As plant species expand their upper limits of distribution under current warming, some retain both traditional climate space and biotic environment while others encounter novel conditions. The latter is the case for
Alpine treelines are expected to shift upward due to recent climate change. However, interpretation of changes in montane systems has been problematic because effects of climate change are frequently confounded with those of land use changes. The eastern Himalaya, particularly Langtang National Park, Central Nepal, has been relatively undisturbed for centuries and thus presents an opportunity for studying climate change impacts on alpine treeline uncontaminated by potential confounding factors. We studied two dominant species, All size classes of
- Award ID(s):
- 1915347
- NSF-PAR ID:
- 10449401
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Ecology and Evolution
- Volume:
- 10
- Issue:
- 3
- ISSN:
- 2045-7758
- Page Range / eLocation ID:
- p. 1209-1222
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Rhododendron campanulatum , a woody shrub that grows both above and below treeline at our study site in the Eastern Himalayas where a very conspicuous, stable treeline was defined by a nearly contiguous canopy of tallAbies spectabilis trees, many of which are over a century old. Prior work showed that treeline had remained static in this region whileR. campanulatum expanded its elevational range limit. We tested local adaptation ofR. campanulatum by performing reciprocal transplants between the species' current elevational range limit (4023 m above sea level [asl]) and just above treeline (3876 m asl). Contrary to expectation, the coldest temperatures of late winter and early mid‐spring were experienced by plants at the lower elevation:R. campanulatum at species' limit (upper site) were covered by snow for a longer period (40 more days) and escaped the coldest temperatures suffered by conspecifics at treeline (lower site). The harsher spring conditions at treeline likely explain why leaves were smaller at treeline (15.3 cm2) than at species limit (21.3 cm2). Contrary to results from equivalent studies in other regions, survival was reduced more by downslope than by upslope movement, again potentially due to extreme cold temperatures observed at treeline in spring. Upslope transplantation had no effect on mortality, but mortality of species limit saplings transplanted downslope was three times higher than that of residents at both sites. A general expectation is that locals should survive better than foreign transplants, but survival of locals and immigrants at our species limit site was identical. However, those species limit saplings that survived the transplant to treeline grew faster than both locals at treeline and the transplants at species limit. Overall, we found asymmetric adaptation: Compared with treeline saplings, those at species limit (147 m above treeline) were more tolerant of extremes in the growing season but less tolerant of extremes in winter and early mid‐spring, displaying local adaptation in a more complex manner than simply home advantage, and complicating predictions about impacts of future regional climate change. -
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Abstract Given the current rates of climate change, with associated shifts in herbivore population densities, understanding the role of different herbivores in ecosystem functioning is critical for predicting ecosystem responses. Here, we examined how migratory geese and resident, non‐migratory reindeer—two dominating yet functionally contrasting herbivores—control vegetation and ecosystem processes in rapidly warming Arctic tundra.
We collected vegetation and ecosystem carbon (C) flux data at peak plant growing season in the two longest running, fully replicated herbivore removal experiments found in high‐Arctic Svalbard. Experiments had been set up independently in wet habitat utilised by barnacle geese
Branta leucopsis in summer and in moist‐to‐dry habitat utilised by wild reindeerRangifer tarandus platyrhynchus year‐round.Excluding geese induced vegetation state transitions from heavily grazed, moss‐dominated (only 4 g m−2of live above‐ground vascular plant biomass) to ungrazed, graminoid‐dominated (60 g m−2after 4‐year exclusion) and horsetail‐dominated (150 g m−2after 15‐year exclusion) tundra. This caused large increases in vegetation C and nitrogen (N) pools, dead biomass and moss‐layer depth. Alterations in plant N concentration and CN ratio suggest overall slower plant community nutrient dynamics in the short‐term (4‐year) absence of geese. Long‐term (15‐year) goose removal quadrupled net ecosystem C sequestration (NEE) by increasing ecosystem photosynthesis more than ecosystem respiration (ER).
Excluding reindeer for 21 years also produced detectable increases in live above‐ground vascular plant biomass (from 50 to 80 g m−2; without promoting vegetation state shifts), as well as in vegetation C and N pools, dead biomass, moss‐layer depth and ER. Yet, reindeer removal did not alter the chemistry of plants and soil or NEE.
Synthesis . Although both herbivores were key drivers of ecosystem structure and function, the control exerted by geese in their main habitat (wet tundra) was much more pronounced than that exerted by reindeer in their main habitat (moist‐to‐dry tundra). Importantly, these herbivore effects are scale dependent, because geese are more spatially concentrated and thereby affect a smaller portion of the tundra landscape compared to reindeer. Our results highlight the substantial heterogeneity in how herbivores shape tundra vegetation and ecosystem processes, with implications for ongoing environmental change. -
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Abstract Climate change is creating phenological mismatches between herbivores and their plant resources throughout the Arctic. While advancing growing seasons and changing arrival times of migratory herbivores can have consequences for herbivores and forage quality, developing mismatches could also influence other traits of plants, such as above‐ and below‐ground biomass and the type of reproduction, that are often not investigated.
