Treeline Ecotone Research Articles

Patterns of snow, deposition, and soil nutrients at multiple spatial scales at a Rocky Mountain tree line ecotone

The forest‐alpine tundra ecotone in the Front Range of Colorado typically occurs as a gradual transition from the treeless tundra to the closed canopy coniferous subalpine forest. We evaluated the patterns of snow, deposition inputs, and soil properties at three spatial scales: across the entire ecotone, with distance from tree limit in the transitional krummholz zone, and around individual trees. Snow depth was deepest in the krummholz zone and lowest in the alpine tundra and upwind of trees near tree limit, but was not predictive of most soil properties except for surface litter decomposition. Inorganic deposition ranged from 0.7 to 7.7 g m−2 yr−1 across the ecotone and tended to be higher downwind than upwind of trees. The exchangeable acid cation concentrations were significantly higher in the forest and increased gradually toward the tundra. The exchangeable base cations (Ca, Mg, and K) were only predictable at the scale of individual trees with greater concentrations downwind of trees. Resin bag available N was rarely predictable at any spatial scale, but was correlated among seasons. While the ecotone is in many ways a gradual transition from alpine tundra to subalpine forest, patterns observed across this vegetation gradient were often replicated on the scale of individual trees: downwind of trees was more similar to the forest, and upwind of trees was more similar to the tundra. No single spatial scale or gradient can provide uniform generalities about the amounts or fluxes of materials that determine the soil properties of this ecotone.

Vegetation patterns at the alpine treeline ecotone: the influence of tree cover on abrupt change in species composition of alpine communities

AbstractAims: The upper elevation limit of forest vegetation in mountain ranges (the alpine treeline ecotone) is expected to be highly sensitive to global change. Treeline shifts and/or ecotone afforestation could cause fragmentation and loss of alpine habitat, and are expected to trigger considerable alterations in alpine vegetation. We performed an analysis of vegetation structure at the treeline ecotone to evaluate whether distribution of the tree population determines the spatial pattern of vegetation (species composition and diversity) across the transition from subalpine forest to alpine vegetation.Location: Iberian eastern range of the Pyrenees.Methods: We studied 12 alpine Pinus uncinata treeline ecotones. Rectangular plots ranging from 940 to 1900 m2 were placed along the forest‐alpine vegetation transition, from closed forest to the treeless alpine area. To determine community structure and species distribution in the treeline ecotone, species variation along the forest‐alpine vegetation transition was sampled using relevés of 0.5 m2 set every 2 m along the length of each plot. Fuzzy C‐means clustering was performed to assess the transitional status of the relevés in terms of species composition. The relation of P. uncinata canopy cover to spatial pattern of vegetation was evaluated using continuous wavelet transform analysis.Results: Vegetation analyses revealed a large degree of uniformity of the subalpine forest between all treeline ecotone areas studied. In contrast, the vegetation mosaic found upslope displayed great variation between sites and was characterized by abrupt changes in plant community across the treeline ecotone. Plant richness and diversity significantly increased across the ecotone, but tree cover and diversity boundaries were not spatially coincident.Conclusions: Our results revealed that no intermediate communities, in terms of species composition, are present in the treeline ecotone. Ecotone vegetation reflected both bedrock type and fine‐scale heterogeneity at ground level, thereby reinforcing the importance of microenvironmental conditions for alpine community composition. Tree cover did not appear to be the principal driver of alpine community changes across the treeline ecotone. Microenvironmental heterogeneity, together with effects of past climatic and land‐use changes on ecotone vegetation, may weaken the expected correlation between species distribution and vegetation structure.

Response of regional tree-line forests to climate change: evidence from the northeastern Tibetan Plateau

Tree-ring width and age structure of Juniperus przewalskii (Qilian juniper) forests were analyzed for four tree-line sites in Qilian and Anyemaqen Mountains, northeastern Tibetan Plateau, to investigate their relationships to climate change. Tree-line growth on Qilian Mountain was mainly limited by temperature at the low-frequency band. However, tree-line growth in the Anyemaqen Mountain was highly correlated with the current growing season temperature at the high-frequency band, and with the previous growing season precipitation at the low-frequency band. A temperature-stressed growth pattern at colder western sites and a moisture-stressed growth pattern at the warm, drier eastern tree-line sites were detected. The number of surviving trees in the tree-line ecotone was not clearly correlated with temperature before the 1900s. An unprecedented rise in the number of trees coincided well with the rapid global warming after the 1900s.

