Showing papers on "Leaf area index published in 2010"
TL;DR: Overall, the current review analysis demonstrates that a suite of chemical and architectural traits at various scales and their plasticity drive plant light harvesting efficiency.
Abstract: Changes in the efficiency of light interception and in the costs for light harvesting along the light gradients from the top of the plant canopy to the bottom are the major means by which efficient light harvesting is achieved in ecosystems. In the current review analysis, leaf, shoot and canopy level determinants of plant light harvesting, the light-driven plasticity in key traits altering light harvesting, and variations among different plant functional types and between species of different shade tolerance are analyzed. In addition, plant age- and size-dependent alterations in light harvesting efficiency are also examined. At the leaf level, the variations in light harvesting are driven by alterations in leaf chlorophyll content modifies the fraction of incident light harvested by given leaf area, and in leaf dry mass per unit area (M
A) that determines the amount of leaf area formed with certain fraction of plant biomass in the leaves. In needle-leaved species with complex foliage cross-section, the degree of foliage surface exposure also depends on the leaf total-to-projected surface area ratio. At the shoot scale, foliage inclination angle distribution and foliage spatial aggregation are the major determinants of light harvesting, while at the canopy scale, branching frequency, foliage distribution and biomass allocation to leaves (F
L) modify light harvesting significantly. F
L decreases with increasing plant size from herbs to shrubs to trees due to progressively larger support costs in plant functional types with greater stature. Among trees, F
L and stand leaf area index scale positively with foliage longevity. Plant traits altering light harvesting have a large potential to adjust to light availability. Chlorophyll per mass increases, while M
A, foliage inclination from the horizontal and degree of spatial aggregation decrease with decreasing light availability. In addition, branching frequency decreases and canopies become flatter in lower light. All these plastic modifications greatly enhance light harvesting in low light. Species with greater shade tolerance typically form a more extensive canopy by having lower M
A in deciduous species and enhanced leaf longevity in evergreens. In addition, young plants of shade tolerators commonly have less strongly aggregated foliage and flatter canopies, while in adult plants partly exposed to high light, higher shade tolerance of foliage allows the shade tolerators to maintain more leaf layers, resulting in extended crowns. Within a given plant functional type, increases in plant age and size result in increases in M
A, reductions in F
L and increases in foliage aggregation, thereby reducing plant leaf area index and the efficiency of light harvesting. Such dynamic modifications in plant light harvesting play a key role in stand development and productivity. Overall, the current review analysis demonstrates that a suite of chemical and architectural traits at various scales and their plasticity drive plant light harvesting efficiency. Enhanced light harvesting can be achieved by various combinations of traits, and these suites of traits vary during plant ontogeny.
452 citations
TL;DR: It is hypothesized that drought could increase EVI by synchronizing leaf flushing via its effects on leaf bud development, and production of new leaves, even when unaccompanied by associated changes in LAI, could play an important role in Basin-wide interannual EVI variability.
Abstract: Drought exerts a strong influence on tropical forest metabolism, carbon stocks, and ultimately the flux of carbon to the atmosphere. Satellite-based studies have suggested that Amazon forests green up during droughts because of increased sunlight, whereas field studies have reported increased tree mortality during severe droughts. In an effort to reconcile these apparently conflicting findings, we conducted an analysis of climate data, field measurements, and improved satellite-based measures of forest photosynthetic activity. Wet-season precipitation and plant-available water (PAW) decreased over the Amazon Basin from 1996-2005, and photosynthetically active radiation (PAR) and air dryness (expressed as vapor pressure deficit, VPD) increased from 2002-2005. Using improved enhanced vegetation index (EVI) measurements (2000-2008), we show that gross primary productivity (expressed as EVI) declined with VPD and PAW in regions of sparse canopy cover across a wide range of environments for each year of the study. In densely forested areas, no climatic variable adequately explained the Basin-wide interannual variability of EVI. Based on a site-specific study, we show that monthly EVI was relatively insensitive to leaf area index (LAI) but correlated positively with leaf flushing and PAR measured in the field. These findings suggest that production of new leaves, even when unaccompanied by associated changes in LAI, could play an important role in Basin-wide interannual EVI variability. Because EVI variability was greatest in regions of lower PAW, we hypothesize that drought could increase EVI by synchronizing leaf flushing via its effects on leaf bud development.
