Soil water content measurements deliver reliable estimates of water fluxes: A comparative study in a beech and a spruce stand in the Tharandt forest (Saxony, Germany)

Previous studies have shown that soil water content can vary considerably within homogeneous sites. This small-scale variability of soil water is often neglected when studying water and carbon fluxes in forest ecosystems. In this paper, the small-scale variability of soil water was analyzed at two contrasting eddy-flux sites, a Norway spruce forest and a European beech forest. Simultaneous measurements of precipitation, eddy covariance, and sap flow, from soil water content readings, were used to answer the question of how representative soil water gain is during rainfall and evapotranspiration is during dry periods.Our study demonstrates that the spatial and temporal variability in soil water under spruce and beech was mainly due to the differences in soil properties and root intensity. This can be concluded from the fact that the pattern of soil moisture distribution and flow paths under the trees were generally stable throughout the season. As a tendency, areas with preferred accumulation of rainwater were mainly characterized by maximum soil water depletion. Therefore, the density of the installed water content sensors should correspond to the variability of soil properties as well as rooting intensity. Based on previous studies and our own results, it can be concluded that a horizontal and vertical distance between 10 and 30 cm is best suited for water content sensors to detect preferential flow paths and deliver reliable estimates of soil water balance.Despite the occurrence of preferential flow, we found that the soil water increase during rainstorm periods and the soil water depletion during dry periods can be estimated relatively well when the small-scale variability of soil properties is considered in the experimental setup. In general, the evaporation estimates based on eddy covariance, sap flow, and soil water balance were consistent. However, compared to the spruce site, at the beech site the gap between the evapotranspiration estimates based on eddy covariance and soil water balance were often relatively large. Differences in the spatial extent of these methods can only explain these discrepancies to a certain extent. We suggest that this might be mainly due to the lack of water content sensors in the immediate vicinity of the beech tree trunk. Thus, stemflow-induced wetting and subsequent drying around the trunk could not be monitored in our study. This may result in an underestimation of evaporation from the soil under beech using the soil water balance method compared to the eddy covariance method. Finally, soil water depletion under spruce led to a significant reduction of transpiration when the actual available plant soil capacity (AWC) was <40% of the potential AWC. In contrast to the spruce stand, a reduction of transpiration of beech due to water shortage was not observed.     

Resolving systematic errors in estimates of net ecosystem exchange of CO2 and ecosystem respiration in a tropical forest biome

The controls on uptake and release of CO₂ by tropical rainforests, and the responses to a changing climate, are major uncertainties in global climate change models. Eddy-covariance measurements potentially provide detailed data on CO₂ exchange and responses to the environment in these forests, but accurate estimates of the net ecosystem exchange of CO₂ (NEE) and ecosystem respiration (R_eco) require careful analysis of data representativity, treatment of data gaps, and correction for systematic errors. This study uses the comprehensive data from our study site in an old-growth tropical rainforest near Santarem, Brazil, to examine the biases in NEE and R_eco potentially associated with the two most important sources of systematic error in Eddy-covariance data: lost nighttime flux and missing canopy storage measurements. We present multiple estimates for the net carbon balance and R_eco at our site, including the conventional “u* filter”, a detailed bottom-up budget for respiration, estimates by similarity with ²²²Rn, and an independent estimate of respiration by extrapolation of daytime Eddy flux data to zero light. Eddy-covariance measurements between 2002 and 2006 showed a mean net ecosystem carbon loss of 0.25 ± 0.04 μmol m⁻² s⁻¹, with a mean respiration rate of 8.60 ± 0.11 μmol m⁻² s⁻¹ at our site. We found that lost nocturnal flux can potentially introduce significant bias into these results. We develop robust approaches to correct for these biases, showing that, where appropriate, a site-specific u* threshold can be used to avoid systematic bias in estimates of carbon exchange. Because of the presence of gaps in the data and the day–night asymmetry between storage and turbulence, inclusion of canopy storage is essential to accurate assessments of NEE. We found that short-term measurements of storage may be adequate to accurately model storage for use in obtaining ecosystem carbon balance, at sites where storage is not routinely measured. The analytical framework utilized in this study can be applied to other Eddy-covariance sites to help correct and validate measurements of the carbon cycle and its components.     