Seedling recruitment, survival and facilitation in alpine Pinus uncinata tree line ecotones. Implications and potential responses to climate warming

ABSTRACTAims Alpine tree line ecotones are harsh environments where low temperatures constrain tree regeneration and growth. However, the expected upward shift of tree line ecotones in response to climate warming has not been ubiquitous. The lack of coupling between tree line dynamics and climate warming might be explained by factors other than climate variation that determine seedling recruitment in these ecotones. We want to assess how the availability of suitable habitat for establishment and the effects of facilitation on seedling survival and growth affect tree recruitment within tree line ecotones and modulate their responses to climate.Location We evaluate the relevance of these factors for Pinus uncinata tree line ecotones in the Catalan Pyrenees (north‐east Spain) and Andorra.Methods We analysed the microhabitat of naturally established seedlings in rectangular plots at the tree line ecotone, assessing the habitat type and the proximity to potentially protective elements that may improve microsite conditions. We tested whether krummholz individuals influence regeneration at the tree line by performing a transplantation field experiment to evaluate the extent of facilitation on seedling survival and growth in height. A total of 820 seedlings were transplanted at different distances and orientations (resulting in 12 positions) from krummholz mats and monitored over 2 years.Results Safe sites for P. uncinata recruits consisted of sparse vegetation covering bare soil, gravel or litter, and close to protective elements that may ameliorate microsite conditions. The field experiment showed that directional positive interactions enhance seedling survival and growth, altering the spatial patterns of recruit survivorship, especially during harsh winter conditions (shallow and irregular snowpack).Main conclusions Our results suggest that scarce availability of safe sites and uneven facilitation by krummholz control seedling recruitment patterns within alpine tree line ecotones. Such constraints may distort or counter the response of tree line ecotones to climate warming at local and regional scales.

The Melting Himalayas: Cascading Effects of Climate Change on Water, Biodiversity, and Livelihoods

The Greater Himalayas hold the largest mass of ice outside polar regions and are the source of the 10 largest rivers in Asia. Rapid reduction in the volume of Himalayan glaciers due to climate change is occurring. The cascading effects of rising temperatures and loss of ice and snow in the region are affecting, for example, water availability (amounts, seasonality), biodiversity (endemic species, predator-prey relations), ecosystem boundary shifts (tree-line movements, high-elevation ecosystem changes), and global feedbacks (monsoonal shifts, loss of soil carbon). Climate change will also have environmental and social impacts that will likely increase uncertainty in water supplies and agricultural production for human populations across Asia. A common understanding of climate change needs to be developed through regional and local-scale research so that mitigation and adaptation strategies can be identified and implemented. The challenges brought about by climate change in the Greater Himalayas can only be addressed through increased regional collaboration in scientific research and policy making.