272 citations
TL;DR: There were differences among varieties of corn and soybean for the vegetation indices during the growing season and these differences were a function of growth stage and vegetative index.
Abstract: The paper investigates the value of using distinct vegetation indices to quantify and characterize agricultural crop characteristics at different growth stages. Research was conducted on four crops (corn, soybean, wheat, and canola) over eight years grown under different tillage practices and nitrogen management practices that varied rate and timing. Six different vegetation indices were found most useful, depending on crop phenology and management practices: (a) simple ratio for biomass, (b) NDVI for intercepted PAR, (c) SAVI for early stages of LAI, (d) EVI for later stages of LAI, (e) CIgreen for leaf chlorophyll, (f) NPCI for chlorophyll during later stages, and (g) PSRI to quantify plant senescence. There were differences among varieties of corn and soybean for the vegetation indices during the growing season and these differences were a function of growth stage and vegetative index. These results clearly imply the need to use multiple vegetation indices to best capture agricultural crop characteristics.
263 citations
TL;DR: In this paper, the applicability of two direct (litterfall, allometry) and five indirect (LAI-2000, TRAC, digital hemispheric photography, digital cover photography, traversing radiometer system) methods to determine leaf area index across a 9 ha domain in an oak-savanna ecosystem in California, USA.
253 citations
TL;DR: In this article, the authors used the apparent clumping index to constrain the true CLF and investigate spatial and temporal variation of clumping effects, which is useful to evaluate a coarse global CLF map and improve land surface models.
189 citations
TL;DR: In this article, an approach to integrate crop stressors and crop descriptors derived from optical remote sensing data with the Monteith radiation use efficiency model was developed for estimating crop aboveground dry biomass and yield.
174 citations
TL;DR: In this article, a comparison of the advantages and drawbacks of the LAI-2000 and hemispherical photography in terms of applicability and accuracy was made, and the best relationship between the LAIs and the Hemisfer photographs was obtained when the hemisherical photographs were overexposed by one to two stops compared with the exposure setting derived from the reading of a spotmeter in a canopy gap.
Abstract: Leaf area index (LAI) was estimated at 15 sites in the Swiss Long-Term Forest Ecosystem Research Programme (LWF) in 2004–2005 using two indirect techniques: the LAI-2000 plant canopy analyzer (Licor Inc.) and digital hemispherical photography, applying several exposure settings. Hemispherical photographs of the canopy were analysed using Hemisfer, a software package that offers several new features, which were tested here: (1) automatic thresholding taking the gamma value of the picture into account; (2) implementation of several equations to solve the gap-fraction inversion model from which LAI estimates are derived; (3) correction for ground slope effects, and (4) correction for clumped canopies. In seven broadleaved stands in our sample set, LAI was also estimated semi-directly from litterfall. The various equations used to solve the gap-fraction inversion model generated significantly different estimates for the LAI-2000 measurements. In contrast, the same equations applied in Hemisfer did not produce significantly different estimates. The best relationship between the LAI-2000 and the Hemisfer estimates was obtained when the hemispherical photographs were overexposed by one to two stops compared with the exposure setting derived from the reading of a spotmeter in a canopy gap. There was no clear general relationship between the litterfall and the LAI-2000 or the hemispherical photographs estimates. This was probably due to the heterogeneity of the canopy, or to biased litterfall collection at sites on steep slopes or sites subject to strong winds. This study introduces new arguments into the comparison of the advantages and drawbacks of the LAI-2000 and hemispherical photography in terms of applicability and accuracy.
159 citations
TL;DR: In this article, the spectral reflectance of cereal canopies measured with a digital camera correlated closely with N status during the vegetative and early stem elongation phases during the harvest season.
