SEASONAL AND INDIVIDUAL VARIATION IN LEAF QUALITY OF TWO NORTHERN HARDWOODS TREE SPECIES

Seasonal trends in five traits of sugar maple (Acer saccharum Marsh.) and yellow birch (Betula allegheniensis Britt.) leaves thought to influence feeding by herbivores were measured from 17 May through 19 September, 1979. Total nitrogen and water contents declined and toughness increased through the growth season. These seasonal changes were more pronounced in sugar maple than in yellow birch. Total polyphenol contents and tanning coefficients of leaf extracts from both species reached a season high by the end of May and changed very little after that date; these patterns differ from those reported by several other investigators. Sugar maple leaf extracts exhibited much higher tanning coefficients than did those of yellow birch, a finding which is consistent with current plant defense theory. Significant differences in total polyphenol content and tanning coefficients were found between individual trees in yellow birch, but not sugar maple. The relationship between successional status, leaf quality traits, and variability in these traits in forest trees is discussed.     

Differential drought responses between saplings and adult trees in four co-occurring species of New England

Tree-ring characteristics in four species were examined to address whether co-occurring mature trees of different successional status respond differently to drought, and whether saplings of these species have a greater response to drought than mature trees. We examined saplings and mature trees of paper birch, yellow birch, red maple and sugar maple, which varied in successional status (shade-tolerance) and co-occurred at Harvard Forest, Petersham, Mass., USA. Three drought events in 1964–1966, 1981 and 1995 were identified using climate data. For mature trees, there was no significant interspecific difference in relative changes in ring-width index (RWI) during the 1964–1966 and 1995 drought events. However, the interspecific difference was significant in the 1981 drought event. Response function analysis for mature trees showed that the radial growth of sugar maple was mainly controlled by spring and summer precipitation, red maple by spring and summer precipitation and temperature, yellow birch by winter and summer precipitation, and spring and summer temperature, and paper birch by spring and summer precipitation and spring temperature. Saplings of sugar maple and yellow birch, but not red maple and paper birch, showed significant positive correlations between RWI and annual total precipitation. In the 1995 drought event, saplings and mature trees of red maple and paper birch differed significantly in drought responses, but this was not true in sugar maple and yellow birch. Our results do not support a generally greater response in saplings than in mature trees, nor an early- versus late successional difference in drought responses.     

Leaf litter quality and decomposition rates of yellow birch and sugar maple seedlings grown in mono-culture and mixed-culture pots at three soil fertility levels

Seedlings of yellow birch (Betula alleghaniensis Britton) and sugar maple (Acer saccharum Marsh.) were grown for 2 years in mono-culture and mixed-culture and at three fertility levels. Following the second growing season, senescent leaves were analysed for N concentration, acid hydrolysable substances (AHS), and nonhydrolysable remains (NHR). A litter sub-sample was then inoculated with indigenous soil microflora, incubated 14 weeks, and mass loss was measured. Litter-N was significantly higher at medium than at poor fertility, as well as in yellow birch than in sugar maple litter. The species effect on litter-N increased with increasing fertility. At medium fertility, litter-N of sugar maple litter was lower in mixed-culture than in mono-culture. AHS, NHR as well the NHR/N ratio were significantly higher in yellow birch than in sugar maple litter. At medium fertility, the NHR/N ratio of sugar maple litter was significantly lower in mono-culture than in mixed-culture. Mass loss was significantly greater at medium and rich fertility than at poor fertility, and in yellow birch than in sugar maple litter. At poor fertility, mixed-litter decomposed at a rate comparable to yellow birch, whereas at medium and rich fertility, mixed-litter decomposed at a rate comparable to sugar maple. There was a significant positive relationship between litter-N and mass loss. A similar positive relationship between NHR and mass loss was presumed to be a species effect on decomposition. Results support the hypothesis that species × fertility and species × mixture interactions can be important determinants of litter quality and, by implication, of site nutrient cycling.     

