Low LAI Research Articles

Competitiveness of chia against brassica weeds improves through a narrow spatial arrangement

Chia (Salvia hispanica L.) is a promising crop valued as a sustainable source of omega-3. However, chia yield and quality are reduced because of weed competition and the lack of registered herbicides. This study evaluated how chia spatial arrangement suppresses weed growth and provided management options for this crop. The performance of chia plants was assessed over three growing seasons in field experiments using a factorial design that considered spatial arrangement (narrow and wide), weed management (control without weeds, soil bank brassica (Brassica rapa L.), and sown brassica), and cover crop (with or without a rye (Secale cereale L.) cover crop during chia fallow). Chia and brassica are not complementary, leading to resource competition (relative yield total around 1) and negative effects on each other. A significant decrease in chia grain yield of 0.9% per unit increase in the percentage of weeds in total biomass was observed. As chia had a competitive advantage over brassica, the grain yield reduction was ameliorated with a narrow spatial arrangement. Narrow spatial arrangement promotes faster canopy closure during the early growth stages, decreasing light penetration and suppressing weed growth. The high extinction coefficient of chia (0.91) and the low critical LAI (3.3) contributed to this light interception behavior. Accordingly, to maximize chia productivity and profitability, a narrow spatial arrangement is an effective management strategy to suppress weeds. This study contributes valuable insights into chia-weed interactions and emphasizes the importance of sustainable crop management practices.

Winter climate preconditioning of summer vegetation extremes in the Northern Hemisphere

Abstract The impact of the spring climate on the Northern Hemisphere’s summer vegetation activity and extremes has been extensively researched, but less attention has been devoted to whether and how the winter climate may additionally influence vegetation extremes in the summer. Here, we provide insights into the influence of winter temperature and precipitation on summer vegetation extremes in the Northern Hemisphere. To do this, we identify positive and negative extremes in the summer leaf area index (LAI, a proxy for vegetation activity) and assess winter effects on those extremes using logistic regression at the regional scale. Over a quarter of the regions in the Northern Hemisphere show strong winter climate preconditioning on summer LAI extremes, which is typically stronger for croplands than forests. In regions with strong winter preconditioning, the spring LAI mediates the link between winter climate and summer LAI extremes through the ecological memory in seasonal legacy effects. Our findings suggest that extremely low summer LAI in both croplands and forests is preconditioned by colder and drier winters, while extremely high summer LAI in forests is associated with warmer and wetter winters. For low summer LAI in croplands, warmer winters are associated with an increased likelihood of extremes in mid-latitude regions and a reduced likelihood in high-latitude regions. Consideration of winter preconditioning effects may improve our understanding of inter-annual variability of vegetation activity and support agricultural and land management practitioners in anticipating the detrimental effects of winter on crop yields and forest conditions.

Utility of L-band and X-band vegetation optical depth to examine vegetation response to soil moisture droughts in South Asia

Vegetation Water Content (VWC) holds essential information about plant photosynthetic activity, phenological changes, ability to withstand extreme events, and soil-plant-atmosphere interactions. However, there is no consensus on the extent to which Soil Moisture (SM) and atmospheric dryness (Vapor Pressure Deficit, VPD) affect VWC during droughts. In this work, we determine the causal relationships with VWC in response to VPD and SM when plants are subjected to SM droughts during the growing season. We use satellite-derived Vegetation Optical Depth (VOD), obtained from two frequencies (L- and X- bands), to represent VWC in different parts of the plant (stem, branches, and leaves). The holistic response of VOD considering VPD and SM interactions is determined using bivariate and multivariate Granger causality tests. The analysis is carried out over South Asia.The results show a statistically significant coupling between VPD and SM at semi-weekly scales over 53.20% of the study area. In these locations, we observe a dominant influence of VPD compared to SM on VODL and VODX while subjected to soil moisture droughts during the growing season. In the areas where VPD and SM are decoupled, we find both VPD and SM to influence VODL and VODX. VODL and VODX largely exhibit contrasting causal responses. We observe a surge in VODL due to water stress while subjected to soil moisture droughts under all land cover conditions. In the case of VODX, we noticed both an increase and a decrease in leaf water content while subjected to SM droughts. The former scenario is noticed predominantly under moderate to severe drought intensity conditions during which plants attempt to maintain a stable water column (through increased stomatal regulation) for their functioning as a response to SM droughts. The latter scenario is noticed in most pixels, including forests and croplands. The observed leaf water content loss in forests is due to their mild drought intensities. We also observe a stronger causal influence of VPD and SM in low LAI areas than in high LAI areas, indicating a pressing plant water stress in sparse vegetation conditions during SM droughts.

