Jason G. Vogel

Vulnerability of permafrost carbon to climate change: Implications for the global carbon cycle

ABSTRACT Thawing permafrost and the resulting microbial decomposition of previously frozen organic carbon (C) is one of the most significant potential feedbacks from terrestrial ecosystems to the atmosphere in a changing climate. In this article we present an overview of the global permafrost C pool and of the processes that might transfer this C into the atmosphere, as well as the associated ecosystem changes that occur with thawing. We show that accounting for C stored deep in the permafrost more than doubles previous high-latitude inventory estimates, with this new estimate equivalent to twice the atmospheric C pool. The thawing of permafrost with warming occurs both gradually and catastrophically, exposing organic C to microbial decomposition. Other aspects of ecosystem dynamics can be altered by climate change along with thawing permafrost, such as growing season length, plant growth rates and species composition, and ecosystem energy exchange. However, these processes do not appear to be able to compensate for C release from thawing permafrost, making it likely that the net effect of widespread permafrost thawing will be a positive feedback to a warming climate.

The effect of permafrost thaw on old carbon release and net carbon exchange from tundra

Permafrost soils in boreal and Arctic ecosystems store almost twice as much carbon as is currently present in the atmosphere. Permafrost thaw and the microbial decomposition of previously frozen organic carbon is considered one of the most likely positive climate feedbacks from terrestrial ecosystems to the atmosphere in a warmer world. The rate of carbon release from permafrost soils is highly uncertain, but it is crucial for predicting the strength and timing of this carbon-cycle feedback effect, and thus how important permafrost thaw will be for climate change this century and beyond. Sustained transfers of carbon to the atmosphere that could cause a significant positive feedback to climate change must come from old carbon, which forms the bulk of the permafrost carbon pool that accumulated over thousands of years. Here we measure net ecosystem carbon exchange and the radiocarbon age of ecosystem respiration in a tundra landscape undergoing permafrost thaw to determine the influence of old carbon loss on ecosystem carbon balance. We find that areas that thawed over the past 15 years had 40 per cent more annual losses of old carbon than minimally thawed areas, but had overall net ecosystem carbon uptake as increased plant growth offset these losses. In contrast, areas that thawed decades earlier lost even more old carbon, a 78 per cent increase over minimally thawed areas; this old carbon loss contributed to overall net ecosystem carbon release despite increased plant growth. Our data document significant losses of soil carbon with permafrost thaw that, over decadal timescales, overwhelms increased plant carbon uptake at rates that could make permafrost a large biospheric carbon source in a warmer world.

Carbon distribution and aboveground net primary production in aspen, jack pine, and black spruce stands in Saskatchewan and Manitoba, Canada

The objectives of this study are to (1) characterize the carbon (C) content, leaf area index, and aboveground net primary production (ANPP) for mature aspen, black spruce, and young and mature jack pine stands at the southern and northern Boreal Ecosystem-Atmosphere Study (BOREAS) areas and (2) compare net primary production and carbon allocation coefficients for the major boreal forest types of the world. Direct estimates of leaf area index, defined as one half of the total leaf surface area, range from a minimum of 1.8 for jack pine forests to a maximum of 5.6 for black spruce forests; stems comprise 5 to 15% of the total overstory plant area. In the BOREAS study, total ecosystem (vegetation plus detritus plus soil) carbon content is greatest in the black spruce forests (445,760–479,380 kg C ha−1), with 87 to 88% of the C in the soil, and is lowest in the jack pine stands (68,370–68,980 kg C ha−1) with a similar distribution of carbon in the vegetation and soil. Forest floor carbon content and mean residence time (MRT) also vary more among forest types in a study area than between study areas for a forest type; forest floor MRT range from 16 to 19 years for aspen stands to 28 to 39 years for jack pine stands. ANPP differs significantly among the mature forests at each of the BOREAS study areas, ranging from a maximum of 3490 to 3520 kg C ha−1 yr−1 for aspen stands to 1170 to 1220 kg C ha−1 yr−1 for jack pine stands. Both net primary production (NPP) and carbon allocation differ between boreal evergreen and deciduous forests in the world, suggesting global primary production models should distinguish between these two forest types. On average, 56% of NPP for boreal forests occurs as detritus and illustrates the need to better understand factors controlling aboveground and below-ground detritus production in boreal forests.

Plant Species Composition and Productivity following Permafrost Thaw and Thermokarst in Alaskan Tundra

Climate warming is expected to have a large impact on plant species composition and productivity in northern latitude ecosystems. Warming can affect vegetation communities directly through temperature effects on plant growth and indirectly through alteration of soil nutrient availability. In addition, warming can cause permafrost to thaw and thermokarst (ground subsidence) to develop, which can alter the structure of the ecosystem by altering hydrological patterns within a site. These multiple direct and indirect effects of permafrost thawing are difficult to simulate in experimental approaches that often manipulate only one or two factors. Here, we used a natural gradient approach with three sites to represent stages in the process of permafrost thawing and thermokarst. We found that vascular plant biomass shifted from graminoid-dominated tundra in the least disturbed site to shrub-dominated tundra at the oldest, most subsided site, whereas the intermediate site was co-dominated by both plant functional groups. Vascular plant productivity patterns followed the changes in biomass, whereas nonvascular moss productivity was especially important in the oldest, most subsided site. The coefficient of variation for soil moisture was higher in the oldest, most subsided site suggesting that in addition to more wet microsites, there were other microsites that were drier. Across all sites, graminoids preferred the cold, dry microsites whereas the moss and shrubs were associated with the warm, moist microsites. Total nitrogen contained in green plant biomass differed across sites, suggesting that there were increases in soil nitrogen availability where permafrost had thawed.

