Does the Normalized Difference Vegetation Index explain spatial and temporal variability in sap velocity in temperate forest ecosystems?
Hoek Van Dijke, Anne J. ; Mallick, Kaniska ; Teuling, Adriaan J. ; Schlerf, Martin ; Machwitz, Miriam ; Hassler, Sibylle K. ; Blume, Theresa ; Herold, Martin - \ 2019
Hydrology and Earth System Sciences 23 (2019)4. - ISSN 1027-5606 - p. 2077 - 2091.
Understanding the link between vegetation characteristics and tree transpiration is a critical need to facilitate satellite-based transpiration estimation. Many studies use the Normalized Difference Vegetation Index (NDVI), a proxy for tree biophysical characteristics, to estimate evapotranspiration. In this study, we investigated the link between sap velocity and 30 m resolution Landsat-derived NDVI for 20 days during 2 contrasting precipitation years in a temperate deciduous forest catchment. Sap velocity was measured in the Attert catchment in Luxembourg in 25 plots of 20×20 m covering three geologies with sensors installed in two to four trees per plot. The results show that, spatially, sap velocity and NDVI were significantly positively correlated in April, i.e. NDVI successfully captured the pattern of sap velocity during the phase of green-up. After green-up, a significant negative correlation was found during half of the studied days. During a dry period, sap velocity was uncorrelated with NDVI but influenced by geology and aspect. In summary, in our study area, the correlation between sap velocity and NDVI was not constant, but varied with phenology and water availability. The same behaviour was found for the Enhanced Vegetation Index (EVI). This suggests that methods using NDVI or EVI to predict small-scale variability in (evapo)transpiration should be carefully applied, and that NDVI and EVI cannot be used to scale sap velocity to stand-level transpiration in temperate forest ecosystems.
Strong contribution of autumn phenology to changes in satellite-derived growing season length estimates across Europe (1982–2011)
Garonna, I. ; Jong, R. de; Wit, A.J.W. de; Mücher, C.A. ; Schmid, B. ; Schaepman, M.E. - \ 2014
Global Change Biology 20 (2014)11. - ISSN 1354-1013 - p. 3457 - 3470.
land-surface phenology - high-resolution radiometer - vegetation index ndvi - spring phenology - climate-change - time-series - monitoring vegetation - trends - avhrr - models
Land Surface Phenology (LSP) is the most direct representation of intra-annual dynamics of vegetated land surfaces as observed from satellite imagery. LSP plays a key role in characterizing land-surface fluxes, and is central to accurately parameterizing terrestrial biosphere–atmosphere interactions, as well as climate models. In this article, we present an evaluation of Pan-European LSP and its changes over the past 30 years, using the longest continuous record of Normalized Difference Vegetation Index (NDVI) available to date in combination with a landscape-based aggregation scheme. We used indicators of Start-Of-Season, End-Of-Season and Growing Season Length (SOS, EOS and GSL, respectively) for the period 1982–2011 to test for temporal trends in activity of terrestrial vegetation and their spatial distribution. We aggregated pixels into ecologically representative spatial units using the European Landscape Classification (LANMAP) and assessed the relative contribution of spring and autumn phenology. GSL increased significantly by 18–24 days decade-1 over 18–30% of the land area of Europe, depending on methodology. This trend varied extensively within and between climatic zones and landscape classes. The areas of greatest growing-season lengthening were the Continental and Boreal zones, with hotspots concentrated in southern Fennoscandia, Western Russia and pockets of continental Europe. For the Atlantic and Steppic zones, we found an average shortening of the growing season with hotspots in Western France, the Po valley, and around the Caspian Sea. In many zones, changes in the NDVI-derived end-of-season contributed more to the GSL trend than changes in spring green-up, resulting in asymmetric trends. This underlines the importance of investigating senescence and its underlying processes more closely as a driver of LSP and global change.
Remote sensing of annual terrestrial gross primary productivity from MODIS: an assessment using the FLUXNET La Thuile data set
Verma, M. ; Friedl, M.A. ; Richardson, A.D. ; Kiely, G. ; Cescatti, A. ; Law, B.E. ; Wohlfahrt, G. ; Gielen, G. ; Roupsard, O. ; Moors, E.J. - \ 2014
Biogeosciences 11 (2014). - ISSN 1726-4170 - p. 2185 - 2200.
net ecosystem exchange - carbon-dioxide exchange - use efficiency model - forests green-up - rain-fed maize - interannual variability - vegetation index - surface temperature - climatic controls - deciduous forest
Gross primary productivity (GPP) is the largest and most variable component of the global terrestrial carbon cycle. Repeatable and accurate monitoring of terrestrial GPP is therefore critical for quantifying dynamics in regional-to-global carbon budgets. Remote sensing provides high frequency observations of terrestrial ecosystems and is widely used to monitor and model spatiotemporal variability in ecosystem properties and processes that affect terrestrial GPP. We used data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and FLUXNET to assess how well four metrics derived from remotely sensed vegetation indices (hereafter referred to as proxies) and six remote sensing-based models capture spatial and temporal variations in annual GPP. Specifically, we used the FLUXNET La Thuile data set, which includes several times more sites (144) and site years (422) than previous studies have used. Our results show that remotely sensed proxies and modeled GPP are able to capture significant spatial variation in mean annual GPP in every biome except croplands, but that the percentage of explained variance differed substantially across biomes (10–80%). The ability of remotely sensed proxies and models to explain interannual variability in GPP was even more limited. Remotely sensed proxies explained 40–60% of interannual variance in annual GPP in moisture-limited biomes, including grasslands and shrublands. However, none of the models or remotely sensed proxies explained statistically significant amounts of interannual variation in GPP in croplands, evergreen needleleaf forests, or deciduous broadleaf forests. Robust and repeatable characterization of spatiotemporal variability in carbon budgets is critically important and the carbon cycle science community is increasingly relying on remotely sensing data. Our analyses highlight the power of remote sensing-based models, but also provide bounds on the uncertainties associated with these models. Uncertainty in flux tower GPP, and difference between the footprints of MODIS pixels and flux tower measurements are acknowledged as unresolved challenges.