Lorentzian model of roots for understory yellow birch and sugar maple saplings

Total 66 small (<50m(2)), 24 medium (101-200m(2)) and 36 large (201-500m(2)) canopy gaps at the three sites of yellow birch (Betula alleghaniensis Britton) and sugar maple (Acer saccharum Marsh) forests were established in southern Québec, Canada. Half of the gaps were covered by 8x8m(2) shading cloths to mimic a closed canopy. From these gaps, 46 understory yellow birch and 46 sugar maple saplings with different tree ages and sizes were sampled. Single- and multi-variable linear and nonlinear models of root biomass and traits (root surface area, volume, length and endings) were developed and examined. Lorentzian model as a multi-variable nonlinear model was firstly applied to the simulations using both base diameter and height, and performed the best fit to total root biomass in both species with the highest correlation coefficients (R(2)=0.96 and 0.98) and smallest root mean squared deviations (RMSD=7.85 and 7.02) among all the examined models. The model also accurately simulated small fine root (2.0mm in diameter), coarse fine root (>2.0-5.0mm) and coarse root (>5.0mm) biomass (R(2)=0.87-0.99; RMSD=2.24-6.41), and the root traits (R(2)=0.71-0.99; RMSD=0.19-19.38). The study showed yellow birch roots were longer, larger, had more endings (tips) and grew faster than sugar maple roots. The root traits were largely distributed to small fine roots, sharply decreased from small fine roots to coarse fine roots, the fewest in coarse roots except for root volume. When trees were large, coarse root biomass increased more rapidly than fine root biomass, but vise versa when the trees were small.     

Tree demography suggests multiple directions and drivers for species range shifts in mountains of Northeastern United States

Climate change is expected to lead to upslope shifts in tree species distributions, but the evidence is mixed partly due to land‐use effects and individualistic species responses to climate. We examined how individual tree species demography varies along elevational climatic gradients across four states in the northeastern United States to determine whether species elevational distributions and their potential upslope (or downslope) shifts were controlled by climate, land‐use legacies (past logging), or soils. We characterized tree demography, microclimate, land‐use legacies, and soils at 83 sites stratified by elevation (~500 to ~1200 m above sea level) across 12 mountains containing the transition from northern hardwood to spruce‐fir forests. We modeled elevational distributions of tree species saplings and adults using logistic regression to test whether sapling distributions suggest ongoing species range expansion upslope (or contraction downslope) relative to adults, and we used linear mixed models to determine the extent to which climate, land use, and soil variables explain these distributions. Tree demography varied with elevation by species, suggesting a potential upslope shift only for American beech, downslope shifts for red spruce (more so in cool regions) and sugar maple, and no change with elevation for balsam fir. While soils had relatively minor effects, climate was the dominant predictor for most species and more so for saplings than adults of red spruce, sugar maple, yellow birch, cordate birch, and striped maple. On the other hand, logging legacies were positively associated with American beech, sugar maple, and yellow birch, and negatively with red spruce and balsam fir – generally more so for adults than saplings. All species exhibited individualistic rather than synchronous demographic responses to climate and land use, and the return of red spruce to lower elevations where past logging originally benefited northern hardwood species indicates that land use may mask species range shifts caused by changing climate.     

Regenerative traits of tree species in a deciduous forest of northeastern North America

