Ecosystem carbon storage and soil organic carbon stability in pure and mixed stands of Cunninghamia lanceolata and Michelia macclurei

Background and aims

Across the world, about 264 million ha forest plantations are monospecific. This practice has been found to cause site productivity and soil fertility decline in the regions where forests have been harvested several times. To mitigate these problems, mixed-species plantations, especially with broadleaved and coniferous species, are preferred. Understanding the effects of introducing broadleaved tree in monospecific coniferous plantation on ecosystem carbon (C) storage and soil organic C (SOC) stability is critical to improve our understanding of forest C sequestration and C cycle.

Methods

Plots were established in subtropical plantations with a randomized block design to examine the influence of introducing Michelia macclurei trees into pure Cunninghamia lanceolata plantation on biomass C storage, SOC storage of total, labile, and recalcitrant fractions (0–40 cm depth), and SOC stability.

Results

Introducing M. macclurei trees increased biomass C by 17.9 % and 14.2 % compared with monospecific C. lanceolata and M. macclurei plantations, respectively. Storage of different SOC fractions was not significantly different between monospecific C. lanceolata and mixed plantations. SOC stability in bulk soils was not affected, although it differed in 10–20 cm and 20–40 cm soil depth among three plantations.

Conclusions

Mixed species plantations can increase C sequestration, and in the subtropical forest ecosystem examined this was mainly attributed to an increase in biomass C.     

柠条人工林幼林与成林细根动态比较研究

以晋西北黄土高原区柠条(Caragana korshinskii)人工林幼林和成林为研究对象,应用微根管技术(Minirhizotron technique)对林地0-100 cm土壤剖面的柠条细根生长动态进行了观测.以2007年生长季(4-9月)观测数据为基础,对两林地的柠条细根生长速率(G,mm cm-3 d-1)和细根死亡速率(M,mm cm-3 d-1)的时空变化格局及其与气温、降水、土壤温度和土壤水分等环境因子的关系进行了研究.结果表明,在年生长季,幼林的G(0.1264 mm cm-3 d-1)和M(0.0354 mm cm-3 d-1)均高于成林(分别为0.0914 mm cm-3 d-1和0.0220 mm cm-3 d-1).在垂直分布上,幼林G出现最大值的土层深度(70-80 cm)较成林(50-60 cm)为深.两林地的G和M具有相似的季节变化特点,即G在4月到7月之间缓慢增大,8月迅速达到峰值,之后迅速减小;M自4月至9月M呈持续增高趋势.配对数据t检验结果显示,幼林与成林的C没有显著差异(P＞0.05),而幼林的M显著高于成林(P＜0.05).Pearson相关系数表明,幼林和成林G的垂直分布与土壤温度和土壤水分的垂直变化没有显著相关性;但是幼林和成林M的垂直分布与土壤温度的相关性显著(幼林地P＜0.01;成林地P＜0.05).在年生长季,幼林G与气温和土壤温度具有显著正相关性(与气温的P＜0.01;与土壤温度的P＜0.05);而成林G与各环境因子的相关性则均不显著(P＞0.05).两林地的M与各环境因子的相关性均不显著(P＞0.05).     

Escape from water or remain quiescent? Lotus tenuis changes its strategy depending on depth of submergence

Background and Aims

Two main strategies that allow plants to cope with soil waterlogging or deeper submergence are: (1) escaping by means of upward shoot elongation or (2) remaining quiescent underwater. This study investigates these strategies in Lotus tenuis, a forage legume of increasing importance in areas prone to soil waterlogging, shallow submergence or complete submergence.

Methods

Plants of L. tenuis were subjected for 30 d to well-drained (control), waterlogged (water-saturated soil), partially submerged (6 cm water depth) and completely submerged conditions. Plant responses assessed were tissue porosity, shoot number and length, biomass and utilization of water-soluble carbohydrates (WSCs) and starch in the crown.

