Dominant Canopy Tree Research Articles

Age structure of wind-felled canopy trees for Siebold's beech (Fagus crenata) in the northernmost population in Karibayama, Hokkaido

To determine the lifespan of Siebold's beech (Fagus crenata), a dominant canopy tree species of the cool temperate forests of Japan, we estimated the age of 33 mature beech trees felled by a large typhoon in September 2004 at Karibayama, in southern Hokkaido. The study site lies within one of the northernmost distributions of Siebold's beech populations in Japan. We counted tree rings from tree disks at a height of 1 m, and diameter growth was measured in annual increments. The sample age revealed two major peaks at 240 and 175 years. The diameter growth was good between 40 and 60 years of age and gradually decreased after 70 years of age. Annual diameter increments were noticeably small in good fruiting years, indicating a large allocation of annual carbon gains to the reproductive organs. Bud scale counts of juvenile trees revealed that the age at which the height reached 1 m was approximately 6–13 years. The lifespan of Siebold's beech in the Karibayama population was therefore estimated to be approximately 250 years, an estimation that is higher than that previously reported for this species in the northernmost populations.

Rapid Inventory of the Ant Assemblage in a Temperate Hardwood Forest: Species Composition and Assessment of Sampling Methods

Ants are key indicators of ecological change, but few studies have investigated how ant assemblages respond to dramatic changes in vegetation structure in temperate forests. Pests and pathogens are causing widespread loss of dominant canopy tree species; ant species composition and abundance may be very sensitive to such losses. Before the experimental removal of red oak trees to simulate effects of sudden oak death and examine the long-term impact of oak loss at the Black Rock Forest (Cornwall, NY), we carried out a rapid assessment of the ant assemblage in a 10-ha experimental area. We also determined the efficacy in a northern temperate forest of five different collecting methods--pitfall traps, litter samples, tuna fish and cookie baits, and hand collection--routinely used to sample ants in tropical systems. A total of 33 species in 14 genera were collected and identified; the myrmecines, Aphaenogaster rudis and Myrmica punctiventris, and the formicine Formica neogagates were the most common and abundant species encountered. Ninety-four percent (31 of 33) of the species were collected by litter sampling and structured hand sampling together, and we conclude that, in combination, these two methods are sufficient to assess species richness and composition of ant assemblages in northern temperate forests. Using new, unbiased estimators, we project that 38-58 ant species are likely to occur at Black Rock Forest. Loss of oak from these forests may favor Camponotus species that nest in decomposing wood and open habitat specialists in the genus Lasius.

Neighbourhood analyses of tree seed predation by introduced rodents in a New Zealand temperate rainforest

House mice Mus musculus and other introduced rodents represent a novel source of predation on tree seeds in New Zealand forests. In the northern temperate forests where these rodents are native, spatial and temporal variation in tree seed production can result in dramatic fluctuations in the distribution and abundance of seed predators, with subsequent feedbacks on the distribution and abundance of seedlings. We use neighbourhood models to examine variation in rodent predation on seeds of 4 tree species of the temperate rainforests of New Zealand as a function of 1) spatial variation in local canopy composition and 2) spatial and temporal variation in mouse activity. We placed seeds throughout mapped stands of mixed forests in alluvial valley bottoms and on elevated marine terraces in the Waitutu Forest, South Island. The risk of predation on seeds of 2 dominant canopy trees – rimu Dacrydium cupressinum and mountain beech Nothofagus solandri var. cliffortioides– peaked in neighbourhoods dominated by those species and by silver beech N. menziesii, particularly in a year of plentiful seed rain from these species. The risk of predation on rimu and beech seed was also related to measures of local mouse activity. These relationships suggest that the highest local abundance of mice was concentrated in rimu and beech neighbourhoods because of the food provided by seed rain from those trees. Predation on seed of miro Prumnopitys ferruginea, which is eaten by rats but not mice, was low in rimu neighbourhoods and where mouse activity was high. These patterns may reflect spatial segregation in the activity of rats versus mice within stands. Our results suggest that the spatial distribution of canopy trees translates into predictable patterns of variation in mouse activity and seed predation. Heterogeneity in rodent activity and seed predation within stands may have important implications for tree population dynamics.

