Multiple processes contribute to methane emission in a riparian cottonwood forest ecosystem.

Methane emission from trees may partially or completely offset the methane sink in upland soils, the only process that has been regularly included in methane budgets for forest ecosystems. Our objective was to analyze multiple biogeochemical processes that influence the production, oxidation and transport of methane in a riparian cottonwood ecosystem and its adjacent river. We combined chamber flux measurements on tree stems, forest soil and the river surface with eddy covariance measurements of methane net ecosystem exchange. In addition, we tested whether methanogens were present in cottonwood stems, shallow soil layers and alluvial groundwater. Average midday peak in net methane emission measured by eddy covariance was c. 12 nmol m-2  s-1 . The average uptake of methane by soils (0.87 nmol m-2  s-1 ) was largely offset by tree stem methane emission (0.75 nmol m-2  s-1 ). There was evidence of methanogens in tree stems but not in shallow soil. Growing season (May-September) cumulative net methane emission (17.4 mmol CH4  m-2 ) included methane produced in cottonwood stems and methane input to the nocturnal boundary layer from the forest and the adjacent river. The multiple processes contributing to methane emission illustrated the linked nature of these adjacent terrestrial and aquatic ecosystems.

Genetic data improves niche model discrimination and alters the direction and magnitude of climate change forecasts.

Ecological niche models (ENMs) have classically operated under the simplifying assumptions that there are no barriers to gene flow, species are genetically homogeneous (i.e., no population-specific local adaptation), and all individuals share the same niche. Yet, these assumptions are violated for most broadly distributed species. Here, we incorporate genetic data from the widespread riparian tree species narrowleaf cottonwood (Populus angustifolia) to examine whether including intraspecific genetic variation can alter model performance and predictions of climate change impacts. We found that (1) P. angustifolia is differentiated into six genetic groups across its range from México to Canada and (2) different populations occupy distinct climate niches representing unique ecotypes. Comparing model discriminatory power, (3) all genetically informed ecological niche models (gENMs) outperformed the standard species-level ENM (3-14% increase in AUC; 1-23% increase in pROC). Furthermore, (4) gENMs predicted large differences among ecotypes in both the direction and magnitude of responses to climate change and (5) revealed evidence of niche divergence, particularly for the Eastern Rocky Mountain ecotype. (6) Models also predicted progressively increasing fragmentation and decreasing overlap between ecotypes. Contact zones are often hotspots of diversity that are critical for supporting species' capacity to respond to present and future climate change, thus predicted reductions in connectivity among ecotypes is of conservation concern. We further examined the generality of our findings by comparing our model developed for a higher elevation Rocky Mountain species with a related desert riparian cottonwood, P. fremontii. Together our results suggest that incorporating intraspecific genetic information can improve model performance by addressing this important source of variance. gENMs bring an evolutionary perspective to niche modeling and provide a truly "adaptive management" approach to support conservation genetic management of species facing global change.

Tree genotype and genetically based growth traits structure twig endophyte communities.

PREMISE OF THE STUDY: Fungal endophytes asymptomatically inhabit plant tissues where they have mutualistic, parasitic, or commensal relationships with their hosts. Although plant-fungal interactions at the genotype scale have broad ecological and evolutionary implications, the sensitivity of endophytes in woody tissues to differences among plant genotypes is poorly understood. We hypothesize that (1) endophyte communities in Populus angustifolia (Salicaceae) twigs vary among tree genotypes, (2) endophyte variation is linked to quantitative tree traits, and (3) tree genotype influences interspecific fungal interactions. METHODS: Endophytes were isolated from twigs of replicated P. angustifolia genotypes in a common garden and characterized with PCR-RFLP and DNA sequencing. Twig length and diameter, aboveground tree biomass, and condensed tannins were also quantified. KEY RESULTS: (1) Aspects of fungal community structure, including composition and total isolation frequency (i.e., abundance), varied among genotypes. (2) Aboveground biomass and twig diameter were positively associated with isolation frequency and covaried with composition, whereas twig length and condensed tannin concentration were not significantly correlated to endophytes. (3) Fungal co-occurrence patterns suggested negative species interactions, but the presence of significant co-occurrences was genotype dependent. CONCLUSIONS: The species is often assumed to be the most important ecological unit; however, these results indicate that genetically based trait variation within a species can influence an important community of associated organisms. Given the dominance of plants as primary producers and the ubiquity of endophytes, the effect of host genetic variation on endophytes has fundamental implications for our understanding of terrestrial ecosystems.

