Influence of stand structure on carbon-13 of vegetation, soils, and canopy air within deciduous and evergreen forests in Utah, United States

Carbon isotope ratios (δ¹³C) were studied in evergreen and deciduous forest ecosystems in semi-arid Utah (Pinus contorta, Populus tremuloides, Acer negundo and Acer grandidentatum). Measurements were taken in four to five stands of each forest ecosystem differing in overstory leaf area index (LAI) during two consecutive growing seasons. The δ¹³C_leaf (and carbon isotope discrimination) of understory vegetation in the evergreen stands (LAI 1.5–2.2) did not differ among canopies with increasing LAI, whereas understory in the deciduous stands (LAI 1.5–4.5) exhibited strongly decreasing δ¹³C_leaf values (increasing carbon isotope discrimination) with increasing LAI. The δ¹³C values of needles and leaves at the top of the canopy were relatively constant over the entire LAI range, indicating no change in intrinsic water-use efficiency with overstory LAI. In all canopies, δ¹³C_leaf decreased with decreasing height above the forest floor, primarily due to physiological changes affecting c _i/c _a (> 60%) and to a minor extent due to δ¹³C of canopy air (< 40%). This intra-canopy depletion of δ¹³C_leaf was lowest in the open stand (1‰) and greatest in the denser stands (4.5‰). Although overstory δ¹³C_leaf did not change with canopy LAI, δ¹³C of soil organic carbon increased with increasing LAI in Pinus contorta and Populus tremuloides ecosystems. In addition, δ¹³C of decomposing organic carbon became increasingly enriched over time (by 1.7–2.9‰) for all deciduous and evergreen dry temperate forests. The δ¹³C_canopy of CO₂ in canopy air varied temporally and spatially in all forest stands. Vertical canopy gradients of δ¹³C_canopy, and [CO₂]_canopy were larger in the deciduous Populus tremuloides than in the evergreen Pinu contorta stands of similar LAI. In a very wet and cool year, ecosystem discrimination (Δ_e) was similar for both deciduous Populus tremulodies (18.0 ± 0.7‰) and evergreen Pinus contorta (18.3 ± 0.9‰) stands. Gradients of δ¹³C_canopy and [CO₂]_canopy were larger in denser Acer spp. stands than those in the open stand. However, ¹³C enrichment above and photosynthetic draw-down of [CO₂]_canopy below tropospheric baseline values were larger in the open than in the dense stands, due to the presence of a vigorous understory vegetation. Seasonal patterns of the relationship δ¹³C_canopy versus 1/[CO₂]_canopy were strongly influenced by precipitation and air temperature during the growing season. Estimates of Δ_e for Acer spp. did not show a significant effect of stand structure, and averaged 16.8 ± 0.5‰ in 1933 and 17.4 ± 0.7‰ in 1994. However, Δ_e varied seasonally with small fluctuations for the open stand (2‰), but more pronounced changes for the dense stand (5‰). Received: 15 April 1996 / Accepted: 19 October 1996     

Within‐stand spatial structure and relation of boreal canopy and understorey vegetation

Question: How do spatial patterns and associations of canopy and understorey vegetation vary with spatial scale along a gradient of canopy composition in boreal mixed‐wood forests, from younger Aspen stands dominated by Populus tremuloides and P. balsamifera to older Mixed and Conifer stands dominated by Picea glauca? Do canopy evergreen conifers and broad‐leaved deciduous trees differ in their spatial relationships with understorey vegetation? Location: EMEND experimental site, Alberta, Canada. Methods: Canopy and understorey vegetation were sampled in 28 transects of 100 contiguous 0.5 m × 0.5 m quadrats in three forest stand types. Vegetation spatial patterns and relationships were analysed using wavelets. Results: Boreal mixed‐wood canopy and understorey vegetation are patchily distributed at a range of small spatial scales. The scale of canopy and understorey spatial patterns generally increased with increasing conifer presence in the canopy. Associations between canopy and understorey were highly variable among stand types, transects and spatial scales. Understorey vascular plant cover was generally positively associated with canopy deciduous tree cover and negatively associated with canopy conifer tree cover at spatial scales from 5–15 m. Understorey non‐vascular plant cover and community composition were more variable in their relationships with canopy cover, showing both positive and negative associations at a range of spatial scales. Conclusions: The spatial structure and relation of boreal mixed‐wood canopy and understorey vegetation varied with spatial scale. Differences in understorey spatial structure among stand types were consistent with a nucleation model of patch dynamics during succession in boreal mixed‐wood forests.     

