A bridge too far: cold and pathogen constraints to assisted migration of riparian forests

Assisted migration of warm‐adapted genotypes to currently cooler climates may reduce maladaptation from future climate change. Few assisted migration trials have considered limitations of the cooler climates and pathogens currently present at transplant sites. This is especially important to consider in riparian ecosystems that are priority targets for restoration in the western United States as they harbor diverse communities. In an effort to validate assisted migration as an effective strategy for mediating the negative impacts of climate change, we used a provenance trial with replicated genotypes from 19 populations of the foundation riparian tree species, Fremont cottonwood (Populus fremontii), transplanted to a cold site to test for genetic variation in growth, mortality, and resistance to shoot blight fungi (Venturia sp.). Populations from cool sites had up to 4 times faster growth, 3 times higher survival, and 8 times higher resistance to Venturia than populations from warm sites, providing evidence of local adaptation to both climate and pathogenic fungi. Budburst phenology and shoot blight were correlated with frost damage, subsequent shrub‐form architecture, and mortality. While climate change models predict 6°C increases, plants transferred distances of 6°C at this time would not perform well; an intermediate transfer distance of less than 3°C would avoid maladaptation to the current environment during assisted migration. Thus, multiple and intermediate transfer phases to supplement local genetic variation will likely be necessary for effective assisted migration to accommodate current environments and large changes in climate.     

小黑杨PsnHB22基因在烟草中的遗传转化研究

HD-Zip转录因子在光信号转导、非生物胁迫、叶片发育等方面发挥重要的作用,HB22转录因子是HD-ZipⅠ亚家族的成员之一。为研究PsnHB22基因的功能,从小黑杨（Populus simonii×P.nigra）cDNA中克隆PsnHB22基因并构建植物表达载体进行烟草（Nicotiana tabacum）的遗传转化,以获得该基因过量表达的转基因株系。对转基因株系进行PCR、qRT-PCR分子检测后观察表型,结果显示在营养生长时期,转基因烟草叶片窄小并且株高显著低于野生型对照。测定转基因烟草及野生型叶片的叶绿素含量,发现转基因烟草叶绿素含量显著高于野生型。由此推测PsnHB22基因在植株高生长、光合作用及叶片的形态建成等过程中起着重要的作用。     

Differing influences of natural and artificial disturbances on riparian cottonwoods from prairie to mountain ecoregions in Alberta, Canada

Aim Ecoregions represent biophysical zones where environmental factors enable the development of particular plant communities. Ecoregions are generally large but abrupt transitions occur in areas with rapid physical change. A particularly abrupt transitional sequence occurs in the Rocky Mountain region of south‐western Alberta where fescue prairie, aspen parkland and mountain ecoregions occur within 15 km. To investigate plant adaptation across ecoregions, our study investigated the influences of a natural disturbance (flooding) and an artificial disturbance (cattle grazing) on reproductive and population processes of black cottonwood (Populus balsamifera subsp. trichocarpa, Torr. & Gray), the dominant riparian tree. Location We studied cottonwoods throughout their elevational range along two free‐flowing, first‐order streams, Yarrow and Drywood creeks. Cottonwood was the only prominent tree in the prairie ecoregion, the dominant riparian tree in the parkland and extended upward through the montane ecoregion where it was a pioneer species for the mixed coniferous–deciduous woodland. Cottonwoods did not occur in the higher elevation sub‐alpine ecoregion. Methods Thirty‐six cross‐sectional sampling transects were located across the three ecoregions with cottonwoods, and in ungrazed and grazed areas of each ecoregion. Rectangular 100 m² tree and 2 m² seedling quadrats were positioned along the transects, and substrate and vegetation were assessed. Historic hydrological data were analysed relative to flood recurrences and seasonal flow patterns. Results Overall, the cottonwoods displayed a sawtooth shaped ‘punctuated progressive age structure’ with many young trees, progressively fewer older trees, and about four pulses of increased recruitment over the past century. This was considered to provide a healthy cottonwood population and recruitment pulses were apparently associated with flood events with appropriate peak timing and magnitude and a gradual post‐flood stage recession. However, analyses of tree, sapling and seedling data indicated that flood‐associated seedling recruitment was less important and clonal processes were more important for cottonwood recruitment in the montane ecoregion, the highest ecoregion with cottonwoods. The correlation between flood events and cottonwood recruitment was strongest in the mid‐elevation parkland ecoregion suggesting greater reliance on flood‐associated seedling recruitment. There was little correlation with flooding and limited recruitment in the fescue prairie ecoregion in recent decades and the disturbed age structure probably results from cattle impacts that have prevented recruitment and produced a decrepit cottonwood forest population. Main conclusions These analyses suggested that a healthy cottonwood population displayed a sawtooth shaped ‘punctuated progressive age structure’ and that cottonwood reproduction processes varied across ecoregions with increased clonality in the highest montane ecoregion. Cattle grazing impacts on reproduction were most severe in the lowest prairie ecoregion that is treeless except for the riparian zone. We conclude that appropriate strategies of instream flow regulation, land‐use policies and practices, and conservation and restoration efforts should be refined according to ecoregion to recognize the differences in cottonwood reproductive and population ecology.     

