Succession in the southern part of the Canadian boreal forest

Forest succession following fire in a forest mosaic of northwestern Quebec has been studied in order to: (1) describe the successional pathways using communities of different ages and (2) evaluate convergence of successional pathways and possible effect of fire suppression on the establishment of steady-state communities. As a first step, ordination and classification techniques were used in order to remove changes in forest composition which are related to abiotic conditions. Then, ordinations based on tree diameter distributions were used to study shifts in species composition in relation to time since the last fire.Even under similar abiotic conditions, successional pathways are numerous. However, regardless of forest composition after fire, most stands show convergence toward dominance of Thuja occidentalis and Picea mariana on xeric sites and dominance of Abies balsamea and Thuja occidentalis on more mesic sites. Stable communities of >300 yr occur on xeric sites while on mesic sites directional succession still occurs after 224 yr. Nearly all species involved in succession are present in the first 50 yr following fire. Only Abies balsamea and Thuja occidentalis increase significantly in frequency during succession. Following initial establishment, successional processes can generally be explained by species longevity and shade tolerance. Early successional species may be abundant in the canopy for more than 200 yr while the rapid decrease of Picea glauca, a late successional species could be related to spruce budworm outbreaks. Considering the short fire rotation observed (about 150 yr), a steady-state forest is unlikely to occur under natural conditions, though it may be possible if fire is controlled.     

Demographic shifts in eastern US forests increase the impact of late-season drought on forest growth

While forest communities are changing as a result of global environmental change, the impacts of tree species shifts on ecosystem services such as carbon storage are poorly quantified. In many parts of the eastern United States (US), more xeric-adapted oak-hickory dominated stands are being replaced with mesic beech-maple assemblages. To examine the possible impacts of this ongoing change in forest composition, we investigated how two wide-ranging and co-occurring eastern US species – Acer saccharum (sugar maple) and Quercus alba (white oak) – respond to interannual climate variability. Using 781 tree cores from 418 individual trees at 18 locations, we found late-growing season drought reduced A. saccharum growth more than that of Q. alba. A gradient in the growth reduction across latitude was also found in A. saccharum, where southern populations of A. saccharum experienced greater reductions in growth during drought. Drought had a legacy effect on growth for both species, with drought occurring later in the growing season having a larger legacy effect. Consequently, as forests shift from oak to maple dominance, drought in the later part of the growing season is likely to become an increasingly important control on forest productivity. Thus, our findings suggest that co-occurring species are responding to environmental conditions during different times in the growing season and, therefore, the timing of drought conditions will play an important role in forest productivity and carbon sequestration as forest species composition changes. These findings are particularly important because the projected increases in potential evapotranspiration, combined with possible changes in the seasonality of precipitation could have a substantial impact on how tree growth responds to future climatic change.     

Canopy disturbance patterns and regeneration of Quercus species in two Ohio old-growth forests

This study was conducted to determine the abundance of Quercus species, the spatial pattern of Quercus regeneration, the current canopy disturbance pattern, and their interrelationship in two old-growth deciduous forests in Ohio (Goll Woods and Sears-Carmean Woods). Acer saccharum and Fagus grandifolia had the greatest density and basal area in both forests, yet the largest trees (by basal area) present at each site were Quercus spp. Quercus spp. appeared to be decreasing in abundance in both sites. Though Quercus seedlings were common, few Quercus saplings or subcanopy trees were present. The current disturbance regimes were dominated by small canopy gaps created by death of 1–2 trees; canopy gaps 100 m² in size were rare and only 2.5–2.8% of the forest area was covered by recognizable canopy gaps. No significant differences in the density of Quercus seedlings or saplings were found between gaps and non-gap areas at either site. Though no significant barrier to seedling establishment appeared to exist, the present disturbance regimes are not well suited for the growth of Quercus into the subcanopy size class or the recruitment of Quercus into the canopy. The most frequent gapmakers in Goll Woods were Tilia americana and Acer saccharum, and those in Sears-Carmean Woods were A. saccharum and F. grandifolia. The species most frequent as gap fillers were A. saccharum (in both sites) and F. grandifolia (in Goll Woods). These results suggest that A. saccharum will continue to increase in abundance, and Quercus decrease in abundance, in these two old-growth stands.     

