Influence of elevated CO2 on canopy development and red:far-red ratios in two-storied stands ofRicinus communis

Vertical structure of plant stands and canopies may change under conditions of elevated CO₂ due to differential responses of overstory and understory plants or plant parts. In the long term, seedling recruitment, competition, and thus population or community structure may be affected. Aside from the possible differential direct effects of elevated CO₂ on photosynthesis and growth, both the quantity and quality of the light below the overstory canopy could be indirectly affected by CO₂-induced changes in overstory leaf area index (LAI) and/or changes in overstory leaf quality. In order to explore such possible interactions, we compared canopy leaf area development, canopy light extinction and the quality of light beneath overstory leaves of two-storied monospecific stands ofRicinus communis exposed to ambient (340 μl l⁻¹) and elevated (610 μl l⁻¹) CO₂. Plants in each stand were grown in a common soil as closed “artificial ecosystems” with a ground area of 6.7 m². LAI of overstory plants in all ecosystems more than doubled during the experiment but was not different between CO₂ treatments at the end. As a consequence, extinction of photosynthetically active radiation (PAR) was also not altered. However, under elevated CO₂ the red to far-red ratio (R:FR) measured beneath overstory leaves was 10% lower than in ecosystems treated with ambient CO₂. This reduction was associated with increased thickness of palisade layers of overstory leaves and appears to be a plausible explanation for the specific enhancement of stem elongation of understory plants (without a corresponding biomass response) under elevated CO₂. CO₂ enrichment led to increased biomass of overstory plants (mainly stem biomass) but had no effect on understory biomass. The results of this study raise the possibility of an important indirect effect of elevated CO₂ at the stand-level. We suggest that, under elevated CO₂, reductions in the R:FR ratio beneath overstory canopies may affect understory plant development independently of the effects of PAR extinction.     

Factors influencing the distribution of two co-occurring dwarf bamboo species (<Emphasis Type="Italic">Sasa kurilensis </Emphasis>and<Emphasis Type="Italic"> S. senanensis</Emphasis>) in a conifer-broadleaved mixed stand in northern Hokkaido

We have investigated the factors influencing the distribution of co-occurring two dwarf bamboo species, Sasa kurilensis (Ruprecht) Makino et Shibata and S. senanensis (Franchet et Savatier) Rehder, within a conifer-broadleaved mixed stand managed with selection cutting in northern Japan. We first sought the possible determinant factors (physical environment and overstory conditions based on 30 years tree census data) deciding the dominant species in the plots (168 3.14m² area). We then examined the effects of these factors on the culm density and height of the dominant species. Linear discriminant analysis indicated that physical environmental conditions are important in determining the distribution; S. kurilensis tended to dominate plots with steep slope, convex shape and deep snow in early spring. Multiple regression analyses showed that culm density and height decreased significantly on steep slopes for both species. Also, the sum of the basal area (BA) of surrounding conifers (7.5 m radius around the plot), as well as the change in BA over the previous 30 years, had a negative influence on the culm height of both species. A reduction in overstory trees, caused by natural or artificial canopy disturbances, would increase the dwarf bamboo biomass. The effects of physical environment and overstory conditions, working through the dominance of the two dwarf bamboo species, should be taken into account in understanding the dynamics of natural forests in this region.     

Overstory Community Composition and Elevated Atmospheric CO2 and O3 Modify Understory Biomass Production and Nitrogen Acquisition

Elevated atmospheric CO₂ and O₃ have the potential to affect the primary productivity of the forest overstory, but little attention has been given to potential responses of understory vegetation. Our objective was to document the effects of elevated atmospheric CO₂ and O₃ on understory species composition and biomass and to quantify nitrogen (N) acquisition by the understory vegetation. The research took place at the aspen free-air CO₂ and O₃ enrichment (FACE) experiment, which has four treatments (control, elevated CO₂, elevated O₃, and elevated CO₂+O₃) and three tree communities: aspen, aspen/birch, and aspen/maple. In June 2003, each FACE ring was uniformly labeled with 15N applied as NH₄Cl. Understory biomass was harvested in June of 2004 for productivity, N, and 15N measurements, and photosynthetically active radiation (PAR) was measured below the canopy. The understory was divided into five species groups, which dominate in this young aggrading forest: Taraxacum officinale (dandelion), Solidago sp. (goldenrod), Trifolium repens and T. pretense (clover), various species from the Poaceae family (grass), and composited minor components (CMC). Understory species composition, total and individual species biomass, N content, and 15N recovery showed overstory community effects, but the direct effects of treatments was masked by the high variability of these data. Total understory biomass increased with increasing light, and thus was greatest under the open canopy of the aspen/maple community, as well as the more open canopy of the elevated O₃ treatments. Species were different from one another in terms of 15N recovery, with virtually no 15N recovered in clover and the greatest amount recovered in dandelion. Thus, understory species composition and biomass appear to be driven by the structure of the overstory community, which is determined by the tree species present and their response to the treatments. However, N acquisition by the understory does not appear to be affected by either the overstory community or the treatments at this point.     

