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1.
Forest harvesting strategies that approximate natural disturbances have been proposed as a means of maintaining natural species’ diversity and richness in the boreal forests of North America. Natural disturbances impact shoreline forests and upland areas at similar rates. However, shoreline forests are generally protected from harvest through the retention of treed buffer strips. We examined bird community responses to forest management guidelines intended to approximate shoreline forest fires by comparing bird community structure in early (1–4 years) post-burned and harvested boreal riparian habitats and the adjacent shoreline forest. We sampled riparian areas with adjacent: (1) burned merchantable shoreline forest (n = 21), (2) burned non-merchantable shoreline forest (n = 29), (3) 10 m treed buffer with 25% retention in the next 30 m (n = 18), and (4) 30 m treed buffer (n = 21). Only minor differences were detected in riparian species’ abundance and bird community composition between treatments with greater differences in these parameters occurring between post-fire and post-harvest upland bird communities. Indicators of all merchantable treatments were dominated by upland species with open-habitat species and habitat generalists being typical upland indicator species of burned merchantable habitats and forest specialists typical upland indicators of harvested treatments. Riparian species indicative of burned riparian habitats were Common Yellowthroat (Geothlypis trichas), Le Conte’s Sparrow (Ammodramus leconteii) and Eastern Kingbird (Tyrannus tyrannus) and indicators of 30 m buffers were Alder Flycatcher (Empidonax alnorum) and Wilson’s Warbler (Wilsonia pusilla). Multivariate Redundancy Analysis (RDA) of the overall (riparian and upland birds) community showed greater divergence than RDA with only riparian species suggesting less effect of fire and forestry on riparian birds than on upland birds. Higher natural range of variability (NRV) of overall post-fire bird communities compared to post-harvest communities emphasizes that harvesting guidelines currently do not achieve this level of variability. However, lack of a large negative effect on common riparian species in the first 4 years post-disturbance allows for the exploration of alternative shoreline forest management that better incorporates bird community composition of post-fire riparian areas and shoreline forests.  相似文献   

2.
Forests accumulate much less carbon than the amount fixed through photosynthesis because of an almost equally large opposing flux of CO2 from the ecosystem. Most of the return flux to the atmosphere is through soil respiration, which has two major sources, one heterotrophic (organisms decomposing organic matter) and one autotrophic (roots, mycorrhizal fungi and other root-associated microbes dependent on recent photosynthate). We used tree-girdling to stop the flow of photosynthate to the belowground system, hence, blocking autotrophic soil activity in a 120-yr-old boreal Picea abies forest. We found that at the end of the summer, two months after girdling, the treatment had reduced soil respiration by up to 53%. This figure adds to a growing body of evidence indicating (t-test, d.f. = 7, p < 0.05) that autotrophic respiration may contribute more to total soil respiration in boreal (mean 53 ± 2%) as compared to temperate forests (mean 44 ± 3%). Our data also suggests that there is a seasonal hysteresis in the response of total soil respiration to changes in temperature. We propose that this reflects seasonality in the tree below-ground carbon allocation.  相似文献   

3.
The establishment of terrestrial buffer zones around vernal pools has been recommended to provide upland habitat for pool-breeding amphibians in areas where forestry practices occur adjacent to breeding sites. However, few studies have empirically tested the effectiveness of buffers. We assessed post-breeding emigration behavior (net emigration distance, rate of movement, proportionate use of available habitats) of radio-tagged adult wood frogs (Lithobates sylvaticus) at nine vernal pools with experimental forest buffer treatments in central Maine, USA. Buffer treatments were either 30-m (N = 3) or 100-m (N = 3) forest buffers surrounded by a 100-m wide clearcut; pools surrounded by uncut forest served as reference sites (N = 3). We tracked 33 individuals in 2004 and 2005, for an average of 41 days, as they emigrated from breeding pools. Recently clearcut habitat was permeable to emigrating adult wood frogs, particularly females. A higher proportion of frogs at 30-m buffer sites than at 100-m buffer sites traveled through the clearcuts to reach intact forest beyond, suggesting that 30-m buffers may not provide sufficient upland habitat to support adult wood frog populations. There was high variability in emigration behavior among frogs, regardless of buffer treatment, and males and females tended to exhibit differential responses to different buffer sizes and to clearcut habitat. Although wood frogs in this study utilized both 30-m and 100-m forest buffers, variability between sexes and density-dependent effects could render small buffers (e.g., 30 m or less) inadequate to support these populations.  相似文献   

