Ross W. Wein

Potential Carbon Losses From Peat Profiles: Effects of Temperature, Drought Cycles, and Fire

Global warming and the resultant increase in evapotranspiration might lead to lowered water tables in peatlands and an increase in fire frequency. The objective of this study was to investigate some of the potential effects of these changes on peat decomposition. Dry mass losses and emissions of CO2 and CH4 from peat samples taken from three depth layers (0-10, 10-20, and 30-40 cm) of a black spruce peatland were measured in the laboratory at 8°, 16°, and 24°C under two moisture treatments. Effects of deep peat fire on decomposition were also simulated by burning the upper layer (0-10 cm) of peat and adding the ash to peat samples from the 10-20 cm layer. CH4 release averaged <1% of total carbon loss in flooded samples. Release of CO2 was 4-9 times greater from the 0-10 cm layer than from the 30-40 cm layer. After 120 d, the 30-40 cm layer had lost <1% of its original dry mass in all treatments. Higher temperatures strongly promoted decomposition of samples exposed to drying cycles but had little effect on decomposition of continuously flooded samples. Ash addition had variable effects on CO2 emissions but may have promoted CH4 production. It is suggested that in certain situations, global warming may not cause appreciable increases in carbon loss from peat deposits. The results indicate that some deeper peats are resistant to decay even when exposed to warm, aerobic conditions. However, further experimental work is needed to predict the long-term response of peat deposits to changes in water levels in different peatland types.

BIOTIC AND ABIOTIC REGULATION OF LIGHTNING FIRE INITIATION IN THE MIXEDWOOD BOREAL FOREST

Lightning fire is the dominant natural disturbance of the western mixedwood boreal forest of North America. We quantified the independent effects of weather and forest composition on lightning fire initiation (a detected and recorded fire start) patterns in Alberta, Canada, to demonstrate how these biotic and abiotic components contribute to ecosystem dynamics in the mixedwood boreal forest. We used logistic regression to describe variation in annual initiation occurrence among 10,000-ha landscape units (voxels) covering a 9 million-ha study region over 11 years. At a voxel scale, forest composition explained more variation in annual initiation than did weather indices. Initiations occurred more frequently in landscapes with more conifer fuels (Picea spp.), and less in aspen-dominated (Populus spp.) ones. Initiations were less frequent in landscapes that had recently burned. Variation in initiation was also influenced by joint weather-lightning indices, but to a lesser degree. For each voxel, these indices quantified the number of days in the fire season when moisture levels were low and lightning was detected. Regional indices of fire weather severity explained substantial interannual variation of initiation, and the effect of forest composition was stronger in years with more severe fire weather. Our study is a conclusive demonstration of biotic and abiotic regulation of lightning fire initiation in the mixedwood boreal forest. The independent effects of forest composition emphasize that vegetation feedbacks strongly regulate disturbance dynamics in the region.

Fire and Acacia seeds: a hypothesis of colonization success

SUMMARY (1) The effects of natural and simulated fire on insect-infested seed germination, seedling emergence and on the viability of seed pools in the soil of Acacia sieberiana were investigated in a savanna in Queen Elizabeth National Park, Uganda. (2) There was a strong potential for the population of A. sieberiana plants to increase from the viable seed pool of about 800 seeds m-2. Seedling emergence was 167 seedlings m-2 after one fire. Seeds collected from the soil surface of burned areas produced significantly higher total germination rates (650%,) than those from unburned areas (10%/,) when tested in the laboratory. As expected, seeds located at shallow depths were stimulated to germinate more readily by fire then seeds located deeper in the soil. (3) The germination was significantly reduced by the high bruchid larvae (Bruchidius spp.) infestation. Bruchid larvae are more sensitive to fire than the seed embryo and, thus, the heat treatment indirectly leads to higher germination. (4) Regression equations were developed for predicting the mortality of seed and bruchid larvae under different fire intensities. (5) A conceptual diagram of germination success of A. sieberiana was developed.

BETULA NANA L. AND BETULA GLANDULOSA MICHX.

