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%).     

Partitioning of dry mass and leaf area within plants of three species grown at elevated CO2

1. We tested the hypothesis that the net partitioning of dry mass and dry mass:area relationships is unaltered when plants are grown at elevated atmospheric CO₂ concentrations.
2. The total dry mass of Dactylis glomerata, Bellis perennis and Trifolium repens was higher for plants in 700 compared to 350 μmol CO₂ mol^–1 when grown hydroponically in controlled-environment cabinets.
3. Shoot:root ratios were higher and leaf area ratios and specific leaf areas lower in all species grown at elevated CO₂. Leaf mass ratio was higher in plants of B. perennis and D. glomerata grown at elevated CO₂.
4. Whilst these data suggest that CO₂ alters the net partitioning of dry mass and dry mass:leaf area relationships, allometric comparisons of the components of dry mass and leaf area suggest at most a small effect of CO₂. CO₂ changed only two of a total of 12 allometric coefficients we calculated for the three species: ν relating shoot to root dry mass was higher in D. glomerata , whilst ν relating leaf area to total dry mass was lower in T. repens .
5. CO₂ alone has very little effect on partitioning when the size of the plant is taken into account.     

Root/Shoot Allocation and Root Architecture in Seedlings: Variation among Forest Sites, Microhabitats, and Ecological Groups1

I analyzed patterns of variation in root mass allocation and root morphology among seedlings of woody species in relation to environmental factors in four Neotropical forests. Among forests, I explored the response of root traits to sites varying in water or nutrient availability. Within each forest, I explored the plastic response of species to different microhabitats: gaps and understory. Additionally, I explored evidence for life history correlation of root and shoot traits by comparing species differing in their successional group (light‐demanding [22 spp.] or shade tolerant [27 spp.]) and germination type (species with photosynthetic cotyledons or species with reserve cotyledons). At each forest site, young seedlings from 10 to 20 species were excavated. A total of 55 species was collected in understory conditions and 31 of them were also collected in gaps. From each seedling, six morphological ratios were determined. Allocation to roots was higher in forest sites with the lowest soil resources. Roots were finer and longer in the most infertile site, while roots were deeper in the site with the longest dry season. Seedling traits did not differ between germination types. Shade tolerant species allocated more to roots and developed thicker roots than light‐demanding species. Light‐demanding species showed stronger plastic responses to habitat than shade tolerant species, and species with photo‐synthetic cotyledons showed lower plasticity than species with reserve cotyledons. Overall, these results suggest that among Neotropical species, root allocation and root morphology of seedlings reflect plant adjustments to water or nutrient availability at geographic and microhabitat scales. In addition, life history specialization to light environments is suggested by differences among groups of species in their allocation to roots and in their root morphology.     

Affinity for CO2 in relation to the ability of freshwater macrophytes to use HCO–3

1. The affinity of photosynthesis for CO₂ is calculated here as the initial slope of net-photosynthetic rate against concentration of CO₂. The affinity for CO₂ for pairs of freshwater macrophytes with similar leaf morphology but able or unable to use HCO^–₃ as a carbon source was compared.
2. Species restricted to CO₂ had a higher affinity for CO₂ than species that were also able to use HCO^–₃ when rates were expressed on the basis of area, dry mass and content of chlorophyll a .
3. Published values for the affinity for CO₂ and the concentration of CO₂ which half-saturated rate of photosynthesis were compiled and compared. Despite a large range of values, affinity for CO₂ was greater for species restricted to CO₂ than for those also able to use HCO^–₃ and statistically different when the slope was expressed on the basis of dry mass and chlorophyll a content.
4. The difference in affinity is consistent with predicted benefits of a high permeability to CO₂ for species relying on passive diffusion of CO₂ and a lower permeability for species able to use HCO^–₃ in order to reduce efflux of CO₂ from a high internal concentration generated by active transport.
5. The implications of the different affinities are discussed in terms of species distribution.     

