Altitudinal zonation of human-disturbed vegetation on Mt. Tianmu,eastern China

Much of the primary vegetation at low altitudes has been greatly altered or destroyed by a long history of human activities. This is particularly true in eastern China, where low-altitude areas are now dominated by secondary forests or plantations. Altitudinal vegetation zonation of this region is often based on these secondary forests, resulting in seral vegetation with an obscure zonal sequence. Here, we deduced the potential climax vegetation according to the regeneration patterns of the dominant species of the secondary forests at low altitudes (below 1,000 m a.s.l.) on Mt. Tianmu (1,506 m a.s.l., 30°18′30″–30°21′37″N, 119°24′11″–119°27′11″E). Based on the potential climax vegetation combined with the floristic composition and community structure, three vegetation zones were identified, viz: (1) evergreen broad-leaved forest zone (400–950 m a.s.l.); (2) evergreen and deciduous broad-leaved mixed forest zone (950–1,100 m a.s.l.); (3) deciduous broad-leaved forest zone (1,100–1,506 m a.s.l.). The altitudinal vegetation zones identified in this study correspond with the thermal conditions on Mt. Tianmu. The distribution of vegetation on Mt. Tianmu was limited by lower temperatures in winter, and the altitudinal thermal vegetation zones on this mountain were more similar to the thermal vegetation of Japan than to that of China. The vertical distributions and roles of conifers were different between the eastern and the western regions along 30°N latitude in humid East Asia. Cryptomeria fortunei formed the emergent layer, towering above the broad-leaved canopy at middle altitudes as C. japonica on Yakushima, but disappeared at high altitudes with hydrothermal limitation on Mt. Tianmu.     

Laurel forests in Tenerife,Canary Islands

 Stand structure and leaf area distribution of a laurel forest in the Agua García mountains of Tenerife are described. The site is situated at 820 m a.s.l., faces NNE, and has a humid mediterranean climate. Summer droughts are mitigated by relatively high air humidity and clouds. The natural mixed hardwood forest is composed of six major tree species: Laurus azorica (Seub.) Franco, Persea indica (L.) Spreng, Myrica faya Ait., Erica arborea L. and two species of Ilex (I. platyphylla Webb & Berth. and I. canariensis Poivet.). The experimental stand had a density of 1693 trees ha^– 1, a basal area of 33.7 m²ha^– 1, and a cumulated volume of above-ground parts of trees of 231 m³ ha^– 1 with a corresponding dry mass of 204 ton ha^– 1. Diameters at breast height ranged from 6 to 46 cm. Mean concentration of plant dry mass per volume was 1.17 kg m^– 3. The vertical pattern of leaf area distribution in individual trees for all tree species was characterized by a Gaussian-like curve. Stand leaf area index was 7.8. These evergreen, broad-leaved (laurisilva or lucidophyllous) forests represent a relic forest that was widespread in the Mediterranean region some 20 million years ago. Our data illustrate some of the structural characteristics of this historically widespread forest type. Received: 2 December 1994 / Accepted: 6 November 1995     

Altitudinal distribution of evergreen broad-leaved trees and their leaf-size pattern on a humid subtropical mountain,Mt. Emei,Sichuan, China

Altitudinal distribution of evergreen broad-leaved trees and changes in their leaf sizes were studied on a humid subtropical mountain, Mt. Emei (3099 m a.s.l., 29°34.5 N, 103°21.5 E), Sichuan, China. Among the total woody flora of ca. 540 species, evergreen broad-leaved trees account for 88 species in 39 genera and 23 families, corresponding to the northern limit of subtropical evergreen broad-leaved trees. The number of evergreen broad-leaved tree species greatly decreased from the low-altitudinal, evergreen broad-leaved forest zone (600–1500 m) to the mid-altitudinal, mixed forest zone (1500–2500 m), and to the high-altitudinal, coniferous forest zone (2500–3099 m). The overall trend of reduced leaf size toward upper zones was analyzed and documented in detail. The 88 species were assigned to three leaf-size classes: notophylls (48%), microphylls (36%), and mesophylls (16%). The leaf size was relatively small and the specific leaf weight (SLW, mg cm^–2) was much larger in high altitude as compared to low altitude. No overall correlation was found between leaf size and SLW, but leaf size decreased as SLW increased toward high altitude for certain species having relatively wide altitudinal ranges. Moreover, leaf size varied with forest stratification: canopy trees were predominantly notophyllous species, while subcanopy and understorey trees were mainly microphyllous species. The tendency is compatible with the trend found in other mountains of East Asia.     

