Annual variation in soil respiration and its components in a coppice oak forest in Central Italy

In order to investigate the annual variation of soil respiration and its components in relation to seasonal changes in soil temperature and soil moisture in a Mediterranean mixed oak forest ecosystem, we set up a series of experimental treatments in May 1999 where litter (no litter), roots (no roots, by trenching) or both were excluded from plots of 4 m². Subsequently, we measured soil respiration, soil temperature and soil moisture in each plot over a year after the forest was coppiced. The treatments did not significantly affect soil temperature or soil moisture measured over 0–10 cm depth. Soil respiration varied markedly during the year with high rates in spring and autumn and low rates in summer, coinciding with summer drought, and in winter, with the lowest temperatures. Very high respiration rates, however, were observed during the summer immediately after rainfall events. The mean annual rate of soil respiration was 2.9 µ mol m^?2 s^?1, ranging from 1.35 to 7.03 µmol m^?2 s^?1. Soil respiration was highly correlated with temperature during winter and during spring and autumn whenever volumetric soil water content was above 20%. Below this threshold value, there was no correlation between soil respiration and soil temperature, but soil moisture was a good predictor of soil respiration. A simple empirical model that predicted soil respiration during the year, using both soil temperature and soil moisture accounted for more than 91% of the observed annual variation in soil respiration. All the components of soil respiration followed a similar seasonal trend and were affected by summer drought. The Q₁₀ value for soil respiration was 2.32, which is in agreement with other studies in forest ecosystems. However, we found a Q₁₀ value for root respiration of 2.20, which is lower than recent values reported for forest sites. The fact that the seasonal variation in root growth with temperature in Mediterranean ecosystems differs from that in temperate regions may explain this difference. In temperate regions, increases in size of root populations during the growing season, coinciding with high temperatures, may yield higher apparent Q₁₀ values than in Mediterranean regions where root growth is suppressed by summer drought. The decomposition of organic matter and belowground litter were the major components of soil respiration, accounting for almost 55% of the total soil respiration flux. This proportion is higher than has been reported for mature boreal and temperate forest and is probably the result of a short‐term C loss following recent logging at the site. The relationship proposed for soil respiration with soil temperature and soil moisture is useful for understanding and predicting potential changes in Mediterranean forest ecosystems in response to forest management and climate change.     

Floristic relationships of the woody component in rocky outcrops savanna areas in Central Brazil

Rocky outcrops usually have a peculiar vegetation which differs from that of the surrounding vegetation matrix. In Brazil, a savanna type (cerrado rupestre) is particularly characterized by rocky outcrops and is one of eleven vegetation types which occur throughout Cerrado biome. It has been scarcely studied in detail. Therefore, the aim of this study was to investigate the phytogeographical patterns of the woody species in ten areas of the rocky outcrop savanna in the Brazilian Central Plateau. The difference in spatial distribution of the species was assessed through indicator species and CCA ordination analyses. Ten indicator species showed significantly different distribution. The first axis of the ordination showed association with the edaphic gradient, based on maximum temperature, pH and physical properties of the soils (eigenvalue 0.63). The second axis showed an eigenvalue 0.46, associated with poor rainfall and land declivity. It appears that the woody flora of the rocky outcrop savanna areas of the Brazilian Central Plateau is relatively homogeneous, with low β diversity. Apparently, the size of the woody species populations is what distinguishes the ten investigated areas. Therefore, initiatives and actions for the Cerrado biome conservation must consider not only the size of the protected areas but also that of the populations to be protected, especially the woody species on rocky outcrops.     