Betula nana. A prostrate shrub up to 1 m in height. Twigs stiff, dull dark brown, pubescent, not warty. Young twigs are covered with resin spots. Leaves orbicular or obovate-orbicular, deeply and regularly crenate all around, rounded at both ends and glabrous (or with a few glands) at maturity. Leaves dark green above and paler beneath with a petiole up to 3 mm. Subspecies exilis (Suk.) Hult. has shorter leaf blades (0.5-1 cm long) than ssp. nana (0.5-2.0 cm). Bracts lack a resiniferous hump on the back, and possess a cuneate base and three parallel (almost equal) narrow lobes at the apex. Male inflorescence is about 0.8 cm long; erect when young and protected by bud scales. Female inflorescence is 0. 5-1.0 cm in fruit; scales with cuneate base and three more or less equal narrow erect lobes at the apex. Nutlet wings very narrow: up to a quarter as broad as the body (although recorded as up to half as broad as the body in Alaska-Hulten 1968).

Long‐term forest floor carbon dynamics after fire in upland boreal forests of western Canada

We examined the long-term dynamics of upland boreal forest floors after disturbance by fire. We selected two important and contrasting upland tree species, Pinus banksiana (jack pine) and Populus tremuloides (trembling aspen), in three distinct climatic zones across the boreal forest of western Canada, and sampled 80 fire-originated stands divided into six chronosequences with ages ranging from 14 to 149 years. The forest floor was a large component of carbon storage. Averaged across ages and zones, it was 1.31 and 2.78 kg C m−2 for P. banksiana and P. tremuloides, respectively, compared with 4.03 and 5.56 kg C m−2 in aboveground trees. These data exclude decomposing coarse woody debris, which was a significant component of the forest floor (0.18/0.13 kg C m−2 ) and requires further study. The contributions from shrubs (0.035/0.151 kg C m−2), ground vegetation (0.019/0.026 kg C m−2), and moss-plus-lichen (0.179/0.004 kg C m−2) were relatively small. An analysis of covariance (ANCOVA) model showed that forest floor carbon was positively related to stand age, as well as being affected by species and climatic zone. Much of the variability was explained by species, and species-specific regression models showed that for P. tremuloides forest floor carbon was strongly related to stand age, mean annual temperature, and mean annual precipitation, and for P. banksiana, forest floor carbon was strongly related to an index of moss dominance. The regression models suggest that the forest floor carbon pool in upland forests of the western Canadian boreal will be sensitive to climate change, but this sensitivity would need to be tested with process-based models.

The Contribution of Typha Components to Floating Mat Buoyancy

The seasonal contribution of living Typha components to the buoyancy of floating mats was investigated in a diked, freshwater impoundment near the head of the Bay of Fundy in New Brunswick, Canada. The objectives were (1) to examine the potential influence of a dominant, mat—building species on hydrologic conditions at the mat surface, and (2) to predict whether a complete killing of the dominant species could cause mats to sink. The Typha component contributing most to mat buoyancy was the rhizomes, which added a net buoyancy pressure of °20 Pa throughout the growing season. The seasonal maximum buoyancy contribution of 28 Pa for all living Typha components combined was reached in spring, followed by a decline to 11 Pa in late summer as the aboveground shoots developed. This positive and seasonally variable contribution of Typha to mat buoyancy is expected to be most important during the early stages of mat development, when mats are thin and composed largely of living, belowground organs. However, on older and thicker mats the living Typha is less important because of the large volumes of gas bubbles from anaerobic decomposition that are trapped in the dead organic material. For the 50 cm thick floating mats under study, it is concluded that trapped gas is the main cause of buoyancy and would lead to the continued flotation of mats even if the living Typha were removed. Implications of the results are discussed in relation to the resiliency of floating mat systems.

Seasonal change in gas content and buoyancy of floating Typha mats

(1) In an embanked, freshwater marsh in New Brunswick, Canada, the buoyancy of experimentally isolated Typha fioating mats and release rate of gas bubbles were monitored. Mats were most buoyant and released the greatest quantity of gas in late summer when water temperature in the mat was highest. (2) Nitrogen and methane were the major components of gas bubbles trapped in the organic material in August, but the percentage of methane was relatively lower in May. (3) In a laboratory experiment, Typha mat samples were incubated in growth chambers at 2, 8, 15 or 22 ?C for twelve weeks, and were then held at 2 ?C for thirteen weeks and 15 ?C for ten weeks to simulate winter and summer temperatures, respectively, in floating mats. Samples incubated at 22 ?C reached an equilibrium gas content of 13 7% of total mat volume, while samples at 2 ?C attained a gas content of 6 2%. The magnitude of seasonal change in gas content was estimated at between 2 5 and 4 7% of total mat volume. (4) Seasonal, temperature-dependent change in the rate of anaerobic decomposition is partly responsible for the observed seasonal variation in the gas content and buoyancy of mats, but physical processes such as changes in gas solubility with temperature are also important. The significance of anaerobic decomposition as a biotic factor influencing successional development is discussed.