Key Results

Lotus tenuis adjusted its strategy depending on the depth of submergence. Root growth of partially submerged plants ceased and carbon allocation prioritized shoot lengthening (32 cm vs. 24·5 cm under other treatments), without depleting carbohydrate reserves to sustain the faster growth. These plants also developed more shoot and root porosity. In contrast, completely submerged plants became quiescent, with no associated biomass accumulation, new shoot production or shoot elongation. In addition, tissue porosity was not enhanced. The survival of completely submerged plants is attributed to consumption of WSCs and starch reserves from crowns (concentrations 50–75 % less than in other treatments).

Conclusions

The forage legume L. tenuis has the flexibility either to escape from partial submergence by elongating its shoot more vigorously to avoid becoming totally submerged or to adopt a non-elongating quiescent strategy when completely immersed that is based on utilizing stored reserves. The possession of these alternative survival strategies helps to explain the success of L. tenuis in environments subjected to unpredictable flooding depths.     

The influence of weed control on foliar δ<Superscript>15</Superscript>N, δ<Superscript>13</Superscript>C and tree growth in an 8 year-old exotic pine plantation of subtropical Australia

Background and aims

The aim of weed control and fertilization in forest plantations was to increase tree growth by reducing competition for available nutrients and water. However, treatments that influence weed biomass can also have significant impacts on soil carbon (C) and nitrogen (N) cycling which can in turn lead to changes in the dynamics of stable C (δ¹³C) and N (δ¹⁵N) isotope compositions in soils and tree foliage.

Methods

We examined the key C and N cycling processes influenced by routine and luxury weed control and fertilization treatments as reflected by soil and foliar δ¹³C and δ¹⁵N and long-term tree growth in an 8-year old F₁ hybrid pine (Pinus elliottii x P. caribaea) plantation in southeast Queensland, Australia. Weed control treatments varied by treatment frequency and intensity while fertilization treatments varied by the application of N, phosphorus (P), potassium (K) and micronutrients. Different soil and canopy sampling positions were assessed to determine if sampling position enhanced the relationships among soil N transformations and tree N use, water use efficiency and carbon gain under the early establishment silviculture.

Results

Routine weed control was associated with increased weed biomass returned to the soil, compared with luxury weed control. Soil δ¹³C increased at the 0–5 cm soil sampling depth in both the inter-planting (IPR) and planting row (PR) as a result of the routine weed control treatments. In addition, soil δ¹³C was significantly higher as a result of fertilisation treatment in the 0–5 cm soil sampling depth in the PR. Soil δ¹³C was negatively correlated to soil δ¹⁵N at the 0–5 cm soil sampling depth in the IPR. Soil δ¹⁵N increased in the 0–5 and 5–10 cm soil sampling depths in the IPR, as a result of more frequent (luxury) weed control. Foliar δ¹⁵N and tree water use efficiency (WUE) (as indicated by foliar δ¹³C) were positively correlated with tree growth at age 8 years. While relationships between δ¹³C and δ¹⁵N in the soil and foliage varied depending on soil sampling depth and position, and with canopy sampling position where there were consistent relationships between soil δ¹³C (or δ¹⁵N) and foliar δ¹⁵N.

Conclusions

This study demonstrates how early establishment silviculture has important implications for soil C and N cycling and how soil δ¹³C and δ¹⁵N were consistent with changes in soil C cycling and N transformations as a result of weed control treatments, while foliar δ¹⁵N was linked to more rapid N cycling as reflected in the soil δ¹⁵N, which increased tree growth and tree WUE (as reflected by foliar δ¹³C).

     