I studied the seed rain, seedling bank, newly emerged seedlings, and seed bank of tree species in a deciduous forest of northeastern North America (Québec, Canada). The main objective was to determine to what extent the tree species present on the site differed in some of their regenerative traits. All the species present on the quadrats as mature trees contributed to the annual seed rain, although seed rain was strongly dominated by yellow birch and sugar maple. The seedling bank was mostly composed of red maple and sugar maple, but inputs from the 1988 cohort were mostly red maple and yellow birch seedlings. Seedling mortality was high in general (from 60% to 100%), but sugar maple had the lowest mortality of the tree species present. Two species maintained a seed bank on the study site: yellow birch and american basswood. Some species not present on the quadrats, but having populations nearby, disseminated seeds onto the quadrats (i.e. white ash and grey birch). Other species, not present on the quadrats as mature individuals, had seeds in the seed bank although they did not participate in the seed rain, did not maintain a seedling bank, and did not establish seedlings in 1988 (i.e. pin- cherry and Rubus spp.). Red maple, sugar maple, and yellow birch showed contagion in the dispersion pattern of their seeds in the seed rain (and the seed bank for yellow birch), and of their seedlings in the seedling bank and the 1988 cohort. Seedlings of american beech and american basswood of the seedling bank and the 1988 cohort were randomly dispersed over the quadrats, although their seeds in the seed rain had a contagious dispersion pattern. It has often been proposed in the literature that regenerative traits come only in “compatible assemblages” and that, consequently, species may be grouped according to their strategy of regeneration: e.g. species that maintain a seed bank vs those with a seedling bank; species with either a seed or a seedling bank vs those with the ability to sprout. In the community studied, it appears that regenerative traits do not come in neatly packaged exclusive assemblages, although the species do seem to possess different combinations of regenerative traits. However, yearly variations in seed production, seed germination, and seedling survival may render difficult the identification of strategies of regeneration.     

Soil Base Saturation Combines with Beech Bark Disease to Influence Composition and Structure of Sugar Maple-Beech Forests in an Acid Rain-Impacted Region

Sugar maple, an abundant and highly valued tree species in eastern North America, has experienced decline from soil calcium (Ca) depletion by acidic deposition, while beech, which often coexists with sugar maple, has been afflicted with beech bark disease (BBD) over the same period. To investigate how variations in soil base saturation combine with effects of BBD in influencing stand composition and structure, measurements of soils, canopy, subcanopy, and seedlings were taken in 21 watersheds in the Adirondack region of NY (USA), where sugar maple and beech were the predominant canopy species and base saturation of the upper B horizon ranged from 4.4 to 67%. The base saturation value corresponding to the threshold for Al mobilization (16.8%) helped to define the species composition of canopy trees and seedlings. Canopy vigor and diameter at breast height (DBH) were positively correlated (P < 0.05) with base saturation for sugar maple, but unrelated for beech. However, beech occupied lower canopy positions than sugar maple, and as base saturation increased, the average canopy position of beech decreased relative to sugar maple (P < 0.10). In low-base saturation soils, soil-Ca depletion and BBD may have created opportunities for gap-exploiting species such as red maple and black cherry, whereas in high-base saturation soils, sugar maple dominated the canopy. Where soils were beginning to recover from acidic deposition effects, sugar maple DBH and basal area increased progressively from 2000 to 2015, whereas for beech, average DBH did not change and basal area did not increase after 2010.     

Patterns of tree replacement: canopy effects on understory pattern in hemlock-northern hardwood forests

The effect of canopy trees on understory seedling and sapling distribution is examined in near-climax hemlock-northern hardwood forests in order to predict tree replacement patterns and assess compositional stability. Canopy trees and saplings were mapped in 65 0.1-ha plots in 16 tracts of old-growth forests dominated by Tsuga canadensis, Acer saccharum, Fagus grandifolia, Tilia americana, and Betula lutea in the northeastern United States. Seedlings were tallied in sub-plots. Canopy influence on individual saplings and sub-plots was calculated, using several indices for canopy species individually and in total. For each species sapling and seedling distributions were compared to those distributions expected if saplings were located independently of canopy influence. Non-random distributions indicated that sapling and seedling establishment or mortality were related to the species of nearby canopy trees. Hemlock canopy trees discriminate against beech and maple saplings while sugar maple canopy favors beech saplings relative to other species. Basswood canopy discourages growth of saplings of other species, but produces basal sprouts. Yellow birch saplings were rarely seen beneath intact canopy. Since trees in these forests are usually replaced by suppressed seedlings or saplings, canopy-understory interactions should influence replacement probabilities and, ultimately, stand composition. I suggest that hemlock and basswood tend to be self-replacing, maple and beech tend to replace each other, and birch survives as a fugitive by occupying occasional suitable gaps. This suggests that these species may co-exist within stands for long periods with little likelihood of successional elimination of any species. There is some suggestion of geographical variation in these patterns.     