Spatial distribution of soil seed banks of three African savanna woody species at two contrasting sites

In southern African savannas, bush encroachment is a major problem for range managers. However, little is understood of the actual regeneration processes leading to it, and in particular the role of soil seed banks. The horizontal (between microsites) and vertical (with depth in litter and soil) distribution of soil seed banks of the microphyllous woody species, Acacia tortilis, A. nilotica and Dichrostachys cinerea (all legumes of the Mimosoideae), were quantified in an area with low intensity grazing (reserve), and a bordering cattle farm with high intensity grazing (farm). Species differed in seed bank densities between microsites and sites. Seed densities for all species were highest below parent tree canopies and decreased with distance beyond the canopy, and with soil depth. D. cinerea had the smallest seed bank associated with parent trees, particularly on the farm (8 vs. 1643 seeds/tree on the reserve), A. tortilis had the largest (6357, 31910), with A. nilotica intermediate (1789, 1906). The proportion of current (recently fallen) versus old (1 year old) seeds differed between species and sites. These species form at least short-term persistent seed banks with the old seeds largely representing the persistent seed bank. Seed densities in the open (inter-canopy) and those dispersed under either of the other two (non-parental) study species were much lower than those associated with parent trees. The latter were mostly found under the acacias (single-stemmed) rather than D. cinerea (multistemmed). Total seed store per parent plant increased with plant size (best fits were mostly power curves of canopy area). A large proportion of intact seeds were viable, namely 81–84% for A. tortilis, 68–77% for A. nilotica and 63–78% for D. cinerea, with no differences between sites. Viability tended to increase with depth of burial, except for A. nilotica seeds at the 3–5 cm depths on the farm. At the landscape scale there were 1.5 million and 140000 A. tortilis seeds/ha on the reserve and farm respectively, with corresponding values of 2000 and 31000 for D. cinerea, and 23000 and 86000 for A. nilotica.     

Photosynthetic parameters from two contrasting woody vegetation types in West Africa

Measurements of leaf gas exchange were made in contrasting wooded ecosystems in West Africa. Measurements were made on 10 species: seven from humid rain forest in Cameroon and three from the semi-arid Sahelian zone in Niger. For each species, two models of photosynthesis were fitted: the first based on a rectangular hyperbolic response to photosynthetic photon flux density (Q), and the second the biochemical model of Farquhar et al. (1980). In both communities, the species studied could be divided into those characteristic of early and late successional stages, but photosynthetic parameters were not closely related to successional stage. The data identified significant relationships between V _cmax and leaf nutrient (N and P) content when expressed on an area basis. Variation in leaf mass per unit area correlated with canopy exposure and dominated the leaf nutrient signal. Statistical analysis suggested weakly that leaf gas exchange was more limited by P than N at the rain forest site.     

木本植物多度在草原和稀树干草原中增加的研究进展

木本植物多度在草原和稀树干草原中增加已经成为全球范围普遍发生的现象。为揭示这一现象发生的原因,从放牧和气候变化与木本植物多度增加的关系、木本植物多度增加过程中的正反馈作用以及木本植物侵入的关键阶段——幼苗的补充和定居,这三个方面综述了目前的研究结果。强调放牧和气候变化之间的相互共同作用,可能引发了木本植物向草原和稀树干草原中的入侵;而生物引起的正反馈作用则进一步促进了木本植物的扩展。从生态系统干扰的角度,讨论了木本植物多度增加机制的复杂性,并指出木本植物幼苗补充和定居的连续性和间断性两种方式,对于草原和稀树干草原木本植物多度增加的贡献。     

Determinants of woody encroachment and cover in African savannas

Savanna ecosystems are an integral part of the African landscape and sustain the livelihoods of millions of people. Woody encroachment in savannas is a widespread phenomenon but its causes are widely debated. We review the extensive literature on woody encroachment to help improve understanding of the possible causes and to highlight where and how future scientific efforts to fully understand these causes should be focused. Rainfall is the most important determinant of maximum woody cover across Africa, but fire and herbivory interact to reduce woody cover below the maximum at many locations. We postulate that woody encroachment is most likely driven by CO₂ enrichment and propose a two-system conceptual framework, whereby mechanisms of woody encroachment differ depending on whether the savanna is a wet or dry system. In dry savannas, the increased water-use efficiency in plants relaxes precipitation-driven constraints and increases woody growth. In wet savannas, the increase of carbon allocation to tree roots results in faster recovery rates after disturbance and a greater likelihood of reaching sexual maturity. Our proposed framework can be tested using a mixture of experimental and earth observational techniques. At a local level, changes in precipitation, burning regimes or herbivory could be driving woody encroachment, but are unlikely to be the explanation of this continent-wide phenomenon.     

