Search

Few guidelines exist for the effective revegetat'on of peatlands following the removal of in-situ oil and gas infrastructure. We conducted a manipulative field study in northeast Alberta, Canada, on a well pad undergoing removal and revegetation to test different management practices for facilitating vegetation establishment and recovery of a sphagnum-dominated peatland. A randomized block design was used to evaluate the effect of various revegetation practices, including augmenting natural recovery with the use of native transplants, acrotelm transfer from a similar intact donor peatland, and the use of variable surface microtopography. Although overall survival of transplants was similar between areas smoothed and left rough, areas th at were rough had greater species richness under natural recovery, including trees, shrubs, and other perennial herbs. Moreover, survival and growth of woody transplants (Picea mariana-black spruce and Ledum groenlandicum-Labrador tea) were greater when planted with in the top and middle microtopographic positions rather than micro-depressions. Survival of transplanted sedges (Carex spp.) was high at all topographic positions, but benefited the most in g rowth from planting in depressions. Contrary to expectations, no benefits of acrotelm application were found on vegetation recovery during the first tw o seasons, and even reduced the presence of some native vegetation. Although transplants directly contributed to revegetation, the recovering peatland remained highly dissimilar (> 90%) in composition relative to the neighboring peatland after two years. [ABSTRACT FROM AUTHOR]

Question and Location: How does soil burn severity and early post-fire tree composition affect long-term understorey vegetation dynamics in the coniferous forests of eastern Canada? Method: Vegetation dynamics were assessed using paleoecological methods and a chronosequence analysis of extant stands. The relation between environmental factors and succession was evaluated using ordination techniques on the chronosequence data. Understorey succession was studied by regression analysis on the chronosequence data and through within-site Markovian transition probabilities between successive 1-cm layers of plant macroremains from soil organic matter profiles. Results: Initial tree composition (Picea mariana and Pinus banksiana) had little effect on understorey composition. Soil burn severity (measured as the thickness of the residual forest floor humus) significantly affected temporal changes in understorey species. Following fires of high severity, stands underwent a gradual paludification with a net increase in Sphagnum and ericaceous shrubs (Ledum groenlandicum), and a decrease in feathermosses. Paludification was accelerated after low severity fires, which led to the dominance of Sphagnum less than 200 years after fire, and of L. groenlandicum shortly after fire. In situ paleo-ecological work confirmed results obtained with the chronosequence analysis. Conclusions: One vegetation gradient related to time after disturbance is insufficient to account for the full complexity of longterm changes in understorey composition following fire. Current forestry practices that protect the forest floor humus may induce a premature paludification. [ABSTRACT FROM AUTHOR]

Early successional competition among boreal forest tree and shrub species and its effects on growth of commercial tree species have been a major source of uncertainty in establishing efficient precommercial thinning and brushing prescriptions. We examined the effect of prethinning competitor density, postthinning competitor regrowth density, prethinning stem diameter, and the timing of thinning operations on the growth response of black spruce (Picea mariana [Mill.] BSP) and jack pine (Pinus banksiana Lamb.). In addition, we examined the mortality rate of hardwoods after thinning and the number of new shoots produced per surviving thinned stem. For jack pine, growth response was greatest when thinning occurred between 4 and 9 years after establishment, whereas for black spruce we observed no significant relationship between growth response and the timing of treatment. For jack pine, growth response was significantly affected by pretreatment competitor density, posttreatment competitor regrowth density, and pretreatment stem diameter. For black spruce, no significant relationship was observed between growth response and any variables. Mortality rates and production of new shoots in hardwoods varied significantly between species. Considering the high regrowth potential of willow (Salix spp.) and alder (Alnus spp.), we recommend that stands exhibiting low densities of these species should be left unthinned. Our results help foresters identify stands that require precommercial thinning and call for modification of currently used thinning strategies. [ABSTRACT FROM AUTHOR]