Treeline shifts in the Ural mountains affect soil organic matter dynamics

AbstractHistorical photographs document that during the last century, forests have expanded upwards by 60–80 m into former tundra of the pristine Ural mountains. We assessed how the shift of the high‐altitude treeline ecotone might affect soil organic matter (SOM) dynamics. On the gentle slopes of Mali Iremel in the Southern Urals, we (1) determined the differences in SOM stocks and properties from the tundra at 1360 m above sea level (a.s.l.) to the subalpine forest at 1260 m a.s.l., and (2) measured carbon (C) and nitrogen (N) mineralization from tundra and forest soils at 7 and 20 °C in a 6‐month incubation experiment. C stocks of organic layers were 3.6±0.3 kg C m−2 in the tundra and 1.9±0.2 kg C m−2 in the forest. Mineral soils down to the bedrock stored significantly more C in the forest, and thus, total soil C stocks were slightly but insignificantly greater in the forest (+3 kg C m−2). Assuming a space for time approach based on tree ages suggests that the soil C sink due to the forest expansion during the last century was at most 30 g C m−2 yr−1. Diffuse reflective infrared spectroscopy and scanning calorimetry revealed that SOM under forest was less humified in both organic and mineral horizons and, therefore, contained more available substrate. Consistent with this result, C mineralization rates of organic layers and A horizons of the forest were two to four times greater than those of tundra soils. This difference was similar in magnitude to the effect of increasing the incubation temperature from 7 to 20 °C. Hence, indirect climate change effects through an upward expansion of forests can be much larger than direct warming effects (Δ0.3 K across the treeline). Net N mineralization was 2.5 to six times greater in forest than in tundra soils, suggesting that an advancing treeline likely increases N availability. This may provide a nutritional basis for the fivefold increase in plant biomass and a tripling in productivity from the tundra to the forest. In summary, our results suggest that an upward expansion of forest has small net effects on C storage in soils but leads to changes in SOM quality, accelerates C cycling and increases net N mineralization, which in turn might stimulate plant growth and thus C sequestration in tree biomass.

Classification of Current Vegetation Cover and Alpine Treeline Ecotone in the Praděd Reserve (Czech Republic), Using Remote Sensing

Abstract The goal of the study was to derive up-to-date and complex information on the current vegetation cover of the Praděd Reserve (Hrubý Jesenik Mountains, Czech Republic), with special regard to the unique alpine treeline ecotone formed by krummholz of Norway spruce. We argue that the data of remote sensing and automated techniques of image processing should be preferably used. Accordingly, a color-infrared orthophoto map was classified in a land cover map employing maximum likelihood spectral classifier, ancillary data, texture analysis, and a knowledge base classification technique. The overall classification accuracy was about 78%, distinguishing 7 land cover classes. Using a reclassified land cover map and the moving window mean filter, a spruce canopy closure map was calculated. The continuous map of the canopy closure was subsequently reclassified in predefined intervals that were used for an automated delimitation and mapping of complex transitional borders of the alpine treeline ecotone. The ...

Early human impact in the forest ecotone of southern High Asia (Hindu Kush, Himalaya)

The vegetation of the treeline ecotone of the southern declivity of arid High Asia (Hindu Kush, northern areas of Pakistan; Himalaya, northern central Nepal) is dominated by hedgehog-like open dwarf shrublands of thorny cushions. Since climatically sensitive ecotones are always also sensitive to human impact, the question arises whether the current lack of forests is a result of the Subboreal climate decline or of human impact. Due to inadequate knowledge of the pollen flora and of ecological indicator values of the plants, pollen analyses in High Asia have mainly been limited to the regional verification of globally known climatic impulses. However, the role of human impact on regional vegetation patterns has been widely neglected. We postulate that today's open dwarf shrublands replace woodlands and forests. Isolated vigorous juniper trees and successful reforestation appear to confirm our hypothesis. An abrupt decline of Pinus forests before 5700 and 5400 ka cal yr BP can be demonstrated. As the first indicator pollen of human impact appeared at both sites synchronous with the forest pollen decline, we infer human impact to be a more decisive cause for this environment change superimposing the effects of a climatic deterioration. The forests were displaced by open dwarf shrublands.

Post‐fire tree establishment patterns at the alpine treeline ecotone: Mount Rainier National Park, Washington, USA

