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In this study, we conducted a controlled experiment to assess the growth and survival of balsam fir (Abies balsamea (L.) Mill.), red spruce (Picea rubens Sarg.), and red maple (Acer rubrum L.) seedlings in response to warming, drought, and elevated CO2, as projected under RCP 8.5 for North America’s Acadian Forest Region. In response to warming, only red spruce increased in height; however, this effect varied by CO2 and soil moisture treatments. Under the drought treatment, red spruce biomass was not affected, but mortality increased by 2%. With warming, increases in balsam fir height growth were only detected under certain soil moisture and CO2 conditions. Balsam fir biomass decreased by 24% under drought, while mortality increased by 5%. Warming did not improve red maple height growth, but it remained 7–50 times greater than that of the conifers and no mortality was observed. Overall, CO2 enrichment increased height growth of droughted seedlings relative to the ambient treatment, demonstrating an amelioration of the negative drought effect. Balsam fir was the least adapted to warming and drought, while red spruce displayed some positive responses. Although growth of red maple seedlings did not increase with warming, they exhibited greater absolute growth and survival, which suggests red maple may outperform both conifers under a warming climate.

Increasing frequencies of severe heat waves and drought are expected to influence the composition and functioning of ecosystems globally. Our ability to predict and mitigate these impacts depends on our understanding of species- and age-specific responses to these stressors. To assess the adaptive capacity of balsam fir to climate change, a cold-adapted boreal tree species, we conducted a climate-controlled greenhouse experiment with four provenances originating from across the species biogeographic range, 12 temperature treatments ensuring a minimum of +11°C warming, and five drought treatment intensities. We found considerable acclimation to temperature and drought treatments across all provenances, with steady gains in biomass under temperatures well-beyond the “worst-case” (RCP 8.5) climate forcing scenario within the species natural range. Acclimation was supported by high phenotypic plasticity in root:shoot ratio (RSR) and photosynthesis, which were greatly increased with warming, but were not affected by drought. Our results suggest that regardless of the observed provenance variation, drought and heat are not limiting factors of the current-year balsam fir seedling growth, instead, these factors may be more impactful on later stages of regeneration or previously stressed individuals, thus highlighting the necessity of incorporating the factors of ontogeny and provenance origin in future research regarding plant and climate interactions.

In the Acadian Forest Region of northeastern North America, forest managers are under increasing public pressure to restore the forest to a more historic, natural condition by reducing in clearcutting and promoting partial-cut treatments that more closely emulate historic, local natural disturbance regimes. However, although numerous studies on the effects of partial-cutting on forest regeneration response have been conducted in surrounding temperate and boreal forest ecosystems, there are few studies that directly explore responses to various forms of harvesting within the Acadian Forest ecosystem, with its unique mixture of northern hardwoods and boreal forest species. Here, we conducted one of the first retrospective studies on forest regeneration following a variety of harvesting methods in the Acadian Forest using univariate and multivariate regression trees to assess regeneration response in 50 naturally-regenerating, harvested forest sites in New Brunswick, Canada. Our study shows that regeneration was highly influenced by harvest type, overstory composition, and environmental conditions as reflected by ecoregion classification. Canopy opening size (as controlled by harvest method) significantly influenced the dominance of regenerating species. The presence of conspecific overstory trees increased the likelihood of their regeneration following disturbance, supporting the direct-regeneration hypothesis, especially for species with limited seed dispersal (e.g., sugar maple (Acer saccharum Marsh.) and American beech (Fagus grandifolia Ehrh.). Despite reported problems elsewhere in eastern North America, neither American beech nor balsam fir (Abies balsamea (L.) Mill.) constituted significant competition for the desired species on a broad scale, but the presence of beech was a significant deterrent for yellow birch (Betula alleghaniensis Britt.).

We used 15 site-years of minirhizotron observations (1998–2006 at one site; 1998–2000 and 2004–2006 at second site) from two mature balsam fir ( Abies balsamea (L.) Mill.) sites to quantify interannual variability in fine root demography and assess the accuracy of estimates from early years of observation. Annual production varied fourfold at Forêt Montmorency (FM) (5.8–26.5 roots·100 cm–2) and twofold at Green River (GR) (7.2–14.2 roots·100 cm–2). Annual mortality varied more than 30-fold at the two sites (FM: 0.7–23.2 roots·100 cm–2; GR: 0.3–10.9 roots·100 cm–2), year-end standing crops varied two- to eight-fold (FM: 3.6–28.4 roots·100 cm–2; GR 8.5–18.6 roots·100 cm–2), and median life-span of annual cohorts varied from 180 to 540 days at FM and from 350 to 577 days at GR. This variation illustrates that root demography estimates from short-term studies may differ widely from long-term means. Annual mortality and standing crops were lowest in the first year of observation and tended to increase for two or more years at both sites, whereas these trends were not observed for annual production. Our results indicate that minirhizotron tubes must be in place for more than 2 years to accurately estimate fine root demography at balsam fir sites.

