Young Balsam Fir Research Articles

Vulnerability of Conifer Regeneration to Spruce Budworm Outbreaks in the Eastern Canadian Boreal Forest

Spruce budworm (Choristoneura fumiferana) is the main defoliator of conifer trees in North American boreal forests, affecting extensive areas and causing marked losses of timber supplies. In 2017, spruce budworm affected more than 7 million ha of Eastern Canadian forest. Defoliation was particularly severe for black spruce (Picea mariana (Mill.) B.S.P.), one of the most important commercial trees in Canada. During the last decades, intensive forest exploitation practices have created vast stands of young balsam fir (Abies balsamea (L.) Mill.) and black spruce. Most research focused on the impacts of spruce budworm has been on mature stands; its effects on regeneration, however, have been neglected. This study evaluates the impacts of spruce budworm on the defoliation of conifer seedlings (black spruce and balsam fir) in clearcuts. We measured the cumulative and annual defoliation of seedlings within six clearcut black spruce stands in Quebec (Canada) that had experienced severe levels of defoliation due to spruce budworm. For all sampled seedlings, we recorded tree species, height class, and distance to the residual forest. Seedling height and species strongly influenced defoliation level. Small seedlings were less affected by spruce budworm activity. As well, cumulative defoliation for balsam fir was double that of black spruce (21% and 9%, respectively). Distance to residual stands had no significant effect on seedling defoliation. As insect outbreaks in boreal forests are expected to become more severe and frequent in the near future, our results are important for adapting forest management strategies to insect outbreaks in a context of climate change.

Plant module size influences the intra‐tree distribution and abundance of a shoot‐boring sawfly in young balsam fir

AbstractField surveys were carried out to assess the effects of intra‐tree variation in developing shoot length within and among crown levels on the density and abundance of the balsam shoot‐boring sawfly, Pleroneura brunneicornis Rohwer (Hymenoptera: Xyelidae), in young balsam fir, Abies balsamea (L.) Mill. (Pinaceae). Overall, cardinal direction had no influence on shoot‐borer density or abundance; however, the highest percentage and abundance of bored shoots occurred on intermediate‐sized shoots within the crown (i.e., in the mid‐crown and on the distal‐lateral and medial‐lateral shoots). Comparatively, few shoot borers occurred in the upper or lower crown levels, or on the relatively large terminal shoots within branches. This distribution appears indicative of the higher suitability of intermediate‐sized shoots within hosts for either egg lay or larval performance. Results of this study are most consistent with predictions of the ‘optimal module size’ hypothesis, which posits that herbivore responses to plant module size should reflect the balance of tradeoffs between utilizing relatively large, nutritious shoots vs. small, more easily exploited shoots.

Effects of fertilization and liming on tree growth, vitality and nutrient status in boreal balsam fir stands

Two fertilization trials were carried out in young balsam fir (Abies balsamea (L.) Mill.) stands showing symptoms of tree decline or foliar chlorosis in boreal forests of eastern Laurentian Highlands in Quebec, Canada. In the first trial, trees showed strong decline symptoms; fertilization treatments consisted of combinations of two nutrients among N, K, and Mg, each at two rates. In the second trial, trees showed some needle chlorosis; various doses of N, K, K–Mg, and K-lime fertilizers were applied. In both trials, N fertilization (200kgNha−1) resulted only in a short-term (2–3years) increase in foliar N concentrations and radial growth. Potassium fertilization (100–400kg K2O ha−1), however, led to an increase of at least 33% in both foliar K concentrations and radial growth, which lasted for the duration of the experiments (7–11years). Liming (1–4t dolomitic lime ha−1) increased the pH, exchangeable calcium and exchangeable magnesium in the humus and top mineral soil horizons, but did not influence balsam fir foliar nutrient status or growth. Tree defoliation rate was mainly related to growth decrease and foliar K concentrations. The trials also show that K was the most limiting nutrient for balsam fir growth, and that the poor K nutrient status and vigor of balsam fir trees can be corrected by K fertilization. These results contrast with past fertilization studies that generally reported N as the most limiting nutrient in these boreal ecosystems. Climatic events may have also played a role in triggering the decline of balsam fir in one trial.