AbstractQuestions: Does tree establishment: (1) occur at a treeline depressed by fire, (2) cause the forest line to ascend upslope, and/or (3) alter landscape heterogeneity? (4) What abiotic and biotic local site conditions are most important in structuring establishment patterns? (5) Does the abiotic setting become more important with increasing upslope distance from the forest line?Location: Western slopes of Mount Rainier, USA.Methods: We performed classification analysis of 1970 satellite imagery and 2003 aerial photography to delineate establishment. Local site conditions were calculated from a LIDAR‐based DEM, ancillary climate data, and 1970 tree locations in a GIS. We used logistic regression on a spatially weighted landscape matrix to rank variables.Results: Considerable establishment after 1970 caused forest line elevation to increase over 150 m in specific locations. Landscape heterogeneity increased with distance from the 1970 forest line. At a broad spatial context, we found establishment was most common near existing trees (0‐50 m) and at low elevations (1250‐1350 m). Slope aspect (W, NW, N, NE, and E), slope angle (40‐60°), and other abiotic factors emerged as important predictors of establishment with increasing upslope distance from the forest line to restricted spatial extents.Conclusions: Favorable climatic conditions likely triggered widespread tree establishment. Readily available seed probably enhanced establishment rates near sexually mature trees, particularly in the less stressful environment at low elevations. The mass effect of nearly ubiquitous establishment in these areas may have obscured the importance of the abiotic setting to restricted spatial extents. Topographic variability apparently produced favorable sites that facilitated opportunistic establishment with increasing upslope distance from the forest line, thereby enabling additional trees to invade the alpine tundra.

Effects of climate variables on intra-annual stem radial increment in Pinus cembra (L.) along the alpine treeline ecotone

Within the alpine treeline ecotone tree growth is increasingly restricted by extreme climate conditions. Although intra-annual stem growth recorded by dendrometers can be linked to climate, stem diameter increments in slow-growing subalpine trees are masked by changes in tree water status.We tested the hypothesis that intra-annual radial stem growth in Pinus cembra is influenced by different climate variables along the treeline ecotone in the Austrian Alps. Dendrometer traces were compared with dynamics of xylem cell development to date onset of cambial activity and radial stem growth in spring.Daily fluctuations in stem radius reflected changes in tree water status throughout the treeline ecotone. Extracted daily radial increments were significantly correlated with air temperature at the timberline and treeline only, where budburst, cambial activity and enlargement of first tracheids also occurred quite similarly. A close relationship was detected between radial increment and number of enlarging tracheids throughout the treeline ecotone.We conclude that (i) the relationship between climate and radial stem growth within the treeline ecotone is dependent on a close coupling to atmospheric climate conditions and (ii) initiation of cambial activity and radial growth in spring can be distinguished from stem re-hydration by histological analysis.

Remote Sensing of Mountain Environments

AbstractLogistical challenges of conducting field research in topographically complex alpine settings, combined with the multi‐scale and multi‐thematic informational needs of earth and landscape scientists and program managers, have contributed to the development and application of remote sensing data, methods, and approaches for the study of mountain environments. This article provides a brief introduction to remote sensing science; illustrates specific challenges unique to remote sensing applications in mountainous environments, along with corresponding solutions; provides several examples of remote sensing applications in the alpine treeline ecotone; and concludes by describing recent and anticipated remote sensing developments that have the potential to improve characterizations of alpine settings through satellite surveillance techniques linked to image processing and analysis.

Bosnian Pine (Pinus Heldreichii) as Geoarchive at the Timberline in the Pirin Mountains and on the Balkan Peninsula

ABSTRACTHigh mountains and their ecosystems offer an outstanding opportunity for studies on the impact of climate change. The Pirin Mountains in Southeast Europe, situated at the transition between temperate and Mediterranean climate, are considered as very sensitive to historical and current global changes. Site specific effects as well as the impact of historical disturbances have been analysed at treeline ecotone testplots. Bosnian Pine (Pinus heldreichii) and Macedonian Pine (Pinus peuce) are the most common species at the recent timberline around 2.100 m a.s.l. in the Pirin Mountains. The results on dendroclimatology provide an insight into the potential of the Bosnian Pine and its chronologies in the Northern Pirin Mountains. First conclusions can be drawn from the chronology and site comparison respectively as well as the climate-growth-analysis. On the one hand, the width growth is humidity limited. On the other hand, the tree-rings similarly reflect high summer temperatures as a negative impact factor at sunny south flanks. At the same time, mild winters have a positive effect.