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

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

Effect of forest canopy composition on soil nutrients and dynamics of the understorey: mixed canopies serve neither vascular nor bryophyte strata

Question: The effect of overstorey composition on above‐ground dynamics of understorey vegetation is poorly understood. This study examines the understorey biomass, production and turnover rates of vascular and non‐vascular plants along a conifer–broadleaf gradient of resource availability and heterogeneity. Location: Canadian boreal forests of northwest Quebec and Ontario. Methods: We sampled mature stands containing various proportions of black spruce (Picea mariana (Mill.) BSP), trembling aspen (Populus tremuloides Michx.) and jack pine (Pinus banksiana Lamb.). Above‐ground biomass of the understorey vegetation was assessed through harvesting; annual growth rates were calculated as the differences between biomass in 2007 and 2008, as estimated by allometric relationships, and turnover rates were estimated as net primary production divided by the biomass in 2007. Results: Higher aspen presence, linked to greater nutrient availability in the forest floor, was generally associated with higher vascular biomass and production in the understorey. This effect was less pronounced in sites of high intrinsic fertility. In contrast, bryophyte biomass was positively associated with conifer abundance, particularly in wet sites of the Quebec study area. Non‐linear responses resulted in total understorey biomass being lower under mixed canopies than under pure aspen or pure conifer canopies. Turnover rates did not differ with overstorey composition. Conclusions: While resource availability is a main driver of understorey productivity, resources as drivers appear to differ with differences in understorey strata components, i.e. vascular versus non‐vascular plants. Resource heterogeneity induced by a mixed canopy had overall negative effects on understorey above‐ground productivity, as this productivity seemed to rely on species adapted to the specific conditions induced by a pure canopy.     

Leaf senescence and decline of end-of-season gas exchange in five temperate deciduous tree species grown in elevated CO2 concentrations

We measured rates of leaf senescence and leaf level gas exchange during autumnal senescence for seedlings of five temperate forest tree species under current and elevated atmospheric CO₂ concentrations and low- and high-nutrient regimes. Relative indices of whole canopy carbon gain, water loss and water use efficiency through the senescent period were calculated based on a simple integrative model combining gas exchange per unit leaf area and standing canopy area per unit time. Seedlings grown under elevated [CO₂] generally had smaller canopies than their current [CO₂]-grown counterparts throughout most of the senescent period. This was a result of smaller pre-senescent canopies or accelerated rates of leaf drop. Leaf-level photosynthetic rates were higher under elevated [CO₂] for grey birch canopies and for low-nutrient red maple and high-nutrient ash canopies, but declined rapidly to values below those of their current [CO₂] counterparts by midway through the senescent period. CO₂ enrichment reduced photosynthetic rates for the remaining species throughout some or all of the senescent period. As a result of smaller canopy sizes and reduced photosynthetic rates, elevated [CO₂]-grown seedlings had lower indices of whole canopy end-of-season carbon gain with few exceptions. Leaf level transpiration rates were highly variable during autumnal senescence and neither [CO₂] nor nutrient regime had consistent effects on water loss per unit leaf area or integrated whole canopy water loss throughout the senescent period. Indices of whole canopy, end-of-season estimates of water use efficiency, however, were consistently lower under CO₂ enrichment, with few exceptions. These results suggest that whole canopy end-of-season gas exchange may be altered significantly in an elevated [CO₂] world, resulting in reduced carbon gain and water use efficiency for many temperate forest tree seedlings. Seedling growth and survivorship, and ultimately temperate forest regeneration, could be reduced in CO₂-enriched forests of the future.     