广西金钟山自然保护区主要植被类型的特征

金钟山自然保护区计有种子植物101科273属514种,落叶栎林分布面积最广。随海拔升高,植被依次呈现出4个分布带:沟谷落叶阔叶林、沟谷常绿阔叶林、常绿落叶阔叶混交林和落叶阔叶林、山地苔藓矮林和山地常绿阔叶林。该区主要有4个分布区类型:世界分布、热带分布、温带分布和中国特有分布,其中热带分布占总属数的75.21%,表明本保护区的植物分布具有热带性质。其天然植被类型可划分为5个植被型组,7个植被型含暖性针叶林、暖性落叶阔叶林、常绿落叶阔叶混交林、常绿阔叶林、竹林和草丛,2个植被亚型含南亚热带山地常绿阔叶林、山顶阔叶矮林,以及33个群系。     

Relationships between photosystem II efficiency and photochemical reflectance index under different levels of illumination: comparison among species grown at high- and low elevations through different seasons

Previously, we found a significant association between photosystem II efficiency (ΦPSII) and photochemical reflectance index (PRI) measured at predawn among different species at different elevations and throughout several seasons. However, this relationship has not been evaluated under varied levels of illumination. Here, we used the Taiwan species Pinus taiwanensis (a conifer distributed at 750–3,000 m a.s.l.), Stranvaesia niitakayamensis (an evergreen tree, 1,700–3,100 m) and two Miscanthus spp. (perennial C₄ Gramineae, coastline–3,200 m) to elucidate the ΦPSII–PRI relationship. We studied six levels of photosynthetic photon flux density (PPFD) (0, 200, 400, 800, 1,200 and 2,000 μmol m⁻² s⁻¹) over several growth seasons at high (2,600 m a.s.l.) and low (800 m a.s.l.) elevation sites. In comparing the same species or genus, ΦPSII and PRI were closely correlated in darkness or under the same level of PPFD, with data obtained from different seasons and elevations pooled for regression analysis. Because both the intercept and slope of the ΦPSII–PRI equation showed a negative curvilinear correlation with PPFD, we could fit an empirical regression model, ΦPSII = c + d·ln(PPFD) + e·[ln(PPFD)]² + f·PRI + g·PRI·ln(PPFD) + h·PRI·[ln(PPFD)]², for multiple regression analysis. Using this model, we found a close correlation between the estimated and measured ΦPSII (r ² = 0.842−0.937, P < 0.001) for all four species examined and for mango (Mangifera indica) measured under both artificial illumination and sunlight (data from Weng et al. 2010). This empirical regression model could simulate both seasonal and diurnal variations of leaf-scale photosynthetic efficiency at high and low elevations.     

Deposition and decomposition of cattle dung in forest grazing in southern Kyushu, Japan

This study monitored deposition and decomposition of cattle dung in a grazed young Chamaecyparis obtusa (an evergreen conifer) plantation in southwestern Japan, as a part of exploring the impacts of livestock in the forest grazing system. Animals defecated 10–19 times hd⁻¹ day⁻¹, producing feces of 2.2–3.5 kg DM and 33–73 g N per animal per day. The DM and N concentrations of feces ranged from 157–207 g DM kg⁻¹ and 14.8−23.1 g (kg DM)⁻¹, respectively. Occurrence of defecation was spatially heterogeneous, with feces being concentrated mainly on areas for resting (forest roads, ridges and valleys) and moving (forest roads and along fence lines). Decomposition of dung pats was considerably slow, showing the rates of 1.37–3.05 mg DM (g DM)⁻¹ day⁻¹ as DM loss. Decomposition was further slower on the basis of N release, 0.51–1.63 mg N (g N)⁻¹ day⁻¹, resulting in steadily increased N concentrations of dung pats with time after deposition. The results show that introduction of livestock into a forest (i.e., forest grazing) may limit nutrient availability to plants, by redistributing nutrients into areas with no vegetation (bare land and streams) and by establishing a large N pool as feces due to an imbalance between deposition and slow release, though further studies are necessary for investigating the occurrence of slow dung decomposition in other forest situations.     

