Dendroecological Analysis Research Articles

A New Workflow for Sampling and Digitizing Increment Cores.

Here we present a new workflow from taking increment cores in the field, storing and transporting them to the lab, to digitizing their tree rings for further analyses for subsequent dendroecological analyses. The procedure involves the use of new sample carriers for increment cores. These new Gärtner Schneider Core (GSC) holders are designed using three-dimensional (3D) modeling software and finally printed with a 3D printer. Using these mounts from the beginning in the field, the cores can directly be cut with a core microtome, and their surface can then be digitized without further rearrangement using a new high-resolution image-capturing system. They are thus available for direct analysis. This system allows digitizing tree rings from cores and disks, and also taking images from long micro sections (up to 40 cm) using transmitted light. This feature is of special interest for dendroecological and geomorphic applications to identify the onset of any disturbance in micro sections cut with a core-microtome.

Identifying the impact of climate extremes on radial growth in young tropical trees: A comparison of inventory and tree-ring based estimates

The loss of tropical forest resilience has been linked to increased climate variability and associated droughts, but the response of tropical trees to climate extremes remains poorly understood. This limits our ability to design effective forest adaptation strategies in the tropics. Here we analyse the potential of using young trees to analyse climate variability and extremes, which opens new avenues given the increasing area of secondary forests and tree plantations. We used annual tree diameter measurements and stem discs from 139 16-year-old trees belonging to five native species planted in the Sardinilla tree diversity experiment in Panama and compared three methods to determine annual radial growth. Employing inventory measurements, visual stem disc analysis, and wood density measurements, series of radial growth were calculated to compare relative growth during wet and dry extreme events, and to compute continuous climate-growth correlations. Our results show that annual radial growth data derived from wood density profiles are best suited for climate-growth relationships, as they could capture a common growth signal within the high intraspecific variability of young trees to seasonal climatic variables. Annual radial growth data derived visually and from inventories are still useful for quantifying growth responses during extreme drought. The highest similarity among methods to determine annual radial growth, combined with the strongest climate-growth relationships, were found for Cedrela odorata, a species that shows a pronounced decrease in water use and cambial dormancy during the dry season. Stem discs from young trees planted in tropical forest plantations may thus offer a suitable source for dendroecological analyses.

Dendroecological analysis of the remote endemic Nothomyrcia fernandeziana forests of Robinson Crusoe Island in the Southeast Pacific

Juan Fernández Archipelago National Park is located in the Southeast Pacific Ocean ∼670 km off the coast of Chile. Composed by three islands, encompasses an ecosystem with 208 native plant species of which 64% are endemic. Robinson Crusoe Island (RCI) is the largest and the only with a permanent human population. Among the tree species of RCI, Nothomyrcia fernandeziana is the most abundant and dominant forming part of this fragile forest ecosystem. Despite this, there is very little information regarding its autoecology and conservation. Therefore, the objective of this work was to determine the radial growth patterns of N. fernandeziana and its longevity using dendrochronological methods. For this purpose, cross sections of trees blown down by wind were collected in conjunction with park rangers from five different sites and analyzed using tree-ring methods. Our results confirm that N. fernandeziana is a shade-tolerant species capable of maintaining extremely low initial growth rates under the canopy for several decades. Individual growth series were found to respond rapidly to changes in environmental conditions, revealed by abrupt positive and negative changes in their annual growth. Our data indicate a longevity of up to 450 years, with one N. fernandeziana individual established in the mid XVII century before the arrival of humans to the pristine island ecosystem. The results of this study are essential to strengthen RCI conservation plans considering restoration actions based on the autoecology of this rare endemic tree species, and the control of exotic invasive species that presently threatens the RCI forests ecosystems.

Rates of Stemwood Carbon Accumulation Are Linked to Hydroclimate Variability in Mexican Conifers

Evapotranspiration demand has increased rapidly as temperatures have risen, affecting forest productivity. Consequently, carbon (C) uptake by forests is being modified; therefore, a more refined knowledge of the relationships between C capture and hydroclimate variability is required, particularly in drought-prone regions. In this study, we analyzed the relationships between climate and C capture as stemwood through the dendroecological analyses of radial growth in 15 conifer species distributed along a broad bioclimatic gradient in Mexico. The C content was calculated using densitometry data; correlations and mixed models were then used to determine the influence of climatic variables (precipitation, mean maximum and minimum temperatures, SPEI drought index) on tree growth. Each species showed specific responses to the climate with Taxodium mucronatum being the most responsive species. Both precipitation and maximum temperature best explained changes in C capture, with minimum temperature and SPEI playing secondary roles. The winter before the growth period was the most important season for C capture, particularly through positive responses to wet-cool conditions. However, the climatic influences of the current fall and summer were also notable. Seasonal climatic influences have implications for C uptake and forest productivity in the face of the severe droughts that repeatedly affect the study region.