温带落叶阔叶林冠层CO2浓度的时空变异

为了研究温带落叶阔叶林CO₂浓度(摩尔分数, [CO₂])的时空变化特征, 利用帽儿山通量塔8层[CO₂]廓线系统分析了[CO₂]的时间动态及垂直梯度, 并结合森林小气候的同步测定数据探讨了影响[CO₂]时空变化的因子。结果表明: 帽儿山温带落叶阔叶林的[CO₂]及其垂直梯度具有明显的日变化和季节变化。在日尺度上, [CO₂]呈“单峰”曲线, 在夜间或日出前后出现最大值, 日出后迅速降低, 在午后达到最低值, 日落时分又开始迅速升高。在季节尺度上, 生长季的[CO₂]日变幅明显大于非生长季, 且冬季(1、2和12月)白天呈“V”型, 其他季节白天呈“U”型, 这与白天对流边界层的持续时间随季节的变化趋势一致。在垂直方向上, [CO₂]及其日变幅随高度增加而降低, 并且在生长季夜间湍流交换较弱时其垂直梯度最显著; 植被冠层的光合作用改变了生长旺季白天的[CO₂]垂直格局, 使冠层高度的[CO₂]最低; 休眠季节该垂直梯度大大减弱。近地层日均[CO₂]与土壤温度的趋势相似, 呈单峰曲线; 而林冠上[CO₂]在5月初和10月各出现一次峰值, 最低值出现在8月初, 与植被光合作用紧密相关。日尺度上[CO₂]及其垂直梯度主要受控于大气边界层和生态系统碳代谢过程; 年尺度上近地层[CO₂]主要受控于土壤呼吸, 而林冠上的[CO₂]则受生态系统光合作用和呼吸作用的共同控制。     

Coexistence mechanisms of evergreen,deciduous and coniferous trees in a mid-montane mixed forest on Mt. Emei,Sichuan, China

Structure and regeneration of a mid-montane (2200 m a.s.l.)mixed forest codominated by evergreen (Lithocarpuscleistocarpus), deciduous (Acer flabellatum)andconiferous (Tsuga chinensis, Abies fabri, andTaxus chinensis) trees were analyzed in a 40m× 60 m plot on Mt. Emei, Sichuan, China. Plant communitystructure and composition varied depending on topographic micro-habitat withinthe plot. Four topographic communities (topo-communities) were distinguishedwith dominant species corresponding to topography: (1)Abies – valley bank, (2) Acer– lower steep slope, (3) Lithocarpus – uppergentle slope, and (4) Tsuga – ridge. The coexistencemechanisms of the evergreen, deciduous, and coniferous trees were determined byidentifying the regeneration process characteristic of each dominant species,asrepresented by their seedling dispersion patterns and seedling establishmentalong the topographic gradients. The saplings and seedlings of the dominantswere distributed differently according to the topography:Lithocarpus under the canopy of parent trees and in gapsofthe upper gentle slope and ridge, Acer mainly in bothwell-lit and shady sites on the lower steep slope and on fallen logs,Abies on the valley bank but only in well-lit sites,Tsuga on the ridge, Taxus mostly onrock and well-lit sites on the slopes. Distribution of surviving saplings andseedlings was also related to the species of nearby canopy trees. We suggestthat Lithocarpus, Tsuga and Acer tendto be self-replacing in their own topographic habitats, andAbies survives as a fugitive by occupying occasionalsuitable gaps. The variation in soil conditions, particularly nutrients, withtopography affected seedling establishment and the growth of trees. Thesuccessional change of quantitative species composition, as predicted by theMarkovian model, shows the mixed forest to be in a sustained climax stage.     