Temporal dynamic of wood formation in Pinus cembra along the alpine treeline ecotone and the effect of climate variables

We determined the temporal dynamic of cambial activity and xylem development of stone pine (Pinus cembra L.) throughout the treeline ecotone. Repeated micro-sampling of the developing tree ring was carried out during the growing seasons 2006 and 2007 at the timberline (1950 m a.s.l.), treeline (2110 m a.s.l.) and within the krummholz belt (2180 m a.s.l.) and the influence of climate variables on intra-annual wood formation was determined.At the beginning of both growing seasons, highest numbers of cambial and enlarging cells were observed at the treeline. Soil temperatures at time of initiation of cambial activity were c. 1.5 °C higher at treeline (open canopy) compared to timberline (closed canopy), suggesting that a threshold root-zone temperature is involved in triggering onset of above ground stem growth.The rate of xylem cell production determined in two weekly intervals during June through August 2006-2007 was significantly correlated with air temperature (temperature sums expressed as degree-days and mean daily maximum temperature) at the timberline only. Lack of significant relationships between tracheid production and temperature variables at the treeline and within the krummholz belt support past dendroclimatological studies that more extreme environmental conditions (e.g., wind exposure, frost desiccation, late frost) increasingly control tree growth above timberline.Results of this study revealed that spatial and temporal (i.e. year-to-year) variability in timing and dynamic of wood formation of Pinus cembra is strongly influenced by local site factors within the treeline ecotone and the dynamics of seasonal temperature variation, respectively.

Short-term signals of climate change along an altitudinal gradient in the South Alps

Short-term changes in plant species number, frequency and composition were studied along an altitudinal gradient crossing four summits from the treeline ecotone to the subnival zone in the South Alps (Dolomites, Italy). Large-scale (summit areas) and small-scale patterns (16 plots of 1 m²/summit) were monitored. After 5 years, a re-visitation of the summit areas revealed a considerable increase of species richness at the upper alpine and subnival zone (10% and 9%, respectively) and relatively modest increases at the lower alpine zone and the treeline ecotone (3% and 1%, respectively). At the small scale, the results were partly different, with species richness decreasing at the lower summits and increasing at the higher summits. The changes can most likely be attributed to climate warming effects and to competitive interactions. The main newcomers at the lower three summits were species from the treeline and the lower altitudinal zones. Only at the highest summit, the newcomers came from the alpine species pool. At the treeline ecotone, the abundance of Pinus cembra, of dwarf shrubs and clonal graminoid species increased. Here, displacements of alpine species may be predicted for the near future. At the higher summits, expansions of the established alpine species and further invasions of species from lower altitudes are forecasted.

Mountain Birch Seedlings in the Treeline Ecotone, Subarctic Finland: Variation in Above- and Below-Ground Growth Depending on Microtopography

The treeline ecotone in northern Finnish Lapland is characterized by a mosaic of sites with highly varying environmental conditions. Density, age structure, growth, and root systems of mountain birch seedlings (Betula pubescens ssp. czerepanovii [Orlova] Hämet-Ahti) were studied in different microsite classes (deflation, lichen heath, dwarf shrub heath, hummock, willow shrub, sedge mire). On wind-exposed convex topography characterized by shortage of moisture and nutrients, seedling establishment is impeded, as is indicated by low seedling densities and lack of very young seedlings as well as by high rooting depths and root/shoot ratios of the few individuals. In sedge mires, birch seedlings occasionally occur in great numbers but die off at an early stage. Extremely shallow root systems point to anoxia as the main reason. Severe shoot damage is common to almost all saplings. This is likely due to grazing by reindeer (Rangifer t. tarandus). As a result, height growth is suppressed even on sites with otherwise relatively suitable conditions (e.g. willow shrub sites). Future effects of climate change which could locally improve conditions for germination and establishment of birch young growth might be overridden by the effects of high reindeer density.