No evidence for cottonwood forest decline along a flow‐augmented western U.S. river

AbstractIn contrast to many other arid region rivers, streamflow in the South Platte River is heavily augmented by trans‐basin water imports and irrigation return flows. Hydrological changes began in the 1880s, resulting in channel narrowing and the development of a continuous Populus‐Salix forest by the mid‐twentieth century. We assessed the composition, structure and regeneration status of the riparian forest and identified environmental variables affecting annual Populus deltoides tree growth. We sampled forest structure at four sites in 2015, and conducted dendroecological analysis at seven additional sites in 2019. The riparian forest was dominated by P. deltoides, which occurred at all sites, comprising 79% of total tree basal area and 62% of total tree density. Age structure data indicated ongoing though episodic recruitment of P. deltoides, at least over the past ~130 years. We tested 14 linear mixed effects models to describe the effect of climate and streamflow on individual tree growth (modeled as the log of BAI, n = 237 trees). The most parsimonious model selected with AICc explained 28.6% of BAI variability, and included hydrology and climate factors during the growing season (i.e., June–August streamflow, June–July PDSI), some aspects of off‐season (i.e., previous November and March) streamflow, along with tree age and study site effects. The riparian forest developed in response to, and has been maintained by, current climate conditions and water management regimes. It may be negatively affected by future climate change and increased urban water demand in the basin.

Woody plant encroachment in granite barrens on the Frontenac Arch, eastern Ontario, Canada

AbstractAimsWoody plant encroachment is a widespread phenomenon affecting treeless or sparsely treed habitats. We aimed to determine the extent and timing of tree and shrub encroachment into rock barrens of eastern Ontario over the last century, and to assess implications for their ongoing management.LocationQueen's University Biological Station in the Frontenac Arch ecoregion.MethodsWe quantified the extent of change in woody vegetation in 290 rock barrens using aerial photography from 1925, 1965, and 2008. Composition and structure of woody plant communities in 10 barrens was subsequently quantified in the field using plot‐based sampling. Cores or cross‐sections were obtained from individuals >1.5 m height and dendrochronological techniques were used to determine their age and identify temporal patterns of any woody encroachment.ResultsAerial photography indicated that the mean proportion of woody plant cover in barrens increased 22.5% from 1925 to 2008. Dendroecological analysis supported this. Few trees were present prior to 1900 and most established since 1960.Fraxinus americana,Juniperus virginiana, andJuniperus communiswere the most common woody species colonizing the barrens. Remnants of largePinus strobusstumps with extensive charring were found in 90% of the sampled barrens at a mean density of 22.6 stumps ha−1.ConclusionsRock barrens on the Frontenac Arch have changed substantially over the past century; gradually being colonized by trees and shrubs and losing their distinctly open character. Active management — including prescribed fire and mechanical thinning — may be necessary if there is a desire to maintain these barrens and the rare species they support as components of the region's biodiversity. However, identification of a reference state for restoration is complicated by the fact that the structure and composition of these habitats were undoubtedly altered by European land clearance in the 19th century, and that some of these areas likely existed as pine woodlands before that.