A free-air enrichment system for exposing tall forest vegetation to elevated atmospheric CO2

A free-air CO₂ enrichment (FACE) system was designed to permit the experimental exposure of tall vegetation such as stands of forest trees to elevated atmospheric CO₂ concentrations ([CO₂]_a) without enclosures that alter tree microenvironment. We describe a prototype FACE system currently in operation in forest plots in a maturing loblolly pine (Pinus taeda L.) stand in North Carolina, USA. The system uses feedback control technology to control [CO₂] in a 26 m diameter forest plot that is over 10 m tall, while monitoring the 3D plot volume to characterize the whole-stand CO₂ regime achieved during enrichment. In the second summer season of operation of the FACE system, atmospheric CO₂ enrichment was conducted in the forest during all daylight hours for 96.7% of the scheduled running time from 23 May to 14 October with a preset target [CO₂] of 550 μmol mol^–1, ≈ 200 μmol mol^–1 above ambient [CO₂]. The system provided spatial and temporal control of [CO₂] similar to that reported for open-top chambers over trees, but without enclosing the vegetation. The daily average daytime [CO₂] within the upper forest canopy at the centre of the FACE plot was 552 ± 9 μmol mol^–1 (mean ± SD). The FACE system maintained 1-minute average [CO₂] to within ± 110 μmol mol^–1 of the target [CO₂] for 92% of the operating time. Deviations of [CO₂] outside of this range were short-lived (most lasting < 60 s) and rare, with fewer than 4 excursion events of a minute or longer per day. Acceptable spatial control of [CO₂] by the system was achieved, with over 90% of the entire canopy volume within ± 10% of the target [CO₂] over the exposure season. CO₂ consumption by the FACE system was much higher than for open-top chambers on an absolute basis, but similar to that of open-top chambers and branch bag chambers on a per unit volume basis. CO₂ consumption by the FACE system was strongly related to windspeed, averaging 50 g CO₂ m^–3 h^–1 for the stand for an average windspeed of 1.5 m s^–1 during summer. The [CO₂] control results show that the free-air approach is a tractable way to study long-term and short-term alterations in trace gases, even within entire tall forest ecosystems. The FACE approach permits the study of a wide range of forest stand and ecosystem processes under manipulated [CO₂]_a that were previously impossible or intractable to study in true forest ecosystems.     

Tree seedling responses to in situ CO2-enrichment differ among species and depend on understorey light availability

Seedlings of six major European temperate forest tree species (Fagus sylvatica, Acer pseudoplatanus, Quercus robur, Taxus baccata, Abies alba, Pinus sylvestris) were exposed to 360, 500, and 660 μL CO₂ L^?1 in the understorey of a 120‐y‐old forest over two growing seasons. Seedlings rooted in the natural forest soil within 36 open‐top chambers (12 OTCs per CO₂ treatment), each with a different known quantum flux density (QFD) ranging from 0.36 to 2.16 mol m^?2 d^?1 (= 0.8% to 4.8% of full sun). In contrast to a frequent assumption the natural CO₂ concentration in the understorey is close to the ambient concentration in the free atmosphere during daytime. The CO₂‐effect on seedling growth differed greatly among species and was strongly codetermined by microsite‐specific QFD. Biomass production in the deep‐shade tolerant species Fagus and Taxus increased by 73% and 37% under elevated CO₂ in low QFD microsites but was not significantly different among CO₂‐treatments in high QFD microsites. The less shade‐tolerant species Acer, Quercus, and Abies showed no significant response to elevated CO₂ in low QFD microsites, but increased their biomass by 39%, 25%, and 55% in high QFD microsites. In the shade‐intolerant Pinus, seedling survival was too low for a safe conclusion. Our data showed that the largest relative responses to increasing CO₂ occurred at a comparatively small increase from 360 to 500 μL L^?1 with only small and non‐significant changes with a further increase to 660 μL L^?1. Subtle shifts in the availability of light can totally reverse interspecific differences in the CO₂ response. Given these different responses, we conclude that increasing atmospheric CO₂ is likely to induce changes in species composition of temperate forests due to altered chances of recruitment. However, these shifts will depend on light patterns in the understorey, and thus on canopy structure, disturbance patterns and forest management.     

