Northern Hemisphere Forests Research Articles

A Multi-Scale Assessment of Solar-Induced Chlorophyll Fluorescence and Its Relation to Northern Hemisphere Forest Productivity

A Multi-Scale Assessment of Solar-Induced Chlorophyll Fluorescence and Its Relation to Northern Hemisphere Forest Productivity

Historical biogeography of Acer L. (Sapindaceae): genetic evidence for Out-of-Asia hypothesis with multiple dispersals to North America and Europe

Biogeography is the study of where, when, and how modern species evolved and diversified. Acer L. (maple) is one of the most diverse and widespread genera in the Northern Hemisphere. It comprises 124–156 species in the world, approximately 80% species of Acer are native in Asia. The current diversity center of Acer is not congruent with the distribution of the oldest fossils of the genus. Therefore, we herein used 84 species and subspecies to reconstruct the phylogeny and investigate the biogeographic history of Acer using nuclear ITS and three cpDNA fragments (psbA-trnH spacer, rpl16 intron, and trnL-trnF spacer) with maximum likelihood, maximum parsimony, and Bayesian inference methods. The analyses showed that the current diversity center and the origin center of Acer is Asia. Additionally, the North American and Euro-Mediterranean species originated from multiple sources from Asia via the North Atlantic Land Bridge and the Bering Land Bridge, and intercontinental migration has mainly occurred since the Miocene. This study not only provides a novel insight of the origin and dispersal routes of Acer but also exemplifies how past climatic changes affect the diversification-rates of Northern Hemisphere forest trees.

Foundation species across a latitudinal gradient in China.

Foundation species structure forest communities and ecosystems but are difficult to identify without long-term observations or experiments. We used statistical criteria--outliers from size-frequency distributions and scale-dependent negative effects on alpha diversity and positive effects on beta diversity--to identify candidate foundation woody plant species in 12 large forest-dynamics plots spanning 26 degrees of latitude in China. We used these data (1) to identify candidate foundation species in Chinese forests, (2) to test the hypothesis--based on observations of a midlatitude peak in functional trait diversity and high local species richness but few numerically dominant species in tropical forests--that foundation woody plant species are more frequent in temperate than tropical or boreal forests, and (3) to compare these results with data from the Americas to suggest candidate foundation genera in northern hemisphere forests. Using the most stringent criteria, only two species of Acer, the canopy tree Acer ukurunduense and the shrubby treelet Acer barbinerve, were identified in temperate plots as candidate foundation species. Using more relaxed criteria, we identified four times more candidate foundation species in temperate plots (including species of Acer, Pinus, Juglans, Padus, Tilia, Fraxinus, Prunus, Taxus, Ulmus, and Corlyus) than in (sub)tropical plots (the treelets or shrubs Aporosa yunnanensis, Ficus hispida, Brassaiopsis glomerulata, and Orophea laui). Species diversity of co-occurring woody species was negatively associated with basal area of candidate foundation species more frequently at 5- and 10-m spatial grains (scale) than at a 20-m grain. Conversely, Bray-Curtis dissimilarity was positively associated with basal area of candidate foundation species more frequently at 5-m than at 10- or 20-m grains. Both stringent and relaxed criteria supported the hypothesis that foundation species are more common in mid-latitude temperate forests. Comparisons of candidate foundation species in Chinese and North American forests suggest that Acer be investigated further as a foundation tree genus.

