Understory Tree Species Research Articles

Tree Ring Analysis for Growth Pattern and Age Structure of the Subtrophic Broad–leaved Forest in Taiwan

In this study, age structure, dynamics and growth pattern of subtropical forest located in central Taiwan were analyzed. The existence of annual rings in the wood of trees was proven by cross–dating and tree ring analysis. 218 individuals which belong to 27 species were investigated in the 0.71 ha plot of Shenmu evergreen broad–leaved forest. It indicates that the age of the trees correlates not strongly with the diameter and the height, while the mean diameter growth rates vary between 0.48 cm per year in understory tree species and 0.76 cm per year in emergent species. Compared with the main canopy tree species in Costa Rica’s tropical forest, the mean annual diameter growth rate 0.66 cm is greater than that of 0.42 cm. Stand dynamic of DBH and age structure fitted by Weibull function indicate that they are in medium status of forest regeneration, but the age’s shape parameter r=1.2 is much close to 1 which stands for the limit boundary of very good regeneration status. The mean age for all individuals is 61 years. According to the age, height distribution and growth rate of the trees, we classify four major types of life strategies and the growth pattern of specie cohorts. These findings lead to the assumption that the investigated stand can be classified as a very late secondary stand in transition to a mature forest. The results also reveal that it is a valuable tool for estimations of growth rate and pattern by using tree ring analysis in subtropical evergreen forest ecology.

Eighty Years of Succession in a Noncommercial Plantation on Hawai'i Island: Are Native Species Returning?

Hawai‘i’s forest ecosystems are changing rapidly due to a high level of species introductions, and it is an open question whether native species will be maintained. Several studies have explored the potential for native species to succeed in future communities dominated by introduced species in Hawai‘i, but the results have been conflicting and most of the studies have been limited to relatively young forest (<30 yr old). I surveyed a remote, 80-yr-old noncommercial plantation on Hawai‘i Island to determine whether any native tree species were able to succeed in the planted forest. I compared abundance and composition of native species in the plantation to that in a relict, native-dominated forest adjacent to the plantation and located on the same substrate type. After 80 yr, native species constituted just 4.5% of basal area and 12.1% of stem density in the plantation. However, I found that the relative success of native species varied strongly by species. Of nine native species encountered in the relict forest, six were rare or absent in the planted forest. A seventh (Metrosideros polymorpha) dominated the relict forest but was unable to recruit in the planted forest. However, two shade-tolerant understory tree species (Psychotria hawaiiensis and Psydrax odoratd) were at least as common in the plantation as in the relict forest, and the latter was significantly more abundant in the plantation. Thus, although I found no evidence that native species will dominate with continued succession, I found that at least two native species may remain important components of plantation-derived Hawaiian forests in the future.

A physical map for the Amborella trichopodagenome sheds light on the evolution of angiosperm genome structure

BackgroundRecent phylogenetic analyses have identified Amborella trichopoda, an understory tree species endemic to the forests of New Caledonia, as sister to a clade including all other known flowering plant species. The Amborella genome is a unique reference for understanding the evolution of angiosperm genomes because it can serve as an outgroup to root comparative analyses. A physical map, BAC end sequences and sample shotgun sequences provide a first view of the 870 Mbp Amborella genome.ResultsAnalysis of Amborella BAC ends sequenced from each contig suggests that the density of long terminal repeat retrotransposons is negatively correlated with that of protein coding genes. Syntenic, presumably ancestral, gene blocks were identified in comparisons of the Amborella BAC contigs and the sequenced Arabidopsis thaliana, Populus trichocarpa, Vitis vinifera and Oryza sativa genomes. Parsimony mapping of the loss of synteny corroborates previous analyses suggesting that the rate of structural change has been more rapid on lineages leading to Arabidopsis and Oryza compared with lineages leading to Populus and Vitis. The gamma paleohexiploidy event identified in the Arabidopsis, Populus and Vitis genomes is shown to have occurred after the divergence of all other known angiosperms from the lineage leading to Amborella.ConclusionsWhen placed in the context of a physical map, BAC end sequences representing just 5.4% of the Amborella genome have facilitated reconstruction of gene blocks that existed in the last common ancestor of all flowering plants. The Amborella genome is an invaluable reference for inferences concerning the ancestral angiosperm and subsequent genome evolution.