Variation in carbon stocks on different slope aspects in seven major forest types of temperate region of Garhwal Himalaya,India

The present study was undertaken in seven major forest types of temperate zone (1500 m a.s.l. to 3100 m a.s.l.) of Garhwal Himalaya to understand the effect of slope aspects on carbon (C) density and make recommendations for forest management based on priorities for C conservation/sequestration. We assessed soil organic carbon (SOC) density, tree density, biomass and soil organic carbon (SOC) on four aspects, viz. north-east (NE), north-west (NW), south-east (SE) and south-west (SW), in forest stands dominated by Abies pindrow, Cedrus deodara, Pinus roxburghii, Cupressus torulosa, Quercus floribunda, Quercus semecarpifolia and Quercus leucotrichophora. TCD ranged between 77.3 CMg ha⁻¹ on SE aspect (Quercus leucotrichophora forest) and 291.6 CMg ha⁻¹ on NE aspect (moist Cedrus deodara forest). SOC varied between 40.3 CMg ha⁻¹ on SW aspect (Himalayan Pinus roxburghii forest) and 177.5 CMg ha⁻¹ on NE aspect (moist Cedrus deodara forest). Total C density (SOC + TCD) ranged between 118.1 CMg ha⁻¹ on SW aspect (Himalayan Pinus roxburghii forest) and 469.1 CMg ha⁻¹ on NE aspect (moist Cedrus deodara forest). SOC and TCD were significantly higher on northern aspects as compared with southern aspects. It is recommended that for C sequestration, the plantation silviculture be exercised on northern aspects, and for C conservation purposes, mature forest stands growing on northern aspects be given priority.     

楚雄花椒园半湿润常绿阔叶林的群落生态学特征

基于20个样方(各样方面积为25 m× 20 m,总面积1 hm2)调查,研究了楚雄花椒园自然保护区元江栲-米饭花林的群落生态学特征.该群落以壳斗科、越桔科、山茶科八角科、山矾科植物为乔木层主要成分,并以壳斗科植物占绝对优势,群落的物种多样性不高.木本植物以非全缘、革质、常绿、小叶等为其特征,反映该群落生境偏干,为典型...     

Aboveground net production and dry matter allocation ofPinus pumila forests in the Kiso mountain range,central Japan

Aboveground net production rates of the subalpine stone pine (Pinus pumila) forests in central Japan were estimated by the summation method; net production was defined as the sum of annual biomass increment and annual loss due to death. In the two pine stands of different scrub heights, P1 (200 cm) and P2 (140 cm), aboveground biomass reached 177 and 126 ton ha⁻¹, respectively. Leaf biomass was about 14 ton ha⁻¹ in each stand. The estimates of aboveground net production during the 2 year period (1987–1989) averaged 4.1 and 3.7 ton ha⁻¹ y⁻¹ in P1 and P2, respectively, which were at the lowest among the pine forests in the world. Two indices of efficiency of energy fixation, that is, the ratio of net production to the total radiation during a growing season and the ratio of net production to total radiation per unit of leaf weight, were evaluated. Both efficiency indices for the twoP. pumila stands fell in the range obtained for other Japanese evergreen conifer forests. This suggested that the low annual net production of the stone pine stands were mainly due to a limitation in the length of the growing season. The pine forests were also characterized by a small allocation (about 17%) of aboveground net production into biomass increment, in comparison with other evergreen conifer forest types. Annual net carbon gain in theP. pumila forests was suggested to be largely invested in leaf production at the expense of the growth of woody parts.     