The Use of Spectral Indices to Recognize Waterlogged Agricultural Land in South Moravia, Czech Republic

The agricultural landscape of the Czech Republic is facing climate change, and drought is among the most severe stress factors. Thousands of small ponds and naturally wet areas have been drained and transformed into agricultural parcels. Their restoration could increase the landscape’s resilience to climate change. Therefore, we describe the possibility of using hyperspectral aerial surveying for the identification of waterlogged areas in the agricultural landscape based on the example of one of the warmest and driest regions of the Czech Republic—the South Moravian region, an area where water retention in the landscape is highly relevant. Within our study, a total of 33 spectral indices related to the waterlogging of soil selected from previous studies were evaluated. The maximum entropy model (MAXENT) was used in the analysis of these indices. The analysis, which was carried out in several locations during different periods of the year (spring and autumn), shows the varying applicability of individual groups of indices. Regardless of the season, chlorophyll-based indices (MCARI—31.8, CARI—26.3, TCARI2—24.3 average percentage contribution) made the most significant contribution to the creation of probability maps of the occurrence of waterlogged areas. However, more accurate results could be achieved in the spring period by using the NVI index (40.5 average percentage contribution). The results show that remote sensing could be used for the identification of waterlogged sites, especially for initial identification, which should then be confirmed by field survey. Furthermore, the research points out the role of the LAI and chlorophyll content. According to the NVI, low LAI contributes the most to the probability of occurrence in the spring season, while chlorophyll-based indices prove to be the best, contributing high values, which is rather contradictory but could be resolved only by subsequent field research.

A Greening Strategy of Mitigation of the Thermal Environment for Coastal Sloping Urban Space

Greening can usually have a cooling effect on urban space; but is this law also applicable to coastal sloping urban space? The coastal urban space of Qingdao Haizhifeng Square, with a sloping topography, was the area we selected to study. The study area contained two parts: a coastal green space and a residential area. ENVI-met was used to create six scenarios. Different lawns, black pine and ash were planted in the two areas to study the cooling effect. The results showed that the closer the area was to the sea, the better the thermal comfort. In both the coastal green area and the residential area, trees increased the PET of the site, and the higher the LAI of the trees, the more obvious the thermal effect. At 15:00, the hottest time during the summer, the highest PET at pedestrian height was lowest in the scenario without trees, reaching 28.3 °C, and the highest was with full ash, reaching 34.3 °C. At the same time, the average difference in PET between the two scenarios was 1.4 °C. The highest PET at pedestrian height was generated in the area of the building away from the sea breeze, especially in the case of the sloping topography behind it or dense street trees on the urban road. Finally, it was concluded that, in urban spaces with a coastal slope topography, lawns should be planted in the coastal green part and low LAI trees in residential areas, and shade trees should not be planted on the coastal walkway. This afforestation strategy can provide a basis to formulate a strategy for promoting the design of regions with similar geographical and climatic conditions in the future.

Short-and long-latency afferent inhibition of the human leg motor cortex by H-reflex subthreshold electrical stimulation at the popliteal fossa

In humans, peripheral sensory stimulation inhibits subsequent motor evoked potentials (MEPs) induced by transcranial magnetic stimulation; this process is referred to as short- or long-latency afferent inhibition (SAI or LAI, respectively), depending on the inter-stimulus interval (ISI) length. Although upper limb SAI and LAI have been well studied, lower limb SAI and LAI remain under-investigated. Here, we examined the time course of the soleus (SOL) muscle MEP following electrical tibial nerve (TN) stimulation at the popliteal fossa at ISIs of 20–220 ms. When the conditioning stimulus intensity was three-fold the perceptual threshold, MEP amplitudes were inhibited at an ISI of 220 ms, but not at shorter ISIs. TN stimulation just below the Hoffman (H)-reflex threshold intensity inhibited MEP amplitudes at ISIs of 30, 35, 100, 180 and 200 ms. However, the relationship between MEP inhibition and the P30 latency of somatosensory evoked potentials (SEPs) did not show corresponding ISIs at the SEP P30 latency that maximizes MEP inhibition. To clarify whether the site of afferent-induced MEP inhibition occurs at the cortical or spinal level, we examined the time course of SOL H-reflex following TN stimulation. H-reflex amplitudes were not significantly inhibited at ISIs where MEP inhibition occurred but at an ISI of 120 ms. Our findings indicate that stronger peripheral sensory stimulation is required for lower limb than for upper limb SAI and LAI and that lower limb SAI and LAI are of cortical origin. Moreover, the direct pathway from the periphery to the primary motor cortex may contribute to lower limb SAI.

