Age-dependency,climate, and environmental controls of recent tree growth trends at subarctic and alpine treelines

Spatial and temporal variability in growth and climate response of trees at and near treeline was investigated in the western Mackenzie Mountains, Northwest Territories, and the Hudson Bay Lowlands of northern Manitoba. Residual ring width chronologies were constructed using cores extracted from 108 trees in the mountains and 170 from the lowlands, and compared to historical climate data. Growth of most trees exhibited significant correlations with summer and autumn temperatures, and the growth–climate relationship did not differ noticeably between trees at and distal to treeline. Most mountain trees had significant positive growth trends from 1851 to 2006 that corresponded with warming over the same period, while growth trends varied among sites and species in the lowlands. Regionally, growth of all species responded positively to warming during the 20th century with the exception of lowland Picea mariana, which exhibited little response. Growth response for most trees was age-dependent, with trees established after 1920 demonstrating improved growth and sensitivity to temperature than older individuals, and growth of most species since the 1990s was greater than any time during the last 250 years, particularly for lowland Larix laricina. This study suggests that site factors and tree age can be more important drivers of local-scale growth trends than regional climate at arctic treelines where temperature is often assumed to be the main constraint on tree growth.     

Reproduction and seedling establishment of Picea glauca across the northernmost forest‐tundra region in Canada

The northern boundary of boreal forest and the ranges of tree species are expected to shift northward in response to climate warming, which will result in a decrease in the albedo of areas currently covered by tundra vegetation, an increase in terrestrial carbon sequestration, and an alteration of biodiversity in the current Low Arctic. Central to the prediction of forest expansion is an increase in the reproductive capacity and establishment of individual trees. We assessed cone production, seed viability, and transplanted seedling success of Picea glauca (Moench.) Voss. (white spruce) in the early 1990s and again in the late 2000s at four forest stand sites and eight tree island sites (clonal populations beyond present treeline) in the Mackenzie Delta region of the Northwest Territories, Canada. Over the past 20 years, average temperatures in this region have increased by 0.9 °C. This area has the northernmost forest‐tundra ecotone in North America and is one of the few circumpolar regions where the northern limit of conifer trees reaches the Arctic Ocean. We found that cone production and seed viability did not change between the two periods of examination and that both variables decreased northward across the forest‐tundra ecotone. Nevertheless, white spruce individuals at the northern limit of the forest‐tundra ecotone produced viable seeds. Furthermore, transplanted seedlings were able to survive in the northernmost sites for 15 years, but there were no signs of natural regeneration. These results indicate that if climatic conditions continue to ameliorate, reproductive output will likely increase, but seedling establishment and forest expansion within the forest‐tundra of this region is unlikely to occur without the availability of suitable recruitment sites. Processes that affect the availability of recruitment sites are likely to be important elsewhere in the circumpolar ecotone, and should be incorporated into models and predictions of climate change and its effects on the northern forest‐tundra ecotone.     

Recent advance of white spruce (Picea glauca) in the coastal tundra of the eastern shore of Hudson Bay (Québec, Canada)

Aim The species‐specific response of tree‐line species to climatic forcing is a crucial topic in modelling climate‐driven ecosystem dynamics. In northern Québec, Canada, black spruce (Picea mariana) is the dominant species at the tree line, but white spruce (Picea glauca) also occurs along the maritime coast of Hudson Bay, and is expanding along the coast and on lands that have recently emerged because of isostatic uplift. Here we outline the present distribution, structure, dynamics and recent spread of white spruce from the tree line up to its northernmost position in the shrub tundra along the Hudson Bay coast. We aimed to obtain a minimum date of the arrival of the species in the area and to evaluate its dynamics relative to recent climate changes. Location White spruce populations and individuals were sampled along a latitudinal transect from the tree line to the northernmost individual in the shrub tundra along the Hudson Bay coast and in the Nastapoka archipelago in northern Québec and Nunavut, Canada (56°06′–56°32′ N). Methods White spruce populations were mapped, and the position, dimension, growth form and origin (seed or layering) of every individual recorded. Tree‐ring analyses of living and dead trees allowed an estimation of the population structure, past recruitment, growth trends and growth rate of the species. A macrofossil analysis was performed of the organic horizon of the northernmost white spruce stands and individuals. Radiocarbon dates of white spruce remains and organic matter were obtained. The rate of isostatic uplift was assessed by radiocarbon dating of drifted wood fragments. Results The first recorded establishment of white spruce was almost synchronous at all sites and occurred around ad 1660. Spruce recruitment was rather continuous at the tree line, while it showed a gap in the northern shrub tundra during the first decades of the 19th century. A vigorous, recent establishment of seedlings was observed in the shrub tundra; only wind‐exposed, low krummholz (stunted individuals) did not show any sexual regeneration. A period of suppressed growth occurred from the 1810s to the 1850s in most sites. A growth increase was evident from the second half of the 19th century and peaked in the 1880s and the 20th century. A shift from stunted to tree growth form has occurred since the mid‐19th century. No sample associated with white spruce remains gave a date older than 300 ¹⁴C years bp [calibrated age (cal.) ad 1430–1690]. Main conclusions White spruce probably arrived recently in the coastal tundra of Hudson Bay due to a delayed post‐glacial spread. The arrival of the species probably occurred during the Little Ice Age. The established individuals survived by layering during unfavourable periods, but acted as nuclei for sexual recruitment almost continuously, except in the northernmost and most exposed sites. Warmer periods were marked by strong seedling recruitment and a shift to tree growth form. Unlike white spruce, black spruce showed no evidence of an ongoing change in growth form and sexual recruitment. Ecological requirements and recent history of tree‐line species should be taken into account in order to understand the present dynamics of high‐latitude ecosystems.     