Diel activity patterns of Paradiplosis tumifex (Diptera: Cecidomyiidae) and its inquiline, Dasineura balsamicola (Diptera: Cecidomyiidae), within young balsam fir

AbstractField studies were carried out to assess diel activity patterns of the balsam gall midge, Paradiplosis tumifex Gagné (Diptera: Cecidomyiidae) in 2012, and its inquiline, Dasineura balsamicola (Lintner) (Diptera: Cecidomyiidae) in 2013, in a young balsam fir (Abies balsamea (Linnaeus) Miller) stand in New Brunswick, Canada. Both gallmaker and inquiline are most active during the afternoon/evening hours (17:00–22:00 hours). Male gallmaker activity was largely confined to the space below the crown and typically involved short periods of flight of <30 seconds. Calling and mating by the gallmaker occurred at ground level and were followed by dispersal of females to the vegetative crown. Female gallmakers were typically observed in the tree crown beginning in early afternoon, with peak oviposition occurring between 20:00 and 21:00 hours. Female inquilines displayed similar activity patterns, although no calling or mating were observed. Moreover, inquiline flight and foraging for oviposition sites were more active than the gallmaker, with shorter rest periods and more buds visited than the gallmaker. Our results indicate that population monitoring should focus on female gallmakers as they fly during the evening. Also, before any treatment application, care should be taken to accurately identify the insects to ensure that the inquiline is not inadvertently killed.

Effects of precommercial thinning on the forest value chain in northwestern New Brunswick: Part 4 – Lumber production, quality and value

Forty-eight years after application, three nominal precommercial thinning (PCT) spacings of 4 ft (1.2 m), 6 ft (1.8 m) and 8 ft (2.4 m) were compared to an unthinned control in six replicate, balsam fir (Abies balsamea [L.] Mill.)-dominated stands. Within three of these replicates chosen for clearcut harvest in 2008, a total of 150 trees were bucked into 479 sawlogs that produced 10 829 board feet of lumber. PCT had a positive impact on Premium lumber grade recovery per tree, increasing it from an average of 9 foot board measure (fbm) in the unthinned control plots, to 22 fbm in plots nominally spaced to 8 ft (p = 0.02). No. 2 & Better grade yields increased from 42 to 76 fbm/tree in these same sets of plots (p < 0.01). PCT to 6 ft and 8 ft increased the production of 2×4 and 2×6 lumber over the control (p ≤ 0.04). Stand-level gross total product value increased between 11% and 23% in thinned stands (estimated at +$3000 to +$6600 per ha, p < 0.01). PCT had no appreciable effect on sawn lumber wood density (p ≥ 0.26) or lumber stiffness (MOE; ≤-6.2%, p ≥ 0.11), but had a negative effect on lumber strength (MOR; ≤ -13.4%, p ≤ 0.03) and wood basic density at stump height (≤- 7%; p < 0.01). MDF panel properties were generally good, regardless of spacing. We recommend thinning young balsam fir stands to a maximum nominal spacing of 6 ft (1.2 m) to maintain satisfactory lumber mechanical properties and maximize product value per hectare.

Impact of precommercial thinning on tree growth, lumber recovery and lumber quality in Abies balsamea

Precommercial thinning (PCT) is often used to improve stand growth and value. While PCT may accelerate tree growth and reduce mortality, it may also have a negative effect on product quality. This study examined the effect of moderate and heavy thinning on tree growth, lumber recovery and quality in a natural balsam fir [Abies balsamea (L.) Mill.] PCT trial 35 years after thinning. Compared with the control, the heavy thinning increased merchantable tree diameter, stem volume per tree and lumber volume recovery per tree by 41.1%, 100.9% and 92.7%, respectively, reduced the Select Structural grade (the best grade) recovery by 33.7%. Thinning did not affect the no. 2 and better grade yield. There was a 12.2% and 15.0% difference, respectively, in the lumber bending modulus of elasticity (MOE) and modulus of rupture (MOR) between the control and heavy thinning. Moderate thinning had little impact on the visual grade recovery, lumber bending MOE and MOR. Heavy thinning is recommended if the goal is to get sizeable sawlogs in the shortest time, whereas moderate thinning is preferable if the intention is to minimize the negative effects on lumber quality while retaining modest tree growth and lumber recovery. Overall, PCT of very dense young balsam fir stands appears to be an effective and viable silvicultural treatment.