158 citations
TL;DR: In this article, the leaf area index, LAI, based thermal performance in distinguishing trees for Cairo's urban developments, ENVI-met plants database was used as platform for a foliage modeling parameter, the leaf areas density, LAD.
137 citations
TL;DR: Yangmai 158 and Yangmai 11 were shaded from jointing to maturity to evaluate the impact of low radiation on crop growth, photosynthesis and yield in the major wheat planting area of China as discussed by the authors.
Abstract: Low radiation reduces wheat grain yield in tree-crop intercropping systems in the major wheat planting area of China. Here, two winter wheat (Triticum aestivum L) cultivars, Yangmai 158 (shading tolerant) and Yangmai 11 (shading sensitive), were shaded from jointing to maturity to evaluate the impact of low radiation on crop growth, photosynthesis and yield. Grain yield losses and leaf area index (LAI) reduction were less than the reduction in solar radiation under both shading treatment in both cultivars. Compared with the control (S0), grain yield only reduced 6.4 % and 9.9 % under 22 % shading treatment (S1), while 16.2 % and 25.8 % under 33 % shading (S2) in Yangmai 158 and Yangmai 11 respectively. The reduction in LAI was 6.0 % and 9.2 % (S1), and 18.2 % and 22.2 % (S2) in Yangmai 158 and Yangmai 11 respectively. However, decline in canopy apparent photosynthetic rate (CAP) was 15.0–22.9 % (S1) and 29.5–49.6 % (S2), which was consistent with the reduction in radiation. The reduction in LAI was partially compensated by increases in the fraction of the top and bottom leaf area to the total leaf area, which facilitated to intercept more solar radiation by the canopy. The decrease in photosynthetic rate (Pn) of flag leaf was partially compensated by the increase in Pn of the third leaf from the top. In addition, an inconsistency between the low Pn and the high Chl content in flag leaf was observed at 30 DAA. This could be explained that more excitation energy was dispersed via the non-photochemical approaches in the photosystem II (PSII) of flag leaf after long-term shading.
135 citations
TL;DR: In this paper, brightness normalization was used to improve the performance of Partial Least Squares Regression (PLSR) under changing canopy and observing conditions, and the spectral data was combined with a canopy radiative transfer model to simulate effects of varying canopy structure and viewing geometry.
Abstract: Developed in the field of chemometrics, Partial Least Squares Regression (PLSR) has become an established technique in vegetation remote sensing. PLSR was primarily designed for laboratory analysis of prepared material samples. Under field conditions in vegetation remote sensing, the performance of the technique may be negatively affected by differences in brightness due to amount and orientation of plant tissues in canopies or the observing conditions. To minimize these effects, we introduced brightness normalization to the PLSR approach and tested whether this modification improves the performance under changing canopy and observing conditions. This test was carried out using high-fidelity spectral data (400–2510 nm) to model observed leaf chemistry. The spectral data was combined with a canopy radiative transfer model to simulate effects of varying canopy structure and viewing geometry. Brightness normalization enhanced the performance of PLSR by dampening the effects of canopy shade, thus providing a significant improvement in predictions of leaf chemistry (up to 3.6% additional explained variance in validation) compared to conventional PLSR. Little improvement was made on effects due to variable leaf area index, while minor improvement (mostly not significant) was observed for effects of variable viewing geometry. In general, brightness normalization increased the stability of model fits and regression coefficients for all canopy scenarios. Brightness-normalized PLSR is thus a promising approach for application on airborne and space-based imaging spectrometer data.
TL;DR: In this article, a histogram-based threshold technique was used to separate green vegetation tissues from background soil and residue materials in order to derive the canopy vertical gap fraction from the digital photos.
TL;DR: The results suggest that evergreen and deciduous woodlands have specific, and similar, ecological costs in mediterranean climates, and that both leaf habits are able to meet these costs.