Forest herbs show species-specific responses to variation in light regime on sites with contrasting soil acidity: An experiment mimicking forest conversion scenarios

Forest conversion from native deciduous forests to coniferous stands has been performed in many European regions and resulted in dramatic shifts in understorey plant community composition. However, the drivers for changes in specific understorey plant species remained unclear.Here, we experimentally determine the species-specific effects of light availability and chemical soil characteristics, on the vegetative and regenerative performance of five herbaceous forest understorey plants. Topsoil samples from both spruce and deciduous stands at four locations, with two levels of soil acidity, were collected and used in a common garden experiment. Additionally, three different light levels were applied, i.e., ‘light deciduous’, ‘dark deciduous’ (extra light reduction during summer) and ‘evergreen’ (light reduction during winter). In a second experiment we evaluated the germination of two of these species against the acidity and tree species at the site of origin of the soil samples.The light regime affected both the vegetative and regenerative performance of the understorey species: compared to light deciduous, Anemone nemorosa had a significantly lower performance under the evergreen light regime, Convallaria majalis under dark deciduous and Luzula luzuloides and Galium odoratum under both light regimes. The vegetative performance was lower in soil from acid sites for the acid-sensitive species G. odoratum and Primula elatior. Differences between the soils sampled under deciduous or spruce stands had no effect on the vegetative, or the regenerative performance of these species. By contrast, the germination of L. luzuloides and P. elatior was higher in soils sampled in deciduous stands and in neutral sites.Species-specific responses in vegetative and regenerative performance of adult plants to a changed light regime and soil acidification could be a reason for the changed vegetation composition in converted stands. Also lower germination and establishment of forest understorey species in spruce stands could influence the species distribution after conversion.     

Differential anatomical responses to elevated CO2 in saplings of four hardwood species

To determine whether an elevated carbon dioxide concentration ([CO₂]) can induce changes in the wood structure and stem radial growth in forest trees, we investigated the anatomical features of conduit cells and cambial activity in 4‐year‐old saplings of four deciduous broadleaved tree species – two ring‐porous (Quercus mongolica and Kalopanax septemlobus) and two diffuse‐porous species (Betula maximowicziana and Acer mono) – grown for three growing seasons in a free‐air CO₂ enrichment system. Elevated [CO₂] had no effects on vessels, growth and physiological traits of Q. mongolica, whereas tree height, photosynthesis and vessel area tended to increase in K. septemlobus. No effects of [CO₂] on growth, physiological traits and vessels were seen in the two diffuse‐porous woods. Elevated [CO₂] increased larger vessels in all species, except B. maximowicziana and number of cambial cells in two ring‐porous species. Our results showed that the vessel anatomy and radial stem growth of Q. mongolica, B. maximowicziana and A. mono were not affected by elevated [CO₂], although vessel size frequency and cambial activity in Q. mongolica were altered. In contrast, changes in vessel anatomy and cambial activity were induced by elevated [CO₂] in K. septemlobus. The different responses to elevated [CO₂] suggest that the sensitivity of forest trees to CO₂ is species dependent.     

Temporal dynamics and spatial variability in the enhancement of canopy leaf area under elevated atmospheric CO2

Increased canopy leaf area (L) may lead to higher forest productivity and alter processes such as species dynamics and ecosystem mass and energy fluxes. Few CO₂ enrichment studies have been conducted in closed canopy forests and none have shown a sustained enhancement of L. We reconstructed 8 years (1996–2003) of L at Duke's Free Air CO₂ Enrichment experiment to determine the effects of elevated atmospheric CO₂ concentration ([CO₂]) on L before and after canopy closure in a pine forest with a hardwood component, focusing on interactions with temporal variation in water availability and spatial variation in nitrogen (N) supply. The dynamics of L were reconstructed using data on leaf litterfall mass and specific leaf area for hardwoods, and needle litterfall mass and specific leaf area combined with needle elongation rates, and fascicle and shoot counts for pines. The dynamics of pine L production and senescence were unaffected by elevated [CO₂], although L senescence for hardwoods was slowed. Elevated [CO₂] enhanced pine L and the total canopy L (combined pine and hardwood species; P<0.050); on average, enhancement following canopy closure was ～16% and 14% respectively. However, variation in pine L and its response to elevated [CO₂] was not random. Each year pine L under ambient and elevated [CO₂] was spatially correlated to the variability in site nitrogen availability (e.g. r²=0.94 and 0.87 in 2001, when L was highest before declining due to droughts and storms), with the [CO₂]‐induced enhancement increasing with N (P=0.061). Incorporating data on N beyond the range of native fertility, achieved through N fertilization, indicated that pine L had reached the site maximum under elevated [CO₂] where native N was highest. Thus closed canopy pine forests may be able to increase leaf area under elevated [CO₂] in moderate fertility sites, but are unable to respond to [CO₂] in both infertile sites (insufficient resources) and sites having high levels of fertility (maximum utilization of resources). The total canopy L, representing the combined L of pine and hardwood species, was constant across the N gradient under both ambient and elevated [CO₂], generating a constant enhancement of canopy L. Thus, in mixed species stands, L of canopy hardwoods which developed on lower fertility sites (～3 g N inputs m^?2 yr^?1) may be sufficiently enhanced under elevated [CO₂] to compensate for the lack of response in pine L, and generate an appreciable response of total canopy L (～14%).     

