Geomorphic principles of terrain organization and vegetation gradients

Abstract. Moisture and nutrient gradients consistently explain much of the variation in plant species composition and abundance, but these gradients are not spatially explicit and only reveal species responses to resource levels. This study links these abstract gradients to quantitative, spatial models of hill‐slope assembly. A gradient analysis in the mixed‐wood boreal forest demonstrates that patterns of upland vegetation distribution are correlated to soil moisture and nutrient gradients. Variation in species abundance with time since the last fire is removed from the gradient analysis in order to avoid confounding the physical environment gradients. The physical‐environment gradients are related to qualitative positions on the hill slope i.e. crest, mid‐slope, bottom‐slope. However, hill‐slope shape can be quantitatively described and compared by fitting allometric equations to the slope profiles. Using these equations, we show that hill‐slope profiles on similar surficial materials have similar parameters, despite coming from widely separated locations. We then quantitatively link the moisture and nutrient gradients to the equations. Moisture and nutrients significantly increase as distance down‐slope from the ridgeline increases. Corresponding vegetation composition changes too. These relationships characterize the general pattern of vegetation change down most hill slopes in the area. Since hill slopes are a universal feature of all landscapes, these principles may characterize landscape scale spatial patterns of vegetation in many environments.     

Long-term Perspectives on Lagged Ecosystem Responses to Climate Change: Permafrost in Boreal Peatlands and the Grassland/Woodland Boundary

Changes in climate could have far-reaching consequences for ecosystems sensitive to changes in temperature and precipitation, such as boreal permafrost peatlands and grassland/woodland boundaries. The long-term data from our studies in these ecosystems suggest that transient responses of permafrost and vegetation to climate change may be difficult to predict due to lags and positive feedbacks related to vegetation and disturbance. Boreal permafrost peatlands comprise an ecosystem with strong local controls on microclimate that influence the formation and thaw of permafrost. These local controls may preserve permafrost during the transient stages of climate warming, producing lagged responses. The prairie–forest border region of the northern Great Plains has experienced frequent change and has complex dynamics involving transitions in the grassland composition of prairie and in the degree of woodiness in bordering forests. Fire frequency interacts with fuel loading and tree recruitment in ways that affect the timing and direction of change. Lags and thresholds could lead to sudden large responses to future climate change that are not readily apparent from current vegetation. The creation of adequate models to characterize transient ecosystem changes will require an understanding of the linkages among processes operating at the scale of 10s of meters and over long time periods. Received 14 December 1999; accepted 7 July 2000.     

Interactive biotic and abiotic regulators of soil carbon cycling: evidence from controlled climate experiments on peatland and boreal soils

Partially decomposed plant and animal remains have been accumulating in organic soils (i.e. >40% C content) for millennia, making them the largest terrestrial carbon store. There is growing concern that, in a warming world, soil biotic processing will accelerate and release greenhouse gases that further exacerbate climate change. However, the magnitude of this response remains uncertain as the constraints are abiotic, biotic and interactive. Here, we examined the influence of resource quality and biological activity on the temperature sensitivity of soil respiration under different soil moisture regimes. Organic soils were sampled from 13 boreal and peatland ecosystems located in the United Kingdom, Ireland, Spain, Finland and Sweden, representing a natural resource quality range of C, N and P. They were incubated at four temperatures (4, 10, 15 and 20 °C) at either 60% or 100% water holding capacity (WHC). Our results showed that chemical and biological properties play an important role in determining soil respiration responses to temperature and moisture changes. High soil C : P and C : N ratios were symptomatic of slow C turnover and long‐term C accumulation. In boreal soils, low bacterial to fungal ratios were related to greater temperature sensitivity of respiration, which was amplified in drier conditions. This contrasted with peatland soils which were dominated by bacterial communities and enchytraeid grazing, resulting in a more rapid C turnover under warmer and wetter conditions. The unexpected acceleration of C mineralization under high moisture contents was possibly linked to the primarily role of fermented organic matter, instead of oxygen, in mediating microbial decomposition. We conclude that to improve C model simulations of soil respiration, a better resolution of the interactions occurring between climate, resource quality and the decomposer community will be required.     