The Effects of Cropping Systems on Avian Communities in Cacao and Banana Agro-Forestry Systems of Talamanca, Costa Rica

Multispecies agro-forestry is generally lauded for providing ecosystem services, especially in tropical environments. Avian communities contribute to services such as biodiversity and pest management. Characterizing and evaluating avian community composition in similar cropping systems will help optimize management for ecosystem services. We examined the relationship between cropping system vegetation and avian communities in four shaded agro-forestry systems common to the Limprovince of Costa Rica: abandoned and managed systems of cacao, cacao with banana, and banana. During two field seasons, we detected 2605 birds from 106 species and identified 2791 trees and shrubs from 62 morphospecies. We compared vegetation and avian species richness across systems with mixed-effects linear models. Canopy, understory, and groundcover vegetation differed among agro-forestry systems. More ground- and understory-foraging forest species were detected in agro-forestry systems lacking banana, whereas richness of agricultural generalist species was highest in systems with banana. Richness of understory- and ground- foraging species correlated with understory tree species richness and leaf litter. Our results indicate that shaded cacao and banana systems can have similar canopy- foraging species richness that includes both agricultural and woodland generalist species, but that interspersing banana with cacao can adversely influence understory forest bird community composition. Agro-forests with diverse understory vegetation support more understory-foraging bird species that have proven valuable in pest management. Abstract in Spanish is available at http: //www.blackwell-synergy.com/loi/btp.

Changes in woody vegetation abundance and diversity after natural disturbances causing different levels of mortality

Questions: How does woody vegetation abundance and diversity differ after natural disturbances causing different levels of mortality? Location: Abies balsamea–Betula papyrifera boreal mixed-wood stands of southeast Quebec, Canada. Methods: Woody vegetation abundance and diversity were quantified and compared among three disturbance-caused mortality classes, canopy gap, moderate-severity disturbances, and catastrophic fire, using redundancy analysis, a constrained linear ordination technique, and diversity indices. Results: Substantial changes in canopy tree species abundance and diversity only occurred after catastrophic fire. Shade-tolerant, late-successional conifer species remained dominant after canopy gap and moderate-severity disturbances, whereas shade-intolerant, early-successional colonizers dominated canopy tree regeneration after catastrophic fire. Density and diversity of mid-tolerant and shade-intolerant understory tree and shrub species increased as the impact of disturbance increased. Highest species richness estimates were observed after catastrophic fire, with several species establishing exclusively under these conditions. Relative abundance of canopy tree regeneration was most similar after canopy gap and moderate-severity disturbances. For the sub-canopy tree and shrub community, relative species abundances were most similar after moderate-severity disturbances and catastrophic fire. Vegetation responses to moderate-severity disturbances thus had commonalities with both extremes of the disturbance-caused mortality gradient, but for different regeneration layers. Conclusions: Current spatio-temporal parameters of natural disturbances causing varying degrees of mortality promote the development of a complex, multi-cohort forest condition throughout the landscape. The projected increase in time intervals between catastrophic fires may lead to reduced diversity within the system.

Can the nutrient dynamics of a northern hardwood forest explain variation in its trophic structure?

Abstract Background/Question/Methods: A time series of Lepidoptera larvae (caterpillar) abundance and biomass in the northern hardwood forests of central New Hampshire, USA, shows large interannual fluctuations in aggregated caterpillar biomass (52-fold variation in mg caterpillars/ 8000 leaves) that were spatially correlated across a regional landscape. Primary production of these mature forests has not varied over this period, and many hundreds of Lepidoptera species share comparatively few host plants. Therefore, interannual changes in nutritional quality of even one host plant species may be capable of generating correlated population dynamics for many species in the Lepidoptera community. We tested the hypothesis that variation in soil nutrient availability influences foliar chemistry and ultimately produces a bottom-up force capable influencing the aggregated biomass of the forest's dominant herbivores. Results/Conclusions: Over a period of four years, measures of nutritional quality for herbivores in three of the forest's four dominant understory tree species (Acer saccharum, Acer pensylvanicum, and Viburnum alnifolium) displayed considerable variation that was consistent across the landscape. Foliar nitrogen (N) content and two metrics of secondary metabolite concentration—total phenolics and condensed tannin—all varied by up to 20% among years. Resin bag extracts of NH4+, NO2- /NO3- and PO4+ showed soil mineral nutrient availability displayed six-fold variation that was consistent with the pattern in foliar N, and a whole tree in situ fertilization experimental manipulation produced rapid effects on foliar chemistry. Finally, variation in the forewing lengths of free-flying moths showed an interannual concordance among species within the group of early season feeders that was also consistent with patterns in foliar quality—suggesting collections of species experience correlated variations in larval growth performance.

