Forest management activities are increasingly analyzed through a lens that quantifies their effects on soil carbon (C) and nitrogen (N) storage because forest soils are an important C sink. Data on ...the longer-term impacts of repeated interventions are often lacking. At the Petawawa Research Forest, Ontario, Canada, treatments to evaluate the effect of repeated thinnings on wood quality of red pine (Pinus resinosa Aiton) and white pine (Pinus strobus L.) were initiated in 1918 with the first experimental plots in Canada: permanent sample plots 1 (thinned) and 2 (control). In 2005, 16 years after the last thinning in 1989, we observed that repeated thinnings reduced soil C and N stocks in the surface L, F, and Ah horizons. Contrary to our hypotheses, concentrations and stocks of C and N increased in the Bm1 horizon, indicating that these elements could be accumulated in deeper horizons after surface disturbance and potentially increased decomposition associated with thinning. However, total C and N accumulation in the profile to 30 cm contributed to reduced storage (-35% for C and -30% for N). Many forest sites in the Great Lakes Forest Region that are selectively cut repeatedly over decades could experience this level of soil C and N decline.
Celotno besedilo
Dostopno za:
BF, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
•Silvicultural treatments oriented vegetation after 25years in Petawawa, Canada.•Surface and subsurface (0–15 and 15–40cm depths) mineral soil layers were analyzed.•The control was characterized by a ...Hardwood shrub-rich vegetation type.•Soil pH, C and N cycling were greater in Hardwood herb-rich, blade-scarified sites.•Scarification and herbicide promoted Softwoods, decreasing soil pH and N cycling.
The combined influence of silvicultural treatments and subsequent forest vegetation succession affects long-term soil organic C (SOC) and nutrient cycling in mineral soil in ways still poorly understood, possibly by influencing soil pH and microbial activity. We investigated how silvicultural disturbance and associated vegetation affected surface (0–15cm) and subsurface (15–40cm) mineral soil biogeochemistry, 25years after establishment of white pine (Pinus strobus) plantations in Petawawa, Ontario, Canada. Three silvicultural treatments led to distinct communities, characterized by contrasting structure and functional composition. The control (CTR) was characterized by a Hardwood shrub-rich forest, blade-scarification treatment (S) by a Hardwood herb-rich forest, while blade-scarification combined with vegetation control (S+V) resulted in a Softwood monoculture. Increasing dominance of the hardwood canopy and herbaceous cover in CTR and S treatments was associated with higher surface mineral soil pH and exchangeable base cations, and dominance of net nitrate (NO3−-N) over ammonium (NH4+-N) production during incubation. In contrast, increased overstory softwood cover in the S+V treatment was associated with lower surface mineral soil pH and NH4+-N concentrations. Across all treatment-induced communities, surface mineral soil pH and exchangeable Ca and Mg were positively correlated with total N and SOC. The relation of Ca with SOC, total N and net nitrification was similarly observed in subsurface soil. Surprisingly, herb cover rather than overstory functional character (hardwood vs softwood) was related to subsurface N cycling; increasing cover in CTR and S treatments was related to higher NH4+-N availability and lower organic-to-inorganic N ratio, indicating accelerated cycling. We conclude that through effects on soil pH and base cations, silvicultural operations combined with subsequent hardwood and herbaceous species cover strongly regulate C and N cycling in the mineral soil of mixedwood forests.
•We exposed seedlings of conifer species to three realistic simulated future climates.•Seedlings of conifer species experienced high mortality (75%) induced by drought.•Conifer species are highly ...vulnerable at the seedling stage to future drought.•Hydraulic safety margins are good indicators of mortality risk for conifer species.
The future climate of northern temperate forests is projected to be drier and warmer by the end of this century. As a result, more drought-induced forest dieback events are anticipated in northeastern North America, and assessing the vulnerability of dominant tree species to drought is critical for understanding the future composition of these forests. In a greenhouse experiment, we exposed two-year-old seedlings of Picea glauca (Moench) Voss, Picea mariana (Mill.) B.S.P. and Pinus strobus L. to three future climate treatments for southern Quebec, Canada, and evaluated their mortality, growth, and foliage water status responses to soil water availability and atmospheric drought. Using a unique approach, climate treatments emulated droughts of different frequencies, durations, and intensities. Treatments closely simulated one growing season, with changes in air temperature and relative humidity every six hours and daily adjustment in the amount of water delivered to the seedlings. The three species experienced high mortality (75%) in all water-limited treatments compared to a control treatment that provided non-limiting soil moisture (0% mortality). The biomass of the seedlings that survived was 40% lower than that of control seedlings. Our results confirmed that the hydraulic safety margins, defined as the difference between seasonal minimum water potential and xylem water potential leading to 12, 50 and 88% of hydraulic conductivity loss, were good predictors of probability of tree mortality. Therefore, hydraulic safety margins are useful functional traits that can be used to compare the vulnerability of various species to drought and then provide crucial information to practitioners and policymakers to adjust forest management to climate change. We showed that three dominant conifer species of northern temperate forests were highly vulnerable to drought in future climates. Because drought is projected to be a significant threat to forests, understanding potentially adaptive physiological responses to drought, such as hydraulic safety margins of tree seedlings, is important for predicting the response of forest regeneration and composition in warmer and drier climates.