Persistence and expansion of ponderosa pine woodlands in the west‐central Great Plains during the past two centuries

Aim Woody plant expansion and infilling in grasslands and savannas are occurring across a broad range of ecosystems around the globe and are commonly attributed to fire suppression, livestock grazing, nutrient enrichment and/or climate variability. In the western Great Plains, ponderosa pine (Pinus ponderosa) woodlands are expanding across broad geographical and environmental gradients. The objective of this study was to reconstruct the establishment of ponderosa pine in woodlands in the west‐central Great Plains and to identify whether it was mediated by climate variability. Location Our study took place in a 400‐km wide region from the base of the Front Range Mountains (c. 105° W) to the central Great Plains (c. 100° W) and from Nebraska (43° N) to northern New Mexico (36° N), USA. Methods Dates for establishment of ponderosa pine were reconstructed with tree rings in 11 woodland sites distributed across the longitudinal and latitudinal gradients of the study area. Temporal trends in decadal pine establishment were compared with summer Palmer Drought Severity Index (PDSI). Annual trends in pine establishment from 1985 to 2005 were compared with seasonal PDSI, temperature and moisture availability. Results Establishment of ponderosa pine occurred in the study area in all but one decade (1770s) between the 1750s and the early 2000s, with over 35% of establishment in the region occurring after 1980. Pine establishment was highly variable among sites. Across the region, decadal pine establishment was persistently low from 1940 to 1960, when PDSI was below average. Annual pine establishment from 1985 to 2005 was positively correlated with summer PDSI and inversely correlated with minimum spring temperatures. Main conclusions Most ponderosa pine woodlands pre‐date widespread Euro‐American settlement of the region around c. ad 1860 and currently have stable tree populations. High variability in the timing of establishment of pine among sites highlights the multiplicity of factors that can drive woodland dynamics, including land use, fire history, CO₂ enrichment, tree population dynamics and climate. Since the 1840s, the influence of climate was most notable across the study area during the mid‐20th century, when the establishment of pine was suppressed by two significant droughts. The past sensitivity of establishment of ponderosa pine to drought suggests that woodland expansion will be negatively affected by predicted increases in temperature and drought in the Great Plains.     

Treeline dynamics in relation to climatic variability in the central Tianshan Mountains, northwestern China