Can the nutrient dynamics of a northern hardwood forest explain variation in its trophic structure?

Background/Question/Methods:A time series of Lepidoptera larvae (caterpillar) abundance and biomass in the northern hardwood forests of central New Hampshire, USA, shows large interannual fluctuations in aggregated caterpillar biomass (52-fold variation in mg caterpillars/ 8000 leaves) that were spatially correlated across a regional landscape. Primary production of these mature forests has not varied over this period, and many hundreds of Lepidoptera species share comparatively few host plants. Therefore, interannual changes in nutritional quality of even one host plant species may be capable of generating correlated population dynamics for many species in the Lepidoptera community. We tested the hypothesis that variation in soil nutrient availability influences foliar chemistry and ultimately produces a bottom-up force capable influencing the aggregated biomass of the forest's dominant herbivores.Results/Conclusions:Over a period of four years, measures of nutritional quality for herbivores in three of the forest's four dominant understory tree species (Acer saccharum, Acer pensylvanicum, and Viburnum alnifolium) displayed considerable variation that was consistent across the landscape. Foliar nitrogen (N) content and two metrics of secondary metabolite concentration—total phenolics and condensed tannin—all varied by up to 20% among years. Resin bag extracts of NH4+, NO2- /NO3- and PO4+ showed soil mineral nutrient availability displayed six-fold variation that was consistent with the pattern in foliar N, and a whole tree in situ fertilization experimental manipulation produced rapid effects on foliar chemistry. Finally, variation in the forewing lengths of free-flying moths showed an interannual concordance among species within the group of early season feeders that was also consistent with patterns in foliar quality—suggesting collections of species experience correlated variations in larval growth performance.

Large-scale synchrony of gap dynamics and the distribution of understory tree species in maple–beech forests

Large-scale synchronous variations in community dynamics are well documented for a vast array of organisms, but are considerably less understood for forest trees. Because of temporal variations in canopy gap dynamics, forest communities-even old-growth ones-are never at equilibrium at the stand scale. This paucity of equilibrium may also be true at the regional scale. Our objectives were to determine (1) if nonequilibrium dynamics caused by temporal variations in the formation of canopy gaps are regionally synchronized, and (2) if spatiotemporal variations in canopy gap formation affect the relative abundance of tree species in the understory. We examined these questions by analyzing variations in the suppression and release history of Acer saccharum Marsh. and Fagus grandifolia Ehrh. from 481 growth series of understory saplings taken from 34 mature stands. We observed that (1) the proportion of stems in release as a function of time exhibited a U-shaped pattern over the last 35 years, with the lowest levels occurring during 1975-1985, and that (2) the response to this in terms of species composition was that A. saccharum became more abundant at sites that had the highest proportion of stems in release during 1975-1985. We concluded that the understory dynamics, typically thought of as a stand-scale process, may be regionally synchronized.