基于集成生物圈模型的南岭不同植被类型碳收支研究

广东南岭保存着世界上同纬度带上最完整的亚热带植被,森林资源丰富,具有巨大的固碳潜力。然而,目前该地区不同森林植被类型的碳收支年积累量特征及月动态规律尚不明确。选择广东南岭国家级自然保护区内沟谷常绿阔叶林、山地常绿阔叶林、针阔叶混交林和山顶常绿阔叶矮林4种典型森林植被为研究对象,运用集成生物圈模型(IBIS)对其2020年总初级生产力(GPP)、净初级生产力(NPP)、净生态系统生产力(NEP)和土壤异养呼吸(R_h)进行模拟,利用样地调查数据对NPP模拟结果进行验证,分析该地区不同植被类型的碳收支年积累量特征及月变化特征。研究结果表明,2020年南岭不同植被类型GPP、NPP、NEP和R_h的平均值分别为1.709、0.718、0.596和0.123 kg C m^-2 a^-1,4种植被类型中GPP最高的是沟谷常绿阔叶林,NPP、NEP最高的是山地常绿阔叶林,山顶常绿阔叶矮林的GPP、NPP和NEP均相对较低。南岭不同植被类型全年各月均表现出碳汇(NEP>0),逐月NPP和NEP均表现为双峰变化规律...     

浙江天童地区常绿阔叶林退化对土壤养分库和碳库的影响

为了解常绿阔叶林退化对土壤碳库和养分库的影响,采用空间代替时间的研究方法,以常绿阔叶林顶级群落为参照,选择了次生常绿阔叶幼年林、次生针阔混交林、次生针叶林、灌丛和灌草丛代表不同的退化类型,分别对其土壤氮磷养分库、碳库进行了调查和分析。结果表明：土壤氮库贮量从大到小依次为,成熟常绿阔叶林、次生常绿阔叶幼年林、灌丛、次生针叶林、灌草丛和次生针阔混交林;土壤总磷含量也是在成熟林最高,次生针阔混交林和次生针叶林的总磷含量显著高于次生常绿阔叶幼年林和灌丛;土壤有机碳含量从高到低依次为：成熟常绿阔叶林,次生针叶林、次生常绿阔叶幼年林、灌丛、灌草丛和次生针阔混交林;土壤铵态氮在成熟林、灌丛和灌草丛的库容量最大,其次分别为次生幼年常绿阔叶林、次生针阔混交林,最小的为次生针叶林;硝态氮则在灌草丛的库容量最大,其次分别为次生针叶林、次生针阔混交林和成熟林针叶林,最小的为次生常绿阔叶幼年林和灌丛。统计显示,常绿阔叶林退化不仅导致土壤有机碳库含量的显著下降,也使得土壤氮磷养分库含量显著下降。可以认为,砍伐导致的大量生物量输出和森林管理措施的影响,植物种类组成的改变,土壤物理性质的改变以及养分和有机碳的主要生物化学转化环节发生改变是导致此类变化的主要因素,常绿阔叶林顶极群落土壤是该地区土壤的最大养分库和碳库。     

Conifer colonization of a 350-year old rock fall at Lassen Volcanic National Park in northern California

Montane conifers of western North America commonly colonize mineral surfaces resulting from landslides, lahars, mudflows, and rock avalanches. This colonization can include shade-tolerant conifers that may eventually dominate the forest in a pattern termed “direct” succession. Documenting examples of this long-term successional process are useful for identifying alternative successional trajectories and indicating potential controlling mechanisms for subsequent experimental analysis. This study (1) analyzes the 1992 status of the conifer colonization on the coarse-textured surface of a 1650 AD rock avalanche in northern California and (2) measures individual growth and survivorship in permanent plots between 1992 and 2003. Increment cores of large trees indicated initial conifer colonization before 1700 AD with continuous subsequent colonization. Mean conifer density in 1992 was 725 (SD = 747) ha⁻¹ with densities increasing with decreasing rock sizes. Densities were not correlated with distances to possible seed sources. Median heights were <1 m, and the mean proportional height growth rate for healthy individuals was 0.0166 year⁻¹. The mortality rate for individuals ≥0.1-m tall was <0.007 year⁻¹. The conifers were more numerous than shrubs, and there was little apparent evidence of facilitation or inhibition of conifers. The species assemblage is mostly (89%) Abies concolor, Pinus monticola, and Pinus ponderosa individuals dispersed across an elevation range of 1870–2040 m a. s. l. This is an atypical species mix for these elevations in this location, and this mix is not readily predicted from species properties such as seed mass, seed flight distance, or drought tolerance.     