<Emphasis Type="Italic">Hedychium gardnerianum</Emphasis> 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.     

Response to CO2 enrichment of understory vegetation in the shade of forests

Responses of forest ecosystems to increased atmospheric CO₂ concentration have been studied in few free‐air CO₂ enrichment (FACE) experiments during last two decades. Most studies focused principally on the overstory trees with little attention given to understory vegetation. Despite its small contribution to total productivity of an ecosystem, understory vegetation plays an important role in predicting successional dynamics and future plant community composition. Thus, the response of understory vegetation in Pinus taeda plantation at the Duke Forest FACE site after 15–17 years of exposure to elevated CO₂, 6–13 of which with nitrogen (N) amendment, was examined. Aboveground biomass and density of the understory decreased across all treatments with increasing overstory leaf area index (LAI). However, the CO₂ and N treatments had no effect on aboveground biomass, tree density, community composition, and the fraction of shade‐tolerant species. The increases of overstory LAI (~28%) under elevated CO₂ resulted in a reduction of light available to the understory (~18%) sufficient to nullify the expected growth‐enhancing effect of elevated CO₂ on understory vegetation.     

Microsite variation in light availability and photosynthesis in a cool-temperate deciduous broadleaf forest in central Japan

An empirical light simulation model was applied to estimate stand scale photosynthesis in a deciduous broadleaved forest in central Japan. Based on diurnal courses of photosynthetically active photon flux density (PPFD), we characterized the components of incoming light within the forest canopy, and found that the instantaneous relative PPFD (PPFD under the canopy relative to that above the canopy) under diffuse light condition was a reliable estimator of the intensity and duration of PPFD. We calculated the daily photosynthesis (A_day) for each PPFD class using photosynthesis–light response curves. Model simulated A_day were corroborated with the estimates obtained from the nearby CO₂ flux tower. The result demonstrated the potential of the light simulation model. The light use efficiency of two dominant species, Betula ermanii as overstory and Sasa senanensis as understory species, were then evaluated. At the forest understory, PPFD under 50 mol m^–2 s^–1 contributed to 77% of the sunshine duration on a completely clear day. Therefore, a higher apparent quantum yield for S. senanensis enhanced the utilization of low PPFD for photosynthesis. On the other hand, at the upper forest canopies, B. ermanii with a higher light-saturated photosynthetic rate used high PPFD efficiently. Consequently, potential of daily net photosynthesis for both B. ermanii and S. senanensis was high under each light condition. Such interspecific difference in the patterns of light utilization was suggested as one of factors allowing coexistence of the two species in the study forest.     

Light limitation creates patchy distribution of an invasive grass in eastern deciduous forests

Species interactions and their indirect effects on the availability and distribution of resources have been considered strong determinants of community structure in many different ecological systems. In deciduous forests, the presence of overstory trees and shrubs creates a shifting mosaic of resources for understory plants, with implications for their distribution and abundance. Determination of the ultimate resource constraints on understory vegetation may aid management of these systems that have become increasingly susceptible to invasions by non-native plants. Microstegium vimineum (Japanese grass) is an invasive annual grass that has spread rapidly throughout the understory of forests across the eastern United States since it was first observed in Tennessee in 1919. M. vimineum occurs as extensive, dense patches in the understory of eastern deciduous forests, yet these patches often exhibit sharp boundaries and distinct gaps in cover. One example of this distributional pattern was observed relative to the native midstory tree Asimina triloba (pawpaw), whereby dense M. vimineum cover stopped abruptly at the drip line of the A. triloba patch and was absent beneath the A. triloba canopy. We conducted field and greenhouse experiments to test several hypotheses regarding the causes of this observed pattern of M. vimineum distribution, including allelopathy, seed dispersal, light limitations, and soil moisture, texture, and nutrient content. We concluded that light reduction by the A. triloba canopy was the environmental constraint that prevented establishment of M. vimineum beneath this tree. Whereas overstory tree canopy apparently facilitates the establishment of this shade-tolerant grass, the interaction of overstory canopy with midstory canopy interferes with M. vimineum by reducing the availability of sunflecks at the ground layer. It is likely that other midstory species influence the distribution and abundance of other herb-layer species, with implications for management of understory invasive plant species.     