Evidence for 40 Years of Treeline Shift in a Central Alpine Valley

Alpine treeline ecosystems are generally expected to advance with increasing temperatures and after land-use abandonment. Multiple interacting factors modify this trend. Understanding the long-term processes underlying treeline advance is essential to predict future changes in structure and function of mountain ecosystems. In a valley in the Central Swiss Alps, we re-assessed a 40-year-old survey of all treeline trees (>0.5 m height) and disentangled climate, topographical, biotic, and disturbance (land use and avalanche risk) factors that have led to treeline advance with a combination of ground-based mapping, decision tree, and dendroecological analyses. Between the first ground survey in 1972/73 and the resurvey in 2012, treeline advanced on average by 10 meters per decade with a maximum local advance of 42 meters per decade. Larch consistently advanced more on south-facing slopes, while pine advance was greater on north-facing slopes. Newly established spruce mostly represented infilling below the previous treeline. The forefront of treeline advance above 2330 m a.s.l. occurred mainly on favorable microsites without competing dwarf shrub vegetation. At slightly lower elevations, treeline advanced mainly on sites that were used for agriculture at the beginning of the 20th century. This study indicates that although treeline advances under the effect of climate warming, a combination of additional ecological factors controls this advance at regional and local scales.

Ecological Dynamics and Regeneration Expansion of Treeline Ecotones in Response to Climate Change in Northern Bhutan Himalayas

The alpine treeline ecotones are an early indicator of vegetation’s response to changes in climate, and the advancement of diffuse treeline ecotones has been associated with mean annual warming temperatures. However, the knowledge of how tree demographic size, age and population distribution, and regeneration decrease with increasing elevation and mean annual temperature remain fragmentary in Bhutan. There was no explanation of how treelines migrate in response to the climate. Therefore, the objectives of this study were to investigate tree demographic size and age and population distribution, as well as the regeneration expansion of treeline ecotones of Abies densa trees in response to climate change. Demographic data from thirty transect bands from treeline ecotones and reconstructed mean annual temperatures from tree-rings were used. Regression analysis was used to establish a relationship between elevation/temperature and demographic tree size and age, as well as to determine recruitment frequency distributions and whether these could be driven by climate change. The tree demography indicated that the treeline ecotone in our sampling site is temperature limited. Hence, cooler temperatures at higher elevations should drive decreases in basal diameter, age and recruitment frequencies. From the dendroecological analysis, the diffuse treeline ecotones appear to be climbing on average 1.00 m per year in Northern Bhutan. We also found that the recruitment frequency has increased over recent years (1850–2017), as temperatures continue to rise. The thermal treeline ecotones will be likely to serve as a line of bioclimatic reference against which other zones of bioclimate can be defined. With documented responses of treeline ecotones toward mean annual temperatures, the expectation is that additional warming will continue to influence regeneration expansion in the future. This dynamic response of treeline ecotones towards the climate acts as an indicator of climate change. Information about climbing treelines and altered ecotones should be a vital part of the material for decision makers to consider, to assess impacts and threats to Himalayan alpine biota.

The effects of fire on Pinus sylvestris L. as determined by dendroecological analysis (Sierra de Gredos, Spain)

Iberian populations of Scots pine (Pinus sylvestris L.) have been declining since the late-glacial period; among those that remain, relict stands have great biological and ecological value. This paper investigates the effects of a 2009 fire on tree growth in one of these small populations in the Sierra de Gredos (Spain) by examining the responses recorded in the tree-ring width series of the surviving trees. The current status and distribution of these surviving trees reveal the severity of the fire; indeed most show scars or other evidence of fire damage. Dendroecological analysis revealed narrower tree rings, indicating negative pointer years for the year of the fire and the following year. A very significant reduction in growth was recorded for the years after the fire, both in terms of tree-ring width and basal area increment; incomplete and even absent rings were also recorded. No relationship was seen between these effects and climatic events. The dates and geographical extension of former possible disturbances were also investigated, using the data from these same trees plus information collected from others in the region. The vulnerability of these populations to past fires was evident. Lastly, given the problems affecting the regeneration of these relict populations, it is strongly suggested to urgently include all these populations in conservation and environmental management programs.

“These Trees Have Stories to Tell”: Linking Dënesǫ́łıné Oral History of Caribou Use with Trample Scar Frequency on Black Spruce Roots at Ɂedacho Kué