We combined year-round eddy covariance with biometry and biomass harvests along a chronosequence of boreal forest stands that were 1, 6, 15, 23, 40, ∼74, and ∼154 years old to understand how ecosystem production and carbon stocks change during recovery from stand-replacing crown fire. Live biomass (C) was low in the 1- and 6-year-old stands, and increased following a logistic pattern to high levels in the 74- and 154-year-old stands. Carbon stocks in the forest floor (C) and coarse woody debris (C) were comparatively high in the 1-year-old stand, reduced in the 6- through 40-year-old stands, and highest in the 74- and 154-year-old stands. Total net primary production (TNPP) was reduced in the 1- and 6-year-old stands, highest in the 23- through 74-year-old stands and somewhat reduced in the 154-year-old stand. The NPP decline at the 154-year-old stand was related to increased autotrophic respiration rather than decreased gross primary production (GPP). Net ecosystem production (NEP), calculated by integrated eddy covariance, indicated the 1- and 6-year-old stands were losing carbon, the 15-year-old stand was gaining a small amount of carbon, the 23- and 74-year-old stands were gaining considerable carbon, and the 40- and 154-year-old stands were gaining modest amounts of carbon. The recovery from fire was rapid; a linear fit through the NEP observations at the 6- and 15-year-old stands indicated the transition from carbon source to sink occurred within 11-12 years. The NEP decline at the 154-year-old stand appears related to increased losses from C by tree mortality and possibly from C by decomposition. Our findings support the idea that NPP, carbon production efficiency (NPP/GPP), NEP, and carbon storage efficiency (NEP/TNPP) all decrease in old boreal stands. [ABSTRACT FROM AUTHOR]

We present a decadal (1994–2004) record of carbon dioxide flux in a 160-year-old black spruce forest/veneer bog complex in central Manitoba, Canada. The ecosystem shifted from a source (+41 g C m−2, 1995) to a sink (−21 g C m−2, 2004) of CO2 over the decade, with an average net carbon balance near zero. Annual mean temperatures increased 1–2° during the period, consistent with the decadal trend across the North American boreal biome. We found that ecosystem carbon exchange responded strongly to air temperature, moisture status, potential evapotranspiration, and summertime solar radiation. The seasonal cycle of ecosystem respiration significantly lagged that of photosynthesis, limited by the rate of soil thaw and the slow drainage of the soil column. Factors acting over long time scales, especially water table depth, strongly influenced the carbon budget on annual time scales. Net uptake was enhanced and respiration inhibited by multiple years of rainfall in excess of evaporative demand. Contrary to expectations, we observed no correlation between longer growing seasons and net uptake, possibly because of offsetting increases in ecosystem respiration. The results indicate that the interactions between soil thaw and water table depth provide critical controls on carbon exchange in boreal forests underlain by peat, on seasonal to decadal time scales, and these factors must be simulated in terrestrial biosphere models to predict response of these regions to future climate. [ABSTRACT FROM AUTHOR]

We deployed a mesonet of year-round eddy covariance towers in boreal forest stands that last burned in ∼1850, ∼1930, 1964, 1981, 1989, 1998, and 2003 to understand how CO2 exchange and evapotranspiration change during secondary succession. We used MODIS imagery to establish that the tower sites were representative of the patterns of secondary succession in the region, and Landsat images to show that the individual stands have changed over the last 22 years in ways that match the spatially derived trends. The eddy covariance towers were well matched, with similar equipment and programs, which maximized site-to-site precision and allowed us to operate the network in an efficient manner. The six oldest sites were fully operational for ∼90% of the growing season and ∼70% of the dormant season from 2001 or 2002 to 2004, with most of the missing data caused by low battery charge or bad signals from the sonic anemometers. The rates of midday growing-season CO2 uptake recovered to preburn levels within 4 years of fire. The seasonality of land–atmosphere exchange and growing-season length changed markedly with stand age. The foliage in the younger stands (1989, 1998, and 2003 burns) was almost entirely deciduous, which resulted in comparatively short growing seasons that lasted ∼65 days. In contrast, the older stands (1850, 1930, 1964, and 1981) were mostly evergreen, which resulted in comparatively long growing seasons that lasted ∼130 days. The eddy covariance mesonet approach we describe could be used within the context of other ecological experimental designs such as controlled manipulations and gradient comparisons. [ABSTRACT FROM AUTHOR]