Scaling relationships for woody tissue respiration in two tropical rain forests

The relationship between gross primary productivity (GPP) and net primary productivity (NPP) is not fully understood. One of the uncertainties relevant to this issue is the magnitude of woody tissue respiration. Although some data exist for temperate and boreal zones, measurements of woody tissue respiration in tropical forests are sparse. We made in situ chamber measurements of woody tissue respiration in two tropical rain forests, one in the Brazilian Amazon (Reserva Jarú) and one in Central Cameroon (Mbalmayo Reserve). We made measurements on a wide range of species at each site and over a range of stem diameters from 0·02 to 1·4 m. The rate of efflux of carbon dioxide (CO₂) from bark at 25 °C, R_t, varied from 0·1 to 5·2 µmol m^?2 s^?1 across the two sites, and the efflux was related to both volume and surface area components of the measured stem sections. The temperature response in R_t was slightly higher at Jarú than at Mbalmayo, with Q₁₀ values of 1·8 (± 0·1 SE) and 1·6 (± 0·1 SE), respectively. A log–log regression showed that R_t was significantly related to stem diameter, D (P < 0·001; r² = 0·58–0·62) and was significantly higher at Mbalmayo than at Jarú (P < 0·001), but that the rate of increase in R_t with stem diameter, D, was similar between sites. At the Mbalmayo site, tree growth measurements made over a 4 month period were used to make two estimates of the maintenance (R_m) and construction (R_c) components of respiration embedded in R_t. The two methods agreed closely, suggesting that R_m was approximately 80% of R_c at this site. R_m could be strongly related to D using a sigmoidal relationship that described both surface area and volume components as sources of respiratory CO₂ (r² = 0·71). This functional model was combined with inventory, growth and climate data for the Mbalmayo site to make a first estimate of annual above‐ground woody tissue respiration, R_A, which was 257 (± 18 SE) g C m^?2 year^?1. This value corresponds to approximately 10% of GPP, slightly lower than that found for another tropical rain forest, but higher than for temperate forests. When combined with data from six other sites in tropical, temperate and boreal settings, a very strong relationship was found between R_A and leaf area index (LAI), and between R_A/GPP and LAI (P < 0·001, r² = 0·98). This indicates that R_A exerts an appreciable constraint on NPP and that this constraint varies closely with LAI across widely differing types of woody vegetation.     

北方森林土壤呼吸和木质残体分解释放出的CO2通量

北方森林因其面积大、土壤碳储量高以及对全球暖化响应敏感而在全球碳平衡和气候系统中起着至关重要的作用。土壤呼吸和木质残体分解释放出的 CO2 通量是北方森林生态系统输入大气圈的最主要的碳源。量化这个通量并深刻理解其中的机理过程 ,是评价和预测北方森林在全球变化中的作用必不可少的内容。综述了北方森林生态系统土壤呼吸和木质残体分解释放出的 CO2 通量随生态系统类型及环境条件而变化的一般格局以及自养呼吸和异氧呼吸在土壤表面 CO2 通量中的相对贡献 ;分析了影响北方森林土壤呼吸的主要生物物理因子 ;讨论了该领域研究存在的问题和今后的研究方向 ;并强调木质残体分解释放出的 CO2 通量虽然在以往的森林生态系统碳平衡研究中常被忽略 ,但在火灾频繁的北方森林中不容忽视     

Grazing and neighborhood interactions limit woody encroachment in wet subtropical savannas