Abstract: We assessed the differential advantages of deciduousness and evergreenness by examining 26 site-years of carbon dioxide, water vapor, and energy flux measurements from five comparable oak woodlands in France, Italy, Portugal, and California (USA). On average, the evergreen and deciduous oak woodlands assimilated and respired similar amounts of carbon while using similar amounts of water. These results suggest that evergreen and deciduous woodlands have specific, and similar, ecological costs in mediterranean climates, and that both leaf habits are able to meet these costs. What are the mechanisms behind these findings? Deciduous oaks compensated for having a shorter growing season by attaining a greater capacity to assimilate carbon for a given amount of intercepted solar radiation during the well-watered spring period; at saturating light levels, deciduous oaks gained carbon at six times the rate of evergreen oaks. Otherwise, the two leaf habits experienced similar efficiencies in carbon use (the change in carbon respired per change in carbon assimilated), water use (the change in carbon assimilation per change in water evaporated), and rainfall use (the change in evaporation per change in rainfall). Overall, leaf area index, rather than leaf habit, was the significant factor in determining the absolute magnitude of carbon gained and water lost by each evergreen and deciduous oak woodland over an annual interval; the closed canopies assimilated and respired more carbon and transpired more water than the open canopies. Both deciduous and evergreen mediterranean oaks survive in their seasonally hot/dry, wet/ cool native range by ensuring that actual evaporation is less than the supply of water. This feat is accomplished by adjusting the leaf area index to reduce total water loss at the landscape scale, by down-regulating photosynthesis, respiration, and stomatal conductance with progressive seasonal soil water deficits, and by extending their root systems to tap groundwater.
TL;DR: Leaf and canopy acclimation to progressive, long-term drought occurred through changes in leaf area index, leaf mass per area and leaf chemical composition, but not through modifications of physiological parameters.
Abstract: Seasonal drought can severely impact leaf photosynthetic capacity. This is particularly important for Mediterranean forests, where precipitation is expected to decrease as a consequence of climate change. Impacts of increased drought on the photosynthetic capacity of the evergreen Quercus ilex were studied for two years in a mature forest submitted to long-term throughfall exclusion. Gas exchange and chlorophyll fluorescence were measured on two successive leaf cohorts in a control and a dry plot. Exclusion significantly reduced leaf water potential in the dry treatment. In both treatments, light-saturated net assimilation rate (A(max)), stomatal conductance (g(s)), maximum carboxylation rate (V(cmax)), maximum rate of electron transport (J(max)), mesophyll conductance to CO2 (g(m)) and nitrogen investment in photosynthesis decreased markedly with soil water limitation during summer. The relationships between leaf photosynthetic parameters and leaf water potential remained identical in the two treatments. Leaf and canopy acclimation to progressive, long-term drought occurred through changes in leaf area index, leaf mass per area and leaf chemical composition, but not through modifications of physiological parameters.
TL;DR: In this article, the authors examined the transpiration, leaf area and growth rates of Eucalyptus globulus Labill plantations aged 2-8 years and found that transpiration increased from 0.4-mm-day−1 at age 2 years to a peak of about 1.6-1.9mm-hour−1 in stands aged 5-7 years.
TL;DR: A review of the sensitivities of one such model, Hybrid6.5, a detailed mechanistic model of terrestrial primary production, is reviewed, finding that leaf phenology has large effects on mean C(3) crop and needleleaved cold deciduous tree production, reducing potential net primary production (NPP) from that obtained using constant maximum annual leaf area index.