Effect of selective logging on tree and understory regeneration in miombo woodland in western Tanzania

Tree regeneration and understory response to selective cutting of Pterocarpus angolensis DC, and Sterculia quinqueloba Sim, was measured outside Katavi National Park, Tanzania. Contrary to expectations, a selective harvest had no effect on tree recruitment for either species in this miombo woodland. In unlogged plots along transects taken at increasing distance from a main road, the stand densities of young trees declined away from the road. Since anthropogenic activity along the road results in a high fire frequency but low native ungulate densities, the increased rate of tree regeneration near the road may result from either reduced grazing and browsing pressure by native ungulates, or reduced fire intensity and severity along roads. These effects appear to mask any possible effect of selective harvest on tree recruitment. Currently, there is no evidence of compensatory recruitment of trees of any species into the canopy to replace logged trees and this will lead to a gradual thinning in overstory stand density.     

Conservation value of timber quality versus associated non-timber quality stands for understory diversity in Nothofagus forests

Conservation strategies of forested landscapes must consider biodiversity of the included site types, i.e. timber-quality forests and associated non-timber-quality stands. The objectives were to characterize forest overstory structure in timber-quality versus associated non-timber-quality stands; and to compare their understory communities. Six forest types were sampled in Nothofagus forests of Tierra del Fuego (Argentina): two timber-quality N. pumilio forests, and four associated non-timber-quality stands (edge, N. antarctica, wetlands and streamside forests). Overstory structure and understory vegetation (species richness, frequencies, cover and biomass) were characterized during spring and summer seasons. Analysis of variance and multivariates were carried out. Overstory structure differed across the site types, with higher tree size, canopy closure and tree volume in timber-quality stands. Fifty-one understory plant species were observed, but understory variables varied with site types, especially wetlands (highest native and exotic richness, cover and biomass, and 25% of exclusive species). Site types were grouped in three: N. antarctica stands, streamside stands and the other N. pumilio forests according to multivariate analysis. Forty three percent of plants were distributed in all site types, and all timber-quality forest understory species were present in some associated non-timber-quality stands. Timber-quality N. pumilio forests have a marginal value for understory conservation compared to associated non-timber-quality stands, because these last include all the plants observed in timber-quality forests and also possess many exclusive species. Therefore, protection of associated non-timber-quality stands during forest management planning could increase understory conservation at landscape level, and these could be better reserves of understory diversity than retentions of timber-quality stands.     

Changes in structure of over‐ and midstory tree species in a Mediterranean‐type forest after an extreme drought‐associated heatwave

Worldwide, extreme climatic events such as drought and heatwaves are associated with forest mortality. However, the precise drivers of tree mortality at individual and stand levels vary considerably, with substantial gaps in knowledge across studies in biomes and continents. In 2010–2011, a drought‐associated heatwave occurred in south‐western Australia and drove sudden and rapid forest canopy collapse. Working in the Northern Jarrah (Eucalyptus marginata) Forest, we quantified the response of key overstory (E. marginata, Corymbia calophylla) and midstory (Banksia grandis, Allocasuarina fraseriana) tree species to the extreme climate event. Using transects spanning a gradient of drought impacts (minimal (50–100 m), transitional (100–150 m) and severe (30–60 m)), tree species mortality in relation to stand characteristics (stand basal area and stem density) and edaphic factors (soil depth) was determined. We show differential mortality between the two overstory species and the two midstory species corresponding to the drought‐associated heatwave. The dominant overstory species, E. marginata, had significantly higher mortality (~19%) than C. calophylla (~7%) in the severe zone. The midstory species, B. grandis, demonstrated substantially higher mortality (~59%) than A. fraseriana (~4%) in the transitional zone. Banksia grandis exhibited a substantial shift in structure in response to the drought‐associated heatwave in relation to tree size, basal area and soil depth. This study illustrates the role of climate extremes in driving ecosystem change and highlights the critical need to identify and quantify the resulting impact to help predict future forest die‐off events and to underpin forest management and conservation.     

