Genetic Identity of Populus tremuloides Litter Influences Decomposition and Nutrient Release in a Mixed Forest Stand

Recent research has shown that genetic variation can directly impact community and ecosystem level processes. Populus tremuloides (trembling aspen) is an extremely widespread and genetically diverse tree species important to many North American forest ecosystems. Using leaf litter from five genotypes grown in a common garden under two nutrient treatments, we tracked litter decomposition in a natural aspen stand for 1 year. Here we show that aspen leaf litter decomposes and releases carbon, nitrogen, and sulfur in relation to its genetic identity. In a secondary experiment, we show that the genetic diversity of aspen litter mixtures can influence decomposition, however weakly so. Overall, nutrient treatments influenced leaf litter decomposition the most, followed by genetic identity, and then by genetic diversity (if at all in some cases). In this widespread, genetically diverse, and dominant species, genetic variation within a single species is important to ecosystem functioning. The relatively weak effect of genetic diversity on the processes measured here does not preclude its importance to ecosystem functioning, but does suggest that genetic identity and composition are more important than genetic diversity per se.     

Soil nutrient status after bamboo flowering and death in a seasonal tropical forest in western Thailand

We have examined the surface (0–10 cm) soil characteristics of sites after bamboo (Cephalostachyum pergracile) mass flowering and death (DB sites) in comparison with sites with living bamboo (Bambusa tulda) (LB sites) in a seasonal tropical forest in Thailand. One year after bamboo flowering the DB sites were acidic with lower concentrations of exchangeable Ca and Mg and soil nitrogen than the LB sites. Therefore, although leaf and root litter of the dead bamboo was deposited in the DB sites after bamboo flowering, soil nutrient status decreased.     

Ectomycorrhizae formed in vitro by quaking aspen: including Inocybe lacera and Amanita pantherina

Ectomycorrhizae formed in synthesis tubes by aspen (Populus tremuloides) seedlings and each of seven fungal isolates are described. Isolates of Amanita muscaria v. formosa, A. pantherina, Inocybe lacera, and Paxillus vernalis, from sporocarps collected in aspen stands in southwestern Montana, developed mantles and Hartig nets on aspen roots, as did the broad-hostrange fungi Cenococcum geophilum and Pisolithus tinctorius from the VPI culture collection. Chalciporus piperatus failed to form mycorrhizae, and Piloderma croceum formed a mantle, but no Hartig net. The first syntheses of I. lacera and A. pantherina with aspen are reported.     

Chemistry and decomposition of litter from Populus tremuloides Michaux grown at elevated atmospheric CO2 and varying N availability

It has been hypothesized that greater production of total nonstructural carbohydrates (TNC) in foliage grown under elevated atmospheric carbon dioxide (CO₂) will result in higher concentrations of defensive compounds in tree leaf litter, possibly leading to reduced rates of decomposition and nutrient cycling in forest ecosystems of the future. To evaluate the effects of elevated atmospheric CO₂ on litter chemistry and decomposition, we performed a 111 day laboratory incubation with leaf litter of trembling aspen (Populus tremuloides Michaux) produced at 36 Pa and 56 Pa CO₂ and two levels of soil nitrogen (N) availability. Decomposition was quantified as microbially respired CO₂ and dissolved organic carbon (DOC) in soil solution, and concentrations of nonstructural carbohydrates, N, carbon (C), and condensed tannins were monitored throughout the incubation. Growth under elevated atmospheric CO₂ did not significantly affect initial litter concentrations of TNC, N, or condensed tannins. Rates of decomposition, measured as both microbially respired CO₂ and DOC did not differ between litter produced under ambient and elevated CO₂. Total C lost from the samples was 38 mg g^?1 litter as respired CO₂ and 138 mg g^?1 litter as DOC, suggesting short‐term pulses of dissolved C in soil solution are important components of the terrestrial C cycle. We conclude that litter chemistry and decomposition in trembling aspen are minimally affected by growth under higher concentrations of CO₂.     

