Forest tree growth response to hydroclimate variability in the southern Appalachians

Climate change will affect tree species growth and distribution; however, under the same climatic conditions species may differ in their response according to site conditions. We evaluated the climate‐driven patterns of growth for six dominant deciduous tree species in the southern Appalachians. We categorized species into two functional groups based on their stomatal regulation and xylem architecture: isohydric, diffuse porous and anisohydric, ring porous. We hypothesized that within the same climatic regime: (i) species‐specific differences in growth will be conditional on topographically mediated soil moisture availability; (ii) in extreme drought years, functional groups will have markedly different growth responses; and (iii) multiple hydroclimate variables will have direct and indirect effects on growth for each functional group. We used standardized tree‐ring chronologies to examine growth of diffuse‐porous (Acer, Liriodendron, and Betula) and ring‐porous (Quercus) species vs. on‐site climatic data from 1935 to 2003. Quercus species growing on upslope sites had higher basal area increment (BAI) than Quercus species growing on mesic, cove sites; whereas, Acer and Liriodendron had lower BAI on upslope compared to cove sites. Diffuse‐porous species were more sensitive to climate than ring porous, especially during extreme drought years. Across functional groups, radial growth was more sensitive to precipitation distribution, such as small storms and dry spell length (DSL), rather than the total amount of precipitation. Based on structural equation modeling, diffuse‐porous species on upslope sites were the most sensitive to multiple hydroclimate variables (r² = 0.46), while ring‐porous species on upslope sites were the least sensitive (r² = 0.32). Spring precipitation, vapor pressure deficit, and summer storms had direct effects on summer AET/P, and summer AET/P, growing season small storms and DSL partially explained growth. Decreasing numbers of small storms and extending the days between rainfall events will result in significant growth reduction, even in regions with relatively high total annual rainfall.     

Demography and matrix analysis on a natural Pterocarya rhoifolia population developed along a mountain stream

The population structures and dynamics of a dominant riparian canopy species, Pterocarya rhoifolia (Juglandaceae), were analyzed based on the census data collected for the 7 years from 1989 through 1996. A study plot was established over an environmental gradient that included the lower hill-slope, river terrace, and low-level floodplain of a riparian area within a 2.8-ha cool-temperate forest stand. Spatial analyses of the demographic data showed that there were significant differences in recruitment, growth, and death processes among the three habitats. The slope and terrace habitats were the severest for the pre-reproductive stage and recruitment process, respectively. A topographically combined projection matrix was constructed for the life-history processes of the three subpopulations, which clearly revealed that the floodplain and terrace subpopulations constituted the mainland source populations, whereas the slope subpopulation was an island sink population. The whole Pterocarya population linked by seed flow showed an increase in population size (λ=1.052). The elasticity analysis showed that the sum of the elasticity values was zero, 0.14, and 0.85 in the slope, terrace and floodplain subpopulations, respectively. This fact clearly indicates that the role of the floodplain subpopulation as a source population is six times as large as that of the terrace subpopulation. Electronic Publication     

淮北相山三种群落中优势树种次生木质部的解剖学特征

对淮北相山侧柏、构树混交林6个优势树种次生木质部的观察表明,其结构表现出一定的旱生特征：高的复孔率和导管分布频率,窄导管,木纤维短,射线低。利用相对输导率和脆性指数对导管水分输导的有效性和安全性进行了评估,结果表明6个优势树种的相对输导率依次为：牡荆〉酸枣〉扁担木〉构树〉柘树〉小叶鼠李,脆性指数依次为：构树〉牡荆〉扁担木〉酸枣〉小叶鼠李〉柘树,植物水分输导的有效性和安全性与其在相山的自然分布相一致。作为广布优势树种,牡荆、酸枣和扁担木的次生木质部在导管分子长度、单孔率、导管频率、相对输导率、脆性指数、多列射线高度、多列射线宽度和单列射线高度等性状上均表现出一定的可塑性。与混交林和人工侧柏林相比,灌丛中植物导管分子短,单孔率低,导管频率大,射线低,相对输导率大,脆性指数小,更倾向于旱生特点。逐步多重回归分析表明,随着郁闭度的增加,导管分子长度和单孔率增加。随着风速的增加,导管频率增加。随着土壤含水量的增加,多列射线变宽。     