Drought effects on water relations in beech: The contribution of exchangeable water reservoirs

The exceptional soil drought and heat wave that occurred in Europe in summer 2003 provided a good opportunity to analyze the response of forest ecosystems to extreme climatic conditions. This work aimed at studying the functioning of exchangeable water reservoirs of beech trees under climatic and edaphic constraints, in a 37-year-old beech stand located in north-eastern France. We characterised the impact of drought on seasonal variations in water fluxes at the tree (sap flow measurements) and the forest (eddy covariance measurements) scales and estimated (i) the daily water storage capacity of beech trees as the difference between stand-scaled sap flow and water vapour flux over the stand and (ii) the contribution of exchangeable water in tree reservoirs to the total tree transpiration, under non-limiting soil water (summer 2002) or severe soil drought conditions (summer 2003). In parallel, daily variations in trunk circumference were analyzed to estimate the contribution of elastic tissues as a compartment for water storage to tree transpiration.Stand transpiration was strongly reduced by soil water shortage (up to 80% at the peak of drought in August 2003). From the beginning of August 2003, we observed daytime contraction of stem circumference, with only partial, or even no night recovery, suggesting that trees were less and less able to refill the elastic and strongly depleted reservoirs. Even if those elastic reservoirs were active throughout the season, the corresponding volume of water withdrawn for tree transpiration remained very low (maximum 1% of the daily transpiration). Thus, elastic tissue reservoirs play a minor role in the total water budget of beech trees.The amount of water depleted daily from the whole reservoirs was much higher than the water extracted from elastic tissues. Furthermore, the contribution of total exchangeable water within trees to the transpiration stream increased during the dry period (from a few percent to 67% at the peak of the drought) and was positively correlated with soil water shortage. Our results thus clearly demonstrated the strong sensitivity of beech to both climate and drought and the major role of whole tree water reservoirs to maintain leaf transpiration under severe drought.     

Converting Australian tropical rainforest to native Araucariaceae plantations alters soil fungal communities

Native rainforest tree plantations are increasingly viewed as potentially important for high value timber production and provision of a range of ecological services in tropical and subtropical areas. In order to determine the extent to which conversion of rainforest to native Araucariaceae plantation influences soil fungi, we compared soil fungal communities under native rainforest and 73-74 year-old Araucaria bidwillii, Araucaria cunninghamii and Agathis robusta plantations at Gadgarra State Forest, Queensland, Australia. Following direct extraction of DNA from soil, terminal restriction fragment length polymorphism (T-RFLP) analysis of rDNA internal transcribed spacer (ITS) regions was conducted. Ordination analysis of the T-RFLP data revealed significant separation of the fungal communities according to forest type along the first canonical axis, with the native rainforest samples separating from the three Araucariaceae plantations along the second axis. Overall, the most abundant ITS sequences in clone assemblages from the four forest types were Ascomycota, followed by Basidiomycota, Zygomycota and Chitridomycota, however their relative importance varied in individual forest types. The results indicate that conversion of tropical rainforest to monoculture plantations of native trees can significantly alter soil fungal diversity.     

Soil development along an altitudinal transect in a Bolivian tropical montane rainforest: Podzolization vs. hydromorphy

Knowledge about soil formation in tropical montane rainforests is scarce and patchy. We examined the altitudinal change of soils in a Bolivian tropical montane rainforest, aiming to illuminate the contribution of podzolization and hydromorphic processes to soil formation. In three transects from 1700 m to 3400 m a.s.l. we determined the pH, exchangeable cation exchange capacity, carbon and nitrogen stocks, and iron and aluminium fractions from 26 soil profiles. Three zones of different dominant soil forming processes were found: In the lower montane forest (LMF, 1700–2200 m a.s.l.), Dystropepts with high nutrient concentration and acidity were common. The pronounced change to the upper montane cloud forest (UMCF, 2200–2700 m a.s.l.) coincided with the appearance of Placorthods with more acidic conditions, deep ectorganic horizons and increasing translocation of sesquioxides. In the sub-alpine forest (SCF, 2700 m–3400 m a.s.l.), hydromorphic processes dominated over podzolization, resulting in Placaquods with low mineralization rate and nutrient concentration. This shows that due to increasing wetness and colder temperatures at high altitudes, dominant soil forming processes change from podzolization to hydromorphism soils with increasing altitude.     

