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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.

Eruptive insect pests have significant impacts on the structure and function of forest ecosystems. Outbreaks of spruce budworm (Choristoneura fumiferana) (Clem.), for instance, occur approximately every 35–40 years, resulting in the loss of millions of hectares of spruce–fir forests in eastern North America. In addition to the density-dependent relationships that drive insect outbreaks, rising populations often coincide with drought events, which are expected to increase in both frequency and intensity in response to climate change. However, as populations approach the eruptive phase, consequences of intraspecific competition may outweigh the benefit of host water stress. The objective of our study was to quantify defoliation and insect performance responses to the interactive effects of drought and density of spruce budworm. To test for these interactions, we established a manipulative field experiment in a mature, balsam fir-dominated forest stand using a combination of single-tree rainout shelters and sleeve-caged insect larvae at four different densities: 0, 25, 50, or 100 individuals. Defoliation of 1-year old shoots, but not current-year shoots, significantly increased in response to higher insect densities. Density also had a significant, negative effect on budworm percent survival, although the total number of recovered adults remained highest in the high density treatment. Adult female body mass was significantly reduced in response to increased density, but only on droughted trees. Lastly, male wing length was significantly decreased in response to increased density. Overall, our results demonstrate that across a broad range of outbreak densities, rain exclusion had a minor impact. Accordingly, we anticipate that as insect pest populations approach epidemic levels, the influence of density on defoliation, insect survival, and body condition is likely to outweigh the impact of moderate drought stress.

Detection of spruce budworm (Choristoneura fumiferana Clem.) defoliation is critical for forest protection strategies aimed at minimizing losses in growth and mortality. However current aerial and ground survey methods of detecting defoliation are imprecise and subjective, limiting their usefulness. We evaluated the use of hemispherical crown canopy images to quantify annual spruce budworm defoliation, comparing images taken before and after defoliation in each of two years in 75 sample plots in Quebec, Canada. Gradient Boosting Machine analysis identified gap fraction change from May-October, gap fraction after defoliation, insecticide spraying, and % balsam fir (Abies balsamea (L.) Mill.) basal area as important explanatory variables of defoliation. Logistic Generalized Linear Model (GLM) and Random Forests (RF) models were trained on a random two-thirds of sample plots combining both years, and defoliation predictions were validated on the remaining one-third of plots. RF predictions consistently resulted in slightly higher correlations and lower root mean squared errors (RMSEs) than GLM predictions. Defoliation models including insecticide spraying, gap fraction change May-October, and % balsam fir basal area had RMSEs of 14–22%, whereas models excluding insecticide spraying had higher RMSEs of 18–24%. Model goodness-of-fit using two-sample Kolmogorov-Smirnov tests indicated that predicted and measured annual defoliation had similar distributions, with the exception of GLM and RF models excluding spray compared to ocular defoliation. Use of hemispherical images to quantify gap fraction change is a feasible, non-destructive, and objective method to assess canopy foliage changes caused by spruce budworm defoliation.

We examined the influence of stand structure surrounding individual dominant and codominant trees on leaf area, tree growth, and growth efficiency (stem growth per unit leaf area) in young white spruce ( Picea glauca (Moench) Voss) plantations. Objectives were to (i) test the hypothesis that individual tree volume increment and growth efficiency increase with increasing growing space, and (ii) determine the relative importance of growth efficiency and leaf area to stem volume increment in young spruce following thinning. Growing space was expressed as area potentially available (APA) to each tree. Relative current annual volume increment (annual increment divided by the mean increment for the 3 years immediately preceding thinning) increased linearly with increasing APA 2 and 3 years after thinning, supporting our hypothesis that tree volume increment increases with APA. Growth efficiency however, was not related to APA. Leaf area was positively related to APA 3 years after thinning, and current annual volume increment was related to leaf area. Leaf area per tree increased from 17.8 m 2 to 29.8 m 2 over the 3 years (2011–2013) following thinning and was 16.5–18.7 m 2 for unthinned trees over the same period. Growth efficiency decreased from 1.35 to 0.64 dm 3 m −2 for thinned trees and from 0.74 to 0.55 dm 3 m −2 for unthinned trees from 2010 to 2013. Growth efficiency did not differ between thinned and unthinned trees, but it was significantly lower in year 3 than year 1 after thinning ( p = 0.0178) and significantly lower than the prethinning growth efficiency ( p = 0.0034). Our results show that thinning increased individual tree volume increment by increasing leaf area of remaining trees and not by increasing their growth efficiency.