Converted vegetation type regulates the vegetation greening effects on land surface albedo in arid regions of China

Land surface albedo (LSA) is a key parameter in the process of vegetation feedback to climate due to its decisive role in land surface radiation budget. However, our current knowledge on the relationship between LSA and vegetation changes is limited by one-sided attention to sole vegetation growth change or land use/land cover change (LULCC). How vegetation growth or LULCC respectively contributes to LSA change under their interactions remains poorly quantified. In this study, the arid and semi-arid areas of China (ASAC) with profound vegetation changes were selected to tackle this problem. The LSA showed a general downward trend (-0.00009 year−1) during 2000-2018 in response to ASAC's wide-range greening (0.0086 m2m−2year−1). Pairwise comparison analysis revealed that under the same unit of vegetation coverage change, the LSA change magnitude was contracted when grassland was converted to cultivated land or forest land, while the conversions of forest land to other vegetations led to an amplified change magnitude of LSA. Vegetation type directly leads to a difference in LSA change magnitude under greening due to their distinct canopy spectral characteristics. Grassland possesses lower LAI and its pixel-level LSA is more prone to be contaminated by background coverage, while LSA of forest land contains more vegetation canopy signal. In most areas where vegetation type converted, greening dominated LSA change with a contribution rate up to 98.14 %, except for the conversion of grassland to forest land, where LULCC accounted for about 66.38 % of LSA change. This study could improve the estimation of LSA from LAI, which is useful for optimizing vegetation-climate interaction models.

Shade and Agronomic Intensification in Coffee Agroforestry Systems: Trade-Off or Synergy?

Despite many years of research, there is still a lack of consensus as to the nature of the relationship between shade trees and agronomic intensification in coffee agroforestry systems. While some studies find unshaded intensively managed coffee is the most productive, other studies find no trade-off between shade characteristics and productivity. Our study of 179 farms from the main coffee growing regions of Costa Rica and Guatemala analyzed how shade affected the productive response of coffee to intensification of agronomic management. Four levels of coffee productivity were differentiated for each country associated with three levels of shade development in Costa Rica and two levels in Guatemala. The highest coffee productivity group was associated with medium shade development in both countries. High shade groups had low productivity, but very low productivity groups were associated with low (Costa Rica) or medium (Guatemala) shade. Medium and high productivity farms were associated with high elevation, lower rainfall and regions with higher coffee prices. Yields with a moderate level of investment (720–1,500 USD−1) and with medium shade (LAI 0.55-1.1) were not significantly different from yields with higher investment or lower shade levels. The increase in yields with increasing N fertilizer application were similar under low, medium and high LAI, but the mean productivity significantly lower with high LAI. Agronomic intensification to increase productivity is equally effective for low and medium shade systems (LAI <1.1); low productivity farms may have high shade or low shade but are mainly limited by low investment. Intensification of production is compatible with medium shade levels that should deliver broader ecosystem services but achieving this depends on coffee prices enabling this investment.