Direct effects of litter decomposition on soil dissolved organic carbon and nitrogen in a tropical rainforest

To clarify how litter decomposition processes affect soil dissolved organic carbon (DOC) and soil dissolved nitrogen (DN) dynamics, we conducted a field experiment on leaf litter and collected DOC and DN from the underlying soil in a tropical rainforest in Xishuangbanna, southwest China. Principal components analysis (PCA) showed the first PCA axis (corresponding to degraded litter quantity and quality) explained 61.3% and 71.2% of variation in DOC and DN concentrations, respectively. Stepwise linear regression analysis indicated that litter carbon mass controlled DOC and hemicellulose mass controlled DN concentrations. Litter decomposition was the predominant factor controlling surface-soil DOC and DN dynamics in this tropical rainforest.     

Physical, chemical and phosphatase activities characteristics in soil-feeding termite nests and tropical rainforest soils

Little is known about the relationship between soil biological function and the physical and chemical characteristics of soil-feeding termite nests in the Lopé tropical rainforest (Gabon). We compared nine soil-feeding termite nests of Cubitermes of different ages (fresh to mature to old) and six surrounding soils that originated from three forests differing with respect to age and vegetative cover according to 14 physical and chemical variables and acid (pH 4) and alkaline (pH 9) phosphatase activities. Physical and chemical variables of the studied samples were influenced by the three factors tested: (1) forest age, (2) termite activity (nest versus soil), (3) termite nest age. Soils from the gallery forest were strongly discriminated from all the other soils studied notably due to their high organic matter contents. All mature nests showed significant increases in K, P, clay and fine silt, pH, and cationic exchange capacity compared to soils. Some nests also had increased amounts of organic matter and larger water retention capacities. Moreover, we observed that with age the termite nests possessed decreased values of these variables from fresh to mature to old. Likewise, phosphatase activities also differed according to the three factors tested. Due to its high organic matter contents, the highest phosphatase activities were noted in the gallery forest. Within each forest, phosphatase activities decreased in mature nests compared to soils and tended to be higher in fresh nests compared to mature nests. These differences might be due to an inhibition by high inorganic P contents, as mature nests were enriched in this element and to the quality of organic matter as nests are built with termite faeces. Termite activity has an important role in influencing physical and chemical variables and phosphatase activities.     

The influence of forest management history on the integrity of the saproxylic beetle fauna in an Australian lowland tropical rainforest

Forest management in temperate and boreal regions is often based on a strong foundation of applied ecological research. Increasingly, this has allowed the needs of saproxylic (dead wood associated) insects to be addressed. However, there has been very little equivalent research in tropical forests, where saproxylic insect faunas are likely to be much richer and where forestry is usually subject to weaker environmental controls. This study compares the saproxylic beetle fauna of old-growth, selectively logged and regrowth rainforest in the Daintree lowlands of northeastern Queensland, Australia. Old-growth levels of abundance, species richness, assemblage composition and guild structure were not maintained in logged and regrowth forest, suggesting that intact assemblages may not survive in the long-term in managed tropical rainforest. However, retaining a continuous supply of commercially overmature trees in the managed stand may prevent a repeat of the widespread extinctions of saproxylic insects witnessed in temperate and boreal forest regions.     