How an Aggressive Weedy Invader Displaces Native Trees

How an Aggressive Weedy Invader Displaces Native Trees

Robinia pseudoacacia (Black Locust) as Day-roosts of Male Myotis septentrionalis (Northern Bats) on the Fernow Experimental Forest, West Virginia

During the summer of 2003, we captured and radiotagged ten male Myotis septentrionalis (northern bats) on the Fernow Experimental Forest (FEF) in the Allegheny Mountains of West Virginia to investigate day-roost selection. Of 16 roosts that were located, 13 were in Robinia pseudoacacia (black locusts), five in snags and eight in live trees. The other three roosts occurred in a Sassafras albidum (sassafras) snag and two live Acer saccharum (sugar maples). All live trees used as roosts were medium to large, canopy-dominant trees with considerable amounts of exfoliating bark and numerous broken limbs and cavities. Snags used as roosts were smaller than trees and other snags in surrounding stands, whereas live trees used as roosts were larger than other trees and snags in surrounding stands. Similar to previous research on female northern bats in the Allegheny Mountains, we observed a strong preference for both live and snag black locust as roosts over other available species. The high abundance of black locust as an important component on the FEF has been a relatively recent development dating to the early 1900s. Use of live canopy-dominant black locust with characteristics of mature forest trees lends support that older forests with decadent conditions provide important day-roost habitat, whereas use of both canopy dominant live trees and long-lasting black locust snags may support the ecological concept of roosting “areas” for northern bats.

Paleoecology of Late Paleozoic pteridosperms from tropical Euramerica1

Abstract DiMichele, W. A. (Department of Paleobiology, NMNH, Smithsonian Institution, Washington, DC 20560), T. L. Phillips (Department of Plant Biology, University of Illinois, Urbana, IL 61801), and H. W. Pfefferkorn (Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19106). Paleoecology of Late Paleozoic pteridosperms from tropical Euramerica. J. Torrey Bot. Soc. 133: 83–118. 2006.— Late Paleozoic pteridosperms are a paraphyletic group of seed plants that were prominent elements of tropical ecosystems, primarily those of wetlands or the wetter portions of seasonally dry environments. Because the group is more a tradition-based, historical construct than a well circumscribed phylogenetic lineage, the wide variance in ecological roles and ecomorphological attributes should not be surprising. Pteridosperms can be the dominant canopy trees in local habitats, prominent understory trees, scrambling ground cover, thicket-formers, or liana-like plants and vines. Some s...

Hedychium gardnerianum Invasion into Hawaiian Montane Rainforest: Interactions Among Litter Quality, Decomposition Rate, and Soil Nitrogen Availability

Few studies have examined the invasion of understory species into closed-canopy forests and, despite inter-specific differences in litter quality and quantity between understory and dominant canopy trees, the influence of understory invasions on soil nitrogen (N) cycling remains unknown. This paper examines litter quality and decomposition of kahili ginger (Hedychium gardnerianum), an invasive understory herb, to determine the influence of this species on N cycling in a Hawaiian montane rainforest. To examine the potential feedback between increased soil N availability and litter decomposition, litter from the invasive ginger, a native tree, and native tree fern was collected from unfertilized and fertilized plots and decomposed in a reciprocal transplant design. Hedychium litter decomposed faster than litter from the two native species. Across species, decomposition rates were negatively correlated with litter lignin content. Despite rapid decomposition rates of Hedychium litter, soil nitrogen availability and rates of net mineralization in the soil were similar in invaded and uninvaded plots. Nitrogen cycling at this site may be more strongly influenced by native species, which contribute the most to overall stand biomass. A negative effect of fertilization on the decomposition of Hedychium litter suggests that a negative feedback between litter quality and soil N availability may exist over longer timescales.