黄土高原水土保持林对土壤水分的影响

黄土高原植被恢复的限制因素主要是土壤水分,植被与土壤水分关系的研究对黄土高原植被恢复具有重要意义.2008年7月1日至2009年10月31日间采用EnviroSMART土壤水分定位监测系统以每30min监测1次的频度,对晋西黄土区刺槐人工林地、油松人工林地、次生林地的土壤水分变化进行了研究.研究得出:次生林地0-150 cm土层中平均蓄水量为331.95mm,刺槐人工林地为233.85 mm,有整地措施的油松人工林地为314.85mm,刺槐人工林比次生林多消耗的98.10mm土壤水分主要来源于80 cm以下土层.次生林主要消耗0-80 cm土层的水分,而人工林不但对0-80 cm土层水分的消耗量大于次生林,对深层土壤的消耗也较次生林大,这将有可能导致人工林地深层土壤的“干化”.在土壤水分减少期(11-1月)刺槐人工林土壤水分的日均损耗量为0.86mm、油松人工林为0.82 mm、次生林为0.84 mm.土壤水分缓慢恢复期(2-5月)刺槐人工林地土壤水分的恢复速度0.90mm/d,油松人工林地为0.53 mm/d、次生林地为0.79 mm/d.土壤水分剧烈变化期(5-10月)刺槐人工林地土壤水分含量的极差为95.71mm,油松人工林地为179.1mm,次生林地为72.03mm.在干旱少雨的黄土高原进行植被恢复时,应多采取封山育林等方式,依靠自然力量形成能够与当地土壤水资源相协调的次生林,是防止人工植被过度耗水形成“干化层”、保障水土保持植被持续发挥生态服务功能的关键.     

四川理县杂谷脑干旱河谷岷江柏造林恢复效果评价

在岷江干旱河谷杂古脑河流域,选择岷江柏(Cupressus chengiana)5个不同年份的造林地,调查了岷江柏的生长情况、植物群落特征与土壤理化性质,总体评价干旱河谷乡土树种的造林成效及造林后的生态效果。结果表明:不同年份造林地岷江柏幼树生长状况良好,2003年造林地生长最好,基径、树高和冠径分别达到4.39cm、4.17m和1.01m,当年生长量超过43cm;2001年和2005年造林地次之,基径为4.19cm和4.52cm,冠径为0.55m和0.61m,当年生长量37cm左右,树高为2.50和2.17m;2007年和2009年造林地由于幼树生长时间较短,其基径、树高和冠径都较小,但当年生长量也都超过20cm。不同年份造林地的植被覆盖度变化相对复杂,除2001年造林地外,灌木层盖度随造林时间的增加而逐渐降低;草本层盖度变化较小,2005造林地最高,其他年份造林地没有显著差异。同时,不同年份造林地的群落结构都比较单一,随造林时间的增长,群落物种数量反而降低。在5个不同年份造林地,土壤pH、有机质和养分含量差异相对较小,土壤含水量尽管存在较大差异,但没有明显的变化趋势。综合分析认为,在杂谷脑干旱河谷中山区岷江柏作为造林树种具有一定的生长优势,但造林地的植被和土壤并没有得到改善,需要更加长期的观测评估。     

Is the simple auger coring method reliable for below-ground standing biomass estimation in Eucalyptus forest plantations?

Background and Aims

Despite their importance for plant production, estimations of below-ground biomass and its distribution in the soil are still difficult and time consuming, and no single reliable methodology is available for different root types. To identify the best method for root biomass estimations, four different methods, with labour requirements, were tested at the same location.

Methods

The four methods, applied in a 6-year-old Eucalyptus plantation in Congo, were based on different soil sampling volumes: auger (8 cm in diameter), monolith (25 × 25 cm quadrate), half Voronoi trench (1·5 m³) and a full Voronoi trench (3 m³), chosen as the reference method.

Key Results

With the reference method (0–1m deep), fine-root biomass (FRB, diameter <2 mm) was estimated at 1·8 t ha⁻¹, medium-root biomass (MRB diameter 2–10 mm) at 2·0 t ha⁻¹, coarse-root biomass (CRB, diameter >10 mm) at 5·6 t ha⁻¹ and stump biomass at 6·8 t ha⁻¹. Total below-ground biomass was estimated at 16·2 t ha⁻¹ (root : shoot ratio equal to 0·23) for this 800 tree ha⁻¹ eucalypt plantation density. The density of FRB was very high (0·56 t ha⁻¹) in the top soil horizon (0–3 cm layer) and decreased greatly (0·3 t ha⁻¹) with depth (50–100 cm). Without labour requirement considerations, no significant differences were found between the four methods for FRB and MRB; however, CRB was better estimated by the half and full Voronoi trenches. When labour requirements were considered, the most effective method was auger coring for FRB, whereas the half and full Voronoi trenches were the most appropriate methods for MRB and CRB, respectively.