4.
Exotic species possess abilities to harm the ecosystems they invade. This study assesses the density, frequency and cover of exotic plants in roadside right-of-ways, logged areas and wildfire sites within mixedwood sections of the southern boreal forest of Saskatchewan. A total of 23 exotic species were observed including nine species of Gramineae, seven species of Leguminosae and five species of Compositae. Average density of exotic species in areas recently disturbed by timber harvesting or wildfire was 0.2 stems m−2 with a frequency of 72%. Exotic species adapted for wind dispersal were best represented including common dandelion (Taraxacum officinale), perennial sow thistle (Sonchus arvensis) and annual hawksbeard (Crepis tectorum). Only two exotic species, T. officinale and Canada bluegrass (Poa compressa), were observed in mature forest; both occurred with a frequency of 13% and an average density of 0.002 stems m−2. A total of 22 exotic species was found in the right-of-ways quadrats with an average density of 117 stems m−2 and a frequency of 94%. The most frequently observed exotic species in the roadside right-of-way areas were T. officinale, alsike clover (Trifolium hybridum), S. arvensis, creeping red fescue (Festuca rubra) and smooth brome grass (Bromus inermis). These species are either common agricultural weeds or were part of the original seed mixture used to establish a plant cover in the roadside right-of-ways.  相似文献   

5.
We addressed the efficacy of stream-side buffers in ameliorating the effects of clearcut timber harvest on Cascade torrent salamanders (Rhyacotriton cascadae), coastal/Cope's giant salamanders (Dicamptodon tenebrosus/D. copei), coastal tailed frogs (Ascaphus truei), and water temperature regimes in the Cascade Range in southern Washington. Forty-one streams in 4 categories were sampled; streams in clearcuts with and without buffers, streams in 35+ year old second-growth forest, and streams in unharvested forest (150+ years old). Tailed frog and Cascade torrent salamander densities were 2–7-fold lower (P < 0.05), respectively, in streams in managed forests than in streams in unharvested forest. In addition, both these species were less abundant (P < 0.05) in unbuffered streams than streams with buffers or in second-growth forest. In contrast, giant salamander densities were 5–50% greater (P > 0.05) in managed streams than unharvested, being greatest in unbuffered and second-growth streams. We used the differences in density estimates of unbuffered streams and unharvested streams to define an ecologically important effect size for each species and then compared the mean effect size and 95% confidence intervals of contrasts between managed stream categories to assess buffer effectiveness. Buffers had a positive ecologically important effect on the density of torrent salamanders and tailed frogs, but had an ecologically negative effect on giant salamanders. Water temperatures were similar among stream categories. However, Cascade torrent salamanders were nearly absent from streams where temperatures were ≥14 °C for ≥35 consecutive hours. Issues that need further study include effective buffer width and longitudinal extent, and confirmation of the water temperature threshold we identified.  相似文献   

6.
Restoration of protected areas in boreal forests frequently includes creating substantial volumes of dead wood. While this benefits a wide range of dead wood dependent invertebrate species, some of these are regarded as forest pests. Therefore, the risk of elevated levels of tree mortality in surrounding commercial forests must be considered. In a large-scale field experiment in southern Finland, we studied the effects of restoration treatments on the abundance of bark beetles within and in the vicinity of restored areas, in particular focusing on Ips typographus and Pityogenes chalcographus. The treatments applied to managed Norway spruce forests were controlled burning and partial harvesting combined with retaining 5, 30 or 60 m3/ha of cut down wood. We found that the abundance of bark beetles increased by both burning and harvesting with down wood retention, being highest where burning and harvesting had been combined. The actual volume of down wood retention had no significant effect. The effect of burning on the number of bark beetles along host tree boles was negative which suggests that burnt spruces provided a less suitable resource for bark beetles than unburnt dead spruces. The abundance of bark beetles along host trees also decreased with increasing volume of down wood retention. The abundance of P. chalographus was slightly elevated up to 50 m outside restored areas but the abundance was very low compared to that within the areas. The abundance of I. typographus was extremely low outside restored areas. We conclude that restoration treatments increase the abundance of bark beetles via increased availability of resources, but that the effect of burning is likely to be counteracted by decreased resource quality. Thus, burning might be the “safest” way to produce large quantities of dead wood. Furthermore, the fact that only few beetles were collected in adjacent areas suggests that restored areas pose little threat of serving as refugia in which bark beetle populations increase in sufficient numbers to attack live trees in adjacent forests. However, restoration actions repeated at consecutive years within a small area might enable the populations to grow to outbreak levels.  相似文献   