Effect of nitrogen fertilization upon the secondary chemistry and nutritional value of quaking aspen (Populus tremuloides Michx.) leaves for the large aspen tortrix (Choristoneura conflictana (Walker))

Summary We investigated the effects of nitrogen fertilization upon the concentrations of nitrogen, condensed tannin and phenolic glycosides of young quaking aspen (Populus tremuloides) leaves and the quality of these leaves as food for larvae of the large aspen tortrix (Choristoneura conflictana), a Lepidopteran that periodically defoliates quaking aspen growing in North America. Nitrogen fertilization resulted in decreased concentrations of condensed tannin and phenolic glycosides in aspen leaves and an increase in their nitrogen concentration and value as food for the large aspen tortrix. These results indicate that plant carbon/nutrient balance influences the quality of aspen leaves as food for the large aspen tortrix in two ways, by increasing the concentrations of positive factors (e.g. nitrogen) and decreasing the concentrations of negative factors (eg. carbon-based secondary metabolites) in leaves. Addition of purified aspen leaf condensed tannin and a methanol extract of young aspen leaves that contained condensed tannin and phenolic glycosides to artificial diets at high and low levels of dietary nitrogen supported this hypothesis. Increasing dietary nitrogen increased larval growth whereas increasing the concentrations of condensed tannin and phenolic glycosides decreased growth. Additionally, the methanol extract prevented pupation. These results indicate that future studies of woody plant/insect defoliator interactions must consider plant carbon/nutrient balance as a potentially important control over the nutritional value of foliage for insect herbivores.     

Independent, Interactive, and Species-Specific Responses of Leaf Litter Decomposition to Elevated CO2 and O3 in a Northern Hardwood Forest

The future capacity of forest ecosystems to sequester atmospheric carbon is likely to be influenced by CO₂-mediated shifts in nutrient cycling through changes in litter chemistry, and by interactions with pollutants like O₃. We evaluated the independent and interactive effects of elevated CO₂ (560 μl l⁻¹) and O₃ (55 nl l l⁻¹) on leaf litter decomposition in trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) at the Aspen free air CO₂ enrichment (FACE) site (Wisconsin, USA). Fumigation treatments consisted of replicated ambient, +CO₂, +O₃, and +CO₂ + O₃ FACE rings. We followed mass loss and litter chemistry over 23 months, using reciprocally transplanted litterbags to separate substrate quality from environment effects. Aspen decayed more slowly than birch across all treatment conditions, and changes in decomposition dynamics of both species were driven by shifts in substrate quality rather than by fumigation environment. Aspen litter produced under elevated CO₂ decayed more slowly than litter produced under ambient CO₂, and this effect was exacerbated by elevated O₃. Similarly, birch litter produced under elevated CO₂ also decayed more slowly than litter produced under ambient CO₂. In contrast to results for aspen, however, elevated O₃ accelerated birch decay under ambient CO₂, but decelerated decay under enriched CO₂. Changes in decomposition rates (k-values) were due to CO₂- and O₃-mediated shifts in litter quality, particularly levels of carbohydrates, nitrogen, and tannins. These results suggest that in early-successional forests of the future, elevated concentrations of CO₂ will likely reduce leaf litter decomposition, although the magnitude of effect will vary among species and in response to interactions with tropospheric O₃.     

Colonisation par les micro-organismes,evolution chimique des feuilles de différentes espèces d'arbres (peuplier,aulne, myrique) dans des eaux oligotrophes du Bouclier canadien et incidence sur leur utilisation par les macro-invertébrés

Myrica gale, Alnus rugosa and Populus tremuloides leaves were incubated in situ in the oligotrophic acid waters of the Canadian Shield (James Bay, Quebec) in order to follow microorganic decomposition, respiration and chemical transformations.Respiratory activities in decomposing speckled alder and trembling aspen leaves were more important than that in sweet gale. In spite of low nutrient concentrations in the water, nitrogen concentration increased in the three species while phosphorus levels increased only in the speckled alder during decomposition.

     

Litter decomposition and nitrogen mineralization of soils in subtropical plantation forests of southern China,with special attention to comparisons between legumes and non-legumes

Litter decomposition and nitrogen mineralization were investigated in subtropical plantation forests in southern China. The CO₂ –C release from incubated litter and the forest floor of Acacia mangium, Acacia auriculaeformis, Eucalyptus citriodora, Pinus elliotii and Schima superba stands were used to estimate relative rates of litter decomposition. Decomposition was not positively correlated with litter nitrogen. E. citridora litter decomposed most rapidly and A. mangium litter most slowly, both with and without the addition of exotic nitrogen. Aerobic incubation and intact soil core incubation at 30 °C over a period of 30 days were used to assess nitrogen mineralization of six forest soils. Although there were differences in results obtained using the two methods, patterns between legume and non-legume species were the same regardless of method. All soils had pH values below 4.5, but this did not prevent nitrification. The dominant form of mineral nitrogen was nitrate for legume species and ammonium for non-legume species. The nitrogen mineralization potential was highest for soils in which legumes were growing.     