Conclusions

As CRB combined with stumps amounted to 78 % of total below-ground biomass, a full Voronoi trench is strongly recommended when estimating total standing root biomass. Conversely, for FRB estimation, auger coring is recommended with a design pattern accounting for the spatial variability of fine-root distribution.     

树种组成对北亚热带11年生常绿阔叶人工林碳储量的影响

以中国北亚热带退化灌木林改造而来的木荷-青冈栎混交林和杜英纯林为对象,研究树种组成对常绿阔叶人工林生态系统碳储量的影响。结果表明:(1)退化灌木林改造成两种人工林生长11年后,生态系统植被、土壤碳储量均显著增加;植被碳储量的增加主要来自乔木层。(2)两种人工林碳积累能力有差异。杜英林植被碳储量比木荷-青冈栎林高99.4%,其中杜英林的乔木层碳储量比木荷-青冈栎林高27.75t·hm-2,是后者的2倍;杜英林土壤有机碳储量(0~50cm)显著高于木荷-青冈栎林10.17t·hm-2,其中在0~10、20~30cm土层杜英林均显著高于木荷-青冈栎林。研究表明,退化灌木林人工改造成常绿阔叶林后生态系统碳储量显著增加,杜英纯林碳蓄积能力明显高于木荷-青冈栎混交林,说明在以增加碳储量为目的的退化生态系统改造过程中,树种选择非常重要。     

Effects of drip irrigation on deep root distribution, rooting depth, and soil water profile of jujube in a semiarid region

Aims

Aimed to understand how soil water was depleted by deep roots, the effects of drip irrigation and stand age on the deep root distribution, rooting depth, and soil water profile dynamics were investigated in a jujube (Ziziphus jujube Mill. CV. Lizao) plantation.

Methods

A soil coring method with a LuoYang shovel was used for sampling until no more roots were found.

Results

It showed that the maximum fine rooting depth (<2 mm in diameter) increased with stand age and it extended deep into the soil rapidly during the first 4 years, but more slowly in the subsequent 4 years. The maximum rooting depth reached 5 m in a 9-year-old jujube plantation, but it stabilized and did not increase thereafter. However, it was 10 m in a 12-year-old jujube plantation that lacked irrigation.

Conclusions

We found that the application of 33.3 mm of irrigation water (equivalent to 7 % of the local annual precipitation) could halve the maximum rooting depth, thereby reducing deep soil water depletion. Our results showed that a low-volume water supply reduced the maximum rooting depth in jujube and prevented the depletion of the deep soil water. Appropriate drip irrigation is an effective water management strategy for sustainable artificial forest development in semiarid regions.     

Biomass Accumulation and Carbon Sequestration in Four Different Aged Casuarina equisetifolia Coastal Shelterbelt Plantations in South China

Thousands of kilometers of shelterbelt plantations of Casuarina equisetifolia have been planted to protect the southeast coastline of China. These plantations also play an important role in the regional carbon (C) cycling. In this study, we examined plant biomass increment and C accumulation in four different aged C. equisetifolia plantations in sandy beaches in South China. The C accumulated in the C. equisetifolia plant biomass increased markedly with stand age. The annual rate of C accumulation in the C. equisetifolia plant biomass during 0–3, 3–6, 6–13 and 13–18 years stage was 2.9, 8.2, 4.2 and 1.0 Mg C ha⁻¹ yr⁻¹, respectively. Soil organic C (SOC) at the top 1 m soil layer in these plantations was 17.74, 5.14, 6.93, and 11.87 Mg C ha⁻¹, respectively, with SOC density decreasing with increasing soil depth. Total C storage in the plantation ecosystem averaged 26.57, 38.50, 69.78, and 79.79 Mg C ha⁻¹ in the 3, 6, 13 and 18- yrs plantation, with most of the C accumulated in the aboveground biomass rather than in the belowground root biomass and soil organic C. Though our results suggest that C. equisetifolia plantations have the characteristics of fast growth, high biomass accumulation, and the potential of high C sequestration despite planting in poor soil conditions, the interactive effects of soil condition, natural disturbance, and human policies on the ecosystem health of the plantation need to be further studied to fully realize the ecological and social benefits of the C equisetifolia shelterbelt forests in South China.     