Relationship between balsam fir sawfly density and defoliation in balsam fir

Field studies were carried out to examine the relationships between balsam fir sawfly, Neodiprion abietis (Harris), density and resultant defoliation on young balsam fir, Abies balsamea (L.) Mill., in western Newfoundland. Densities of early- and late-instar larvae explained 57 and 70% of variation in defoliation in a sleeve-cage study. Densities of both early- and late-instar larvae were significantly related to defoliation on 1 to 5-year-old, but not always on 6 and 7-year-old foliage, due to the preference of larvae for young, but not current-year, foliage. Defoliation was strongly related to the densities of eggs and larvae in field surveys. Sawfly density and whether or not a population had increased or decreased in density during the previous year explained 77% (eggs), 64% (early-instar larvae) and 83% (mid-instar larvae) of the annual variation in defoliation. Our results suggest that establishment of robust density–damage relationships are possible for N. abietis on balsam fir, using egg densities to estimate defoliation and previously established relationships to predict both the amount of foliage remaining and growth loss.

Temporal patterns of balsam fir sawfly defoliation and growth loss in young balsam fir

A 2-year field study was carried out with the balsam fir sawfly, Neodiprion abietis (Harris), on balsam fir, Abies balsamea (L.) Mill., to determine: (1) the temporal patterns of defoliation and foliage weight loss associated with early- and late-larval instars; (2) the losses in branch and tree stem volume growth as a result of defoliation by larvae; and (3) to determine the extent to which individual branches are autonomous, and their utility for predicting stem growth losses due to defoliation. In addition, this study also permitted us to determine the relationships between defoliation or foliage weight and branch and tree stem volume growth. Defoliation, foliage weight loss and larval development occurred more quickly in the first year due to warmer temperatures, but final levels of defoliation and foliage weight loss were similar in both years. Cumulative defoliation levels increased from <5% at the beginning of the study to 35 and 63%, respectively, in fall 1999 and 2000. Defoliation was greatest on 1-year-old foliage in both years and most defoliation and foliage weight loss occurred after the initiation of third-instar. Based upon individual year–volume increment relationships for each branch and tree, mean volume increment loss in 1999 and 2000, respectively, was 20 and 42% for observation branches and 12 and 35% for tree stems. Similar growth loss estimates of 12 and 46%, respectively, for 1999 and 2000, were obtained when comparing the mean specific volume increment for control and observation branches. Foliage weight after sawfly attack was related to branch and stem volume growth, but defoliation was only related to branch volume loss. Although branch and tree stem growth were strongly related, growth losses in a branch were not related to those in a tree stem. Branch removal and measurement is a practical alternative to cutting trees to estimate stem volume growth in balsam fir.

Spruce budworm defoliation and growth loss in young balsam fir: relationships between volume growth and foliage weight in spaced and unspaced, defoliated and protected stands

Foliage weight (FW) and tree volume, volume increment (VI), and specific volume increment (SVI) were examined throughout a spruce budworm ( Choristoneura fumiferana (Clem.)) outbreak cycle from 1976 to 1984, for young balsam fir ( Abies balsamea (L.) Mill.). Treatments were spaced and unspaced, defoliated and protected (annual insecticide spraying to prevent defoliation). Six years into the outbreak, FW in defoliated plots reached its minimum levels, representing a reduction of 83.3 and 84.3% for spaced and unspaced stands, respectively, compared with the protected stand. For surviving trees, FW recovery was rapid and substantial (threefold to fivefold over 3 years), due to the release of suppressed buds. Still, at the end of the outbreak, FW was reduced by 37.5 and 68.5% for spaced and unspaced stands, respectively. VI was reduced by 92.5 and 91.1% and SVI by 92.0 and 89.3% for spaced and unspaced stands, respectively, compared with the protected stand. This severe budworm outbreak reduced growth rates by about the same level, regardless of initial differences in growth rates and density. By the end of the outbreak, volume was reduced by 32.6 and 54.1 m 3/ha for spaced and unspaced stands, respectively, or a decrease of 51.7 and 35.7% compared with the protected stand. Relationships between VI and FW were, in general, linear for spaced trees and a mixture of linear and non-linear for unspaced trees. On an individual-tree basis, VI: FW relationships were strong at the beginning and end of the spruce budworm outbreak, but weaker in the middle. Combining all years, on a per hectare basis, the relationships between VI and FW were strong ( r 2=0.78 for spaced and 0.80 for unspaced stands), and still held despite defoliation.