Effects of genotype and nutrient availability on phytochemistry of trembling aspen (Populus tremuloides Michx.) during leaf senescence

We documented temporal patterns in phytochemical composition of Populus tremuloides Michx. during leaf senescence, and the influence of genotype and soil nutrient availability on such patterns. Levels of foliar nitrogen, carbohydrates, phenolic glycosides and condensed tannins were quantified for four aspen genotypes grown in a common garden, with low and high levels of soil nutrients. Levels of all compounds tended to decline over time, although the magnitude of change was influenced by plant genotype and nutrient availability. Genetic variation in concentrations of phytochemicals was much greater for phenolic glycosides and tannins than for nitrogen and carbohydrates, and these phenolic signatures generally persisted through leaf abscission. Our results suggest that genotypic and nutrient effects on patterns of chemical change during senescence will likely influence the performance of late-season herbivores on aspen. Moreover, nutrient and especially genotypic variation in phytochemistry of abscised leaves is likely to affect litter decomposition rates.     

模拟N沉降下不同林龄马尾松林凋落叶分解-土壤C、N化学计量特征

模拟N沉降对森林生态系统的影响是当今全球变化生态学研究的一个热点问题,土壤碳库对N沉降比较敏感,N沉降增加了凋落叶分解过程中外源N含量,间接影响凋落叶分解的化学过程并改变凋落叶分解速率,因此,研究模拟N沉降下凋落叶分解-土壤C-N关系对预测森林C吸存有重要意义。利用原位分解袋法研究了模拟N沉降下三峡库区不同林龄马尾松林(Pinus massoniana)凋落叶分解过程中凋落叶-土壤C、N化学计量响应及其关系;N沉降水平分对照(CK,0 g m~(-2)a~(-1))、低氮(LN,5 g m~(-2)a~(-1))、中氮(MN,10 g m~(-2)a~(-1))和高氮(HN,15 g m~(-2)a~(-1))。结果表明:分解540 d后,N沉降促进20年生和30年生马尾松林凋落叶分解,46年生马尾松林中仅低氮处理促进凋落叶分解,4种处理均是30年生分解最快,说明同一树种起始N含量低的凋落叶对N沉降呈正响应,N沉降处理促进起始N含量低的凋落叶分解,起始N含量高的凋落叶分解过程中易达到"N饱和"。N沉降抑制20年生和46年生凋落叶C释放(低于对照0.62%—6.69%),促进30年生C释放(高于对照0.28%—5.55%);30年生和46年生林分N固持量均高于对照(高于对照0.15%—21.34%),20年生则低于对照(5.70%—13.87%),说明模拟N沉降处理促进起始C含量低的凋落叶C释放和起始N含量低的凋落叶N固持。N沉降处理下仅30年生马尾松林土壤有机碳较对照增加,且土壤有机质与凋落叶C、N和分解速率呈正相关,与凋落叶C/N比呈显著负相关;土壤总氮与凋落叶分解速率、凋落叶N含量呈正相关,土壤有机碳/总氮比与凋落叶C、N含量呈正相关;对照处理中凋落叶分解指标对土壤养分影响顺序是分解速率凋落物C含量凋落物C/N比凋落物N含量,低、中、高氮处理中则是凋落物C含量分解速率凋落物N含量凋落物C/N比。研究表明低土壤养分含量马尾松林对N沉降呈正响应,N沉降促进低土壤养分马尾松林凋落叶分解并提高土壤肥力;凋落叶质量和土壤养分含量低的生态系统土壤C对N沉降响应更显著。     

Competition and facilitation between tree species change with stand development

Processes governing tree interspecific interactions, such as facilitation and competition, may vary in strength over time. This study tried to unveil them by performing dendrometrical analyses on black spruce Picea mariana, trembling aspen Populus tremuloides and jack pine Pinus banksiana trees from pure and mixed mature boreal forest stands in the Clay Belt of northwestern Quebec and on the tills of northwestern Ontario. We cored 1430 trees and cut 120 for stem analysis across all stand composition types, tree species and study regions. Aspen annual growth rate was initially higher when mixed with conifers, but then progressively decreased over time compared to pure aspen stands, while jack pine growth rate did not differ with black spruce presence throughout all stages of stand development. When mixed with aspen, black spruce showed a contrary response to aspen, i.e. an initial loss in growth but a positive gain later. On the richer clay soil of the Quebec Clay Belt region, however, both aspen and spruce responses in mixed stands reversed between 37 and 54 years. Overall, our results demonstrate that interspecific interactions were present and tended to change with stand development and among species. Our results also suggest that the nature of interspecific interactions may differ with soil nutrient availability.     