Plant and soil seed bank diversity across a range of ages of Eucalyptus grandis plantations afforested on arable lands

Aims

Plantation forests are often assumed to have reduced biodiversity relative to unmanaged forests. However, existing knowledge is based on studies of rotation-aged tree crops. We investigated how Eucalyptus afforestation of agricultural land affected plant species composition and biodiversity across a range of plantation ages (1–10 years). We also studied whether the soil seed bank could contribute to regeneration of existing vegetation in such plantations.

Methods

We used a chronosequence approach to evaluate plant and seed species composition and diversity in forests and soil seed banks. We also quantified the similarity of seed banks and aboveground vegetation within plantation sites of a given age. Plantation sites were also compared to a nearby, mature pine forest.

Results

Total plant species number, density and diversity in Eucalyptus grandis plantations increased for the first 3 years plantation establishment, then stabilized or decreased for the next 1–2 years and then increase significantly over the following years. Species number and density in soil seed bank increased significantly with plantation age only after an initial 6-year decrease. Shannon–Wiener index of total species diversity did not significantly differ with plantation age. The understory vegetation and soil seed bank were dominated by pioneer species in the first 3 years, but intermediate-successional and shade-tolerant species gradually invaded as plantations developed further. After 7 years, E. grandis plantation understories were composed of mainly shade-tolerant species. Nevertheless, the diversity of the diversity of intermediate-successional in soil seed banks were higher than that of shade-tolerant species in soil seed banks at this age range (7–10 year). Among species successfully germinated from soil seed banks, 48 % were not found in the aboveground plant community. Similarities between the species in the soil seed bank and the aboveground vegetation were low for both plantation and control forests and did not significantly change with plantation ages.

Conclusions

E. grandis likely produces a changing microclimate during plantation development, which in turn drives composition and diversity dynamics in understory vegetation and soil seed banks after the afforestation of agricultural land. The first 4 years after plantation establishment is associated with lower plant and soil seed bank diversity, meriting a greater focus on biodiversity stabilization and possibly longer rotation periods.     

黄土丘陵区刺槐与油松人工林生态系统生态化学计量特征

为阐明不同人工林生态系统间生态化学计量特征的差异,采用野外采样与室内分析相结合的方式分析了陕北黄土丘陵区落叶阔叶树种刺槐和常绿针叶树种油松人工林乔木、灌草、枯落物和土壤(土层深度0—100cm)C、N、P化学计量特征。结果表明:1)刺槐乔木各器官(叶、枝、干、皮、根)C含量显著低于油松,但N和P含量显著高于油松。因此,油松的C∶N和C∶P显著大于刺槐,而N∶P小于刺槐。2)刺槐林下枯落物N和P含量显著高于油松,但C含量显著小于油松。此外,油松林下枯落物C∶N(70.21)大于刺槐林下枯落物C∶N(19.71),说明油松林下枯落物分解较慢,有利于养分的存储。3)刺槐和油松人工林土壤C、N含量均随土壤深度增加而减少,P含量则基本保持不变。刺槐人工林土壤中C含量低于油松,N、P含量在两者之间无显著差异。4)刺槐人工林内乔灌草叶、枯落物与土壤C、N、P及其计量比的相关性多集中在10—20、20—30cm土层,而油松林中各组分与土壤营养元素的相关性相对较小,其中20—30cm土层中无显著相关性,说明相比刺槐人工林而言,油松人工林内土壤层N、P供应量对植物叶片N、P含量影响不显著。本研究为深入了解黄土丘陵区生态系统养分耦合循环机制奠定了基础,同时也为黄土丘陵区的植被恢复工作提供了一定的指导意义。     

Harvest residue management effects on tree growth and ecosystem carbon in a Chinese fir plantation in subtropical China

Aims

This study aimed to determine the influence of different harvest residue management strategies on tree growth, soil carbon (C) concentrations, soil nitrogen (N) availability and ecosystem C stocks 15 years after replanting second rotation Chinese fir (Cunninghamia lanceolata), an important plantation species in subtropical China. Such information is needed for designing improved management strategies for reforestation programmes in subtropical environments aimed at mitigating CO₂ emissions.