Satellite microwave detection of boreal forest recovery from the extreme 2004 wildfires in Alaska and Canada

The rate of vegetation recovery from boreal wildfire influences terrestrial carbon cycle processes and climate feedbacks by affecting the surface energy budget and land‐atmosphere carbon exchange. Previous forest recovery assessments using satellite optical‐infrared normalized difference vegetation index (NDVI) and tower CO₂ eddy covariance techniques indicate rapid vegetation recovery within 5–10 years, but these techniques are not directly sensitive to changes in vegetation biomass. Alternatively, the vegetation optical depth (VOD) parameter from satellite passive microwave remote sensing can detect changes in canopy biomass structure and may provide a useful metric of post‐fire vegetation response to inform regional recovery assessments. We analyzed a multi‐year (2003–2010) satellite VOD record from the NASA AMSR‐E (Advanced Microwave Scanning Radiometer for EOS) sensor to estimate forest recovery trajectories for 14 large boreal fires from 2004 in Alaska and Canada. The VOD record indicated initial post‐fire canopy biomass recovery within 3–7 years, lagging NDVI recovery by 1–5 years. The VOD lag was attributed to slower non‐photosynthetic (woody) and photosynthetic (foliar) canopy biomass recovery, relative to the faster canopy greenness response indicated from the NDVI. The duration of VOD recovery to pre‐burn conditions was also directly proportional (P < 0.01) to satellite (moderate resolution imaging spectroradiometer) estimated tree cover loss used as a metric of fire severity. Our results indicate that vegetation biomass recovery from boreal fire disturbance is generally slower than reported from previous assessments based solely on satellite optical‐infrared remote sensing, while the VOD parameter enables more comprehensive assessments of boreal forest recovery.     

A new method for restoring ditches in peatlands: ditch filling with fiber bales

Ditching is a common practice to dewater wetlands, including peatlands, and ditch blocking is a common method for restoring wetlands because substrate is often unavailable for filling the ditches. However, filling has many advantages compared to blocking ditches. Our goal was to test whether ditches could be filled in a Colorado sloping fen (Chattanooga Fen) using bales created from shredded aspen (Populus tremuloides) tree‐fiber. We monitored water table levels before and after we filled two ditches (combined length of approximately 165 m × 3 m wide) as well as an undisturbed reference portion of Chattanooga fen. The reference site had stable water tables that rarely dropped more than 20 cm below the soil surface. The ditches had been dewatering large areas of the fen for at least 100 years. Filling the ditches with fiber bales resulted in a water table increase between 2 and 22 cm in an area up to 150 m below the ditch. Native sedges now cover the area where we filled the ditches, with no erosion or compression/settling of the ground observed and no water backing up behind the filled ditches. Filling the ditches with shredded fiber bales is a good option for restoration in wilderness areas, or areas lacking peat or mineral soil fill because it is a natural material that is easily transported and placed in the ditches.     

Oxidation of methane in boreal forest soils: a comparison of seven measures

Methane oxidation rates were measured in boreal forest soils using seven techniques that provide a range of information on soil CH₄ oxidation. These include: (a) short-term static chamber experiments with a free-air (1.7 ppm CH₄) headspace, (b) estimating CH₄ oxidation rates from soil CH₄ distributions and (c)²²²Rn-calibrated flux measurements, (d) day-long static chamber experiments with free-air and amended (+20 to 2000 PPM CH₄) headspaces, (e) jar experiments on soil core sections using free-air and (f) amended (+500 ppm CH₄) headspaces, and (g) jar experiments on core sections involving tracer additions of¹⁴CH₄. Short-term unamended chamber measurements,²²²Rn-calibrated flux measurements, and soil CH₄ distributions show independently that the soils are capable of oxidizing atmospheric CH₄ at rates ranging to < 2 mg m^–2 d^–1. Jar experiments with free-air headspaces and soil CH₄ profiles show that CH₄ oxidation occurs to a soil depth of 60 cm and is maximum in the 10 to 20 cm zone. Jar experiments and chamber measurements with free-air headspaces show that CH₄ oxidation occurs at low (< 0.9 ppm) thresholds. The¹⁴CH₄-amended jar experiments show the distribution of end products of CH₄ oxidation; 60% is transformed to CO₂ and the remainder is incorporated in biomass. Chamber and jar experiments under amended atmospheres show that these soils have a high capacity for CH₄ oxidation and indicate potential CH₄ oxidation rates as high as 867 mg m^–2 d^–1. Methane oxidation in moist soils modulates CH₄ emission and can serve as a negative feedback on atmospheric CH₄ increases.     