Seasonal variability in the effect of elevated CO2 on ecosystem leaf area index in a scrub-oak ecosystem

We report effects of elevated atmospheric CO₂ concentration (C_a) on leaf area index (LAI) of a Florida scrub‐oak ecosystem, which had regenerated after fire for between three and five years in open‐top chambers (OTCs) and was yet to reach canopy closure. LAI was measured using four nondestructive methods, calibrated and tested in experiments performed in calibration plots near the OTCs. The four methods were: PAR transmission through the canopy, normalized difference vegetation index (NDVI), hemispherical photography, and allometric relationships between plant stem diameter and plant leaf area. Calibration experiments showed: (1) Leaf area index could be accurately determined from either PAR transmission through the canopy or hemispherical photography. For LAI determined from PAR transmission through the canopy, ecosystem light extinction coefficient (k) varied with season and was best described as a function of PAR transmission through the canopy. (2) A negative exponential function described the relationship between NDVI and LAI; (3) Allometric relationships overestimated LAI. Throughout the two years of this study, LAI was always higher in elevated C_a, rising from, 20% during winter, to 55% during summer. This seasonality was driven by a more rapid development of leaf area during the spring and a relatively greater loss of leaf area during the winter, in elevated C_a. For this scrub‐oak ecosystem prior to canopy closure, increased leaf area was an indirect mechanism by which ecosystem C uptake and canopy N content were increased in elevated C_a. In addition, increased LAI decreased potential reductions in canopy transpiration from decreases in stomatal conductance in elevated C_a. These findings have important implications for biogeochemical cycles of C, N and H₂O in woody ecosystems regenerating from disturbance in elevated C_a.     

湿地松林叶面积指数测算

对湿地松（Pinus elliotii）当年生和多年生两针一束、三针一束叶片的长度、宽度、厚度和重量分别进行量测,据此探讨不同类型叶片的叶形和比叶面积差异,并结合样地调查数据对中科院千烟洲试验站湿地松人工林的叶面积指数进行计算。结果表明：湿地松三针一束叶片合拢后横切面基本呈圆形,当年生叶和多年生叶的平均直径分别为1．688mm和1.706mm;两针一束叶片合拢后从统计学上讲横切面不是圆形,而是椭圆形,叶片厚度方向直径大于宽度方向（当年生叶厚度和宽度方向直径分别为1．580ram和1．422mm,多年生叶分别为1．568mm和1.410mm）,但如果把厚度和宽度方向直径的平均值近似成圆柱体直径计算时误差在3％以内;如果只用厚度或宽度方向直径代表平均直径计算结果会有2％。10％的误差;当年生叶和多年生叶、两针一束叶和三针一束叶之间比叶面积差别很大,计算的三种比叶面积（投影比叶面积、圆柱面比叶面积和比表面积）中,当年生叶的比叶面积明显大于多年生叶,三针一束叶片的投影比叶面积和比表面积都大于两针一束叶片,但圆柱面比叶面积恰好相反。湿地松林的叶面积指数若按投影叶面积算为3．61,按圆柱面的外表面算为5．12,按总表面积的一半算为4．52,比利用冠层分析仪测量的结果略大。     

多效唑在蔓性千斤拔上应用的初步研究

为了解生长调节剂提高蔓性千斤拔产量的方法及机理,进行多效唑对蔓性千斤拔的化控试验。在蔓性千斤拔初花期,设30、60和90株·m~(-2)3个植株密度,用浓度为0、300、600、900 mg·L~(-1)的多效唑对蔓性千斤拔进行叶面喷施,测定叶绿素、叶面积、叶面积指数及药材产量。结果表明,植株密度为60株·m~(-2)、多效唑喷施浓度为600 mg·L~(-1)的处理,蔓性千斤拔叶绿素含量显著高于其它处理,叶面积和叶面积指数适度缩小,药材干重提高了16.7%。初步确定蔓性千斤拔增产效果最好的植株密度为60株·m~(-2)、多效唑喷施浓度为600 mg·L~(-1)。     