7.
Ammonium nitrate (NH4NO3) was applied monthly (from June to October) for 3 years in a balsam fir (Abies balsamea (Linné) Miller) and a black spruce (Picea mariana (Mill.) BSP) boreal forest in Québec (Canada). The design was composed of nine experimental units of 10 m × 10 m for each site. Application rates were 3 and 10 times the atmospheric N deposition measured at each site which was 6 and 3 kg ha−1 year−1 for the fir and the spruce sites, respectively. Soil solution composition (30 and 60 cm), tree growth, and foliar concentrations were analysed. The inorganic N in the soil solution of the control plots of both sites was low, particularly at the spruce site indicating that these forests are actively accumulating the atmospheric deposited N. Nitrogen additions regularly caused sudden and large inorganic N increases in the soil solution at both sites, both treatments and both sampling depths. However, these increases were transitory in nature and no persistent changes in inorganic N were observed. It was estimated that more than 95% of the added N was retained above the rooting zone at both sites. Nitrogen addition increased N, Ca, Mg and Mn foliar concentrations at the black spruce site but had no effects at the balsam fir site. After 3 years of N application, tree growth was similar in the control and the treated plots at both sites. Our results show that slow growing black spruce boreal forests with low ambient N deposition are responsive (in term of foliar N, Ca, Mg and Mn concentrations) to even small increases in N inputs, compared to higher growth balsam fir boreal forests with higher N deposition.  相似文献   

8.
The aim of this study was to describe the dynamics of seedling establishment and development in spruce-dominated uneven-aged boreal forests. The study was based on empirical data from 15 stands with permanent plots, which had been intensively monitored for 10 years in southern Finland. All trees (height > 1.3 m) were measured every fifth year. Regeneration was measured on 64 permanent sample plots (4 m2) in each stand. The establishment of first-year seedlings was analysed on a yearly basis. The survival and development of older Norway spruce (Picea abies (L.) H. Karst.) seedlings were analysed based on observations made every five years. The establishment of spruce seedlings was closely correlated with the abundance of seed crops. Seedling cohorts originating from abundant seed crops were clearly detectable in the development of seedling height distributions over time. It took about 15 years for spruce seedlings to reach a height level of 15-30 cm. Local basal area had hardly any effect on the emergence or survival of small spruce seedlings, while the number of higher spruce seedlings decreased with increasing local basal area.  相似文献   

9.
In boreal forests, historical variations in the area disturbed by natural disturbances or harvesting have rarely been compared. We measured temporal and spatial variations in areas affected by severe fires and clearcutting throughout the 20th century in a 57, 332 km2 section of the eastern Canadian boreal forest. We examined the effects of these disturbances on spatio-temporal variations in the abundance of forests >60 years. Natural variability for the abundance of forests >60 years was estimated from simulations of natural disturbance regimes. We also measured compositional and structural differences between three categories of stands originating from relatively recent disturbances (∼50 years; clearcutting, fires, and clearcutting followed by fires), and one category of stands that were undisturbed for at least 200 years. At the regional level, we observed that forests >60 years gradually became scarcer throughout the 20th century due to a gradual expansion of harvested areas, an effect most pronounced in the southern part of the region, where mature and old forest abundance was clearly outside the range of natural variability at the end of the studied period. At the stand level, forest composition and structure differed between stand-origin categories: clearcutting-origin stands contained more balsam fir (Abies balsamea), fire-origin stands more black spruce (Picea mariana), and fire/clearcutting-origin stands more hardwoods (Betula papyrifera and Populus tremuloides). Overall, we estimate that strict forest management targets based on natural disturbance regimes will be difficult to achieve in eastern North-American boreal forests, most notably because contemporary disturbance rates, including both clearcutting and fire, have gradually become higher than the fire rates observed during the preindustrial period.  相似文献   