Spatial patterns of tree seedling establishment and their relationship to environmental variables in a cold-temperate deciduous forest of eastern North America

Patterns of seedling recruitment may have persistent effects on population and community processes. Assuming seed availability is not limiting, the environmental sieve (i.e., the suite of factors influencing seed germination and seedling emergence and survival) determines how many seedlings establish and, most importantly, where they do so. In this study, we identify the spatial structure of some resources and abiotic conditions known to be significant for tree seedling emergence and survival and determine how these environmental factors influence the establishment of Fagus grandifolia, Acer saccharum, Fraxinus americana, and Ostrya virginiana in a deciduous forest of southern Québec (Canada). We expect an increase from Fagus, through Acer and Fraxinus, to Ostrya in the control of environmental variables on seedling emergence and survival, because of differences in the seed size of these species. Density of newly-emerged seedlings of all four species showed positive spatial autocorrelation at distances of up to ca. 10 m. Environmental variables were also structured at the same spatial scale, except for soil moisture. Acer seedling emergence pattern was positively correlated to photosynthetic photon flux density (PPFD), and the pattern of Fraxinus to soil N and moisture. Seedling survival was not spatially autocorrelated for any of the four species, although it was positively density-dependent in Acer and Fagus. In only Ostrya was seedling survival correlated (positively) to one of the environmental variables studied, i.e., PPFD. Overall, environmental variables were spatially less heterogeneous than seedling emergence and survival. Either seed availability was not saturating or factors not considered here, such as competition and predation (the intensity of which often varies with resources and/or abiotic conditions), modified the influence that the physical environment had on patterns of seedling establishment. Our prediction of a greater environmental control on seedling emergence and survival in small-seed species was not totally confirmed.     

Variations among woody plants in stomatal conductance and photosynthesis during and after drought

Acer saccharum, Fraxinus americana, Juglans nigra, Acer rubrum, Cornus amomum, and Ulmus americana seedlings were subjected to a soil drying cycle and then rewatered. At frequent intervals during the drying cycle and following rewatering, determinations were made of equilibrium photosynthesis rates, leaf conductances and leaf water potentials. As the drying cycle progressed, leaf water potentials decreased, stomata closed, and rates of transpiration and photosynthesis were reduced. Stomata of the two Acer species initially were more sensitive to water stress than were those of the other species. At low leaf water potentials, stomata of Juglans and Cornus were more open than those of the other species. Photosynthesis of Acer saccharum, Fraxinus and Juglans was significantly reduced by plant water stress, while photosynthetic water use efficiency of Cornus and Juglans was most unfavourable. Photosynthesis/leaf conductance ratios in water stressed leaves were higher in Fraxinus than in the other species. Immediately after rewatering, only limited stomatal opening occurred in Acer saccharum and Cornus with recovery of stomatal opening most protracted in Fraxinus and Ulmus. There was extended reduction of photosynthesis of all species as a result of the soil drying treatment. This effect was most significant in Acer saccharum and Juglans. Survival of plants on moist and dry sites is discussed in relation to stomatal control of transpiration and metabolic responses to water stress. Research supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison and the International Shade Tree Conference. The cooperation of the Wisconsin Department of Natural Resources is acknowledged. Research supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison and the International Shade Tree Conference. The cooperation of the Wisconsin Department of Natural Resources is acknowledged.     

Impacts of tree species diversity on litter decomposition in northern temperate forests of Wisconsin, USA: a multi-site experiment along a latitudinal gradient

Over the past decade an increasing amount of research has sought to understand how the diversity of species in an ecosystem can influence fluxes of biologically important materials, such as the decomposition of organic matter and recycling of nutrients. Generalities among studies have remained elusive, perhaps because experimental manipulations have been performed at relatively small spatial scales where site-specific variation generates patterns that appear idiosyncratic. One approach for seeking generality is to perform parallel experiments at different sites using an identical species pool. Here we report results from a study where we manipulated the diversity of leaf litter from the same six dominant tree species in the litter layer of three forested ecosystems. These ecosystems spanned a 300 km latitudinal transect in Wisconsin, USA, and were characterized by a large gradient in temperature and moisture, and thus, rates of decomposition. After allowing combinations of one, two, four, and six species of leaf litter to decompose for 1 year, we found that increasing leaf litter richness led to slower rates of decomposition and higher fractions of nitrogen lost from litter. Across all sites, climate and initial litter chemistry explained more of the variation in decomposition rates than did litter richness. Effects of leaf litter diversity were non-additive, meaning they were greater than expected from the impacts of individual species, and appeared to be strongly influenced by the presence/absence of just 1–2 species (Tilia americana and Acer saccharum). The rate of decomposition of these two species was highly site-specific, which led to strong negative effects of litter richness only being observed at the southernmost sites where T. americana and A. saccharum decomposed more quickly. In contrast, litter diversity increased nitrogen loss at the northernmost sites where decomposition of T. americana was notably slowed. Our study shows that species diversity affected at least one of the two litter processes at each site along this 300-km gradient, but the exact nature of these effects were spatially variable because the performance of individual species changed across the heterogeneous landscape.     