Tree mortality from an exceptional drought spanning mesic to semiarid ecoregions.

Significant areas of the southern USA periodically experience intense drought that can lead to episodic tree mortality events. Because drought tolerance varies among species and size of trees, such events can alter the structure and function of terrestrial ecosystem in ways that are difficult to detect with local data sets or solely with remote-sensing platforms. We investigated a widespread tree mortality event that resulted from the worst 1-year drought on record for the state of Texas, USA. The drought affected ecoregions spanning mesic to semiarid climate zones and provided a unique opportunity to test hypotheses related to how trees of varying genus and size were affected. The study was based on an extensive set of 599 distributed plots, each 0.16 ha, surveyed in the summer following the drought. In each plot, dead trees larger than 12.7 cm in diameter were counted, sized, and identified to the genus level. Estimates of total mortality were obtained for each of 10 regions using a combination of design-based estimators and calibrated remote sensing using MODIS 1-yr change in normalized difference vegetation index products developed by the U.S. Forest Service. As compared with most of the publicized extreme die-off events, this study documents relatively low rates of mortality occurring over a very large area. However, statewide, regional tree mortality was massive, with an estimated 6.2% of the live trees perishing, nearly nine times greater than normal annual mortality. Dead tree diameters averaged larger than the live trees for most ecoregions, and this trend was most pronounced in the wetter climate zones, suggesting a potential re-ordering of species dominance and downward trend in tree size that was specific to climatic regions. The net effect on carbon storage was estimated to be a redistribution of 24-30 Tg C from the live tree to dead tree carbon pool. The dead tree survey documented drought mortality in more than 29 genera across all regions, and surprisingly, drought resistant and sensitive species fared similarly in some regions. Both angiosperms and gymnosperms were affected. These results highlight that drought-driven mortality alters forest structure differently across climatic regions and genera.

Long-term effects of weed control and fertilization on the carbon and nitrogen pools of a slash and loblolly pine forest in north-central Florida

The effects of fertilization, weed control, and fertilization plus weed control on vegetation and soil C and N pools were examined for a loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii var. elliottii Engelm.) forest at ages 18 and 26 years (at the end of rotation). The total C accumulated in fertilized forests without weed control was 20% (slash pine) and 40% (loblolly pine) greater than in the control forests at the end of rotation. Weed control increased pine C pools at 18 years, but by the end of rotation, weed control effectively resulted in no gain in ecosystem C. When the two treatments were combined, weed control slightly subtracted from the net C benefit produced by fertilization. This result occurred because of decreased forest floor and soil C in the weed control plots. Fertilization significantly increased stem, foliage, forest floor, and soil N pools, and N retention was 63% and 103% of the applied N in the slash and loblolly pine forests, respectively. Weed control with fertilization reduced ecosystem N retention efficiency, but weed control alone did not negatively affect ecosystem N accumulation. These results suggest that the optimal treatment for increasing C accumu- lation and N retention in these ecosystems is fertilization without weed control.

Association of Soil Aggregation with the Distribution and Quality of Organic Carbon in Soil along an Elevation Gradient on Wuyi Mountain in China

Forest soils play a critical role in the sequestration of atmospheric CO2 and subsequent attenuation of global warming. The nature and properties of organic matter in soils have an influence on the sequestration of carbon. In this study, soils were collected from representative forestlands, including a subtropical evergreen broad-leaved forest (EBF), a coniferous forest (CF), a subalpine dwarf forest (DF), and alpine meadow (AM) along an elevation gradient on Wuyi Mountain, which is located in a subtropical area of southeastern China. These soil samples were analyzed in the laboratory to examine the distribution and speciation of organic carbon (OC) within different size fractions of water-stable soil aggregates, and subsequently to determine effects on carbon sequestration. Soil aggregation rate increased with increasing elevation. Soil aggregation rate, rather than soil temperature, moisture or clay content, showed the strongest correlation with OC in bulk soil, indicating soil structure was the critical factor in carbon sequestration of Wuyi Mountain. The content of coarse particulate organic matter fraction, rather than the silt and clay particles, represented OC stock in bulk soil and different soil aggregate fractions. With increasing soil aggregation rate, more carbon was accumulated within the macroaggregates, particularly within the coarse particulate organic matter fraction (250–2000 μm), rather than within the microaggregates (53–250μm) or silt and clay particles (< 53μm). In consideration of the high instability of macroaggregates and the liability of SOC within them, further research is needed to verify whether highly-aggregated soils at higher altitudes are more likely to lose SOC under warmer conditions.