10.
Coastal swamps are among the rapidly vanishing wetland habitats in Louisiana due to accelerated sea-level rise and hydrological alterations that alter the natural flooding regime. In particular, the swamp forests of Lake Maurepas, Louisiana, have degraded considerably, and research regarding their condition might suggest approaches for their restoration. We measured forest structure, species composition, tree mortality, annual aboveground net primary production (ANPP) of woody species, and aboveground biomass allocation to leaf litter and wood, and soil strength at forty study plots within the Lake Maurepas basin over 5 years to evaluate the current condition of this coastal forested wetland. Local measures of salinity and regional measures of flooding were used to predict ANPP and aboveground biomass allocation. The 5-year study period included an intense drought as well as years characterized by hurricane-induced flooding. The forty study plots could be divided into four distinct habitat clusters based on standing biomass, structural variables, and salinity. The majority of the plots were co-dominated by Taxodium distichum and Nyssa aquatica. Acer rubrum var. drummondii and Fraxinus pennsylvanica were common mid-story species throughout the western and southern parts of the study area, while Salix nigra, Morella cerifera, and Triadica sebiferum were more important at the more degraded plots in the eastern part of the basin. Annual mean soil salinity reached unprecedented level (2–5 psu) during the drought and cumulative tree mortality reached up to 85% in areas characterized by frequent saltwater intrusions. The ANPP was higher during the drought period in 2000–2001 than during subsequent years, and was dominated by T. distichum. At most sampling plots, litter production exceeded wood production annually. A negative correlation between aboveground biomass allocation to litter and flooding indicated that biomass allocation shifted from litter toward wood during wet years. Overall, the majority of the plots sampled produced less than 400 g m−2 yr−1 of aboveground biomass annually due to the interacting negative effects of saltwater intrusion and prolonged flooding with nutrient-poor water. Reintroduction of Mississippi River water to the Maurepas system has the potential to benefit these swamps greatly by restoring a greater flow of nutrients, sediments, and fresh water through the wetlands. The historically slow (i.e., multi-decadal) process of swamp deterioration was greatly sped by low salinity (i.e., 2–5 psu) saltwater intrusions during a drought in 1999–2000. The majority of the coastal swamps in the Pontchartrain Basin are deteriorating, and most of this swamp area will be lost to open water in the foreseeable future if no restoration action is taken.  相似文献   

11.
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12.
Establishing white spruce (Picea glauca (Moench) Voss) by planting it under established aspen (Populus tremuloides Michx.), stands has substantial potential as a technique for regenerating boreal mixedwood stands. The presence of an aspen overstory serves to ameliorate frost and winter injury problems and suppresses understory vegetation that may compete with white spruce. In this study we examine the growth of white spruce during the first 10 years after being planted underneath a 39-year-old stand of trembling aspen following thinning and fertilization. Results indicate successful establishment and reasonable growth rates of white spruce planted under thinned and unthinned aspen stands, even with aspen basal area of 51 m2 ha−1. Thinning of overstory aspen to 1000 or 2000 stems ha−1 did not increase light reaching seedlings, but did result in improvements in light above the shrub layer and in diameter and height growth of the underplanted seedlings. However, these increases in growth of underplanted spruce may not justify the expense of thinnings. Fertilization of these stands prior to planting had no effect on spruce growth. Growth of spruce underplanted at this site near Fort Nelson was similar to that at two other stands near Dawson Creek, B.C.  相似文献   

