This work assesses the seed biometry of Stryphnodendron adstringens, S. obovatum and S. polyphyllum species, the effectiveness of some pre-germination treatments to break physical coat dormancy, and the effects of different temperatures and light qualities on the germination of these seeds. The biometry data show that the seeds of S. polyphyllum are bigger than those of the other two species, whose seed characteristics are very similar. Stryphnodendron adstringens and S. polyphyllum present the highest germination percentage and emergence speed index when their seeds are scarified with sulfuric acid for 40 min and then soaked in GA₃ for 24 h. For S. obovatum, the highest percentage and emergence speed index were reached when seeds were scarified with sulfuric acid for 40 min or scarified with sulfuric acid for 40 min and then soaked in water, but these values did not differ statistically from the other treatments. Seeds of the three evaluated Stryphnodendron species proved insensitive to light. The temperatures used were within the range of optimum temperatures for the species.

Despite the economic and therapeutic importance of Mexican oregano essential oil, there is no information regarding secretory structures and the effect of environmental factors on their density is unknown. The main goal of this study was to describe the morphology of glandular trichomes in Lippia graveolens H.B.K. (Verbenaceae), and evaluate the hypothesis that glandular trichome density on leaves of L. graveolens would be greatest for populations found in the most arid locations. Morphology and trichome density were investigated using a combination of scanning electron and stereoscopic microscopy in nine wild oregano populations. Emberger's index of aridity was calculated to characterize the populations located in a bio-climatic gradient. Correlation analysis was used to assess the relation between trichome density and aridity. Lippia graveolens presented two types of glandular trichome on both sides of the leaf: peltate and capitate, of which the former presented higher density. Geographic variation in trichome density was observed at three spatial scales: among bio-climatic regions, among populations within regions and among plants within populations. We found that plant populations at the most arid sites had, on average, lower trichome density. Our results showed a trend for a greater trichome density in more humid sites. Nevertheless, the importance of inter-population variation in trichome density suggests that micro-environmental and genetic factors should be taken into account, in order to better explain the observed differences.

The genus Ariocarpus comprises seven endangered species endemic to the Chihuahuan Desert. We studied the reproductive ecology of Ariocarpus fissuratus at Cuatrociénegas region, Mexico, in order to explore if reproductive traits influence population viability. The size of perianth, pistils and stamens, floral behavior and controlled pollination treatments were assessed to determine Cruden's outcrossing index (OCI), the outcrossing rate and the mating system. The breeding system was estimated with pollen/ovule ratios (P/O) and outcrossing rate was estimated by comparing progeny from self- and outcross-pollination treatments. Frequency and behavior of floral visitors was determined, and nectar production recorded. Ariocarpus fissuratus blooms synchronously during three weeks in autumn. Flowers display herkogamy and homogamy, and OCI and P/O ratio suggest xenogamy. Generalized linear mixed models were fitted to evaluate the effect of pollination treatment on fruit set and number of viable seeds yielded per fruit. Fruit set was significantly higher in the outcrossing treatment than in the selfing treatments. The mating system indicates that A. fissuratus is mainly an outcrosser, showing a partial self-incompatibility with low fruit set for hand self-pollination. Floral visitors are native solitary and introduced bees, and beetles cause high levels of florivory damage. Reproductive ecology suggests that the need for pollinators, presence of exotic pollinators, partial self-incompatibility and florivory negatively affect sexual reproduction and thus the persistence of this endangered species.

Individual species are expected to shift their distributions in response to global climate change. Species within existing communities may respond to climate change individualistically, resulting in the formation of novel communities, or may instead shift as intact communities. We examined whether montane plant communities in the northeastern United States have shifted their elevational range as intact assemblages or individualistically in response to recent regional climatic and environmental change. We used non-metric multidimensional scaling (NMDS) to examine changes in plant community composition and species distributions using vegetation surveys repeated five times between 1964 and 2006 across an elevational gradient (549–1158 m) on Camels Hump Mountain, Vermont, USA, in conjunction with an analysis of local climate change. We found evidence that species elevational distributions and community compositions have shifted in response to a 0.49 °C per decade warming. These species responses were complex and largely individualistic at some elevations, while at other elevations species in a given community tended to respond similarly. The magnitude of community compositional change was largely dependent on location with respect to the ecotone between northern hardwood and boreal forests. While climate change likely contributed to the large shifts in species within NMDS space, these shifts may also be a response to invasive earthworms at low elevations and to prolonged exposure to acid deposition at high elevations. Though we found evidence of shared species responses within communities, future species responses may become increasingly divergent as the magnitude of climate change increases causing species-specific environmental thresholds to be reached and as the synergistic effects of multiple anthropogenic perturbations rise.

To understand how plant communities respond to fluctuations in hydrology, we conducted a natural study on five riverine islands located within the lower Mississippi River. Specifically, we addressed whether species richness and diversity peak at intermediate elevations of flooding and whether plant assembly on these islands follow a niche-derived continuum pattern or develop due to chance processes. Elevations of islands were standardized into 1 m zones providing replicate island gradients of flood frequency and intensity. We examined effects of changing frequency and duration along the gradient and between years, and changes in intensity due to flow based on island side (main channel sides had greater intensity floods). We analyzed diversity data using nonparametric general linear models and compositional dissimilarity using Sorenson's index. Less frequent, more intense disturbances decreased both landscape level and local diversity with the frequency and intensity (alone) of floods affecting community composition. Both temporally and spatially, ‘average’ disturbances beget higher diversity, supporting the intermediate disturbance hypothesis. Compositional data support multiple controls of community assembly. Clear gradients due to niche drivers were found in relation to typical flooding regimes and so specific species have adapted to various frequencies and intensities of disturbance. However, chance played some role throughout the gradient, resulting in either very high similarity or very low similarity where diversity was low and generally low similarity (not different from random), albeit also more permutations of communities, where diversity was high along the gradient.