Abstract: The likely future increase in atmospheric CO(2) and associated changes in climate will affect global patterns of plant production. Models integrate understanding of the influence of the environment on plant physiological processes and so enable estimates of future changes to be made. Moreover, they allow us to assess the consequences of different assumptions for predictions and so stimulate further research. This paper is a review of the sensitivities of one such model, Hybrid6.5, a detailed mechanistic model of terrestrial primary production. This model is typical of its type, and the sensitivities of the global distribution of predicted production to model assumptions and possible future CO(2) levels and climate are assessed. Sensitivity tests show that leaf phenology has large effects on mean C(3) crop and needleleaved cold deciduous tree production, reducing potential net primary production (NPP) from that obtained using constant maximum annual leaf area index by 32.9% and 41.6%, respectively. Generalized Plant Type (GPT) specific parameterizations, particularly photosynthetic capacity per unit leaf N, affect mean predicted NPP of higher C(3) plants by -22.3% to 27.9%, depending on the GPT, compared to NPP predictions obtained using mean parameter values. An increase in atmospheric CO(2) concentrations from current values to 720 ppm by the end of this century, with associated effects on climate from a typical climate model, is predicted to increase global NPP by 37.3%. Mean increases range from 43.9-52.9% across different C(3) GPTs, whereas the mean NPP of C(4) grass and crop increases by 5.9%. Significant uncertainties concern the extent to which acclimative processes may reduce any potential future increase in primary production and the degree to which any gains are transferred to durable, and especially edible, biomass. Experimentalists and modellers need to work closely together to reduce these uncertainties. A number of research priorities are suggested. 'The green leaf or, to be more precise, the microscopic green grain of chlorophyll, is the focus, the point in the world to which solar energy flows on one side while all the manifestations of life on earth take their source on the other side.' Kliment Arkadievich Timiryazev The conclusions of a century of plant physiology, speech at Moscow University, 12 January 1901.
TL;DR: In this paper, the authors investigated the effects of different plant spacings on rice plant characteristics and yield by comparing the plants grown with different methods of cultivation, focusing on the impact of different spacings.
Abstract: The System of Rice Intensification (SRI) reportedly enhances the yields of rice (Oryza sativa L.) through synergy among several agronomic management practices. This study was conducted to investigate the effects on rice plant characteristics and yield by comparing the plants grown with different methods of cultivation – SRI vs. recommended management practices (RMP) focusing on the impact of different plant spacings.
Performance of individual hills was significantly improved with wider spacing compared with closer-spaced hills in terms of root growth and xylem exudation rates, leaf number and leaf sizes, canopy angle, tiller and panicle number, panicle length and grain number per panicle, grain filling and 1000-grain weight and straw weight, irrespective of whether SRI or RMP was employed. Both sets of practices gave their highest grain yield with the spacing of 20 × 20 cm; however, SRI yielded 40 % more than the recommended practice. At this spacing, canopies also had the highest leaf area index (LAI) and light interception during flowering stage. The lowest yield was recorded at 30 × 30 cm spacing under both the practices, as a result of less plant population (11 m−2), despite improved hill performance.
During the ripening stage, hills with wider spacing had larger root dry weight, produced greater xylem exudates, and transported these towards shoot at faster rates. These features contributed to the maintenance of higher chlorophyll levels, enhanced fluorescence and photosynthesis rates of leaves and supported more favourable yield attributes and grain yield in individual hills than in closely-spaced plants.
Moreover, these parameters further improved in SRI, apart from the enhanced percentage of effective tillers and showed substantial and positive impacts on grain yield (17 %) compared with recommended practice. In conclusion, wide spacing beyond optimum plant density, however, does not give higher grain yield on an area basis and for achieving this, a combination of improved hills with optimum plant population must be worked out for SRI.
TL;DR: In this article, the authors present a sensitivity analysis for the estimation of chlorophyll content in woody coniferous canopies using radiative transfer modeling, and use the modeled top-of-canopy reflectance data to analyze the contribution of woody elements, leaf area index (LAI), and crown cover (CC) to the retrieval of foliar Cab content.
TL;DR: In this article, the authors examined whether the process-based growth model, 3-PG, is sufficiently sensitive to climatic variation to provide a virtual record of changes in growing stock across 180,000 ha eucalypt plantations.
TL;DR: In this article, the authors used a set of 16 Landsat TM and Landsat ETM+ images from 1984 to 2006 to test whether climate trends in the last three decades differentially controlled the vegetal activity of eight Pinus halepensis forests located across a marked bioclimatic gradient.
TL;DR: Estimates of genetic differences in soil evaporation within the four populations suggest that there is sufficient genetic variation to increase water harvest through targeting faster ground cover, and implications for improved wheat yields and breeding are discussed.