13.
In Sweden, whole tree harvest is common practice, possibly leading to the depletion of mineral nutrients. Furthermore, the increased use of forestry residues for heat production has caused an increasingly growing amount of by-product consisting of wood ash. Therefore, the Swedish Forest Agency has recommended wood ash application (WAA) to replace the mineral nutrients removed by whole tree harvesting, as well as a means to mitigate the acidification of boreal forests and surface waters. In a multidisciplinary study during 2003–2006 in Bispgården (Sweden), we have investigated the limnological effects on a first order stream after WAA (conducted in 2004; 3000 kg ha−1) to a 50-ha forested catchment. In general, no significant effects on an annual basis were found for acidification parameters, such as pH, alkalinity and toxic forms of aluminum (Al). There was, however, evidence of an increased pH during the spring flood, accompanied by a simultaneous decrease in the frequency of low pH-values (<5.6). Moreover, alkalinity increased in the years 2005 and 2006 compared to that of 2003, although the increase in 2006 was not statistically different from that in 2005 or 2003. High concentrations of Al repeatedly occurred in the stream, and the WAA did not affect the frequencies of high concentrations of toxic Al forms (>50 μg l−1). The benthic diatom community did not change as a result of the wood ash treatment and the diatom-based index IPS (Indice de PulluoSensibilité) indicated no nutrient enrichment or organic pollution of the stream water. There were, however, indices of elevated concentrations of potassium (K) in the aquatic moss Fontinalis antipyretica and in leaves from Alder (Alnus incana). We conclude that wood ash treatment of a forested catchment with the dose and form of ash applied in this study did not modify the freshwater ecosystem of a first order stream.  相似文献   

14.
Establishing white spruce (Picea glauca (Moench) Voss) by planting it under established aspen (Populus tremuloides Michx.), stands has substantial potential as a technique for regenerating boreal mixedwood stands. The presence of an aspen overstory serves to ameliorate frost and winter injury problems and suppresses understory vegetation that may compete with white spruce. In this study we examine the growth of white spruce during the first 10 years after being planted underneath a 39 year-old stand of trembling aspen following thinning and fertilization. Results indicate successful establishment and reasonable growth rates of white spruce planted under thinned and unthinned aspen stands, even with aspen basal area of 51 m2 ha−1. Thinning of overstory aspen to 1000 or 2000 stems ha−1 did not increase light reaching seedlings, but did result in improvements in light above the shrub layer and in diameter and height growth of the underplanted seedlings. However, these increases in growth of underplanted spruce may not justify the expense of thinnings. Fertilization of these stands prior to planting had no effect on spruce growth. Growth of spruce underplanted at this site near Fort Nelson was similar to that at two other stands near Dawson Creek, B.C.  相似文献   

15.
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16.
Black spruce (Picea mariana (Mill.) B.S.P.) is the dominant tree species in the Canadian province of Québec’s boreal ecosystem, particularly in the black spruce-feathermoss (BSFM) domain (between the 49th and the 52nd parallels). While black spruce is generally well adapted to regenerate after wildfires, regeneration failure can sometimes occur, resulting in the irreversible conversion of closed-crown BSFM to open black spruce-lichen woodlands (OW). With OWs representing approximately 7% (1.6 M ha) of Québec’s BSFM domain, the afforestation of OWs carries significant theoretical potential for carbon (C) sequestration, which has not yet been evaluated. The main objectives of the study were then: (i) to estimate the theoretical C balance of OW afforestation within the closed-crown BSFM domain in Québec’s boreal forest; (ii) to calculate, using the life cycle analysis (LCA) method, all the GHG emissions related to black spruce OW afforestation in the closed-crown BSFM domain of Québec. The CO2FIX v. 3.1 model was used to calculate the biological C balance between the baseline (natural OW of site index 9 at age 50) and afforestation (black spruce plantation of site index 6 at age 25) scenarios, using the best estimates available for all five recommended C compartments (aboveground biomass, belowground biomass, litter, deadwood, and soil). The simulation revealed a biological C balance of 77.0 t C ha−1, 70 years following afforestation, for an average net sequestration rate of 1.1 t C ha−1 year−1. Biological C balance only turns positive after 27 years. When integrating the uncertainties related to both the plantation growth yield and the wildfire disturbance, the average sequestration rate varies between 0.2 and 1.9 t C ha−1 year−1. GHG emissions are 1.3 t CO2 equiv. ha−1 for all afforestation-related operations, which is less than 0.5% of the biological C balance after 70 years. Thus, GHG emissions do not significantly affect the net C balance of the afforestation project simulated. Several recommendations are made, mostly centered on the factors influencing the growth rate of carbon stocks and the impact of natural disturbances, to minimize the range of uncertainties associated to the sequestration potential and maximize the mitigation benefits of an OW afforestation project.  相似文献   