We measured respiratory fluxes of carbon dioxide by aboveground tree components and soil respiration with chambers in 2005 and scaled up these measurements over space and time to estimate annual ecosystem respiration (R(e)) at a mature black spruce (Picea mariana (Mill.) B.S.P.) ecosystem in Quebec, Canada. We estimated periodic annual net primary production (NPP) for this ecosystem also. R(e) was estimated at 10.32 Mg C ha(-)(1) year(-)(1); heterotrophic respiration (R(h)) accounted for 52% of R(e) and autotrophic respiration (R(a)) accounted for the remainder. We estimated NPP at 3.02 Mg C ha(-1) year(-1), including production of bryophyte biomass but not including shrub NPP. We used these estimates of carbon fluxes to calculate a carbon use efficiency [CUE = NPP/(NPP + R(a))] of 0.38. This estimate of CUE is similar to those reported for other boreal forest ecosystems and it is lower than the value frequently used in global studies. Based on the estimate of R(h) being greater than the estimate of NPP, the ecosystem was determined to emit approximately 2.38 Mg C ha(-1) year(-1) to the atmosphere in 2005. Estimates of gross primary production (GPP = NPP + R(a)) and R(e) differed substantially from estimates of these fluxes derived from eddy covariance measurements during 2005 at this site. The ecological estimates of GPP and R(e) were substantially greater than those estimated for eddy covariance measurements. Applying a correction for lack of energy balance closure to eddy covariance estimates reduces differences with ecological estimates. We reviewed possible sources of systematic error in ecological estimates and discuss other possible explanations for these discrepancies.

We are investigating responses of ecosystem processes to climate and silvicultural practices to improve predictions of climate change impacts and to assess opportunities for adapting to impending changes. This involves studies of fine- and coarse-root dynamics, the distribution of net primary production between aboveground and belowground components of balsam fir (Abies balsamea [L.] Mill.) and sugar maple (Acer saccharum [Marsh.]) and photosynthetic capacity of balsam fir during winter and spring. Ongoing studies are described. Key words: fine roots, coarse roots, conifer, hardwood, net primary production, biomass dynamics, photosynthetic recovery, root lifespan, root turnover, soil temperature

Root systems of 31 balsam fir ( Abies balsamea (L.) Mill.) trees were excavated at five sites in central New Brunswick. Sites ranged from 10 to >80 years in stand age; most had been thinned at least once, and densities ranged from 750 to 44 000 trees·ha–1. The relationship between diameter at breast height and coarse root biomass in the present investigation was similar to one reported for balsam fir in northern New Brunswick but differed from two other published biomass equations. Coarse root biomass per hectare ranged from 3 to 30 Mg·ha–1 among studied sites. Coarse root biomass in the smaller root diameter classes was similar at all sites, but biomass in diameter classes >50 mm increased with stand age. Precommercial thinning added 3 Mg coarse root biomass·ha–1 to the detrital pool. The ratio of coarse root biomass to stem biomass averaged 0.36 (±0.02 SE). These results show that there is more belowground biomass in balsam fir ecosystems than would be expected from the generic ratio of belowground to aboveground biomass often applied to coniferous ecosystems, pointing to a need for using ratios that are more specific.

Aboveground net primary productivity (ANPP) was measured in three balsam fir (Abies balsamea (L.) Mill.) forests on a climatic transect extending from southern New Brunswick ("warm" study area) to central Quebec ("cool" study area). Annual foliar production was estimated with a relationship between cross-sectional area at breast height of the current-year annual xylem ring and the mass of current-year foliage, using data obtained by harvesting trees at the beginning of the study. This relationship differed among study areas. Annual branch production was determined from annual foliar production and the ratio of annual branch production to annual foliar production. The ratio of branch to foliar production was estimated from intensive measurement of a sample of branches collected at the end of the study period; it varied among years but was similar for all study areas. ANPP was 3.36 Mg C·ha–1·year–1 at the warm study area, 3.73 Mg C·ha–1· year–1 at the mid-transect study area, and 3.04 Mg C·ha–1·year–1 at the cool study area. These estimates of ANPP were greater than those estimated using a conventional method of summing up increment and litterfall. On average, the conventional estimate of ANPP was 83% of the estimate using relationships described above. Because net ecosystem productivity is the difference between NPP and heterotrophic respiration, a 17% underestimate of NPP can have a substantial effect on the estimate of carbon-sink activity of a forest.