Aim Climate variability may be an important mediating agent of ecosystem dynamics in cold, arid regions such as the central Tianshan Mountains, north‐western China. Tree‐ring chronologies and the age structure of a Schrenk spruce (Picea schrenkiana) forest were developed to examine treeline dynamics in recent decades in relation to climatic variability. Of particular interest was whether tree‐ring growth and population recruitment patterns responded similarly to climate warming. Location The study was conducted in eight stands that ranged from 2500 m to 2750 m a.s.l. near the treeline in the Tianchi Nature Reserve (43°45′?43°59′ N, 88°00′?88°20′ E) in the central Xinjiang Uygur Autonomous Region, northwestern China. Methods Tree‐ring cores were collected and used to develop tree‐ring chronologies. The age of sampled trees was determined from basal cores sampled as close as possible to the ground. Population age structure and recruitment information were obtained using an age–d.b.h. (diameter at breast height) regression from the sampled cores and the d.b.h. measured on all trees in the plots. Ring‐width chronologies and tree age structure were both used to investigate the relationship between treeline dynamics and climate change. Results Comparisons with the climatic records showed that both the radial growth of trees and tree recruitment were influenced positively by temperature and precipitation in the cold high treeline zone, but the patterns of their responses differed. The annual variation in tree rings could be explained largely by the average monthly minimum temperatures during February and August of the current year and by the monthly precipitation of the previous August and January, which had a significant and positive effect on tree radial growth. P. schrenkiana recruitment was influenced mainly by consecutive years of high minimum summer temperatures and high precipitation during spring. Over the last several decades, the treeline did not show an obvious upward shift and new recruitment was rare. Some trees had established at the treeline at least 200 years ago. Recruitment increased until the early 20th century (1910s) but then decreased with poor recruitment over the past several decades (1950–2000). Main conclusions There were strong associations between climatic change and ring‐width patterns, and with recruitments in Schrenk spruce. Average minimum temperatures in February and August, and total precipitation in the previous August and January, had a positive effect on tree‐ring width, and several consecutive years of high minimum summer temperature and spring precipitation was a main factor favouring the establishment of P. schrenkiana following germination within the treeline ecotone. Both dendroclimatology and recruitment analysis were useful and compatible to understand and reconstruct treeline dynamics in the central Tianshan Mountains.     

Influences of 20th‐century warming at the upper tree line contingent on local‐scale interactions: evidence from a latitudinal gradient in the Rocky Mountains,USA

Aims Twentieth‐century climate, the spatial pattern of tree establishment and positive feedback influence upper tree line ecotones. Here, I investigate how these factors interact to gain a more holistic understanding of how broad‐scale abiotic and local‐scale site conditions regulate tree establishment within upper tree line ecotones. Location A latitudinal gradient (c. 35–45° N) in the US Rocky Mountains. Study sites (n= 22) were located in the Bighorn (BH), Medicine Bow (MB), Front Range (FR) and Sangre de Cristo (SDC) mountain ranges. Methods Dendroecological techniques were used to reconstruct tree establishment dates that were compared with 20th‐century climate data using correlation and regime shift analyses. Spatial patterns of tree establishment were analysed by Ripley's K and used to determine local‐scale interactions capable of ameliorating broad‐scale climate inputs through positive feedback. Results Significant correlations (P < 0.01) between tree establishment and climate were confined to the FR, where a positive correlation was found with summer (June–August) and cool season (November–April) temperature range (T_max?T_min). These trees were almost exclusively situated in a random spatial pattern. Similar patterns exist in the BH, yet their establishment was contingent on the availability of local shelter in the lee of boulders. Trees in the MB and SDC were primarily clustered in space and had no significant correlations with climate. Considerable lag times exist between regime shift changes in climate towards more favourable growing conditions and corresponding shifts in tree establishment in all mountain ranges except the FR, where synchronous shifts occurred in the early 1950s. Main conclusions These results suggest that the influence of broad‐scale climate on upper tree line dynamics is contingent on the local‐scale spatial patterns of tree establishment and related influences of positive feedback. This research has important implications for understanding how vegetation communities will respond to global climate change.     

Primary succession of subarctic vegetation and soil on the fast‐rising coast of eastern Hudson Bay,Canada

Aim The objectives of the study are: (1) to evaluate the dynamics of the maritime tree line and forest limit of white spruce, Picea glauca, within the dual framework of primary succession induced by the rapid post‐glacial land emergence on the eastern coast of Hudson Bay and the impacts of recent and past climate changes; and (2) to determine the time lapse between land emergence and seedling, tree, and forest establishment in the context of the primary chronosequence occurring on rising, well‐drained sandy beaches and terraces. Location The study area was located on the eastern coast of Hudson Bay (56°20′ N, 76°32′ W) in northern Québec, Canada. Methods We evaluated the colonization dynamics of white spruce as seedlings, tree‐line trees and primary‐forest trees at eight sites distributed along a 200‐km latitudinal gradient based on a mean land emergence rate of 1.2 m century^?1. A 30‐m wide by 140–300‐m long quadrat was positioned at random at the centre of each site. The elevation above sea level, position and age of all individuals of spruce present in the quadrat areas were determined, and the soils of each chronosequence were described. Results The main stages of primary succession along the emerging coast were common to all the sites, regardless of latitude, but occurred at different elevations above sea level (a.s.l.). White spruce seedlings colonized near‐shore beaches 2 m a.s.l., whereas the tree line and forest limit tended to form only at about 3–4 m and 4–8 m a.s.l., corresponding approximately to 180–825 years and 310–1615 years after land emersion, respectively. White spruce establishment at the tree line occurred about 50 years ago. Climatic conditions at this time were probably more favourable to tree colonization than when the species established at the forest limit. Soil formation was influenced primarily by distance from the seashore and elevation above sea level, with podzolization being accelerated by white spruce cover. Main conclusions The current tree‐line and forest‐limit positions on the rising coast of eastern Hudson Bay correspond to ecological limits established during the course of primary succession within a context of changing climatic conditions. The recent establishment of trees at the tree line and forest limit at relatively old coastal sites is associated with warmer conditions over the last 100 years. Although white spruce was present nearby, coastal sites were devoid of trees before the 20th century.     