天目山常绿阔叶林空间结构动态变化特征

采用固定样地重复监测的方法,2005-2015年每5年1次对天目山国家级自然保护区内1hm²常绿阔叶林固定样地进行3次样地调查,按优势度分析法确定群落优势树种,根据树种类别将优势树种分为常绿阔叶树种、常绿针叶树种和落叶阔叶树种3个优势树种组,利用聚集指数、树种多样性混交度和Hegyi竞争指数3个空间结构指数,分析群落和优势树种在2005-2015年期间的空间结构动态变化特征。结果表明：（1）群落表现为低度聚集、中度混交和中度竞争状态。群落聚集程度和竞争强度降低,混交程度保持稳定。群落空间结构动态变化总体特征是混交度稳定性最高,其次是分布格局,竞争态势稳定性最低。（2）群落优势种组成变化不大,3个优势树种组均呈低度聚集分布;常绿阔叶树种表现为低度混交、中度竞争状态;常绿针叶和落叶阔叶树种均表现为中度混交、低度竞争状态。常绿阔叶树种的聚集程度和竞争强度降低,混交程度保持稳定,与群落空间结构动态变化特征基本一致。常绿针叶和落叶阔叶树种的3种空间结构均保持稳定。在常绿阔叶林经营或人工林近自然改造过程中,应重视群落和优势树种的空间结构变化规律,确定阶段性空间结构优化目标,逐步促进群落进展演替。     

广州三种森林粗死木质残体（CWD）的储量与分解特征

粗死木质残体(Coarse Woody Debris, CWD)对森林生态系统的稳定性具有不可忽视的贡献.对广州3种森林CWD的储量与分解特征进行了调查分析,结果表明:(1)CWD储量及其与相应森林总生物量比值均表现为常绿阔叶林>针阔混交林>针叶林;枯立木与倒木为CWD的主要成分,其中,针叶CWD主要物种为马尾松(Pinus massonianai),阔叶CWD物种主要为荷木(Schima superba)与黄杞(Engelhardtia chrysolepis)等.(2)CWD径级主要集中在<10cm的范围内,存在状态主要为中级腐烂状态,干扰与竞争是3种森林CWD产生的主要因素.(3)针叶林、针阔混交林与常绿阔叶林CWD的分解常数k分别为0 0244、0.0407和0.0487,即分解速率为常绿阔叶林>混交林>针叶林;随着CWD的分解,N、P与木质素的含量逐渐升高,C、C/N与木质素/N呈降低趋势.     

Nitrogen Fixation by Termites in Tropical Forests, Thailand

Nitrogen (N) fixed by termites was evaluated as a N input to decomposition processes in two tropical forests, a dry deciduous forest (DDF) and the neighboring dry evergreen forest (DEF), Thailand. A diverse group of termite species were assayed by acetylene reduction method and only the wood/litter-feeding termites were found to fix N. More intensive samplings of two abundant species, Microcerotermes crassus and Globitermes sulphureus, were done across several seasons, suggesting N fixation rates of 0.21 and 0.28 kg ha⁻¹ y⁻¹ by termites in the DDF and DEF, respectively. Also, estimates of asymbiotic N fixation rates were 0.75 and 3.95 kg ha⁻¹ y⁻¹. N fixed by termites and by asymbiotic fixers is directly supplied to decomposers breaking down dead plant material and could be a major source of their N. N fixed by termites was 7–22% of that fixed by termites and asymbiotic fixers. Although N fixed by termites is a small input compared to other inputs, this N is likely important for decomposition processes.     