Secondary Xylem Anatomic Characteristics of Dominant Plant Species in Three Communities in Xiangshan Mountain,Huaibei , China

The results of anatomic observations on secondary xylem of 6 dominant species in a Platycladus orientalis + Broussonetia papyrifera mixed forest in Xiangshan mountain, Huaibei, China, indicated that these species share some xeromorphic features in secondary xylem such as high vessel frequency, less percentage of single porous, thinner vessel, shorter fibers, and lower rays. The availability and safety of the vessel associated with water conduction was evaluated through relative conductivity and the vulnerability index, the result shown that relative conductivity of the 6 dominant species is ordered as Vitex negundo var. cannabifolia, Ziziphus jujuba var. spinosa, Grewia biloba var. parviflora, Broussonetia papyrifera, Cudrania tricuspuidata, Rhamnus parvifolia , while the vulnerability index is ordered as B. papyrifera, V. negundo var. cannabifolia, G. biloba var. parviflora, Z. jujuba var. spinosa, C. tricuspuidata, R. parvifolia, and the availability and safety of water conduction of these plants is coincided with their distribution in Xiangshan mountain. The three widely distributed dominant species, Z. jujuba var. spinosa, V. negundo var. cannabifolia and G. biloba var. parviflora, show great plasticity in some characters, including vessel element length, vessel frequency, relative conductivity, vulnerability index, percentage of single porous, multiserate ray height, multiserate ray width, and single ray height, etc. Compared to those in mixed forest and P. orientalis forest, the species in shrub tend to have shorter vessel elements, higher vessel frequency, less single porous percentage, lower rays, higher relative conductivity, and less vulnerabilityindex, thus indicated that they are inclined to more xeromorphic anatomically. The result from the stepwise multiple regressions show that both of the vessel element length and percentage of single porous increased with the canopy density, and the vessel frequency increased with the wind velocity, multiserate ray width increased with the soil water content.     

On the Conductivity of the Xylem in Trees

The xylem of three tree species, the ring porous Fraxinus excelsiorL., and the diffuse porous Acer pseudoplatanus L., and Salixatrocinerea Brot., has been examined to determine how closelyflow through it obeys the Poiseuille equation. The equationwas most closly obeyed in the relationship between flow-rateand pressure difference between the ends of the xylem cylinder,though there were differences between the ring and diffuse poroustypes. The relationship of flow-rate and specific conductivityto the radius of the xylem cylinder was complex.Fraxinus showeda rise with increase of radius up to a given size (0.54cm),followed by a steep decline. This decline was shown to be causedby a progressive increase in the number of vessels which wereunable to conduct water. Small increases in the viscosity ofthe xylem liquid produced a disproportionately large fall inthe rate of flow, and the specific conductivity of the xylemdecreased. With viscosities greater than 2.5 centipoise, thespecific conductivity remained constant. An effect of alternatelyraising and lowering the hydrostatic pressure of the xylem wateron the specific conductivity of the wood is described. These results are discussed in relation to the structure ofthe xylem, and particularly with regard to the possibility thatthe conduction channels in the wood consist of a system of microcapillariessurrounding the vessel cavities.     

Wood structure and function change with maturity: Age of the vascular cambium is associated with xylem changes in current‐year growth

Xylem vessel structure changes as trees grow and mature. Age‐ and development‐related changes in xylem structure are likely related to changes in hydraulic function. We examined whether hydraulic function, including hydraulic conductivity and vulnerability to water‐stress‐induced xylem embolism, changed over the course of cambial development in the stems of 17 tree species. We compared current‐year growth of young (1–4 years), intermediate (2–7 years), and older (3–10 years) stems occurring in series along branches. Diffuse and ring porous species were examined, but nearly all species produced only diffuse porous xylem in the distal branches that were examined irrespective of their mature xylem porosity type. Vessel diameter and length increased with cambial age. Xylem became both more conductive and more cavitation resistant with cambial age. Ring porous species had longer and wider vessels and xylem that had higher conductivity and was more vulnerable to cavitation; however, these differences between porosity types were not present in young stem samples. Understanding plant hydraulic function and architecture requires the sampling of multiple‐aged tissues because plants may vary considerably in their xylem structural and functional traits throughout the plant body, even over relatively short distances and closely aged tissues.     