Effect of increased Populus cover on Abies regeneration in the Picea–feathermoss boreal forest

Question: Does the increase in Populus tremuloides cover within the Picea mariana–feathermoss domain enhance establishment and growth conditions for Abies balsamea regeneration? Location: Boreal forest of northwest Quebec, Canada. Method: To document the effect of Populus tremuloides on A. balsamea regeneration, mixed stands with a heterogeneous presence of P. tremuloides adjacent to Picea mariana‐dominated stands were selected. Abies balsamea regeneration, understorey environment and canopy composition were characterized from 531 sampling units distributed along transects covering the mixed–coniferous gradient. Abundance of understorey A. balsamea regeneration was described using three height groups: seedling (<30 cm), small sapling (30 to <100 cm) and tall sapling (100 to 300 cm). Growth characteristics were measured from 251 selected individuals of A. balsamea (<3 m). Results: Results showed that A. balsamea regeneration was generally more abundant when P. tremuloides was present in the canopy. Differences between seedling and sapling abundance along the mixed–coniferous gradient suggest that while establishment probably occurs over a wide range of substrates, the better growth conditions found under mixed stands ensure a higher survival rate for A. balsamea seedlings. Conclusions: The abundant A. balsamea regeneration observed within mixed stands of the Picea mariana–feathermoss domain suggests that the increase in P. tremuloides cover, favoured by intensive management practices and climatic change, could contribute to acceleration of the northward expansion of the A. balsamea–Betula papyrifera domain into the northern boreal forest dominated by Picea mariana.     

Growth of Eastern Cottonwoods (Populus deltoides) in elevated [CO2] stimulates stand-level respiration and rhizodeposition of carbohydrates, accelerates soil nutrient depletion, yet stimulates above- and belowground biomass production

We took advantage of the distinctive system‐level measurement capabilities of the Biosphere 2 Laboratory (B2L) to examine the effects of prolonged exposure to elevated [CO₂] on carbon flux dynamics, above‐ and belowground biomass changes, and soil carbon and nutrient capital in plantation forest stands over 4 years. Annually coppiced stands of eastern cottonwoods (Populus deltoides) were grown under ambient (400 ppm) and two levels of elevated (800 and 1200 ppm) atmospheric [CO₂] in carbon and N‐replete soils of the Intensive Forestry Mesocosm in the B2L. The large semiclosed space of B2L uniquely enabled precise CO₂ exchange measurements at the near ecosystem scale. Highly controllable climatic conditions within B2L also allowed for reproducible examination of CO₂ exchange under different scales in space and time. Elevated [CO₂] significantly stimulated whole‐system maximum net CO₂ influx by an average of 21% and 83% in years 3 and 4 of the experiment. Over the 4‐year experiment, cumulative belowground, foliar, and total aboveground biomass increased in both elevated [CO₂] treatments. After 2 years of growth at elevated [CO₂], early season stand respiration was decoupled from CO₂ influx aboveground, presumably because of accelerated fine root production from stored carbohydrates in the coppiced system prior to canopy development and to the increased soil carbohydrate status under elevated [CO₂] treatments. Soil respiration was stimulated by elevated [CO₂] whether measured at the system level in the undisturbed soil block, by soil collars in situ, or by substrate‐induced respiration in vitro. Elevated [CO₂] accelerated depletion of soil nutrients, phosphorus, calcium and potassium, after 3 years of growth, litter removal, and coppicing, especially in the upper soil profile, although total N showed no change. Enhancement of above‐ and belowground biomass production by elevated [CO₂] accelerated carbon cycling through the coppiced system and did not sequester additional carbon in the soil.     

