Alpine Treeline Research Articles

Experimental evidence for a thermal limitation of plant cell wall lignification at the alpine treeline

In their recent article in Alpine Botany (133:63-67, 2023a), Körner et al. revisit the outcome of an interesting experiment from 2009 (Lenz et al., Plant Ecolog Divers 6:365–375, 2013). Although I appreciate the new focus on cell wall lignification, I disagree with their main conclusion. Rather than questioning the role cold temperatures play in cell wall lignification, the authors provide experimental evidence for a thermal threshold under which the secondary cell walls of mountain pines (Pinus uncinata) at the upper treeline in the Swiss Alps exhibit a reduced lignin content.

Role of LiDAR remote sensing in identifying physiognomic traits of alpine treeline: a global review

Role of LiDAR remote sensing in identifying physiognomic traits of alpine treeline: a global review

Windows of opportunity: a new tree-shrub dynamic at alpine treeline?

Windows of opportunity: a new tree-shrub dynamic at alpine treeline?

Mapping and Estimating Aboveground Biomass in an Alpine Treeline Ecotone under Model-Based Inference

Due to climate change, treelines are moving to higher elevations and latitudes. The estimation of biomass of trees and shrubs advancing into alpine areas is necessary for carbon reporting. Remotely sensed (RS) data have previously been utilised extensively for the estimation of forest variables such as tree height, volume, basal area, and aboveground biomass (AGB) in various forest types. Model-based inference is found to be efficient for the estimation of forest attributes using auxiliary RS data, and this study focused on testing model-based estimations of AGB in the treeline ecotone using an area-based approach. Shrubs (Salix spp., Betula nana) and trees (Betula pubescens ssp. czerepanovii, Sorbus aucuparia, Populus tremula, Pinus sylvestris, Picea abies) with heights up to about five meters constituted the AGB components. The study was carried out in a treeline ecotone in Hol, southern Norway, using field plots and point cloud data obtained from airborne laser scanning (ALS) and digital aerial photogrammetry (DAP). The field data were acquired for two different strata: tall and short vegetation. Two separate models for predicting the AGB were constructed for each stratum based on metrics calculated from ALS and DAP point clouds, respectively. From the stratified predictions, mean AGB was estimated for the entire study area. Despite the prediction models showing a weak fit, as indicated by their R2-values, the 95% CIs were relatively narrow, indicating adequate precision of the AGB estimates. No significant difference was found between the mean AGB estimates for the ALS and DAP models for either of the strata. Our results imply that RS data from ALS and DAP can be used for the estimation of AGB in treeline ecotones.

Temperature sensitivity of tree recruitment at alpine treelines increases along latitudinal gradients

The alpine treeline is highly sensitive to temperature change, and treelines worldwide have been rapidly responding to temperature warming by increasing in tree density. However, often inconsistent or even conflicting results, regarding the response of tree recruitment to temperature change across regions, still challenge our profound comprehension of warming‐induced treeline dynamics. Here, we quantified the temperature sensitivity of tree recruitment within alpine treeline ecotones (i.e. the strength of relationship between tree recruitment and temperature change), and tested whether and how the temperature sensitivity changed along latitudinal gradients by examining 124 alpine treelines across North America and Asia. Our analyses demonstrated that temperature changes could significantly influence tree recruitment at treeline ecotones and the sensitivity of tree recruitment to temperature change was significantly and positively associated with latitude, showing that tree recruitment was more sensitive to temperature change at high latitudes than at low latitudes. Our findings highlighted the heterogeneous responses of tree recruitment at alpine treelines to temperature changes across latitudinal gradients, which should be incorporated into assessment models to improve future projections of global warming impacts across alpine ecosystems.

Warming-induced phenological mismatch between trees and shrubs explains high-elevation forest expansion.

