The arboreal ants of a Neotropical rain forest show high species density and comprise one third of the ant fauna

In tropical rain forests, the ant community can be divided into ground and arboreal faunas. Here, we report a thorough sampling of the arboreal ant fauna of La Selva Biological Station, a Neotropical rain forest site. Forty-five canopy fogging samples were centered around large trees. Individual samples harbored an average of 35 ant species, with up to 55 species in a single sample. The fogging samples yielded 163 observed species total, out of a statistically estimated 199 species. We found no relationship between within-sample ant richness and focal tree species, nor were the ant faunas of nearby trees more similar to each other than the faunas of widely spaced trees. Species density was high, and beta diversity was low: A single column of vegetation typically harbors at least a fifth of the entire arboreal ant fauna. Considering the entire fauna, based on 23,326 species occurrence records using a wide variety of collecting methods, 182 of 539 observed species (196 of 605, estimated statistically) were entirely arboreal. The arboreal ant fauna is thus about a third of the total La Selva ant fauna, a robust result because inventory completeness was similar for ground and arboreal ants. The taxonomic history of discovery of the species that make up the La Selva fauna reveals no disproportionately large pool of undiscovered ant species in the canopy. The "last biotic frontier" for tropical ants has been the rotten wood, leaf litter, and soil of the forest floor.     

抚育间伐对海南热带次生林地上生物量恢复的影响

多次刀耕火种弃耕后自然恢复的热带次生林恢复速度通常较为缓慢。抚育是提高森林恢复和木材生产速度的重要营林措施,因此利用抚育间伐的方式加快热带次生林的恢复速度是当前森林经营和保护的重要议题。基于海南岛60个0.25 hm²热带次生林样地开展了抚育间伐对比试验。研究表明,经过5年的自然恢复,30个抚育样地和30个对照样地的地上生物量分别提高了24.5%和13.4%,而且抚育样地中减少的地上生物量迅速接近对照样地。抚育主要减少了清除种的地上生物量,而提高了保留种的地上生物量。次生林经过抚育处理后,其地上生物量的绝对增长量显著提高了58.74%,相对增长率显著提高了67.93%。在抚育样地中,地上生物量的绝对增长量和相对增长量均随着抚育强度呈现单峰曲线变化的趋势,抚育强度在（10±2.5）%时地上生物量的相对和绝对增长量最高。抚育强度是影响地上生物量增长量的重要因素,而物种多样性和功能离散度的作用较小。决定地上生物量的相对增长量最重要的因素（负作用）是初始生物量。本研究为我国热带次生林的未来管理提供了重要的理论基础和实践证据。     

东北林区主要森林类型土壤有机碳密度及其影响因素

2011年在东北林区研究了5种主要森林类型(针叶混交林、针阔混交林、阔叶混交林、落叶松林和白桦林)4个林龄(幼龄林、中龄林、近熟林和过熟林)的土壤碳密度.结果表明: 东北林区不同森林类型的土壤有机碳含量和有机碳密度均以表层土壤最高,随土壤深度的增加逐渐减少,而随森林类型和林龄的变化并不显著;森林土壤有机碳主要集中在表层土壤,其中大兴安岭、小兴安岭和长白山森林0～20 cm土壤贮存了其剖面总有机碳密度的847%～86.1%、51.7%～59.8%和51.2%～53.4%.随纬度的增加,森林土壤总有机碳密度明显下降,可能与东北林区土壤发生层的厚度密切相关.     

秦岭火地塘林区油松林下主要灌木碳吸存

林下灌木是森林生态系统的重要组成部分,在维持森林碳平衡中发挥着重要作用.为估算林下灌木的固碳功能,采用TOC-VT H-2000A型TOC/TON分析仪,测定了油松林下主要灌木不同器官的含碳率;根据野外实测资料,建立了油松群落内主要灌木测树因子与其器官生物量的回归模型;计算了2006年和2007年,灌木层CO2年吸存量.结果表明:5种灌木叶、茎、根和皮的平均含碳率为:41.80%～46.25%,39.24%～49.22%,39.56%～46.71%和36.65%～48.23%;刚毛忍冬(Lonicera Hispida pall)各器官的含碳率最高,栓翅卫矛(Euonymus alatus)的叶和茎,白檀 (Symplocos paniculata)的根和皮含碳率最低;不同灌木同一器官和同一灌木不同器官的含碳率均存在显著差异;不同灌木同一器官平均含碳率差值最高达10.58%,同一灌木不同器官平均含碳率差值最高达6.47%;模拟显示:灌木器官的生物量和测树因子间的关系可用复合式、幂、二次方程、三次方程、对数方程、指数方程和倒数方程来描述;残差和误差分析表明,均方差根不大于1.70,模型有效性指数均接近于1,残差系数均接近于0,灌木根、茎、叶和皮的生物量模型估计值与实测值间相对误差的绝对值分别为3.89%～8.58%,0.57%～6.84%,4.69%～9.09%和4.50%～7.03%.回归模型的决定系数(R2)都在0.90以上,估计精度在95%以上,建立的油松群落内主要灌木测树因子与其器官生物量的回归模型,具有较高的估计精度和较好的适用性;研究区2006～2007年,主要灌木CO2年吸存量为10.138 Mghm-2.     