Fungal Community Composition as Affected by Litter Chemistry and Weather during Four Years of Litter Decomposition in Rainshadow Coastal Douglas-Fir Forests.

Climate and litter chemistry are major factors influencing litter decay, a process mediated by microbes, such as fungi, nitrogen-fixing bacteria and ammonia-oxidizing bacteria. Increasing atmospheric CO2 concentrations can decrease nitrogen (N) and increase condensed tannin (CT) content in foliar litter, reducing litter quality and slowing decomposition. We hypothesized that reduced litter quality inhibits microbes and is the mechanism causing decomposition to slow. Litterbags of Douglas-fir needles and poplar leaves with a range of N (0.61-1.57%) and CT (2.1-29.1%) treatment and natural acid unhydrolyzable residue (35.3-41.5%) concentrations were placed along climatic gradients in mature Douglas-fir stands of coastal British Columbia rainshadow forests. The structure (diversity, richness and evenness) and composition of microbial communities were analyzed using DGGE profiles of 18S, NifH-universal and AmoA PCR amplicons in foliar litter after 7, 12, 24 and 43 months of decay. High CT and low N concentrations in leaf litter were associated with changes in microbial community composition, especially fungi. Contrary to our hypothesis, high CT and low N treatments did not inhibit microbial colonization or diversity. The joint effects of air temperature and soil moisture on microbial community composition at our sites were more important than the effects of initial litter chemistry.

How trees thrive in a dry climate: diurnal and seasonal hydrology and water relations in a riparian cottonwood grove.

In semi-arid ecoregions, trees are restricted to river valley floodplains where river water supplements the limited precipitation. To characterize the associated diurnal and seasonal dynamics in hydrology and water relations, we studied narrowleaf cottonwoods (Populus angustifolia) along a prairie river in Canada. From June through August, the shallow soil moisture was depleted but moisture remained higher above the alluvial groundwater table, which dropped to 2.3 m along with river recession. Throughout the summer, with the daily rise in temperature and insolation, foliar stomatal conductance (gs) and transpiration (E) increased to midday and then fell, thus maintaining the midday leaf water potential (Ψmd) above ~-1.7 MPa. This Ψmd approximated the water potential associated with 12% loss of xylem conductivity due to cavitation for branches (P12); the Ψmd and P12 varied independently across eight trees, providing differences in relative hydraulic risk. Sap flux density (Fd) was measured with thermal dissipation probes, and revealed diurnal patterns similar to foliar E. In contrast to our expectation, the daily Fd maxima were consistent through the summer despite the seasonal recession in water supply. Canopy conductances (GS), derived from Fd, sapwood area and canopy area, declined with vapor pressure deficit (D) and fell slightly in late summer along with stomatal sensitivity, which reflects the magnitude of decrease in GS with increasing D. For spatial up-scaling, satellite-derived near-infrared reflectance of vegetation revealed the woodland phenology, with leaf expansion from May through June and gradual decline in photosynthetic productivity through the summer. Thus, the phreatophytic cottonwoods: (i) sustained substantial water use and productivity through the warm and dry summer, by (ii) accessing shallow soil moisture and then deeper alluvial groundwater, and (iii) providing diurnal stomatal regulation, to (iv) avoid xylem cavitation and (v) maintain fairly constant hydraulic conductance.

Agrobacterium-Mediated Stable Genetic Transformation of Populus angustifolia and Populus balsamifera.

The present study demonstrates Agrobacterium tumefaciens-mediated stable genetic transformation of two species of poplar - Populus angustifolia and Populus balsamifera. The binary vector pCAMBIA-Npro-long-Luc containing the luciferase reporter gene was used to transform stem internode and axillary bud explants. Putative transformants were regenerated on selection-free medium using our previously established in vitro regeneration method. Explant type, genotype, effect of pre-culture, Agrobacterium concentration, a time period of infection and varying periods of co-culture with bacteria were tested for the transformation frequency. The highest frequency of transformation was obtained with stem internode explants pre-cultured for 2 days, infected with Agrobacterium culture at the concentration of OD600 = 0.5 for 10 min and co-cultivated with Agrobacterium for 48 h. Out of the two genotypes tested, P. balsamifera exhibited a higher transformation rate in comparison to P. angustifolia. The primary transformants that exhibited luciferase activity in a bioluminescence assay under the CCD camera when subjected to polymerase chain reaction and Southern blot analysis revealed a stable single-copy integration of luc in their genomes. The reported protocol is highly reproducible and can be applied to other species of poplar; it will also be useful for future genetic engineering of one of the most important families of woody plants for sustainable development.