Despite the importance of species interaction in modulating the range shifts of plants, little is known about the responses of coexisting life forms to a warmer climate. Here, we combine long-term monitoring of cambial phenology in sympatric trees and shrubs at two treelines of the Tibetan Plateau, with a meta-analysis of ring-width series from 344 shrubs and 575 trees paired across 11 alpine treelines in the Northern Hemisphere. Under a spring warming of +1°C, xylem resumption advances by 2-4 days in trees, but delays by 3-8 days in shrubs. The divergent phenological response to warming was due to shrubs being 3.2 times more sensitive than trees to chilling accumulation. Warmer winters increased the thermal requirement for cambial reactivation in shrubs, leading to a delayed response to warmer springs. Our meta-analysis confirmed such a mechanism across continental scales. The warming-induced phenological mismatch may give a competitive advantage to trees over shrubs, which would provide a new explanation for increasing alpine treeline shifts under the context of climate change.

The composition of non-structural carbohydrates affects the respiration and morphology of tree fine roots in Japan Alps

Non-structural carbohydrates (NSC), such as soluble sugars and starch, are important drivers of physiological function and survival in woody plants. The soluble sugar and starch in the organs of woody plants might affect their resource acquisition, but little is known about their direct association with the fine root physiological function. Here, we aimed to quantify the storage of soluble sugar and starch in the fine roots of two woody species, found under the alpine treeline, namely, Maries' fir (Abies mariesii) and Erman's birch (Betula ermanii). We also set out to identify the relationship between each NSC and both fine root respiration and morphology of the plants. To this end, the concentrations of soluble sugar and starch, root respiration rate, and specific root length (SRL) were measured in three root diameter classes: <0.5, 0.5–1.0, and 1.0–2.0 mm. In both species, we found that the sugar concentration of <0.5 mm roots was lower than that in both 0.5–1.0 and 1.0–2.0 mm roots, whereas there were no clear differences in starch concentration among the three classes. Additionally, our results showed that the sugar concentration was negatively correlated with the root respiration rate and SRL, whereas starch concentration did not show any correlation with these factors. Altogether, these findings suggest that the soluble sugar in fine roots can act as a direct source of energy for the roots to aid soil resource acquisition, whereas the starch can serve as an indirect, longer-term storage reservoir. Such distinct differences in the storage and function of soluble sugar and starch in fine roots are likely to improve our understanding of the sink-size of each process in terms of carbon consumption for root respiration and root morphology for soil resource acquisition.

Loss of growth resilience towards the alpine shrubline

The loss of resilience and prolonged recovery times after extreme climate events can be used as early warning signals of impending tipping points or abrupt irreversible changes. However, evidence of such critical slowing down in growth series is still lacking in terrestrial ecosystems under harsh environmental conditions such as alpine shrublands. Alpine shrublines, at which shrublands shift into grasslands under climate thresholds, constitute unique settings to look for critical slowing down phenomena. On the northeastern Tibetan Plateau, we sampled Salix oritrepha Schneid. (willow shrubs) in three elevational sites along each of two transects (QL, Qilian; HSX, Huashixia) running from alpine treeline position to shrubline. We then quantified the radial growth resilience and recovery indices of alpine willow shrubs to cold spells along elevational gradients by using dendrochronology. Willow growth was primarily constrained by low winter temperatures in QL and low summer temperatures in HSX. Cold spell events were detected in 2008 and 2011 in HSX and QL sites, respectively. Because temperature decreases with elevation, both growth resilience and recovery indices differed within each transect, showing a decreasing trend with elevation. Growth in 78% and 56% willow shrubs recovered back to the level before cold spell within two years in QL and HSX sites, respectively. Moreover, the average growth recovery period was longer at the shrubline (ca. 2–4 years) than near the treeline position (ca. 1–2 years). Critical slowing down theory could thus be applied to assess growth resilience towards the alpine shrubline, a thermal threshold for shrub growth. Our results and framework highlight the priority to directly measure the resilience metrics and also offer a promising perspective that the critical slowing down phenomenon may be universal in various ecosystems.

Climate Overrides the Influence of Microsite Conditions on Radial Growth of the Tall Multi-Stemmed Shrub Alnus alnobetula at Treeline.