Light environment under Rhododendron maximum thickets and estimated carbon gain of regenerating forest tree seedlings

Canopy tree recruitment is inhibited by evergreen shrubs in many forests. In the southern Appalachian mountains of the USA, thickets of Rhododendron maximum L. restrict dominant canopy tree seedling survival and persistence. Using R. maximum as a model system, we examined available light under the thickets and the photosynthetic responses of seedlings of canopy tree species. We tested the hypothesis that the additional shading from under R. maximum drives carbon gain in seedlings below the threshold for growth and survival. A reduction in light under the thicket was found where canopy openness (derived from canopy photographs) under R. maximum was half the amount measured in forest without R. maximum. R.␣maximum also reduced direct radiation by 50% and diffuse radiation by 12–29% compared to forest without the shrub layer. Mean mid-day PPFD (photosynthetically active photon flux density between 1000 and 1400 h) under R. maximum (obtained from quantum sensors) was below 10 $$\upmu$$ mol m−2 s−1 on both clear and overcast days and the amount of sunflecks greater than 10 $$\upmu$$ mol m−2 s−1 PPFD was only 0–20 min per day. In contrast, forest without R. maximum received a mean PPFD of 18–25 $$\upmu$$ mol m−2 s−1 on clear days and a cumulative sunfleck duration of 100–220 min per day in all sky conditions. Consistent with light availability between the sites, daily carbon gain in Quercus rubra L. seedlings was lower in forest with R. maximum compared to forest where the shrub was absent. The presence of the shrub layer also significantly suppressed average mid-day photosynthesis of both Q. rubra and Prunus serotina Ehrt. seedlings on 8 out of 11 measurement dates. However, parameters derived from light response curves between seedlings growing in forest sites with or without a thicket of R. maximum was significantly different only in A max (maximum photosynthetic rate), indicating a lack of further acclimation to the deeper shade under R. maximum. While the additional shade cast by R. maximum is sufficient to prevent the regeneration of tree seedlings under this shrub, there was sufficient heterogeneity in light under the thicket to imply that deep shade only partially explains the complete inhibition of regenerating canopy trees under R. maximum.

Stomatal conductance, transpiration and sap flow of tropical montane rain forest trees in the southern Ecuadorian Andes

We investigated tree water relations in a lower tropical montane rain forest at 1950-1975 m a.s.l. in southern Ecuador. During two field campaigns, sap flow measurements (Granier-type) were carried out on 16 trees (14 species) differing in size and position within the forest stand. Stomatal conductance (g(s)) and leaf transpiration (E(l)) were measured on five canopy trees and 10 understory plants. Atmospheric coupling of stomatal transpiration was good (decoupling coefficient Omega = 0.25-0.43), but the response of g(s) and E(l) to the atmospheric environment appeared to be weak as a result of the offsetting effects of vapor pressure deficit (VPD) and photosynthetic photon flux (PPF) on g(s). In contrast, sap flow (F) followed these atmospheric parameters more precisely. Daily F depended chiefly on PPF sums, whereas on short time scales, VPD impeded transpiration when it exceeded a value of 1-1.2 kPa. This indicates an upper limit to transpiration in the investigated trees, even when soil water supply was not limiting. Mean g(s) was 165 mmol m(-2) s(-1) for the canopy trees and about 90 mmol m(-2) s(-1) for the understory species, but leaf-to-leaf as well as tree-to-tree variation was large. Considering whole-plant water use, variation in the daily course of F was more pronounced among trees differing in size and crown status than among species. Daily F increased sharply with stem diameter and tree height, and ranged between 80 and 120 kg day(-1) for dominant canopy trees, but was typically well below 10 kg day(-1) for intermediate and suppressed trees of the forest interior.

Effects of natural and human disturbances on the dynamics and spatial structure of Nothofagus glauca in south‐central Chile