For thousands of years Ɂedacho Kué (Artillery Lake, Northwest Territories) has been a key water crossing site for barren-ground caribou (Rangifer tarandus groenlandicus). Human disturbance of barren-ground caribou habitat in northern Canada has emerged as an important focus of study in the last decade; particularly in the Bathurst range of the Northwest Territories where caribou populations have declined by more than 95% since the 1980s. Guided by local Indigenous leaders and Elders, a collaborative research project was developed with the Dënesǫ́łıné people of Łutsël K’e Dëne First Nation (2012 – 14). This paper describes linkages between knowledge derived from Dënesǫ́łıné oral history and quantitative dendroecological analysis of trample scars on black spruce (Picea mariana) root samples collected at Ɂedacho Kué to provide a better understanding of caribou use at this location. Findings from oral histories and dendroecology analysis were consistent with one another and with previous dendroecology study in the region, although some discrepancies were detected in data from 1995 – 2006 that require further study to elucidate. Key findings include relatively low caribou use at Ɂedacho Kué during the 1930s and late 1960s, with use increasing into the 1970s and peaking in the late 1980s, as well as Elder and hunter reports of no caribou in some years between 2005 and 2012. This work addresses a gap in scientific data about barren-ground caribou movements at Ɂedacho Kué prior to satellite collar use in 1996 and corroborates previously documented oral histories about the enduring value of Ɂedacho Kué as critical habitat to barren-ground caribou. Given the drastic decline of the Bathurst caribou over the last two decades, more research is needed to understand movements and their relationship to population dynamics. In this context, the research approach described in this paper could be used as an example of how to meaningfully bring together place-based Indigenous knowledge and science in addressing an urgent issue of Arctic sustainability.

Between Pocket Forest Wilderness and Restored Rural Arcadia: Optimizing the Use of a Feral Woodland Enclave in Urban Environment

An eight-hectare forest reserve in the centre of the metropolitan area in SE Poland was investigated for the effects of its management in relation to its purpose—protection of old Quercus robur L. trees. This local issue corresponds to a wider debate on the role of urban forests in contemporary societies. The study embraced (1) oak stand history—stand mapping and dendrochronology, historical maps’ analysis; (2) vegetation trends—undergrowth and ground layer analysis; and (3) visitors’ opinions on the reserve’s use—interview. The dendroecological analysis corroborates the landscape’s history emerging from the 1700s–1900s maps. The reserve is a feral park established in the early 1900s on the abandoned agricultural, partly wooded landscape. Under the current regime, shade-tolerant trees continue gaining advantage over older oaks, preventing their regeneration, whilst the herb layer, due to the reserve’s isolation remains poor and polluted by alien species. The non-intervention approach does not allow the reserve’s objective to be met. We recommend the restoration of features of semi-open silvopastoral landscape. This would correspond with both local eco-history and the dominating preference of the interviewed visitors. Such “bio-cultural refugia” should become key knowledge centres of the natural, cultural, and economic importance of the “working rural landscape”, fostering urban care for the countryside.

Silver fir (Abies alba Mill.) is able to thrive and prosper under meso-Mediterranean conditions

The potential ecological envelope of silver fir (Abies alba Mill.) based on its present distribution suggests a high suitability for moist rather than warm and dry environments. This contrasts with paleoecological evidence reporting its former presence at low elevations under meso-Mediterranean conditions. In this study, we evaluated the growth performance of silver fir at low elevation (20–60 m a.s.l.) under meso-Mediterranean climatic conditions in Tuscany (central Italy). We conducted a dendroecological analysis on Abies alba trees along a geomorphological gradient (from depression to upper slope conditions). Climate-growth relationships were assessed by means of correlations, response functions, pointer years, and superposed epoch analysis.Silver fir was found to grow and regenerate well in these stands mixed with evergreen (e.g., Quercus ilex L.) and thermophilous deciduous Mediterranean tree species (e.g., Q. cerris L.). Summer drought was the most growth-influencing factor, with immediate (i.e., current season) negative impacts on tree-ring widths (TRW). No significant impacts were observed in the four years following extreme summer droughts, but the TRW series (which started between the 1930s and 1950s) showed a growth decline since the mid-1990s that is likely drought-related.Our results show that, provided there is a sufficiently large soil water holding capacity, silver fir provenances exist which are able to withstand Mediterranean summer droughts, can naturally and regularly regenerate in these systems, and may even dominate over typical meso-Mediterranean species. As long as annual precipitation is not too low (i.e., >850 mm) and summer drought conditions not too extreme (i.e., less than three months), silver fir has thus the potential to thrive under warm Mediterranean conditions.

How Does Radial Growth of Water-Stressed Populations of European Beech (Fagus sylvatica L.) Trees Vary under Multiple Drought Events?