Riverine wetland vegetation: importance of small-scale and large-scale environmental variation

Abstract. The purpose of this study was to evaluate the relative importance of small-scale variation in abiotic factors and large-scale spatio-temporal variation on the distribution of wetland vegetation of a section of the Upper St. Lawrence River in Québec. Vegetation data have been classified with agglomerative clustering into 11 community types, from Acer rubrum, Acer saccharinum and Fraxinus swamps, to scrubs dominated by Salix petiolaris, Alnus rugosa var. americana or Myrica gale, to Typha, Typha/Lythrum, Carex lacustris and Calamagrostis canadensis marshes. Canonical Correspondence Analysis (CANOCO) suggested that peat thickness and water level are the most important abiotic variables correlated with plant community composition. As a whole, small-scale variation accounts for 25.6 % of the species variation. Another 21.6% is explained by large-scale variation based on location data summarizing large-scale spatial distribution, and historical landscape dynamics differentiated into (a) no net loss of wetlands, (b) net loss of wetland, and (c) changes within wetlands, which are correlated with the actual variation in herbaceous and scrubby vegetation. The interaction between small-scale and large-scale variations explains another 1.7 %. In total, 48.9% of the species variation is explained by the two data sets, leaving 51.1% unexplained. Whereas the omission of some abiotic variables is possible, it is hypothesized that the abiotic conditions measured in this study play an important role, especially in the distribution of forested swamps. In addition, past history, particularly that of human interventions, becomes another important factor leading to the observed importance of large-scale spatio-temporal variables. This is particularly true for Alnus rugosa var. americana shrublands. Time lag between a relative stabilization of species distribution and the reduction of natural disturbances (water level fluctuations and fires) could be a possible cause of the importance of spatio-temporal variables and the undetermined portion of species variation.     

Structure and spatial patterns of trees in old-growth northern hardwood and mixed forests of northern Maine

Stand structure including spatial patterns was studied in northern hardwood and mixed forest types in the 2000-ha old-growth Big Reed Forest Reserve in northern Maine using complete stem mapping, dendrochronology, and spatial analyses on 0.5 plots. The inclusion of saplings, dead wood, age distributions, spatial pattern, and interactions provided some idea of underlying processes and temporal change. Structural characteristics were most comparable to spruce-northern hardwood forests of northern New England and New York, and most characteristics matched expected patterns for old-growth forests of the region.Results indicated smaller maximum-tree sizes, lower basal areas (26–34 ) and downed-wood volumes (29–64), higher densities (475–649), but similar species longevities compared to other mesic old-growth forests further south and in the Lake States. The stands were dominated by very shade-tolerant tree species, including Fagus grandifolia Ehrh., Acer saccharum Marsh.,Picea rubens Sarg. and Abies balsamea(L.) Mill, with each species found in many crown positions and age and size classes. The sapling layer was dominated by Fagus grandifolia followed by Picea rubens. Most species had reverse-J shaped diameter distributions, but age distributions were indicative of synchronous, episodic recruitment. In most plots, Acer saccharum diameter distributions were skewed towards the mid-larger size classes. Lack of young and small Acer saccharum stems suggested change in forest composition towards Fagus grandifolia dominance. Most species formed small-scale clusters (15 ) perhaps in response to small gap disturbances. Snags were the dominant dead wood type and were randomly to regularly distributed in most plots. Logfall directions were unrelated to hurricane paths. Recent small-scale disturbance events and topographic position appear to be important in explaining current structure and dynamics of the hardwood and mixed forests of Big Reed Forest Reserve in northern Maine. The continued effects of beech bark disease had a greater effect on hardwood plots, whereas a recent spruce budworm outbreak had a greater effect on plots with higher conifer density.The dominance of very shade tolerant tree species in small-scale clusters, and randomly distributed snags rather than clustered uproots were indicative of the prevalence of small scale gap disturbance regimes in the hardwood and mixed forests of Big Reed Forest Reserve in northern Maine. Varying topographic position may allow for slight changes in disturbance regime leading to consequent variation in structure and dynamics. H1, a more open plot on upper exposed slopes, had distinctly different characteristics such as lower live and dead tree and sapling densities than the other plots, but more uprooted trees and Acer saccharum saplings. Such small scale gap disturbance regimes operating on an episodic basis, and effects of slight variations in this regime on stand composition and structure have significant implications for silvicultural interventions and management of these forest types.     