Woody encroachment in savannas is a worldwide concern, and there is growing consensus that anthropogenic activities play a central role in changing tree – grass interactions. We evaluated the influence of livestock grazing and neighborhood interactions on seedling emergence and survival of the native tree Acacia caven in wet savannas of northeastern Argentina. We hypothesized that grazing and grass competition act as biotic barriers limiting tree recruitment, but the relative magnitude of such barriers differs according to grass patch type. In two consecutive years (cohort 1 and 2) we sowed seeds and transplanted seedlings of Acacia in two grass patch types (prostrate/palatable and tussock/unpalatable grasses) in both, grazed and ungrazed plots. Each grass patch type was further manipulated to create three levels of grass competition (unclipped control, above-ground biomass removal and total biomass removal).Cattle grazing diminished seedling emergence of both cohorts and seedling survival of cohort 1. The effect of grass competition changed according to grass patch type. Prostrate grass cover enhanced emergence but lowered early survival, while tussock grass cover and also its total biomass removal facilitated early survival. During the second year, a severe drought drastically reduced Acacia recruitment, and it was strong enough to eliminate any grazing effects although the effect of grass competition on seedling establishment remained significant.Our results suggest that grazing and grass competition additively diminished the risk of woody establishment in this wet savanna. However, the stocking rate should be carefully balanced, thus contributing to the maintenance of a competitive grass cover to limit tree recruitment.     

Rainfall and topo-edaphic influences on woody community phenology in South African savannas

Woody community phenology was studied in the central lowveld, South Africa, over a twelve month period at three sites along a rainfall gradient, with both toplands and bottomlands sampled at each site. Each month, individual plants, in replicated samples, were scored into a number of categories describing their phenological state. Position on the rainfall gradient influenced: (1) onset and magnitude of leaf emergence, (2) onset and duration of mature leaves, and (3) the proportion of leafless trees. Generally, the moist site demonstrated earlier leaf growth than the intermediate or arid sites. Emergent and mature leaves were recorded earlier, and in the case of mature leaves, retained longer. Overall, there was a lower proportion of leafless trees during the dry season at the moist site, followed by the semi-arid site, followed by the arid site. Differences with respect to catenal position were evident for the proportion of trees in winter with mature leaves, and the proportion of trees with senescent leaves. Bottomlands had a greater proportion of trees with leaves during winter, but a lower proportion of trees recorded with senescent leaves. Both of these findings were a result of the greater proportion of evergreen species in bottomlands, as well as increased leaf retention by the deciduous species. Phenological activity of leaves was related to plant stem size. In particular, there was greater leaf retention during the dry period by small stems, relative to large stems.     

玉米生长季土壤呼吸的时间变异性及其影响因素

基于东北地区玉米生态系统土壤呼吸连续2个生长季的观测,阐明了土壤呼吸日、季节变化特征,综合分析了水热因子、土壤性质、生物量及叶面积指数（LAI）对土壤呼吸的影响。结果表明：玉米地土壤呼吸日变化为不对称的单峰型曲线,最小值和最大值分别出现在6：00-7：00和13：00左右。2005年玉米生长季土壤呼吸速率均值为3．16μmol CO2&#183;m^-2&#183;s^-1,最大值为4．77μmol CO2&#183;m^-2&#183;s^-1,出现在7月28日;最小值为1．31μmol CO2&#183;m^-2&#183;s^-1,出现在5月4日。统计分析表明：土壤温度是玉米生态系统土壤呼吸日变化的驱动因素;土壤温度和土壤水分是影响土壤呼吸季节变化的关键因素,二者可以解释玉米生长季土壤呼吸时间变异的87％;LAI和根系生物量与土壤呼吸速率呈正相关,说明生物因子对土壤呼吸季节变化也有影响;土壤有机质、全氮和碳氮比等土壤理化特性与土壤呼吸速率的关系较弱;玉米生长季追施氮肥明显促进土壤呼吸速率。     