Species shift drives decomposition rates following invasion by hemlock woolly adelgid

Insect and disease outbreak is an important cause of selective species removal and accompanying functional change in North American forests. Outbreak of hemlock woolly adelgid, Adelgies tsugae– HWA, is causing selective removal of eastern hemlock Tsuga canadensis at a regional scale. Impacts of outbreak‐caused canopy mortality and shifts in dominant species on litter decay were compared across sites that range in HWA‐caused canopy damage and subsequent canopy dominance by black birch Betula lenta. Senescent litter from eastern hemlock, black birch, and equal litter mixes were decomposed in the field for 36 months within nine sites in Connecticut and Massachusetts USA. Mass loss and % N accumulation of black birch was 65% and 52% greater compared to eastern hemlock. In contrast, outbreak related canopy damage increased litter mass loss by 11.5% in high mortality stands relative to uninfested stands but canopy damage had no impact on % N dynamics. Non‐additive effects of litter mixing influenced chemical dynamics of decaying litter; black birch accumulated less N and eastern hemlock accumulated more N compared to each species decaying alone. However, these changes offset and mixed litter bags overall showed no differences in N dynamics compared to values from each species decaying alone. In eastern hemlock stands invaded by hemlock woolly adelgid, canopy damage influences the rates and dynamics of decay but species differences between hemlock and black birch leaf litter are the dominant mechanisms of decomposition changes and a long‐lasting driver of increased N cycling rates. Species shifts may be the dominant driver of altered ecosystem processes for other insect outbreaks, particularly when replacement species have very different characteristics regulating decomposition and N cycling.     

Relationship between vegetation,hydrology and fluvial landforms on an unregulated sand‐bed stream in the Hunter Valley,Australia

A survey of fluvial landforms was conducted at Widden Brook, an unregulated sand‐bed stream in the Hunter Valley, New South Wales (NSW), Australia, to investigate the physical factors associated with vegetation pattern in Riverine Oak Forest. Groundwater depth and chemistry (pH, dissolved oxygen and electrical conductivity) were measured using piezometers and submersible data loggers on three fluvial landforms (i.e. toe of bank, top of bank and floodplain) along five transects. Floristic composition, canopy cover, bare ground and leaf litter were assessed within 45 quadrats on the three landforms along the five transects. Elevation above the bed and flood return period were determined by cross‐sectional survey and flood frequency analysis, while flow duration was determined from the gauge record. Canonical correspondence analysis demonstrated that vegetation composition was associated with average watertable depth and flood variables to a similar extent. The relative importance of these factors would be expected to vary with flood‐ and drought‐dominated climatic periods on a scale of several decades. Floristic composition was moderately associated with the canopy cover of the dominant woody species, Casuarina cunninghamiana (Miq.), but weakly correlated with bare ground and groundwater chemistry. Suites of species were associated with particular fluvial landforms and their corresponding flood and watertable conditions. The reach examined has characteristics similar to both the semi‐arid and mesic riparian ecosystems of the USA. The coarse sediments, high flood variability, short flood duration and dominance by a pioneer tree that relies on groundwater are similar to riparian ecosystems in the western USA, while the relatively broad floodplain and the development of a forest canopy that is associated with the distribution of understorey plants are similar to the mesic riparian systems in the eastern USA.     