Regional tree line dynamics in response to global change in the Pyrenees

Summary 1. Many studies in northern Europe, North and South America, describe regional trends of population densification at altitudinal and polar tree lines during the 20th century. The purpose of this study was (1) to ascertain if this regeneration enhancement is present across the alpine ecotones of the Pyrenees, (2) if synchronous recruitment trends are common among the studied populations and (3) to determine the tree limit stability during recent decades. 2. Twelve Pinus uncinata tree line populations were studied on the Iberian eastern range of the Pyrenees. Rectangular plots ranging from 940 to 7600 m2 were set along the forest-alpine grassland transition; more than 3600 P. uncinata individuals were mapped. Tree size and age were used to establish the demographic structure at each stand, and to characterize abrupt or smooth transition patterns along the tree line ecotone. A new procedure for estimating missing rings in off-centre cores was developed to ensure a correct interval for the age-classes distribution analysis. 3. Past and recent synchronous recruitment trends (mid 19th century, second half of the 20th century) were apparent at the tree line over the studied area of the Pyrenean range. The ecotone densification since the 1950s occurred in the context of climatic warming and substantial land use abandonment. Both gradual and step-like transition patterns in tree age and size along the ecotone were observed. 4. Regeneration enhancement in the last approximately 30 years appears as an abrupt change in population age structures, which could indicate the importance of feedback mechanisms for tree line recruitment dynamics. In 50% of the surveyed tree lines ecotone densification has been coupled to tree limit shifts in the recent past. This indicates both great tree limit sensitivity to short-term climatic changes and the presence of differential tree line dynamics at a regional scale. 5. Synthesis. The observed past and recent synchronous recruitment trends suggest the presence of regional climatic factors modulating tree line structure and dynamics. However, tree line dynamics in the Pyrenees have been widely affected by local anthropogenic activities. We suggest that the presence of step-like tree line transitions in tree age can be considered an evidence of recent human induced disturbances when no other major natural disturbances affect the tree line dynamics.

Timberline structure and limited tree recruitment in the Catalan Pyrenees

Background: Pyrenean timberlines, lowered to various extents through ancient land use, are expected to rise due to abandonment of land use and climate warming. Aims: To provide a comprehensive survey of the timberline in the Catalan Pyrenees, at the landscape, plant community, and tree population levels. Methods: We assessed the present location of timberlines by means of GIS techniques and characterised the treeline ecotone in 12 representative plots, where we took small phytosociological relevés (surface cover type, vegetation structure and species cover) and studied the regeneration of Pinus uncinata (seedlings and saplings). Results: The potential timberline was mostly found between 2300 and 2400 m a.s.l., with complex variation in these values due to bioclimatic and topographic diversity. Most of the present timberline was found at far lower altitudes than its potential location. Present forests reached the potential timberline about 25% along its total length. At natural or semi-natural treelines, the transition from subalpine forest to open alpine landscapes was relatively sharp. It was formed by the irregular alternation of two or three contrasting plant community types (forest, dwarf-shrub, grassland). Most Pinus regeneration was recorded where vegetation mosaics occurred on a fine scale. The highest numbers of Pinus seedlings and saplings were found emerging from bare ground or litter in small vegetation gaps. Over recent years Pinus recruitment showed very slow growth and a strong limitation in reaching the pole stage. Conclusions: In Pyrenean treeline ecotones situated near to their potential altitudes, recruitment and growth of existing Pinus uncinata seedlings and saplings is unlikely to produce pronounced infilling and thus perceptible changes in high-altitude forest cover.

Factors structuring the treeline ecotone in Fennoscandia

Many hypotheses have been put forward to explain the structure and position of alpine treelines. The spatial complexity of the ecotone, ranging from sharp boundaries to networks of tree patches within a heath matrix, may explain why no consensus has been reached. In this paper, we discuss factors from abiotic disturbances to herbivory that may help understand the spatial structure of the alpine treeline ecotone in Fennoscandia. The ecotone is dominated by mountain birch (Betula pubescens ssp. tortuosa), and may show a wide range of spatial structures. We discuss the influence of topography, seed limitations, seedling establishment, growth limitations, abiotic disturbances and herbivory as structuring factors. All of these factors may operate, but their relative importance in space and time is unknown. There is a basic difference between factors that prevent the establishment of trees, and thus act on early life history stages, and factors that thin out a previously dense forest, and thus act on adult trees. Mortality caused directly or indirectly by geometrid moths may belong to the latter category. We suggest that seedling and sapling mortality is more important than seed limitation for the establishment of new individuals in the treeline ecotone. Important mortality factors may be abiotic disturbances, competition (or allelopathy) from field layer plants and herbivory. The relative role of these factors needs to be examined further.