Effects of light and intraspecific competition on growth and crown morphology of two size classes of understory balsam fir saplings

This paper characterizes the growth and crown morphology of young balsam fir saplings naturally regenerated under a gradient of understory light environments and intraspecific competition densities for two size classes (50–100 cm and 100–200 cm). Most growth and crown morphological parameters investigated were strongly related to the natural light gradient investigated (3–83% full sunlight), but the relationship tended to plateau at around 25% full sunlight. The relationships were generally better for the larger size class. Intraspecific competition did not significantly affect growth and crown morphology of saplings receiving less than 25% full sunlight, but it affected relative height growth, relative radial growth and the apical dominance ratio for those receiving more than 25% full sunlight ( R 2=0.506; p<0.001; R 2=0.403; p<0.002; R 2=0.348; p<0.001, respectively). These results suggest that live crown ratio, apical dominance ratio and the number of internodal branches can provide, alone or in combination, useful indicators of vigour for understory fir. Such a study provides the basic data inputs required for the development of empirically-derived mechanistic models that can predict understory tree growth and survival.

Impact of precommercial thinning in balsam fir stands on soil nitrogen dynamics, microbial biomass, decomposition, and foliar nutrition

Precommercial thinning is being tested in Quebec as a preventive silvicultural treatment to reduce vulnerability of young balsam fir (Abies balsamea (L.) Mill.) stands to spruce budworm (Choristoneura fumiferana (Clem.)) damage and to shorten rotations. As part of a larger study of ecosystem response to thinning, we have examined the impact of this treatment on soil nitrogen dynamics, microbial biomass, cellulose decomposition, and foliar nutrition across a range of drainage conditions (good, imperfect, and poor). In the first year after thinning, initial early season ammonium (NH4+-N) pools in the mineral horizon were significantly higher in the thinned plots (P = 0.019), while net nitrogen mineralization (NH4+-N plus NO3--N) decreased in these same plots (P = 0.052). The thinning treatment significantly increased microbial biomass nitrogen (Nmic) in the organic horizon (P = 0.051). Simple regression analysis indicated the importance of soil temperature in controlling Nmic. Decomposition of cellulose substrate in the organic horizon was significantly increased by thinning, and mass loss was related to soil temperature. Increased decomposition and nutrient availability after thinning were reflected in improved N, P, and K nutrition in current, 1- and 2-year-old balsam fir needles. The temporal extent of this improved fertility will be verified by longer term monitoring.

Spruce budworm defoliation and growth loss in young balsam fir: patterns of shoot, needle and foliage weight production over a nine-year outbreak cycle

Patterns of shoot numbers, shoot length, needle length, and foliage weight were examined throughout a spruce budworm ( Choristoneura fumiferana (Clem.)) outbreak cycle, for young balsam fir ( Abies balsamea (L.) Mill.), representing four spacing and defoliation conditions. Treatments were defoliated spaced and unspaced, and protected (control, by annual insecticide spraying to prevent defoliation) spaced and unspaced. Foliage parameters were measured on one branch from every second whorl of 20–38 trees per treatment, each year from 1976 to 1984. We found that the number of shoots for protected spaced trees was about three times higher (1140 vs. 400 shoots/tree) than for unspaced trees. There was, in general, no significant difference between protected spaced and defoliated spaced trees, and between protected unspaced and defoliated unspaced trees. For protected trees, the number of shoots increased from the top toward mid-crown, and then decreased or stabilized toward the base, whereas defoliated trees had the greatest number of shoots in the lower crown. Shoots were longer on protected than on defoliated trees (means of 3.9–4.0 vs. 2.2 cm/shoot), but there was no effect of spacing. Shoot length was greatest in the uppermost whorls of protected trees and decreased toward the base, while defoliated trees had similar shoot lengths throughout the crown, but were converging to the non-defoliated pattern by 1984. Defoliation resulted in longer needles in one year and shorter needles in three years; there were no consistent relationships with crown position while spacing affected needle lengths in three and two years for protected and defoliated trees, respectively. Foliage weight per tree was 2.7–3.3 times higher for the protected spaced than unspaced trees, while by 1981, foliage weight of defoliated trees was only 7–12% that of the protected trees. However, by 1984, the rate of foliage weight increase was similar between protected and defoliated trees. Regardless of treatment, the maximum foliage weight was in the lower crown. For protected, unspaced suppressed trees, there was a shift in the foliage weight distribution over time, with increased weight in the upper crown.

