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Treeline Ecotone Dynamics

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A special issue of Forests (ISSN 1999-4907). This special issue belongs to the section "Forest Ecophysiology and Biology".

Deadline for manuscript submissions: closed (20 September 2017) | Viewed by 50324

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Dr. Constance Millar

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USDA Forest Service, Pacific Southwest Research Station, 800 Buchanan St, Albany, CA 94710, USA

Special Issue Information

Dear Colleagues,

Alpine and arctic treelines are thermally-influenced boundaries between forested communities and more cold-tolerant, non-arboreal vegetation. Treeline research has been advancing rapidly, motivated in part by the need to predict land surface feedbacks to regional and global climate change, water resources, and impacts of environmental change on high latitude and high elevation ecosystems. Concern about loss of mountain biodiversity under global warming makes treeline a worldwide focus for regime shifts. Treeline ecotones are structured by complex interactions among vegetation, soils, climate, snow, topography, and disturbance regimes. They are assumed to be sensitive to climate change, but decadal to millennial responses are complex, with poorly understood lags and feedbacks. This issue seeks contributions to improve basic understanding of treeline dynamics under conditions of climate change, focusing on the role of ecological and systems theory to interpret ecotone processes, including seedling establishment and recruitment, demography and population ecology, mature forest growth responses, and interactions of treeline ecotones to disturbance and climate variability. Additionally invited are manuscripts that elaborate implications for biodiversity conservation and resource adaptation under changing climates. Submissions are encouraged that reflect treeline studies from mountain and arctic regions worldwide, with the objective to bridge the historic divide between arctic and alpine treeline research communities.

Dr. Constance Millar
Guest Editor

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Keywords

Ecotones
Alpine
Subalpine
Treeline
Climate change
Climate adaptation
Mountain ecosystems
Arctic ecosystems
Spatial phase transitions
Gradient analysis

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Published Papers (9 papers)

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Research

4975 KiB

Open AccessArticle

Responses of Tree Seedlings near the Alpine Treeline to Delayed Snowmelt and Reduced Sky Exposure

by Maaike Y. Bader, Hannah Loranger, Gerhard Zotz and Glenda Mendieta-Leiva

Forests 2018, 9(1), 12; https://doi.org/10.3390/f9010012 - 25 Dec 2017

Cited by 8 | Viewed by 5348

Abstract

Earlier snowmelt changes spring stress exposure and growing-season length, possibly causing shifts in plant species dominance. If such shifts involve trees, this may lead to changes in treeline position. We hypothesized that earlier snowmelt would negatively affect the performance of tree seedlings near the treeline due to higher spring stress levels, but less so if seedlings were protected from the main stress factors of night frosts and excess solar radiation. We exposed seedlings of five European treeline tree species: Larix decidua, Picea abies, Pinus cembra, Pinus uncinata, and Sorbus aucuparia to two snow-cover treatments (early and late melting, with about two weeks difference) combined with reduced sky exposure during the day (shading) or night (night warming), repeated in two years, at a site about 200 m below the regional treeline elevation. Physiological stress levels (as indicated by lower Fv/Fm) in the first weeks after emergence from snow were higher in early-emerging seedlings. As expected, shade reduced stress, but contrary to expectation, night warming did not. However, early- and late-emerging seedlings did not differ overall in their growth or survival, and the interaction with shading was inconsistent between years. Overall, shading had the strongest effect, decreasing stress levels and mortality (in the early-emerging seedlings only), but also growth. A two-week difference in snow-cover duration did not strongly affect the seedlings, although even smaller differences have been shown to affect productivity in alpine and arctic tundra vegetation. Still, snowmelt timing cannot be discarded as important for regeneration in subalpine conditions, because (1) it is likely more critical in very snow-rich or snow-poor mountains or landscape positions; and (2) it can change (sub)alpine vegetation phenology and productivity, thereby affecting plant interactions, an aspect that should be considered in future studies. Full article

(This article belongs to the Special Issue Treeline Ecotone Dynamics)

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5004 KiB

Open AccessFeature PaperArticle

Site- and Species-Specific Influences on Sub-Alpine Conifer Growth in Mt. Rainier National Park, USA

by Myesa Legendre-Fixx, Leander D. L. Anderegg, Ailene K. Ettinger and Janneke HilleRisLambers