AbstractAim The general model for Nothofagus regeneration dynamics states that the species of this genus tend to occur in frequent, coarse‐scale disturbance sites and/or in harsh conditions. The aim of this study is to test this regeneration model in deciduous mediterranean Nothofagus glauca forests at the northern limit of range of this genus, which include some of the most diverse but least well understood forests in the region. The study also aimed to advance general understanding of the ecological role of N. glauca in these forests, and its response to disturbance, by examining the composition, structure and regeneration patterns of N. glauca forests in the mediterranean zone following three different disturbance histories: mature, post‐logging, and post‐landslide second growth.Location The study was conducted at the southern limit of the mediterranean climatic zone, in the foothills of the Andes (36°35′ S, 71°28′ W) in south‐central Chile, near the south limit of range of N. glauca. Study sites were located at a range of altitudes between 660 and 850 m a.s.l.Methods Two fully mapped 0.12–0.14‐ha plots were located at each of three sites in order to sample age, composition, structure, canopy density and spatial patterns of the stands. The spatial description of sites was estimated with point‐pattern analysis using Ripley's L‐function (univariate and bivariate versions). Shannon's index was used to account for stand structural diversity by height class. Leaf area index (LAI) and canopy density were assessed with fisheye digital photos.Results Nothofagus glauca was the dominant canopy tree species at all sites. In the mature site there was abundant regeneration of N. glauca saplings, leading to a complex open canopy and an all‐aged structure with a total basal area (BA) of 21–33 m2 ha−1 (including all tree species), despite the presence of shade‐tolerant tree species and the bamboo Chusquea cumingii. The canopy was complex (Shannon's index up to 1.7), but was relatively open (16%). There was no evidence of a shift in regeneration from shade‐intolerant Nothofagus species towards more shade‐tolerant species. In the post‐logging secondary growth stand an evenly aged structure prevailed, with patchy, more open canopies (15–21%, total BA = 9–38 m2 ha−1), but with a less diverse canopy structure. The last site, located in the debris fan of a landslide, presented the lowest LAI and highest canopy opening (23–30%) where the site occupancy is not yet complete (total BA = 14–23 m2 ha−1). At the harshest sites, saplings had a positive spatial association with adult trees, which suggests facilitation from adult trees to saplings.Main conclusions Our study supports the general model for Nothofagus regeneration dynamics, except that N. glauca does not attain sufficient canopy density to require disturbance for regeneration, even in the best sites. Nothofagus glauca can be both a pioneer in harsh site conditions, as well as a gap strategist in mature forests. In these generally harsh sites, the most species, the densest canopies and the most diverse structures occur in the oldest forests. Low canopy density allowed for diverse understorey species at all sites.

Disturbance and regeneration dynamics of an old‐growth North Patagonian rain forest in Chiloé Island, Chile

Summary 1 Few studies have addressed the mechanisms of coexistence of shade-tolerant and intolerant tree species in the canopy of old-growth, lowland rain forests of southern South America. We explored the hypothesis that their forest dynamics result from frequent, single tree-fall gap episodes. 2 We analysed the disturbance regime and assessed the regeneration modes of shadetolerant and intolerant canopy trees in a lowland, old growth North Patagonian rain forest in Chiloe Island (42 ° S) using dendroecological methods. 3 Dominant canopy trees were the shade-intolerant Nothofagus nitida (Fagaceae), Drimys winteri (Winteraceae) and the tolerant Podocarpus nubigena (Podocarpaceae). The oldest individuals, however, were represented by Saxegothaea conspicua , Podocarpaceae (shade tolerant > 498 years) and Weinmannia trichosperma , Cunoniaceae (intolerant > 382 years). Shade-tolerant species have regenerated continuously for the past 400 years, but recruitment of shade-intolerant species has increased only in the past 50 years. These regeneration patterns suggest a lack of catastrophic disturbance, at least since 1650. 4 Analysis of tree radial growth patterns revealed frequent moderate releases of both shade-tolerant and intolerant tree species, consistent with frequent small-scale disturbances, such as individual tree-fall gaps. Increased releases, peaking in 1940 and followed by enhanced recruitment, may be a consequence of individual tree-falls induced by lowintensity windstorms or limited selective logging. Such disturbances have occurred for at least 250 years, but have had little overall effect on stand structure and canopy composition. 5 The replacement and coexistence of shade-tolerant and intolerant tree species in the canopy of this North Patagonian forest can be explained by frequent small-scale disturbances (i.e. gap-phase dynamics) and by the absence of large-scale natural or anthropogenic disturbances over the past four centuries.