Background A novel approach to modelling individual tree growth dynamics is proposed. The approach combines multiple imputation and copula sampling to produce a stochastic individual tree growth and yield projection system. Methods The Nova Scotia, Canada permanent sample plot network is used as a case study to develop and test the modelling approach. Predictions from this model are compared to predictions from the Acadian variant of the Forest Vegetation Simulator, a widely used statistical individual tree growth and yield model. Results Diameter and height growth rates were predicted with error rates consistent with those produced using statistical models. Mortality and ingrowth error rates were higher than those observed for diameter and height, but also were within the bounds produced by traditional approaches for predicting these rates. Ingrowth species composition was very poorly predicted. The model was capable of reproducing a wide range of stand dynamic trajectories and in some cases reproduced trajectories that the statistical model was incapable of reproducing. Conclusions The model has potential to be used as a benchmarking tool for evaluating statistical and process models and may provide a mechanism to separate signal from noise and improve our ability to analyze and learn from large regional datasets that often have underlying flaws in sample design.

Simultaneous biometric measurements of aboveground net primary production (ANPP) and eddy-covariance measurements of gross primary production (GPP) were made at 18 forest stands with 80 site-years of data across Canada – to assess the fraction of photosynthesis that is used to produce plant tissues and the consistency of carbon allocation patterns across forest ecosystems. The stands included boreal and temperate forests and spanned very young to mature stand ages. Across all sites, ANPP averaged 298 ± 138 g C m−2 yr−1 (mean ± 1 s.d.), with the highest values for temperate white pine plantations (307–630 g C m−2 yr−1) and harvested Douglas-fir stands (219–459 g C m−2 yr−1), and the lowest values for boreal harvested jack pine stands (97–185 g C m−2 yr−1). ANPP more than doubled from newly established (≤12 years) to young (13–25 years) stands, then stabilized in young to mature (≥51 years) stands but with diverging trends among species. Inter-site variations in ANPP and GPP were closely related to site characteristics, in particular, to leaf area index, which explained 66% of the variation in ANPP and 80% of the variation in GPP, and absorbed photosynthetically active radiation, which explained 80% of the variation in ANPP and 82% of the variation in GPP. Both ANPP and GPP were also positively correlated with mean annual air temperature, mean annual precipitation, and total soil nitrogen in the upper 10 cm of the mineral soil. ANPP was strongly, positively correlated with GPP and the ANPP/GPP ratio was relatively constant (0.29 ± 0.06), with no consistent differences among species or age classes. The results support the use of a constant ANPP/GPP ratio as a reasonable assumption in models of forest productivity for boreal and northern temperate forests. A similar conclusion is reached for the NPP/GPP ratio when published values of belowground NPP are considered.

A new stand-level dynamics model based on observed stand growth trajectories is presented. This stand-level dynamics model uses the trajectories of observed plots through Reineke’s quadratic mean diameter-density space to predict the change in quadratic mean diameter, ingrowth and mortality over time. The model uses the collection of observed trajectories as a probability distribution that is used to guide an informed random walk. An imputation model is used to select k nearest neighbors (bandwidth) which are then used to build joint kernel distributions. From these kernel distributions, m random samples (sample intensity) are averaged to predict the change in quadratic mean diameter, ingrowth and mortality. All levels of k tested (10, 20, 30) performed well as long as sampling intensity was above 1. Variability in predictions was reduced at sampling intensities above 1, but no significant differences were visible among sampling intensities 5 and above.

Fine roots play an important role in forest productivity and carbon cycling, however, little is known about their dynamics during stand development or their response to forestry practices. We used the minirhizotron method to measure fine-root production ( NPP fr ), mortality ( M fr ), and year-end standing crop ( SC fr ) for 5 years at balsam fir ( Abies balsamea (L.) Mill.) stands representing a chronosequence after clearcutting that also incorporated thinning of younger and older stands. We used seemingly unrelated regression to maintain mass balance among components of fine-root dynamics in our investigation of relationships with stand development and responses to thinning. The final statistical model explained 58% of the variation in fine-root dynamics, did not use stand age to explain variations in NPP fr , and used stand age to explain variations in M fr . In the years immediately after clearcutting, SC fr increased rapidly, reaching a maximum in approximately 10 years, and declining slowly thereafter as a stand approached maturity. In the early years as SC fr increased, NPP fr increased and remained approximately constant thereafter. We ascribed the rapid early increase in SC fr to the existence of vigorous advanced growth and high relative fine-root production rates, and the slow decline after reaching a maximum to increasing relative mortality rates. In a pole-stage stand, SC fr recovered approximately 5 years after thinning due to increased relative fine-root production rate and lower mortality rates. In contrast, SC fr did not recover after thinning in a semi-mature stand because relative fine-root production did not increase. Elevated mortality after thinning due to death of root systems of cut trees was observed for several years in thinned stands. The lack of response of fine roots in older thinned stands may limit the response of aboveground components to thinning.

