Explore the vast range of titles available.

Ecosystem responses to past climate change can provide insight into plausible scenarios of response to future change and can elucidate factors that may influence the overall predictability of such responses. I explore the utility of paleoecological studies for addressing questions about the predictability of ecosystem responses to climate change using Alaskan tree line ecosystems as a case study. Published studies were used to develop a regional analysis of patterns of recent tree line advance, and to estimate lags between recruitment onset and forest development beyond tree line. Tree line advance is ubiquitous, but asynchronous in time, occurring significantly earlier in the White Mountains in interior Alaska than in western Alaska or the Alaska Range. The mean lag between initiation of recruitment and forest development was estimated at approximately 200 years, similar to what modeling studies have found. Although continued advance of white spruce forests is the most likely scenario of future change, variability in the rate of forest response to warming may be likely due to limitation of spruce establishment in highly permafrost-affected sites, changes in seed dispersal and early establishment, and recent changes in the growth responses of individual trees to temperature. All of these factors may cause spruce populations to exhibit nonlinear responses to future warming, and uncritical extrapolation from recent trends is thus unwarranted.

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 Methods We used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics.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.

Questions: Tree line ecotone regions are expected to respond swiftly to climate changes. In this paper, remote sensing- and ground-based tree population data are used to examine past and on-going changes of the tree line ecotone in a subarctic region characterized by precipitation increase. Questions addressed are: (1) at what rate has the tree line ecotone changed since the mid-20th century; (2) can specific temporal dynamics be identified; and (3) do combined remote sensing and tree population analyses add essential knowledge for the interpretation of tree line changes? Location: Khibiny Mountains, Kola Peninsula, northwest Russia. Methods: Aerial photos from 1958, high-resolution satellite imagery from 2006/2008 and age structure data for dominant tree line species (birch and pine) were used to analyse rate of change and temporal and species-specific tree line recruitment patterns. This was accomplished using digital elevation models, resolution-merging procedures, visual interpretation and dendroecological methods. Results: Mean tree line advance for birch and pine was recorded as 29 and 27 altitudinal metres (0.6 and 0.5 m·yr -1 ), respectively. The advance was accompanied by an apparent infilling of pre-established tree populations and by recruitment beyond the tree line. Evident increased recruitment occurred in the late 1980s for birch and in the 1970s and 1990s for pine. Establishment showed no strong correlations with climate variables, but the importance of non-growing season variables was indicated. Conclusions: The recorded tree line advance is modest compared to global model predictions for advance at high latitudes, but in accordance with results from a number of high-latitude areas. Concomitantly, the apparent increased recruitment is indicative of a more rapidly advancing tree line zone. Studies combining remote sensing and ground-based data minimize the risk of under-or overestimating potential tree line advance. Low detectability of small seedlings and saplings by remote sensing can cause underestimation of the current potential, while ground-based data used alone can overestimate potential advance. A balance between the two approaches is beneficial and enhances quality in production of change scenarios related to high latitudinal tree line areas at local to large regional scales.

The expansion of shrubs and trees across high-latitude ecosystems is one of the most dramatic ecological manifestations of climate change. Most of the work quantifying these changes has been done in small areas and over relatively recent time scales. These land-cover transitions are highly spatially variable, and we have limited understanding of the factors underlying this variation. We use repeat photography to generate a data set of land-cover changes in Denali National Park and Preserve, Alaska, stretching back a century and spanning a range of edaphic, topographic, and climatic conditions. Most land-cover classes were quite stable, with low probabilities of transitioning to other land-cover types. The advance of woody vegetation into low-stature tundra, and the spread of conifer trees into shrub-dominated areas, were both more likely at low elevations and in areas without permafrost. Permafrost also reduced the likelihood of herbaceous vegetation transitioning to woody cover. Exceptions to the general trend of relative stability included nearly all (96%) of the broadleaf forest–dominated areas being invaded by conifers, an expected successional trajectory, and many open gravel river bars (17.8%) transitioning to thick shrubs. These floodplain areas were distinctly not at equilibrium, as only 0.1% of shrub-dominated areas converted to gravel. Warming temperatures in coming decades and concomitant declines in the extent of permafrost are predicted to enhance the spread of woody vegetation in Denali further, but only by ∼3%. Land-cover transitions, notably the rapid advance of trees and shrubs observed in other studies, could be less likely and more spatially heterogeneous here than in other high-latitude systems.