European beech (Fagus sylvatica L.) trees are becoming vulnerable to drought, with a warming climate. Existing studies disagree on how radial growth varies in European beech in response to droughts. We aimed to find the impact of multiple droughts on beech trees’ annual radial growth at their ecological drought limit created by soil water availability in the forest. Besides, we quantified the influence of competition and canopy openness on the mean basal area growth of beech trees. We carried out this study in five near-natural temperate forests in three localities of Germany and Switzerland. We quantified available soil water storage capacity (AWC) in plots laid in the transition zone from oak to beech dominated forests. The plots were classified as ‘dry’ (AWC < 60 mL) and ‘less-dry’ (AWC > 60 mL). We performed dendroecological analyses starting from 1951 in continuous and discontinuous series to study the influence of climatic drought (i.e., precipitation-potential evapotranspiration) on the radial growth of beech trees in dry and less-dry plots. We used observed values for this analysis and did not use interpolated values from interpolated historical records in this study. We selected six drought events to study the resistance, recovery, and resilience of beech trees to drought at a discontinuous level. The radial growth was significantly higher in less-dry plots than dry plots. The increase in drought had reduced tree growth. Frequent climatic drought events resulted in more significant correlations, hence, increased the dependency of tree growth on AWC. We showed that the recovery and resilience to climatic drought were higher in trees in less-dry plots than dry plots, but it was the opposite for resistance. The resistance, recovery, and resilience of the trees were heterogeneous between the events of drought. Mean growth of beech trees (basal area increment) were negatively impacted by neighborhood competition and positively influenced by canopy openness. We emphasized that beech trees growing on soil with low AWC are at higher risk of growth decline. We concluded that changes in soil water conditions even at the microsite level could influence beech trees’ growth in their drought limit under the changing climate. Along with drought, neighborhood competition and lack of light can also reduce beech trees’ growth. This study will enrich the state of knowledge about the ongoing debate on the vulnerability of beech trees to drought in Europe.

Growth sensitivity to climate varies with soil moisture regime in spruce\u2013fir forests in central British Columbia

Key messageGrowth sensitivity to climate varies with soil moisture regime in spruce–fir forests in central British Columbia. Stands growing at their dry edaphic limits displayed especially strong and unique climatic sensitivities.Soil moisture regime is an important influence of productivity, process, and structure in forested ecosystems. In western North America, projected warming trends may result in decreasing available soil moisture; however, the potential effects on forest growth remain unclear. This study aimed to determine the influence of stand-level soil moisture regime on the climatic sensitivity of mature hybrid white spruce (Picea glauca (Moensch) Voss x Picea engelmannii Parry) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) forests in central British Columbia, Canada. We collected and analyzed tree-ring data from 51 stands spanning a range of soil moisture regimes. Dendroecological analyses of climate–growth relationships indicated that warm summer temperatures and drought limit growth for both species across all soil moisture regimes; however, responses were strongest on the driest sites. Spruce populations across the gradient of soil moisture regimes displayed unique climate-growth relationships; growth in populations on wetter sites was more correlated with summer climate from the year prior to growth. Radial growth responses to prior summer temperatures strengthened over the past ca. 80 years in both species and across most sites, suggesting that climate–growth relationships are shifting in this region. This study presents evidence of the importance of considering site-level ecological factors such as soil moisture regime when studying forest growth responses to climate.

Climatic and anthropogenic influence on tree-ring growth in riparian lake forest ecosystems under contrasting disturbance regimes

In the Mediterranean region, the coupled effect of recent changes in climatic features and an increase in human-related water demand induced a progressive reduction in the water level of many lakes. Consequently, intense withdrawals for public supply can affect the health of lake ecosystems and increase trees’ vulnerability to predicted drought intensification effects.This study aimed to evaluate, through a dendroecological analysis of riparian trees, the strength of the climatic and hydrological signals retained in the tree-ring widths and to detect a critical water-level threshold affecting tree resilience and growth recovery. Our study area is located within the Regional Natural Park of Bracciano-Martignano in central Italy. In recent years, the lake water-level drop observed at Bracciano after intensive water withdrawals raised concerns about the ecological impacts on the ecosystem's vulnerability.The main distinctive pattern in the climate-growth relations was the influence of previous years precipitation and drought conditions up to five years at Bracciano. At Martignano, where lake water withdrawals are absent, long-term climatic signals disappeared after two years. The intensive human water use at Bracciano Lake most likely induced trees to be more dependent on groundwater raising or soil water content stored from the previous rainy periods especially on an annual scale. On the other hand, tree-ring growth was strongly correlated with short-term water-level fluctuations (monthly – seasonal).Tree-rings of riparian species appear to be a useful tool to detect critical water levels after the intensive water withdrawals and the increase in dry conditions occurred in the past few decades. Yearly values ranging from -108 to -139 cm may be considered as potential thresholds affecting riparian species. This finding may support lake water withdrawal regulation for riparian ecosystem protection and conservation under future drought intensification.