Topographic control of vegetation in a mountain big sagebrush steppe

Mountain big sagebrush steppes in Wyoming have strong spatial patterning associated with topography. We describe the spatial variability of vegetation in a sagebrush steppe, and test the relationship between topography and vegetation using canonical correlation. Results of the analysis suggest that the main control over vegetation distribution in this system is wind exposure. Exposed sites are characterized by cushion plant communities and Artemisia nova, and less exposed sites by the taller sagebrush species Artemisia tridentata ssp. vaseyana. Topographic depressions and leeward slopes are characterized by aspen stands and nivation hollows. Measurements of soil microclimate suggest that a major influence of topographic position on vegetation is snow redistribution and its effect on soil moisture and temperature.Abbreviations ARNO Artemisia nova - ARTRW Artemisia tridentata ssp. wyomingensis - ARTRV Artemisia tridentata ssp. vaseyana - PUTR Purshia tridentata - RIP riparian community - POTR Populus tremuloides - NIV nivation hollow community     

Effects of leaf litter on the growth of boreal feather mosses: Implication for forest floor development

Question: What is the effect of leaf litter on growth and mortality of feather mosses. Three experiments were conducted to isolate the shading effect from the effect of leaching of soluble compounds from the leaf litter effect on feather moss mortality and growth. Location: Edmonton, Alberta and the northern boreal coniferous forest, west‐central Alberta, Canada. Methods: In a field experiment Populus tremuloides (aspen) leaf litter was applied to beds of feather mosses dominated by Hylocomium splendens. Treatments were one layer of leaves held in place with netting; one layer of leaves coarsely ground, sprinkled over the moss layer; a shade cloth equivalent to one layer of leaves; and a control. In two growth chamber experiments, the application of aqueous extracts of P. tremuloides and Pinus contorta leaf litter and the effect of shade and soluble carbohydrates were tested on the growth and mortality of Ptilium crista‐castrensis. Results: Mortality of Hylocomium was greatest under the intact leaves, followed by the treatments using shade cloth and ground leaves and finally the control. The application of aqueous extracts of aspen leaf litter resulted in senescence or death of nearly all Ptilium shoots compared to no effects for the control or for similar extracts of pine needle litter. Extracts from aspen litter had greater concentrations of phenolic compounds and soluble sugars than pine extracts. Addition of sugars to Ptilium allowed it to grow and accumulate carbohydrates, even in low light conditions. Conclusions: Results suggest that broad‐leaved deciduous overstory species can limit the growth of feather mosses through their leaf litter and by implication affect the humus form, nutrient cycling, and understory composition of these forests.     

Forest regrowth as the controlling factor of soil nutrient availability 75 years after fire in a deciduous forest of Southern Quebec

The effects of secondary succession on soil nutrient availability following fire in the 1920s was investigated in a hardwood forest of southern Quebec by correlation analyses between soil and solution chemistry, tree species composition, tree growth, litterfall nutrient fluxes and presence of charcoal monitored in six 300 m² plots between 1994 and 1998. The results suggests that the pioneer stand of largetooth aspen, paper birch and yellow birch that followed fire enriched the forest floor and upper mineral soil with its K-rich litter, but lowered solution NH₄, NO₃ and Mg concentrations through its high nutrient demand. High woody biomass primarily observed in the maple-dominated stands was associated with low exchangeable base cation concentrations in the forest floor, suggesting that nutrient immobilisation in trees is also a significant process leading to spatial variation in base cation availability in the forest floor. Finally, charcoal was positively correlated with exchangeable base cations in the forest floor which leads to believe that charcoal has a high affinity for base cations and that it can increase base cation availability decades after forest fire disturbance.     