Water source partitioning among trees growing on shallow karst soils in a seasonally dry tropical climate

The sources of water used by woody vegetation growing on karst soils in seasonally dry tropical regions are little known. In northern Yucatan (Mexico), trees withstand 4–6 months of annual drought in spite of the small water storage capacity of the shallow karst soil. We hypothesized that adult evergreen trees in Yucatan tap the aquifer for a reliable supply of water during the prolonged dry season. The naturally occurring concentration gradients in oxygen and hydrogen stable isotopes in soil, bedrock, groundwater and plant stem water were used to determine the sources of water used by native evergreen and drought-deciduous tree species. While the trees studied grew over a permanent water table (9–20 m depth), pit excavation showed that roots were largely restricted to the upper 2 m of the soil/bedrock profile. At the peak of the dry season, the δ¹⁸O signatures of potential water sources for the vegetation ranged from 4.1 ± 1.1‰ in topsoil to −4.3 ± 0.1‰ in groundwater. The δ¹⁸O values of tree stem water ranged from −2.8 ± 0.3‰ in Talisia olivaeformis to 0.8 ± 1‰ in Ficus cotinifolia, demonstrating vertical partitioning of soil/bedrock water among tree species. Stem water δ¹⁸O values were significantly different from that of groundwater for all the tree species investigated. Stem water samples plotted to the right of the meteoric water line, indicating utilization of water sources subject to evaporative isotopic enrichment. Foliar δ¹³C in adult trees varied widely among species, ranging from −25.3 ± 0.3‰ in Enterolobium cyclocarpum to −28.7 ± 0.4‰ in T. olivaeformis. Contrary to initial expectations, data indicate that native trees growing on shallow karst soils in northern Yucatan use little or no groundwater and depend mostly on water stored within the upper 2–3 m of the soil/bedrock profile. Water storage in subsurface soil-filled cavities and in the porous limestone bedrock is apparently sufficient to sustain adult evergreen trees throughout the pronounced dry season.     

Plant Biomass,Nutrient Concentration and Nutrient Storage in a Tropical Dry Forest in the South–west of Madagascar

Plant biomass, mineral composition and the amounts of nutrients in the different fractions of the vegetation were determined for a dense dry deciduous forest growing on light red sands in south-western Madagascar. Complete harvesting and soil coring were used to determine the above- and below-ground biomass respectively. The above-ground biomass, weighing 118 t ha⁻¹ (dry matter), was mostly (96%) made up of phanerophytes (woody trees and shrubs >25 cm tall). Dead material (litter and dead wood on the soil surface) represented 13.8 t ha⁻¹. These results fit well into the range of values reported for other tropical ecosystems. The below-ground biomass was 17.8 t ha⁻¹ giving a root/shoot ratio of 0.15. Rooting is superficial. The nutrient concentration in this dry forest on light reddish-brown sands is, as in other dry forests, considerably higher than that usually found for humid forests. Calcium is the most abundant element. The plant biomass Ca/K ratio is much higher than that of humid tropical forests. In spite of its high originality, this Madagascan dry forest has the same behaviour as other dry forests of the world.     

水文过程对缙云山径流盐基离子变化特征的影响

通过选取缙云山针阔混交林(马尾松×四川大头茶)和常绿阔叶林(四川大头茶×四川山矾)内2个标准径流小区,结合野外观测和室内分析研究降雨及径流的化学性质径流盐基离子变化特征,为该地区水环境保护提供依据。结果表明:1)针阔混交林与常绿阔叶林中p H值变化趋势相同均为:地表径流壤中流大气降水。p H与降雨量之间存在显著相关关系。2)在大气降雨-地表径流-壤中流过程中,NH+4的地表径流及壤中流输出总量小于降雨输入量,表明了土壤对NH+4有一定的吸附持留能力。其他盐基离子径流输出量均高于大气降雨的输入量,且主要以壤中流输出为主,元素发生流失。从盐基离子总量来看,径流输出量均增加:针阔混交林(2.86 kg/hm2),常绿阔叶林(2.47 kg/hm2),说明在两种林分内养分均流失。(3)K+、Ca2+、Mg2+和Na+盐基离子输出量与径流量之间存在相关关系,随着径流量的增大径流输出量增加。而NH+4输出量与径流量之间的相关性不明显,说明了NH+4的变化除受径流量的影响外,可能还受微生物、季节等其他因素的影响。     