Question: What are the drivers of bryophyte succession in paludifying boreal Picea mariana forests? Location: The Clay-Belt of Québec and Ontario, Canada. Methods: The bryophyte community and habitat variables (forest floor thickness, water table, stand density, canopy openness micro-climate and presence of ericaceous species) were analysed in a chronosequence of 13 stands from 50 to more than 350 years since fire. Results: Across the chronosequence, feathermosses were replaced by shade and desiccation tolerant slower growing hummock Sphagna and then by faster growing hollow Sphagna. These changes were linked with both increasing light availability and the movement of the water table into the forest floor. Conclusions: As water table rise is dependent on forest floor thickness, which is in turn influenced by the presence of Sphagna, this successional sequence represents an example of facilitation. Furthermore, it emphasizes the importance of water table rise in determining stand level, and landscape level variables such as carbon balance. [ABSTRACT FROM AUTHOR]

Specific leaf area (SLA) and leaf area index (LAI) were estimated using site-specific allometric equations for a boreal black spruce (Picea mariana (Mill.) BSP) fire chronosequence in northern Manitoba, Canada. Stands ranged from 3 to 131 years in age and had soils that were categorized as well or poorly drained. The goals of the study were to: (i) measure SLA for the dominant tree and understory species of boreal black spruce-dominated stands, and examine the effect of various biophysical conditions on SLA; and (ii) examine leaf area dynamics of both understory and overstory for well- and poorly drained stands in the chronosequence. Overall, average SLA values for black spruce (n = 215), jack pine (Pinus banksiana Lamb., n = 72) and trembling aspen (Populus tremuloides Michx., n = 27) were 5.82 ± 1.91, 5.76 ± 1.91 and 17.42 ± 2.21 m2 kg−1, respectively. Foliage age, stand age, vertical position in the canopy and soil drainage had significant effects on SLA. Black spruce dominated overstory LAI in the older stands. Well-drained stands had significantly higher overstory LAI (P < 0.001), but lower understory LAI (P = 0.022), than poorly drained stands. Overstory LAI was negligible in the recent (3–12 years old) burn sites and highest in the 70-year-old burn site (6.8 and 3.0 in the well- and poorly drained stands, respectively), declining significantly (by 30–50%) from this peak in the oldest stands. Understory leaf area represented a significant portion (> 40%) of total leaf area in all stands except the oldest. [ABSTRACT FROM PUBLISHER]

Measurements of the photosynthetic response to midsummer irradiance were made for 11 species representing the dominant trees, understory shrubs, herbaceous plants and moss species in an old black spruce (Picea mariana (Mill.) B.S.P.) boreal forest ecosystem. Maximum rates of photosynthesis per unit foliage area at saturating irradiance, Amax, were highest for aspen (Populus tremuloides Michx.), reaching 16 μmol m−2 s−1. For tamarack (Larix laricina (Du Roi) K. Kock) and P. mariana, Amax was only 2.6 and 1.8 μmol m−2 s−1, respectively. Values of Amax for understory shrubs and herbaceous plants were clustered between 9 and 11 μmol m−2 s−1, whereas Amax of feather moss (Pleurozium schreberi (Brid.) Mitt.) reached only 1.9 μmol m−2 s−1. No corrections were made for differences in shoot structure, but values of photosynthetic light-use efficiency were similar for most species (70–80 mmol CO2 mol−1); however, they were much lower for L. laricina and P. mariana (15 mmol CO2 mol−1) and much higher for P. schreberi (102 mmol CO2 mol−1). There was a linear relationship between Amax and foliage nitrogen concentration on an area basis for the broad-leaved species in the canopy and understory, but the data forP. mariana, L. laricina and P. schreberi fell well below this line. We conclude that it is not possible to scale photosynthesis from leaves to the canopy in this ecosystem based on a single relationship between photosynthetic rate and foliage nitrogen concentration. [ABSTRACT FROM AUTHOR]