Methods

Four harvest residue management treatments including slash burning, whole tree, stem-only and double residue retention were applied to sixteen 20 m?×?30 m plots in a randomized complete block design with four replicates. Tree growth was measured annually and soil properties were measured at 3 year intervals over a 15 year period after re-planting.

Results

Cumulative diameter growth at age 15 years was significantly smaller in the slash burning than the whole tree and double residue harvest treatments. Hot water extractable N concentrations increased with the increased organic residue retention levels and significant differences were observed between double residue and slash burning treatments. Harvest residue management had no significant effect on the soil C concentrations to 40 cm depth. ANOVA showed that harvest residue management had no significant effect on total biomass carbon at age 15, but the plantation ecosystem (soil C at 0–40 cm depth plus forest biomass C) had significantly lower C mass in the slash burning treatment compared with whole tree, stem only harvest and double residue harvest treatments.

Conclusions

These observations suggest that organic residue retention during the harvesting could improve the growth and ecosystem C stocks of Chinese fir in second rotation forest plantations in subtropical China and highlight the importance of viewing the ecosystem as a whole when evaluating the impact of harvest residue management on C stocks.     

Does tree species composition affect soil CO<Subscript>2</Subscript> emission and soil organic carbon storage in plantations?

Key message

Mixed tree plantations are potential silvicultural systems to increase soil carbon storage through altering litter and root inputs and soil physiochemical properties.

Abstract

Afforestation and reforestation are major strategies for global climate change mitigation. Different tree species composition can induce diverse changes in soil CO₂ emission and soil carbon sequestration in tree plantation. This study employed three plantations of monoculture and mixed Pinus yunnanensis and Eucalyptus globulus to estimate the effect of tree species composition on soil CO₂ emission and soil organic carbon storage in subtropical China. We found that tree species composition had a significant effect on the soil CO₂ emission and soil organic carbon storage. Soil CO₂ emission was lower in the mixed plantation than in the P. yunnanensis plantation, whereas it was higher than in the E. globulus plantation. Differences in soil CO₂ emission among the three plantations were determined by leaf litterfall mass, fine root biomass, soil available nitrogen, pH, soil bulk density, and soil C:N ratio. Soil organic carbon storage was 34.5 and 23.2 % higher in the mixed plantation than in the P. yunnanensis and E. globulus plantations, respectively. Higher soil organic carbon stock in the mixed plantation was attributed to lower C/N ratio of leaf litter and soil, greater fine root biomass and soil organic carbon content, and lower soil CO₂ emission. We conclude that mixed tree plantation can enhance soil carbon sequestration, but can decrease or increase soil CO₂ emission through altering litter and root inputs and soil physiochemical properties.

     

采伐剩余物管理措施对二代杉木人工林土壤全碳、全氮含量的长期效应

根据福建省南平市峡阳国有林场二代杉木(Cunninghamia lanceolata(Lamb.) Hook)人工林5种采伐剩余物管理措施(收获采伐剩余物和地被层、全树收获、仅收获树干和树皮以及加倍采伐剩余物、炼山)下0-40 cm深度土壤全碳、全氮含量15a的监测数据,研究了采伐剩余物管理措施对杉木林土壤碳氮含量的影响.结果显示,加倍采伐剩余物处理样地5次取样年份(造林第3年、第6年、第9年、第12年和第15年)0-10 cm土层土壤全碳、全氮含量均高于其他处理样地,但单因素方差分析显示,采伐剩余物管理措施在5次取样年份对0-10 cm、10-20 cm和20-40 cm土层全碳、全氮含量均没有显著影响(P＞0.05).重复测量方差分析显示,杉木造林15a期间土壤全碳、全氮含量随年份显著变化(P＜0.01),但处理措施以及处理措施与取样年份的交互作用对3个土层土壤全碳、全氮含量影响不显著(P＞0.05).杉木林15年生时,不同处理样地3个土层碳储量差异不显著(P＞0.05),0-40 cm土层平均值为88.71 Mg/hm2.表明采伐剩余物管理措施对亚热带杉木人工林土壤全碳、全氮含量的长期效应并不显著.     