Green alder (Alnus alnobetula), a tall multi-stemmed deciduous shrub, is widespread at high elevations in the Central European Alps. Its growth form frequently leads to asymmetric radial growth and anomalous growth ring patterns, making development of representative ring-width series a challenge. In order to assess the variability among radii of one shoot, among shoots belonging to one stock and among stocks, 60 stem discs were sampled at treeline on Mt. Patscherkofel (Tyrol, Austria). Annual increments were measured along 188 radii and analyzed in terms of their variability by applying dendrochronological techniques. Results revealed a high agreement in ring-width variation among radii of one shoot, among shoots of one stock and largely among stocks from different sites, confirming the pronounced limitation of radial stem growth by climate forcing at the alpine treeline. In contrast to this, a high variability in both absolute growth rates and long-term growth trends was found, which we attribute to different microsite conditions and disturbances. These factors also override climate control of radial growth under growth-limiting environmental conditions. Based on our findings we provide recommendations for the number of samples needed to carry out inter- and intra-annual studies of radial growth in this multi-stemmed clonal shrub.

Foliar uptake of persistent organic pollutants at alpine treeline

Previous studies highlighted the role of temperature on the foliar uptake of persistent organic pollutants (POPs) based on their physicochemical properties. However, few studies have focused on the indirect impacts of low temperature on the foliar uptake of POPs due to the changed physiology of leaves. We measured the concentrations and temporal variations of foliar POPs at the treeline on the Tibetan Plateau, the highest-altitudinal treeline on Earth. The leaves at the treeline showed high uptake efficiencies and reservoir capacity of dichlorodiphenyltrichloroethanes (DDTs), which were two times to one order of magnitude higher than those in forests worldwide. Enhanced surface adsorption due to the increased thickness of the wax layer in a colder climate was found to be the primary contributor (>60 %) to the high uptake of DDTs at the treeline, and slow penetration controlled by temperature contributed 13 %−40 %. The relative humidity, related negatively to temperature, also influenced the uptake rates of DDTs by foliage at the treeline (contribution: <10 %). The uptake rates of small molecular-weight POPs (hexachlorobenzene and hexachlorocyclohexanes) by foliage at the treeline were quite lower than those of DDTs, relating probably with the weak penetration of these compounds into leaves and/or low-temperature-induced precipitation washout from leaf surface.

Asymmetric effects of daytime and nighttime warming on alpine treeline recruitment.

Trees at their upper range limits are highly sensitive to climate change, and thus alpine treelines worldwide have changed their recruitment patterns in response to climate warming. However, previous studies focused only on daily mean temperature, neglecting the asymmetric influences of daytime and nighttime warming on recruitments in alpine treelines. Here, based on the compiled dataset of tree recruitment series from 172 alpine treelines across the Northern Hemisphere, we quantified and compared the different effects of daytime and nighttime warming on treeline recruitment using four indices of temperature sensitivity, and assessed the responses of treeline recruitment to warming-induced drought stress. Our analyses demonstrated that even in different environmental regions, both daytime and nighttime warming could significantly promote treeline recruitment, and however, treeline recruitment was much more sensitive to nighttime warming than to daytime warming, which could be attributable to the presence of drought stress. The increasing drought stress primarily driven by daytime warming rather than by nighttime warming would likely constrain the responses of treeline recruitment to daytime warming. Our findings provided compelling evidence that nighttime warming rather than daytime warming could play a primary role in promoting the recruitment in alpine treelines, which was related to the daytime warming-induced drought stress. Thus, daytime and nighttime warming should be considered separately to improve future projections of global change impacts across alpine ecosystems.

Improvement in the delineation of alpine treeline in Uttarakhand using spaceborne light detection and ranging data

Advanced remote sensing technologies, such as light detection and ranging (LiDAR), offer significant potential to mapping the alpine treeline ecotone (ATE) based on its actual definition (tree height ≥ 3 m) and contribute to the generation of baseline data for future change detection investigations. We propose an approach for combining LiDAR-derived absolute tree height data with elevation data to delineate the ATE in Uttarakhand, India. The approach was implemented using observations from the recently launched Global Ecosystem Dynamics Investigation system and validated with field measurements. The LiDAR-derived treeline was compared with the traditional normalized difference vegetation index (NDVI) treeline. The treeline derived from LiDAR was found to have root mean square error of ∼60 m with respect to the ground verified treeline location. The NDVI treeline was overestimated in comparison to the LiDAR treeline by an average surface distance of 290, 232, 257, and 237 m in the south, north, west, and east aspects, respectively. It is observed that the overestimation was higher at the lowest and highest elevation zones. We prove that LiDAR-based treeline mapping is an efficient method to delineate alpine treelines at a landscape scale.