Straw mulching improves soil water content, increases flag leaf photosynthetic parameters and maintaines the yield of winter wheat with different irrigation amounts

In North China Plain, straw mulching is increasingly applied to the succeeding crops with winter wheat-summer maize double cropping system. However, research results on the effect of maize straw mulching on the yield of winter wheat are inconsistent, and the underlying mechanism of the effect remains unclear. Such information is helpful to guide the field water management for higher yield. In this study, soil moisture, plant growth, photosynthesis and yield of winter wheat field treated with no-till and straw mulching (SM) and non- mulching (N) and three irrigation amounts (high, middle and low irrigation, HI, MI and LI) in four growing seasons in 2013–2016. The results showed that straw mulching improved soil moisture by reducing the days of soil moisture less than 60% field capacity for 2–10 days. Straw mulching increased net photosynthetic rate, stomatal conductance, maximum carboxylation rate (Vcmax), and the maximum rate of photosynthetic electron transport (Jmax) of flag leaves, especially at the post-anthesis measurements, during which, the above parameters increased by 20.6%, 21.9%, 28.7% and 25.2%, respectively. Compared with pre-anthesis measurements, the post-anthesis Vcmax and Jmax values decreased, but the decrease percentage of the SM treatments (10.8–25.7% for Vcmax and 22.0%−49.6% for Jmax) was less than those of N treatments (19.6%−41.4% for Vcmax and 29.3%−61.3% for Jmax). Vcmax and Jmax were significantly linear correlating to leaf nitrogen content and the relationships between them were not affected by straw mulching. The yield was not affected by straw mulching. However, straw mulching significantly increased the intercept of the linear relationship between yield and canopy potential photosynthetic capacity (the productions of leaf area index and Vcmax, or Jmax), indicating that straw mulching may improve the harvest index of winter wheat. The overall net effect of straw mulching was a favorable environment that stimulated plant photosynthesis, which compensated the lower LAI and tiller density, in turn, maintained wheat yield. This study elucidated the physiological basis of straw mulching and no-till effect on yield and provided photosynthetic parameters of winter wheat under such soil management.

Optimizing production by varying leaf number and stem density in high-wire cucumber

As earlier leaf removal experiments in tomato had shown that this improved dry matter partitioning to the fruits and production, a pilot experiment with high-wire cucumber was conducted in the spring and summer of 2016 at the experimental research facilities of Wageningen UR Greenhouse Horticulture. The five treatments were combinations of 3 or 3.5 stems m-2, and 0, 16 or 33% leaf removal in head of the crop. The combination 3.5/0% was lacking. The experimental data were analysed with the INTKAM crop growth model. The total number of fruits m-2 was the most important factor for production, which is determined by the optimal combination of stem density and assimilate supply. It is therefore important to grow the crop such that the maximum number of fruits with sufficient weight can be realized. Leaf removal in itself led to a slightly higher abortion, a lower development rate and therefore less axils, and a lower LAI with less light interception, and therefore produced less fruits and kilos. This, however, can be compensated with a higher stem density. It results in a crop with a normal LAI and architecture, combined with more axils m-2 that can bear fruits. Production and light use efficiency increase. On the other hand, labour and the risk to harvest small fruits will increase.

Joint Use of PROSAIL and DART for Fast LUT Building: Application to Gap Fraction and Leaf Biochemistry Estimations over Sparse Oak Stands

Gap Fraction, leaf pigment contents (content of chlorophylls a and b (Cab) and carotenoids content (Car)), Leaf Mass per Area (LMA), and Equivalent Water Thickness (EWT) are considered relevant indicators of forests’ health status, influencing many biological and physical processes. Various methods exist to estimate these variables, often relying on the extensive use of Radiation Transfer Models (RTMs). While 3D RTMs are more realistic to model open canopies, their complexity leads to important computation times that limit the number of simulations that can be considered; 1D RTMs, although less realistic, are also less computationally expensive. We investigated the possibility to approximate the outputs of a 3D RTM (DART) from a 1D RTM (PROSAIL) to generate in very short time numerous extensive Look-Up Tables (LUTs). The intrinsic error of the approximation model was evaluated through comparison with DART reference values. The model was then used to generate LUTs used to estimate Gap Fraction, Cab, Car, EWT, and LMA of Blue Oak-dominant stands in a woodland savanna from AVIRIS-C data. Performances of the approximation model for estimation purposes compared to DART were evaluated using Wilmott’s index of agreement (dr), and estimation accuracy was measured with coefficients of determination (R2) and Root Mean Squared Error (RMSE). The low approximation error of the proposed model demonstrated that the model could be considered for canopy covers as low as 30%. Gap Fraction estimations presented similar performances with either DART or the approximation (dr 0.78 and 0.77, respectively), while Cab and Car showed improved performances (dr increasing from 0.65 to 0.77 and 0.34 to 0.65, respectively). No satisfying estimation methods were found for LMA and EWT using either models, probably due to the high sensitivity of the scene’s reflectance to Gap Fraction and soil modeling at such low LAI. Overall, estimations using the approximated reflectances presented either similar or improved accuracy. Our findings show that it is possible to approximate DART reflectances from PROSAIL using a minimal number of DART outputs for calibration purposes, drastically reducing computation times to generate reflectance databases: 300,000 entries could be generated in 1.5 h, compared to the 12,666 total CPU hours necessary to generate the 21,840 calibration entries with DART.