Losses of seeds of temperate trees to soil fungi: effects of habitat and host ecology

Evidence suggests that impacts of fungal pathogens on tree recruitment tend to be greater in the forest understory than in openings, and that shade-tolerant trees are less vulnerable than shade-intolerant species. To investigate the role that harmful soil fungi may have in reducing regeneration of temperate trees, we applied fungicide to buried seeds of matched pairs of species differing in their relative shade tolerance and/or successional status (Acer negundo versus Acer saccharum, Prunus virginiana versus Prunus serotina, and Pinus strobus versus Tsuga canadensis), in three habitats that differed in their degree of openness (old field, forest gap, and forest understory). Our results indicated that soil fungi reduced germination of A. negundo, P. virginiana, P. serotina. and T. canadensis, and reduced viability of ungerminated seeds of P. strobus; no significant effects of fungi on seeds of A. saccharum were detected. However, we found seeds were not less likely to survive following burial in forest understory than in gaps. As well, results for only one species pair (A. negundo versus A. saccharum) were consistent with the prediction that shade-intolerant or successional species should be more susceptible to fungal attack than mature forest species. These results contrast with other studies of temperate and especially tropical forest trees.     

Fine-root production in small experimental gaps in successional mixed boreal forests

Abstract. The effects of small 10m × 10m experimental above-ground gaps on fine-root production for the first two years were studied in three fire-initiated stands of the northwestern mixed broad-leaf-conifer boreal forest of Québec. The 48-yr-old forest is dominated by Populus tremuloides (Trembling aspen), the 122-yr-old forest by a mixture of P. tremuloides, Abies balsamea (balsam fir) and Picea glauca (white spruce), and the 232-yr-old forest by Thuja occidentalis (eastern white cedar) and A. balsamea, with some P. tremuloides. 40 root-ingrowth bags were installed in different locations in and around each gap (at gap center, 1 to 2 m either side of gap edge and in adjacent control plots). Half of the ingrowth bags were harvested after one year following gap creation, the other half after two years. Roots were sorted into different species grouping. Fine-root production was statistically (P < 0.05) larger in the youngest forest compared to the two older ones after one year, but not after two years. The individual species or groups of species increased, decreased or showed no change in fine-root production in gaps, but overall we did not observe a major shift in species proportion between gap and control plots after two years. Some herbs and also Taxus canadensis seemed to benefit in terms of fine-root growth from such small openings after two years. No statistical differences (P > 0.10) in total fine-root production were found among locations within and outside gaps in either year. However, there was a clear tendency for fine-root production to be smaller in gap center than in the other locations for the two younger successional forests the first year after gap creation. We conclude that small above-ground gaps (i.e. < 100 m²) do not produce a significant and long-lasting below-ground gap in terms of total fine-root production in the successional forests investigated.     

Comparing the Importance of Seedbed and Canopy Type in the Restoration of Upland Thuja occidentalis Forests of Northeastern Minnesota

In cold‐temperate ecosystems of the upper Great Lakes Region, evergreen conifer‐dominated forests were once common. As a result of past management practices, early‐successional deciduous forests now dominate the landscape. Embedded in this matrix are stands of shade‐tolerant conifers, including Thuja occidentalis. For the past several decades, large‐scale T. occidentalis regeneration in remnant T. occidentalis and adjacent Betula papyrifera forests has not occurred. Using a combination of restoration experiments and field surveys at three study sites on the Lake Superior Highlands, Minnesota, U.S.A., we examined safe sites for T. occidentalis regeneration under both Thuja and Betula canopy types. This study focused on the colonization and establishment phases of regeneration, differentiating among safe‐site components for T. occidentalis. Seedbed type determined colonization success, with higher rates on conifer logs than on B. papyrifera logs, mounds, or pits. Mortality rates for seedlings on natural decayed wood seedbeds were higher under Thuja canopy than under Betula canopy, but the reverse was true for seedlings on manipulated seedbeds, suggesting that seedbed type was more important than the dominant canopy type. Growth rates for seedlings on moved log segments were greater under the Betula than the Thuja canopy type, but seedlings on natural decayed wood seedbeds did not exhibit this difference. Results indicated that T. occidentalis regeneration was more limited in Betula forest by seedbed availability, while in Thuja forest canopy conditions were more limiting.     