Impacts of Falcataria moluccana Invasion on Decomposition in Hawaiian Lowland Wet Forests: The Importance of Stand-level Controls

Invasive species have the capacity to substantially alter soil processes, including rates of litter decomposition. Currently, the few remaining native-dominated lowland wet forests in Hawai’i are being invaded by Falcataria moluccana, a large, fast-growing, N₂-fixing tree. In this study, we sought to determine the extent to which Falcataria invasion alters decomposition in these lowland wet forests, and whether changes resulted from differences in litter substrate type, lava flow age and type, forest stand type and associated soil biota, or some combination of these factors. We measured decomposition rates and nitrogen (N) and phosphorus (P) dynamics of Metrosideros polymorpha and Falcataria leaf litter in native-dominated and Falcataria-invaded stands on 48- and 300-year-old a’a lava flows and a 213-year-old pāhoehoe flow in the Puna district of eastern Hawai’i. Despite significant differences in the initial quality of Metrosideros and Falcataria litter, in nearly all cases mass remaining of the two litter types did not differ within a given forest stand, whether native-dominated or invaded. Instead, stand type accounted for large differences in the decomposition of both litter types, and litter decomposed two to 10 times faster in Falcataria-invaded stands than it did in their native-dominated counterparts on each lava flow. Dynamics of N (that is, immobilization or release) during decomposition were affected by stand, litter, and lava flow type; P dynamics were affected by stand and flow type, but not litter type. Although not definitive proof of causality, the decay rates of both species were positively correlated to previously measured inputs of N mass and P mass via litterfall as well as availability of soil N and P, characteristics that all increased substantially with Falcataria invasion. Given the degree of change to a host of ecosystem processes, including decomposition, after invasion by Falcataria, these transformed forest ecosystems may best be viewed as fundamentally new and different, in both structure and function, from the native ecosystems they have replaced.     

Biomass,nutrient distribution and litterfall in Populus,Pinus and Picea stands on two different soils in Minnesota

Summary Pole sized stands ofPopulus tremuloides Michx.,Picea glauca (Moench.) Voss,Pinus resinosa Ait., andPinus banksiana Lamb., were sampled on both a very fine sandy loam and a loamy sand. Relative species ranking in above-ground tree biomass (Pinus resinosa>Populus>Picea>Pinus banksiana) and above-ground tree nutrient (N, P, K, Ca, Mg) weights (Populus>Picea>Pinus resinosa>Pinus banksiana) were similar on both soils. Particularly large proportions of biomass and nutrients were found in aspen bark and spruce foliage and branches on both soils. Harvesting entire above-ground trees would remove up to three times more nutrients than would harvesting only the bole.Herbs and shrubs had less than 3% of the total vegetation organic matter but contributed as much as one-half of the total annual litterfall nutrients. Litterfall weights and nutrient concentrations, and especially forest floor nutrients, were all less on the loamy sand. Nutrients in the rooting zone of the loamy sand were 12 to 29% less than in the very fine sandy loam except for P which averaged 24% higher. On both soils, exchangeable Ca in the surface soil was much lower under Populus and Picea than under the pines, owing to species differences in uptake and apparently slow release of Ca by weathering.Ca in the above-ground Populus amounted to 18% (very fine sandy loam) to 25% (loamy sand) of the exchangeable Ca in the total complex. Intensive utilization of this species in particular could stress the Ca economy of these sites.This article was written and prepared by U.S. Government employees on official time; it is therefore in the public domain.Principal Silviculturist and Research Soil Scientist, resp.     

Fire Interval Effects on Successional Trajectory in Boreal Forests of Northwest Canada

Although succession may follow multiple pathways in a given environment, the causes of such variation are often elusive. This paper describes how changes in fire interval mediate successional trajectory in conifer-dominated boreal forests of northwestern Canada. Tree densities were measured 5 and 19 years after fire in permanent plots and related to pre-fire vegetation, site and fire characteristics. In stands that were greater than 75 years of age when they burned, recruitment density of conifers was significantly correlated with pre-fire species basal area, supporting the expectation of stand self-replacement as the most common successional pathway in these forests. In contrast, stands that were under 25 years of age at the time of burning had significantly reduced conifer recruitment, but showed no change in recruitment of trembling aspen (Populus tremuloides). As a result, young-burned stands had a much higher probability of regenerating to deciduous dominance than mature-burned stands, despite the dominance of both groups by spruce (Picea mariana and Picea glauca) and pine (Pinus contorta) before the fire. Once initiated, deciduous-dominated stands may be maintained across subsequent fire cycles through mechanisms such as low on-site availability of conifer seed, competition with the aspen canopy, and rapid asexual regeneration of aspen after fire. We suggest that climate-related increases in fire frequency could trigger more frequent shifts from conifer to deciduous-dominated successional trajectories in the future, with consequent effects on multiple ecosystem processes.     