Summary Knowing the abundance of small trees is necessary for accurate calculation of gross production, total carbon and/or biomass of forest stands. The abundance of small trees can also be used to predict ingrowth into larger tree diameter classes. We present a method of predicting numbers of trees in small diameter classes using diameter distributions of larger trees in stands. A truncated two-parameter Weibull distribution was fit to large tree diameters (diameter at breast height (d.b.h.) ≥ 9.0 cm). These parameters were then used to predict the number of small trees in d.b.h. classes smaller than 9.0 cm. Three methods of predicting densities of small trees were used: (1) an extrapolation of the truncated Weibull to a full two-parameter Weibull distribution; (2) a modification of the Weibull using an empirical estimate and (3) a combined approach. While the full two-parameter Weibull distribution generally fitted the distribution of small trees, densities were typically under-predicted. The empirical method (i.e. method 2) produced the best predictions of small tree densities, with a root mean square error of 132 trees h 21 (28 per cent of mean small tree density). Overall, predicting the distribution of small trees using the distribution of large trees worked very well in this study.

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 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.

To investigate the effect of defoliation by the balsam fir sawfly (Neodiprion abietis) on current-year growth and photosynthesis, all 1-year-old and older foliage were removed by manual clipping before the start of the growing season from balsam fir seedlings cultivated at two levels of fertilization. Measurements carried out late in the same growing season showed that both defoliation and withholding fertilizer reduced current-year foliar dry weight, shoot length, needle size, and stem xylem radial width, as well as total foliar dry weight, and root dry weight. Defoliation increased the projected area to weight ratio, nitrogen concentration, net photosynthetic rate per unit of dry weight, and ratio of photosynthesis to internal CO2 concentration in current-year needles. In contrast, withholding fertilizer did not alter the projected area to weight ratio and decreased the nitrogen concentration, net photosynthetic rate per unit of dry weight, and ratio of photosynthesis to internal CO2 concentration. Both defoliation and withholding fertilizer also decreased the number of current-year needles produced in the growing season following treatment. The results indicate that (1) the reduction in total photosynthesizing biomass caused by removing old foliage is much greater than the biomass of old foliage removed due to decreased production of current-year foliage, and (2) old foliage removal stimulates net photosynthesis in current-year shoots, probably by improving foliar nitrogen relations. Removing old foliage reduced the total photosynthesizing biomass more, and stimulated net photosynthesis of remaining foliage less, than the removal of current-year foliage measured in earlier studies, which helps explain why tree recovery is slower following defoliation by the balsam fir sawfly than after spruce budworm (Choristoneura fumiferana) attack.

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.

Satellite remote sensing has long held promise as a powerful method of detecting forest canopy changes and mapping landscape structure over vast, often multi-jurisdictional forest areas. Landsat Thematic Mapper (TM) spectral response, for example, can be related accurately to changes in physiology and cover at a range of small to intermediate mapping scales. These data have been available continuously for almost 20 years; many areas have earlier satellite image archives stretching back to the 1970s. When considering spatially-explicit changes to landscapes—caused by natural and human disturbances—over this time period, digital, synoptic, and repeatable satellite remotely-sensed data are emerging as the observational media of choice that forest managers must possess and use wisely. In this paper, successful use of satellite remote sensing in two of Canada’s Model Forests is described. First, in the Fundy Model Forest in southeastern New Brunswick, a 15-year TM image sequence was used to detect area changes associated with different harvesting and silvicultural practices. Second, in the Foothills Model Forest in west-central Alberta, grizzly bear habitat maps have been created from multi-scene TM land cover mosaics. These map products constitute critical information on landscape change and configuration required to answer key management questions. The paper concludes with a prognosis for the future role of satellite remote sensing in sustainable forest management as data quality continues to improve (i.e., increasing spatial, spectral, temporal, and radiometric resolutions), and methods are brought into the purview of forest managers and practitioners. Key words: remote sensing, landscape change, forest fragmentation, human disturbance, management information needs, habitat

We used the Landsat Thematic Mapper (TM) enhanced wetness difference index (EWDI) to detect forest changes in the Fundy Model Forest, a large, multi-jurisdictional forest management unit in southeastern New Brunswick. The average annual change on the landscape was approximately 3068 ha over a 15 year interval (1984‐1999). The estimate of total change (almost 50 000 ha) as a percentage of the total Fundy Model Forest available productive forest land (approximately 240 000 ha) translates into a rate of change of approximately 1.3% annually.