Un gisement a plantes des formations pliocènes des environs de Rasteau près Vaison-la-Romaine (Vaucluse): Stratigraphie,faune, étude des macrorestes végétaux,palynologie

A new Pliocene deposit with vegetal impressionswas discovered at Rasteau (near Vaison-la-Romaine, Vaucluse. The study of these impression and the sporopollinic analysis from many levels allows us to specify its stratigraphical position in the Pliocene of the Rhone valley. So, the above mentioned levels settled just before the Hautimagne deposit (Neogene zone MN 14 for the mammals). The macroflora and the microflora appear to be related with a temperate warm and wet climate.     

Foliar nutrient concentrations and hardwood growth influenced by cultural treatments

Summary Six species of hardwoods were planted at a 3 by 3 m spacing on a slackwater clay soil (Vertic Haplaquept) in western Mississippi and subjected to three intensities of cultural treatments. Periodic disking significantly increased heights, diameters, and survival of trees. Cultural treatments during the 4 years of the study did not cause any significant changes in soil nutrient levels. Disking eliminated vines and weeds and thus made more water available to trees. Trees growing on disked plots had significantly higher N and Ca and significantly lower P and Mg concentrations in foliage than trees in mowed and control plots. Trees in mowed plots had significantly lower K than control plots, while disked plots had K concentrations that were intermediate between mowed and control. Cottonwood had the highest nutrient concentrations and sycamore the lowest for most elements tested. Other species were intermediate, and no ranking was apparent.     

Morphometrics,Illustration, and Histopathology of Sphaeronema rumicis on Cottonwood in Utah

The morphology of a population of Sphaeronema rumicis Kir''yanova found on cottonwood in Utah is illustrated by light and scanning electron micrographs, as well as by drawings. This is the first report of males of S. rumicis, a species also not known previously to occur in North America. S. rumicis females on cottonwood in the United States were smaller than those found by Kir''yanova on sorrel in the USSR. Females and second-stage juveniles (J2) from the United States had slightly shorter stylets than did females and J2 from the USSR. Males were vermiform and had degenerate esophagi. On secondary cottonwood roots S. rumicis induces formation of a syncytium originating from proliferated pericyclic cells. Thick outer walls, wall protuberances, absence of cell wall ingrowths, dense cytoplasm, and hypertrophied nuclei were the main characteristics of syncytia observed in S. rumicis-infected cottonwood roots.     

Genetic consequences of glacial survival: the late Quaternary history of balsam poplar (Populus balsamifera L.) in North America

Aim Beringia, the unglaciated region encompassing the former Bering land bridge, as well as the land between the Lena and Mackenzie rivers, is recognized as an important refugium for arctic plants during the last ice age. Compelling palaeobotanical evidence also supports the presence of small populations of boreal trees within Beringia during the Last Glacial Maximum. The occurrence of balsam poplar (Populus balsamifera) in Beringia provides a unique opportunity to assess the implications of persistence in a refugium on present‐day genetic diversity for this boreal tree species. Location North America. Methods We sequenced three variable non‐coding regions of the chloroplast genome (cpDNA) from 40 widely distributed populations of balsam poplar across its North American range. We assessed patterns of genetic diversity, geographic structure and historical demography between glaciated and unglaciated regions of the balsam poplar’s range. We also utilized a coalescent model to test for divergence between regions. Results Levels of genetic diversity were consistently greater for populations at the southern margin (θ_W = 0.00122) than in the central (θ_W = 0.00086) or northern (θ_W = 0.00034) regions of the current distribution of balsam poplar, and diversity decreased with increasing latitude (R² = 0.49, P < 0.01). We detected low, but significant, structure (F_CT = 0.05, P = 0.05), among regions of P. balsamifera’s distribution. The cpDNA genealogy was shallow, however, showing an absence of highly differentiated chloroplast haplotypes. Coalescent analyses supported a model of divergence between the southern ice margin and the northern unglaciated region of balsam poplar’s distribution, but analyses of other regional comparisons did not converge. Main conclusions The palaeobotanical record supports the presence of a Beringian refugium for balsam poplar, but we were unable to definitively identify the presence of known refugial populations based on genetic data alone. Balsam poplar populations from Beringia are not a significant reservoir of cpDNA diversity today. Unique alleles that may have been present in the small, isolated populations that survived within Beringia were probably lost through genetic drift or swamped by post‐glacial, northward migration from populations south of the ice sheets.