Abstract: Rapid development of leaf area and/or aboveground biomass has the potential to improve water harvest of rain fed wheat in Mediterranean-type environments through reduced soil evaporation. However, quantitative relationships between genetic differences in early ground cover and soil water evaporation have not been established. Furthermore, accurate phenotyping of ground cover and early vigour have typically been achieved via destructive sampling methods, which are too time-consuming to undertake within breeding programs. Digital image analysis has previously been identified as an alternative indirect method of analysis, whereby computer analysis is ued to determine percentage ground cover. This study uses a digital ground cover (DGC) analysis tool for high throughput screening of four large wheat populations. The DGC methodology was validated via comparisons with alternative measures of canopy cover, such as normalised difference vegetation index (NDVI) (r2 = 0.69), biomass (r2 = 0.63), leaf area index (r2 = 0.80) and light penetration through the canopy (r2 = 0.70). The wheat populations were utilised to estimate the potential variation in soil evaporation associated with genetic differences in early ground cover, which was validated using established models. Estimates of genetic differences in soil evaporation within the four populations (6.90–24.8 mm) suggest that there is sufficient genetic variation to increase water harvest through targeting faster ground cover. Implications for improved wheat yields and breeding are discussed.
TL;DR: The Normalized Area Over reflectance Curve (NAOC), proposed as a new index for remote sensing estimation of the leaf chlorophyll content of heterogeneous areas with different crops, different canopies and different types of bare soil, was proven to exhibit a linear correlation to chlorophylla content.
TL;DR: This simulation based on in situ ecophysiological research suggests the importance of intensive consideration and understanding of the phenology of leaf photosynthetic capacity and LAI to analyze and predict carbon fixation in forest ecosystems.
Abstract: Revealing the seasonal and interannual variations in forest canopy photosynthesis is a critical issue in understanding the ecological mechanisms underlying the dynamics of carbon dioxide exchange between the atmosphere and deciduous forests. This study examined the effects of temporal variations of canopy leaf area index (LAI) and leaf photosynthetic capacity [the maximum velocity of carboxylation (Vcmax)] on gross primary production (GPP) of a cool-temperate deciduous broadleaf forest for 5 years in Takayama AsiaFlux site, central Japan. We made two estimations to examine the effects of canopy properties on GPP; one is to incorporate the in situ observation of Vcmax and LAI throughout the growing season, and another considers seasonality of LAI but constantly high Vcmax. The simulations indicated that variation in Vcmax and LAI, especially in the leaf expansion period, had remarkable effects on GPP, and if Vcmax was assumed constant GPP will be overestimated by 15%. Monthly examination of air temperature, radiation, LAI and GPP suggested that spring temperature could affect canopy phenology, and also that GPP in summer was determined mainly by incoming radiation. However, the consequences among these factors responsible for interannual changes of GPP are not straightforward since leaf expansion and senescence patterns and summer meteorological conditions influence GPP independently. This simulation based on in situ ecophysiological research suggests the importance of intensive consideration and understanding of the phenology of leaf photosynthetic capacity and LAI to analyze and predict carbon fixation in forest ecosystems.
TL;DR: In this paper, the authors investigated the use of physiological indices calculated from hyperspectral remote sensing imagery as potential indicators of wine grape quality assessment in vineyards affected by iron deficiency chlorosis.
TL;DR: In this paper, the authors decompose the MODISLAI time series of each pixel into several components: trend, intra-annual variations, seasonal cycle, and stochastic stationary or irregular parts.
TL;DR: The CO2-driven stimulation of photosynthesis in these trees may persist in the upper canopy under future atmospheric CO2 concentrations without reductions in photosynthetic capacity, however, given the lack of growth stimulation, the fate of the additionally assimilated carbon remains uncertain.