Early post-hurricane stand development in Fringe mangrove forests of contrasting productivity

We examined the immediate effects of a hurricane (Hurricane Andrew, August 1992) in a coastal landscape in sub-tropical Florida, and then monitored stand recovery in Fringe mangrove sites of different productive capacity for 9 years after the disturbance. Structural impacts of the hurricane were confined almost entirely to forests within 200–300 m of the coast. Mortality and damage were concentrated on canopy individuals. Following the hurricane, rapid canopy recovery and the early onset of competition among Fringe forest stems, as evidenced by relatively high mortality of smaller individuals, magnified the initial dominance of hurricane survivors and early-established seedlings over later cohorts, and limited recruitment to the brief period prior to canopy closure. Changes in the relative abundance of the two dominant mangrove species following disturbance varied strongly along the productivity gradient. The shade-tolerant Rhizophora mangle L. generally became the overwhelming canopy dominant in the competitive environment of the recovering Coastal Fringe forest following hurricane, but the shade-intolerant Laguncularia racemosa (L.) C.F. Gaertn was better represented in less productive Interior Fringe sites, where canopy closure was delayed. Site productivity is an important determinant of the success of mangrove species during post-hurricane stand development, and consequently of the zonation of communities in the coastal landscape.     

Peculiarities of the biotope distribution of click beetle larvae (Coleoptera, Elateridae) in the Irtysh River valley

The spatial distribution of Elateridae larvae in the Lower Irtysh valley has been analyzed. The larvae analyzed belonged to 19 Elateridae species inhabiting soils of the floodplain and the river terrace. It has been pointed out that the larvae of Paraphotistus nigricornis, Agriotes lineatus, Limonius parallelus, and Aplotarsus incanus are most tolerant to the regular spring flooding. It has been found that the species compositions of floodplains and terrace meadows are significantly different. As a rule, species with a wide tropical spectrum inhabit floodplain soils, including bush and forest habitats. At the same time, the terrace was dominated by species characteristic of open habitats, including the steppe zone. It has been revealed that the values of species diversity vary significantly in the floodplain and terrace plots, but the higher species richness (Menhinick index) was registered in the floodplain.     

Population biology of Aesculus turbinata Blume: A demographic analysis using transition matrices on a natural population along a riparian environmental gradient

Population structure and dynamics of a riparian canopy species, Aesculus turbinata (Japanese horse chestnut), were analyzed based on the census data collected for the 8 years from 1989 to 1996 in temperate deciduous forests in Ashiu, Kyoto Prefecture, Japan. The censuses were conducted in three permanently established study plots over an environmental gradient that included the lower hill slope, river terrace, and floodplain of a riparian area within a forest stand of approximately 3 ha. Transition matrix based on the data from 1989 to 1996 was provided for the total population made by pooling population data from all subpopulations in three different habitats (i.e. slope, terrace, floodplain). The total Aesculus population showed positive population growth (λ = 1.0298). From the elasticity analysis, larger elasticity values were obtained with increasing size- or stage-classes. A combined transition matrix was also constructed for the life-history processes consisting of three subpopulations developed on an environmental gradient. This whole population linked by seed flow showed an increase in population size (λ = 1.0286). The elasticity matrix showed the relative importance of the slope subpopulation, suggesting its significant role as a mainland source population. Log-linear analyses were carried out to examine spatiotemporal variations of life-history parameters; significant effects of stage and plot were recognized, while no effect of year was detected on any life-history parameters except for fecundity.     

Growth of Valley Oak (<Emphasis Type="Italic">Quercus Lobata Nee</Emphasis>) in Four Floodplain Environments in the Central Valley of California

Although there have been numerous studies of California upland oak regeneration and growth there has been no research investigating oak sapling growth in riparian environments. This study looks at the growth response of young valley oak (Quercus lobata Nee), a dominant late successional riparian species in California, across environmental variation on a floodplain. Growth was measured over the course of three years at four different sites. The sites were chosen to represent the range of successional stages and surface age. Growth was significantly higher on younger, unforested sites. There was no difference in growth rate in the two forested sites (early successional cottonwood willow forest and late successional mature oak forest). Herbivory was highest in the cottonwood willow forest, where density of young oaks was also highest. The impact of flooding was measured on the youngest floodplain surface, an open floodplain restoration site where acorns were planted the year our study began. There was a significant negative impact of flooding on sapling growth in all but the first year of growth. Taken together these results suggest that existing forest trees and flooding both inhibit the growth of valley oaks on the floodplain, and that restoration in open sites may be more successful than restoration under an existing canopy.     