Selective logging of lowland evergreen rainforests in Chiloé Island, Chile: Effects of changing tree species composition on soil nitrogen transformations

Lowland evergreen rainforests in southern Chile growing on highly productive soils and accessible sites have been subjected to traditional and industrial logging of valuable timber trees. Old-growth rain forests in this area are characterized by highly conservative N cycles, which results in an efficient N use of ecosystems. We hypothesize that different logging practices, by changing forest structure and species composition, can alter the quantity and quality (i.e. C/N ratio) of litterfall and soil organic matter and soil microbial processes that determine N storage and availability. To test this hypothesis we investigated chemical properties, microbial N transformations, N fluxes and N storage in soils of lowland evergreen rainforests of Chiloé Island after 10 years since industrial selective logging (ISL) and in stands subjected to traditional selective logging (TSL) by landowners in small properties. We compared them to reference unlogged old-growth stands (OG) in the same area. Tree basal area was more reduced in the stands subjected to ISL than to TSL. Litterfall inputs were similar in both logging treatments as in OG stands. This was due to greater biomass of understory species after logging. In TSL understory tree species determined a higher litterfall C/N ratio than ISL. We found higher soil N availability and content of base cations in surface soils of logged forests than in OG. The litter horizon of OG forest had significantly higher rates of non-symbiotic N fixation than logged forests. In the ISL treatment there was a trend toward increasing soil denitrification and significantly higher NO 3–N/N t ratio in spring waters, which led to a stronger δ 15N signal in surface and deep soils. We conclude that massive understory occupation by the shade-intolerant native bamboo Chusquea quila in ISL led to enhanced litter quality (lower C/N ratios) relaxing the tightness of the N cycle, which increased soil N availability leading to a higher proportion of nitrate in spring waters and higher gaseous N losses. In contrast, under TSL a higher litterfall C/N ratio slowed decomposition and net N mineralization rates thus reducing the chances for N losses, and enhancing C and N storage in soil. We suggest that sustainable logging practices in these rain forests should be based on lower rates of canopy removal to enhance colonization of the understory by shade-tolerant trees, which are associated with a more efficient N cycle.

Seasonal variation in the maximum rate of leaf gas exchange of canopy and understory tree species in an Amazonian semi-deciduous forest

Leaf gas exchange, water potential, and specific leaf area of two tropical semi-deciduous tree species, Brosimum lactescens S. Moore and Tovomita schomburgkii Planch & Triana, were quantified to establish how these properties were affected by seasonal variations in rainfall and leaf canopy position. The study was conducted at a site near Sinop Mato Grosso, Brazil, which is located within the ecotone of savanna and tropical rain forest. Both species exhibited significant declines in leaf water potential (ΨL), specific leaf area, area- and mass-based light saturated photosynthesis and dark respiration, and maximum stomatal conductance during the dry-season, suggesting that leaf structural properties and gas exchange are significantly altered by drought that develops during the 4-month dry season. Internal leaf CO2 concentrations (Ci) were consistently lower during the dry season suggesting that the decline in maximum photosynthesis was due in part to a decline in stomatal conductance. However, seasonal variations in leaf gas exchange were larger for upper-canopy leaves, indicating an important interaction between drought stress and canopy position. The seasonal variation in leaf gas exchange and morphology was presumably due to a combination of drought stress and leaf lifespan. The results of this study suggest that drought has important implications for the leaf physiology and morphology of semi-deciduous Amazonian forest trees.

Delayed tree mortality in the Atchafalaya Basin of southern Louisiana following Hurricane Andrew

Hurricanes can damage trees in forested wetlands, and the potential for mortality related to these storms exists due to the effects of tree damage over time. In August 1992, Hurricane Andrew passed through the forested wetlands of southern Louisiana with winds in excess of 225 kph. Although more than 78% of the basal area was destroyed in some areas, most trees greater than 2.5 cm dbh were alive and resprouting prolifically the following year (98.8%). Survival of most tree species was similarly high two years after the hurricane, but mortality rates of some species increased dramatically. For example, Populus heterophylla (swamp cottonwood) mortality increased from 7.8 to 59.2% (n = 76) and Salix interior (sandbar willow) mortality increased from 4.5 to 57.1% (n = 21). Stem sprouts on many up-rooted hardwood trees of other species were still alive in 1998, 6 years after the hurricane. Due to the understory tree species composition, regeneration, and high levels of resprouting, there was little change in species composition or perhaps a slight shift toward more shade and flood tolerant species six years following the hurricane event. Triadica sebifera (Chinese tallow) was found on some of the sites heavily disturbed by Hurricane Andrew, and may proliferate at the expense of native tree species.