Establishment of boreal forest species in alpine dwarf-shrub heath in subarctic Sweden

Background: Saplings of mountain birch ( Betula pubescens ssp. czerepanovii ) have established in pockets of dwarf-shrub heath approximately 250 m above the treeline in the Latnjavagge Valley, northern Sweden. Aim: We examined if the establishment of these mountain birch outposts was related to favourable local microclimate, and if birch establishment has affected the surrounding vegetation, changing it from dwarf-shrub heath to more akin to birch forest floor. Methods: Daily mean and mean maximum temperatures were compared for two sites in the dwarf-shrub heath (990 m a.s.l., few birch saplings; 1060 m a.s.l., numerous birch saplings) between January 2005 and June 2006. The cover-abundance of vascular plants was estimated in sample plots in dwarf-shrub heath with mountain birch, dwarf-shrub heath without mountain birch, in heath between the upper limit of closed mountain birch forest and the treeline, and in closed mountain birch forest. Species composition and diversity were statistically compared. The presence in the dwarf-shrub heath of boreal montane species other than birch was also noted. Results: The higher elevation site, containing a higher density of birch, had a significantly higher growing season temperature than the lower elevation site. There was a significant difference in plant community composition between the alpine heath plots containing mountain birch saplings and plots without mountain birch, alpine heath with birch being more similar to the plots of the treeline ecotone and the birch forest than to alpine heath without birch. No significant difference in species diversity among plots in dwarf-shrub heath was found and species diversity increased with altitude. A number of montane species were observed in the dwarf-shrub heath, however, their distribution was not associated with that of the mountain birch. Conclusions: This study provides evidence for favourable microclimate being a key driver for the establishment of mountain birch above the treeline. In addition, the results imply that the composition of the dwarf-shrub heath changes after the establishment of mountain birch to a plant community whose composition points towards a birch forest.

CONSTRAINTS ON TREE SEEDLING ESTABLISHMENT IN MONTANE GRASSLANDS OF THE VALLES CALDERA, NEW MEXICO

Montane and subalpine grasslands are prominent, but poorly understood, features of the Rocky Mountains. These communities frequently occur below reversed tree lines on valley floors, where nightly cold air accumulation is spatially coupled with fine soil texture. We used field experiments to assess the roles of minimum temperature, soil texture, grass competition, and ungulate browsing on the growth, photosynthetic performance, and survival of transplanted ponderosa pine (Pinus ponderosa) seedlings at 32 sites straddling such reversed tree lines in the Valles Caldera National Preserve (VCNP) of the Jemez Mountains, New Mexico (USA). Seedling growth increased most strongly with increasing nighttime minimum temperatures away from the valley bottoms; seedlings experiencing the coldest temperatures on the caldera floor exhibited stunted needles and often no measurable height growth. Based on the chlorophyll fluorescence ratios PhiPSII and Fv/Fm, we found that low minimum temperatures, low soil moisture, and fine soil texture all contributed to photoinhibition. Neighboring herbs had only minor negative effects on seedlings. We found no effect of ungulates, but golden-mantled ground squirrels (Spermophilus lateralis) caused substantial seedling mortality. Second-year seedling survival was highest on sandy soils, and third-year survival was highest at sites with higher minimum temperatures. We conclude that differential tree seedling establishment driven by low minimum temperatures in the valley bottoms is the primary factor maintaining montane grasslands of the VCNP, although this process probably operated historically in combination with frequent surface fire to set the position of the tree line ecotone. As at alpine tree lines, reversed tree lines bordering montane and subalpine grasslands can represent temperature-sensitive boundaries of the tree life form.