EVALUATION OF THIRD CYCLE PROGENY TEST FOR COMPACT CHARACTER IN IOPRI’S OIL PALM BREEDING PROGRAM

One of the secondary characters desired by oil palm planters is compact palm. Compact characters are characterized by slow height increment rate and short rachis length. Both characters are able to extend the economic life of oil palm and increase palm productivity. This paper will discuss the results of the evaluation of compact characters in several crosses of the third cycle progeny test of RRS (Reciprocal Recurrent Selection) program. The field trial was carried out on 21 crosses planted in 2008 at Dolok Sinumbah Estate. The observations showed that cross no. BJ62/05 and BJ05/05 have the potential to produce compact palm with the slowest height increment rates, the shortest rachis, and low LAI values. With these characters, these crosses can be planted at higher planting densities. Parents no. BJ5633D and 93-133-4T also have the potential to produce offspring with compact characters.

Importance of canopy closure and dry matter partitioning for yield formation of sugar beet varieties

Phenotyping of sugar beet canopy for yield estimate requires a close relationship between canopy characteristics and final sugar yield. The objective of the study was to identify the importance of early canopy closure and of dry matter partitioning for high sugar yields, or more general, to study whether sugar beet yield is limited more by the source or by the sink capacity. Randomized field trials were conducted with two varieties (sugar type, yield type) in three sowing dates and four harvest dates at two sites in three years (2012–2014). Results showed very stable variety characteristics for all parameters and a linear increase of sugar yield with thermal time from September to November. Although variety 2 had reached canopy closure 100 GDD later and had a lower LAI and leaf yield than variety 1 throughout the growing season, it reached a higher sugar yield. That could be attributed to a higher RUE and a higher fraction of dry matter partitioned to sugar storage, while total dry matter did not differ distinctly. In the storage root, more cambial rings were formed, indicating a higher sink strength. Hence, sugar yield is more determined by dry matter partitioning (sink) than by canopy formation (source), making phenotyping difficult.

AN INSIGHT INTO THE VOLUME COMPONENT GENERATED FROM RISAT-1 HYBRID POLARIMETRIC SAR DATA FOR CROP BIOPHYSICAL PARAMETERS RETRIEVAL

Abstract. Polarimetric parameters have been extensively used for target parameters retrieval than backscattering coefficients. In previous studies, volume component generated from polarimetric SAR data has been considered as the return signal component from vegetation and intern used this for biophysical parameters retrieval. Un-polarized component of the return signal has been considered as volume component. The present study is mainly focused to analyze the volume component generated from C-band RISAT-1 hybrid polarimetric SAR data from wheat crop. Three temporal datasets acquired at ∼31° central incidence angle between Jan and Mar 2016 over parts of Bharatpur and Mathura districts located in Rajasthan and Uttar Pradesh (India) have been used in this study. Water Cloud Model with Gaps has been considered for modeling the first Stokes parameter (g0), which represents total intensity of return signal, from wheat crop using LAI and Interaction factor as vegetation descriptors. The vegetation component derived using calibrated Water Cloud Model with Gaps has been analyzed with volume component derived from RISAT-1 hybrid polarimetric SAR data. The analyses observed that a significant difference during lower LAI values and shown comparably during higher LAI values. The higher values of volume component derived from RISAT-1 SAR data than modeled vegetation component indicates that the volume component can also be generated by underneath soil. It is also observed the difference in derived un-polarized component and modeled vegetation component has shown higher correlation with underneath soil moisture than directly correlating with derived un-polarized component. This study indicates that the volume component derived from hybrid polarimetric SAR data has return signals from vegetation as well as underneath soil.