Functional Group Identity Does not Predict Invader Impacts: Differential Effects of Nitrogen-fixing Exotic Plants on Ecosystem Function

The introduction of exotic plants can have large impacts on ecosystem functions such as soil nutrient cycling. Since these impacts result from differences in traits between the exotic and resident species, novel physiological traits such as N cycling may cause large alterations in ecosystem function. It is unclear, however, whether all members of a given functional group will have the same ecosystem effects. Here we look at a within functional group comparison to test whether an annual (Lupinus luteus) and a perennial (Acacia saligna) N-fixing exotic species cause the same effects on soil N cycling in the fynbos vegetation of South Africa. We measured litterfall quantity and quality, and soil total nitrogen and organic matter for each vegetation type as well. Available nitrogen was quantified using ion exchange resin bags monthly for 1 year. We used microcosms to evaluate litter decomposition. Although both exotic species increased the available nitrogen in the soil, only Acacia increased the total soil N and organic matter. This could be explained by the slow decomposition of Acacia litter in the microcosm study, despite the fact that Acacia and Lupinus litter contained equivalent N concentrations. Presumably, low carbon quality of Acacia litter slows its decomposition in soil, resulting in retention of organic nitrogen in Acacia stands after clearing for restoration purposes. The differences in long term impacts of these annual and perennial species highlight the fact that not all N-fixing exotic species exert equivalent impacts. Ecologists should consider multiple traits rather than broadly defined functional groups alone when predicting invader impacts.     

Aspen (<Emphasis Type="Italic">Populus tremuloides</Emphasis>) stands and their contribution to plant diversity in a semiarid coniferous landscape

We conducted a field study to determine the relative contributions of aspen (Populus tremuloides), meadow, and conifer communities to local and landscape-level plant species diversity in the Sierra Nevada and southern Cascade Range, northeastern California, USA. We surveyed plant assemblages at 30 sites that included adjacent aspen, conifer, and meadow communities across a 10,000-km² region. We statistically investigated patterns in local and landscape-scale plant diversity within and among the three vegetation types. Summing across sites, aspen stands supported more plant species overall and more unique plant species than either meadow or conifer communities. Local richness and diversity did not differ between aspen and meadow plots; conifer forest plots were significantly lower in both measures. Heterogeneity in species composition was higher for aspen forest than for meadows or conifer forest, both within sites and between sites. Plant communities in aspen stands shared less than 25% of their species with adjacent vegetation in conifer and meadow plots. Within aspen forest, we found a negative relationship between total canopy cover and plant diversity. Our results strongly support the idea that plant communities of aspen stands are compositionally distinct from adjacent meadows and conifer forest, and that aspen forests are a major contributor to plant species diversity in the study region. Current patterns of aspen stand succession to conifer forest on many sites in the semiarid western US are likely to reduce local and landscape-level plant species diversity, and may also have negative effects on other ecosystem functions and services provided by aspen forest.     

Spatial leaf distribution and self-thinning exponent of<Emphasis Type="Italic"> Pinus banksiana</Emphasis> and<Emphasis Type="Italic"> Populus tremuloides</Emphasis>

A hypothesis that the pattern of spatial leaf distribution in forest canopies is numerically related to the exponent of the self-thinning relationship in even-aged monocultures was tested by evaluating the crown fractal dimension of Pinus banksiana (jack pine) and Populus tremuloides (quaking aspen) in Wood Buffalo National Park, Canada. Pure species stands that were considered the most dense for a given mean tree size were measured to establish the empirical self-thinning relationships. The value of the self-thinning exponent was estimated as –1.42 with 95% Confidence Interval (CI) (–1.47, –1.36) for Pinus banksiana, and –1.29 with 95% CI (–1.45, –1.14) for Populus tremuloides. For each species the box dimension of spatial leaf distribution was estimated from unit cylinders described by sequentially lowering in forest canopies, horizontal flaps of one of various diameters attached to the top of a height-measuring pole. The box dimension appeared as 1.95 (1.84, 2.06) for Pinus banksiana, and 2.24 (2.05, 2.43) for Populus tremuloides. By assuming that the box dimension is equivalent to the fractal dimension at the inter-population level, the self-thinning exponent was predicted to be –1.53 (–1.62, –1.45) for Pinus banksiana, and –1.33 (–1.45, –1.23) for Populus tremuloides. The empirical exponent was equivalent to that predicted from the box fractal dimension, as judged by the 95% CI of the dimensions. We conclude that spatial patterns of leaf distribution in forest canopies, as being characterized by the box fractal dimension, are closely related to the value of the self-thinning exponent in the dense monocultures of the species we examined.