Greenhouse Gas Budget of a Cool-Temperate Deciduous Broad-Leaved Forest in Japan Estimated Using a Process-Based Model

A terrestrial ecosystem model, called the Vegetation Integrative Simulator for Trace gases model (VISIT), which fully integrates biogeochemical carbon and nitrogen cycles, was developed to simulate atmosphere–ecosystem exchanges of greenhouse gases (CO₂, CH₄, and N₂O), and to determine the global warming potential (GWP) taking into account the radiative forcing effect of each gas. The model was then applied to a cool-temperate deciduous broad-leaved forest in Takayama, central Japan (36°08′N, 137°25′E, 1420 m above sea level). Simulations were conducted at a daily time step from 1948 to 2008, using time-series meteorological and nitrogen deposition data. VISIT accurately captured the carbon and nitrogen cycles of this typical Japanese forest, as validated by tower and chamber flux measurements. During the last 10 years of the simulation, the model estimated that the forest was a net greenhouse gas sink, having a GWP equivalent of 1025.7 g CO₂ m⁻² y⁻¹, most of which (1016.9 g CO₂ m⁻² y⁻¹) was accounted for by net CO₂ sequestration into forest biomass regrowth. CH₄ oxidation by the forest soil made a small contribution to the net sink (11.9 g CO₂-eq. m⁻² y⁻¹), whereas N₂O emissions were a very small source (3.2 g CO₂-eq. m⁻² y⁻¹), as expected for a volcanic soil in a humid climate. Analysis of the sensitivity of GWP to changes in temperature, precipitation, and nitrogen deposition indicated that warming temperatures would decrease the size of the sink, mainly as a result of increased CO₂ release due to increased ecosystem respiration.     

Seasonal variation in photosynthesis in six woody species with different leaf phenology in a valley savanna in southwestern China

During 2003–2005, we examined the effect of seasonal drought on water status, gas exchange, δ¹³C, chlorophyll fluorescence and spectral reflectance in six woody species in a valley savanna near the Yuanjiang River (the upper Red River) in southwestern China. Three different phenological types of these woody species were compared, i.e., an evergreen species, Cyclobalanopsis helferiana, two winter-deciduous (WD) species, Buchanania latifolia and Symplocos racemosa, and three drought-deciduous (DD) species, Terminthia paniculata, Wendlandia tinctoria and Woodfordia fruticosa. We aimed to test the following three hypotheses: (1) the evergreen and WD species employ a drought avoidance strategy, whereas DD species employ a drought tolerance strategy; (2) the evergreen and WD species have a more economical water use strategy than the DD species and (3) the evergreen and WD species have a stronger photoprotection capacity through thermal dissipation than the DD species. At the end of a prolonged drought, the predawn leaf water potential (Ψ_pd) in C. helferiana and S. racemosa dropped to ca. −0.8 MPa, whereas the Ψ_pd in B. latifolia remained close to zero and DD species were leafless. In the rainy seasons, maximal photosynthetic rates of the evergreen (18.4 μmol m⁻² s⁻¹) and W. fruticosa (18.0 μmol m⁻² s⁻¹) were higher than those of the other four species (12.2−13.8 μmol m⁻² s⁻¹). The evergreen and WD species responded to drought by closing stomata and thus maintained a constant relative water content (RWC), which is a typical drought avoidance strategy; however, it is at the expense of carbon gain. DD species maintained a high photosynthetic capacity with a decrease in both stomatal conductance and RWC until the driest period, and then shifted from the drought tolerance strategy to the avoidance mechanism by shoot dieback. There was no significant difference in the means of δ¹³C across the phenological groups. The evergreen and WD species had stronger heat dissipation than the DD species in dry seasons. All species increased leaf spectral reflectance, probably because of degradation of chlorophyll as indicated by the leaf reflectance index, which should reduce light harvesting. All species showed a strong increase in the ratio of red to green spectral reflectance of leaves during dry seasons, indicating the accumulation of anthocyanin, which may contribute to screening sunlight and scavenging reactive oxygen species. Different responses to drought of savanna woody species with different leaf phenologies may facilitate the partitioning of resource use and hence their co-existence.