Differences in Fine-Root Biomass of Trees and Understory Vegetation among Stand Types in Subtropical Forests

Variation of total fine-root biomass among types of tree stands has previously been attributed to the characteristics of the stand layers. The effects of the understory vegetation on total fine-root biomass are less well studied. We examined the variation of total fine-root biomass in subtropical tree stands at two sites of Datian and Huitong in China. The two sites have similar humid monsoon climate but different soil organic carbon. One examination compared two categories of basal areas (high vs. low basal area) in stands of single species. A second examination compared single-species and mixed stands with comparable basal areas. Low basal area did not correlate with low total fine-root biomass in the single-species stands. The increase in seedling density but decrease in stem density for the low basal area stands at Datian and the quite similar stand structures for the basal-area contrast at Huitong helped in the lack of association between basal area and total fine-root biomass at the two sites, respectively. The mixed stands also did not yield higher total fine-root biomasses. In addition to the lack of niche complementarity between tree species, the differences in stem and seedling densities and the belowground competition between the tree and non-tree species also contributed to the similarity of the total fine-root biomasses in the mixed and single-species stands. Across stand types, the more fertile site Datian yielded higher tree, non-tree and total fine-root biomasses than Huitong. However, the contribution of non-tree fine-root biomass to the total fine-root biomass was higher at Huitong (29.4%) than that at Datian (16.7%). This study suggests that the variation of total fine-root biomass across stand types not only was associated with the characteristics of trees, but also may be highly dependent on the understory layer.     

Forest plant community as a driver of soil biodiversity: experimental evidence from collembolan assemblages through large‐scale and long‐term removal of oak canopy trees Quercus petraea

Plant–soil interactions are increasingly recognized to play a major role in terrestrial ecosystems functioning. However, few studies to date have focused on slow dynamic ecosystems such as forests. As they are vertically stratified by multiple vegetation strata, canopy tree removal by thinning operations could alter forest plant community through tree canopy opening. Very little is known about cascading effects on soil biodiversity. We conducted a large‐scale, multi‐site assessment of collembolan assemblage response to long‐term canopy tree removal in sessile oak Quercus petraea temperate forests. A total of 33 experimental plots were studied covering a large gradient of canopy tree basal area, stand age and local abiotic contexts. Collembolan abundance strongly declined with canopy tree removal in early forest successional stage and this was mediated by negative effect of understory plant community composition changes, i.e. shift from moss and forb to tree seedling, fern, shrub and grass species. Negative effect of this composition shift on collembolan species richness was largely offset by positive effect of the increase in understory plant species richness. This gives support to both the plant mass‐ratio and functional diversity hypotheses. Collembolan functional groups had contrasting response patterns, which were mediated by different ecological factors. Epedaphic (r‐strategist) abundance and species richness increased with canopy tree removal in relation with the increase in understory plant species richness. In contrast, euedaphic (K‐strategist) abundance and species richness declined with canopy tree removal in early forest successional stage in relation with changes in understory plant community composition and species richness, as well as microclimatic conditions. Overall, our study provides experimental evidence that forest plant community can be a strong driver of collembolan assemblages. It also emphasizes the role of trees as foundation species of forest ecosystems that can shape soil biodiversity through their regulation of understory plant community and ecosystem abiotic conditions.     