17.
Information on roosting requirements and responses to forest management is integral to effectively conserve and manage bat populations. Tree hollows are especially important for roosting bats given the long time taken for hollows to form. We used radiotelemetry to compare roost site selection in two species, Vespadelus regulus and Nyctophilus gouldi, in logged jarrah forests of south-western Australia. We compared characteristics of roost trees and forest structure around roost trees (n = 48) with randomly located plots at a local roost tree level (n = 90) in February and March 2009. For landscape features, we compared roost trees with randomly selected trees in the broader landscape that had cavities or exfoliating bark (n = 204). V. regulus roosted solely in hollows that were located predominantly in contemporarily unlogged buffers and mature forest while N. gouldi used a broader range of roost types, located in contemporarily unlogged buffers and mature forest and in retained habitat trees in gap release and shelterwood creation silvicultural treatments. In contrast with N. gouldi, which selected hollows or crevices under bark near the ground and close to vegetation, V. regulus used hollows that were high above ground and had little surrounding vegetation. Both species preferred large trees, in intermediate or advanced stages of decay and crown senescence. Bats changed roosts frequently, with short distances between subsequent roosts, suggesting a degree of spatial fidelity. Contemporarily unlogged buffers and mature forest contained higher densities of trees with hollows than gap release and shelterwood creation areas, potentially providing more alternate bat roosts. Our results demonstrate the importance of mature forest and unlogged buffers as bat roost sites in logged jarrah forests of south-western Australia, but the area of old forest required by these and co-occurring bat species remains to be determined.  相似文献   

18.
Laterally cast Picea albertiana ssp. albertiana (western white spruce) shadows were analyzed to determine their effect on understory plant abundance in two high-latitude (62.7°N) boreal Populus tremuloides (trembling aspen) forest stands. Each stand had a uniform and continuous overstory, and occurred on level to gently sloping terrain with a submesic moisture regime. Picea >1 m tall had <20% cover in each stand, with few trees equalling or exceeding the height of the P. tremuloides canopy. Understory vegetation composition was sampled in 30-m × 30-m plots that were subdivided into 1.5-m × 1.5-m cells (200 sampled per plot). Picea shadow locations and their areal extent were determined on an hourly basis (7:00-19:00 h Pacific Standard Time on the summer solstice) for individual plot cells using silhouette diagrams constructed from tree height and canopy-related data (n = 140 trees). Shadow data were analyzed using the lower- (QL, minimum to first-quartile values) and upper-most (QU, third-quartile values to maximum) portions of each species’ abundance distribution. Kruskal-Wallis tests (P < 0.001) indicated that greater Arctostaphylos uva-ursi (bearberry) abundance occurred where shadow cover was the least (daytime average ∼24%); whereas Geocaulon lividum (toadflax), Hylocomium splendens (stairstep moss), and Shepherdia canadensis (buffaloberry) incurred the most shadows (>34% cover) and had the shortest periods of continuous (<6 h) sunlight exposure with <30% Picea shadow cover. Hylocomium and Shepherdia also occurred nearer Picea than Arctostaphylos. Rosa acicularis (wild rose), Linnaea borealis (twinflower), Vaccinium vitis-idaea (bog cranberry), Chamerion angustifolium (fireweed), and Calamagrostis purpurascens (purple reedgrass) incurred intermediate amounts of shadow. Differences in hourly shadow abundance values (QU minus QL plot cells) were greatest for Arctostaphylos (−14.7%) and Rosa (−10.8%), but H. splendens (+3.8%) and Geocaulon had the least (+1.7%). Greater Hylocomium and Shepherdia abundance occurred in plot cells with more shadow indicating a tolerance for shade, which was contrary to the other species. These differences may represent examples of niche partitioning based on relative light availability. Individual understory species based on percent cover and species richness were more strongly correlated with Picea shadow cover than canopy cover. As a direct representation of impeded light transmittance, assessment of lateral tree shadows may represent a viable approach for investigating within stand compositional variation and temporal change among forest understory species, when a distinct physiognomic difference occurs between seral and climax overstory species.  相似文献   