A century of tree line changes in sub‐Arctic Sweden shows local and regional variability and only a minor influence of 20th century climate warming

Aim Models project that climate warming will cause the tree line to move to higher elevations in alpine areas and more northerly latitudes in Arctic environments. We aimed to document changes or stability of the tree line in a sub‐Arctic model area at different temporal and spatial scales, and particularly to clarify the ambiguity that currently exists about tree line dynamics and their causes. Location The study was conducted in the Torneträsk area in northern Sweden where climate warmed by 2.5 °C between 1913 and 2006. Mountain birch (Betula pubescens ssp. czerepanovii) sets the alpine tree line. Methods We used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics. Results Since 1912, only four out of eight tree line sites had advanced: on average the tree line had shifted 24 m upslope (+0.2 m year^?1 assuming linear shifts). Maximum tree line advance was +145 m (+1.5 m year^?1 in elevation and +2.7 m year^?1 in actual distance), whereas maximum retreat was 120 m downslope. Counter‐intuitively, tree line advance was most pronounced during the cooler late 1960s and 1970s. Tree establishment and tree line advance were significantly correlated with periods of low reindeer (Rangifer tarandus) population numbers. A decreased anthropozoogenic impact since the early 20th century was found to be the main factor shaping the current tree line ecotone and its dynamics. In addition, episodic disturbances by moth outbreaks and geomorphological processes resulted in descent and long‐term stability of the tree line position, respectively. Main conclusions In contrast to what is generally stated in the literature, this study shows that in a period of climate warming, disturbance may not only determine when tree line advance will occur but if tree line advance will occur at all. In the case of non‐climatic climax tree lines, such as those in our study area, both climate‐driven model projections of future tree line positions and the use of the tree line position for bioclimatic monitoring should be used with caution.     

Quaking aspen (Populus tremuloides Michx.) at treeline: a century of change in the San Juan Mountains, Colorado, USA

During a repeat photography study quaking aspen (Populus tremuloides Michx.) was observed invading conifer stands at treeline in the San Juan Mountains of south‐western Colorado. Aspen tree core samples were collected from nine plots ranging in elevation from 3192 to 3547 m, and estimated dates of establishment of aspen were grouped into 10‐year intervals for analysis. Estimated periods of establishment were compared with century‐long climate data records to derive any correlations with aspen invasion. Other disturbance agents, such as fire and livestock grazing were also considered. Quantitative analysis of climate variables suggests that decreased mean spring precipitation and increased mean summer maximum temperature provide optimal conditions for aspen establishment. Episodes of invasion were non‐synchronous, but all occurred after 1900, and are likely from seed germination, considered unusual in aspen. Different climate variables explain stand initiation from seed and subsequent peak establishment from vegetative reproduction. Long‐term climate records indicate a general warming since the beginning of the 20th century and explain the continued invasion and persistence of aspen at treeline, resulting from asexual reproduction. Short‐term climate records identify anomalously cool, moist years that explain rarely observed sexual reproduction in aspen.     

Microsite variation and climate result in stand dynamics variation in pristine Pinus sylvestris mires: Evidence from a stand structure study

Questions: How do climate conditions and the site's ecohy‐ drological properties affect the age and size structure of natural Pinus sylvestris stands on pristine boreal mires? How do the long‐term stand dynamics on mires proceed as stands age? Do the mire stands reach a balanced, old‐growth stage? Location: Boreal mire forests in southern and northern Finland. Methods: Tree age and diameter distributions were analysed in 52 stands in two climate areas and in two mire site types with different ecohydrological properties. Temporal stand dynamics were examined by (1) comparing the graphs of the stands’ mean tree ages by diameter at breast height (1.3 m) classes and (2) describing the changes in stand characteristics and stand age and size structures as a function of stand dominant age in a chronosequence. Results: In the south, the DBH distributions were mostly unimodal and bell‐shaped in both site type groups. Age distributions were multimodal and flat in fully‐stocked sites but more uneven in sparsely forested composite sites. In the north, both the age and size distributions were clearly uneven in both site type groups. Tree age and size variation increased with stand age, but levelled out in the long term. Particularly in the south, the abundance of small trees decreased as stand age increased. Conclusions: The pine stands on pristine boreal mires are more dynamic than anticipated and are generally not characterised by a balanced, self‐perpetuating structure. Their dynamics reflect differences in climate and ecohydrology: on stocked sites in favourable boreal conditions, the stands showed structures typically resultant of inter‐tree competition processes that control tree growth and regeneration, whereas in harsh boreal climates, the tree regeneration process is ongoing diversifying the stand structure.     