We investigated effects of nutrient addition on several physiological characteristics of 60-cm-tall black spruce (Picea mariana Mill. B.S.P.) layers (i.e., rooted branches of overstory trees) and 20-cm-tall planted seedlings on a clear-cut, N-limited boreal site. After two growing seasons, current-year and one-year-old needles of fertilized trees (layers and seedlings combined) had higher net photosynthetic rates (An) and maximum capacity of Rubisco for CO2 fixation (Vmax) than unfertilized trees. One-year-old needles of fertilized trees had higher stomatal conductance (gs), higher water-use efficiency, and lower intercellular to ambient CO2 ratio than unfertilized trees. Additionally, fertilized trees had higher predawn and midday shoot water potentials than unfertilized trees. [ABSTRACT FROM PUBLISHER]

Measurements of net ecosystem CO exchange by eddy correlation, incident photosynthetically active photon flux density (PPFD), soil temperature, air temperature, and air humidity were made in a black spruce ( Picea mariana) boreal woodland near Schefferville, Quebec, Canada, from June through August 1990. Nighttime respiration was between 0.5 and 1.5 kg C ha h, increasing with temperature. Net uptake of carbon during the day peaked at 3 kg C ha h, and the daily net uptake over the experiment was 12 kg C ha day. Photosynthesis dropped substantially at leaf-to-air vapor pressure deficit (VPD) greater than 7 mb, presumably as a result of stomatal closure. The response of ecosystem photosynthesis to incident PPFD was markedly non-linear, with an abrupt saturation at 600 μmol m s. This sharp saturation reflected the geometry of the spruce canopy (isolated conical crowns), the frequently overcast conditions, and an increase in VPD coincident with high radiation. The ecosystem light-use efficiency increased markedly during overcast periods as a result of a more even distribution of light across the forest surface. A mechanistic model of forest photosynthesis, parameterized with observations of leaf density and nitrogen content from a nearby stand, provided accurate predictions of forest photosynthesis. The observations and model results indicated that ecosystem carbon balance at the site is highly sensitive to temperature, and relatively insensitive to cloudiness. [ABSTRACT FROM AUTHOR]

Prior research has demonstrated the importance of water limitations and increasing temperatures on upland black spruce (Picea mariana [Mill.] B.S.P.) tree growth, which is a dominant component of the North American boreal forest. However, little work has been done to investigate the connectivity between growth and hydro‐climate in peatland black spruce systems. The boreal forest is the largest global terrestrial biome and is highly threatened due to current and projected increases in temperatures for the northern latitudes. Here we explore the dynamics among annual black spruce growth, climate, and water table elevations using 45 years of in situ precipitation, temperature, and water table elevation coupled with dendrochronological analysis from six research peatlands at the Marcell Experimental Forest, MN, USA. From 1963 to 2010, we found weak relationships between water table elevation and black spruce growth at the six study sites. Instead, annual black spruce growth was most favourable during three climatic periods: (a) cool, moist conditions in mid‐summer; (b) warm mid‐spring temperatures; and (c) cool temperatures in the fall prior to the current growing season. The disconnect between black spruce growth and water table dynamics was surprising and suggests that either annual black spruce growth is minimally responsive to hydrological fluctuations at the timescale we analysed or there is great elasticity of black spruce growth to peatland water table and evapotranspiration dynamics under the range of hydrological fluctuations contained in our record. [ABSTRACT FROM AUTHOR]