Patterns of rhizosphere carbon flux in sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) saplings

Despite its importance in the terrestrial C cycle rhizosphere carbon flux (RCF) has rarely been measured for intact root–soil systems. We measured RCF for 8‐year‐old saplings of sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) collected from the Hubbard Brook Experimental Forest (HBEF), NH and transplanted into pots with native soil horizons intact. Five saplings of each species were pulse labeled with ¹³CO₂ at ambient CO₂ concentrations for 4–6 h, and the ¹³C label was chased through rhizosphere and bulk soil pools in organic and mineral horizons for 7 days. We hypothesized yellow birch roots would supply more labile C to the rhizosphere than sugar maple roots based on the presumed greater C requirements of ectomycorrhizal roots. We observed appearance of the label in rhizosphere soil of both species within the first 24 h, and a striking difference between species in the timing of ¹³C release to soil. In sugar maple, peak concentration of the label appeared 1 day after labeling and declined over time whereas in birch the label increased in concentration over the 7‐day chase period. The sum of root and rhizomicrobial respiration in the pots was 19% and 26% of total soil respiration in sugar maple and yellow birch, respectively. Our estimate of the total amount of RCF released by roots was 6.9–7.1% of assimilated C in sugar maple and 11.2–13.0% of assimilated C in yellow birch. These fluxes extrapolate to 55–57 and 90–104 g C m^?2 yr^?1 from sugar maple and yellow birch roots, respectively. These results suggest RCF from both arbuscular mycorrhizal and ectomycorrhizal roots represents a substantial flux of C to soil in northern hardwood forests with important implications for soil microbial activity, nutrient availability and C storage.     

Light and tree size influence belowground development in yellow birch and sugar maple

The effects of light and tree size on the root architecture and mycorrhiza of yellow birch (Betula alleghaniensis Britton) and sugar maple (Acer saccharum Marsh) growing in the understory of deciduous forests in southern Québec, Canada were studied. At the study site, small (<50 m²), medium (101–200 m²) and large (201–500 m²) canopy gaps were investigated. From within these gaps, 17 yellow birch and 23 sugar maple saplings from 40 to 600 cm in height were sampled. In both species, root biomass and morphological traits were strongly correlated with tree size, but only weakly with light availability. Increased root biomass was primarily allocated to coarse roots and secondarily to fine roots. Yellow birch roots were longer, had a larger area, more endings and branches and grew more rapidly than sugar maple roots. Mycorrhizal colonization increased with available light and declined with tree age in sugar maple and was positively associated with tree size in yellow birch. The study demonstrates that tree size is a very important determinant of how belowground systems acclimate to understory conditions.     

Fertilization effects on fineroot biomass, rhizosphere microbes and respiratory fluxes in hardwood forest soils

Fertilizer-induced reductions in CO(2) flux from soil ((F)CO(2)) in forests have previously been attributed to decreased carbon allocation to roots, and decreased decomposition as a result of nitrogen suppression of fungal activity. Here, we present evidence that decreased microbial respiration in the rhizosphere may also contribute to (F)CO(2) reductions in fertilized forest soils. Fertilization reduced (F)CO(2) by 16-19% in 65-yr-old plantations of northern red oak (Quercus rubra) and sugar maple (Acer saccharum), and in a natural 85-yr-old yellow birch (Betula allegheniensis) stand. In oak plots, fertilization had no effects on fine root biomass but reduced mycorrhizal colonization by 18% and microbial respiration by 43%. In maple plots, fertilization reduced root biomass, mycorrhizal colonization and microbial respiration by 22, 16 and 46%, respectively. In birch plots, fertilization reduced microbial respiration by 36%, but had variable effects on root biomass and mycorrhizal colonization. In plots of all three species, fertilization effects on microbial respiration were greater in rhizosphere than in bulk soil, possibly as a result of decreased rhizosphere carbon flux from these species in fertile soils. Because rhizosphere processes may influence nutrient availability and carbon storage in forest ecosystems, future research is needed to better quantify rhizo-microbial contributions to (F)CO(2).     

