Dramatic Increases of Soil Microbial Functional Gene Diversity at the Treeline Ecotone of Changbai Mountain.

The elevational and latitudinal diversity patterns of microbial taxa have attracted great attention in the past decade. Recently, the distribution of functional attributes has been in the spotlight. Here, we report a study profiling soil microbial communities along an elevation gradient (500-2200 m) on Changbai Mountain. Using a comprehensive functional gene microarray (GeoChip 5.0), we found that microbial functional gene richness exhibited a dramatic increase at the treeline ecotone, but the bacterial taxonomic and phylogenetic diversity based on 16S rRNA gene sequencing did not exhibit such a similar trend. However, the β-diversity (compositional dissimilarity among sites) pattern for both bacterial taxa and functional genes was similar, showing significant elevational distance-decay patterns which presented increased dissimilarity with elevation. The bacterial taxonomic diversity/structure was strongly influenced by soil pH, while the functional gene diversity/structure was significantly correlated with soil dissolved organic carbon (DOC). This finding highlights that soil DOC may be a good predictor in determining the elevational distribution of microbial functional genes. The finding of significant shifts in functional gene diversity at the treeline ecotone could also provide valuable information for predicting the responses of microbial functions to climate change

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 Tornetrask area in northern Sweden where climate warmed by 2.5 degrees 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 Methods We used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics.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

Recruitment and facilitation in Pinus hartwegii, a Mexican alpine treeline ecotone, with potential responses to climate warming

Alpine treelines in Mexico are represented by high-elevation forests dominated by P. hartwegii Ldl. To address the degree to which the presence of suitable microsite facilitators are factors for successful recruitment within the treeline ecotone of P. hartwegii and modulate their responses to climate warming, year of natural tree establishment, number of trees recruited, and the presence of shelter elements at different exposures of Monte Tlaloc (in the Trans-Mexican Volcanic System) were recorded. For tree recruitment and microsite facilitation we recorded each tree and the type of potentially protective elements that may improve microsite conditions within a total of 32 circular plots (r = 18 m) in the alpine treeline ecotone (above 4000 m). Temperatures for Monte Tlaloc at 4000 m were estimated using the thermal gradient for the study area, and standard dendrochronological methods and a regression model were used to date tree recruitment. Vector generalized linear models show that maximum growing season temperatures have significantly influenced the temporal pattern of tree recruitment in this system over the past 50 years, but this influence was mediated by the presence (or absence) of specific shelter elements (shrubs, soil depressions, rocks or bare soil) within a specific treeline ecotone exposure, also shaping the spatial pattern of tree recruitment. The response of the treeline ecotone to climate warming at local scales is qualitatively modified by the presence of microscale features, requiring sufficient soil moisture to be available on the site of recruitment

Effects of climate and land-use change on establishment and growth of cembran pine (Pinus cembra L.) over the timberline-treeline ecotone in the Central Swiss Alps

Tree growth is generally limited by temperature in cold climates and by water availability in and zones. Establishment in altitudinal treeline ecotones depends on the temperature, but may be very sensitive to water availability as well. We studied the effect of climate and land use on the colonization and growth of Pinus cembra in the treeline ecotone of the dry Central Swiss Alps; one site was influenced by timber harvest and cattle activity and another one was undisturbed. Stands were sampled at three elevations: in the forest and the lower and upper parts of the treeline ecotone. The age structure was similar in all sites, ranging from uneven-aged (forest) to more even-aged, with recent densitication and upslope expansion of the treeline ecotone. However, recruitment started at the treeline around 1850 (the end of the Little Ice Age) in the undisturbed site, simultaneously with an increase of tree-ring growth, but about 60 years later at the other site, after cattle grazing decreased. These results, and the positive correlation of radial growth with summer and previous autumn temperatures indicated that, in this altitudinal treeline ecotone, growth and establishment are mainly linked to temperature. However, drought stress was visible in the lowest stands, with a positive correlation of growth with rainfall during the previous autumn and December, and in August of the growing season. This could limit growth in a future warmer climate

Land cover classification of treeline ecotones along a 1100 km latitudinal transect using spectral- and three-dimensional information from UAV-based aerial imagery