Characteristics of free air carbon dioxide enrichment of a northern temperate mature forest

In 2017, the Birmingham Institute of Forest Research (BIFoR) began to conduct Free Air Carbon Dioxide Enrichment (FACE) within a mature broadleaf deciduous forest situated in the United Kingdom. BIFoR FACE employs large‐scale infrastructure, in the form of lattice towers, forming ‘arrays’ which encircle a forest plot of ~30 m diameter. BIFoR FACE consists of three treatment arrays to elevate local CO₂ concentrations (e[CO₂]) by +150 µmol/mol. In practice, acceptable operational enrichment (ambient [CO₂] + e[CO₂]) is ±20% of the set point 1‐min average target. There are a further three arrays that replicate the infrastructure and deliver ambient air as paired controls for the treatment arrays. For the first growing season with e[CO₂] (April to November 2017), [CO₂] measurements in treatment and control arrays show that the target concentration was successfully delivered, that is: +147 ± 21 µmol/mol (mean ± SD) or 98 ± 14% of set point enrichment target. e[CO₂] treatment was accomplished for 97.7% of the scheduled operation time, with the remaining time lost due to engineering faults (0.6% of the time), CO₂ supply issues (0.6%) or adverse weather conditions (1.1%). CO₂ demand in the facility was driven predominantly by wind speed and the formation of the deciduous canopy. Deviations greater than 10% from the ambient baseline CO₂ occurred <1% of the time in control arrays. Incidences of cross‐contamination >80 µmol/mol (i.e. >53% of the treatment increment) into control arrays accounted for <0.1% of the enrichment period. The median [CO₂] values in reconstructed three‐dimensional [CO₂] fields show enrichment somewhat lower than the target but still well above ambient. The data presented here provide confidence in the facility setup and can be used to guide future next‐generation forest FACE facilities built into tall and complex forest stands.     

Distinct seasonal dynamics of responses to elevated CO2 in two understorey grass species differing in shade‐tolerance

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Canopy manipulations of exotic Bitter Willow (Salix elaeagnos) forest for indigenous seedling recruitment: A pilot study

The invasive exotic tree species Bitter Willow (Salix elaeagnos; Salicaceae) has colonised areas of rank exotic grassland and has been found to contain indigenous seed, dispersed by frugivorous birds into the monospecific stands. This small pilot study examined whether indigenous seedlings that have germinated in the understorey of exotic Bitter Willow stands could be stimulated to establish through the creation of small‐scale canopy gaps. In Bitter Willow forest, four single Bitter Willow trees were poisoned to create canopy gaps. Light transmission and seedling regeneration of tree and shrub species were assessed beneath both the four manipulated and three comparable intact Bitter Willow canopies. Over 3 years, seedling height and density increased more beneath opened compared to intact Bitter Willow canopies. These results suggest that Bitter Willow can fill the roles of both a facilitative nurse and a perch tree. Larger‐scale canopy manipulation experiments of both Bitter Willow and other Salix species are needed to determine the full potential of canopy manipulations for forest restoration.     