Abstract: Carbon uptake by forests constitutes half of the planet’s terrestrial net primary production; therefore, photosynthetic responses of trees to rising atmospheric CO2 are critical to understanding the future global carbon cycle. At the Swiss Canopy Crane, we investigated gas exchange characteristics and leaf traits in five deciduous tree species during their eighth growing season under free air carbon dioxide enrichment in a 35-m tall, ca. 100-year-old mixed forest. Net photosynthesis of upper-canopy foliage was 48% (July) and 42% (September) higher in CO2-enriched trees and showed no sign of down-regulation. Elevated CO2 had no effect on carboxylation efficiency (Vcmax) or maximal electron transport (Jmax) driving ribulose-1,5-bisphosphate (RuBP) regeneration. CO2 enrichment improved nitrogen use efficiency, but did not affect leaf nitrogen (N) concentration, leaf thickness or specific leaf area except for one species. Non-structural carbohydrates accumulated more strongly in leaves grown under elevated CO2 (largely driven by Quercus). Because leaf area index did not change, the CO2-driven stimulation of photosynthesis in these trees may persist in the upper canopy under future atmospheric CO2 concentrations without reductions in photosynthetic capacity. However, given the lack of growth stimulation, the fate of the additionally assimilated carbon remains uncertain.
TL;DR: In this article, the authors report sap flux measurements during the 2007 and 2008 growing seasons for dominant tree species in a suburban neighborhood of Minneapolis-Saint Paul, Minnesota, USA, and find that tree species vary in the magnitude and seasonality of transpiration owing to differences in physiology, response to climate, and biophysical characteristics, thereby complicating efforts to manage evapotranspiration at city scales.
Abstract: [1] Tree transpiration provides a variety of ecosystem services in urban areas, including amelioration of urban heat island effects and storm water management. Tree species vary in the magnitude and seasonality of transpiration owing to differences in physiology, response to climate, and biophysical characteristics, thereby complicating efforts to manage evapotranspiration at city scales. We report sap flux measurements during the 2007 and 2008 growing seasons for dominant tree species in a suburban neighborhood of Minneapolis–Saint Paul, Minnesota, USA. Evergreen needleleaf trees had significantly higher growing season means and annual transpiration per unit canopy area (1.90 kg H2O m−2 d−1 and 307 kg H2O m−2 yr−1, respectively) than deciduous broadleaf trees (1.11 kg H2O m−2 d−1 and 153 kg H2O m−2 yr−1, respectively) because of a smaller projected canopy area (31.1 and 73.6 m2, respectively), a higher leaf area index (8.8 and 5.5 m2 m−2, respectively), and a longer growth season (8 and 4 months, respectively). Measurements also showed patterns consistent with the species' differences in xylem anatomy (conifer, ring porous, and diffuse porous). As the growing season progressed, conifer and diffuse porous genera had increased stomatal regulation to high vapor pressure deficit, while ring porous genera maintained greater and more constant stomatal regulation. These results suggest that evaporative responses to climate change in urban ecosystems will depend in part on species composition. Overall, plant functional type differences in canopy structure and growing season length were most important in explaining species' differences in midsummer and annual transpiration, offering an approach to predicting the evapotranspiration component of urban water budgets.
TL;DR: In this article, the authors performed simulations of future biophysical vegetation-climate feedbacks with a regional Earth System Model, RCA-GUESS, interactively coupling a regional climate model and a process-based model of vegetation dynamics and biogeochemistry.
Abstract: We performed simulations of future biophysical vegetation-climate feedbacks with a regional Earth System Model, RCA-GUESS, interactively coupling a regional climate model and a process-based model of vegetation dynamics and biogeochemistry. Simulated variations in leaf area index and in the relative coverage of evergreen forest, deciduous forest, and open land vegetation in response to simulated climate influence atmospheric state via variations in albedo, surface roughness, and the partitioning of the land-atmosphere heat flux into latent and sensible components. The model was applied on a similar to 50 x 50 km grid over Europe under a future climate scenario. Three potential "hot spots" of vegetation-climate feedbacks could be identified. In the Scandinavian Mountains, reduced albedo resulting from the snow-masking effect of forest expansion enhanced the winter warming trend. In central Europe, the stimulation of photosynthesis and plant growth by "CO2 fertilization" mitigated warming, through a negative evapotranspiration feedback associated with increased vegetation cover and leaf area index. In southern Europe, increased summer dryness restricted plant growth and survival, causing a positive warming feedback through reduced evapotranspiration. Our results suggest that vegetation-climate feedbacks over the European study area will be rather modest compared to the radiative forcing of increased global CO2 concentrations but may modify warming projections locally, regionally, and seasonally, compared with results from traditional "off-line" regional climate models lacking a representation of the relevant feedback mechanisms.