The alpine treeline ecotone is expected to move upwards in elevation with global warming. Thus, mapping treeline ecotones is crucial in monitoring potential changes. Previous remote sensing studies have focused on the usage of satellites and aircrafts for mapping the treeline ecotone. However, treeline ecotones can be highly heterogenous, and thus the use of imagery with higher spatial resolution should be investigated. We evaluate the potential of using unmanned aerial vehicles (UAVs) for the collection of ultra-high spatial resolution imagery for mapping treeline ecotone land covers. We acquired imagery and field reference data from 32 treeline ecotone sites along a 1100 km latitudinal gradient in Norway (60–69°N). Before classification, we performed a superpixel segmentation of the UAV-derived orthomosaics and assigned land cover classes to segments: rock, water, snow, shadow, wetland, tree-covered area and five classes within the ridge-snowbed gradient. We calculated features providing spectral, textural, three-dimensional vegetation structure, topographical and shape information for the classification. To evaluate the influence of acquisition time during the growing season and geographical variations, we performed four sets of classifications: global, seasonal-based, geographical regional-based and seasonal-regional-based. We found no differences in overall accuracy (OA) between the different classifications, and the global model with observations irrespective of data acquisition timing and geographical region had an OA of 73%. When accounting for similarities between closely related classes along the ridge-snowbed gradient, the accuracy increased to 92.6%. We found spectral features related to visible, red-edge and near-infrared bands to be the most important to predict treeline ecotone land cover classes. Our results show that the use of UAVs is efficient in mapping treeline ecotones, and that data can be acquired irrespective of timing within a growing season and geographical region to get accurate land cover maps. This can overcome constraints of a short field-season or low-resolution remote sensing data.publishedVersio

Factors Structuring Treeline Dynamics of the Nepal Himalaya

The alpine treeline ecotone is an important component of mountain ecosystems of the Nepal Himalayas; it plays a vital role in the livelihood of indigenous people and provides ecosystem services. However, the region faces a problem of paucity of data on treeline characteristics at the regional, landscape, and local scales. Therefore, I applied remote sensing and geographic information science approaches to investigate the treeline ecotone at the regional (entire Nepal) and landscape (Barun and Manang Valleys) scales. Treeline elevation ranges from 3300–4300 m. Abies spectabilis, Betula utilis, and Pinus wallichiana are the main treeline-forming species in the Nepal Himalayas. There is an east to west treeline elevation gradient at the regional scale. No slope exposure is observed at the regional scale; however, at the landscape scale, slope exposure is present only in a disturbed area. From the landscape scale study, I found that topography and human disturbance are the main treeline-controlling factors in Barun and Manang, respectively. Diverse treeline-forming species and treeline nature observed in the landscape and regional scale study suggested more investigation was needed at the local scale. Therefore, I established two transects of 20 m width and 120 m length (100 m above and 20 m below the forestline) in the Betula utilis sub-alpine forest of the Dhorpatan Hunting Reserve in western Nepal to understand the local scale treeline dynamics. Poor regeneration was observed above the forestline in both transects compared to below the forestline. Low regeneration at the treeline ecotone suggested site-specific biotic and abiotic controlling factors. Seedling and sapling establishment above the forestline is limited by a lack of moisture, an absence of suitable microsites, and the presence of herbivores. I found the treeline stable at the local scale. I used the Maxent species distribution modeling approach to predict the likelihood of treeline advance in the Nepal Himalayas by modeling the habitat suitability of three dominant treeline species—A. spectabilis, B. utilis, and P. wallichiana—under present and alternative future climates. Temperature-related climatic variables and elevation explained the greatest amount of variance in the distribution of the study species. Under future climate models, I found a regional increase in habitat suitability of all three treeline species that predicted a potential for northward and upslope advance

Soil carbon stocks in different vegetation classes across the treeline ecotone in central- and southern Norway