The effects of tropospheric ozone on oaks: A global meta-analysis

Tropospheric ozone (O3) levels are still elevated in many regions of the world including Northern Hemisphere forests areas, and are predicted to increase further due to anthropogenic activities and climate change. Oaks are major woody angiosperms in the Northern Hemisphere in terms of biodiversity, ecological dominance, and economic values. This meta-analysis shows overwhelming evidence of the O3 effects on 51 growth, anatomical, biomass, physiological and biochemical parameters of 14 deciduous or evergreen oak species distributed all around the Northern Hemisphere. Although no large impacts were observed on biomass, suggesting an O3 tolerance by oaks, some impairments were found at physiological level that might negatively affect carbon sequestration and water vapour transfer to the atmosphere. This outcome suggests the need to incorporate this phenomenon into future projection studies dealing with how atmospheric change and forest biomes will interact in effecting climatic change. Among the antioxidants used by oaks to respond to O3, phenols seem to have a crucial role. Deciduous species resulted more affected by O3 than evergreen ones, as well as oaks native to Eurasia, in comparison with those from North-America. Experiments performed in less controlled environments showed more O3 deleterious effects, especially under higher AOT40 levels, but negative impacts were also reported for acute O3 exposures. Most of the reviewed studies with additional treatments to O3 exposure investigated the interaction(s) between O3 and drought, but the negative effects induced by drought seemed not to be exacerbated by the pollutant. However, more combined experiments on the impact of O3 and co-occurring stressors on woody species are necessary. Another major issue is the lack of experiments on adult trees. To better understand O3 impacts, and to reinforce the strength of O3 impact predictions, O3 controlled experiments on young individuals should be combined with long-term experiments on mature trees grown in open-air conditions.

Current and future impacts of drought and ozone stress on Northern Hemisphere forests.

Rising ozone (O3 ) concentrations, coupled with an increase in drought frequency due to climate change, pose a threat to plant growth and productivity which could negatively affect carbon sequestration capacity of Northern Hemisphere (NH) forests. Using long-term observations of O3 mixing ratios and soil water content (SWC), we implemented empirical drought and O3 stress parameterizations in a coupled stomatal conductance-photosynthesis model to assess their impacts on plant gas exchange at three FLUXNET sites: Castelporziano, Blodgett and Hyytiälä. Model performance was evaluated by comparing model estimates of gross primary productivity (GPP) and latent heat fluxes (LE) against present-day observations. CMIP5 GCM model output data were then used to investigate the potential impact of the two stressors on forests by the middle (2041-2050) and end (2091-2100) of the 21st century. We found drought stress was the more significant as it reduced model overestimation of GPP and LE by ~11%-25% compared to 1%-11% from O3 stress. However, the best model fit to observations at all the study sites was obtained with O3 and drought stress combined, such that the two stressors counteract the impact of each other. With the inclusion of drought and O3 stress, GPP at CPZ, BLO and HYY is projected to increase by 7%, 5% and 8%, respectively, by mid-century and by 14%, 11% and 14% by 2091-2100 as atmospheric CO2 increases. Estimates were up to 21% and 4% higher when drought and O3 stress were neglected respectively. Drought stress will have a substantial impact on plant gas exchange and productivity, off-setting and possibly negating CO2 fertilization gains in future, suggesting projected increases in the frequency and severity of droughts in the NH will play a significant role in forest productivity and carbon budgets in future.

Organic carbon distribution and budget of dominant woody plant community in the subalpine zone at volcanic Jeju Island, Korea

BackgroundThe Northern Hemisphere forest ecosystem is a major sink for atmospheric carbon dioxide, and the subalpine zone stores large amounts of carbon; however, their magnitude and distribution of stored carbon are still unclear.ResultsTo clarify the carbon distribution and carbon budget in the subalpine zone at volcanic Jeju Island, Korea, we report the C stock and changes therein owing to vegetation form, litter production, forest floor, and soil, and soil respiration between 2014 and 2016, for three subalpine forest ecosystems, namely, Abies koreana forest, Taxus cuspidata forest, and Juniperus chinensis var. sargentii forest. Organic carbon distribution of vegetation and NPP were bigger in the A. koreana forest than in the other two forests. However, the amount of soil organic carbon distribution was the highest in the J. chinensis var. sargentii forest.Compared to the amount of organic carbon distribution (AOCD) of aboveground vegetation (57.15 t C ha−1) on the subalpine-alpine forest in India, AOCD of vegetation in the subalpine forest in Mt. Halla was below 50%, but AOCD of soil in Mt. Halla was higher. We also compared our results of organic carbon budget in subalpine forest at volcanic island with data synthesized from subalpine forests in various countries.ConclusionsThe subalpine forest is a carbon reservoir that stores a large amount of organic carbon in the forest soils and is expected to provide a high level of ecosystem services.