TL;DR: In this paper, the authors used ground observations and Satellite Pour l'Observation de la Terre SPOT4 and SPOT5 time series acquired monthly over a 2-year period over Reunion Island and Guadeloupe French West Indies.
Abstract: Sugarcane is a semi-perennial grass whose cultivation is characterized by an extended harvest season lasting several months leading to very high spatio-temporal variability of the crop development and radiometry. The objective of this paper is to understand this variability in order to propose appropriate spectral indicators for yield forecast. To do this, we used ground observations and Satellite Pour l‘Observation de la Terre SPOT4 and SPOT5 time series acquired monthly over a 2-year period over Reunion Island and Guadeloupe French West Indies. We showed that variations in the Normalized Difference Vegetation Index NDVI of sugarcane at the field scale are the result of the interaction between the sugarcane crop calendar and plant phenology in a given climatic environment. We linked these variations to crop variables measured in the field leaf area index and leaf colour, and derived simple, appropriate NDVI-based indicators of sugarcane yield components at the field scale cane yield and sugar content. For biomass forecast, the best correlation R2 = 0.78 was obtained with images acquired about 2 months before the harvest season, when all the fields are fully developed but before the maturation stage. For sugar content, a polynomial relationship R2 = 0.75 was observed between the field NDVI acquired during the maturation stage and sugar content in the stalk.
TL;DR: In this paper, four alternative airborne laser scanning (ALS) canopy penetration variables were compared for their suitability for mapping of gap fraction, leaf area index and disturbances in a Scots pine forest.
Abstract: Four alternative airborne laser scanning (ALS) canopy penetration variables were compared for their suitability for mapping of gap fraction, leaf area index and disturbances in a Scots pine forest. The variables were based on either echo counting or intensity, and on either first or first and last echoes. ALS data and field-measured gap fraction and effective leaf area index (LAIe) were gathered before and after a severe insect defoliation by pine sawflies. LAIe is a commonly used form of leaf area index that is mathematically derived from gap fraction, and includes the areas of foliage, branches and trunks, and which is not corrected for the clumping of foliage. The ALS penetration variables were almost equally strongly related to field-measured gap fraction and LAIe. The estimated slopes in the LAIe models varied from 0.94 to 2.71, and had coefficient of determination R2 values of 0.92-0.94. They were strongly correlated to each other (R2 values of 0.95-0.98) and agreed fairly well for temporal changes of LAIe during the summer and the insect defoliation (R2 values of 0.82-0.95). Counting of first and last echoes produced penetration rates close to the gap fraction, and this penetration variable was able to penetrate tree crowns. Ground-only echoes represented mostly between-tree gaps, and canopy-first-ground-last pulses represented mostly within-canopy gaps. However, the penetration variables based on first and last echoes suffered from the problem that a second echo might be impaired both in low and in tall canopies. In low canopies, two adjacent echoes from the same pulse would be too close in time to be separated by the sensor, while in tall canopies the pulse might apparently be fragmented down through the canopy. The intensity-based penetration variables needed to be supplemented with reflectance values, or at least the ratio between reflectance of the canopy and the ground, and this ratio was estimated from the data. The study demonstrated that one might be able to distinguish between disturbance types, e.g. between defoliation and cutting, by comparing alternative ALS penetration variables. Insect defoliation was dominated by an increase in within-canopy gaps and, correspondingly, the fraction of partly penetrating canopy-first-ground-last pulses. Tree removals from cutting were dominated by increases in between-tree gaps and the corresponding fraction of ground-only pulses.