Arctic and subarctic ecosystems are changing rapidly in species composition and functioning as they warm twice as fast as the global average. It has been suggested that tree-less boreal landscapes may shift abruptly to tree-dominated states as climate warms. Yet, we insufficiently understand the conditions and mechanisms underlying tree establishment in the subarctic and arctic regions to anticipate how climate change may further affect ecosystem structure and functioning. We conducted a field experiment to assess the role of permafrost presence, micro-topography and shrub canopy on tree establishment in almost tree-less subarctic peatlands of northern Finland. We introduced seeds and seedlings of four tree-line species and monitored seedling survival and environmental conditions for six growing seasons. Our results show that once seedlings have emerged, the absence of permafrost can enhance early tree seedling survival, but shrub cover is the most important driver of subsequent tree seedling survival in subarctic peatlands. Tree seedling survival was twice as high under an intact shrub canopy than in open conditions after shrub canopy removal. Under unclipped control conditions, seedling survival was positively associated with dense shrub canopies for half of the tree species studied. These strong positive interactions between shrubs and trees may facilitate the transition from today’s treeless subarctic landscapes towards tree-dominated states. Our results suggest that climate warming may accelerate this vegetation shift as permafrost is lost, and shrubs further expand across the subarctic.

1. Snowpatches are disjunct arctic ecosystems scattered across the subarctic, particularly on wind-protected lee slopes, where a thick snow cover accumulates during the winter. These snow-rich treeless ecosystems are affected by delayed snowmelt, causing shorter growing seasons. Snow-tolerant plants occupy the centre of subarctic snowpatches, whereas black spruce trees grow at the margins. Snowpatches have shown sporadic expansion and shrinkage phases from tree establishment and mortality linked to climatic trends. Field surveys in the subarctic of eastern Canada are showing an afforestation process occurring in snowpatches. The origin and nature of this afforestation were investigated based on the hypothesis that tree colonization and growth were closely associated with recent changes in climate. 2. Snowpatches were categorized into three types based on border trees: forested, semi-forested and tundra-like. In eight randomly selected snowpatches of each type, snowpatch borders and areas covered by deciduous shrubs or trees were mapped and chionophilous plants and frost-associated soil disturbances were recorded. Radial (tree-ring) and vertical growth were measured on stems to evaluate the impact of climate and local factors on snowpatch black spruce populations. 3. Two waves of spruce colonization occurred in all forested and semi-forested snowpatches in the 1960s and in the 1980s to present. Spruce establishment was more frequent and abundant in recent years, near the forest margins and on moss and barren seedbeds. Expanding shrub (dwarf birch) cover inhibited spruce seedling establishment. Tree establishment and growth were positively correlated with growing season temperature and negatively correlated with annual maximum snow depth. More recently established spruce seedlings exhibited faster vertical growth than those established in the 20th century. 4. Synthesis. Due to warmer conditions and earlier snowmelt in eastern Canada, black spruce trees and dwarf birch shrubs are racing through subarctic snowpatches. This afforestation could change biodiversity in the subarctic and affect watershed dynamics through a change in snowmelt pattern. Subarctic snowpatches are climatesensitive ecosystems of the forest-tundra landscape, forming ideal biotopes for snow-tolerant, arctic-alpine species. Further tree encroachment in snowpatches in this century is potentially a threat to plant diversity, especially chionophilous species that have no corridors to migrate towards arctic ecosystems.

The possible effects of climate change on the advance of the tree line are considered. As temperature, elevated CO2 and nitrogen deposition co‐vary, it is impossible to disentangle their impacts without performing experiments. However, it does seem very unlikely that photosynthesis per se and, by implication, factors that directly influence photosynthesis, such as elevated CO2, will be as important as those factors which influence the capacity of the tree to use the products of photosynthesis, such as temperature. Moreover, temperature limits growth more severely than it limits photosynthesis over the temperature range 5–20 °C. If it is assumed that growth and reproduction are controlled by temperature, a rapid advance of the tree line would be predicted. Indeed, some authors have provided photographic evidence and remotely sensed data that suggest this is, in fact, occurring. In regions inhabited by grazing animals, the advance of the tree line will be curtailed, although growth of trees below the tree line will of course increase substantially.