Bark type reflects growth potential of yellow birch and sugar maple at the northern limit of their range

We carried out a study to determine if bark type could reflect the growth potential of yellow birch (Betula alleghaniensis Britt.) and sugar maple (Acer saccharum Marsh.) at the northern limit of their range in Québec, Canada (47°N, 75°W). We measured a large sample of 266 trees that ranged in size from 95 to 712 mm in diameter at breast height, on two independent study sites. Our results suggest that trees with smooth bark type had mean 5-year diameter increment 8 and 11 mm higher than trees with rough bark type, depending upon the study site. Differences in growth of 8 and 11 mm represented 85% of the overall rough bark type increment in the first site and 78% of the overall rough bark type increment in the second site. The rapid identification of a tree’s growth potential using bark type could be of great use to practitioners because it avoids the need to bore trees to collect increment cores, which injures trees and may serve as an entry point for disease. Moreover, the proposed method helps protect or release the smallest trees with high growth potential and remove trees with low growth potential. While the proposed method is valuable to practitioners operating in uneven-aged forests, its applicability still needs to be tested in even-aged forests.     

Effect of snow removal on leaf water potential,soil moisture,leaf and soil nutrient status and leaf peroxidase activity of sugar maple

Effect of removal of snow cover in winter was investigated in an 80-year-old sugar maple (Acer saccharum Marsh.) stand in southern Quebec. We hypothesized that winter soil frost would induce some of the decline symptoms observed in sugar maple stands in southern Quebec in the early 1980's. Snow was continuously removed from around trees for a one week (partial removal) or for a four-month period (complete removal) during the 1990–1991 winter. Foliage and soils were sampled periodically during the summer of 1991. The complete snow removal treated trees showed decreased leaf water potential and increased peroxidase activity over most of the growing season. Foliar Ca was reduced in both snow removal treatments early in the growing season while foliar N was reduced in the complete snow removal trees late in the growing season. Soil NO ₃ ^– and K⁺ were elevated in both snow removal treatments at various times throughout the growing season. Prolonged soil frost in a sugar maple stand can induce lower leaf water potential, higher leaf peroxidase activity and early leaf senescence during the following growing season. Soil frost may have reduced nutrient uptake without affecting significantly the leaf nutrient status.     

Compositional vegetation changes and increased red spruce abundance during the Little Ice Age in a sugar maple forest of north-eastern North America

In north-eastern North America, the recent red spruce decline has been linked to atmospheric pollution, notably acid rain, although climate was also advocated as a potential factor. A high resolution lake sediment pollen stratigraphy was obtained to elucidate long-term trends in tree-species abundance in a sugar maple??yellow birch forest. The reconstructed history (~250?C1996?A.D.) showed a steady increase of red spruce after 1300?A.D., with a peak between 1600 and 1900?A.D. followed by a strong decline in the last century, while sugar maple and yellow birch experienced an opposite trend. Red spruce abundance reached its apogee during the cool Little Ice Age (LIA) and decreased abruptly when annual temperature in the region increased by 2?°C in the last 125?years. American Beech was much more abundant in the forest before the LIA, typifying a sugar maple??American beech forest as the dominant forest type during the Late Holocene. Our results suggest that climate warming has played an important role in the current red spruce decline, the latter having been initiated well before acidic depositions reached deleterious potential effects on red spruce. Climate warming probably acted as a long-term predisposing factor that was aggravated by atmospheric pollution, in the last decades.     