Modelling the effect of temperature on carbon mineralization rates across a network of European forest sites (FORCAST)

Temperature is a major environmental variable influencing microbial respiration in soils. Thus, understanding how heterotrophic processes in soils may respond to potential increases in temperature is crucial for the prediction of the response of forest carbon budgets to climate change. We investigated carbon mineralization rates from eight European forest soils in relation to soil temperature. Mineral soil samples were collected from eight mature forest sites in the European network CARBOEUROFLUX and were incubated in the laboratory for ca. 270 days at four temperatures: 4, 10, 20 and 30°C. In all soils, carbon mineralization rates decreased over time when incubated at high temperatures of 20 and 30°C. In this study, we explore the different models available to analyse long-term incubation data. Carbon mineralization rates were best predicted by a first-order, two-compartment model that predicted carbon mineralization as a function of time and temperature using all of the incubation data. We found very small fractions (1–9%) of labile carbon in the upper mineral soils. Despite large differences among sites, we found higher carbon mineralization rates and larger amounts of labile carbon in the broadleaf than in the conifer forest soils. No significant differences in temperature sensitivity among the sites (average Q ₁₀ of 2.88 using the two-compartment model) were observed, as estimated with all methods used. Although not statistically significant, the sensitivities of the rate constant of the labile fractions tended to be higher than those for the rate constant of the recalcitrant fractions. Thus, the results of this modelling exercise suggest that despite large variation among sites, a single temperature sensitivity parameter can be used for a range of soils over the range of temperatures we used (4–30°C).     

Long-term CO2 production from deeply weathered soils of a tropical rain forest: evidence for a potential positive feedback to climate warming

Currently, it is unknown what role tropical forest soils will play in the future global carbon cycle under higher temperatures. Many tropical forests grow on deeply weathered soils and although it is generally accepted that soil carbon decomposition increases with higher temperatures, it is not known whether subsurface carbon pools are particularly responsive to increasing soil temperatures. Carbon dioxide (CO₂) diffusing out of soils is an important flux in the global carbon. Although soil CO₂ efflux has been the subject of many studies in recent years, it remains difficult to deduct controls of this flux because of the different sources that produce CO₂ and because potential environmental controls like soil temperature and soil moisture often covary. Here, we report results of a 5‐year study in which we measured soil CO₂ production on two deeply weathered soil types at different depths in an old‐growth tropical wet forest in Costa Rica. Three sites were developed on old river terraces (old alluvium) and the other three were developed on old lava flows (residual). Annual soil CO₂ efflux varied between 2.8–3.6 μmol CO₂‐C m^?2 s^?1 (old alluvium) and 3.4–3.9 μmol CO₂‐C m^?2 s^?1 (residual). More than 75% of the CO₂ was produced in the upper 0.5 m (including litter layer) and less than 7% originated from the soil below 1 m depth. This low contribution was explained by the lack of water stress in this tropical wet forest which has resulted in very low root biomass below 2 m depth. In the top 0.5 m CO₂ production was positively correlated with both temperature and soil moisture; between 0.6 and 2 m depth CO₂ production correlated negatively with soil moisture in one soil and positively with photosynthetically active radiation in the other soil type. Below 2 m soil CO₂ production strongly increased with increasing temperature. In combination with reduced tree growth that has been shown for this ecosystem, this would be a strong positive feedback to ecosystem warming.     

Substrate conditions,foliar nutrients and the distributions of two canopy tree species in a Costa Rican secondary rain forest