Climate warming differently affects Larix decidua ring formation at each end of a French Alps elevational gradient

The ongoing global warming tends to reduce Larix decidua radial growth and ring wood density at the bottom of an elevational gradient in the French Alps, while it has a less marked effect at the top. The ongoing climate warming affects most tree species across their biogeographic distribution range. The bottom and the top of an elevational gradient are appropriate environments to observe the effect of climate warming in contrasted temperature conditions. To retrospectively study the effect of the ongoing climate warming on Larix decidua trees located in warm and cold conditions, i.e., at the bottom (1200 m) and the top (2300 m) of an elevational gradient, respectively. Dendroecological analysis of two groups of Larix decidua trees located at very low (1200 m) and very high (2300 m) elevations. Construction of climatic response curves for annual ring variables. Decomposition of the variance of the response models into their high and low frequency components. At Briancon, in the French Alps, the increase of the daily maximum temperature during the last 50 years is already almost 3 °C for the March–October period. The results reveal contrasted behaviors at both ends of the investigated elevational gradient. Basal area increment, ring width, and their earlywood and latewood components increase or level off at the top of the gradient, while they all strongly decrease at the bottom. At the bottom, the low frequency warming effect explains the recent ring width decrease, with no influence of the high frequency temperature variation. At the top, both the low frequency warming effect and the high frequency temperature effect explain each about half of the ring width variation encompassed by the model. Latewood density displays the same trend as ring width, while earlywood density decreases at both ends of the gradient. Such opposed trends of the radial growth time trends between the top (2300 m) and the bottom (1200 m) of the gradient reflects the respectively favorable and unfavorable effects of climate warming at such ends of this Larix decidua elevational gradient. We propose that the strong ring width decrease observed at the warmer bottom announces a coming dieback. The corresponding wood density decrease will affect wood hydraulic properties in a way that is unclear.

Effects of extreme drought and wet events for tree mortality: Insights from tree‐ring width and carbon isotope ratio in a Siberian larch forest

AbstractRecent studies suggest that forest vulnerability to tree mortality has increased as a result of extreme climate events such as severe drought and heavy rain. However, little is known about the mortality processes, particularly when they are induced by heavy rain. A dendroecological analysis was conducted on living and dead trees in a Siberian larch forest using the chronologies of 30 years of radial growth and carbon isotope (Δ13C) discrimination. The analysis demonstrates the relationship between heavy rain and larch tree mortality. Radial growth and Δ13C patterns in dead trees at a wet subsite diverged from those of living trees only during the heavy rain period in 2005–2007. On the other hand, the radial growth and Δ13C patterns in dead trees at the dry subsite diverged from those of living trees after severe drought events in 1998 and in 2002–2003. At the wet subsite, the heavy rain was intense enough to push the larch trees past their mortality threshold, whereas previous drought conditions also contributed to tree mortality at the dry subsite. Our results demonstrated that the process of larch tree mortality differs with site‐specific topographical soil moisture conditions, even when synchronous tree death occurs in a specific year of climatic extremes.

Dendroecological Analysis of Xeric, Upland, Quercus-Dominated Old-Growth Forest within the Ridge and Valley Province of Maryland, USA