Ecological impacts of a widespread frost event following early spring leaf‐out

In the spring of 2010, temperatures averaged ~3 °C above the long‐term mean (March–May) across the northeastern United States. However, in mid‐to‐late spring, much of this region experienced a severe frost event. The spring of 2010 therefore provides a case study on how future spring temperature extremes may affect northeastern forest ecosystems. We assessed the response of three northern hardwood tree species (sugar maple, American beech, yellow birch) to these anomalous temperature patterns using several different data sources and addressed four main questions: (1) Along an elevational gradient, how was each species affected by the late spring frost? (2) How did differences in phenological growth strategy influence their response? (3) How did the late spring frost affect ecosystem productivity within the study domain? (4) What are the potential long‐term impacts of spring frost events on forest community ecology? Our results show that all species exhibited early leaf development triggered by the warm spring. However, yellow birch and American beech have more conservative growth strategies and were largely unaffected by the late spring frost. In contrast, sugar maples responded strongly to warmer temperatures and experienced widespread frost damage that resulted in leaf loss and delayed canopy development. Late spring frost events may therefore provide a competitive advantage for yellow birch and American beech at the expense of sugar maple. Results from satellite remote sensing confirm that frost damage was widespread throughout the region at higher elevations (>500 m). The frost event is estimated to have reduced gross ecosystem productivity by 70–153 g C m^?2, or 7–14% of the annual gross productivity (1061 ± 82 g C m^?2) across 8753 km² of high‐elevation forest. We conclude that frost events following leaf out, which are expected to become more common with climate change, may influence both forest composition and ecosystem productivity.     

Assessment of the potential role of metals in sugar maple (Acer saccharum Marsh) decline in Ontario, Canada

Spatial and temporal differences in the crown condition of sugar maple (Acer saccharum Marsh) in Ontario remain largely unexplained. In this study, the potential role of metals in sugar maple dieback was explored by measuring metal concentrations in foliage and forest floor (LFH) at 35 forest stands in south-central Ontario that exhibit varying levels of decline and span a climatic, soil acidity and acid deposition gradient. Foliar and forest floor metal concentrations varied among sites by between two and ten-fold, with acidic sites exhibiting the highest concentrations of many metals in the forest floor and foliage. Sites with moderate decline symptoms (decline index (DI)?>?10, averaged between 1986 and 2004) had significantly greater Cd, Zn, Cu, Pb and Mn concentrations and lower Ca concentrations in the forest floor compared with healthy sites (DI?<?10). Foliar concentrations of Cd, Sr and Mn were also significantly greater and Ca was significantly lower in sites with moderate decline symptoms compared with healthy sites. However the highest metal concentrations in foliage and the forest floor found in this study are lower than critical values reported in the literature. The notable exception is Mn where values at acidic sites may be high enough to negatively impact sugar maple.     

Effects of Experimental Soil Warming and Water Addition on the Transpiration of Mature Sugar Maple

Sugar maple (Acer saccharum), an economically important timber and syrup species, is not expected to flourish under projected future climates. The objectives of our study were to (1) tease apart the effects of warming and soil moisture availability on transpiration rates of mature sugar maple trees with a full-factorial soil warming × water addition experiment and (2) determine the primary environmental driver(s) of sugar maple transpiration in the upper Midwestern United States. Over three growing seasons, we monitored sap flux of 33 trees in eight 100-m² plots, two replicates each of four treatments: (1) heat (soil warmed +4°C), (2) water (1.3 × ambient growing season precipitation, (3) heat + water, and (4) control. As expected, sugar maple transpiration decreased under the heat treatments in all years and increased in water treatments in years 1 and 2, all mediated primarily by soil moisture. However, under the heat + water treatment, supplemental water compensated for the warming-induced soil evaporation only in year 1 (2011), which was the driest year. Despite clear evidence of a soil moisture-mediated treatment effect, light was the dominant driver of seasonal variation in sap flux in this sugar maple-dominated ecosystem with relatively short growing seasons. However, sap flux was reduced with decreases in soil moisture, and therefore, net C gain likely was as well. Overall, our results suggest that even though this northern temperate species is primarily limited by light, sugar maple productivity may be reduced by a warming climate on drier sites within its current range, if warming is not accompanied by a sufficient increase in precipitation.     