Plastic and adaptive response of carbon allocation to temperature change in alpine treeline trees

Understanding how tree individuals allocate nonstructural carbohydrate (NSC) between storage and growth is key to reveal the mechanism of alpine treeline formation as well as its response to climate warming. Yet, little is known about whether carbon allocation of treeline trees responds to temperature change by means of phenotypic plasticity and/or ecotypic adaptation. Here, we employed a reciprocal transplant experiment and a growth chamber warming treatment to explore the effects of temperature change on functional traits and carbon allocation between NSC storage and growth in treeline species Larix chinensis. In the reciprocal transplant experiment, we observed that the individuals grown at high elevation always displayed relatively lower specific leaf area (SLA) than those grown at lower elevations irrespective of species provenance, demonstrating the phenotypic plasticity of functional traits in relation to elevation. Higher growth rate and late-season NSC concentration in the individuals at their provenance elevation than at the elevations away from home, indicated that trees were locally adapted to their home environments in terms of NSC storage and growth. Additionally, individuals from high-elevation provenance showed considerably less phenotypic plasticity in carbon allocation (i.e., the ratio of NSC storage to growth) in response to temperature change compared to those from low-elevation provenance. In the chamber experiment, warming greatly accelerated shoot growth but not accompanied by a down-regulation in NSC storage in cold-edge trees, resulting in a lower ratio of NSC storage to growth. It was noteworthy that there was a time lag in variations of carbon allocation, especially in variations of NSC storage when cold-edge trees were grown in warmed climates. Our findings demonstrate that the responses of NSC storage and growth to temperature change might be a combination of short-term plastic and long-term adaptive reactions, and suggest a limited response of cold-edge populations in carbon allocation to climate warming.

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

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

High-frequency depositional sequence of the upper Quaternary deposits controlled by sediment supply: A case study of an inland basin in central Japan

Depositional sequences in inland basins are regulated by environmental changes, such as climate change, sediment supply, water input/output, and tectonics. Their mechanisms and driving factors are diverse in each geological setting. This study provides a short-term (millennial to ten thousand years timescales) overview of the sedimentologic evolution in an inland basin located at a high altitude and characterized by fast sedimentation rates (Suwa Basin, central Japan). We employed a multi-disciplinary approach combining sedimentology, paleopedology, and high-resolution sequence stratigraphic analysis of the upper Pleistocene and Holocene succession to clarify the driving factors of the basin's sedimentologic evolution. The depositional systems and systems tracts were divided into meandering fluvial (lowstand systems tract; 26.7–16.5 cal kyr BP), pond/marsh (early transgressive systems tract; 16.5–13.9 cal kyr BP), lacustrine (transgressive systems tract; 13.9–5.8 cal kyr BP), and delta systems (highstand systems tract; 5.8 cal kyr BP–). The boundary between the meandering fluvial and pond/marsh systems corresponds to a transgressive surface appearing as a rising in the lake level (ca. 16.5 cal kyr BP). The boundary between the lacustrine and delta systems corresponds to a maximum flooding surface of the lake (ca. 5.8 cal kyr BP). The lake transgression occurs simultaneously with a decrease in the sediment supply, as suggested by a decrease in the sedimentation rate, the dense development of paleosols, and high TOC and TN showing a lower decomposition rate. The timing coincides with the intensification of chemical weathering, plant succession, and the ascending alpine tree line in the source region during the Northern Hemisphere deglaciation. The depositional sequence in the interior basin might have been easily controlled by changes in the sediment supply due to vegetation/soil cover successions during the glacial-interglacial cycle. Our findings imply that low-frequency depositional sequences observed in such inland basins in the East Asian coastal region may have also been forced by global ice volume changes.

