that increased size and competitive ability might be necessary but not sufficient to turn an exotic into a successful invader. Aim: To assess size and competitive ability of the exotic non-invasive weed Centaurea sulphurea, and of inter-regional hybrids between native and non-native populations. Methods: We experimentally produced a cohort of C. sulphurea individuals from the native range of the species in Spain, from its non-native range in California, as well as hybrids between the two regions. We grew these plants in pots in competition with the grass Bromus hordeaceus, or alone in control pots. Results: Individuals from California were larger and better competitors than individuals from Spain. Furthermore, inter-regional hybrids showed competitive responses similar to that of individuals from California. Conclusions: Our results confirm that increased competitive ability might be more frequent than previously thought among introduced species, since it can be detected in at least some exotic non-invasive species. They also illustrate the importance of maternal effects, how locally adapted traits are conserved and spread in the non-native ranges of exotic species, and suggest that plant size and competitive ability are not directly associated in this species.

Diminishing importance of elaiosomes for acacia seed removal in non-native ranges

Myrmecochorous plants produce seeds with lipid-rich appendages (elaiosomes) which act as a reward for seed-dispersing ants. Seed dispersal is important for exotic species, which often need to establish new mutualistic interactions in order to colonize new non-native habitats. However, little is known about the importance of elaiosomes for seed removal in many of their non-native ranges. We studied ant–seed interactions of elaiosome-bearing and elaiosome-removed seeds of the Australian trees Acacia dealbata and Acacia longifolia in order to assess the relative importance of elaiosomes for seed removal between their native (Australia) and non-native (Portugal) ranges. In Portugal, we also studied the co-occurring native plant species with myrmecochorous seeds, Pterospartum tridentatum and Ulex europaeus, across three contiguous levels of acacia invasion: control (i.e. no acacia), low, and high acacia tree density. Acacia seeds were successfully removed by ants in their non-native region by a diversified assemblage of ant species, even in sites where native plants interacted with only one specialized ant species. In the invaded range, diminishing relative importance of elaiosomes was associated with changes in the ant community due to acacia invasion, and for A. dealbata, ant species richness decreased with increasing acacia tree density. Although seed removal was high for both acacia species, the importance of elaiosomes was proportionally lower for A. dealbata in the non-native region. Native plant species experienced significant reductions in seed removal in areas highly invaded by acacia, identifying another mechanism of displacement of native plants by acacias.