Shrubs and trees are expected to expand in the sub-Arctic due to global warming. Our study was conducted in Abisko, sub-arctic Sweden. We recorded the change in coverage of shrub and tree species over a 32- to 34-year period, in three 50 × 50 m plots; in the alpine-tree-line ecotone. The cover of shrubs and trees (<3.5 cm diameter at breast height) were estimated during 2009–2010 and compared with historical documentation from 1976 to 1977. Similarly, all tree stems (≥3.5 cm) were noted and positions determined. There has been a substantial increase of cover of shrubs and trees, particularly dwarf birch (Betula nana), and mountain birch (Betula pubescens ssp. czerepanovii), and an establishment of aspen (Populus tremula). The other species willows (Salix spp.), juniper (Juniperus communis), and rowan (Sorbus aucuparia) revealed inconsistent changes among the plots. Although this study was unable to identify the causes for the change in shrubs and small trees, they are consistent with anticipated changes due to climate change and reduced herbivory.

1. When tree seedlings establish beyond the current tree line due to climate warming, they encounter existing vegetation, such as bryophytes that often dominate in arctic and alpine tundra. The stress gradient hypothesis (SGH) predicts that plant interactions in tundra become increasingly negative as climate warms and conditions become less harsh. However, for seedlings, climate warming might not result in lower winter stress, if insulating snow cover is reduced. 2. We aimed to understand if bryophytes facilitate seedling survival in a changing winter climate and if these effects of bryophytes on tree seedlings comply with the SGH along elevational gradients under contrasting snow conditions. 3. In the Swedish subarctic, we transplanted intact bryophyte cores covered by each of three bryophyte species and bryophyte-free control soil from above the tree line to two field common garden sites, representing current and future tree line air temperature conditions (i.e. current tree line elevation and a lower, warmer, elevation below the tree line). We planted seedlings of Betula pubescens and Pinus sylvestris into these cores and subjected them to experimental manipulation of snow cover during one winter 4. In agreement with the SGH, milder conditions caused by increased snow cover enhanced the generally negative or neutral effects of bryophytes on seedlings immediately after winter. Furthermore, survival of P. sylvestris seedlings after one full year was higher at lower elevation, especially when snow cover was thinner. However, in contrast with the SGH, impacts of bryophytes on over-winter survival of seedlings did not differ between elevations, and impacts on survival of B. pubescens seedlings after 1 year was more negative at lower elevation. Bryophyte species differed in their effect on seedling survival after winter, but these differences were not related to their insulating capacity. 5. Synthesis. Our study demonstrates that interactions from bryophytes can modify the impacts of winter climate change on tree seedlings, and vice versa. These responses do not always comply with SGH, but could ultimately have consequences for large-scale ecological processes such as tree line shifts. These new insights need to be taken into account in predictions of plant species responses to climate change.

Background and aims Facilitative plant-plant interactions are common in harsh environments such as Arctic and alpine tree lines. In Fennoscandia, mountain birch dominates tree lines, but mixes with Scots pine in less severe areas. Using over 30-yr. old Scots pine common gardens, established at three locations near the present Scots pine tree line, we tested (1) if mountain birch can facilitate Scots pine numbers and (2) if improved soil fertility under mountain birch canopies has a role in facilitation. Methods We counted the number of pines within 1-m and 3-m radii of the tallest mountain birch vs. a random spot in 70–75 planting plots and sampled soil for nutrients at 0.3-, 1- and 3-m distance to the birch in ten plots in each location. Results Number of Scots pines was 29% higher within a 1-m radius of a mountain birch than of a random spot. This effect did not depend on location, although the locations differed significantly in soil fertility, and no effect was detected within a 3-m radius. Concentrations of water, NH4, NO3 and PO4 decreased significantly with increasing distance to a mountain birch, but only in the least fertile location. Conclusions Mountain birch can significantly facilitate Scots pine in tree line conditions. However, unlike we expected, improved soil fertility under birch canopies may not have a general role in facilitation.