喜旱莲子草对喀斯特三种不同生境的可塑性反应

喜旱莲子草自入侵以来在我国各地广为传播蔓延,对人们生产生活产生了重要影响。该研究通过野外采样和室内理化指标分析,探讨喀斯特不同生境中喜旱莲子草的可塑性。结果表明:(1)在三种生境中喜旱莲子草的节间长度、叶面积、单株重和叶干重差异显著。(2)在岩石环境中其无性繁殖器官——茎的投入比例占其生物量的比例最大,其次是行道生境和湘江河道。(3)节间距随环境因子从岩石环境、绿化行道和水生生境的变化,依次增加,叶质比(叶面积/质量)也随水分环境的增加而增加;(4)在岩石环境中,喜旱莲子草通过缩短节间距并增加无性繁殖器官——茎的质量来增加其在水分匮乏生境的繁殖能力。该研究结果为喜旱莲子草今后的防治对策提供了科学依据。     

Shade‐tolerance as a predictor of responses to elevated CO2 in trees

Evidence from 10 studies comparing angiosperm trees and 5 studies comparing conifers of differing shade‐tolerance was analysed. The number of intraphyletic comparisons in which the more shade‐tolerant species showed the greater relative increase of biomass in elevated CO₂ was significantly higher than would be expected by chance alone. It is suggested that more shade‐tolerant species are inherently better disposed, in terms of plant architecture and partitioning of biomass and nitrogen, to utilise resources (light, water, nutrients) that are potentially limiting in elevated CO₂ and that these traits are responsible for the interaction between shade‐tolerance and CO₂ concentration. Compared with less shade‐tolerant angiosperm trees, more shade‐tolerant angiosperm species generally have a lower leaf area ratio in ambient CO₂ and show a smaller relative reduction in elevated CO₂. Furthermore, leaf nitrogen content is usually lower in more shade‐tolerant angiosperm species and tends to be more strongly reduced by elevated CO₂ in those species. Within angiosperm trees, more shade‐tolerant species showed a stronger stimulation of net leaf photosynthetic rate in most experiments, but this trend was not significant.     

广西桉树人工林叶面积指数的估测及其校正

叶面积指数(Leaf area index, LAI)是森林生态系统重要的结构参数,通过遥感技术可反演区域LAI,但其可靠性需要地面准确的实测数据进行验证。选取广西国有高峰林场不同林龄的桉树(Eucalyptus robusta)人工林为对象,以异速生长法(Allometry)为对照,综合利用植物冠层分析仪法(LAI-2200)、跟踪辐射和冠层结构分析仪法(TRAC)、半球摄影法(DHP)以及地基激光雷达法(TLS)等间接法估测样地的LAI,并考虑木质成分以及聚集效应影响,进行相应的校正处理,为地面快速、准确测量桉树人工林LAI提供参考。结果表明：桉树人工林的比叶面积为125.37±13.38 cm~2/g,通过Allometry获得的LAI变化范围在1.65—3.84,平均为2.73,不同林龄间的差异均显著(P<0.05),随着林龄的增加呈现先增加后减少的趋势。在未校正情况下,LAI-2200、TRAC、DHP、TLS估算的LAI存在显著差异(P<0.05)。与对照相比,LAI-2200在幼龄林和过熟林中估算误差最小,TRAC在成熟林中估算误差最小。相对于完全去除法,利用...     

Leaf area is stimulated in Populus by free air CO2 enrichment (POPFACE), through increased cell expansion and production

The effects of free‐air CO₂ enrichment (FACE) on leaf growth in Populus, was studied. For the first time in field conditions, both the production and expansion of leaf cells were shown to be sensitive to atmospheric carbon dioxide. Leaf area expansion rate and final leaf size were stimulated under FACE for three species (Populus x euramericana (I‐214), P. nigra (Jean Pourtet) and P. alba (2AS‐11), with the largest effect observed for P. x euramericana (61%). In this species and in P. nigra, both epidermal cell size and cell number were increased, whereas for P. alba, only cell production was increased in FACE. Two findings suggest that changes in the cell wall may be important in explaining larger leaf cells in FACE: (i) Leaf cell wall extensibility of rapidly growing leaves increased in all species in FACE; and (ii) an increase in xyloglucan endotransglycosylase activity, a cell wall‐loosening enzyme, was increased in FACE and associated with leaf growth rate. The results suggest that the mechanisms by which FACE promotes leaf growth differ, depending on species. Despite this, increases in final leaf size provide an important component driving increased biomass accumulation in POPFACE, during this first year of rapid growth, prior to canopy closure. The question as to whether these effects are the result of a direct response to CO₂, or are driven indirectly through substrate availability remains unresolved, although evidence from the literature suggests that the latter mechanism is most likely.     

