Thermokarst acceleration in Arctic tundra driven by climate change and fire disturbance

Nitrogen (N2) fixation performed by moss-associated cyanobacteria is one of the main sources of new N in pristine, high latitude ecosystems like boreal forests and arctic tundra. Here, mosses and associated cyanobacteria can contribute more than 50% to total ecosystem N input. However, N2 fixation in mosses is strongly influenced by abiotic factors, in particular moisture and temperature. Hence, climate change will significantly affect this key ecosystem process in pristine ecosystems. Here,&#160;I will present a synthesis of several field and laboratory assessments of moss-associated N2 fixation in response to climate change by manipulating moisture and temperature in subarctic and arctic tundra.Both in a long-term climate warming experiment in the arctic, and along a continental climate gradient, spanning arctic, subarctic and temperate ecosystems, increased temperatures (up to 30 &#176;C) lead to either no effect or decreased N2 fixation rates in different moss species. Yet, N2 fixation rates were strongly dependent on moss-moisture, which seems to be a more important driver of N2 fixation in mosses than temperature.In another set of studies, two dominant moss species (Hylocomium splendens, Pleurozium schreberi) were collected from a steep precipitation gradient (400-1200 mm mean annual precipitation, MAP) in the Subarctic close to Abisko, Northern Sweden, and were incubated at different moisture and temperature levels in the laboratory. Nitrogen fixation, cyanobacterial abundance (via qPCR) and cyanobacterial community composition (via sequencing) on the mosses were assessed. Moisture and temperature interacted strongly to control moss-associated N2 fixation rates, and the highest activity was found at the wet end of the precipitation gradient. Although cyanobacterial abundance was higher in one of the investigated mosses (H. splendens), translating into higher N2 fixation rates, cyanobacterial community composition did not differ between the two moss species. Nostoc was the most common cyanobacterial genera on both mosses, and hardly any methanotrophic N2 fixing bacteria were found on the mosses along the precipitation gradient. Increased temperatures lead to increased abundances of certain cyanobacterial genera (Cylindrospermum and Nostoc), while others declined in response to warming. Hence, cyanobacterial communities colonizing mosses will be dominated by a few cyanobacteria species in a warmer climate, and temperature and moisture interact strongly to affect their activity. Thus, these two major climate change factors should be considered in unison when estimating climate change effects on key ecosystem processes such as N2 fixation. Further, host identity determines cyanobacterial abundance, and thereby, N2 fixation rates.&#160;&#160;&#160;

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Spatiotemporal patterns of tundra fires: late-Quaternary charcoal records from Alaska

Biogeosciences Discussions ◽

10.5194/bgd-12-3177-2015 ◽

2015 ◽

Vol 12 (3) ◽

pp. 3177-3209 ◽

Cited By ~ 3

Author(s):

M. L. Chipman ◽

V. Hudspith ◽

P. E. Higuera ◽

P. A. Duffy ◽

R. Kelly ◽

...

Keyword(s):

Climate Change ◽

Late Quaternary ◽

Rotation Period ◽

Arctic Tundra ◽

Ecosystem Processes ◽

Anthropogenic Climate Change ◽

The Arctic ◽

Spatial And Temporal Patterns ◽

The Past ◽

Tundra Ecosystem

Abstract. Anthropogenic climate change has altered many ecosystem processes in the Arctic tundra and may have resulted in unprecedented fire activity. Evaluating the significance of recent fires requires knowledge from the paleo-fire record because observational data in the Arctic span only several decades, much shorter than the natural fire rotation in Arctic tundra regions. Here we report results of charcoal analysis on lake sediments from four Alaskan lakes to infer the broad spatial and temporal patterns of tundra fire occurrence over the past 35 000 years. Background charcoal accumulation rates are low in all records (range = 0–0.05 pieces cm-2 year-1), suggesting minimal biomass burning across our study areas. Charcoal peak analysis reveals that the mean fire return interval (FRI; years between consecutive fire events) ranged from 1648 to 6045 years at our sites, and that the most recent fire events occurred from 882 to 7031 years ago, except for the CE 2007 Anaktuvuk River Fire. These mean FRI estimates are longer than the fire rotation periods estimated for the past 63 years in the areas surrounding three of the four study lakes. This result suggests that the frequency of tundra burning was higher over the recent past compared to the late Quaternary in some tundra regions. However, the ranges of FRI estimates from our paleo-fire records overlap with the expected values based on fire-rotation-period estimates from the observational fire data, and thus quantitative differences are not significant. Together with previous tundra-fire reconstructions, these data suggest that the rate of tundra burning was spatially variable and that fires were extremely rare in our study areas throughout the late Quaternary. Given the rarity of tundra burning over multiple millennia in our study areas and the pronounced effects of fire on tundra ecosystem processes such as carbon cycling, dramatic tundra ecosystem changes are expected if anthropogenic climate change leads to more frequent tundra fires.

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Species adaptation to both fire and climate change in tropical montane heath: Can Melaleuca uxorum (Myrtaceae) survive?

Pacific Conservation Biology ◽

10.1071/pc120319 ◽

2012 ◽

Vol 18 (4) ◽

pp. 319 ◽

Cited By ~ 1

Author(s):

Andrew J Ford ◽

Britta Denise Hardesty

Keyword(s):

Climate Change ◽

Soil Seed Bank ◽

Seed Storage ◽

Fire Disturbance ◽

Cloud Base ◽

Widespread Species ◽

History Of ◽

In Fire ◽

Local Burning ◽

Impacts Of Climate Change

Resprouting following fire is an effective and well utilized strategy for tropical montane heath species which have had a long evolutionary history of intermittent fire disturbance. Research conducted in both burnt and unburnt heath suggests that species richness is related to fire, however actual species presence is dependent upon local burning regimes. Taxa that persist in fire-adapted environments may survive through mechanisms including seed storage in the soil seed bank, resprouting from basal, axillary or epicormic buds, roots/rhizomes or terminal aerial buds and/or through migration of seed. We investigated the montane endemic Melaleuca uxorum’s response to fire to understand local adaptation and persistence to fire in fire prone heath and to understand potential impacts of climate change on montane heath ecosystems. We found that the species resprouts at the stem base, along stems from epicormic buds and from axillary buds. The species forms small colonies which appear to be a mixture of sexual and asexual (clonal) reproduction. We predict that the effects of climate change will conspire against tropical montane heath below 1000 m, and those communities away from maritime influences will be under threat of increasingly reduced population numbers and extent as the dry season cloud base is expected to rise in elevation with anticipated rising temperatures. Furthermore, as evaporation rates increase, such communities are anticipated to lose their local specialized flora and to be replaced by more common unspecialized, widespread species.

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