Red Spruce Stand Dynamics, Simulations, and Restoration Opportunities in the Central Appalachians

Red spruce (Picea rubens)–dominated forests occupied as much as 600,000 ha in West Virginia prior to exploitive logging era of the late nineteenth and early twentieth centuries. Subsequently, much of this forest type was converted to northern hardwoods. As an important habitat type for a number of rare or sensitive species, only about 12,000 ha of red spruce forests presently remain in the state. In order to assess the prospects for restoration, we examined six northern hardwood stands containing understory red spruce to (1) characterize stand dynamics and regeneration patterns and (2) simulate the effectiveness of restoration silviculture to enhance red spruce overstory recruitment. Stands originated in the late 1800s to early 1900s and are currently in the (late) stem exclusion or understory reinitiation stages. Five of the six stands had even‐aged overstories that originated after clear‐cutting. Tree‐ring chronologies show high initial growth rates consistent with stand initiation. One stand, partially harvested in 1915, was uneven aged with older, legacy residuals in the canopy. Most stands had two cohorts of understory red spruce, with more than 40% of these individuals showing prior release. Our 100‐year growth simulation suggested that a 50% basal area thinning from above could double red spruce basal area to support a mixed spruce–hardwood stand in approximately 20–40 years. These results indicate that restoration silviculture could be an effective tool for increasing the amount and quality of this reduced forest type in the central Appalachians.     

Vegetative Characteristics of Recently Reforested Bottomlands in the Lower Cache River Watershed, Illinois, U.S.A.

Abstract Interest in restoring native ecosystems is resulting in conversion of marginal agricultural lands to bottomland hardwood‐dominated forests in the midwestern and midsouthern United States. Growing stock for these efforts typically consists of planted oak (Quercus spp.) and volunteer vegetation. Reports of mixed reforestation success and the lack of post‐establishment tree growth data prompted this evaluation of vegetation characteristics of 5‐ to 7‐year‐old operational restorations in the Lower Cache River Watershed in southernmost Illinois, U.S.A. Fraxinus pennsylvanica (green ash), Acer negundo (box‐elder), and Liquidambar styraciflua (sweetgum) together comprised 77% of all tree stems observed. Full stocking of overstory tree species can be expected to produce a closed canopy stand within 160 m of a forested edge, due primarily to the abundance of rapidly growing volunteer‐origin trees. Planted oaks contributed minimally to total tree stocking but were present in sufficient numbers to eventually improve wildlife habitat, and therefore satisfied restoration objectives. Oak height was 23% greater when in the presence of a non‐oak tree species. Herbaceous cover was dominated by Solidago gigantea (late goldenrod) and Juncus spp. (rushes). Solidago gigantea was associated with poor growth and low density of non‐oak stems, whereas Juncus dudleyi (Dudley's rush) was associated with taller non‐oak stems. These results suggest that the presence of volunteer‐origin trees is crucial for the creation of full stand stocking that will result in rapid development of a closed canopy forest. Improved success of future reforestation efforts will require more intensive methods to establish adequate stocking beyond 160 m of a forest edge. Methods described here could be adapted for agricultural field to forest restorations in other regions to predict critical distances from volunteer seed sources within which supplemental planting would be unnecessary to meet tree stocking objectives.     

Effects of different Rhododendron control methods in eastern beech (Fagus orientalis Lipsky) ecosystems in the western Black Sea region of Turkey

Intensive weed control and plot preparation practices have become a critical and integral part of productive beech forest management in Turkey’s coastal Black Sea region (BSR). This study was conducted in an eastern beech forest of 100+ year old in the BSR to evaluate ecosystem effects of three different experimental Rhododendron ponticum understory control methods with a randomised block design, including manual grubbing, foliar and cut stump spraying with imazapyr (Arsenal) and foliar and cut stump spraying with triclopyr (Garlon). Untreated vegetation plots served as controls. Evaluation of these treatments included their effects on understory and forest floor biomass and nutrients (C, N, P, S, K, Ca and Mg) and effects on soils, including bulk density, pH, soil nutrients (C, N, P and S), exchangeable cations (K, Ca and Mg) and soil cation exchange capacity (CEC). Grubbing and imazapyr treatments had greatly reduced the amount of understory biomass 5 years after application (P = 0.002). Triclopyr treatment also had a major effect on understory vegetation control, but by 5 years later, about 10% of the rhododendron originally present on these plots had gradually re‐sprouted and partially covered the plots. Five years after woody vegetation control treatments, at the 0‐ to 20‐cm depth, treatments did not appear to affect soil bulk density, pH and CEC. For the upper 20‐cm soil depth, the exchangeable soil K concentration at the 10‐ to 20‐cm depth on triclopyr‐treated plots was 33% higher than on grubbing plots, and it was twice that of imazapyr application plots. Imazapyr plots had almost 11 times more dead organic matter on the forest floor than there was on grubbing plots. Forest floor C concentrations on imazapyr plots were 26 and 14% greater than those on grubbing and triclopyr plots, respectively. Total ecosystem (forest floor + understory + soil exchangeable) Ca content was 50% higher on imazapyr plots than that on triclopyr plots, while the ecosystem K pool on imazapyr treatment plots was 27% lower than that on triclopyr plots. Herbicides can be used as an alternative for achieving some forest management objectives when other vegetation control methods are not feasible or economical. It is recommended that vegetation control not be used on steep slopes because of greater risk of soil erosion. There may be benefits in encouraging slash disposal by fire after imazapyr treatments, thus removing recalcitrant understory residues left on the forest floor and releasing the essential nutrients within them.     