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.

THE EFFECTS OF HOST-PLANT GENOTYPE, HYBRIDIZATION, AND ENVIRONMENT ON GALL-APHID ATTACK AND SURVIVAL IN COTTONWOOD: THE IMPORTANCE OF GENETIC STUDIES AND THE UTILITY OF RFLPS.

Using restriction fragment length polymorphisms (RFLPs) we show how host-plant genotype and hybridization in cotton wood, Populus sp., affects the attack and survival of the gall-forming aphid, Pemphigus betae. Fremont cottonwoods, hybrid ∗∗∗F1's and backcross ∗∗∗1's were found to be highly resistant, while backcross 2's, 3's, 4's and pure narrowleafs ranged from highly resistant to highly susceptible with only a few trees being highly resistant. Results from our genetic analysis also suggest that resistance is likely polygenic, not the result of single gene resistance. In addition, genetic analysis showed that studies based upon leaf morphology alone give an inaccurate assessment of host-plant genotype ∗∗∗and the extent of hybridization, altering previous views of the relationship between plant hybridization and pest attack. Previous studies assumed that narrowleafs were more resistant than backcross genotypes based upon comparisons of overall levels of resistance between the hybrid zone and the "pure" narrowleaf zone. Results from RFLP analyses, however, show that there are no significant differences in the levels of resistance between backcross genotypes (BC2's-4's) and pure narrowleafs. Furthermore, results show that the "pure" narrowleaf zone is in fact a mixture of pure and backcross genotypes, extending the zone of introgression previously reported. Experiments in combination with RFLP analyses suggest that resistance traits are differentially expressed along an environmental gradient partially explaining the previously reported differences in resistance between these two regions. In light of our results it is clear that genetic studies will be necessary to discern the true relationship between hybridization and pest resistance. Until such studies are widely conducted generalizations regarding the effects of hybridization on the structure and dynamics of pest populations will be premature at best.

MITOCHONDRIAL INHERITANCE PATTERNS ACROSS A COTTONWOOD HYBRID ZONE: CYTONUCLEAR DISEQUILIBRIA AND HYBRID ZONE DYNAMICS.

In this study we examine the cytoplasmic inheritance patterns of an interspecific hybridizing population of Fremont and narrowleaf cottonwoods, using mitochondrial DNA. Three mitochondrial probes showing polymorphisms were used to distinguish between trees of known nuclear inheritance. Every tree screened had only one cytoplasmic genotype, either Fremont or narrowleaf. Thus, these results demonstrate that mitochondria are uniparentally inherited in these trees. Previous studies of the nuclear inheritance of this interspecific hybridizing population of cottonwood trees indicated an asymmetry in the frequency of parental genes. Using mitochondrial markers we tested one hypothesis potentially responsible for this asymmetric distribution (i.e., trees of mixed genotypes will be sterile or will not survive if their cytoplasm is derived from one or the other parent). Our results, however, show that both Fremont and narrowleaf mitochondrial markers are found in trees with mixed nuclear genotypes. Thus, nuclear-cytoplasmic incompatibilities do not appear to account for the asymmetric distribution of nuclear genotypes within the hybrid swarm. An alternative explanation for the observed asymmetric distribution of nuclear genotypes is advanced. Although nuclear-cytoplasmic incompatibilities do not appear to explain the asymmetric distribution of nuclear alleles within the hybrid zone, nonrandom associations between nuclear and cytoplasmic genotypes do exist. For example, all F1 hybrids had Fremont mitochondrial genotypes. Furthermore, backcrosses between F1 hybrid and narrowleaf trees have a higher than expected proportion of heterozygous loci and a higher than expected proportion of Fremont mitochondria. We propose that seeds, seedlings, or trees with high proportions of heterozygous loci are at a disadvantage unless they also have the Fremont mitochondrial genotype. While it is generally difficult to infer dynamic processes from static patterns, studies such as ours enable one to gain new insights to the dynamics of plant hybrid zones. A hybridization pattern of decreasingly complex backcrosses as one proceeds from higher to lower elevation within the hybrid swarm, a residue of Fremont cytoplasmic DNA within the pure narrowleaf population, and the unidirectional nature of these crosses suggest that the narrowleaf population may be spreading down the canyon and the Fremont population receding. The eventual fate of the hybrid zone, in relation to these processes, is discussed.