The expansion of the treeline into the currently treeless alpine tundra may have considerable impacts on carbon storage. Recent studies have shown that the expansion of trees into the alpine tundra could lead to an increase in CO2 emissions, having positive feedback on global warming. However, few studies have assessed differences in carbon storage between vegetation classes in the forest-tundra ecotone. Here, I have done that by classifying the vegetation according to the Nature Types in Norway (NiN) classification system. Top organic soil was sampled in five different vegetation classes: forest, lee side, ridge, subxeric heath, and xeric heath along a 600-kilometer latitudinal gradient from the south to the middle part of Norway from late July to late August 2021. From the soil samples both carbon stock and carbon concentration were estimated in the lab, and statistical analyses were carried out to see if there were any differences in carbon stock and carbon concentration between the vegetation classes in the alpine tundra and treeline ecotone. To account for spatial dependencies caused by the sampling design (study sites and sample lines), a linear mixed effects model was used. Elevation was also included in the model to account for the possible effect of elevation on soil carbon storage. All vegetation classes found in the alpine tundra had larger soil carbon stock sizes and higher carbon concentration value than the vegetation class in the treeline ecotone, but not all were significantly different. Elevation had only a very minor influence on carbon concentration, and non-significant effect on carbon stock. Altogether, the results indicate that carbon stock and carbon concentration in the organic soil layer is larger for the vegetation classes in the alpine tundra than the vegetation class in the treeline. An expansion of the treeline could further lead to higher CO2 emissions and have positive feedback on global warming. The study illustrates how the expansion of trees into alpine tundra could affect the carbon storage in the top organic soil layer.En utvidelse av den nåværende tregrensen til den alpine tundraen kan få ytterligere påvirkninger på karbonlagring. Ny forskning viser at en utvidelse av trær til den alpine tundraen kan føre til høyere CO2 utslipp, og dermed gi en positiv tilbakemelding på global oppvarming. Da de fleste studier undersøker de mulige påvirkningene en økt tregrense vil få for karbonlagring for alle økosystemer i tundraen, er det ennå få studier som undersøker karbonlagring i de ulike vegetasjonsklassene vi finner i tregrensen og den alpine sonen. Denne studien undersøker eventuelle forskjeller i karbonlager og karbonkonsentrasjon mellom vegetasjonsklasser i tregrensen og den alpine tundraen etter kartleggingsverktøyet Natur i Norge (NiN). I studiet ble jordprøver fra det øvre organiske laget samlet inn. Totalt fem forskjellige vegetasjonsklasser i tregrenseøkotonen ble undersøkt: skog, leside, rabbe, lavhei, og lynghei langs en 600-kilometer transekt i Midt- og Sør-Norge. Feltarbeidet ble gjennomført fra sen juli til sen august i 2021. Fra jordprøvene ble både karbonlager og karbonkonsentrasjon estimert. Videre statistiske analyser ble gjennomført for å finne eventuelle forskjeller mellom karbonlager- og konsentrasjon for de forskjellige vegetasjonsklassene i tregrenseøkotonen. For å ta hensyn til om variasjon i karbonlager- og konsentrasjon ble forårsaket av prøvetakingsdesignet (studiested og lokasjon for hver prøve), ble en lineær «mixed model» brukt. Høydemeter over havet ble også inkludert for å undersøke mulige endringer i karbonlager- og konsentrasjon ved økt høydemeter. Alle vegetasjonsklasser i den alpine tundraen hadde et større karbonlager og en høyere verdi for karbonkonsentrasjon sammenlignet med vegetasjonsklassen i tregrensen, men ikke alle hadde en signifikant forskjell. En økning i høydemeter hadde ingen påvirkning for karbonlager, men svak tendens for lavere karbonkonsentrasjon ved økt høydemeter. I alt, indikerer resultatene på at karbonlager og karbonkonsentrasjon er høyere i alle vegetasjonsklasser over tregrensen. En utvidelse av tregrensen inn til den alpine sonen kan derfor føre til et høyere utslipp av CO2 og dermed gi en positiv tilbakemelding på global oppvarming. Studien illustrerer hvor viktig det er å undersøke hvilke konsekvenser en økt tregrense kan føre til for karbonlagring, og hvordan ulike vegetasjonsklasser har forskjellig karbonlagring i det organiske jordlageret.M-S

Fine-scale Topoclimate Modeling and Climatic Treeline Prediction of Great Basin Bristlecone Pine (Pinus longaeva) in the American Southwest