Effects of Water Level on Three Wetlands Soil Seed Banks on the Tibetan Plateau

Background

Although the effect of water level on germination in soil seed banks has been documented in many ecosystems, the mechanism is not fully understood, and to date no empirical studies on this subject exist. Further, no work has been done on the effect of water level on seed banks of drying and saline-alkaline wetlands in alpine areas on the Tibetan Plateau.

Methodology

We examined the effects of water level (0 cm, 5 cm and 10 cm) on seed germination and seedling establishment from soil seed banks at 0–5 cm and 5–10 cm depths in typical, drying, and saline-alkaline wetlands. We also explore the potential role of soil seed bank in restoration of drying and saline-alkaline wetlands.

Principal Findings

Species richness decreased with increase in water level, but there almost no change in seed density. A huge difference exists in species composition of the seed bank among different water levels in all three wetlands, especially between 0 cm and 5 cm and 0 cm and 10 cm. Similarity of species composition between seed bank and plant community was higher in 0 cm water level in drying wetland than in the other two wetlands. The similarity was much higher in 0 cm water level than in 5 cm and 10 cm water levels in all three wetlands. Species composition of the alpine wetland plant community changed significantly after drying and salinization, however, species composition of the seed bank was unchanged regardless of the environment change.

Conclusions/Significance

Water level greatly affects seed bank recruitment and plant community establishment. Further, different water levels in restored habitats are likely to determine its species composition of the plant community. The seed bank is important in restoration of degraded wetlands. Successful restoration of drying and salinization wetlands could depend on the seed bank.     

木麻黄纯林及其混交林对土壤剖面理化性质的影响

为明确混交林对木麻黄林地土壤肥力的改善作用,该文选取海南岛北部滨海沙地木麻黄纯林、木麻黄-琼崖海棠混交林、木麻黄-大叶相思混交林3种林分类型,通过采集土壤剖面样品,分析腐殖质层、0～100 cm土壤各层次的理化性质、分布特征及其林分间差异。结果表明：(1)与纯林相比,木麻黄-琼崖海棠混交林可显著提高腐殖质层以及20～40 cm土壤层pH,增幅分别为6.11%和5.97%。(2)与纯林相比,木麻黄-琼崖海棠混交林和木麻黄-大叶相思混交林均可显著提高各层土壤的有机碳和全氮含量,有机碳含量增幅分别为69.8%～358.3%和90.2%～908.3%,全氮含量增幅分别为44.1%～160.7%和31.4%～210.7%;另外,木麻黄-琼崖海棠混交林还可显著提高0～100 cm各土层的全磷含量,增幅为20.8%～39.6%,而木麻黄-大叶相思混交林可显著提高20～100 cm各土层的全磷含量,增幅为25.0%～39.6%;木麻黄-大叶相思混交林对腐殖质层速效养分的改善效果较好,而两种混交林均可显著提高各土层的速效钾含量。(3)方差分析表明,林分类型和土层深度对林下有机碳、全氮、全磷、有效磷、硝态...     

The influence of the bolus-surface distance on the dose distribution in the build-up region

Aim

The aim of the paper is to examine the relation between the increase of the photon dose in water in the region of electronic disequilibrium – so-called build-up region – and the distance of the bolus from the water surface for the applied parameters of X-ray beams.

Materials and methods

PDD measurements were carried out using the plane-parallel ionization chamber Markus in the automatic water phantom IBA BluePhantom with OmniPro-Accept V7 (IBA Dosimetry GmbH, Schwarzenbruck, Germany). All measurements were performed for different field sizes and for 6 MV and 15 MV X-ray beams, respectively. A water-equivalent RW3 slab (Goettingen White Water) produced by PTW was used as a bolus.