We examined the effect of position with respect to the soil surface, species, and piece size on the decomposition rate of fine woody debris (< 15 cm diameter) in a North Carolina forest disturbed by hurricane. To examine year-to-year trends, pieces of two species (Carya tomentosa ((Lam.) Nutt.) and Quercus alba (Lam.)) in four size classes were placed on the forest floor and collected annually for ten years. In addition, to examine position effects samples of the same species and sizes were suspended in the air and buried underground at a depth of 20 cm and collected at years 2, 4, and 8. Nine other species were placed on the forest floor and collected at years 2, 4, and 8 to determine the range of variability among species. Decomposition was slower the first year than subsequent years, therefore the lag exponential equation was used to determine time trends and an integrated decomposition rate-constant (k_I) reflecting the overall decomposition rate-constant was calculated. The k_I for C. tomentosa and Q. alba ranged from 0.17–0.25 year⁻¹ with a significant interaction between species and size. The buried and suspended samples generally decomposed more slowly than the samples on the surface and k_I ranged from 0.11–0.24 year⁻¹ and from 0.10–0.18 year⁻¹, respectively. There was a significant interaction between position and size; while drying limited decomposition of suspended pieces regardless of size, high moisture may have limited decomposition in the largest buried pieces. The k_I for all eleven species and sizes averaged over all size classes ranged from 0.06–0.33 year⁻¹. There was a highly significant interaction between species and size with the smaller sizes tending to decompose faster than the larger sizes and in general species with the most decay-resistant heartwood having the largest response to increases in size. Our experiments and comparison to other studies suggests that the interactions between species, size, and position relative to soil surface are highly complex and dependent on site climate.

Canopy gaps create distinct microenvironments within the broader forest environment and provide a mechanism for regeneration and recruitment. In this study, we investigated patterns of sapling richness and composition in 40 canopy gaps in a secondary, mixed Quercus forest on the Cumberland Plateau in Tennessee. We found that sapling richness was high, with 34 species represented in canopy gaps. A species-gap area curve revealed that sapling richness increased steadily with gap sizes up to ca. 200 m², but then tended to level off. While minor components in the main forest canopy, Acer saccharum, Fagus grandifolia, and Acer rubrum were the most widespread and abundant species in gaps. Our analyses of floristic composition using ordination (NMS) and randomization techniques (MRPP) clarified the mechanisms responsible for structuring patterns of gap composition and species-environment relationships, including the effects of individual factors (gap formation mechanism, gap size, aspect, and soils) and their collective impacts. Snag-formed gaps were smaller in size and most common on southern exposures with stony loam soils. Gaps formed by uprooted trees were larger and most common on north-facing slopes with shallow, gravelly loam soils. Gap composition varied along these gradients. Saplings of current canopy dominant genera (Quercus and Carya) were largely restricted to small gaps on xeric sites, suggesting that gap-phase succession will result in a transition from a Quercus–Carya system to one with a much stronger Acer–Fagus component.

We evaluated the impact of chronic deer over browsing on the diversity and abundance of understory forbs and shrubs within a forest stand in the Allegheny High Plateau Region of Pennsylvania by comparing vegetation inside a 60-year-old exclosure to vegetation within an adjacent reference site. This is the oldest known exclosure in the Eastern Deciduous Forest. Browsing caused the formation of an extremely low diversity herbaceous understory dominated by a single fern species, caused the local extirpation of shrubs, and drove forbs to extremely low abundance (< 0.2% cover m⁻² vs. 43% inside the exclosure). Our results confirm previous findings that demonstrate that browsing has caused 60–80% declines in herb and shrub richness regionally. Because many of these species have low dispersal and reproductive rates, we predict long-term legacy effects if deer numbers are ever reduced. Our results combined with other studies provide information on shrub and herb abundance in the absence of browsing that may serve as a baseline to compare potential community recovery in the future.

Species ranges are expected to shift northward with climate warming. Southern Appalachian Magnolia species have never been recorded in oak-dominated forests on Long Island. This study presents data for the occurrence of Magnolia acuminata, M. macrophylla and M. tripetala, in a few stands at Tiffany Creek Preserve, Oyster Bay Cove, Nassau County, Long Island, New York. A tree core datum suggests that the largest Magnolia acuminata in our oak-dominated stands was established in the 1930's; the population has proliferated in the most recent two decades. The largest population of M. tripetala in our area occurs in a complex of swamp forests dominated by Acer rubrum. We compare the climate Normals of Asheville, NC, in the core area of Magnolia acuminata distribution, with those of Mineola, L. I., NY, near its northeastern limit. We find that the winters (average January temperatures) on Long Island have been colder, and with a longer freezing period than in the mountains and piedmont of North Carolina. This suggests that the microclimate of Oyster Bay Cove, north central (North Shore) Long Island, now resembles that of Asheville, and has become more equable.

In order to increase the understanding of the diversity, distribution and abundance of liverworts and hornworts on the Atlantic Coastal Plain, a six year study (2004–2009) was conducted focusing on the Delmarva Peninsula (a land mass comprising portions of the states of Delaware, Maryland, and Virginia). Our previous knowledge of the liverworts and hornworts of the Delmarva were minimal. Collections were few and usually localized, and published and unpublished reports were scarce. Results from this study are presented in the format of an annotated checklist and represents the first comprehensive study on the liverworts and hornworts of the Delmarva Peninsula. A total of 65 species, subspecies, and varieties were documented during this study with 22 being reported as new to the bryoflora of the Delmarva.