RESUMEOn decrit des zones de coupes a blanc et de coupes partielles ainsi que les conditions sylvicoles des peuplements forestiers au Nouveau-Brunswick au cours d'une periode d'un an (1997–1998) au moyen de l'interpretation visuelle des differences observees suite a l'application de la transformation «Tasseled Cap» sur deux images Landsat Thematic Mapper. Une image rehaussee des differences d'humidite (EWDI) a ete utilisee pour creer des exemples de changements relies a des conditions de changements connues au sol; nous avons interprete des patrons distinctifs dans les compositions colorees normales et l'image EWDI associes avec les coupes a blanc, les coupes progressives et les coupes d'ensemencement, les coupes par bandes et les coupes associees au jardinage par groupe, et les eclaircies precommerciales et commerciales. Notre interpretation suggere que ces traitements au niveau des peuplements forestiers et les patrons spectraux qu'ils entrainent pourraient servir de base pour le developpement d'une met...

ECOLEAP is a Canadian Forest Service research project that is aimed at improving our understanding of the environmental controls on boreal and sub-boreal forest productivity and at developing tools for predicting stand-level forest productivity over large areas. This interdisciplinary project combines, in a coordinated manner, ecophysiological and soils research, remote sensing research, development of scaling up procedures and process modelling of net primary productivity on a common set of field sites. The process research is carried out in different forest types across large climatic and productivity gradients. Remote sensing provides timely stand information such as composition, leaf area, and absorbed radiation that are not currently available in existing spatial databases. A geographic information system is used to integrate the diverse sources of data. Models serve both as integrators of knowledge and as vehicles for the transfer of the information and methodologies to resource managers. Currently there are three interrelated modelling exercises being carried out within ECOLEAP to address different objectives of the project: an empirical, spatially explicit model of site index, a site-specific process model of productivity, and a spatially explicit process model of productivity. Application of the spatially explicit models will be conducted on extensive pilot regions, the first of which is located north of Quebec City. Key words: modelling, GIS, remote sensing, Canadian Forest Service

RESUMELes modeles regionaux actuels de production primaire nette (PPN) requierent des estimations precises de la surface foliaire du couvert et des classifications precises des couverts forestiers. Ces deux intrants aux modeles peuvent etre difficiles ou impossibles a extraire a partir de bases de donnees SIG mais peuvent etre obtenus par teledetection. Dans le cas des couverts forestiers, une base de donnees SIG d'inventaire forestier type contiendra generalement, sur une base polygonale ou au niveau du peuplement, certaines informations comme par exemple, la description des especes dominantes-codominantes. La structure polygonale est generalement deduite par photointerpretation aerienne et permet d'organiser le paysage en fonction de plusieurs activites de gestion, telles que la coupe d'arbres ou les operations sylvicoles. Toutefois, cette structure polygonale peut s'averer moins utile pour d'autres applications telles que l'estimation de la croissance forestiere, notamment dans le cas des petites surfa...

RESUMEL'indice de surface foliaire LAI (Leaf Area Index) est relie au fonctionnement atmosphere-biosphere et constitue un important attribut structurel des ecosystemes et des peuplements forestiers. Generalement, le LAI est estime pour des zones de grande etendue au moyen d'un ratio ou d'un indice de teledetection multibande tel que le NDVI (Normalized Difference Vegetation Index/Indice d'activite vegetale normalise). De tels ratios sont relies au LAI dans la mesure ou le LAI et le rayonnement photosynthetiquement actif absorbe (APAR - Absorbed Photosynthetically Active Radiation) sont interrelies. Le NDVI ou une mesure semblable est calcule pour des pixels individuels et les valeurs sont ensuite regroupees par rapport aux zones d'interet, par exemple, un ecosysteme entier, un polygone de peuplement forestier dans un SIG ou dans des parcelles individuelles. Les donnees de teledetection a haute resolution spatiale combinees aux techniques avancees de traitement d'image, telle que l'analyse de texture, peuv...

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.

Mixed stands are difficult to manage silviculturally because each species has a different growth habit and mixed stands frequently grow on the most productive sites (Smith 1986). These same characteristics make it difficult to predict the development of mixed stands, but knowledge of the growth of these stands is essential for designing silvicultural practices. The high potential productivity of many mixed stands make it important to address these problems.

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.