Shade adaptation and shade tolerance in saplings of three Acer species from eastern North America

Summary Saplings of three, co-occurring maple species in a mature maple-beech forest differed in a suite of structural and physiological characters that separated the canopy species, Acer, saccharum, from the two subcanopy species, A. pensylvanicum and A. spicatum. Acer saccharum had both more dense wood and tougher and heavier but thinner leaves than the subcanopy species. Acer pensylvanicum had the largest, lightest leaves with high stomatal density and its canopy architecture was the most effective in terms of leaf display for light interception. Acer spicatum had weaker wood similar to that of A. pensylvanicum but also small, soft and relatively poorly displayed leaves. Both subcanopy species maintained marginally higher average rates of photosynthesis over the growing season in the understory environment. We consider juvenile A. saccharum only shade-tolerant, capable of persisting through long periods in the closed canopy until a gap occurs but not specifically adapted to the understory environment. Juvenile A. sacchrum appears to be constrained functionally by the requirements set by the canopy environment that adults will occupy. Characters such as high wood density are already expressed in the understory sapling; this investment in denser wood slows the growth of saplings, but is necessary for structural reasons in the adult. Juvenile A. saccaharum have morphological and photosynthetic characters better suited to gas exchange and extension growth under the increased photon flux densities in large forest gaps, characteristics that will also be advantageous in the sunlit canopy environment of adults.Both subcanopy maples appear to be more truly shade-adapted, although in somewhat different ways. Acer pensylvanicum has characteristics that enhance the potential for capture and utilization of sunflecks and is able to sustain higher growth rates than A. saccharum in the shaded subcanopy environment. Acer spicatum shares some shade-adapted features with A. pensylvanicum, and its habit of lateral spread through stem layering may confer an additional advantage in foraging for small light gaps.     

Multiple successional pathways of boreal forest stands in central Canada

Predicting forest composition change through time is a key challenge in forest management. While multiple successional pathways are theorized for boreal forests, empirical evidence is lacking, largely because succession has been inferred from chronosequence and dendrochronological methods. We tested the hypotheses that stands of compositionally similar overstory may follow multiple successional pathways depending on time since last stand‐replacing fire (TSF), edaphic conditions, and presence of intermediate disturbances. We used repeated measurements from combining sequential aerial photography and ground surveys for 361 boreal stands in central Canada. Stands were measured in 8–15 yr intervals over a ~ 60 yr period, covering a wide range of initial stand conditions. Multinomial logistic regression was used to analyze stand type transitions. With increasing TSF, stands dominated by shade‐intolerant Pinus banksiana, Populus sp., and Betula papyrifera demonstrated multiple pathways to stands dominated by shade‐tolerant Picea sp., Abies balsamea, and Thuja occidentalis. Their pathways seemed largely explained by neighborhood effects. Succession of stands dominated by shade‐tolerant species, with an exception of stands dominated by Picea sp., was not related to TSF, but rather dependent on edaphic conditions and presence of intermediate disturbances. Varying edaphic conditions caused divergent pathways with resource limited sites being dominated by nutrient‐poor tolerant species, and richer sites permitting invasion of early successional species and promoting species mixtures during succession. Intermediate disturbances promoted deciduous persistence and species diversity in A. balsamea and mixed‐conifer stands, but no evidence was detected to support “disturbance accelerated succession”. Our results demonstrate that in the prolonged absence of stand‐replacing disturbance boreal forest stands undergo multiple succession pathways. These pathways are regulated by neighborhood effects, resource availability, and presence of intermediate disturbance, but the relative importance of these regulators depends on initial stand type. The observed divergence of successional pathways supports the resource‐ratio hypothesis of plant succession.     