Dispersal and germination of big and small nuts of Quercus serrata in a subtropical broad-leaved evergreen forest

Few studies have concerned with the differences in seed dispersal, germination and seedling establishment between big and small seeds within a tree species. In this study, the dispersal and fates of big and small nuts (labeled with coded tin-tags) of Quercus serrata (a dominant canopy tree species) were monitored at two stands (a primary stand and a secondary stand) in a subtropical broad-leaved evergreen forest in the Dujiangyan Region, Sichuan Province, China. We also examined the differences in germination of big and small nuts of Q. serrata without predation by sowing nuts on the ground surface or in the soil. We found that, the big nuts were better survived with longer cache lifetime, longer dispersal distances and higher proportion of emerged seedlings (2% at primary stand, 0.5% at secondary stand) than small nuts. We also found that, there was a marginally physiological difference in germination (germination proportion and seedling height) between big and small nuts. Burial obviously improves germination of both big and small nuts. It is suggested that small rodents may act as an important selective force to affect seed fates, seed shadows and seedling establishment for big and small nut of Q. serrata.

DENSITY-DEPENDENT POPULATION DYNAMICS OF A DOMINANT RAIN FOREST CANOPY TREE

Intraspecific, negative density dependence may contribute to the maintenance of diversity by limiting the dominance of common species. Shorea quadrinervis Slooten (Dipterocarpaceae) is one of the dominant canopy trees in a species-rich tropical rain forest in Southeast Asia. We test whether juvenile density and performance and overall population growth rate of S. quadrinervis decline with increasing local abundance of conspecific adult trees. We mapped the 357 S. quadrinervis adults (≥15 cm dbh) in 75 ha at Gunung Palung National Park, Indonesian Borneo, and compared the dynamics of juveniles <1 cm dbh between eight areas (80 m diameter plots) with high densities of conspecific adults and eight containing low densities. Within these 16 areas, we mapped 5215 S. quadrinervis juveniles and followed their growth and survival for two years. Plots with high and low density of S. quadrinervis adults did not differ in understory light, slope, aspect, or soil properties nor in the total density (all species combined) of either adults or juveniles. We evaluated four kinds of evidence. All were consistent with the hypothesis of negatively density-dependent population dynamics: (1) Density and size: Although the density of juveniles 75 cm tall. This suggests positive density-dependent seedling recruitment followed by negative density-dependent juvenile dynamics. (2) Foliar condition: Juveniles had more leaves per unit height where adult density was low. Growth and survival increased more with leaf number than with height. (3) Juvenile dynamics: Growth and survival were higher where adult density was low. Previous work suggests that herbivores may drive this negative density dependence in juvenile performance. (4) Population dynamics: We modeled population growth rate (λ) for each of the 16 areas using separate matrix models parameterized with empirical juvenile transition rates. We found that λ declined with local adult density (r2 = 0.40), and this trend was robust to the inclusion of El Nino conditions. We conclude that replacement of adults in the canopy is less likely in areas where conspecific adults are more abundant, tending to limit the dominance of S. quadrinervis in the canopy.

Temporal, spatial, and structural patterns of adult trembling aspen and white spruce mortality in Quebec's boreal forest

Temporal, spatial, and structural patterns of adult trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) mortality were studied in intact 150-year-old stands in the southwestern boreal forest of Quebec. For both species, mortality decreases (number of dead trees/total number of trees) with distance from the lake edge until 100–150 m, from which point it slightly increases. Strong peaks in mortality were found for 40- to 60-year-old aspen mainly between 1974 and 1992. Such mortality in relatively young aspen is likely related to competition for light from the dominant canopy trees. Also, the recruitment of this young aspen cohort is presumably the result of a stand breakup that occurred when the initial aspen-dominated stand was between 90 and 110 years old. For spruce, strong peaks in mortality were found in 110- to 150-year-old trees and they occurred mainly after 1980. No clear explanation could be found for these peaks, but we suggest that they may be related to senescence or weakening of the trees following the last spruce budworm outbreak. Suppressed and codominant aspen had a much higher mortality ratio than spruce in the same height class, while more surprisingly, no difference in mortality rate was found between dominant trees of the two species. Most spruce trees were found as standing dead, which leads us to reject the hypothesis that windthrow is an important cause of mortality for spruce in our forests.