Tree rings provide no evidence of a CO2 fertilization effect in old‐growth subalpine forests of western Canada

Atmospheric CO₂ concentrations are now 1.7 times higher than the preindustrial values. Although photosynthetic rates are hypothesized to increase in response to rising atmospheric CO₂ concentrations, results from in situ experiments are inconsistent in supporting a CO₂ fertilization effect of tree growth. Tree‐ring data provide a historical record of tree‐level productivity that can be used to evaluate long‐term responses of tree growth. We use tree‐ring data from old‐growth, subalpine forests of western Canada that have not had a stand‐replacing disturbance for hundreds of years to determine if growth has increased over 19th and 20th centuries. Our sample consisted of 5,858 trees belonging to five species distributed over two sites in the coastal zone and two in the continental climate of the interior. We calculated annual increments in tree basal area, adjusted these increments for tree size and age, and tested whether there was a detectable temporal trend in tree growth over the 19th and 20th centuries. We found a similar pattern in 20th century growth trends among all species at all sites. Growth during the 19th century was mostly stable or increasing, with the exception of one of the coastal sites, where tree growth was slightly decreasing; whereas growth during the 20th century consistently decreased. The unexpected decrease in growth during the 20th century indicates that there was no CO₂ fertilization effect on photosynthesis. We compared the growth trends from our four sites to the trends simulated by seven Earth System Models, and saw that most of the models did not predict these growth declines. Overall, our results indicate that these old‐growth forests are unlikely to increase their carbon storage capacity in response to rising atmospheric CO₂, and thus are unlikely to contribute substantially to offsetting future carbon emissions.     

Long‐term variation in Amazon forest dynamics

Questions: Have forest dynamics changed significantly in intact Amazon rainforests since the early 1980s? If so, what environmental drivers might potentially be responsible? Location: Central Amazonia, north of Manaus, Brazil. Methods: Within 20 1‐ha plots scattered over ～300 km², all trees (≥10 cm diameter at breast height) were marked, identified, and measured five times between 1981 and 2003. We estimated stand‐level dynamics (mortality, recruitment, and growth) for each census interval and evaluated weather parameters over the study period. Results: We observed a widespread, significant increase in tree mortality across our plots. Tree recruitment also rose significantly over time but lagged behind mortality. Tree growth generally accelerated but varied considerably among census intervals, and was lowest when mortality was highest. Tree basal area rose 4% overall, but stem number exhibited no clear trend. In terms of climate variation, annual maximum and minimum temperatures increased significantly during our study. Rainfall anomalies were strongly and positively associated with ENSO events. Conclusions: The increasing forest dynamics, growth, and basal area observed are broadly consistent with the CO₂ fertilization hypothesis. However, pronounced shorter‐term variability in stand dynamics might be associated with climatic vicissitudes. Tree mortality peaked, and tree recruitment and growth declined during atypically wet periods. Tree growth was fastest during dry periods, when reduced cloudiness might have increased available solar radiation. Inferences about causality are tenuous because tree data were collected only at multi‐year intervals. Mean temperatures and rainfall seasonality have both increased over time in central Amazonia, and these could potentially have long‐term effects on forest dynamics and carbon storage.     

Large‐scale disturbance legacies and the climate sensitivity of primary Picea abies forests