Growth analysis and yield performance of four potato (Solanum tuberosum L.) Varieties

An experiment with four potato varieties, namely Asterix, Cardinal, Diamant and Granola was conducted at the Horticulture Research Field of Bangabandhu Sheikh Mujibur Rahman Agricultural University, Gazipur during November 2013 to February 2014 to observe the behaviour of growth parameters and yield potentiality of potato. Final harvest of tuber was done at 90 DAP. LAI, CGR, RGR and NAR were not significantly influenced by the varieties. Irrespective of varieties, LAI increased progressively with time reaching a peak at 60 DAP and thereafter declined. Granola maintained the lower LAI throughout all growth stages. Numerically, the maximum CGR (19.99 g m-2 day-1) was found in Asterix at 50-60 DAP and the highest RGR (107.33 mg g-1 day-1) as well as NAR (9.54 g m-2 day-1) was recorded in Cardinal at 30-40 DAP. At 60-70 DAP, Cardinal recorded the highest TBR (84.35 g m-2 day-1) while Asterix, the highest TGR (18.54 g m-2 day-1) but Granola gave the lowest values of TBR (65.21 g m-2 day-1) and TGR (14.92 g m-2 day-1) at this growth stage. At all growth stages except 30 and 40 DAP, Diamant showed the best performance in SLA while Granola showed the lowest. The maximum yield was produced by Asterix (31.46 t/ha) closely followed by Diamant (29.91 t/ha) and Cardinal (27.27 t/ha) while the lowest yield was produced by Granola (24.82 t/ha). Granola showed the capacity of producing 95% or more of its final yield within 60 to 70 DAP. Therefore, Granola can be used as an early variety for potato cultivation.Bangladesh J. Agril. Res. 43(2): 267-280, June 2018

Analysis of evapotranspiration components of a rainfed olive orchard during three contrasting years in a semi-arid climate

Evapotranspiration is one of the most important fluxes of the water budget in semi-arid areas. The estimation of actual crop transpiration is a major issue in those regions due to its remarkable impacts on the precision of irrigation scheduling, crop growth and yield. Rainfed olive trees are adapted to the southern part of the Mediterranean basin even though they are vulnerable to an increased number of drought spells that might occur under current climate change scenarios. This present paper studies both water and energy exchanges over a rainfed olive grove in semi-arid conditions. The hydrological functioning of sparse olive trees is difficult to characterize because of its low LAI. To better understand water exchanges within the Soil–Plant–Atmosphere continuum and better evaluate the evapotranspiration and its components, we combine data arising from eddy covariance, soil water content measurements and the sap flow method. First, we check the consistency of the evapotranspiration partitioning and water balance over three contrasted years: one wet and two dry. Total evapotranspiration (ET) from eddy covariance method compares well with the sum of the evaporation (E) generated from the surface soil moisture measurements and the transpiration derived from the sap flow method. The top meter soil water balance corresponds roughly to ET during the wet year but for the dry years there is an evidence of extraction by roots below the first meter of soil. Inter-annual variations of the transpiration and associated water stress levels are analyzed by the combined use of different types of eco-physiological (sap flow) as well as remotely sensed variables that can be monitored through proxi-detection (albedo, surface temperature, surface soil moisture). The amount and timing of vegetation stress are consistent throughout the various indicators. Consequently, this consistent set of data can be used to constrain a SVAT land-surface model capable of representing the various features of the water and energy budget for this specific land cover.

Performance evaluation of biomass sorghum in Hawaii and Texas

Although biomass sorghum [Sorghum bicolor (L.) Moench] has been identified as a high yielding bioenergy feedstock crop on the continental USA, there is however lack of conclusive data on its performance in Hawaii (HI). The objectives of this study were to (i) determine the adaptability and productivity of two biomass sorghum hybrids, and (ii) identify the associated crop parameter attributes and environmental factors for high biomass yields. Two comparative trials were conducted, one on Maui island, HI and the other at Temple, Texas (TX). At Temple, the biomass sorghum hybrids responded as expected, growing to heights in excess of 3m and producing average biomass yields of 37.4Mgha−1. The high leaf area indices (LAI, 7.8–9.8) intercepted over 90% incident photosynthetically active radiation (PAR) at LAI >4, while the computed average light extinction coefficient (k) was 0.48. An average plant growth rate (PGR) of 189kg dry biomass ha−1 day-1 was achieved, while the average radiation use efficiency (RUE) was 2.75gMJ−1. In sharp contrast to the results obtained at Temple, the two sorghum hybrids planted on Maui behaved like photoperiod insensitive short day grain sorghums; lower plant heights (<2.0m), LAI (average, 3.8) and PGR prior to heading (average, 41kgha−1 day-1), flowered early (∼90days after planting), and had aboveground biomass yields reduced by 76%. The study underscored the importance of not only choosing the right bioenergy crop species, but also the suitability of target environments, planting date and management practices. The crop parameters determined at Temple can be incorporated into crop and environmental simulation models such as ALMANAC, EPIC/APEX and SWAT to model various biomass sorghum cropping systems and their associated environmental impacts.