Bird Diversity in Urban Ecosystems: The Role of the Biome and Land Use Along Urbanization Gradients

Fil: Filloy, Julieta. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Ecologia, Genetica y Evolucion de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecologia, Genetica y Evolucion de Buenos Aires; Argentina

Phylogenetic classification of the world’s tropical forests

Knowledge about the biogeographic affinities of the world's tropical forests helps to better understand regional differences in forest structure, diversity, composition, and dynamics. Such understanding will enable anticipation of region-specific responses to global environmental change. Modern phylogenies, in combination with broad coverage of species inventory data, now allow for global biogeographic analyses that take species evolutionary distance into account. Here we present a classification of the world's tropical forests based on their phylogenetic similarity. We identify five principal floristic regions and their floristic relationships: (i) Indo-Pacific, (ii) Subtropical, (iii) African, (iv) American, and (v) Dry forests. Our results do not support the traditional neo- versus paleotropical forest division but instead separate the combined American and African forests from their Indo-Pacific counterparts. We also find indications for the existence of a global dry forest region, with representatives in America, Africa, Madagascar, and India. Additionally, a northern-hemisphere Subtropical forest region was identified with representatives in Asia and America, providing support for a link between Asian and American northern-hemisphere forests.

Сучасний стан і перспективи збереження генетичних ресурсів тополі в Україні

Узагальнено інформацію про наявні площі природних і штучних деревостанів, розподіл за категоріями лісів, а також про видове різноманіття представлених в Україні тополь за результатами аналізу бази даних ВО "Укрдержліспроект". Встановлено сучасний стан лісових генетичних ресурсів тополі та пріоритети їхнього збереження і використання.
 З’ясовано, що у лісовому фонді рівнинної частини України представлено 7 видів тополь, які включено до секцій Aigeiros, Populus, Tacamahaca, зокрема: P. nigra, P. deltoides, P. alba, P. balsamifera, P. laurifolia, P. simonii, P. tremula, а також три культивари: P. nigra var. Pyramidalis, P. alba var. Bolleana і P. х canescens. Загальна площа тополевих лісів сягає 29071,1 га, зокрема природного походження – 15522,7 га.
 Визначено такі пріоритети зі збереження та сталого використання генетичних ресурсів тополі в Україні: поширення інформації про розповсюдження та використання, біологію та генетичну мінливість видів і популяцій тополі в Україні; розробка та реалізація стратегії та відповідної нормативно-правової бази зі збереження, відтворення та використання генофонду Populus sp.; відбір та паспортизація нових об'єктів, зокрема із застосуванням різних методів геномної та генної селекції, а також ДНК-маркування; дослідження міжвидової та внутрішньовидової мінливості тополь в умовах кліматичних змін та визначення значущості тополь у виконанні екосистемних послуг; укріплення раціонального використання ресурсів тополь, розвитку та управління ними.
 Для ефективного культивування тополі в Україні необхідно виконати такі ключові програмні та наукові заходи, як пошук міжнародних дослідницьких програм для розроблення інноваційного застосування тополь і створення нових клонів з підвищеною стійкістю проти посухи, засолення ґрунту та шкідників і хвороб.

Simulating the onset of spring vegetation growth across the Northern Hemisphere.