Carbon isotope composition of boreal plants: functional grouping of life forms

 We tested the hypothesis that life forms (trees, shrubs, forbs, and mosses; deciduous or evergreen) can be used to group plants with similar physiological characteristics. Carbon isotope ratios (δ¹³C) and carbon isotope discrimination (Δ) were used as functional characteristics because δ¹³C and Δ integrate information about CO₂ and water fluxes, and so are useful in global change and scaling studies. We examined δ¹³C values of the dominant species in three boreal forest ecosystems: wet Picea mariana stands, mesic Populus tremuloides stands, and dry Pinus banksiana stands. Life form groups explained a significant fraction of the variation in leaf carbon isotope composition; seven life-form categories explained 50% of the variation in δ¹³C and 42% of the variation in Δ and 52% of the variance not due to intraspecific genetic differences (n=335). The life forms were ranked in the following order based on their values: evergreen trees<deciduous trees=evergreen and deciduous shrubs=evergreen forbs<deciduous forbs=mosses. This ranking of the life forms differed between deciduous (Populus) and evergreen (Pinus and Picea) ecosystems. Furthermore, life forms in the Populus ecosystem had higher discrimination values than life forms in the dry Pinus ecosystem; the Picea ecosystem had intermediate Δ values. These correlations between Δ and life form were related to differences in plant stature and leaf longevity. Shorter plants had lower Δ values than taller plants, resulting from reduced light intensity at lower levels in the forest. After height differences were accounted for, deciduous leaves had higher discrimination values than evergreen leaves, indicating that deciduous leaves maintained higher ratios of intracellular to ambient CO₂ (c _i/c _a) than did evergreen leaves in a similar environment within these boreal ecosystems. We found the same pattern of carbon isotope discrimination in a year with above-average precipitation as in a year with below-average precipitation, indicating that environmental fluctuations did not affect the ranking of life forms. Furthermore, plants from sites near the northern and southern boundaries of the boreal forest had similar patterns of discrimination. We concluded that life forms are robust indicators of functional groups that are related to carbon and water fluxes within boreal ecosystems. Received: 15 April 1996 / Accepted: 16 November 1996     

Soil CO2 efflux in contrasting boreal deciduous and coniferous stands and its contribution to the ecosystem carbon balance

Similar nonsteady‐state automated chamber systems were used to measure and partition soil CO₂ efflux in contrasting deciduous (trembling aspen) and coniferous (black spruce and jack pine) stands located within 100 km of each other near the southern edge of the Boreal forest in Canada. The stands were exposed to similar climate forcing in 2003, including marked seasonal variations in soil water availability, which provided a unique opportunity to investigate the influence of climate and stand characteristics on soil CO₂ efflux and to quantify its contribution to the net ecosystem CO₂ exchange (NEE) as measured with the eddy‐covariance technique. Partitioning of soil CO₂ efflux between soil respiration (including forest‐floor vegetation) and forest‐floor photosynthesis showed that short‐ and long‐term temporal variations of soil CO₂ efflux were related to the influence of (1) soil temperature and water content on soil respiration and (2) below‐canopy light availability, plant water status and forest‐floor plant species composition on forest‐floor photosynthesis. Overall, the three stands were weak to moderate sinks for CO₂ in 2003 (NEE of ?103, ?80 and ?28 g C m^?2 yr^?1 for aspen, black spruce and jack pine, respectively). Forest‐floor respiration accounted for 86%, 73% and 75% of annual ecosystem respiration, in the three respective stands, while forest‐floor photosynthesis contributed to 11% and 14% of annual gross ecosystem photosynthesis in the black spruce and jack pine stands, respectively. The results emphasize the need to perform concomitant measurements of NEE and soil CO₂ efflux at longer time scales in different ecosystems in order to better understand the impacts of future interannual climate variability and vegetation dynamics associated with climate change on each component of the carbon balance.     

Community dynamics of evergreen broadleaf forests in southwestern Japan

The process and rate of revegetation in gaps in an evergreen oak forest were studied by comparing the species composition, tree density, frequency distribution of tree height, and relation between diameter at breast height and tree height among different aged stands. For estimating stand ages, the ages of gap indicators, such as,Symplocos prunifolia andAcer rufinerve, were very useful. It took about 70 years for gaps to be filled by large fully-grown trees. Since the mean residence time of the forest canopy was 180 years, the trees that attain the forest canopy were expected to be canopy trees for 110 years on the average. Tree densities of all broadleaved evergreens exceptS. prunifolia, were independent of stand age. On the other hand, densities of gap indicators,S. prunifolia andA. rufinerve, decreased as stand age increased. Other deciduous broadleaf and coniferous species were scarce as a whole. According to the frequency distributions of height of live and dead trees in different aged stands, it was suggested that shorter trees were more susceptible to death than taller trees. The self-thinning in revegetation process in gaps approximately followed the 3/2 power law, though the power was larger (−1.32) than expected from the law.