Variações da reflectância e dos índices de vegetação em função dos parâmetros da modelagem topográfica no Parque Estadual do Turvo, Rio Grande do Sul, Brasil

Remote sensing techniques have been widely used in forestry studies as they allow evaluation and monitoring of large forested areas. The Parque Estadual do Turvo (pet) (17 491 ha) is the largest remaining tract of well-preserved subtropical deciduous forest in Southern Brazil, it constitutes the northern-most portion of the Misiones forest of Argentina (10 000 km2). The area is of great environmental importance and is suitable for conducting remote sensing studies using high or even coarse-to-moderate spatial resolution data and related vegetation indices. Both, reflectance values and vegetation indices are affected by external factors that modify the spectral response of surface elements. Among the factors that can induce errors in image interpretation are topographic effects that add spectral variability to satellite products. In addition, previous studies in subtropical forests have shown that the geometry of data acquisition also affects significantly the estimates of vegetation parameters derived from images acquired at off-nadir viewing or by sensors with large field-of-view (FOV).This study aimed to evaluate the magnitude of variations in bidirectional reflectance values and in vegetation indices derived from those, as a function of local topography, using high spatial resolution data acquired by the RapidEye satellite constellation.The method included the use of a digital elevation model (dem) from the Advanced Space-borne Thermal Emission and Reflection Radiometer – Global Digital Elevation Map version 2 (aster gdem v2) and two RapidEye scenes. From the dem, topographical parameters including slope aspect (eight classes), elevation (nine classes with 120-m spacing interval) and shaded relief (shaded, intermediate and sunlit surfaces) were derived. These data provide information on areas with or without direct exposure to solar radiation, depending on topographic features. RapidEye data were acquired on June 28, 2012 and October 17, 2012, corresponding to dates when the forest shows low or high leaf area index (lai), respectively. Both scenes were acquired with a view angle close to nadir. Solar elevation angles for the June and October images were 37.93° and 71.25°, respectively. The RapidEye data were corrected for atmospheric effects using the Fast Line-of-sight Atmospheric Analysis of Hypercubes (flaash®). The next step was to perform topographic modeling in order to extract slope, aspect and elevation data. From the topographic variables and the RapidEye metadata, shaded relief was calculated for both scenes. After segmenting the scenes based on the topographic variables, we evaluated the spectral reflectance and vegetation indices, as measured by RapidEye, as a function of topographic parameters. For this purpose, we randomly sampled 1000 pixels from each topographic class. The Normalized Difference Vegetation Index (ndvi) and Enhanced Vegetation Index (evi) were calculated from the RapidEye data. The samples were averaged and analyzed using graphics and descriptive statistics. Select transects were analyzed in more detail to evaluate the effects of local topographic parameters on the remote sensing products. In addition, we conducted forest surveys in 14 plots (20x50m) to produce a floristic-structural characterization of the deciduous forest.The field inventories identified a total of 74 plant species (in 31 families) distributed in three strata and showed the presence of discontinuities within the forest. The results showed that, in addition to seasonal phenological variations, local illumination conditions caused by the relief in the pet contributed to explain the forest spectral response. The topographic variable that most importantly affected the pet spectral response, as measured by RapidEye, was slope aspect. Despite being affected by varying illumination conditions in the two dates, the reflectance and vegetation indices of the June image were most impacted by the shaded relief caused by the low solar elevation and large amount of shadows in the scene. The illumination effects were compounded by the seasonal leaf shedding in the deciduous forests, especially in the upper canopy layer. The winter time (June) scene showed more shaded relief and lower lai values. In the October scene the shade fraction was substantially reduced and leaf shedding reduced the forest canopy anisotropy. A per band analysis showed that the red and near infrared bands were the ones with the highest dependence on aspect and shaded relief. Increases in elevation caused a positive change in near infrared reflectance and a lower reflectance in the visible bands. The ndvi showed lower dependence on topographical conditions than the evi. The evi showed higher sensitivity to illumination conditions, shade and seasonal lai variations.We concluded that the analysis of remote sensing data (reflectance values or vegetation indices such as ndvi and evi) should take into account local topographic effects. In general, the spectral anisotropy in the June scene was higher than in the October scene due the combined effect of deciduousness (leaf shedding), the lower solar elevation and larger amount of shadows.