Determining the drivers of shifting forest disturbance rates remains a pressing global change issue. Large‐scale forest dynamics are commonly assumed to be climate driven, but appropriately scaled disturbance histories are rarely available to assess how disturbance legacies alter subsequent disturbance rates and the climate sensitivity of disturbance. We compiled multiple tree ring‐based disturbance histories from primary Picea abies forest fragments distributed throughout five European landscapes spanning the Bohemian Forest and the Carpathian Mountains. The regional chronology includes 11,595 tree cores, with ring dates spanning the years 1750–2000, collected from 560 inventory plots in 37 stands distributed across a 1,000 km geographic gradient, amounting to the largest disturbance chronology yet constructed in Europe. Decadal disturbance rates varied significantly through time and declined after 1920, resulting in widespread increases in canopy tree age. Approximately 75% of current canopy area recruited prior to 1900. Long‐term disturbance patterns were compared to an historical drought reconstruction, and further linked to spatial variation in stand structure and contemporary disturbance patterns derived from LANDSAT imagery. Historically, decadal Palmer drought severity index minima corresponded to higher rates of canopy removal. The severity of contemporary disturbances increased with each stand's estimated time since last major disturbance, increased with mean diameter, and declined with increasing within‐stand structural variability. Reconstructed spatial patterns suggest that high small‐scale structural variability has historically acted to reduce large‐scale susceptibility and climate sensitivity of disturbance. Reduced disturbance rates since 1920, a potential legacy of high 19th century disturbance rates, have contributed to a recent region‐wide increase in disturbance susceptibility. Increasingly common high‐severity disturbances throughout primary Picea forests of Central Europe should be reinterpreted in light of both legacy effects (resulting in increased susceptibility) and climate change (resulting in increased exposure to extreme events).     

Regeneration dynamics of subalpine fir (Abies fargesii) forest across the altitudinal range in the Shennongjia Mountains,central China

Aims Age structure and regeneration dynamics have been used to infer population response to environmental events and reconstruct forest development history. The aim of this study was to characterize and examine the differences of the age structure and regeneration dynamics of subalpine fir (Abies fargesii) forest across the altitudinal range in the north and south aspects in the Shennongjia Mountains, central China.Methods Ten plots (20×20 m) at each altitudinal zone (i.e. the low elevation, the middle elevation and the high elevation) were established in both the north and south aspects of the Shennongjia Mountains, central China. Dendroecological techniques were applied to obtain information about ages of the trees in the plots. The population age structure was analyzed to investigate the regeneration dynamics across the altitudinal range.Important findings Fir regeneration dynamics and age structure were similar in both aspects, and a unimodal population age structure was found at different altitudinal sites of both aspects, indicating that environmental factors might play an important role in shaping the regeneration dynamics and age structure of A. fargesii across its altitudinal range. There was a sustained recruitment during the 19th century, but the regeneration was rarer in the last century at low and midelevations. A significant greater number of fir seedlings and saplings recruited at high elevations in the last century, and fir tree density at high elevations was significantly higher than that at low elevations. Thus, the fir population at the high elevations showed a significant increase in recruitment and stem density in the last century, and we propose that the gradual infilling of fir seedlings might result in changes in regeneration dynamics and stand structure of the subalpine fir forest at high elevations in the Shennongjia Mountains, central China.     

High and dry: post‐fire tree seedling establishment in subalpine forests decreases with post‐fire drought and large stand‐replacing burn patches

Aim Climate warming and increased wildfire activity are hypothesized to catalyse biogeographical shifts, reducing the resilience of fire‐prone forests world‐wide. Two key mechanisms underpinning hypotheses are: (1) reduced seed availability in large stand‐replacing burn patches, and (2) reduced seedling establishment/survival after post‐fire drought. We tested for regional evidence consistent with these mechanisms in an extensive fire‐prone forest biome by assessing post‐fire tree seedling establishment, a key indicator of forest resilience. Location Subalpine forests, US Rocky Mountains. Methods We analysed post‐fire tree seedling establishment from 184 field plots where stand‐replacing forest fires were followed by varying post‐fire climate conditions. Generalized linear mixed models tested how establishment rates varied with post‐fire drought severity and distance to seed source (among other relevant factors) for tree species with contrasting post‐fire regeneration adaptations. Results Total post‐fire tree seedling establishment (all species combined) declined sharply with greater post‐fire drought severity and with greater distance to seed sources (i.e. the interior of burn patches). Effects varied among key species groups. For conifers that dominate present‐day subalpine forests (Picea engelmannii, Abies lasiocarpa), post‐fire seedling establishment declined sharply with both factors. One exception was serotinous Pinus contorta, which did not vary with either factor. For montane species expected to move upslope under future climate change (Larix occidentalis, Pseudotsuga menziesii, Populus tremuloides) and upper treeline species (Pinus albicaulis), establishment was unrelated to either factor. Greater post‐fire tree seedling establishment on cooler/wetter aspects suggested local topographic refugia during post‐fire droughts. Main conclusions If future drought and wildfire patterns manifest as expected, post‐fire tree seedling establishment of species that currently characterize subalpine forests could be substantially reduced. Compensatory increases from lower montane and upper treeline species may partially offset these reductions, but our data suggest important near‐ to mid‐term shifts in the composition and structure of high‐elevation forests under continued climate warming and increased wildfire activity.     