Forest management activities are increasingly analyzed through a lens that quantifies their effects on soil carbon (C) and nitrogen (N) storage because forest soils are an important C sink. Data on ...the longer-term impacts of repeated interventions are often lacking. At the Petawawa Research Forest, Ontario, Canada, treatments to evaluate the effect of repeated thinnings on wood quality of red pine (Pinus resinosa Aiton) and white pine (Pinus strobus L.) were initiated in 1918 with the first experimental plots in Canada: permanent sample plots 1 (thinned) and 2 (control). In 2005, 16 years after the last thinning in 1989, we observed that repeated thinnings reduced soil C and N stocks in the surface L, F, and Ah horizons. Contrary to our hypotheses, concentrations and stocks of C and N increased in the Bm1 horizon, indicating that these elements could be accumulated in deeper horizons after surface disturbance and potentially increased decomposition associated with thinning. However, total C and N accumulation in the profile to 30 cm contributed to reduced storage (–35% for C and –30% for N). Many forest sites in the Great Lakes Forest Region that are selectively cut repeatedly over decades could experience this level of soil C and N decline.
Celotno besedilo
Dostopno za:
BF, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Effects of shoot water potential water and leaf-to atmosphere vapor pressure difference (VPD) on gas exchange of jack pine (Pinus banksiana Lamb.), black spruce (Picea mariana (Mill.) B.S.P.), and ...aspen (Populus tremuloides Michx.) were investigated at the northern edge of the boreal forest in Manitoba, Canada. Laboratory measurements on cut branches showed that net photosynthesis (An) and mesophyll conductance (gm) of jack pine and gm of black spruce did not respond to water potential until a threshold water potential was reached below which they decreased linearly. Photosynthesis of black spruce decreased slowly with decreasing water potential above the threshold and declined more rapidly thereafter. The threshold water potential was lower in black spruce than in jack pine. However, stomatal conductance (gs) of black spruce deceased continuously with decreasing water potential, whereas gs of jack pine showed a threshold response. Mesophyll limitations were primarily responsible for the decline in An at low water potential for jack pine and black spruce in the middle of the growing season, but stomatal limitations became more important later in the season. Field measurements on in situ branches on warm sunny days showed that both conifer species maintained water potential above the corresponding threshold and there was no evidence of water potential limitation on An of jack pine, black spruce or aspen. Vapor pressure difference was important in regulating gas exchange in all three species. An empirical model was used to quantify the gs response to VPD. When parameterized with laboratory data for the conifers, the model also fit the corresponding field data. When parameterized with field data, the model showed that stomata of aspen were the most sensitive of the three species to VPD, and stomata of black spruce were the least sensitive. For jack pine and aspen, stomata of foliage in the upper canopy were significantly more sensitive than stomata of foliage in the lower canopy. Vapor pressure difference had a greater impact on An of aspen than on An of the conifers as a result of aspen's greater stomatal sensitivity to VPD and greater slope of the relationship between An and intercellular CO2 concentration (Ci). During the 1994 growing season, VPD averaged 1.0 kPa, corresponding to ratios of Ci to ambient CO2 of 0.77, 0.71 and 0.81 for jack pine, black spruce and aspen, respectively. We conclude that increases in VPD at the leaf surface in response to climate change should affect the absolute CO2 and H2O fluxes per unit leaf area of the aspen component of a boreal forest landscape more than those of the conifer component.
Profiles of photosynthetically active radiation (PAR), leaf nitrogen per unit leaf area (Narea), and photosynthetic capacity (Amax) were measured in an aspen, two jack pine, and two black spruce ...stands in the BOREAS northern study area. Narea decreased with decreasing %PAR in each stand, in all conifer stands combined (r=0.52) and in all stands combined (r=0.46). Understory alder had higher Narea for similar %PAR than did aspen early in the growing season. Amax decreased with decreasing Narea, except for the negative correlation between Narea and Amax during shoot flush for jack pine. For the middle and late growing season data, Narea and Amax had r values of 0.51 for all stands combined and 0.60 for all conifer stands combined. For similar Narea the aspen stand had higher Amax than did the conifer stands. Photosynthetic capacity expressed as a percentage of Amax at the top of the canopy (%Amax0) decreased with %PAR similarly in all stands, but %Amax0 decreased at a much slower rate than did %PAR. To demonstrate the implications of the vertical distribution of Amax, three different assumptions were used to scale leaf Amax to the canopy (Acan‐max): (1) constant Amax with canopy depth, (2)Amax scaled proportionally to %PAR, and (3) a linear relationship between Amax and cumulative leaf area index derived from our data. The first and third methods resulted in similar Acan‐max; the second was much lower. All methods resulted in linear correlations between normalized difference vegetation indices measured from a helicopter and Acan‐max (r=0.97, 0.93, and 0.97, respectively), but the slope was strongly influenced by the scaling method.