Sugar maple (Acer saccharum Marsh.) establishment in boreal forest: results of a transplantation experiment

Aim To evaluate whether seedlings of sugar maple (Acer saccharum Marsh.) can establish beyond the species northern range limit in adjacent boreal forest. Location The hardwood–boreal forest transition zone on the north‐east shore of Lake Superior, Ontario, Canada. Methods Seed fall of sugar maple was monitored for 5 years in a stand of this species at its northern range limit, and seed from this stand was transplanted to five micro‐habitat types in an adjacent boreal forest. The establishment and survival of sugar maple seedlings there, and in the seed‐source stand, was monitored for the following 7–11 years. Soil‐surface light levels were measured in both forest types. Results Most seed fell in the final year of monitoring, when c. 250 seeds m^?2 were recorded. First‐year seedling establishment rates in the maple stand, deriving from this mast seed year, was approximately double that deriving from seed transplanted to the boreal forest sites; this is tentatively attributed to seed predator satiation in the maple stand. However, subsequent seedling survivorship in the boreal forest was greater than that in the maple stand, resulting in comparable seedling densities by the end of 6 years. This difference is tentatively attributed to better illumination in the boreal forest sites, and canopy‐opening disturbances appear to be especially facilitative of seedling survival. Main conclusions There is no fundamental impediment to sugar maple seedlings establishing in boreal forest communities if climate warming occurs and seed is available. If management intervention is needed to accelerate seed availability in a rapidly warming boreal forest, then diffuse seed application to disturbed boreal forest sites during mast years of local boreal tree species is recommended as the most effective way of avoiding seed predation and increasing seedling survival.     

Tree diversity in 30-year chronosequences of cool-humid forests

Tree data collected in four inventories of twelve permanent sampling plots during 30 years were analyzed to determine to what extent the abundance of the ubiquitous balsam fir (Abies balsamea) drives tree diversity in mixed stands of Eastern Cape Breton Island, Canada. Dominant deciduous species comprised yellow birch (Betula allegheniensis), sugar maple (Acer saccharum), and American beech (Fagus grandifolia). Tree species richness ranged from 2 to 6. The Shannon–Wiener diversity index indicated that high fir proportion decreased tree species diversity (H’) and structural evenness (J’). The basal area ratio for year 30 over year 1 provided an index of resilience (R) for forest stands subject to natural self-thinning and epidemic insect infestations. There was no significant effect of fir admixture on R since basal area losses due to fir decline were offset within the 30-year period largely by hardwood increases. Stand dynamics were however, structurally stratified due to low R-values for low-diameter trees. The indices H’, J’, and R are recommended for assessing the naturalness and apparent resilience function of tree diversity and large trees in mixed forests of low species richness.     

Differences in the success of sugar maple and red maple seedlings on acid soils are influenced by nutrient dynamics and light environment

Variation in tolerance to nutrient limitations may contribute to the differential success of sugar maple ( Acer saccharum Marsh.) and red maple ( Acer rubrum L.) on acid soils. The objectives of this study were to examine these relationships as influenced by light environment and test whether sensitivity to nutrient stress is mediated by oxidative stress. First-year sugar maple and red seedlings were grown on forest soil cores contrasting in nutrient availability under high or low light intensity. Foliar nutrition, photosynthesis, growth and antioxidant enzyme activity were assessed. Photosynthesis and growth of sugar maple were significantly lower on nutrient-poor soils and were correlated with leaf nutrient status with Ca and P having the strongest influence. For red maple, only chlorophyll content showed sensitivity to the nutrient-poor soils. High light exacerbated the negative effects of nutrient imbalances on photosynthesis and growth in sugar maple. Antioxidant enzyme activity in sugar maple was highest in seedlings growing on nutrient-poor soils and was inversely correlated with photosynthesis, Ca, P, and Mg concentrations. These results suggest that: (1) sugar maple is more sensitive to nutrient stresses associated with low pH soils than red maple; (2) high light increases sugar maple sensitivity to nutrient stress; (3) the negative effects of nutrient imbalances on sugar maple may be mediated by oxidative stress.