A 28 yr old secondary lowland rain forest in Costa Rica was dominated by two tree species of contrasting ecologies, Vochysia ferruginea - a species typical of well-drained but infertile soils of high Al saturation, and Cordia alliodora, a species requiring soils of at least moderate fertility. The two species exhibited markedly different spatial distributions in the forest and we sought to determine whether or not these different distributions were related to variation in substrate conditions and the nutrient requirements of the two species. Two soil types were present: Ultisols (Typic Haplohumults) and Inceptisols (Typic Dystropepts) and topography was of low hills. The study formed part of an analysis of site quality for timber production in the two species and therefore used standard forestry methods for such analyses. 36 plots of 20 m×20 m were established using predetermined criteria of site uniformity, presence of at least four dominant trees of at least one of the study species, and absence of disturbance. The abundance (number of individuals 10 cm dbh) of each species was determined in each plot. Soil samples were taken in 27 of the 36 plots, 9 plots being considered to replicate conditions already sampled, and 14 soil chemical and physical variables were measured using standard methods. Foliar nutrient analyses were carried out for trees of both species with crowns fully exposed to sunlight, distributed across the gradient of soil conditions. Variations in species abundances were not related to soil type, but plots in which Vochysia ferruginea was more abundant tended to be on steeper slopes with soils of higher exchangeable acidity and lower concentrations of Mn; Cordia alliodora was more abundant on gentler topography where soils had much lower exchangeable acidity but higher Mn. Differences between the two species in foliar nutrient concentrations were marked and supported previous interpretations of their nutrient requirements; foliar nutrients of Vochysia ferruginea were typical of tree species of moist tropical forest on infertile soils, and those of Cordia alliodora typical of a species requiring more fertile soils. Factors such as the distributions of seed trees at site abandonment may affect the spatial distributions of tree species in secondary forests such as that studied. The relationships of the distributions of the two species to substrate variation, however, and their evidently different nutrient requirements, support the hypothesis that variation in the composition and structure of the forest studied is at least partially related to exchangeable acidity and its dominant cation, Al.     

Multi‐scale effects of habitat structure and landscape context on a vertebrate with limited dispersal ability (the brown‐throated sloth,Bradypus variegatus)

As human population, food consumption, and demand for forest products continue to rise over the next century, the pressures of land‐use change on biodiversity are projected to intensify. In tropical regions, countryside habitats that retain abundant tree cover and structurally complex canopies may complement protected areas by providing suitable habitats and landscape connectivity for a significant portion of the native biota. Species with low dispersal capabilities are among the most at risk of extinction as a consequence of land‐use change. We assessed how the spatial distribution of the brown‐throated sloth (Bradypus variegatus), a model species for a vertebrate with limited dispersal ability, is shaped by differences in habitat structure and landscape patterns of countryside habitats in north‐central Costa Rica using a multi‐scale framework. We quantified the influence of local habitat characteristics and landscape context on sloth occurrence using mixed‐effects logistic regression models. We recorded 27 sloths within countryside habitats and found that both local and landscape factors significantly influenced their spatial distribution. Locally, sloths favored structurally complex habitats, with greater canopy cover and variation in tree height and basal area. At the landscape scale, sloths demonstrated a preference for habitats with high proportions of forest and nearby large tracts of forest. Although mixed‐use areas and tree plantations are not substitutes for protected forests, our results suggest they provide important supplemental habitats for sloths. To promote the conservation and long‐term viability of sloth populations in the tropical countryside, we recommend that land managers retain structurally complex vegetation and large patches of native habitat.     

Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment

The impact of anthropogenic CO₂ emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO₂ concentrations stimulate primary productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO₂ concentrations, the Duke Forest Free‐Air CO₂ Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO₂ concentrations 200 μL L^?1 above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9‐year period (1996–2005). During the first 6 years of the experiment, forest‐floor C and N pools increased linearly under both elevated and ambient CO₂ conditions, with significantly greater accumulations under the elevated CO₂ treatment. Between the sixth and ninth year, forest‐floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of ～30 g C m^?2 yr^?1 was sequestered in the forest floor of the elevated CO₂ treatment plots relative to the control plots maintained at ambient CO₂ owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO₂ effects on the rate of decomposition or on the chemical composition of forest‐floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO₂ suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment × time interaction indicates that N is being transferred at a higher rate under elevated CO₂ (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO₂.     

Assessing the value of secondary forest for amphibians: <Emphasis Type="Italic">Eleutherodactylus</Emphasis> frogs in a gradient of forest alteration

Secondary forests constitute a growing portion of forested areas worldwide. They might have a substantial role for the conservation of biodiversity in tropical areas, but there is little information on their potential to support forest species and the recovery of faunal communities. We studied two forest frogs (Eleutherodactylus diastema and E. fitzingeri) in an area of Costa Rica composed of a mosaic of primary forest, young secondary forest and pasture, and we compared the density of calling males in areas with different forest alteration. Autoregressive models were used to compensate for potentially undesired effects of spatial autocorrelation and pseudoreplication. Both species were most abundant in riparian, primary forest. However, E. fitzingeri was also abundant in riparian secondary forests, and its density far from the river was similar in primary and secondary forest, suggesting that river proximity can influence the recovery of secondary forest for amphibians. Conversely, the density of E. diastema was similar in secondary forest and pasture, stressing interspecific differences for recovery rate. These frogs have a keystone role in nutrient cycling and food webs, and their prompt recovery might represent an important step for the functional recovery of forests. Nevertheless, the strong interspecific differences stress the complexity of these processes.