长白山阔叶红松林不同强度择伐后关键树种的竞争关系

天然林择伐改变了林分的树种组成和结构,导致林木竞争关系发生变化,进而影响树木的生长和种群的动态.关键树种在维持群落结构和生态系统功能方面具有至关重要的作用.为了弄清长白山区阔叶红松林典型林型关键树种竞争关系的特点,本研究以红松、紫椴和水曲柳为研究对象,运用Hegyi竞争指数分析了受不同程度采伐干扰后形成的原始林(未受干扰)、轻度择伐林、中度择伐林和重度择伐林的林分结构和竞争关系.结果表明: 与原始林相比,轻度择伐林中关键树种的种群结构没有显著变化;中度择伐林和重度择伐林中3个关键树种大树(胸径DBH≥20 cm)的密度和平均胸径均显著减少,但幼树(2 cm≤DBH<10 cm)的数量增加.所有样地中关键树种的竞争指数均随径级的增加而减小,且二者服从幂函数分布,而林木胸径生长到20 cm后,竞争指数进入稳定状态.在原始林、轻度和中度择伐林中,3个树种的幼树的竞争指数受到非冠层树种的影响最大,而红松小树(10 cm≤DBH<20 cm)和大树主要受红松种内和非冠层树种的影响;水曲柳主要受红松和非冠层树种的影响,紫椴主要受种内和红松的影响.重度择伐林中,白桦是3个树种的主要竞争来源,贡献率均超过50%.根据以上关键树种竞争关系的特点,抚育非冠层树种有利于3个关键种幼树的更新和生长;对于小树来说,还需要根据树种类型来采取相应措施,而大树不需要采用抚育措施.本研究对关键树种培育和天然林择伐后快速恢复具有指导意义.     

Vertical distribution and seasonal pattern of fine-root dynamics in a cool–temperate forest in northern Japan: implication of the understory vegetation, <Emphasis Type="Italic">Sasa</Emphasis> dwarf bamboo

We measured the vertical distribution and seasonal patterns of fine-root production and mortality using minirhizotrons in a cool–temperate forest in northern Japan mainly dominated by Mongolian oak (Quercus crispula) and covered with a dense understory of dwarf bamboo (Sasa senanensis). We also investigated the vertical distribution of the fine-root biomass using soil coring. We also measured environmental factors such as air and soil temperature, soil moisture and leaf area indices (LAI) of trees and the understory Sasa canopy for comparison with the fine-root dynamics. Fine-root biomass to a depth of 60 cm in September 2003 totaled 774 g m⁻², of which 71% was accounted for by Sasa and 60% was concentrated in the surface soil layer (0–15 cm), indicating that understory Sasa was an important component of the fine-root biomass in this ecosystem. Fine-root production increased in late summer (August) when soil temperatures were high, suggesting that temperature partially controls the seasonality of fine-root production. In addition, monthly fine-root production was significantly related to Sasa LAI (P<0.001), suggesting that fine-root production was also affected by the specific phenology of Sasa. Fine-root mortality was relatively constant throughout the year. Fine-root production, mortality, and turnover rates were highest in the surface soil (0–15 cm) and decreased with increasing soil depth. Turnover rates of production and mortality in the surface soil were 1.7 year⁻¹ and 1.1 year⁻¹, respectively.     