Changes in the spring onset of vegetation growth in response to climate change can profoundly impact climate-biosphere interactions. Thus, robust simulation of spring onset is essential to accurately predict ecosystem responses and feedback to ongoing climate change. To date, the ability of vegetation phenology models to reproduce spatiotemporal patterns of spring onset at larger scales has not been thoroughly investigated. In this study, we took advantage of phenology observations via remote sensing to calibrate and evaluated six models, including both one-phase (considering only forcing temperatures) and two-phase (involving forcing, chilling, and photoperiod) models across the Northern Hemisphere between 1982 and 2012. Overall, we found that the model that integrated the photoperiod effect performed best at capturing spatiotemporal patterns of spring phenology in boreal and temperate forests. By contrast, all of the models performed poorly in simulating the onset of growth in grasslands. These results suggest that the photoperiod plays a role in controlling the onset of growth in most Northern Hemisphere forests, whereas other environmental factors (e.g., precipitation) should be considered when simulating the onset of growth in grasslands. We also found that the one-phase model performed as well as the two-phase models in boreal forests, which implies that the chilling requirement is probably fulfilled across most of the boreal zone. Conversely, two-phase models performed better in temperate forests than the one-phase model, suggesting that photoperiod and chilling play important roles in these temperate forests. Our results highlight the significance of including chilling and photoperiod effects in models of the spring onset of forest growth at large scales, and indicate that the consideration of additional drivers may be required for grasslands.

Linking plant functional trait plasticity and the large increase in forest water use efficiency

AbstractElevated atmospheric CO2 concentrations are expected to enhance photosynthesis and reduce stomatal conductance, thus increasing plant water use efficiency. A recent study based on eddy covariance flux observations from Northern Hemisphere forests showed a large increase in inherent water use efficiency (IWUE). Here we used an updated version of the same data set and robust uncertainty quantification to revisit these contemporary IWUE trends. We tested the hypothesis that the observed IWUE increase could be attributed to interannual trends in plant functional traits, potentially triggered by environmental change. We found that IWUE increased by ~1.3% yr−1, which is less than previously reported but still larger than theoretical expectations. Numerical simulations with the Tethys‐Chloris ecosystem model using temporally static plant functional traits cannot explain this increase. Simulations with plant functional trait plasticity, i.e., temporal changes in model parameters such as specific leaf area and maximum Rubisco capacity, match the observed trends in IWUE. Our results show that trends in plant functional traits, equal to 1.0% yr−1, can explain the observed IWUE trends. Thus, at decadal or longer time scales, trait plasticity could potentially influence forest water, carbon, and energy fluxes with profound implications for both the monitoring of temporal changes in plant functional traits and their representation in Earth system models.

Biotic disturbances in Northern Hemisphere forests – a synthesis of recent data, uncertainties and implications for forest monitoring and modelling

AbstractAimBiotic disturbances (BD, including insects, pathogens and wildlife herbivory) can alter forest structure and the capability of forests to deliver ecosystem services. Impact assessments, however, are limited by the lack of reliable and timely disturbance data at large spatial scales. This review synthesizes empirical data on the magnitude and distribution of spatio‐temporal impacts of BD.LocationNorthern Hemisphere.MethodsBased on large‐scale, multi‐year BD data sets, covering c. 46% of the global forest, we calculated annual disturbance fractions Df (percentage of forest area affected) and their inter‐annual variability at a grid cell resolution of 1°. The impact of BD on forest carbon pools was determined by overlaying Df with data on forest cover and carbon density.ResultsOverall, 43.9 million hectares (Mha) (Df = 2.6%) of forests were affected annually by BD, particularly by insects (36.5 Mha, Df = 2.2%). Our synthesis demonstrates that fractions affected by BD (1) vary greatly over space and time, mainly in response to ephemeral bark beetle and defoliator outbreaks, (2) show temporal trends that are inconsistent across regions, yet are largely increasing over recent decades, and (3) are substantially higher than Df caused by fire and other abiotic disturbances. Tree mortality was estimated over an area of 3.3 Mha year−1 (medium estimate which assumed mortality at 7.5% of the affected area), with associated committed carbon fluxes from living biomass to litter and the atmosphere at 129.9 Mt C year−1.Main conclusionsBD are key drivers of forest dynamics, making a contribution to tree mortality of a similar magnitude to fire. Despite inherent uncertainties, the data reported can be used to improve the representation of BD in global ecosystem models. Our findings call for future forest monitoring approaches to provide accessible, precise and consistent data on the occurrence and severity of BD which are harmonized across jurisdictions.