CHANGES IN SPRUCE BUDWORM DEFOLIATION WITH CROWN LEVEL

AbstractDetailed estimates of defoliation caused by spruce budworm [Choristoneura fumiferana (Clem.)] over the crown length of young balsam fir [Abies balsamea (L.) Mill.] were made throughout a spruce budworm outbreak from 1976 to 1984 in the Cape Breton Highlands, Nova Scotia. The results show no clear tendency for a particular level of the crown to be damaged more heavily than any other. Thus, there is no reason to continue the common practice of taking samples from the mid-crown level on the assumption that they represent an ‘average’ level of defoliation either for high or low populations. Sampling from the bottom of the crown should provide a more convenient and cost-effective approach for estimating defoliation.

Spruce Budworm Defoliation and Growth Loss in Young Balsam Fir: Spacing Effects on Needlefall in Protected Trees

Abstract Age-dependent Weibull models were used to describe precise measurements of needlefall from a pair of protected plots of young, unspaced, balsam fir (Abies balsamea [L.] Mill.). These models showed that the age-specific rates of needlefall tended to increase with needle age throughout the observed crown. Where crown level effects were evident, needle lifespans were shorter in the lower crown than in the upper crown. The influence of crown level in the unspaced plots, however, was too inconsistent to be useful as a general predictor of needlefall rates. Comparing the results with previous work on spaced sample plots showed that spacing affected many aspects of needlefall. The mean needle lifespan of 5.3 yr (SE = 0.17) on the spaced sample plots was significantly greater than the 4.7 yr (SE = 0.22) estimated for the unspaced plots. These differences were most extreme in the lower crown. Spacing also influenced the earliest phase of needlefall. For needles under 2 yr, the mean age-specific rates of fall were negligible in the spaced plots, but were about 6%/yr in the unspaced plots. Thereafter, annual increases in the age-specific rates of needlefall were similar in the spaced and unspaced plots. Spacing also affected the pattern of year-to-year variation in needlefall. There was evidence of statistically significant trends in mean needle lifespans over time in the spaced plots, but not in the unspaced plots. For. Sci. 42(3):282-289.

Spruce Budworm Defoliation and Growth Loss in Young Balsam Fir: Estimation of Volume Growth Based on Stem Analysis and Increment Cores at Breast Height

Abstract Comparisons of estimates of volume and volume increment, based on increment cores sampled at breast height and on stem analysis, were made in a young balsam fir stand that had been defoliated by spruce budworm. Use of increment cores is not recommended to estimate individual tree growth, because large errors, ranging from 20.9 to 57.2%, and error variability can be expected. The inability, when using increment cores, to estimate basal area is the major source of this variation. Although large uncertainties are associated with future growth predictions, on a per ha basis, changes over time based on increment cores may in some instances give comparable results to those based on stem analysis due to the canceling effect of trees being approximately equally overestimated and underestimated. However, a prerequisite is that the defoliation history and the year of individual tree death are known in detail to aid in the correct dating of ring widths. North. J. Appl. For. 13(2):73-78.

Habitat suitability for marten of second-growth balsam fir forests in Newfoundland

The marten subspecies on the island of Newfoundland, Martes americana atrata, is threatened. Survey data suggest that most of the extant marten population lives in old uncut balsam fir (Abies balsamea) forests, but a very few live in adjacent 40- to 60-year-old second-growth stands of balsam fir. We compared habitat structure and composition and prey abundance in old forest and second-growth stands to test the hypotheses that either food abundance or habitat quality, or both, limit use of the 40- to 60-year-old forests by marten. Snowshoe hares (Lepus americana) were most abundant in 40-year-old forests and also occurred in old forests, but field voles (Microtus pennsylvanicus) were not found in second-growth stands. A multivariate discriminant model indicated that older, uncut forests contained more structure than younger forests at ground level, because there was more woody debris, more young balsam fir, less litter, more mosses, and more low shrubs. Canopy cover was similar in all forest types, and subnivean access did not differ among the three age-classes when snow was about 1 m deep. We suggest that marten did not use 40- or 60-year-old forest stands because of the lack of the meadow voles that form a necessary part of their diet. Meadow voles likely respond to ground-level forest structure in selecting habitat, and this structure is unavailable in young forests. We recommend a management strategy for marten that would preserve current old forests as long as possible and allow sufficient second-growth balsam fir forest to become old forest with the required characteristics to maintain a viable marten population.