Warming drives a front of white spruce establishment near western treeline,Alaska

Regional warming has led to increased productivity near the boreal forest margin in Alaska. To date, the effects of warming on seedling recruitment have received little attention, in spite of forecasted forest expansion. Here, we used stand structure and environmental data from 95 white spruce (Picea glauca) plots sampled across a longitudinal gradient in southwest Alaska to explore factors influencing spruce establishment and recruitment near western treeline. We used total counts of live seedlings, saplings, and trees, representing five life stages, to evaluate whether geospatial, climate, and measured plot covariates predicted abundance, using current abundance distributions as a surrogate for climate conditions in the past. We used generalized linear models to test the null hypothesis that conditions favorable for recruitment were similar along the environmental gradient represented by longitude, by exploring relationships between per‐plot counts of each life stage and the covariates hypothesized to affect abundance. We also examined the relationship between growing degree days (GDD) and seedling establishment over a period of three decades using tree‐ring chronologies obtained from cores taken at a subset of our sites (n = 30). Our results indicated that seedling, sapling, and tree abundance were positively correlated with temperature across the study area. The response to longitude was mixed, with earlier life stages (seedlings, saplings) most abundant at the western end of the gradient, and later life stages (trees) most abundant to the east. The differential relationship between longitude and life‐stage abundance suggests a moving front of white spruce establishment through time, driven by changes in environmental conditions near the species’ western range limit. Likewise, we found a positive relationship between periods of seedling establishment and GDD, suggesting that longer summers and/or greater heat accumulation might enhance establishment, consistent with the positive relationship we found between life‐stage abundance and temperature.     

Current and Potential Tree Locations in Tree Line Ecotone of Changbai Mountains,Northeast China: The Controlling Effects of Topography

Tree line ecotone in the Changbai Mountains has undergone large changes in the past decades. Tree locations show variations on the four sides of the mountains, especially on the northern and western sides, which has not been fully explained. Previous studies attributed such variations to the variations in temperature. However, in this study, we hypothesized that topographic controls were responsible for causing the variations in the tree locations in tree line ecotone of the Changbai Mountains. To test the hypothesis, we used IKONOS images and WorldView-1 image to identify the tree locations and developed a logistic regression model using topographical variables to identify the dominant controls of the tree locations. The results showed that aspect, wetness, and slope were dominant controls for tree locations on western side of the mountains, whereas altitude, SPI, and aspect were the dominant factors on northern side. The upmost altitude a tree can currently reach was 2140 m asl on the northern side and 2060 m asl on western side. The model predicted results showed that habitats above the current tree line on the both sides were available for trees. Tree recruitments under the current tree line may take advantage of the available habitats at higher elevations based on the current tree location. Our research confirmed the controlling effects of topography on the tree locations in the tree line ecotone of Changbai Mountains and suggested that it was essential to assess the tree response to topography in the research of tree line ecotone.     

Tree recruitment at the treeline across the Continental Divide in the Northern Rocky Mountains,USA: the role of spring snow and autumn climate

ABSTRACT

Background: Topoclimate can influence tree establishment within treeline ecotones. Yet much less is known about how regional topography, such as the Continental Divide, Rocky Mountains, mediates the role of climate in governing treeline dynamics.

Aims: To utilise the Continental Divide to test whether contrasts in growing-season moisture regimes to the west (summer-dry) and east (summer-wet) impact the spatio-temporal patterns of tree establishment and rates of treeline advance in the Northern Rocky Mountains.

Methods: We sampled trees at sites on north- and south-facing slopes, west and east of the Continental Divide. We used dendroecological techniques to reconstruct patterns of tree establishment. Age-structure data were quantitatively compared with climate to evaluate possible mechanistic linkages.

Results: Across all sites, 96% of trees established after 1950. There was a treeline advance (range = 39–140 m) accompanied by increases in tree density. Significantly more trees established during wet springs on both sides of the Divide.

Conclusions: Overall, snow duration in spring and autumn temperatures appear to influence patterns of tree recruitment at the treeline. Continued warming will likely amplify the role of autumn climate in regulating tree establishment throughout treeline ecotones in the Northern Rocky Mountains, particularly west of the Divide where summer-dry conditions persist.     