Understory structure in a 23-year-old Acacia koa forest and 2-year growth responses to silvicultural treatments

Restoration of degraded Acacia koa forests in Hawaii often involves mechanical scarification to stimulate germination of seed buried in the soil and to suppress vegetation that competes with shade intolerant A. koa. Resulting even-age stands are gradually colonized by other plant species, but understory structure is poorly quantified, and the effects of management on understory vegetation are unknown. We examined the structure of the dominant understory species 23 years after stand initiation and determined their subsequent 2-year growth responses to silvicultural treatments prescribed to improve growth of koa. Release thinning, chemical control of introduced grasses, and phosphorus fertilization were applied in a split-plot experimental design. Results for DBH and height class distributions indicated that recruitment for most tree species began within a few years of stand initiation. By 23 years understory trees made up only 14% of total stand basal area, but they added greatly to native plant biodiversity. Alien grasses covered 92% of the forest floor and averaged 1.35 Mg ha −1. Of the four most abundant native understory tree species, only shade intolerant Myoporum sandwicense grew faster in response to the treatments. P-fertilizer combined with grass control significantly increased production of the native shrub, Rubus hawaiiensis. Notably, alien grass biomass did not increase in response to thinning, and actually declined in P-fertilized plots. Only, where P-fertilizer was applied were P concentrations of understory leaves elevated. Foliar concentrations of other nutrients were generally unaffected by treatments. Our findings suggest that the conservative silvicultural treatments we used can be applied without adversely impacting the capacity of aggrading A. koa forests to support a diversity of native understory plants. Lack of an increase in alien grass biomass, and in the case of P fertilization, a reduction in grass biomass, indicates that treatments should not increase competition with native species.

Photosynthetic responses of forest understory tree species to long-term exposure to elevated carbon dioxide concentration at the Duke Forest FACE experiment

We examined the photosynthetic responses of four species of saplings growing in the understory of the Duke Forest FACE experiment during the seventh year of exposure to elevated CO2 concentration ([CO2]). Saplings of these same species were measured in the first year of the Duke Forest FACE experiment and at that time showed only seasonal fluctuations in acclimation of photosynthesis to elevated [CO2]. Based on observations from the Duke Forest FACE experiment, we hypothesized that after seven years of exposure to elevated [CO2] significant photosynthetic down-regulation would be observed in these tree species. To test our hypothesis, photosynthetic CO2-response and light-response curves, along with chlorophyll fluorescence, chlorophyll concentration and foliar N were measured twice during the summer of 2003. Exposure to elevated [CO2] continued to increase photosynthesis in all species measured after seven years of treatment with the greatest photosynthetic increase observed near saturating irradiances. In all species, elevated [CO2] increased electron transport efficiency but did not significantly alter carboxylation efficiency. Quantum yield estimated by light curves, chlorophyll concentration, and foliar nitrogen concentrations were unaffected by elevated [CO2]. Contrary to our hypothesis, there is little evidence of progressive N limitation of leaf-level processes in these understory tree species after seven years of exposure to elevated [CO2] in the Duke Forest FACE experiment.

The Influence of Flowering Dogwood (Cornus florida) on Land Snail Diversity in a Southern Mixed Hardwood Forest

In many areas, soil calcium is an important limiting factor for land snail distributions. Some tree species are able to extract calcium from lower soil depths, concentrate it in their leaves and deliver it to the surface upon leaf fall. Flowering dogwood (Cornus florida) is an example of a “calcium-pumping” species and has been suggested to promote land snail diversity in woodland ecosystems. In this study, leaf tissue calcium, O- and B-horizon soil calcium and land snail diversity were compared for three common understory tree species: flowering dogwood, American beech (Fagus grandifolia) and blackgum (Nyssa sylvatica). Using a litter sieving technique, land snails were sampled underneath these three focal tree species. Land snail density, species richness and diversity were estimated for a total of 19 trees. As expected, B-horizon soil calcium was similar across all tree species, while leaf tissue calcium and O-horizon soil calcium were significantly higher for flowering dogwood. Land snail density, species richness and diversity (Shannon-Wiener Index) were also significantly higher for flowering dogwood. Calcium-pumping tree species are important ecosystem components in that they increase soil fertility and promote invertebrate diversity. Land snails are an important food and calcium source for a variety of woodland species including passerine birds, woodland salamanders, cychrine beetles, and wild turkeys. Flowering dogwood is currently seriously threatened in the southeastern United States by the parasitic dogwood anthracnose fungus (Discula destructiva). In light of the findings of this study, the implications of dogwood anthracnose may reach beyond the target species and impact multiple taxa.