Concern exists that soil respiration (Rs) will increase with global warming, thereby reducing the sink strength of forest ecosystems for carbon (C) or making them sources. We measured Rs at three balsam fir (Abies balsamea (L.) Mill.) sites on a climatic gradient from 1997 to 2000 to assess potential responses to global warming. Mean annual Rs varied from 0.67 kg C·m-2·year-1 at the cool site, to 1.19 kg C·m-2·year-1 at the midtransect site, to 1.52 kg C·m-2·year-1 at the warm site. Trenched plots were installed in 1999 to partition Rs into root respiration (Rr) and heterotrophic respiration (Rh). Differences in trenched-plot respiration (Rt) among sites were much less than differences in Rs, because trenching reduced Rs at the warm site more than at the cool site. Annual Rt was 22% less than annual Rs at the cool site, 36% less at the midtransect site, and 50% less at the warm site. After making adjustments for extraordinary decomposition of killed roots in trenched plots, it was estimated that annual Rh varied little among sites (0.40 kg C·m-2·year-1 at the cool site to 0.54 kg C·m-2·year-1 at the warm site), but annual Rr varied substantially among sites (0.24 kg C·m-2·year-1 at the cool site to 0.98 kg C·m-2·year-1 at the warm site). We conclude that Rs varied among sites, because climatically driven differences in primary productivity caused greater Rr and greater detrital production for Rh.

Net primary production (NPP) and growth efficiency (wood growth per leaf area) decline with stand age for forest ecosystems. Three hypotheses have been suggested: high respiration to photosynthesis ratios, hydraulic limitations on water transport in trees, and nutrient limitations of photosynthesis. Recent work indicates that hydraulic or nutrient limitations probably are the mechanism underlying the decline of growth efficiency in Douglas fir (Pseudotsuga menziesii ) and lodgepole pine (Pinus contorta), both of which are native to western North America. However, balsam fir (Abies balsamea) grows in the cool-humid boreal forest and has very high stem respiration rates, suggesting that the mechanism may be different. We predicted measured aboveground NPP of 24 out of 25 balsam fir stands reasonably well. These stands were selected for having large differences in leaf area index and wood mass. Simulations indicate that increased wood respiration is the major factor causing the decline of NPP and growth efficiency with stand age; a secondary factor for mature stands is nitrogen limitation. For simulations in which balsam fir was fertilized, final growth efficiency was less than that of unfertilized forest stands because of increased stem respiration. From values in the literature, we calculated the potential difference conductance to water flow in the xylem from the soil to the leaves for a mean tree in each stand. Twigs had the smallest conductance; however, because the twigs conductances are added in parallel, the bole was the smallest conductance in the series: bole, branch and twigs. The overall drop of water potential with very high transpiration flux densities was generally less than 2.5 MPa. Therefore, we conclude that, for balsam fir, the decline in NPP and growth efficiency with stand age may be caused by the high respiration to photosynthesis ratios. This conclusion differs from recent work, so there is not a single unifying mechanism of age-related decline for all forest ecosystems. © 1999 Elsevier Science B.V. All rights reserved.

The use of sapwood cross-sectional area at breast height (S) was compared with the use of diameter at breast height (D) and total height (H) for predicting branch weight per tree (B). Data obtained by destructive sampling of 58 balsam fir (Abiesbalsamea (L.) Mill.) trees growing in western and northern Newfoundland were used to fit a simple linear and a nonlinear model using D2H and to fit a simple linear and two nonlinear models using S. Coefficients of determination and standard error of estimates were similar for all equations. There were large differences in estimates of branch weight per hectare between equations for 25 cruised stands. The relationship between B and leaf area was used to evaluate the accuracy of stand estimates. The allometric equation using S as the independent variable provided the most accurate estimates.