Tree-ring analysis of white cedar (Thuja occidentalis L.) archaeological and historical wood in Québec City (Québec,Canada)

In Québec City, a segment of an old wooden palisade built for protection was found buried in situ. The palisade was excavated by the Laval University archaeology field school in 2004–2005, and 29 posts were recovered and sampled. The palisade enclosed the Intendant's Palace compound, which was the residence of the governor of New France at the end of the 17th century. Tree-ring analysis was performed on wood excavated from the Intendant's Palace archaeological site (PDI) and additional wood from two historical military buildings, the Artillery Park (ART) (early 18th century) and the Québec Citadel (CIT) (17th–19th centuries). Wood identification revealed that white cedar (Thuja occidentalis L.) was used for wood construction at the three sites. Many trees used for the construction of the PDI palisade were felled after the 1690 growing season, likely in September/October 1690. Posts probably came from trees growing close to the site on the banks of the Saint-Charles River. A white cedar ring-width chronology from the Rimouski area, approximately 300 km northeast of Québec City, along the St. Lawrence River, was used for cross-dating. Archaeological and historical wood samples from two of the three sites (PDI and ART) were first assembled in a 235-year tree-ring chronology, called the Québec chronology, extending from 1489 to 1723. The two master chronologies (Québec and Rimouski) were merged into a single 513-year tree-ring chronology (1489–2001), called the Saint-Laurent chronology.     

Influence of northern limit range on genetic diversity and structure in a widespread North American tree,sugar maple (Acer saccharum Marshall)

Due to climate change, the ranges of many North American tree species are expected to shift northward. Sugar maple (Acer saccharum Marshall) reaches its northern continuous distributional limit in northeastern North America at the transition between boreal mixed‐wood and temperate deciduous forests. We hypothesized that marginal fragmented northern populations from the boreal mixed wood would have a distinct pattern of genetic structure and diversity. We analyzed variation at 18 microsatellite loci from 23 populations distributed along three latitudinal transects (west, central, and east) that encompass the continuous–discontinuous species range. Each transect was divided into two zones, continuous (temperate deciduous) and discontinuous (boreal mixed wood), based on sugar maple stand abundance. Respective positive and negative relationships were found between the distance of each population to the northern limit (D_north), and allelic richness (A_R) and population differentiation (F_ST). These relations were tested for each transect separately; the pattern (discontinuous–continuous) remained significant only for the western transect. structure analysis revealed the presence of four clusters. The most northern populations of each transect were assigned to a distinct group. Asymmetrical gene flow occurred from the southern into the four northernmost populations. Southern populations in Québec may have originated from two different postglacial migration routes. No evidence was found to validate the hypothesis that northern populations were remnants of a larger population that had migrated further north of the species range after the retreat of the ice sheet. The northernmost sugar maple populations possibly originated from long‐distance dispersal.     

Leaf Litter Disappearance in Earthworm-Invaded Northern Hardwood Forests: Role of Tree Species and the Chemistry and Diversity of Litter

Earthworm invasion in North American temperate forest reduces forest floor mass, yet the interactions between litter composition, invasive earthworm community composition, and forest floor structure and composition are not well understood. For 2?years, we compared disappearance of leaf litter in field mesocosms in which we manipulated litter composition (monocultures of Quercus rubra, Acer saccharum, and Tilia americana litter, and an equal mixture of all three) and thereby the initial litter chemistry (C, C fractions, N, Ca) in sites with and without the major litter-feeding invasive earthworm species. The disappearance of litter mass followed the same ranking at both the sites: T. americana?>?equal mixtures?>?A. saccharum?≥?Q. rubra. However, differences in disappearance rate between the sites depended on litter composition and time. The differences in mass loss among litters of different compositions were greatest at the site invaded by the large litter-feeding earthworm, Lumbricus terrestris, and especially for T. americana and the mixture. Similarly, observed disappearance of the litter mixture was faster than predicted by an additive model at the site with L. terrestris, especially for the higher quality litter component in early summer. Initial litter calcium content was the best predictor (R ²?≥?0.90) of overall litter mass remaining each year, supporting the idea of the importance of calcium in forest floor dynamics, especially in the presence of calciferous, invasive earthworms.     