In the North American boreal forest, black spruce (Picea mariana) forests have historically regenerated after the passage of large-scale wildfires. Over the past 30 years, tree harvesting has ...replaced wildfire as the predominant agent of disturbance in black spruce-feathermoss communities in the Lac St. Jean-Chibougamau region of Quebec, Canada. This study addressed how natural and anthropogenic disturbances altered soil and fine root carbon (C), nitrogen (N), and phosphorus (P) stocks and dynamics under four disturbance types in central Quebec including: (1) recently burned sites, (2) sites recently harvested using a practice called "cutting with protection of tree regeneration and soils" (CPRS), (3) sites that were burned 75-85 yr ago, and (4) sites that were horse-logged 55 yr ago. Soil N contents in the surface organic layer of the recently burned sites (91 g/m2) were significantly lower than under the old burn sites (146 g/m2). Using equivalent soil masses, total N stores in the mineral soils under the recently harvested stands (76 g/m2) were significantly lower than N stores under the old burn sites (114 g/m2). In field incubations from June to October 1997, net N mineralization rates in the organic horizons ranged from 3.3 kg/ha (recent burn) to 17.1 kg/ha (old harvest). The largest pools of labile inorganic-P (Bray 2 extractable) were in the organic horizons of the old harvest and old burn sites (59.1 and 55.3 mg/kg, respectively), and these pools increased up to 24% from the beginning to the end of the growing season. Pools of dissolved organic nitrogen in surface organic horizons in all of the disturbance types were highest just after snowmelt and declined steadily through the growing season. The older disturbance types had higher N stores in fine roots compared to the recently disturbed sites, and fine root length in the old harvest sites (1623 m/m2) was higher than in the stands burned 75-85 yr ago (917 m/m2).
The ratio between projected leaf area (LA) and cross-sectional sapwood area (SA) of dominant and codominant balsam fir trees (Abies balsamea (L.) Mill.) was determined in 24 forest stands across the ...province of Quebec. Various physical factors proposed in the Whitehead hydraulic model, and some of the easily measured surrogates of these factors, were tested for their influence on LA:SA ratios. Average growing season vapor pressure deficit, temperature, precipitation, and stand drainage class did not significantly influence LA:SA ratios. On the other hand, LA:SA ratios were positively influenced by sapwood permeability (k), tree height, and crown length. As suggested by the model, there was a positive correlation between sapwood permeability and LA:SA ratio and a negative correlation between tree height or crown length and LA/(SA k). Increases in sapwood permeability with tree age were associated with longer tracheids having larger lumen diameters. Of the various empirical factors tested, only site quality, 5-year basal area growth, and age had a significant influence on LA:SA ratios. Sapwood cross-sectional area at breast height by itself was a reasonable linear predictor of LA for all stands (LA = -0.158 + 0.709 SA(BH), R(2) = 0.75). Using the variables that were previously determined to influence LA:SA ratios, stepwise regressions revealed that only crown length and 5-year basal area growth significantly improved linear predictions of LA based on sapwood area. However, the increase in R(2) was relatively modest, i.e., 0.83 for all three independent variables versus 0.75 for SA alone. The results from this study will be useful in integrating physiologically based measurements, such as growth efficiency, into standard forest inventory practices for balsam fir and thus could be beneficial in developing new silvicultural strategies for protecting Quebec's forest resource.
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 (A(n)) and maximum capacity of Rubisco for CO(2) fixation (V(max)) than unfertilized trees. One-year-old needles of fertilized trees had higher stomatal conductance (g(s)), higher water-use efficiency, and lower intercellular to ambient CO(2) ratio than unfertilized trees. Additionally, fertilized trees had higher predawn and midday shoot water potentials than unfertilized trees. Stomatal conductance of 1-year-old needles was 23% higher in seedlings than in layers, but there were no significant differences in g(s) of current-year needles between the regeneration types. For both needle age-classes, A(n) and V(max) of layers were 25 and 40% higher, respectively, than the corresponding values for seedlings. The higher values of A(n), V(max) and foliar N concentration of layers compared with seedlings after two growing seasons may be associated with the larger root systems of the layers compared with the transplanted seedlings.
Growth and physiology of layers versus naturally established seedlings of boreal black spruce (Picea mariana (Mill.) BSP) were compared 15 years after a cutover in Quebec. During the first 8 years, ...height growth of seedlings was greater than that of layers, averaging 10.4 and 7.0 cm/year, respectively. For the last 5 years, annual height growth of layers and seedlings did not differ (25 cm/year; p > 0.05). Over the entire 15-year period, total height growth of seedlings (251 cm) was greater than that of layers (220 cm), although total height did not differ (p > 0.05) over the last 6 years. During the 15th growing season, there were no differences (p > 0.05) for predawn shoot water potential, stomatal conductance, net photosynthesis, intercellular to ambient CO2 ratio, water use efficiency, and hydraulic conductance between layers and seedlings. For diurnal shoot water potential, seedlings showed slightly less stress than layers on two of the four sampling dates. Thus, in the first few years following the cutover, the slower growth observed for layers indicated that they had a longer acclimation period following the cutover. Afterwards, similar height growth, total height, and physiological characteristics of the two regeneration types indicated that layers can perform as well as naturally established seedlings.
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