Environmental factors influencing spatial patterns of shrub diversity in chaparral,Santa Ynez Mountains,California

We examined patterns of shrub species diversity relative to landscape‐scale variability in environmental factors within two watersheds on the coastal flank of the Santa Ynez Mountains, California. Shrub species richness and dominance was sampled at a hierarchy of spatial units using a high‐powered telescope from remote vantage points. Explanatory variables included field estimates of total canopy cover and percentage rock cover, and modeled distributions of slope, elevation, photosynthetically active radiation, topographic moisture index, and local topographic variability. Correlation, multiple regression, and regression tree analyses showed consistent relationships between field‐based measurements of species richness and dominance, and topographically‐mediated environmental variables. In general, higher richness and lower dominance occurred where environmental conditions indicated greater levels of resource limitation with respect to soil moisture and substrate availability. Maximum richness in shrub species occurred on high elevation sites with low topographic moisture index, rocky substrate, and steep slopes. Maximum dominance occurred at low elevation sites with low topographic variability, high potential solar insolation, and high total shrub canopy cover. The observed patterns are evaluated with respect to studies on species‐environment relations, resource use, and regeneration of shrubs in chaparral and coastal sage scrub. The results are discussed in the context of existing species‐diversity hypotheses that hinge on reduced competitive dominance and increased resource heterogeneity under conditions of resource limitation.     

Clonal plants—What is their role in succession?

Fifteen successional seres from man-made habitats in central Europe were compared and the occurrence of clonal species assessed on the basis of cover data. The effects of soil moisture and nitrogen (expressed using Ellenberg indicator values) on the performance of clonal plants were also considered. Clonal species formed the dominant component of vegetation cover in the majority of the seres studied. In moist sites, their dominance was more pronounced and the peak in their relative cover occurred earlier in succession. The relative importance of species with guerilla type growth tended to increase with time in most seres and after 10 years these were mostly more important than those exhibiting phalanx type growth. The prevalence of guerilla species after 10 years was more obvious in moist seres. Clonal species were able to become dominant regardless of soil conditions, whereas the dominance of non-clonal species tended to be restricted to very wet and nutrient-poor sites. Clonal plant species appeared to maintain their dominance for a longer period than non-clonal plants.     

Effects of deforestation on fish community structure in Ecuadorian Amazon streams

SUMMARY 1. There is little information on the impacts of deforestation on the fish fauna in neotropical streams, and on parameters influencing species diversity and community structure of fish. We analysed these aspects in 12 stream sites in the Ecuadorian Amazon. The stream sites represented a large gradient in canopy cover and were located in an area of fragmented forest. While some streams had been deforested, they had not suffered gross degradation of the habitat.
2. The species richness of stream fish was not related to deforestation. Local fish diversity (Fisher's Alpha) was positively related to the surface area of stream pools (m²). Beta diversity was higher among forested than deforested sites, indicating greater heterogeneity in species composition among forested than deforested sites. The percentage of rare species was positively correlated with canopy cover.
3. Total fish density increased with deforestation, and the fish community changed from dominance by omnivorous and insectivorous Characiformes at forested sites to dominance of periphyton-feeding loricariids at deforested sites.
4. Multidimensional statistical analysis of fish community structure showed that six environmental variables (the area of stream bottom covered by leaves, relative pool area, particulate organic matter, mean depth, conductivity and suspended solids) were related to the ordination axes. The presence of leaves, which was strongly correlated to canopy cover, was the variable most closely related to fish community structure, while relative pool area was the second strongest variable. Thus, fish community structure was strongly affected by deforestation.     