Impacts of droughts on the growth resilience of Northern Hemisphere forests

AbstractAimThe intensity and frequency of drought have increased considerably during recent decades in some Northern Hemisphere forested areas, and future climate warming could further magnify drought stress. We quantify how forests resist drought events and recover after them, i.e. we determine their growth resilience.LocationNorth America and Europe.MethodsWe use a large tree‐ring database to study how drought influences forest growth resilience. We selected 775 tree‐ring width chronologies and studied the occurrence of years with extremely dry conditions (low soil moisture and/or high evaporative stress; hereafter ‘drought’) in these forests. For each drought in each forest we calculated three indices that represent different components of growth resilience to drought: resistance (Rt), recovery (Rc) and resilience (Rs). We related the variation in these indices with geographical, topographic, climatic and ecological conditions from each region.ResultsThe three components of forest growth resilience were interrelated. Resistance and recovery were negatively related, and both were positively and nonlinearly related to resilience. Drought resistance increased with latitude, soil moisture and slope, whereas drought recovery decreased with latitude, soil moisture and summer normalized difference vegetation index. Drought resilience increased with elevation and decreased with the variation in soil moisture. Temperate broadleaf forests from wet regions showed a greater growth resistance (e.g. north‐eastern USA, central Europe) while conifer forests from dry to semi‐arid regions (e.g. south‐western USA, southern Europe) presented a greater growth recovery.Main conclusionsThe geographical patterns of growth resilience indices confirm the existence of different strategies among forests to cope with droughts, depending on the biome, the tree species and the prevailing climatic conditions. Geographical patterns in soil moisture availability tend to override species‐specific responses to drought.

Multisite analysis of land surface phenology in North American temperate and boreal deciduous forests from Landsat

Abstract Forests play important roles in the Earth's climate system and global carbon cycle. Therefore, a critical need exists to improve our understanding of how the growing seasons of forests are changing, and by extension, how the composition and function of forests will respond to future climate change. Coarse spatial resolution satellite remote sensing has been widely used to monitor and map the phenology of terrestrial ecosystems at regional to global scales, and despite widespread agreement that the growing season of Northern Hemisphere forests is changing, the spatial resolution of these data sources imposes significant limitations on the character and quality of inferences that can be drawn from them. In particular, the spatial resolution afforded by instruments such as MODIS does not resolve ecologically important landscape-scale patterns in phenology. With this issue in mind, here we evaluate the ability of a newly developed Landsat phenology algorithm (LPA) to reconstruct a 32-year time series for the start and end of the growing season in North American temperate and boreal forests. We focus on 13 “sidelap” regions located between overlapping Landsat scenes that span a large geographic range of temperate and boreal forests, and evaluate the quality and character of LPA-derived start and end of growing season (SOS and EOS) dates using several independent data sources. On average, SOS and EOS dates were detected for about two-thirds of the 32 years included in our analysis, with the remaining one-third missing due to cloud cover. Moreover, there was generally better agreement between ground observations and LPA-derived estimates of SOS dates than for EOS across the 13 sites included in our study. Our results demonstrate that, despite the presence of time series gaps, LPA provides a robust basis for retrospective analysis of long-term changes in spring and autumn deciduous forest phenology over the last three decades. Finally, our results support the potential for monitoring land surface phenology at 30 m spatial resolution in near real-time by combining time series from multiple sensors such as the Landsat Operational Land Imager and the Sentinel 2 MultiSpectral Instrument.

Real-Time PCR Detection of Dogwood Anthracnose Fungus in Historical Herbarium Specimens from Asia.