19.
To investigate the interactive effects of CO2 concentration ([CO2]) and nitrogen supply on the growth and biomass of boreal trees, white birch seedlings (Betula papyrifera) were grown under ambient (360 μmol mol−1) and elevated [CO2] (720 μmol mol−1) with five nitrogen supply regimes (10, 80, 150, 220, and 290 μmol mol−1) in greenhouses. After 90 days of treatment, seedling height, root-collar diameter, biomass of different organs, leaf N concentration, and specific leaf area (SLA) were measured. Significant interactive effects of [CO2] and N supply were found on height, root-collar diameter, leaf biomass, stem biomass and total biomass, stem mass ratio (SMR), and root mass ratio (RMR), but not on root mass, leaf mass ratio (LMR), leaf to root ratio (LRR), or leaf N concentration. The CO2 elevation generally increased all the growth and biomass parameters and the increases were generally greater at higher levels of N supply or higher leaf N concentration. However, the CO2 elevation significantly reduced SLA (13.4%) and mass-based leaf N concentration but did not affect area-based leaf N concentration. Increases in N supply generally increased the growth and biomass parameters, but the relationships were generally curvilinear. Based on a second order polynomial model, the optimal leaf N concentration was 1.33 g m−2 for height growth under ambient [CO2] and 1.52 g m−2 under doubled [CO2]; 1.48 g m−2 for diameter under ambient [CO2] and 1.64 g m−2 under doubled [CO2]; 1.29 g m−2 for stem biomass under ambient [CO2] and 1.43 g m−2 under doubled [CO2]. The general trend is that the optimal leaf N was higher at doubled than ambient [CO2]. However, [CO2] did not affect the optimal leaf N for leaf and total biomass. The CO2 elevation significantly increased RMR and SMR but decreased LMR and LRR. LMR increased and RMR decreased with the increasing N supply. SMR increased with increase N supply up to 80 μmol mol−1 and then leveled off (under elevated [CO2]) or stated to decline (under ambient [CO2]) with further increases in N supply. The results suggest that the CO2 elevation increased biomass accumulation, particularly stem biomass and at higher N supply. The results also suggest that while modest N fertilization will increase seedling growth and biomass accumulation, excessive application of N may not stimulate further growth or even result in growth decline.  相似文献   

20.
Neighbourhood competition indices (NCI), where position and species identity of neighbours are known, have been used to investigate growth and competitive interactions among adult trees. In this study, we used NCI in 8–15-year-old stands following clear-cutting in a boreal mixedwood forest of eastern Canada to improve our understanding of early successional forest dynamics. Trees of increasing diameter from the center (≥1 cm) to the edge (≥5 cm) were mapped in twenty-five circular 450 m2 plots. Target trees (DBH ≥ 1 cm) were sampled in plot center to determine their annual radial stem growth. For each species, we compared a set of growth models using either a spatially explicit NCI or a non-spatial competition index. Both types of indices estimated a species-specific competition coefficient for each pair of competitor–target species. NCI were selected as the best competition model for all target species although differences in variance explained relative to the non-spatial index were small. This likely indicates that competition occurs at the local level but that the high density and the relative uniformity of these young stands creates similar neighbourhoods for most trees in a given stand. The effective neighbourhood radius for competitors varied among species and was smaller for shade tolerant species. Intraspecific neighbours were the strongest competitors for most species. Aspen (Populus tremuloides) was a weak competitor for all species as opposed to balsam fir (Abies balsamea) which was a strong competitor in all cases. These results are in contradiction with some widely used forest policies in North America (e.g. free-to-grow standards) that consider broadleaf species, such as aspen, as the strongest competitors. For these early successional forests, the decision regarding the use of spatial or non-spatial competition indices should rest on the intended use. For even-age management, spatial indices might not justify their use in high-density stands but they are needed for the simulation of novel harvest techniques creating complex stand structure.  相似文献   

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