Forests 2018, 9(1), 1; https://doi.org/10.3390/f9010001 - 22 Dec 2017

Cited by 6 | Viewed by 5763

Abstract

Identifying the factors that influence the climate sensitivity of treeline species is critical to understanding carbon sequestration, forest dynamics, and conservation in high elevation forest/meadow ecotones. Using tree cores from four sub-alpine conifer species collected from three sides of Mt. Rainier, WA, USA, we investigated the influences of species identity and sites with different local climates on radial growth–climate relationships. We created chronologies for each species at each site, determined influential plant-relevant annual and seasonal climatic variables influencing growth, and investigated how the strength of climate sensitivity varied across species and location. Overall, similar climate variables constrained growth on all three sides of the mountain for each of the four study species. Summer warmth positively influenced radial growth, whereas snow, spring warmth, previous summer warmth, and spring humidity negatively influenced growth. We discovered only a few subtle differences in the climate sensitivity of co-occurring species at the same site and between the same species at different sites in pairwise comparisons. A model including species by climate interactions provided the best balance between parsimony and fit, but did not lead to substantially greater predictive power relative to a model without site or species interactions. Our results imply that at treeline in moist temperate regions like Mt. Rainier, the same climatic variables drive annual variation in growth across species and locations, despite species differences in physiology and site differences in mean climates. Full article

(This article belongs to the Special Issue Treeline Ecotone Dynamics)

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1519 KiB

Open AccessArticle

Lab and Field Warming Similarly Advance Germination Date and Limit Germination Rate for High and Low Elevation Provenances of Two Widespread Subalpine Conifers

by Lara M. Kueppers, Akasha Faist, Scott Ferrenberg, Cristina Castanha, Erin Conlisk and Jennifer Wolf

Forests 2017, 8(11), 433; https://doi.org/10.3390/f8110433 - 11 Nov 2017

Cited by 19 | Viewed by 4703

Abstract

Accurately predicting upslope shifts in subalpine tree ranges with warming requires understanding how future forest populations will be affected by climate change, as these are the seed sources for new tree line and alpine populations. Early life history stages are particularly sensitive to climate and are also influenced by genetic variation among populations. We tested the climate sensitivity of germination and initial development for two widely distributed subalpine conifers, using controlled-environment growth chambers with one temperature regime from subalpine forest in the Colorado Rocky Mountains and one 5 °C warmer, and two soil moisture levels. We tracked germination rate and timing, rate of seedling development, and seedling morphology for two seed provenances separated by ~300 m elevation. Warming advanced germination timing and initial seedling development by a total of ~2 weeks, advances comparable to mean differences between provenances. Advances were similar for both provenances and species; however, warming reduced the overall germination rate, as did low soil moisture, only for Picea engelmannii. A three-year field warming and watering experiment planted with the same species and provenances yielded responses qualitatively consistent with the lab trials. Together these experiments indicate that in a warmer, drier climate, P. engelmannii germination, and thus regeneration, could decline, which could lead to declining subalpine forest populations, while Pinus flexilis forest populations could remain robust as a seed source for upslope range shifts. Full article

(This article belongs to the Special Issue Treeline Ecotone Dynamics)

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2309 KiB

Open AccessArticle

Soil Preferences in Germination and Survival of Limber Pine in the Great Basin White Mountains

by Brian V. Smithers

Forests 2017, 8(11), 423; https://doi.org/10.3390/f8110423 - 5 Nov 2017

Cited by 10 | Viewed by 4080

Abstract

In the Great Basin, limber pine is a sub-alpine tree species that is colonizing newly available habitat above treeline in greater numbers than treeline-dominating Great Basin bristlecone pine, especially on dolomite soil, where few plants are able to grow and where limber pine adults are rare. To examine the role of soil type on germination and establishment of limber pine, I sowed limber pine seeds in containers of the three main White Mountains soil types in one location while measuring soil moisture and temperature. I found that dolomite soil retains water longer, and has higher soil water content, than quartzite and granite soils and has the coolest maximum growing season temperatures. Limber pine germination and survival were highest in dolomite soil relative to quartzite and granite where limber pine adults are more common. While adult limber pines are rare on dolomite soils, young limber pines appear to prefer them. This indicates that limber pine either has only recently been able to survive in treeline climate on dolomite or that bristlecone pine has some long-term competitive advantage on dolomite making limber pine, a species with 1500 year old individuals, an early succession species in Great Basin sub-alpine forests. Full article

(This article belongs to the Special Issue Treeline Ecotone Dynamics)