Latitudinal response of subarctic tree lines to recent climate change in eastern Canada

Aim The predictions from biogeographical models of poleward expansion of biomes under a warmer 2 × CO2 scenario might not be warranted, given the non‐climatic influences on vegetation dynamics. Milder climatic conditions have occurred in northern Québec, Canada, in the 20th century. The purpose of this study was to document the early signs of a northward expansion of the boreal forest into the subarctic forest‐tundra, a vast heterogeneous ecotone. Colonization of upland tundra sites by black spruce (Picea mariana (Mill.) BSP.) forming local subarctic tree lines was quantified at the biome scale. Because it was previously shown that the regenerative potential of spruce is reduced with increasing latitude, we predicted that tree line advances and recent establishment of seedlings above tree lines will also decrease northwards. Location Black spruce regeneration patterns were surveyed across a > 300‐km latitudinal transect spanning the forest‐tundra of northern Québec, Canada (55°29′–58°27′ N). Methods Elevational transects were positioned at forest–tundra interfaces in two regions from the southern forest‐tundra and two regions from the northern forest‐tundra, including the arctic tree line. The surroundings of stunted black spruce, forming the species limit in the shrub tundra, were also examined. Position, total height and origin (seed or layer) of all black spruce stems established in the elevational transects were determined. Dendrochronological and topographical data allowed recent subarctic tree line advances to be estimated. Age structures of spruce recently established from seed (< 2.5 m high) were constructed and compared between forest‐tundra regions. Five to 20‐year heat sum (growing degree‐days, > 5 °C) and precipitation fluctuations were computed from regional climatic data, and compared with seedling recruitment patterns. Results During the 20th century, all tree lines from the southern forest‐tundra rose slightly through establishment of seed‐origin spruce, while some tree lines in the northern forest‐tundra rose through height growth of stunted spruce already established on the tundra hilltops. However, the rate of rise in tree lines did not slow down with latitude. The density of < 2.5‐m spruce established by seed declined exponentially with latitude. While the majority of < 2.5‐m spruce has established since the late 1970s on the southernmost tundra hilltops, the regeneration pool was mainly composed of old, suppressed individuals in the northern forest‐tundra. Spruce age generally decreased with increasing elevation in the southern forest‐tundra stands, therefore indicating current colonization of tundra hilltops. Although spruce reproductive success has improved over the twentieth century in the southern forest‐tundra, there was hardly any evidence that recruitment of seed‐origin spruce was controlled by 5‐ to 20‐year regional climatic fluctuations, except for winter precipitation. Main conclusions Besides the milder 20th century climate, local topographic factors appear to have influenced the rise in tree lines and recent establishment by seed. The effect of black spruce's semi‐serotinous cones in trapping seeds and the difficulty of establishment on exposed, drought‐prone tundra vegetation are some factors likely to explain the scarcity of significant correlations between tree establishment and climatic variables in the short term. The age data suggest impending reforestation of the southernmost tundra sites, although the development of spruce seedlings into forest might be slowed down by the harsh wind‐exposure conditions.     

Changes in fire regime break the legacy lock on successional trajectories in Alaskan boreal forest

Predicting plant community responses to changing environmental conditions is a key element of forecasting and mitigating the effects of global change. Disturbance can play an important role in these dynamics, by initiating cycles of secondary succession and generating opportunities for communities of long‐lived organisms to reorganize in alternative configurations. This study used landscape‐scale variations in environmental conditions, stand structure, and disturbance from an extreme fire year in Alaska to examine how these factors affected successional trajectories in boreal forests dominated by black spruce. Because fire intervals in interior Alaska are typically too short to allow relay succession, the initial cohorts of seedlings that recruit after fire largely determine future canopy composition. Consequently, in a dynamically stable landscape, postfire tree seedling composition should resemble that of the prefire forest stands, with little net change in tree composition after fire. Seedling recruitment data from 90 burned stands indicated that postfire establishment of black spruce was strongly linked to environmental conditions and was highest at sites that were moist and had high densities of prefire spruce. Although deciduous broadleaf trees were absent from most prefire stands, deciduous trees recruited from seed at many sites and were most abundant at sites where the fires burned severely, consuming much of the surface organic layer. Comparison of pre‐ and postfire tree composition in the burned stands indicated that the expected trajectory of black spruce self‐replacement was typical only at moist sites that burned with low fire severity. At severely burned sites, deciduous trees dominated the postfire tree seedling community, suggesting these sites will follow alternative, deciduous‐dominated trajectories of succession. Increases in the severity of boreal fires with climate warming may catalyze shifts to an increasingly deciduous‐dominated landscape, substantially altering landscape dynamics and ecosystem services in this part of the boreal forest.