Physiological Characteristics and Cold Resistance of Five Woody Plants in Treeline Ecotone of Sygera Mountains

Investigating the distribution of internal physiological indicators and the cold resistance of woody plants in the alpine treeline ecotone is of great ecological importance to explain the mechanism of alpine treeline formation. Less research has been conducted on the cold resistance mechanisms of alpine treeline woody plants than on commercial crops. In this paper, five different tree species in the alpine treeline ecotone of the Sygera Mountains were used as the research objects and the leaves, branches, and roots of 19 woody plants were collected in the non-growing season (November) of 2019. Their non-structural carbohydrate content (soluble sugar and starch), malondialdehyde, hydrogen peroxide (H2O2), proline, superoxide dismutase, and peroxidase levels were measured. The contents of C, N, P, and K elements were analyzed, along with the distribution characteristics of physiological indices and organs of various woody plants and their relationship to plant nutrients. Results showed that the MDA (5.46 ± 1.95 μg·g−1) and H2O2 (4.11 ± 0.76 mmol·g−1) of tree root organs and the MDA (3.03 ± 2.05 μg·g−1) and H2O2 (4.25 ± 1.03 mmol·g−1) of shrub leaf organs were higher than those of other organs, indicating that under the stress of low temperatures, the root organ of arbor species and the leaf organ of shrub species experienced the most damage. Osmotic substances, particularly soluble sugars, play a crucial role in the response of the woody plants in Sygera Mountains to low-temperature stress. Plant nutrients could enhance plant stress resistance by further activating the activity of the antioxidant system and increasing the synthesis of osmotic substances. This study hypothesized that the stress on the root organs of the arbor species in the treeline ecotone may not be repaired in time, which may be a key mechanism for the formation of the alpine treeline in the Sygera Mountains.

Global patterns of mobile carbon partitioning in mountain trees in response to elevation

Non-structural carbohydrate (NSC), a major substrate of primary metabolism, has been implicated in mediating cold-induced termination of tree growth, particularly in the alpine environment. Despite its functional significance, NSC pattern as well as its partitioning between sugar and starch in response to elevation across the globe remains unclear. Here, we compiled data from 67 studies encompassing 90 elevational transects worldwide and conducted a meta-analysis to evaluate the responses of NSC (including sugar, starch, and total NSC concentrations) as well as sugar:starch ratio in mountain trees to increasing elevation at organ scale. Our results showed that declining temperature with increasing elevation promoted NSC accumulation in aboveground organs, but did not affect the sugar:starch ratio in all organs analysed, implying that the remarkably significant role of NSC in both short-term cellular metabolism and long-term recalcitrant storage. Trees grown within warmer mountain regions (i.e., regions with relatively higher mean temperature during the growing season) stored more NSC in leaf in response to elevation than those grown within colder mountain regions. We also found that the observed increasing pattern of NSC with elevation occurred only in treeline tree species but not in non-treeline trees (i.e., tree taxa that do not reach the alpine treeline). Additionally, a significant negative effect of elevation on NSC revealed an unbalanced carbon supply in the roots of angiosperms tree species, indicating that the growth inhibition of belowground organ with increasing elevation occurred mainly in angiosperms rather than in gymnosperms. Our findings suggest that a sugar-starch partitioning pattern, which remains essentially stable over a wide range of climates and functional type species, predominately modulates the balance of carbon assimilates between supply and demand in responding to cold stress in the mountain trees worldwide.