Laboratory assessment of the workable range of soils in the tropical zone of Veracruz, Mexico

Information on workability limits is highly valuable at the farm level to help the farmer in deciding when and possibly how to carry out tillage. On higher levels (e.g. on regional scale) this knowledge will form a basis for obtaining guidelines with respect to required number of tractors, equipment etc. and so help in planning and policy making. Data of this kind are often lacking. The study reported here is the laboratory component of research carried out in the tropical area of Veracruz, Mexico. In this study, laboratory methods to determine workability limits were assessed for a typical clay and loam soil. For the dry workability limit (where energy for producing specifically sized aggregates is the decisive factor), the drop test was applied. Results in terms of soil moisture tension at the point of lowest energy expenditure were comparable with the field results, although energy levels were different. For the determination of the wet workability limit (WWL), an air permeability test and a compression test were used. Both tests yielded limits that were very close to the ones determined in the field. The compressibility test allows the assessment of other soil characteristics such as prediction of modification of pF curve and aeration properties under compressive forces. For field or farm scale, workability limits can be obtained from representative areas and then used in combination with water balance models to determine the number of workable days under a certain weather (rainfall) regime. The applicability on a regional scale is as yet more difficult, soil physical information from soil maps is usually not sufficient to allow reliable interpretations. The use of existing pedo-transfer functions to obtain input for models to predict workable days did not produce satisfactory results.     

紫色土坡耕地农田生态系统蒸散发与水分利用效率及其影响因素

蒸散发与水分利用效率是农田生态系统碳水循环的重要衡量指标。本研究利用涡度相关技术对紫色土坡耕地生态系统进行连续观测,获取2014—2018年碳水通量数据,分析紫色土冬小麦-夏玉米轮作下的雨养坡耕地农田生态系统蒸散发和水分利用效率变化特征及其对主要环境因子的响应规律。结果表明:紫色土坡耕地农田生态系统蒸散发日变化规律呈单峰型趋势,最大值均在14:00前后出现;一年中8月日蒸散发最高,1月最低;夏季日变化幅度最大,春季次之,冬季和秋季变化较为平缓。叶面积指数、温度为影响紫色土坡耕地蒸散发的最主要因子,其次为饱和水汽压差。水分利用效率在9:00—17:00期间基本呈先下降后回升的变化规律,冬季水分利用效率为全年最高;叶面积指数、CO2通量为影响水分利用效率的主要因子,其次为温度,相对湿度、饱和水汽压差等水分条件也显著影响了水分利用效率。年际差异分析结果表明,紫色土坡耕地夏季玉米生长盛期的水分利用效率对降雨响应更为敏感,同时冬季土壤水分为冬季蒸散发和水分利用效率的关键影响因子。未来仍需对紫色土坡耕地农田生态系统生长盛期蒸散发与水分利用效率动态进行深入研究,从而为探明当地主要作物应对春夏季季节性干旱威胁的系统性策略提供科学依据。     

Effect of forest structure on the spatial variation in soil respiration in a Bornean tropical rainforest

This study was undertaken to identify critical and practical factors explaining spatial variations in soil respiration and to estimate stand-scale soil respiration in an aseasonal tropical rainforest on Borneo Island. To this aim, we conducted soil respiration measurements at 25 points in a 40 m × 40 m subplot of a 4 ha study plot between 2002 and 2006, and examined the spatial variation in soil respiration averaged over the 4 years in relation to soil, root, and forest structural factors. In addition, we examined the spatial representativeness of soil respiration measured in the subplot using a specific scaling procedure. Consequently, we found significant positive correlation between the soil respiration and forest structural parameters such as the mean diameter at breast height (DBH), total basal area, and maximum DBH within 6 m of the measurement points. The most important factor was the mean DBH within 6 m of the measurement points, which had a significant linear relationship with soil respiration. Using the derived linear regression and an inventory dataset, we estimated the 4 ha plot-scale soil respiration. The 4 ha plot-scale estimation (6.0 μmol m⁻² s⁻¹) was nearly identical to the subplot-scale measurements (5.7 μmol m⁻² s⁻¹), which were roughly comparable to the nocturnal CO₂ fluxes calculated using the eddy covariance technique. In addition, we discuss characteristics of the stand-scale soil respiration at this site by comparing with those of other forests reported in previous literature in terms of the soil C balance. Soil respiration at our site was noticeably greater, relative to the incident litterfall amount, than soil respiration in other tropical and temperate forests probably owing to the larger total belowground C allocation by emergent trees. Overall, this study suggests the arrangement of emergent trees with larger DBH and their belowground C allocation could be primary factors controlling spatial variations in soil respiration in the tropical rainforest.     