There is high potential for the formation of common mycorrhizal networks between understorey and canopy trees in a mixed evergreen forest

1 The patterns of ectomycorrhizal (ECM) host specificity between understorey and canopy trees were investigated in three mixed evergreen forest stands in northern coastal California. ECM root tips from the dominant canopy (Pseudotsuga menziesii) and understorey (Lithocarpus densiflora) trees were sampled from 18 soil cores (six per stand) and identified using molecular techniques (PCR, RFLP, and DNA sequencing of the rDNA ITS region). 2 We found 56 ECM taxa; 17 on both hosts, 27 solely on Pseudotsuga and 12 on Lithocarpus. There were no significant differences in ECM taxon richness or diversity across stands, although ECM taxon richness was significantly higher on Pseudotsuga than Lithocarpus. ECM taxa similarity across stands was low. 3 Multiple-host ECM taxa had significantly higher abundance than single-host ECM taxa and 13 of the 17 multiple-host ECM taxa were present on both hosts within at least one core. Twelve of the 14 cores had at least one ECM taxon that was present on both hosts, although the specific taxon varied between cores and stands. In addition, shared ECM taxa often had unequal relative abundances on the two hosts. 4 Taken together, our results suggest that there is high potential for common mycorrhizal networks to form between Lithocarpus understories and Pseudotsuga canopies in mixed evergreen forests.

Understorey vegetation in boreal Picea mariana and Populus tremuloides stands in British Columbia

Abstract. We compared the species composition and species density of vascular plants in the understorey vegetation of boreal forest between Picea mariana (Black spruce) and Populus tremuloides (Trembling aspen) stands in British Columbia, Canada, and related differences in species composition and species density between the two forest types to dominant canopy tree species as well as a wide variety of environmental factors. We analysed 231 stands, distributed in three different climatic regions representing drier, wetter, and milder variations of montane boreal climate. Of these stands 118 were dominated by P. mariana and 113 by P. tremuloides. P. tremuloides stands had higher species density than P. mariana stands in all climatic regions, but species density of each dominance type varied among climatic regions. The floristic composition of the understorey vegetation was markedly different for P. mariana and P. tremuloides dominated stands.A detailed study on the effect of canopy dominance and local environmental factors on the understorey vegetation of the boreal forest was conducted using 88 stands from one of the three climatic regions. Using a combination of ordination and variation partitioning by constrained ordination we demonstrated a small but unique effect of canopy dominance type on the understorey vegetation, while a larger amount of compositional variation was shared with other factors. Our results accord with a scenario in which differences in primary environmental factors and humus form properties, the latter accentuated by the canopy dominants themselves, are the most important causes of higher species density in P. tremuloides stands than in P. mariana stands, as well as differences in species composition among the two canopy dominance types. Processes and time scales involved in the small but significant direct and indirect effects of the canopy dominant on understo‐ rey species composition are discussed.

Heterozygote advantage in the American chestnut, Castanea dentata (Fagaceae)

The American chestnut (Castanea dentata; Fagaceae) was a dominant canopy tree in the Appalachian Mountains of North America. Since the introduction of the chestnut blight fungus (Cryphonectria parasitica; Valsaceae) in America, the American chestnut has been reduced to a predominantly clonal, understory species. Our objective was to determine whether the ecological changes and absence of new recruits have influenced the population genetics of American chestnut. Leaf samples were collected from four populations in southwestern Virginia. Electrophoretic data from five polymorphic loci were used to determine the genetic diversity and population structure of the populations and subpopulations. Growth data and infection status were recorded for one of the populations to determine their relationship with heterozygosity. F statistics revealed a significant amount of differentiation among subpopulations and an excess of heterozygotes within subpopulations. Heterozygous individuals also had higher rates of vegetative growth. The superior performance and excess of heterozygotes suggests that selection favors heterozygous individuals. The prolonged absence of sexual reproduction in C. dentata has allowed subtle fitness differences to accumulate to the extent that they have had significant effects on the genetics of chestnut populations.