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2579 KiB

Open AccessArticle

Growth and Its Relationship to Individual Genetic Diversity of Mountain Hemlock (Tsuga mertensiana) at Alpine Treeline in Alaska: Combining Dendrochronology and Genomics

by Jeremy S. Johnson, Parveen K. Chhetri, Konstantin V. Krutovsky and David M. Cairns

Forests 2017, 8(11), 418; https://doi.org/10.3390/f8110418 - 2 Nov 2017

Cited by 10 | Viewed by 5472

Abstract

Globally, alpine treelines are characterized as temperature-limited environments with strong controls on tree growth. However, at local scales spatially heterogeneous environments generally have more variable impacts on individual patterns of tree growth. In addition to the landscape spatial heterogeneity there is local variability in individual tree genetic diversity (level of individual heterozygosity). It has been hypothesized that higher individual heterozygosity will result in more consistent patterns of growth. In this article, we combine genomics and dendrochronology to explore the relationship between individual genetic diversity and tree growth at a mountain hemlock (Tsuga mertensiana Bong. Carr) alpine treeline on the Kenai Peninsula, Alaska, USA. We correlated average observed individual heterozygosity with average tree-ring width and variance in tree-ring width within individuals to test the hypothesis that trees with higher individual heterozygosity will also have more consistent growth patterns, suggesting that they may be more resilient to climate and environmental fluctuations at the alpine treeline. Our results showed that there was no significant relationship between tree growth and individual heterozygosity. However, there was a significant positive relationship between average tree-ring width and variance in tree-ring width implying that overall, fast growing trees in stressful environments, such as the alpine treeline, grow unstably regardless of the level of individual heterozygosity. Full article

(This article belongs to the Special Issue Treeline Ecotone Dynamics)

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4455 KiB

Open AccessArticle

Population Spatial Dynamics of Larix potaninii in Alpine Treeline Ecotone in the Eastern Margin of the Tibetan Plateau, China

by Jia’nan Cui, Jihong Qin and Hui Sun

Forests 2017, 8(10), 356; https://doi.org/10.3390/f8100356 - 22 Sep 2017

Cited by 8 | Viewed by 4499

Abstract

The high-altitude treeline is known to be sensitive to climate variability, and is thus considered as a bio-monitoring indicator of climate change. However, our understanding of the population dynamics and the cumulative climate-change effects on the alpine treeline ecotone in recent decades is limited. Here, we investigated the population dynamics of Larix potainii on the south- and north-facing slopes in the alpine treeline ecotone in the eastern margin of the Tibetan Plateau, China, including treeline position, population density, and tree recruitment. Results showed that on both south- and north-facing slopes, the treeline did not show a significant advancement in the past four decades. The population was dominated by young individuals, which tend to be established in the lower areas. Larix, here, tends to be clustered, especially in the upper areas. However, population density increased dramatically only on north-facing slopes. Larix here suffer from the stressful environment, but the warmer winter due to climate warming could facilitate the vertical growth of seedlings and saplings. Aggregated spatial patterns also provide a positive feedback in ameliorating the harsh environment. The slope-climate-moisture interactions have a pronounced impact on tree recruitment, including snow-limited tree establishment on the north-facing slopes and moisture-limited tree establishment on the south-facing slopes. Full article

(This article belongs to the Special Issue Treeline Ecotone Dynamics)

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2623 KiB

Open AccessArticle

Modeled Shifts in Polylepis Species Ranges in the Andes from the Last Glacial Maximum to the Present

by Brian R. Zutta and Philip W. Rundel

Forests 2017, 8(7), 232; https://doi.org/10.3390/f8070232 - 29 Jun 2017

Cited by 27 | Viewed by 6638

Abstract

Polylepis woodlands, the dominant high-elevation woodland species of the Andes of South America, are an increasingly important focus of conservation and restoration efforts as a buffer to the regional effects of climate change. However, the natural extent of these woodlands before the arrival of human populations is still debated. One significant approach to this question is an assessment of the change in woodland extent from a hypothetical peak at the time of the Last Glacial Maximum (LGM) to the present where distributions have been altered by both Holocene climate oscillations and anthropogenic pressures of pre-Colombian and modern communities. LGM and present distributions for 21 Polylepis species were modeled using Maxent with environmental data obtained from the WorldClim database. Overall, potential woodland extent is 36% smaller today than at LGM, however a few species have experienced a projected increase in potential range of 180%. This has occurred at the interface of the southern Amazonian Basin with the Altiplano where Polylepis species richness is highest. Bioclimatically stable areas for each species averaged 20 ± 4% of the modeled range and mostly occurred in disjunct pockets from central Peru to northern Argentina and Chile. Full article