Results

Placing a bolus in an irradiated field changes the shape of the PDD curve in the build-up region in comparison with the one obtained for an open field. All results has been inserted in tables and figures.

Conclusion

The closer the bolus is to the water surface, the smaller the depth of the maximum dose in the phantom for all investigated fields and energies. The changes in the build-up region are important, even if the bolus does not touch the surface of the water phantom. The influence of the bolus can be ignored when the bolus-surface distance equals 25 cm for 6MV X-ray beams and 39 cm for 15 MV X-ray beams.     

Above- and Below-Ground Carbon Stocks in an Indigenous Tree (Mytilaria laosensis) Plantation Chronosequence in Subtropical China

More than 60% of the total area of tree plantations in China is in subtropical, and over 70% of subtropical plantations consist of pure stands of coniferous species. Because of the poor ecosystem services provided by pure coniferous plantations and the ecological instability of these stands, a movement is under way to promote indigenous broadleaf plantation cultivation as a promising alternative. However, little is known about the carbon (C) stocks in indigenous broadleaf plantations and their dependence on stand age. Thus, we studied above- and below-ground biomass and C stocks in a chronosequence of Mytilaria laosensis plantations in subtropical China; stands were 7, 10, 18, 23, 29 and 33 years old. Our assessments included tree, shrub, herb and litter layers. We used plot-level inventories and destructive tree sampling to determine vegetation C stocks. We also measured soil C stocks by analyses of soil profiles to 100 cm depth. C stocks in the tree layer dominated the above-ground ecosystem C pool across the chronosequence. C stocks increased with age from 7 to 29 years and plateaued thereafter due to a reduction in tree growth rates. Minor C stocks were found in the shrub and herb layers of all six plantations and their temporal fluctuations were relatively small. C stocks in the litter and soil layers increased with stand age. Total above-ground ecosystem C also increased with stand age. Most increases in C stocks in below-ground and total ecosystems were attributable to increases in soil C content and tree biomass. Therefore, considerations of C sequestration potential in indigenous broadleaf plantations must take stand age into account.     

Root morphology, hydraulic conductivity and plant water relations of high-yielding rice grown under aerobic conditions

Background and Aims

Increasing physical water scarcity is a major constraint for irrigated rice (Oryza sativa) production. ‘Aerobic rice culture’ aims to maximize yield per unit water input by growing plants in aerobic soil without flooding or puddling. The objective was to determine (a) the effect of water management on root morphology and hydraulic conductance, and (b) their roles in plant–water relationships and stomatal conductance in aerobic culture.

Methods

Root system development, stomatal conductance (g_s) and leaf water potential (Ψ_leaf) were monitored in a high-yielding rice cultivar (‘Takanari’) under flooded and aerobic conditions at two soil moisture levels [nearly saturated (> –10 kPa) and mildly dry (> –30 kPa)] over 2 years. In an ancillary pot experiment, whole-plant hydraulic conductivity (soil-leaf hydraulic conductance; K_pa) was measured under flooded and aerobic conditions.

Key Results

Adventitious root emergence and lateral root proliferation were restricted even under nearly saturated conditions, resulting in a 72–85 % reduction in total root length under aerobic culture conditions. Because of their reduced rooting size, plants grown under aerobic conditions tended to have lower K_pa than plants grown under flooded conditions. Ψ_leaf was always significantly lower in aerobic culture than in flooded culture, while g_s was unchanged when the soil moisture was at around field capacity. g_s was inevitably reduced when the soil water potential at 20-cm depth reached –20 kPa.

Conclusions

Unstable performance of rice in water-saving cultivations is often associated with reduction in Ψ_leaf. Ψ_leaf may reduce even if K_pa is not significantly changed, but the lower Ψ_leaf would certainly occur in case K_pa reduces as a result of lower water-uptake capacity under aerobic conditions. Rice performance in aerobic culture might be improved through genetic manipulation that promotes lateral root branching and rhizogenesis as well as deep rooting.