Seedling survival in a northern temperate forest understory is increased by elevated atmospheric carbon dioxide and atmospheric nitrogen deposition

AbstractWe tested the main and interactive effects of elevated carbon dioxide concentration ([CO2]), nitrogen (N), and light availability on leaf photosynthesis, and plant growth and survival in understory seedlings grown in an N‐limited northern hardwood forest. For two growing seasons, we exposed six species of tree seedlings (Betula papyrifera, Populus tremuloides, Acer saccharum, Fagus grandifolia, Pinus strobus, and Prunus serotina) to a factorial combination of atmospheric CO2 (ambient, and elevated CO2 at 658 μmol CO2 mol−1) and N deposition (ambient and ambient +30 kg N ha−1 yr−1) in open‐top chambers placed in an understory light gradient. Elevated CO2 exposure significantly increased apparent quantum efficiency of electron transport by 41% (P&lt;0.0001), light‐limited photosynthesis by 47% (P&lt;0.0001), and light‐saturated photosynthesis by 60% (P&lt;0.003) compared with seedlings grown in ambient [CO2]. Experimental N deposition significantly increased light‐limited photosynthesis as light availability increased (P&lt;0.037). Species differed in the magnitude of light‐saturated photosynthetic response to elevated N and light treatments (P&lt;0.016). Elevated CO2 exposure and high N availability did not affect seedling growth; however, growth increased slightly with light availability (R2=0.26, P&lt;0.0001). Experimental N deposition significantly increased average survival of all species by 48% (P&lt;0.012). However, seedling survival was greatest (85%) under conditions of both high [CO2] and N deposition (P&lt;0.009). Path analysis determined that the greatest predictor for seedling survival in the understory was total biomass (R2=0.39, P&lt;0.001), and that carboxylation capacity (Vcmax) was a better predictor for seedling growth and survival than maximum photosynthetic rate (Amax). Our results suggest that increasing [CO2] and N deposition from fossil fuel combustion could alter understory tree species recruitment dynamics through changes in seedling survival, and this has the potential to alter future forest species composition.

Dogwood Anthracnose in Eastern Hardwood Forests: What Is Known and What Can Be Done?

Abstract Flowering dogwood (Cornus florida L.), one of the most common understory tree species in eastern North America, currently is threatened throughout its native range by a fungus that causes dogwood anthracnose. This aggressive disease rapidly kills dogwood trees and mortality hasexceeded 90% in some forest types. The health and ecological integrity of forest ecosystems throughout the eastern United States are threatened by the rapid decline of dogwood populations. The severity and rate of infection are reported to vary with several environmental factors. Within individualstands, disease severity increases with decreased light availability and increased moisture. Management techniques to control anthracnose have received little attention; however, recent findings suggest that prescribed burning may offer an effective management tool for controlling anthracnose.

Biomass and nutrient distribution of pinus koraiensis seedlings invading a mixed forest dominated by quercus mongolica

This study was conducted to estimate the biomass and nutrient distribution in understory P. koraiensis seedlings invading a mixed forest dominated by Quercus mongolica in central Korea. Total above and belowground biomass for the three overstory and understory dominant tree species were 2.70 t/ha for P. koraiensis, 210.56 t/ha for Q. mongolica, and 28.39 t/ha for Q. variabilis. P. koraiensis seedlings seemed to allocate more biomass to foliage (31.0% of total biomass) than overstory Quercus spp. (0.9%), so as to exploit the low light flux found in the shady conditions. Furthermore, the high specific leaf area of P. koraiensis seedlings indicated morphological characteristics of shade leaves in low light conditions. Total nitrogen (N) and phosphorus (P) contents for P. koraiensis seedlings and Quercus spp. were 19.6 and 1.9 kg/ha, and 94.7–626.5 and 9.4–137.6 kg/ha, respectively. Our results suggested that understory P. koraiensis seedlings exerted only a minor influence on the biomass and nutrient distribution for the study forest.