Paired-tower measurements of carbon and energy fluxes following disturbance in the boreal forest

Disturbances by fire and harvesting are thought to regulate the carbon balance of the Canadian boreal forest over scales of several decades. However, there are few direct measurements of carbon fluxes following disturbances to provide data needed to refine mathematical models. The eddy covariance technique was used with paired towers to measure fluxes simultaneously at disturbed and undisturbed sites over periods of about one week during the growing season in 1998 and 1999. Comparisons were conducted at three sites: a 1‐y‐old burned jackpine stand subjected to an intense crown fire at the International Crown Fire Modelling Experiment site near Fort Providence, North‐west Territories; a 1‐y‐old clearcut aspen area at the EMEND project near Peace River, Alberta; and a 10‐y‐old burned, mixed forest near Prince Albert National Park, Saskatchewan. Nearby mature forest stands of the same types were also measured as controls. The harvested site had lower net radiation (R_n), sensible (H) and latent (LE) heat fluxes, and greater ground heat fluxes (G) than the mature forest. Daytime CO₂ fluxes were much reduced, but night‐time CO₂ fluxes were identical to that of the mature aspen forest. It is hypothesized that the aspen roots remained alive following harvesting, and dominated soil respiration. The overall effect was that the harvested site was a carbon source of about 1.6 gC m^?2 day^?1, while the mature site was a sink of about ?3.8 gC m^?2 day^?1. The one‐year‐old burn had lower R_n, H and LE than the mature jackpine forest, and had a continuous CO₂ efflux of about 0.8 gC m^–2 day^?1 compared to the mature forest sink of ? 0.5 g C m^?2 day^?1. The carbon source was likely caused by decomposition of fire‐killed vegetation. The 10‐y‐old burned site had similar H, LE, and G to the mature mixed forest site. Although the diurnal amplitude of the CO₂ fluxes were slightly lower at the 10‐y‐old site, there was no significant difference between the daily integrals (? 1.3 gC m^?2 day^?1 at both sites). It appears that most of the change in carbon flux occurs within the first 10 years following disturbance, but more data are needed on other forest and disturbance types for the first 20 years following the disturbance event.     

Permafrost thaw causes large carbon loss in boreal peatlands while changes to peat quality are limited

Rapid, ongoing permafrost thaw of peatlands in the discontinuous permafrost zone is exposing a globally significant store of soil carbon (C) to microbial processes. Mineralization and release of this peat C to the atmosphere as greenhouse gases is a potentially important feedback to climate change. Here we investigated the effects of permafrost thaw on peat C at a peatland complex in western Canada. We collected 15 complete peat cores (between 2.7 and 4.5 m deep) along four chronosequences, from elevated permafrost peat plateaus to saturated thermokarst bogs that thawed up to 600 years ago. The peat cores were analysed for peat C storage and peat quality, as indicated by decomposition proxies (FTIR and C/N ratios) and potential decomposability using a 200-day aerobic laboratory incubation. Our results suggest net C loss following thaw, with average total peat C stocks decreasing by ~19.3 ± 7.2 kg C m⁻² over <600 years (~13% loss). Average post-thaw accumulation of new peat at the surface over the same period was ~13.1 ± 2.5 kg C m⁻². We estimate ~19% (±5.8%) of deep peat (>40 cm below surface) C is lost following thaw (average 26 ± 7.9 kg C m⁻² over <600 years). Our FTIR analysis shows peat below the thaw transition in thermokarst bogs is slightly more decomposed than peat of a similar type and age in permafrost plateaus, but we found no significant changes to the quality or lability of deeper peat across the chronosequences. Our incubation results also showed no increase in C mineralization of deep peat across the chronosequences. While these limited changes in peat quality in deeper peat following permafrost thaw highlight uncertainty in the exact mechanisms and processes for C loss, our analysis of peat C stocks shows large C losses following permafrost thaw in peatlands in western Canada.