Monodominance in an African Rain Forest: Is Reduced Herbivory Important?1

Tropical monodominant forests in which one tree species dominates the canopy occur in all three major tropical regions, but few studies have focused on the mechanisms responsible for dominance. This study tests the hypothesis that relative to other species in the community, dominant species are well defended and escape herbivore and pathogen damage. We surveyed the rate of damage on young expanding leaves of seedlings and saplings belonging to eight species within both monodominant Gilbertiodendron dewevrei forests and adjacent mixed–species forests in eastern Congo. Results showed that escape from herbivore and pathogen damage is not a mechanism by which Gilbertiodendron achieves dominance, as it suffered the highest damage level of any species surveyed. Similarly, other sub–dominant common species also suffered high rates of damage. These results are discussed in relation to the phenolic, fiber, and nitrogen content of leaves, and in the context of current theories pertaining to plant–herbivore interactions.     

SPECIES RESPONSE TO A MOISTURE GRADIENT IN CENTRAL ILLINOIS FORESTS

Polar and Gaussian ordination applied to data collected from 37 forest sites in central Illinois resulted in a continuous and gradual change in species composition along a moisture gradient. A series of overlapping species success curves formed by plotting Importance Values over stands ordered along the gradient varied continuously in modal location and habitat width. Blackjack oak and black oak dominated upland sandy sites. Black oak, white oak, and shagbark hickory were the most important species on exposed, upper slope positions or ridge tops with silt-loam soils. Red oak, sugar maple, American elm, and bur oak dominated sheltered locations on lower slope positions and stream terraces. Sycamore, silver maple, and cottonwood were leading tree species in floodplain forests. Conversion of black, white, and red oak forests on silt-loam sites to sugar maple, white ash, and red elm dominance is evident by high densities of these shade tolerant species in the understory. Composition of forests at the extreme ends of the moisture gradient is more stable than the mesic sites. Maximum tree diversity occurred on mesic sites and decreased toward the extreme ends of the moisture gradient. However, competitive exclusion of shade intolerant species by sugar maple and other species has caused a decrease in understory diversity on mesic sites. Diversity decreased from canopy to understory strata in lowland forests and increased on xeric sites.     

广西涠洲岛造礁珊瑚种群结构的空间分布

对2007年和2008年调查的涠洲岛沿岸海域垂直海岸方向的6条主剖面上20条分断面的影像资料、样品鉴定等资料的分析,涠洲岛石珊瑚共出现10科22属46种,9个未定种;角蜂巢珊瑚属(Favites)、滨珊瑚属(Porites)、蔷薇珊瑚属(Montipora)为优势类群,其属级重要值百分比分别为25.78%、17.47%、15.11%;各主剖面珊瑚种群优势类群分布各有差异,以西南部的W2、东北部的W5的珊瑚属种为较多,均为8科13属,其石珊瑚属种分布均匀度略高;涠洲岛珊瑚分布自岸向海分为砂砾及珊瑚断枝带、石珊瑚稀疏带、石珊瑚繁盛带、柳珊瑚繁盛带4 个生物地貌带,石珊瑚分布较多的石珊瑚繁盛带主要分布于-1.39--5 m的水深范围内。石珊瑚形态组合以块状/亚块状与板块状为主要类型,覆盖程度以块状/亚块状占优。调查区受1998年的"厄尔尼诺现象"引起的全球性极高温事件和2008年1-2月的极低温气候事件及地域性不合理人类活动胁迫影响较大,枝状形态石珊瑚覆盖度、优势程度下降,已退出属种优势组合。     

大亚湾大鹏澳水域春季浮游植物优势种的演替

根据2002年4月28日～5月27日大亚湾大鹏澳内每日采样的现场连续观测资料,使用主分量分析方法和多元回归方法,分析浮游植物优势种类之间的关系及影响其生长与演替的主要理化因子,建立春季浮游植物优势种类的演替模型。模型表明,中肋骨条藻和柔弱菱形藻之间为竞争关系;与浮游植物(以硅藻为主)生长密切相关的理化因子为温度(T)、溶解氧饱和度(DOsat)、铵氮(NH4-N)、亚硝酸氮(NO2-N)、磷酸盐(PO4-P)、硅酸盐(SiO3-Si)。影响中肋骨条藻的理化因子还有盐度(S)和氮磷比值(NO3／PO4)。并根据优势种类发生演替期间的环境变化(降雨),分析了浮游植物优势种由中肋骨条藻演替为柔弱菱形藻的过程。     

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.