Vegetation development of boreal riparian plant communities after flooding, fire, and logging, Peace River, Canada

Abstract In this study we compare and contrast vegetation development following natural and logging disturbances in a major boreal river valley. Permanent sample plots and releves were established and sampled for vegetation and landscape attributes in June and July of 1993 and 1994 in the Peace River Lowlands, Wood Buffalo National Park, Canada. In the Peace River Lowlands, primary succession is a flood-origin process. Secondary succession may be either autogenic through gap dynamics mediated by nursery logs, buried wood, and suckering, or allogenic, following fire or logging. Flood origin accounts for 72% and fire origin for 29% of the undisturbed forests. From 1951–1995, 24% of the forest land burned, yielding a fire return interval of 186 years. Forest successional trajectories are set soon after flood, logging, or fire, with little evidence of gradual replacement of one forest type by another. Vegetation composition and relative species abundance are strongly correlated with living moss depth, moss-lichen total cover, total tree cover, herb cover, and canopy height. Species with high indicator value are Hylocomium splendens, Picea glauca, Pyrola chlorantha, Equisetum pratense , and Epilobium angustifolium . Strong correlations exist between white spruce tree density and canopy height, total tree cover and canopy height, total tree cover and basal area per hectare, basal area and canopy height, and between canopy height and surface age. Clearcuts are initially dominated by rose-raspberry followed by balsam poplar (with lesser amounts of Alaska birch and aspen). After logging, temporal changes in composition and dominance occur more rapidly than during natural succession. There is no evidence of post-logging convergence toward the original white spruce and mixedwood forests; a long-term deciduous disclimax is predicted. Vegetation associations, successional pathways, landscape relationships, and ecological benchmarks are identified.

Exotic vascular plant invasiveness and forest invasibility in urban boreal forest types.

The riverine forests of the northern city of Edmonton, Alberta, Canada display strong resilience to disturbance and are similar in species composition to southern boreal mixedwood forest types. This study addressed questions such as, how easily do exotic species become established in urban boreal forests (species invasiveness) and do urban boreal forest structural characteristics such as, native species richness, abundance, and vertical vegetation layers, confer resistance to exotic species establishment and spread (community invasibility)? Eighty-four forest stands were sampled and species composition and mean percent cover analyzed using ordination methods. Results showed that exotic tree/shrub types were of the most concern for invasion to urban boreal forests and that exotic species type, native habitat and propagule supply may be good indicators of invasive potential. Native forest structure appeared to confer a level of resistance to exotic species and medium to high disturbance intensity was associated with exotic species growth and spread without a corresponding loss in native species richness. Results provided large-scale evidence that diverse communities are less vulnerable to exotic species invasion, and that intermediate disturbance intensity supports species coexistence. From a management perspective, the retention of native species and native forest structure in urban forests is favored to minimize the impact of exotic species introductions, protect natural succession patterns, and minimize the spread of exotic species.

Fire as an agent in redistributing fallout137Cs in the Canadian boreal forest

The presence of fallout137Cs in the boreal forest and the effect of fire in redistributing137Cs were studied in the remote region of Wood Buffalo National Park, N.W.T., Canada. Results of a preliminary study of five burned (the fire occurred in 1981) and five unburned stands conducted in 1986 revealed that137Cs concentrations were higher in the surface soil of the burned stands than in the unburned ones. In 1989, a comprehensive study was conducted, in which one burned and one unburned white spruce stand were sampled in greater detail. The latter investigation also revealed a difference in the distribution of137Cs within the burned stand compared to the unburned one. Specifically, in the unburned stand, the highest137Cs concentration was identified in the epiphytic lichens and in the mosses, whereas in the burned stand, the highest concentration was measured in the surface organic soil. These results indicate that fire caused the mobilization of part of the137Cs bound to the above-ground matter and concentrated it in the ash layer of the burned surface soil. An additional ecologically important finding in our study was that significantly lower total137Cs load was observed in the burned stand compared to the unburned one. Hence, our data not only provide evidence that137Cs is being redistributed within the burned stand to the surface soil, but also that part of the137Cs is lost due to fire, presumably contaminating other ecosystems. Volatilization and fly-ash during the fire, and runoff (e.g. from snow melt) after the fire are the most likely mechanisms for the137Cs removal. These findings point to fire as an agent of137Cs secondary contamination for initially unaffected systems, as well as for those previously contaminated.