Influence of the decoupling degree on the estimation of canopy stomatal conductance for two broadleaf tree species

Abstract Canopy stomatal conductance (G S ) estimation is a critical aspect of evapotranspiration research. Selecting an appropriate model for scaling-up is necessary to study plant water regulation. Two monoculture plantations of broadleaf tree species, Eucalyptus urophylla and Schima superba, were selected to study G S via a widely recognized equation simplified by Kostner (G S1 ) and an inverse Penman–Monteith equation (G S2 ). The decoupling coefficient (Ω) was estimated to quantify the decoupling extent of stomata from the atmosphere. We found that both species were well coupled with environmental conditions (0.1 ± 0.06 and 0.22 ± 0.09 for E. urophylla and S. superba ). Ω increased exponentially with canopy conductance (G C ) and was depressed by the increase of G a , which implied that there was a combined climatic and physiological regulation on Ω. G S1 for both species (63.7 ± 33.9 mmol m −2 s −1 and 48.4 ± 18.1 mmol m −2 s −1 for E. urophylla and S. superba, respectively) was underestimated with the simplified equation compared to G S2 (77.0 ± 52.4 mmol m −2 s −1 and 112.0 ± 52.5 mmol m −2 s −1 for E. urophylla and S. superba , respectively). The ratio of G S1 /G S2 linearly decreased with Ω by a slope of −1.92 and −1.31 for E. urophylla and S. superba , respectively. The increase of the LAI tended to increase the decoupling extent, which further reduced the accuracy of the estimation of G S1 . According to our results, the ratio of G S1 /G S2 had a mean of 0.94 for 85.3% and 0.6 for 37.6% of all of the data for E. urophylla and S. superba , respectively, implying a better estimation of G S1 for the stand that had a lower LAI. Based on the relationship between the LAI and the ratios of G S1 /G S2 (ratio = 1.0759 × 0.02547 (0.053LAI + 0.054) − 0.0078), we can shed some light on the prediction error of stomatal conductance derived from the simplified equation in different forest types.

Warm and Fertile Sub-Humid Conditions Enhance Litterfall to Sustain High Soil Respiration Fluxes in a Mediterranean Cork Oak Forest

Soil respiration is a major component of the global carbon budget and Mediterranean ecosystems have usually been studied in locations with shallow soils, mild temperatures, and a prolonged dry season. This study investigates seasonal soil respiration rates and underlying mechanisms under wetter, warmer, and more fertile conditions in a Mediterranean cork oak forest of Northern Tunisia (Africa), acknowledged as one of the most productive forests in the Mediterranean basin. We applied a soil respiration model based on soil temperature and relative water content and investigated how ecosystem functioning under these favorable conditions affected soil carbon storage through carbon inputs to the soil litter. Annual soil respiration rates varied between 1774 gC m−2 year−1 and 2227 gC m−2 year−1, which is on the highest range of observations under Mediterranean climate conditions. We attributed this high soil carbon flux as a response to favorable temperatures and soil water content, but this could be sustained only by a small carbon allocation to roots (root/shoot ratio = 0.31–0.41) leading to a large allocation to leaves with a multiannual leaf production, enhanced annual twig elongation (11.5–28.5 cm) with a reduced leaf life span (&lt;1 year) maintaining a low LAI (1.68–1.88) and generating a high litterfall (386–636 gC m−2 year−1). Thus, the favorable climatic and edaphic conditions experienced by these Mediterranean cork oak forests drove high soil respiration fluxes which balanced the high carbon assimilation leading to a relatively small overall contribution (10.96–14.79 kgC m−2) to soil carbon storage.