The effect of dense understory dwarf bamboo (Sasa senanensis) on soil respiration before and after clearcutting of cool temperate deciduous broad-leaved forest

To evaluate the effect of understory dwarf bamboo (Sasa senanensis) on soil respiration in forest ecosystems, we compared soil respiration rates between four deciduous broad-leaved forest sites representing two levels of understory Sasa (with and without) and two levels of forest stand age (50-year-old stand and 1-year-old stand after clearcut). The understory Sasa enhances the soil respiration rate both before and after the clearcutting of deciduous broad-leaved forest. The Sasa sites had larger total belowground biomass compared with the non-Sasa sites, which could be attributed to Sasa presence. Our results also suggest that clearcutting decreases temperature-normalized soil respiration rates (R ₁₅) and temperature sensitivity (Q ₁₀) in both Sasa and non-Sasa ecosystems. Clearcutting significantly reduced the fine root biomass of trees and Sasa. The fine roots of trees and Sasa had high specific respiration rates compared with larger roots and rhizomes at Sasa and non-Sasa sites, respectively. Therefore, we hypothesize that the loss of fine roots after clearcutting is responsible for the reduction in soil respiration rate. A comparison with other studies revealed a positive linear relationship between total (tree and Sasa) fine root biomass and R ₁₅, suggesting that fine root biomass controls soil respiration at the landscape scale. The Q ₁₀ value is also likely to be related to fine root biomass, although the relationship was not significant. We conclude that understory Sasa increases belowground biomass, especially fine roots, and the spatial variation in soil respiration at the landscape scale.     

Overstory-Understory Relationships along Forest Type and Environmental Gradients in the Spring Mountains of Southern Nevada, USA

Isolated forested mountains in deserts have numerous ecological and societal values, but land-management practices (e.g., fire-regime alteration) and climate change can affect forest composition. We analyzed tree overstory-understory relationships on 123 sites in the Spring Mountains within the Mojave Desert near Las Vegas, Nevada, USA to assess three hypotheses. We hypothesized that: the tree species comprising understories are less tolerant of fire than species in overstories, reflecting land-management practices of fire exclusion; mid-elevation forests have the lowest overstory:understory similarity because this zone could have maximum species mixing; and overstory:understory similarity is correlated with environmental gradients (consisting of 14 topographic and soil variables). We found that Pinus monophylla comprised greater relative canopy cover in understories of juniper (32% relative cover) and pinyon-juniper (78%) forests than it did in overstories of these forests (0% and 53%). Similarly, fire-intolerant Abies concolor had 6-fold greater understory than overstory cover in forests with overstories dominated by the fire-tolerant Pinus ponderosa. Overstory:understory S?rensen similarity averaged 43%?77% among six forest types, and there was little support for the supposition that similarity was lowest in mid-elevation forests. Distributions of individual overstory and understory species more closely corresponded with environmental gradients than did overstory:understory similarity. Results suggest that there is high potential for change in at least two of the six dominant forest types of the Spring Mountains. The direction of change (species of moist, higher elevation sites establishing in understories of drier forests) is the opposite of what would be expected for forest adaptation to the warmer, drier, more fire-prone conditions projected for the next century in the southwestern USA.     

DIFFERENCES IN LEAF STRUCTURE,CHLOROPHYLL, AND NUTRIENTS FOR THE UNDERSTORY TREE ASIMINA TRILOBA

To investigate differences in leaf structure, chlorophyll and nutrients on terminal branches of the understory tree Asimina triloba, the first (proximal) and the last (distal) leaves to develop in the spring were compared. Proximal leaf expansion was completed before the overstory canopy was fully closed but distal leaf expansion occurred during and after the development of the overstory canopy. Fully expanded proximal leaves were 76% smaller in area, were 18% thicker and had 36% more stomates per m of leaf area when compared to distal leaves. In addition, maximum stomatal conductance to water vapor was greater (150 vs. 120 mmol m^−-2s^−-1) and the minimum PPFD required for maximum conductance was higher (200 vs. 150 μmol m^−-2s^−-1) for the proximal leaves. Chlorophyll content was also greater for proximal leaves, but nitrogen and phosphorus contents were lower throughout the entire summer. Seasonal measurements indicated an increase in chlorophyll a content and reductions in nitrogen content throughout the summer growth period for leaves from both positions. The results suggest that distal and proximal leaves differed physiologically and that the measured differences were related to the changing irradiance environment during leaf development. The time of leaf expansion, as indicated by leaf position on the branch, may be an important consideration when examining the water and photosynthetic relations of understory trees.