Disturbances are important for renewal of North American forests. Here we summarize more than 180 site years of eddy covariance measurements of carbon dioxide flux made at forest chronosequences in North America. The disturbances included stand-replacing fire (Alaska, Arizona, Manitoba, and Saskatchewan) and harvest (British Columbia, Florida, New Brunswick, Oregon, Quebec, Saskatchewan, and Wisconsin) events, insect infestations (gypsy moth, forest tent caterpillar, and mountain pine beetle), Hurricane Wilma, and silvicultural thinning (Arizona, California, and New Brunswick). Net ecosystem production (NEP) showed a carbon loss from all ecosystems following a stand-replacing disturbance, becoming a carbon sink by 20 years for all ecosystems and by 10 years for most. Maximum carbon losses following disturbance (g C m−2y−1) ranged from 1270 in Florida to 200 in boreal ecosystems. Similarly, for forests less than 100 years old, maximum uptake (g C m−2y−1) was 1180 in Florida mangroves and 210 in boreal ecosystems. More temperate forests had intermediate fluxes. Boreal ecosystems were relatively time invariant after 20 years, whereas western ecosystems tended to increase in carbon gain over time. This was driven mostly by gross photosynthetic production (GPP) because total ecosystem respiration (ER) and heterotrophic respiration were relatively invariant with age. GPP/ER was as low as 0.2 immediately following stand-replacing disturbance reaching a constant value of 1.2 after 20 years. NEP following insect defoliations and silvicultural thinning showed lesser changes than stand-replacing events, with decreases in the year of disturbance followed by rapid recovery. NEP decreased in a mangrove ecosystem following Hurricane Wilma because of a decrease in GPP and an increase in ER. An edited version of this paper was published by AGU. Copyright 2010 American Geophysical Union.

The current annual growth of foliage, branches, and stems per unit of foliar weight was approximately the same in unthinned stands of young balsam fir (Abiesbalsamea (L.) Mill.) and stands thinned 4–6 years earlier. Foliage, branches, and stems accounted for approximately 30, 30, and 40%, respectively, of aboveground growth in the thinned and unthinned stands. The allocation of photosynthates to stems was estimated by adding the carbon used for respiration to that used for growth. The proportion of photosynthetic production allocated to stems was greater in an unthinned stand than in a thinned stand. The additional proportion of net photosynthetic production allocated to stems in the unthinned stand was used for maintenance respiration. Since the photosynthetic production per unit of foliar weight of thinned stands should equal or exceed that of unthinned stands, differences in the proportion of photosynthates allocated to stems between these stands imply differences in the proportion of photosynthates allocated to other tree components. Therefore, it appears that a larger proportion of photosynthates was allocated to branch respiration and (or) roots in thinned stands than in unthinned stands.

Argon gas was incorporated into the sap flowing through xylem of cut branches, or added to the air in a sealed cuvette surrounding a needle-free portion of the branch to investigate lateral movement of gases in Pinus strobus. Microdialysis was used to sample air in the xylem and evacuated vials were used to collect samples of air from cuvettes attached to branches. Argon concentrations of samples of air were measured by GC-MS. When argon was added through the sap, concentrations of argon in the xylem and in air of chambers enclosing needle stumps was greater than that of controls, but argon concentrations of air in cuvettes enclosing a needle-free portion of the branch were not greater than controls. When argon was added to cuvettes enclosing a needle-free portion of the branch, the argon did not enter the xylem and it was not emitted by needle stumps.

Average respiration rate per square decimetre of stem surface area was greater in a thinned stand of balsam fir (Abiesbalsamea (L.) Mill.) than in an unthinned stand. While respiration rates of internodes were highly correlated to their rates of stem growth, the differences in respiration among internodes and stands could not be explained by the unequal use of respiration products to support synthesis alone. Maintenance respiration rates were greater for internodes with higher rates of stem growth, and consequently the efficiency of using photosynthates for growth was reduced in the thinned stand. In addition, breast-height internodes respired at higher rates than upper stem internodes with equal growth rates. It is argued that lower stem internodes expended more respiration products for translocation than upper stem internodes. The rate of stem growth per unit of foliar weight was less in the thinned than in the unthinned stand, indicating that the higher rate of respiration per unit of stem surface area counteracted the beneficial effects of thinning on the rate of photosynthesis per unit of foliar weight.

Current annual stem growth rates per hectare of unthinned young stands of balsam fir (Abiesbalsamea (L.) Mill.) were 2–3 times greater than in adjacent stands thinned 4–6 years earlier, largely because unthinned stands contained 2.5–4.0 times more foliar weight per hectare. Stem growth rates per tree in thinned stands were 3–8 times greater than those in unthinned stands because of the reduced competition and the lower ratio of heterotrophic to autotrophic tissues (S/F). However, the foliar efficiencies, as measured by annual stem growth per unit of foliar weight (E), of thinned plots were not significantly greater than those of unthinned plots, in spite of the reduced competition and lower S/F ratio. The lack of response of foliar efficiency to thinning is discussed in relation to published information about photosynthetic rates and stem respiration rates. In all stands, the E values of dominant trees were equal to, or less than, those of subordinate trees, in spite of their superior competitive position and lower S/F ratio. The lack of correlation between competitive position and foliar efficiency is also discussed.