(This article belongs to the Special Issue Treeline Ecotone Dynamics)

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7166 KiB

Open AccessArticle

Tree Height-Diameter Relationships in the Alpine Treeline Ecotone Compared with Those in Closed Forests on Changbai Mountain, Northeastern China

by Xiaoyu Wang, Dapao Yu, Shoule Wang, Bernard J. Lewis, Wangming Zhou, Li Zhou, Limin Dai, Jing-Pin Lei and Mai-He Li

Forests 2017, 8(4), 132; https://doi.org/10.3390/f8040132 - 21 Apr 2017

Cited by 9 | Viewed by 6857

Abstract

Height-diameter relationship is one of the most important stature characteristics of trees. It will change with climatic conditions because height and diameter growth displays different sensitivities to climatic factors such as temperature. Detecting and understanding changes in the stature of trees growing along altitudinal gradients up to their upper limits can help us to better understand the adaptation strategy of trees under global warming conditions. On Changbai Mountain in northeastern China, height-diameter datasets were collected for 2723 Erman’s birch (Betula ermanii Cham.) in the alpine treeline ecotone in 2006 and 2013, and for 888 Erman’s birch, spruce (Picea jezoensis Siebold & Zucc. Carr.), larch (Larix olgensis A. Henry), and fir (Abies nephrolepis Trautv. ex Maxim.) along an altitudinal gradient below the alpine treeline in 2006. These datasets were utilized to explore both changes in the stature of birch at the alpine treeline over time and variations in tree stature of different tree species across altitudes at a given time point (2006). Results showed that birch saplings (<140 cm in height) became stunted while birches with a height of >140 cm became more tapered in the alpine treeline ecotone. The stature of birch along the altitudinal gradient became more tapered from 1700 to 1900 m above see level (a.s.l.) and then became more stunted from 1900 to 2050 m a.s.l., with 1900 m a.s.l. being the altitudinal inflection point in this pattern. The treeline birch, due to its great temperature magnitude of distribution, displayed higher stature-plasticity in terms of its height-diameter ratio than the lower elevation species studied. The stature of birch is strongly modulated by altitude-related temperature but also co-influenced by other environmental factors such as soil depth and available water, wind speed, and duration and depth of winter snow cover. The high stature-plasticity of birch makes it fare better than other species to resist and adapt to, as well as to survive and develop in the harsh alpine environment. Full article

(This article belongs to the Special Issue Treeline Ecotone Dynamics)

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3402 KiB

Open AccessArticle

The Coupling of Treeline Elevation and Temperature is Mediated by Non-Thermal Factors on the Tibetan Plateau

by Yafeng Wang, Eryuan Liang, Shalik Ram Sigdel, Bo Liu and J. Julio Camarero

Forests 2017, 8(4), 109; https://doi.org/10.3390/f8040109 - 5 Apr 2017

Cited by 21 | Viewed by 6000

Abstract

Little is known about the relationships between treeline elevation and climate at regional and local scales. It is compelling to fill this research gap with data from the Tibetan Plateau where some of the highest alpine treelines in the world are found. This research question partially results from the lack of in situ temperature data at treeline sites. Herein, treeline variables (e.g., elevation, topography, tree species) and temperature data were collected from published investigations performed during this decade on the Tibetan Plateau. Temperature conditions near treeline sites were estimated using global databases and these estimates were corrected by using in situ air temperature measurements. Correlation analyses and generalized linear models were used to evaluate the effects of different variables on treeline elevation including thermal (growing-season air temperatures) and non-thermal (latitude, longitude, elevation, tree species, precipitation, radiation) factors. The commonality analysis model was applied to explore how several variables (July mean temperature, elevation of mountain peak, latitude) were related to treeline elevation. July mean temperature was the most significant predictor of treeline elevation, explaining 55% of the variance in treeline elevation across the Tibetan Plateau, whereas latitude, tree species, and mountain elevation (mass-elevation effect) explained 30% of the variance in treeline elevation. After considering the multicollinearity among predictors, July mean temperature (largely due to the influence of minimum temperature) still showed the strongest association with treeline elevation. We conclude that the coupling of treeline elevation and July temperature at a regional scale is modulated by non-thermal factors probably acting at local scales. Our results contribute towards explaining the decoupling between climate warming and treeline dynamics. Full article

(This article belongs to the Special Issue Treeline Ecotone Dynamics)

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