Mesophytic species invasion and the loss of oaks from previously oak-dominated forest communities are two well-documented changes currently affecting the deciduous forests of the Appalachian Mountains. Several previous assessments of these changes prescribe active management to maintain oak. Currently, historically white oak (Quercus alba) dominated mesic stands documented in both the Allegheny Plateau and Ridge and Valley regions show increasing dominance of mesophytic species such as red maple (Acer rubrum) and black birch (Betula lenta). We describe a unique near-xeric, 72-ha tract of old-growth forest within the Ridge and Valley Region. This upland oak-pine, formerly oak-chestnut community, has maintained oak dominance without silvicultural management, despite being greatly impacted from the loss of American chestnut (Castanea dentata), fire suppression, high deer densities, and European gypsy moth (Lymantria dispar) defoliation. A dendroecological analysis of these 200-year-old stands shows persistence of oak through centuries of documented disturbances. These unmanaged old-growth stands have resisted mesophytic species invasion and therefore may provide insight into future compositional patterns and changes of similar upland and ridgeline forests within the Ridge and Valley. The combination of low site quality of ridgelines and ecological disturbances such as defoliation events, which may act as a moderate basal area reduction, allows oaks to persist and maintain dominance without silvicultural intervention. However, fire is recommended for the maintenance of co-dominant fire-dependent species such as Table Mountain pine (Pinus pungens) and herbaceous diversity along these ridge forests.

Functional assessment of invasive Salix fragilis L. in north-western Patagonian flood plains: A comparative approach

European willows (Salicaceae) are pioneer species in temperate zone floodplains. The species are considered invasive and introduction can lead to substantial alteration of floodplain vegetation communities and ecosystem functioning. Invasive spread of different Salicaceae have been attributed to differences in flood tolerance, growth and dispersal between it and co-occurring native species, suggesting a comparative approach examining key traits to be a profitable strategy in elucidating and predicting patterns of Salicaceae invasion. Here we study incipient Salix fragilis invasions in Chilean Patagonia on the western slopes of the Andes. Our approach combines vegetation transect data with dendroecological analysis to compare patterns of niche position along the hydrogeomorphic gradient, niche breadth, and growth rate among S. fragilis and co-occurring species. We evaluated these traits because they are thought to be key to species’ capacity to colonize, survive, grow and reproduce in dynamic floodplain habitats. Transect data showed that relative to the most common native trees and shrubs, S. fragilis has the broadest environmental tolerance along the hydrogeomorphic gradient and has a mean niche position closest to severely disturbed and highly flooded channel margins. Moreover, growth models showed that S. fragilis grows faster relative to native species, especially at juvenile stages. Our data suggest that a unique set of trait values allows S. fragilis to occupy a previously under-occupied and abundantly distributed niche, and will continue to colonize, survive, grow and reproduce in morphodynamically disturbed river margins. This is the first report of invasive S. fragilis in Chile, and we comment on potential impacts to native vegetation communities, soil development and ecosystem function.

Tree-ring analysis of larch sawfly (Pristiphora erichsonii (Hartig)) defoliation events and hydrological growth suppression in a peatland

Insect defoliation events are a major forest disturbance in the boreal forest in Canada. Reconstructing previous events are crucial to understanding natural factors that lead to insect defoliation periods, improving our ability to predict future infestations and increasing the reliability of forest management plans and pest control programs. Researchers have often been limited in their ability to draw accurate conclusions regarding the history of larch sawfly (Pristiphora erichsonii (Hartig)) infestation events in North America. It is well known that floods can affect survival of larch sawfly populations, as well as suppress radial growth of eastern larch (Larix laricina (Du Roi) K. Koch) trees. Eastern larch often inhabits peatlands where high water-table levels can lead to a decrease in tree-ring widths. Water-table level increases result in similar radial-growth patterns to when trees are defoliated by larch sawfly, making accurate diagnoses of larch sawfly events a challenge. This fact becomes more accentuated when non-host species used for standard dendroecological analyses (often black spruce (Picea mariana (Mill.) Britton) present an opposite relationship to water-table variability, enhancing chances of drawing erroneous conclusions when using program OUTBREAK.This paper introduces a novel case study to advance the understanding of the complex landscapes where these larch sawfly events are taking place. It stresses the necessity of using upland sites less influenced by the hydrological regime to accurately reconstruct larch sawfly defoliation events. We found that where investigations are taking place solely in peatlands or lowlands, and where they are heavily influenced by hydrological conditions, the use of other lines of evidence such as pale rings and long-term hydrological records are crucial to understand the dynamics in the system. In our case study, program OUTBREAK identified five defoliation events in a peatland and upland site; however, hydrological data and pale ring evidence indicated that the last event identified in the peatland site by the host/non-host analysis was confounded by hydrological growth suppression.