Spruce Budworm Defoliation and Growth Loss in Young Balsam Fir: Cohort Models of Needlefall Schedules for Spaced Trees

Abstract The relationship between the age-specific rates of needlefall, q(x), and needle age, x, for cohorts of needles from young, spaced balsam fir (Abies balsamea [L.] Mill.) was described by the discrete hazard function of the Weibull model, q(x) = 1 - exp{(xs - [x + 1]s)/us}. Here u = 6.32 yr (SE = 0.122 yr), s = 3.03 (SE = 0.109), and R² = 0.982. These age specific rates increased with needle age, demonstrating a type I pattern of needlefall. Three phases of needlefall were distinguished. An "early life" phase in which the mean rates of needlefall remained under 10% per year occurred during the first 2 years of existence. Later, up to about needle age 4, the rates climbed steadily upwards with age (at 5.5% per year per year). After age 4, the rate of increase in needlefall with age increased to over 8% per year per year. This may be due to the onset of old-age or wear-out effects in the progressive deterioration of the needles with age. Use of crown level and year of observation as independent variables in addition to needle age did little to extend the explanatory power of our models. These results are consistent with the period analysis performed by Fleming and Piene (1992) on the same stands. Because of the large year-to-year variability in age-specific rates of needlefall, fits of the Weibull's discrete hazard function to individual cohorts usually left over half the variation in the rates unexplained. This contrasted with the period analysis in which the Weibull model explained over 80% of the variation each year. Hence, despite the success of the period analysis, the large year-to-year variability drastically restricts our ability to predict age-specific rates of needlefall. Although the Weibull model may describe future needle age distributions within a given year accurately, large fluctuations in parameter values, possibly reflecting weather-caused variations or long-term cycles and trends in needlefall, can be expected. For. Sci. 38(3):678-604.

Spruce Budworm Defoliation and Growth Loss in Young Balsam Fir: Period Models of Needle Survivorship for Spaced Trees

Abstract The age-dependent Weibull model described detailed observations of needle survivorship taken in two protected plots of young, spaced, balsam fir (Abies balsamea [L.] Mill.) on the Cape Breton Highlands, Nova Scotia, Canada. Throughout the midcrown, the rates of age-specific needlefall increased with needle age, indicating a Type I survivorship curve typical of foliage. It was shown that the needle populations experienced distinct stages of needlefall as they aged. Few needles surviving a period of "infant mortality" in their first year fell in their second. There was a low, but steady, increase in the rates up to age 4-5, with a further increase in the rates for older needles. The mean longevity of the needle populations ranged from 4.1 to 6.1 years and generally increased with tree age. Differences between trees of the same age and differences between crown levels explained little more of the variation in needle fall than that already explained by needle age alone. For. Sci. 38(2):287-304.

Effects of thinning on the allocation of growth and respiration in young stands of balsam fir

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.

Changes in the water relations of balsam fir and white birch saplings after thinning

In the autumn of 1987, young balsam fir (Abies balsamea (L.) Mill.) and white birch (Betula papyrifera Marsh.) trees were thinned and their water relations followed during the next two growing seasons. At the beginning of the first summer following treatment, thinned trees of both species had lower osmotic potentials at full saturation (Psi(pi,sat)) and at turgor loss point (Psi(pi,tlp)) compared with controls. At this time, Psi(pi,sat) was linearly related to the percentage of full sunlight reaching the trees. A higher sugar concentration in leaves was an important component of the lower Psi(pi,sat) of thinned trees. For the other two sampling dates during the first growing season after treatment and all three sampling dates during the second growing season after treatment, little osmotic adjustment of the thinned trees relative to the control tress was observed in either species. The absence of osmotic adjustment during the second growing season following thinning suggests that other mechanisms were responsible for the acclimation of the treated trees to the higher atmospheric evaporative demand. Sapwood permeability (k) of white birch was higher than that of balsam fir, but no differences in k or in sapwood area were found between treated and control trees of either species. Predawn water potentials (Psi(pred)) of treated trees were less negative than those of controls.