Carbon dioxide assimilation of hardwood seedlings in relation to community dynamics in central illinois

Summary Field measurements of net assimilation and respiration for seedlings of four hardwood species were made periodically over a growing season with soil moisture tension maintained between 0 and 0.75 bar. Total net assimilation per day was significantly greater for Acer saccharum than either Quercus rubra or Quercus alba and for Quercus macrocarpa as compared with Q. rubra, when measurements were made under natural shade conditions and light intensity varied from 80 to 120 ft-c. Mean light compensation points determined under canopy shade were 50.3, 53.5, 87.2, and 102.5 ft-c., respectively, for Acer saccharum, Quercus macrocarpa, Q. rubra, and Q. alba. In a 0.04-hectare canopy opening, total net assimilation per day was not significantly different between Q. rubra, Q. alba, and A. saccharum but was significantly greater for Q. macrocarpa than Q. alba and A. saccharum. Relationships between photosynthetic efficiency and successional characteristics of these species are inferred from the data.     

Population age structure of Pinus banksiana at the southern edge of the Canadian boreal forest

Abstract. To assess the effects of site type, forest initiation periods and fire regimes on the dynamics of Pinus banksiana (Jack pine), the age structure of 69 populations of the species was analyzed. Two landscapes with different fire regimes were selected in the southern part of the Canadian boreal forest in Québec: the ‘mainland landscape’ is characterized by a fire regime of large lethal fires, the ‘island landscape’ is affected by a complex fire regime including lethal and non-lethal fires. Age structure was compared between forest initiation periods and site types (mesic mainland, xeric mainland and xeric island) using the Shannon regularity index. An even-aged population structure was found within the first 100 yr following a lethal fire, while after that period the population structure becomes more uneven-aged. Under mesic conditions, populations tend to have an even-aged structure, under xeric conditions an uneven-aged structure. Natural openings present in xeric sites allow for recruitment in the absence of fire. This permits the self-maintenance of Pinus banksiana. Xeric island populations show more uneven-aged structures than xeric mainland populations. The occurrence of non-lethal fires on the islands creates uneven-aged structures. Further, the results suggest that the selection pressure of the island fire regime, favouring non-serotinous and mixed P. banksiana individuals, is one of the factors responsible for a higher recruitment in the absence of fire on islands than on the mainland.     

Do Woody Plants Operate Near the Point of Catastrophic Xylem Dysfunction Caused by Dynamic Water Stress? : Answers from a Model

We discuss the relationship between the dynamically changing tension gradients required to move water rapidly through the xylem conduits of plants and the proportion of conduits lost through embolism as a result of water tension. We consider the implications of this relationship to the water relations of trees. We have compiled quantitative data on the water relations, hydraulic architecture and vulnerability of embolism of four widely different species: Rhizophora mangle, Cassipourea elliptica, Acer saccharum, and Thuja occidentalis. Using these data, we modeled the dynamics of water flow and xylem blockage for these species. The model is specifically focused on the conditions required to generate `runaway embolism,' whereby the blockage of xylem conduits through embolism leads to reduced hydraulic conductance causing increased tension in the remaining vessels and generating more tension in a vicious circle. The model predicted that all species operate near the point of catastrophic xylem failure due to dynamic water stress. The model supports Zimmermann's plant segmentation hypothesis. Zimmermann suggested that plants are designed hydraulically to sacrifice highly vulnerable minor branches and thus improve the water balance of remaining parts. The model results are discussed in terms of the morphology, hydraulic architecture, eco-physiology, and evolution of woody plants.     

The Finnish forest site type approach: ordination and classification studies of mesic forest sites in southern Finland

A.K. Cajander's forest site type classification system is based on definition of plant communities typical to certain climatical and edaphical site conditions, but the structure and composition of the tree stands in Finland are considered sensitive to random variation and are therefore not used as primary classification criteria. The system has often received criticism, usually that the effects of the tree stand and successional stage of the stand have been underestimated. Most of the present-day forest stands in Finland represent young successional stages and are subjected to intensive management. This should result in an additional difficulty in the application of the forest site types in the field.The present study is based on three independent data sets representing forests on mineral soil in southern part of Finland. TWINSPAN classification, DCA ordination and canonical correspondence analysis (CCA) techniques were applied in successive stages of the data analysis. It was found that the definition of the intermediately fertile, mesic site types was clearly confused by the effects of the tree species and age of the stand. The analyses also revealed that the succession pathways on mesic forest sites are largely determined by the tree species composition. In stands dominated by Pinus sylvestris, the succession follows the competitive hierarchy model, whereas in stands dominated by Picea abies, severe shading of the tree canopy governs the development of understorey vegetation.Abbreviations CCA Canonical correspondence Analysis - DCA Detrended correspondence Analysis - TWINSPAN Two-way indicator species analysis