In coastal western Alaska, we conducted a 3‐year factorial experiment that simulated scenarios of phenological mismatch by manipulating the start of the growing season (3 weeks early and ambient) and grazing times (3 weeks early, typical, 3 weeks late, or no‐grazing) of Pacific black brant (
Branta bernicla nigricans ), to examine how the timing of these events influence a primary goose forage species,Carex subspathacea .After 3 years, an advanced growing season compared to a typical growing season increased stem heights, standing dead biomass, and the number of inflorescences. Early season grazing compared to typical season grazing reduced above‐ and below‐ground biomass, stem height, and the number of tillers; while late season grazing increased the number of inflorescences and standing dead biomass. Therefore, an advanced growing season and late grazing had similar directional effects on most plant traits, but a 3‐week delay in grazing had an impact on traits 3–5 times greater than a similarly timed shift in the advancement of spring. In addition, changes in response to treatments for some variables, such as the number of inflorescences, were not measurable until the second year of the experiment, while other variables, such as root productivity and number of tillers, changed the direction of their responses to treatments over time.
Synthesis . Factors affecting the timing of migration have a larger influence than earlier springs on an important forage species in the breeding and rearing habitats of Pacific black brant. The phenological mismatch prediction for this site of earlier springs and later goose arrival will likely increase above‐ and below‐ground biomass and sexual reproduction of the often‐clonally reproducingC. subspathacea . Finally, the implications of mismatch may be difficult to predict because some variables required successive years of mismatch to respond. -
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Abstract Aim Alpine treeline ecotones are influenced by environmental drivers and are anticipated to shift their locations in response to changing climate. Our goal was to determine the extent of recent climate‐induced treeline advance in the northeastern United States, and we hypothesized that treelines have advanced upslope in complex ways depending on treeline structure and environmental conditions.
Location White Mountain National Forest (New Hampshire) and Baxter State Park (Maine), USA.
Taxon High‐elevation tree species—
Abies balsamea, Picea mariana andBetula cordata. Methods We compared current and historical high‐resolution aerial imagery to quantify the advance of treelines over the last four decades, and link treeline changes to treeline form (demography) and environmental drivers. Spatial analyses of the aerial images were coupled with ground surveys of forest vegetation and topographical features to ground‐truth treeline classification and provide information on treeline demography and additional potential drivers of treeline locations. We used multiple linear regression models to examine the importance of both topographic and climatic variables on treeline advance.
Results Regional treelines have significantly shifted upslope over the past several decades (on average by 3 m/decade). Gradual diffuse treelines (characterized by declining tree density) showed significantly greater upslope shifts (5 m/decade) compared to other treeline forms, suggesting that both climate warming and treeline demography are important correlates of treeline shifts. Topographical features (slope, aspect) as well as climate (accumulated growing degree days, AGDD) explained significant variation in the magnitude of treeline advance (
R 2 = 0.32).Main Conclusions The observed advance of treelines is consistent with the hypothesis that climate warming induces upslope treeline shifts. Overall, our findings suggest that gradual diffuse treelines at high elevations may be indicative of climate warming more than other alpine treeline ecotones and thus they can inform us about past and ongoing climatic changes.
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Abstract Understanding how climate warming will affect the demographic rates of different ecotypes is critical to predicting shifts in species distributions. Here, we present results from a common garden, climate change experiment in which we measured seedling recruitment of lodgepole pine, a widespread North American conifer that is also planted globally. Seeds from a low‐elevation provenance had more than three‐fold greater recruitment to their third year than seeds from a high‐elevation provenance across sites within and above its native elevation range and across climate manipulations. Heating halved recruitment to the third year of both low‐ and high‐elevation seed sources across the elevation gradient, while watering more than doubled recruitment, alleviating some of the negative effects of heating. Demographic models based on recruitment data from the climate manipulations and long‐term observations of adult populations revealed that heating could effectively halt modeled upslope range expansion except when combined with watering. Simulating fire and rapid postfire forest recovery at lower elevations accelerated lodgepole pine expansion into the alpine, but did not alter final abundance rankings among climate scenarios. Regardless of climate scenario, greater recruitment of low‐elevation seeds compensated for longer dispersal distances to treeline, assuming colonization was allowed to proceed over multiple centuries. Our results show that ecotypes from lower elevations within a species’ range could enhance recruitment and facilitate upslope range shifts with climate change.