Variation in the herb species response and the humus quality across a 200‐year chronosequence of beech and oak plantations in Belgium

The present study aimed at exploring the long‐term impact of pure and mixed beech Fagus sylvatica and oak Quercus robur stands on the forest floor by documenting changes in the herb species’ behaviour and in humus index across a 200‐yr chronosequence of forest stands. The research was conducted in central Belgium, in a 4383 ha beech‐dominated forest. Analyses were carried out in stands which are replicated, of the same age, managed in the same way, and growing on the same soil type with the same land‐use history. The results of this study indicate that stand aging is an important determinant of herb species occurrence in the studied area. Most of the species studied show a different response to stand age in pure compared to mixed stands. Our results clearly show a decrease of the humus quality with age in pure stands (beech as well as oak). On the other hand, we found that mixing beech and oak maintained or improved the humus status along the chronosequence according to the proportion of each tree. So the addition of some oak to the beech made it possible to keep a constant quality of the humus. We found that, even if the understory tree species is very scarce, it may be sufficient to maintain the humus status on the long term. In the present study, a cover of 1% oak in a beech stand was sufficient to show an effect of the minor species on these soils. This pattern contrasts with the widespread idea that substantial effects of the minor tree species on soils might not develop if the ratio of major/minor species is low.

Gypsy Moth Defoliation of Oak Trees and a Positive Response of Red Maple and Black Cherry: An Example of Indirect Interaction

We calculated relative growth rates of all species over 10 cm circumference at breast height from 1997–1999, 1999–2001 and 2001–2003 in a 70 by 100 m plot in an oak forest in southeastern Michigan. Red maple and black cherry represented most of the understory species while white and black oaks dominated the canopy. During the summer of 1999, outbreak levels of gypsy moths preferentially defoliated oaks on the plot, often resulting in 100% defoliation. The relative growth rates (RGR) of red maple and black cherry were significantly higher during the time period where gypsy moths were at outbreak levels. Oaks had lower RGR during the gypsy moth attack than in the subsequent time period. Consequently, the preference gypsy moths exhibited towards oak species indirectly resulted in a higher growth rate for understory tree species, possibly due to increased light availability and the decreased competitive capacity of the defoliated oaks. This indirect effect could accelerate the succession at which oak dominated communities become red maple forests.

Facilitating natural regeneration in Saccharum spontaneum (L.) grasslands within the Panama Canal Watershed: effects of tree species and tree structure on vegetation recruitment patterns

To counteract the escalating rates of tropical deforestation, it is essential that we not only minimize forest loss, but that we create effective reforestation strategies. This study investigates understory recruitment patterns in mixed native species plantations along the Panama Canal that were established in grasslands dominated by the invasive exotic species Saccharum spontaneum (L.) Graminae. We test the hypothesis that regeneration rates vary significantly by overstory tree species and overstory tree structure, and explore the mechanisms generating such patterns. Of the seven tree species sampled, Inga spp. recruits significantly more tree seedlings than any other species. Additionally, crown foliage density appears to be the most significant structural factor determining rates of understory tree species regeneration. A survey of bird activity in the plantations and in unplanted areas indicates that birds generally visit large trees and that those tree species most frequently visited by birds also have the greatest density of understory tree seedlings. These results support the hypothesis that tree structure significantly affects regeneration patterns, and suggests that bird dispersal may be a fundamental driver in seedling recruitment. Furthermore, results indicate that the presence of any tree species in a reforestation plot increases the understory species richness and species cover relative to non-reforested areas, while significantly reducing the degree of S. spontaneum dominance. While active reforestation appears to facilitate forest regeneration in areas occupied in S. spontaneum, tree species and tree structure are important factors to consider when designing reforestation programs that are intended to facilitate natural regeneration.