Monitoring tree occupancy and height in the Norwegian alpine treeline using a time series of airborne laser scanner data

The main objective of this study was to demonstrate a method for monitoring tree occupancy and height in the alpine treeline ecotone using a time series of ALS data. We applied data collected in a longitudinal survey, comprising three spatially consistent campaigns from the years 2008, 2012 and 2018, on 25 sites along the Scandinavian Mountain Range (60–69°N). We compared ALS-based estimates of tree occupancy and height with corresponding field-based estimates and provided ALS-based estimates of uncertainty. Cross validation of a longitudinal model for predicting tree occurrence probability from ALS data revealed an overall accuracy of 83 %. ALS data were useful for predicting the height of pioneer trees, despite sparse laser points. Both models needed to account for the time of measurement. ALS-based estimates of tree occupancy were 4.6, 6.7 and 6.0 % for the three measurement occasions, respectively, and corresponding field-based estimates were 4.3, 4.6 and 5.0 %. ALS-based estimates of tree height were 2.2, 2.1 and 2.2 m, respectively, and corresponding field-based height estimates were 2.3, 2.2 and 2.2 m. Overlapping confidence intervals of ALS-based estimates for both variables and for all three measurement occasions indicated no statistically significant changes in either of the studied variables. The proposed method can be used to monitor alpine treeline ecotones and to provide accompanying uncertainty estimates to inform whether changes are significant.

A new method for quantifying treeline-ecotone change based on multiple spatial pattern dimensions

ContextTreeline-ecotone spatial patterns and their dynamics reflect underlying processes. Changes in ecotone pattern may reflect changes in natural drivers or land-use practices. However, characterizing these dynamics presents a major challenge, limiting our ability to map, understand and predict changes in the upper limits of mountain forests.ObjectiveThis paper proposes a new method using multiple pattern dimensions to describe treeline-ecotone spatial pattern shifts. This standardized protocol should be able to (i) distinguish different types of treeline-ecotone patterns within a large study area, (ii) characterize temporal pattern shifts in spatial pattern between two or more dates.MethodWe mapped alpine treeline ecotones (ATE) at 648 sites in the eastern French Pyrenees using aerial images from ~ 1955 and ~ 2015, identifying forest and non-forest areas at the hillslope scale. Extracted patch metrics were summarized using a Principle Component Analysis (PCA) and spatial pattern change was quantified from the shift in the PCA space and compared to elevational shifts.ResultsThree clusters of patterns were distinguished: diffuse, discrete and island-forming ATEs. Between 1955 and 2015, about half of the sites changed from one pattern cluster to another. Shifts into discrete ATEs were associated with smaller and negative elevational shifts, while shifts into diffuse ATEs coincided with the highest positive elevational shifts.ConclusionThe proposed method allows a standardized and repeatable quantification of vegetation pattern change in alpine treeline ecotones based on historical aerial imagery. Seeing the importance of treeline-ecotone shifts for alpine biodiversity, we encourage the use of this protocol to better understand treeline dynamics at treelines globally.

Decomposing niche components reveals simultaneous effects of opposite deterministic processes structuring alpine small mammal assembly

IntroductionSpecies distribution in alpine areas is constrained by multiple abiotic and biotic stressors. This leads to discrepant assembly patterns between different locations and study objects as opposite niche-based processes—limiting similarity and habitat filtering—simultaneously structure communities, masking overall patterns. We aimed to address how these processes structure small mammal communities in the alpine tree line transition zone, one of the most distinct vegetation transitions between alpine and montane habitats.MethodsWe compiled a dataset of species checklist, phylogeny, and functional traits from field collection and published sources spanning 18 mountains in southwest China. We first examined hypothetical niche-based processes with frequently used phylogenetic and trait approaches using this dataset. The species traits were decomposed into different niche components to explore the respective effects of specific stressors. Indices representing evolutionary history, trait space, and pairwise species distance were estimated and compared with null model expectations. Linear mixed-effect models were used to assess the association patterns between diversity indices and elevation.ResultsThe results indicated that phylogenetic and functional richness were positively correlated with species richness. In contrast, distance-based indices were either negatively or weakly positively correlated with species richness. Null model analyses suggested no evidence of non-random phylogenetic or overall trait patterns. However, the resource acquisition niche tended to be more overdispersed (positive slopes), while the habitat affinity niche tended to be more clustered (negative slopes) beyond the high elevation tree line.DiscussionThese findings show that opposite niche-based processes simultaneously structure small mammal communities in alpine areas. Overall, the present study provides vital insights into the complexity of assembly processes in these habitats. It also highlights the importance of relating relevant traits to distinguish the influences of specific abiotic and biotic stressors.