Understorey vegetation in boreal Picea mariana and Populus tremuloides stands in British Columbia

We compared the species composition and species density of vascular plants in the understorey vegetation of boreal forest between Picea mariana (Black spruce) and Populus tremuloides (Trembling aspen) stands in British Co- lumbia, Canada, and related differences in species composi- tion and species density between the two forest types to dominant canopy tree species as well as a wide variety of environmental factors. We analysed 231 stands, distributed in three different climatic regions representing drier, wetter, and milder variations of montane boreal climate. Of these stands 118 were dominated by P. mariana and 113 by P. tremuloides. P. tremuloides stands had higher species density than P. mariana stands in all climatic regions, but species density of each domi- nance type varied among climatic regions. The floristic compo- sition of the understorey vegetation was markedly different for P. mariana and P. tremuloides dominated stands. A detailed study on the effect of canopy dominance and local environmental factors on the understorey vegetation of the boreal forest was conducted using 88 stands from one of the three climatic regions. Using a combination of ordination and variation partitioning by constrained ordination we dem- onstrated a small but unique effect of canopy dominance type on the understorey vegetation, while a larger amount of com- positional variation was shared with other factors. Our results accord with a scenario in which differences in primary envi- ronmental factors and humus form properties, the latter accen- tuated by the canopy dominants themselves, are the most important causes of higher species density in P. tremuloides stands than in P. mariana stands, as well as differences in species composition among the two canopy dominance types. Processes and time scales involved in the small but significant direct and indirect effects of the canopy dominant on understo- rey species composition are discussed.

Post-logging acorn production and oak regeneration in a tropical montane forest, Costa Rica

We assessed the effects of controlled selective logging on stand-level acorn production and early tree recruitment in a tropical montane oak–bamboo forest in Costa Rica (2600–2800 m elevation). For this, we took advantage of a mast fruiting event during year 2000 in a population ( N=870 individuals ≥10 cm dbh) of one of the dominant canopy tree species, Quercus costaricensis. The forest was selectively logged for timber between 1991 and 1992 under two harvesting intensities (20 and 30% basal area removal of trees ≥10 cm dbh). We found that more sound acorns fell per unit area in plots subjected to the highest harvesting intensity than in adjacent, unlogged plots (180,000 and 100,000 total acorns/ha, respectively); acorn production in the lightest harvesting intensity plots was marginally higher than in unlogged plots. We suggest that this result is largely due to the presence of small (10–30 cm dbh) trees that commenced their reproduction only in the logged plots. Our results also suggest that logging may have enhanced stand-level acorn production in Q. costaricensis by augmenting crown illumination levels in retained individuals: within plot types, the crowns of fruiting individuals were better illuminated than those not in fruit; among plot types, and for a given size class, trees in the unlogged plots had their crowns less well-illuminated than those in the logged plots. Because more sound acorns fell per unit area in the more intensively-logged plots, vertebrate predators apparently became satiated and left proportionally more acorns to germinate on the forest floor with respect to unlogged plots. Seedling densities from the 2000 acorn cohort mirrored the observed differences in acorn production and acorn survival between the more intensively-logged and unlogged plots. Acorn germination and seedling establishment success, however, did not vary among these plot types. Our results point out to the potentially beneficial role of controlled logging on stand regeneration in this montane forest, being detectable even one decade after its implementation.

Development of oak plantations established for wildlife

Extensive areas that are currently in agricultural production within the Mississippi Alluvial Valley are being restored to bottomland hardwood forests. Oaks ( Quercus sp.), sown as seeds (acorns) or planted as seedlings, are the predominant trees established on most afforested sites. To compare stand development and natural invasion on sites afforested by planting seedlings or by sowing acorns, we sampled woody vegetation on ten 14- to 18-year-old oak plantations established to provide wildlife habitat. Stem densities of about 900 oaks/ha were comparable between stands established by sowing 4000 acorns/ha and stands established by planting 900 seedlings/ha. Densities of oaks in stands established from seedlings increased 38% from densities detected when these stands were 4- to 8-year-old. Densities of oaks established from field-sown acorns increased >100% during this same 10-year span. Oaks that were planted as seedlings were larger than those established from acorns, but trees resulting from either afforestation method were larger than trees naturally colonizing these sites. Natural invasion of woody species varied greatly among afforested sites, but was greater and more diverse on sites sown with acorns. Afforested stands were dominated by planted species, whereas naturally invading species were rare among dominant canopy trees. When afforestation objectives are primarily to provide wildlife habitat, we recommend, sowing acorns rather than planting seedlings. Additionally, planting fewer seeds or seedlings, diversifying the species planted, and leaving non-planted gaps will increase diversity of woody species and promote a more complex forest structure that enhances the suitability of afforested sites for wildlife.