Cornus species (dogwoods) are popular ornamental trees and important understory plants in natural forests of northern hemisphere. Dogwood anthracnose, one of the major diseases affecting the native North American Cornus species, such as C. florida, is caused by the fungal pathogen Discula destructiva. The origin of this fungus is not known, but it is hypothesized that it was imported to North America with its host plants from Asia. In this study, a TaqMan real-time PCR assay was used to detect D. destructiva in dried herbarium and fresh Cornus samples. Several herbarium specimens from Japan and China were detected positive for D. destructiva, some of which were collected before the first report of the dogwood anthracnose in North America. Our findings further support that D. destructiva was introduced to North America from Asia where the fungus likely does not cause severe disease.

The oldestPinusand its preservation by fire

Pinus (Pinaceae) is a diverse conifer genus that dominates Northern Hemisphere forests today and is noteworthy for its fire-adapted traits. Here we describe the oldest known fossils attributable to the genus from the Lower Cretaceous (Valanginian, ca. 133–140 Ma) part of the Chaswood Formation of Nova Scotia, Canada. Pinus mundayi sp. nov. comprises charred long-shoots, which show a constellation of derived characters including (1) axial resin ducts with thin-walled epithelial cells in the secondary xylem and phloem, (2) fenestriform or pinoid cross-field pits, and (3) helically arranged short-shoots that pass through growth ring boundaries before distally diverging into two separate needle bases. The fossils, which are interpreted as remains of an evergreen two-needle pine, provide a new constraint on timetrees of Pinaceae evolution. Their preservation as charcoal and the occurrence of resin ducts, which produce flammable terpenes in modern pines, show that Pinus has co-occurred with fire since its Mesozoic origin.

Can we set a global threshold age to define mature forests?

Globally, mature forests appear to be increasing in biomass density (BD). There is disagreement whether these increases are the result of increases in atmospheric CO2 concentrations or a legacy effect of previous land-use. Recently, it was suggested that a threshold of 450 years should be used to define mature forests and that many forests increasing in BD may be younger than this. However, the study making these suggestions failed to account for the interactions between forest age and climate. Here we revisit the issue to identify: (1) how climate and forest age control global forest BD and (2) whether we can set a threshold age for mature forests. Using data from previously published studies we modelled the impacts of forest age and climate on BD using linear mixed effects models. We examined the potential biases in the dataset by comparing how representative it was of global mature forests in terms of its distribution, the climate space it occupied, and the ages of the forests used. BD increased with forest age, mean annual temperature and annual precipitation. Importantly, the effect of forest age increased with increasing temperature, but the effect of precipitation decreased with increasing temperatures. The dataset was biased towards northern hemisphere forests in relatively dry, cold climates. The dataset was also clearly biased towards forests <250 years of age. Our analysis suggests that there is not a single threshold age for forest maturity. Since climate interacts with forest age to determine BD, a threshold age at which they reach equilibrium can only be determined locally. We caution against using BD as the only determinant of forest maturity since this ignores forest biodiversity and tree size structure which may take longer to recover. Future research should address the utility and cost-effectiveness of different methods for determining whether forests should be classified as mature.

Woody biomass production lags stem-girth increase by over one month in coniferous forests.

Wood is the main terrestrial biotic reservoir for long-term carbon sequestration(1), and its formation in trees consumes around 15% of anthropogenic carbon dioxide emissions each year(2). However, the seasonal dynamics of woody biomass production cannot be quantified from eddy covariance or satellite observations. As such, our understanding of this key carbon cycle component, and its sensitivity to climate, remains limited. Here, we present high-resolution cellular based measurements of wood formation dynamics in three coniferous forest sites in northeastern France, performed over a period of 3 years. We show that stem woody biomass production lags behind stem-girth increase by over 1 month. We also analyse more general phenological observations of xylem tissue formation in Northern Hemisphere forests and find similar time lags in boreal, temperate, subalpine and Mediterranean forests. These time lags question the extension of the equivalence between stem size increase and woody biomass production to intra-annual time scales(3, 4, 5, 6). They also suggest that these two growth processes exhibit differential sensitivities to local environmental conditions. Indeed, in the well-watered French sites the seasonal dynamics of stem-girth increase matched the photoperiod cycle, whereas those of woody biomass production closely followed the seasonal course of temperature. We suggest that forecasted changes in the annual cycle of climatic factors(7) may shift the phase timing of stem size increase and woody biomass production in the future.