Great Basin bristlecone pine (Pinus longaeva) and foxtail pine (Pinus balfouriana) are valuable paleoclimate resources due to the climatic sensitivity of their annually-resolved rings. Recent treeline research has shown that growing season temperatures limit tree growth at and just below the upper treeline. In the Great Basin, the presence of precisely dated remnant wood above modern treeline shows that this ecotone shifts at centennial timescales tracking long-term changes in climate; in some areas during the Holocene climatic optimum treeline was 100 meters higher than at present. Such phenomena has motivated this analysis; regional treeline position models built exclusively from climate data may identify characteristics specific to Great Basin treelines and inform future physiological studies, and provide a measure of climate sensitivity specific to bristlecone and foxtail pine treelines. This study implements a topoclimatic analysis—using topographic position to explain patterns in surface temperatures across complex mountainous terrain—to model treeline position of three semi-arid bristlecone and/or foxtail pine treelines in the Great Basin as a function of topographically modified climate variables calculated from in situ measurements. Results indicate: (1) the treelines used in this study require a growing season length of between 143 - 152 days and average temperature ranging from 5.5 - 7.6 °C, (2) site-specific treeline position models may be improved through topoclimatic analysis—specifically the inclusion of an integrated measure of climate rather than a growing season isotherm measured in degrees, (3) treeline position in the Great Basin is likely out of equilibrium with the current climate indicating a potential shift in the primary growth-limiting factor at the highest elevations where trees are found

Mapping alpine treeline with high resolution imagery and LiDAR data in North Cascades National Park, Washington

We evaluated several approaches for the automated detection and mapping of trees and treeline in an alpine environment. Using multiple remote sensing platforms and software programs, we evaluated both pixel-based and object-based classification approaches in combination with high-resolution multispectral imagery and LiDAR-derived tree height data. The study area in North Cascades National Park included over 10,000 hectares of some of the most rugged terrain in the conterminous U.S. Through the use of the Normalized Difference Vegetation Index (NDVI), differences in illumination conditions created by steep slopes and tall trees were minimized. Data fusion of the multispectral imagery, NDVI, and LiDAR-derived tree height data produced the highest percent accuracies using both the pixel-based (88.4%) and the object-based classifications (92.9%). These results demonstrate that either method will produce an acceptable level of accuracy, and that the availability of a near-infrared band to calculate NDVI is extremely important. The NDVI used in conjunction with the multispectral imagery helped to minimize issues with shadows caused by rugged terrain. Furthermore, LiDAR-derived tree heights were used to augment classification routines to achieve even greater accuracy; where shadows were too dark to produce meaningful NDVI values, the LiDAR-derived tree height data was instrumental in helping to distinguish trees from other land cover types. Both the pixel-based and the object-based approaches hold considerable promise for automated mapping and monitoring of the treeline ecotone; however, the pixel-based approach may be superior because it is more straightforward and easily replicable compared to the object-based approach. These treeline mapping efforts will enhance future ecological treeline research by producing more accurate detections of trees and estimations of treeline position, and will be instrumental in building time series of imagery for future scientists conducting change detection studies at treeline

Accelerating upward treeline shift in the Altai Mountains under last-century climate change

Treeline shift and tree growth often respond to climatic changes and it is critical to identify and quantify their dynamics. Some regions are particularly sensitive to climate change and the Altai Mountains, located in Central and East Asia, are showing unequivocal signs. The mean annual temperature in the area has increased by 1.3–1.7 °C in the last century. As this mountain range has ancient and protected forests on alpine slopes, we focus on determining the treeline structure and dynamics. We integrated in situ fine-scale allometric data with analyses from dendrochronological samples, high-resolution 3D drone photos and new satellite images to study the dynamics and underlying causal mechanisms of any treeline movement and growth changes in a remote preserved forest at the Aktru Research Station in the Altai Mountain. We show that temperature increase has a negative effect on mountain tree growth. In contrast, only younger trees grow at higher altitudes and we document a relatively fast upward shift of the treeline. During the last 52 years, treeline moved about 150 m upward and the rate of movement accelerated until recently. Before the 1950s, it never shifted over 2150–2200 m a.s.l. We suggest that a continuous upward expansion of the treeline would be at the expense of meadow and shrub species and radically change this high-mountain ecosystem with its endemic flora. This documented treeline shift represents clear evidence of the increased velocity of climate change during the last century