Decline of photosynthetic capacity with leaf age in relation to leaf longevities for five tropical canopy tree species

The effect of leaf aging on photosynthetic capacities was examined for upper canopy leaves of five tropical tree species in a seasonally dry forest in Panama. These species varied in mean leaf longevity between 174 and 315 d, and in maximum leaf life span between 304 and 679 d. The light-saturated CO2 exchange rates of leaves produced during the primary annual leaf flush measured at 7-8 mo of age were 33-65% of the rates measured at 1-2 mo of age for species with leaf life span of < 1 yr. The negative regression slopes of photosynthetic capacity against leaf age were steeper for species with shorter maximum leaf longevity. In all species, regression slopes were less steep than the slopes predicted by assuming a linear decline toward the maximum leaf age (20-80% of the predicted decline rate). Maximum oxygen evolution rates and leaf nitrogen content declined faster with age for species with shorter leaf life spans. Statistical significance of regression slopes of oxygen evolution rates against leaf age was strongest on a leaf mass basis (r = 0.49-0.87), followed by leaf nitrogen basis (r = 0.48-0.77), and weakest on a leaf area basis (r = 0.35-0.70).     

桂北岩溶区青冈栎-青檀群落植物叶片比叶面积研究

为揭示岩溶地区植物叶片比叶面积变化规律和叶片形态之间的相关关系,研究了桂北岩溶区青冈栎-青檀群落的叶片长/宽(LL/LW)、叶片厚度(LT)和比叶面积(SLA)及其之间的关系。结果表明:群落中常绿树种的LL/LW和LT显著高于落叶树种,而常绿树种的SLA显著低于落叶树种;乔木和灌木之间的LL/LW存在显著差异,但SLA和LT不存在显著差异。总体上看,SLA与LL/LW、LT之间是显著负相关关系,SLA随着LL/LW、LT的变大而逐渐减小。但LT与SLA负相关趋势比LL/LW与SLA的明显。因此,LT比LL/LW对叶片SLA的影响大,其明显影响了叶片的SLA。SLA的大小是衡量叶片获取光照能力的指标,因此岩溶区植物较厚的叶片对光照的获取能力具有较大的影响,并可导致光合作用效率的降低。     

A Comparison of Photosynthesis in Endangered and Non-Endangered Plants Changium smyrnioides and Anthriscus sylvestris

Changium smyrnioides Wolff. and Anthriscus sylvestris (L.) Hoffm. have similar photosynthetic characters; they use radiant energy in winter and early spring effectively, but cannot take full advantage of higher irradiance after spring. The specific leaf area (SLA), leaf area ratio (LAR), and leaf mass ratio (LMR) of C. smyrnioides were lower than those of A. sylvestris. The photosynthetic period of C smyrnioides was about 160 d shorter than that of A. sylvestris, causing the total photosynthetic production of C. smyrnioides to be lower than that of A. sylvestris. Hence if C. smyrnioides is disturbed, it could not recover within a short period.     

Scale‐dependent trait correlations in a temperate tree community

Recent investigations of relationships among plant traits have generated important insights into plant form and function. However, relationships involving leaf area, leaf shape and plant height remain poorly resolved. Previous work has also focused on correlations between average trait values for individual species. It is unclear whether similar relationships occur within species. We searched for novel plant trait correlations by comparing leaf area, leaf circularity, specific leaf area (SLA) and plant height among 16 common woody plant species from a temperate forest in New Zealand. Analyses were conducted both within species (intra‐specifically) and among species (inter‐specifically) to determine whether trait correlations were scale dependent. Leaf area was unrelated to other leaf traits inter‐specifically. However, leaf area declined with plant height and increased with SLA intra‐specifically. Leaf circularity decreased with plant height inter‐specifically, but increased with plant height intra‐specifically. SLA increased with plant height both inter‐ and intra‐specifically. Leaf circularity increased with SLA inter‐specifically, but decreased with SLA intra‐specifically. Overall results showed that leaf shape, SLA and plant height are interrelated. However, intra‐specific relationships often differed substantially from inter‐specific relationships, suggesting that the processes shaping relationships between this suite of plant traits are scale‐dependent.     