Functional trait variation along environmental gradients in temperate and Mediterranean trees

AbstractAimCharacterizing the variation of functional traits in nature is a first step towards linking environmental changes to changes in ecosystem function. Here we aim to characterize the spatial variability of major plant functional traits along wide environmental gradients in Mediterranean and temperate forests, and assess to what extent this variability differs between two dominant families in Northern Hemisphere forests: Fagaceae and Pinaceae.LocationCatalonia (north‐east Iberian Peninsula).MethodsFour functional traits were selected to incorporate information on both the leaf and the wood economic spectra: maximum tree height (Hmax), wood density (WD), leaf mass per area (LMA) and nitrogen content of leaves (Nmass). We quantified the variance distribution of each functional trait across three nested ecological scales: population, species and family. Through such scales, we explored the spatial variation of functional traits through climatic and biotic gradients, as well as the covariation among traits.ResultsFunctional trait variability was distributed across all the ecological scales considered, but mostly at the family level, with functional traits differing markedly between Fagaceae and Pinaceae. Within families, variation in functional traits was similar or higher within species than between species. The spatial variability in functional traits was related to biotic and abiotic gradients, although this effect was quantitatively small compared with differences between families. Covariation among functional traits was not necessarily conserved across ecological scales. Trait covariation across all species was structured along the Hmax−WD and LMA−Nmass axes, but this structure was partially lost within families, where variation was mostly structured along the Hmax−LMA and WD−Nmass axes.Main conclusionsIntraspecific variation emerges as a fundamental component of functional trait structure along wide environmental gradients. Understanding the sources of intraspecific variation, as well as how it contributes to community assembly and ecosystem functioning, thus becomes a primary research question.

Mitochondrial introgression and complex biogeographic history of the genus Picea

Biogeographic history of plants is much more complex in the Northern Hemisphere than in the Southern Hemisphere due to that both the Bering and the North Atlantic land bridges contributed to floristic exchanges in the Cenozoic, which led to hybridization between congeneric species from different continents. It would be interesting to know how intercontinental gene flow and introgression have affected plant phylogenetic reconstruction and biogeographic inference. In this study, we reinvestigated the phylogenetic and biogeographic history of Picea, a main component of the Northern Hemisphere forest with many species that originated from recent radiation, using two chloroplast (cp), one mitochondrial (mt) and three single-copy nuclear gene markers. The generated gene trees are topologically highly discordant and the geographically closely related species generally show a close affinity of mtDNA rather than cp- or nuclear DNA, suggesting that inter- and intra-continental gene flow and mtDNA introgression might have occurred commonly. However, all gene trees resolved Picea breweriana as the basal-most lineage, which, together with fossil evidence, supports the North American origin hypothesis for the genus. Both dispersal and vicariance have played important roles in the evolution of Picea, and the Bering Land Bridge could have mediated the “North America to Eurasia” dispersal at least two times during the Miocene and Pliocene. Our study again demonstrates the importance of applying data from three genomes for a clear understanding of evolutionary histories in the pine family. Any markers from a single genome alone will not reveal a clear picture of the phylogenetic relationships among closely related congeneric species. In particular, mtDNA markers should be cautiously used, considering that introgression of the maternally inherited mtDNA with a lower rate of gene flow (by seeds) could have occurred much more frequently than that of the paternally inherited cpDNA with a higher rate of gene flow (by pollen) in Pinaceae.