Vertical structure of evapotranspiration at a forest site (a case study)

The components of ecosystem evapotranspiration of a Norway spruce forest (Picea abies L.) as well as the vertical structure of canopy evapotranspiration were analyzed with a combination of measurements and models for a case study of 5 days in September 2007. Eddy-covariance and sap flux measurements were performed at several heights within the canopy at the FLUXNET site Waldstein-Weidenbrunnen (DE-Bay) in the Fichtelgebirge mountains in Germany. Within and above canopy fluxes were simulated with two stand-scale models, the 1D multilayer model ACASA that includes a third-order turbulence closure and the 3D model STANDFLUX. The soil and understory evapotranspiration captured with the eddy-covariance system in the trunk space constituted 10% of ecosystem evapotranspiration measured with the eddy-covariance system above the canopy. A comparison of transpiration measured with the sap flux technique and inferred from below and above canopy eddy-covariance systems revealed higher estimates from eddy-covariance measurements than for sap flux measurements. The relative influences of possible sources of this mismatch, such as the assumption of negligible contribution of evaporation from intercepted water, and differences between the eddy-covariance flux footprint and the area used for scaling sap flux measurements, were discussed. Ecosystem evapotranspiration as well as canopy transpiration simulated with the two models captured the dynamics of the measurements well, but slightly underestimated eddy-covariance values. Profile measurements and models also gave us the chance to assess in-canopy profiles of canopy evapotranspiration and the contributions of in-canopy layers. For daytime and a coupled or partly coupled canopy, mean simulated profiles of both models agreed well with eddy-covariance measurements, with a similar performance of the ACASA and the STANDFLUX model. Both models underestimated profiles for nighttime and decoupled conditions. During daytime, the upper half of the canopy contributed approximately 80% to canopy evapotranspiration, whereas during nighttime the contribution shifted to lower parts of the canopy.     

大孔径闪烁仪与涡度相关系统对灌溉农田蒸散量的对比观测

蒸散是地表能量平衡的重要组成部分, 在土壤-植物-大气连续体的能量、质量、动量交换过程中起着重要作用。大孔径闪烁仪(LAS)是近时期兴起的观测跨像元尺度地表通量的地面仪器, 为验证其观测数据的可靠性, 本文把专家认可且在中国生态系统研究网络(CERN)中广泛应用的涡度相关仪(EC)的观测数据作为参考依据, 于2010 年8 月在中国科学院栾城农业生态系统试验站夏玉米田对大孔径闪烁仪计算结果进行验证。验证结果表明, 两种仪器测定的地表感热通量日变化和月变化观测结果基本一致。由于下垫面属性、环境因子及观测范围等因素的影响, 大孔径闪烁仪与涡度相关仪观测的蒸散量日变化之间有一定的差异,但日蒸散总量的测定基本一致。试验证明了大孔径闪烁仪观测数据的准确性。将大孔径闪烁仪计算的蒸散量与涡度相关仪观测值进行线性回归分析, 二者的吻合度较高(R²=0.800 4)。研究结果显示, 大孔径闪烁仪在地表水热通量的数据监测中具有很大的使用价值, 为日后用大孔径闪烁仪验证遥感估算蒸散值奠定了基础。     

Determination of actual evapotranspiration and crop coefficients of broad bean (Vicia Faba L.) grown under field conditions in the jordan valley,jordan: Bestimmung der aktuellen evapotranspiration und pflanzenbestandskoeffizienten von bohnen (Vicia Faba L.) unter feldbedingungen im jordantal,jordanien

This study was conducted to determine actual evapotranspiration and crop coefficients at different growth stages of broad bean (Vicia faba L.) grown in an open field in the Jordan Valley, Jordan using a precise and accurate approach. The study involved 30-min fluxes measurements of energy budget components over broad bean crop using a complete setup of an Eddy Correlation (EC) system. The measurements were conducted during the three main crop growth stages namely initial, development, and midseason growth stages following the Food and Agriculture Organization of the United Nations (FAO) crop coefficient model for green harvested broad bean crop. The average crop coefficients during the initial (K_{C ini}), development (K_{C dev}) and midseason (K_{C mid}) growth stages were 0.37, 0.8 and 1.05, respectively. The measured weighted average crop coefficient over the entire growing season K_{C GS} was 9.5% lower than the FAO corresponding value.