Functional diversity of photosynthesis during drought in a model tropical rainforest – the contributions of leaf area, photosynthetic electron transport and stomatal conductance to reduction in net ecosystem carbon exchange

The tropical rainforest mesocosm within the Biosphere 2 Laboratory, a model system of some 110 species developed over 12 years under controlled environmental conditions, has been subjected to a series of comparable drought experiments during 2000–2002. In each study, the mesocosm was subjected to a 4–6 week drought, with well‐defined rainfall events before and after the treatment. Ecosystem CO₂ uptake rate (A_eco) declined 32% in response to the drought, with changes occurring within days and being reversible within weeks, even though the deeper soil layers did not become significantly drier and leaf‐level water status of most large trees was not greatly affected. The reduced A_eco during the drought reflected both morphological and physiological responses. It is estimated that the drought‐induced 32% reduction of A_eco has three principal components: (1) leaf fall increased two‐fold whereas leaf expansion growth of some canopy dominants declined to 60%, leading to a 10% decrease in foliage coverage of the canopy. This might be the main reason for the persistent reduction of A_eco after rewatering. (2) The maximum photosynthetic electron transport rate at high light intensities in remaining leaves was reduced to 71% for three of the four species measured, even though no chronic photo‐inhibition occurred. (3) Stomata closed, leading to a reduced ecosystem water conductance to water vapour (33% of pre‐drought values), which not only reduced ecosystem carbon uptake rate, but may also have implications for water and energy budgets of tropical ecosystems. Additionally, individual rainforest trees responded differently, expressing different levels of stress and stress avoiding mechanisms. This functional diversity renders the individual response heterogeneous and has fundamental implications to scale leaf level responses to ecosystem dynamics.     

浙江天童常绿植物当年生与往年生叶片性状的变异与关联

植物叶片性状随叶龄的变化是植物生活史策略的体现, 反映了植物叶片的物质投资和分配方式。该研究通过在个体和物种2个水平, 比较浙江天童1 hm ²样地内常绿阔叶树种的平均叶面积(MLA)、比叶面积(SLA)和叶片干物质含量(LDMC)在当年生和往年生叶片间的差异和关联, 探究叶片物质分配策略在异龄叶间的变化, 并分析叶龄对植物叶片性状, 特别是叶片面积建成消耗的影响。结果显示: 1)在个体和物种水平上, MLA变异系数最大(个体: 79.5%; 物种: 66.5%), SLA次之(个体: 28.1%; 物种: 24.7%), LDMC较低(个体: 17.0%, 物种: 14.1%); 当年生叶片MLA、LDMC和SLA的变异系数均高于往年生叶片; 2)往年生叶MLA显著大于当年生叶(t = -38.53, p < 0.001), 往年生叶SLA显著小于当年生叶(t = 45.30, p < 0.001), 往年生叶LDMC显著大于当年生叶(t = -9.71, p < 0.001); 3)在个体水平, 当年生叶片MLA、SLA和LDMC值分别解释了往年生叶片MLA、SLA和LDMC变异的86%、48%和41%; 在物种水平, 当年生叶片MLA、SLA和LDMC值分别解释了往年生叶片MLA、SLA和LDMC变异的97%、83%和85%; 4) SLA在异龄叶间的变化表明, 与往年生叶片相比, 投资相同干物质, 当年生叶片可形成较大的叶面积, 其叶片面积建成消耗较小。研究认为, 植物叶性状在异龄叶间具有较大的变异性和关联性, 叶面积形成过程中生物量建成与消耗的协调可能影响植物叶片的发育。