杉木人工林去除根系土壤呼吸的季节变化及影响因子

2007年1月至2008年12月,在长沙天际岭国家森林公园内,采用挖壕法研究杉木人工林去除根系后土壤呼吸速率季节动态及其与5 cm土壤温、湿度的相关关系。结果表明:去除根系与对照5 cm土壤温度的差异性不显著(P=0.987),5 cm土壤湿度差异显著(P=0.035)。杉木林去除根系处理后土壤呼吸速率明显降低,2007至2008两年实验期间去除根系与对照处理变化范围分别为0.19-2.01μmol.m-2s-1和0.26-2.61μmo.lm-2s-1,年均土壤呼吸速率分别为0.90μmo.lm-2s-1和1.30μmol.m-2s-1。去除根系土壤呼吸速率降低幅度为9.4%-59.7%,平均降低了30.4%。去除根系和对照的土壤呼吸速率与5 cm土壤温度之间均呈显著指数相关,模拟方程分别为:y=0.120e0.094t(R2=0.882,P=0.000),y=0.291e0.069t(R2=0.858,P=0.000)。Q10值分别为2.56和2.01。     

Tree-girdling to separate root and heterotrophic respiration in two Eucalyptus stands in Brazil

The release of carbon as CO₂ from belowground processes accounts for about 70% of total ecosystem respiration. Insights about factors controlling soil CO₂ efflux are constrained by the challenge of apportioning sources of CO₂ between autotrophic tree roots (and mycorrhizal fungi) and heterotrophic microorganisms. In some temperate conifer forests, the reduction in soil CO₂ efflux after girdling (phloem removal) has been used to separate these sources. Girdling stops the flow of carbohydrates to the belowground portion of the ecosystem, which should slow respiration by roots and mycorrhizae while heterotrophic respiration should remain constant or be enhanced by the decomposition of newly dead roots. Therefore, the reduction in CO₂ efflux after girdling should be a conservative estimate of the belowground flux of C from trees. We tested this approach in two tropical Eucalyptus plantations. Tree canopies remained intact for more than 3 months after girdling, showing no reduction in light interception. The reduction in soil CO₂ efflux averaged 16–24% for the 3-month period after girdling. The reduction in CO₂ efflux was similar for plots with one half of the trees girdled and those with all of the trees girdled. Girdling did not reduce live fine root biomass for at least 5 months after treatment, indicating that large reserves of carbohydrates in the root systems of Eucalyptus trees maintained the roots and root respiration. Our results suggest that the girdling approach is unlikely to provide useful insights into the contribution of tree roots and heterotrophs to soil CO₂ efflux in this type of forest ecosystem.     

西双版纳热带季节雨林与橡胶林土壤呼吸的季节变化

采用挖壕沟法与红外气体分析法,研究了西双版纳热带季节雨林和人工橡胶林内土壤呼吸包括根系呼吸、异养呼吸的干湿季动态变化.结果表明：季节雨林内土壤呼吸和异养呼吸速率均显著大于橡胶林(P＜0.01),但根系呼吸差异不显著;土壤温湿度是呼吸速率变化的主要影响因子,季节雨林和橡胶林内土壤呼吸和异养呼吸速率均为雨季＞干热季＞雾凉季,但季节雨林内根系呼吸为雨季＞雾凉季＞干热季,而橡胶林内为雾凉季＞雨季＞干热季;季节雨林内根系呼吸对土壤呼吸的贡献率(29%)小于橡胶林(42%,P＜0.01),而季节雨林内异养呼吸对土壤呼吸的贡献率为71%、橡胶林为58%;当5 cm土壤温度在12 ℃～32 ℃范围内变化时,季节雨林内土壤呼吸及根系呼吸、异养呼吸的Q₁₀值均大于橡胶林,且异养呼吸的Q₁₀值最大而根系呼吸的Q₁₀值最小.