Results showed that there was a clear decrease of (bulk) surface resistance (r_s) as crop canopy developed. Daily average r_s values were 855, 337, and 166?s/m for initial, development, and midseason growth stages, respectively. Moreover, r_s was found to be highly correlated to crop height (h_c). A simple linear relation between r_s and h_c with R² of 0.91 was found. This relation will enable future direct determination of crop evapotranspiration (ET_C) using Penman-Monteith equation without the need to calculate both grass reference evapotranspiration (ET_O) and crop coefficient (K_C) values.     

Evaluation of alternative approaches to rainforest restoration on abandoned pasturelands in tropical North Queensland,Australia

The lag time for natural recruitment of tropical rainforest species in abandoned pastureland is very long, therefore artificial restoration techniques have been employed to accelerate natural seedling recruitment. The objectives of this study were to investigate: (1) the success/failure of establishment 502 seedlings belonging to 15 species from 11 families planted approximately ten years ago; and (2) the influence of different restoration techniques on enhancing natural recruitment during this period. The study was conducted in the wet tropical rainforest region of northeast Queensland, Australia as a completely randomized block design involving five treatments with two replicates. In each plot, 63 tropical rainforest seedlings from one or a combination of species were planted randomly. Two control plots were laid out where no seedlings were planted. Survival, height and diameter data were taken on the seedlings ten years after planting. Each 11×17 m² plot was further divided into 187, 1×1 m² subplots. Within each subplot all seedlings recruited were located and identified. Canopy cover was estimated using belt transects 1 m apart that ran in an east–west direction across the plots. Within each plot the percentage of grass, and the crown cover were estimated using the Braun‐Blanquet cover abundance scale. Survival rate of planted seedlings varied across the treatment plots. The survival rate ranged from 65 to 75 per cent for primary‐promoter species, 85 to 100 per cent in middle‐phase species and 42 to 57 per cent for mature‐phase species. No Pilidiostigma tropicum seedlings survived in any treatment. Fourteen species recruited naturally across the treatment plots. A total of 410 seedlings were naturally recruited from 11 different families in the ten‐year‐old reforested site. The highest natural recruitment (236 seedlings) occurred in Treatment 3, where Omalanthus novo‐guineensis seedlings were planted with eight primary‐promoter species, followed by 99 in Treatment 5 where a group of primary‐promoters, middle phase species and mature‐phase species were planted together, 36 in Treatment 4 (Alphitonia petriei planted with eight primary‐promoter species), 10 in Treatment 2 where only Omalanthus novo‐guineensis seedlings were planted, and 13 in control plots. Grass cover declined with increasing species diversity and increased canopy cover. The results indicate that the diversity of species used in restoration had a major influence on natural recruitment. Copyright © 2004 John Wiley & Sons, Ltd.     

Strong seasonal variations in net ecosystem CO2 exchange of a tropical pasture and afforestation in Panama

Pasture and afforestation are land-use types of major importance in the tropics, yet, most flux tower studies have been conducted in mature tropical forests. As deforestation in the tropics is expected to continue, it is critical to improve our understanding of alternative land-use types, and the impact of interactions between land use and climate on ecosystem carbon dynamics. Thus, we measured net ecosystem CO₂ fluxes of a pasture and an adjacent tropical afforestation (native tree species plantation) in Sardinilla, Panama from 2007 to 2009. The objectives of our paired site study were: (1) to assess seasonal and inter-annual variations in net ecosystem CO₂ exchange (NEE) of pasture and afforestation, (2) to identify the environmental controls of net ecosystem CO₂ fluxes, and (3) to constrain eddy covariance derived total ecosystem respiration (TER) with chamber-based soil respiration (R_Soil) measurements. We observed distinct seasonal variations in NEE that were more pronounced in the pasture compared to the afforestation, reflecting changes in plant and microbial activities. The land conversion from pasture to afforestation increased the potential for carbon uptake by trees vs. grasses throughout most of the year. R_Soil contributed about 50% to TER, with only small differences between ecosystems or seasons. Radiation and soil moisture were the main environmental controls of CO₂ fluxes while temperature had no effect on NEE. The pasture ecosystem was more strongly affected by soil water limitations during the dry season, probably due to the shallower root system of grasses compared to trees. Thus, it seems likely that predicted increases in precipitation variability will impact seasonal variations of CO₂ fluxes in Central Panama, in particular of pasture ecosystems.     

用涡度相关法测定网室内香蕉树蒸散量

为了研究涡度相关法在网室内的适用性,于2005年6月在一个长、宽、高分别为352m、228m和6m的香蕉树种植网室内进行试验,测定的参数主要有网室内的潜热和感热、网室内的净辐射,土壤热通量和表层土壤热量.研究发现涡度相关法测定的能量和和小气候仪器测定的能量一致,说明涡度相关法可用来准确测定网室内作物的蒸散量.在试验期间(6月7日-20日),涡度相关法测定的网室内香蕉树的蒸散量在4.83～6.50 mm/d之间,考虑到当地每年4～10月之间气象要素变化较小,则该文测定的蒸散量可用来指导当地网室内香蕉树的灌溉.     

Litter mixture effects on decomposition in tropical montane rainforests vary strongly with time and turn negative at later stages of decay

In a litterbag study in a tropical montane rainforest in Ecuador we assessed the impact of leaf litter species identity and richness on decomposition. We incubated leaf litter of six native tree species in monocultures and all possible two and four species combinations and analysed mass loss over a period of 24 months. Mass loss in monocultures averaged 30.7% after 6 month and differed significantly between species with variations being closely related to initial concentrations of lignin, Mg and P. At later harvests mass loss in monocultures averaged 54.5% but did not vary among leaf litter species and, unexpectedly, did not increase between 12 and 24 months suggesting that litter converged towards an extremely poor common quality retarding decomposition. After 6 months mass loss of leaf litter species was significantly faster in mixtures than in monocultures, resulting in synergistic non-additive mixture effects on decomposition, whereas at later harvests mass loss of component litter species was more variable and leaf litter mixture effects differed with species richness. Mass loss in the two species mixtures did not deviate from those predicted from monocultures, while we found antagonistic non-additive mixture effects in the four species mixtures. This suggests that litter species shared a poor common quality but different chemistry resulting in negative interactions in chemically diverse litter mixtures at later stages of decomposition. Overall, the results suggest that interspecific variations in diversity and composition of structural and secondary litter compounds rather than concentrations of individual litter compounds per se, control long term leaf litter decomposition in tropical montane rainforests. Plant species diversity thus appears to act as a major driver for decomposition processes in tropical montane rainforest ecosystems, highlighting the need for increasing plant conservation efforts to protect ecosystem functioning of this threatened biodiversity hotspot.     

Influence of evapotranspiration assessment on the accuracy of moisture transport modeling under the conditions of sprinkling irrigation in the south of Ukraine

ABSTRACT

The paper presents the results of computational experiments for evaluating the impact of different evapotranspiration assessment methods usage on the accuracy of soil moisture dynamics simulation for the conditions of sprinkling irrigation. We consider a differential one-dimensional model of moisture transport stated in terms of water head which uses as an initial data the estimates of evapotranspiration according to the methods of Ivanov, Shtoiko, Penman–Monteith and SEBAL. The differential moisture transport model was discretized using the finite-difference scheme. Its unknown parameters values were found by solving an inverse problem using the metaheuristic particle swarm optimization algorithm. We perform water head modeling accuracy assessment using different methods of evapotranspiration evaluation along with their linear combination. The Ivanov’s method was found to be the most accurate in the conditions of soybean growing in the south of Ukraine under sprinkling irrigation while linear combination of four considered methods gives an additional increase of accuracy by 6.5%.