Trends in spring phenology of Western European deciduous forests
Hamunyela, E. ; Verbesselt, J. ; Roerink, G.J. ; Herold, M. - \ 2013
Remote Sensing 5 (2013)12. - ISSN 2072-4292 - p. 6159 - 6179.
image time-series - green-up date - climate-change - temporal resolution - plant phenology - vegetation - ndvi - responses - season - china
Plant phenology is changing because of recent global warming, and this change may precipitate changes in animal distribution (e.g., pests), alter the synchronization between species, and have feedback effects on the climate system through the alteration of biogeochemical and physical processes of vegetated land surface. Here, ground observations (leaf unfolding/first leaf separation of six deciduous tree species) and satellite-derived start-of-growing season (SOS) are used to assess how the timing of leafing/SOS in Western European deciduous forest responded to climate variability between 2001 and 2011 and evaluate the reliability of satellite SOS estimates in tracking the response of forest leafing to climate variability in this area. Satellite SOS estimates are derived from the Normalized Difference Vegetation Index (NDVI) time series of the Moderate Resolution Imaging Spectroradiometer (MODIS). Temporal trends in the SOS are quantified using linear regression, expressing SOS as a function of time. We demonstrated that the growing season was starting earlier between 2001 and 2011 for the majority of temperate deciduous forests in Western Europe, possibly influenced by regional spring warming effects experienced during the same period. A significant shift of up to 3 weeks to early leafing was found in both ground observations and satellite SOS estimates. We also show that the magnitude and trajectory of shifts in satellite SOS estimates are well comparable to that of in situ observations, hence highlighting the importance of satellite imagery in monitoring leaf phenology under a changing climate
Differentiation of plant age in grasses using remote sensing
Knox, N. ; Skidmore, A.K. ; Werff, H.M.A. van der; Groen, T.A. ; Boer, W.F. de; Prins, H.H.T. ; Kohi, E. ; Peel, M. - \ 2013
International Journal of applied Earth Observation and Geoinformation 24 (2013)10. - ISSN 0303-2434 - p. 54 - 62.
difference water index - monitoring vegetation - nitrogen concentration - imaging spectroscopy - hyperspectral data - boreal regions - time-series - green-up - phenology - reflectance
Phenological or plant age classification across a landscape allows for examination of micro-topographical effects on plant growth, improvement in the accuracy of species discrimination, and will improve our understanding of the spatial variation in plant growth. In this paper six vegetation indices used in phenological studies (including the newly proposed PhIX index) were analysed for their ability to statistically differentiate grasses of different ages in the sequence of their development. Spectra of grasses of different ages were collected from a greenhouse study. These were used to determine if NDVI, NDWI, CAI, EVI, EVI2 and the newly proposed PhIX index could sequentially discriminate grasses of different ages, and subsequently classify grasses into their respective age category. The PhIX index was defined as: (An VNIR+ log(An SWIR2))/(An VNIR- log(An SWIR2)), where An VNIRand An SWIR2are the respective nor- malised areas under the continuum removed reflectance curve within the VNIR (500-800 nm) and SWIR2 (2000-2210 nm) regions. The PhIX index was found to produce the highest phenological classification accuracy (Overall Accuracy: 79%, and Kappa Accuracy: 75%) and similar to the NDVI, EVI and EVI2 indices it statistically sequentially separates out the developmental age classes. Discrimination between seedling and dormant age classes and the adult and flowering classes was problematic for most of the tested indices. Combining information from the visible near infrared (VNIR) and shortwave infrared region (SWIR) region into a single phenological index captures the phenological changes associated with plant pigments and the ligno-cellulose absorption feature, providing a robust method to discriminate the age classes of grasses. This work provides a valuable contribution into mapping spatial variation and monitoring plant growth across savanna and grassland ecosystems.
Phenological response of vegetation to upstream river flow in the Heihe Rive basin by time series analysis of MODIS data
Jia, L. ; Shang, H.L. ; Hu, G. ; Menenti, M. - \ 2011
Hydrology and Earth System Sciences 15 (2011)3. - ISSN 1027-5606 - p. 1047 - 1064.
fourier-analysis - ejina basin - northwestern china - ndvi composites - lower reaches - tarim river - avhrr data - groundwater - noaa - cover
Liquid and solid precipitation is abundant in the high elevation, upper reach of the Heihe River basin in northwestern China. The development of modern irrigation schemes in the middle reach of the basin is taking up an increasing share of fresh water resources, endangering the oasis and traditional irrigation systems in the lower reach. In this study, the response of vegetation in the Ejina Oasis in the lower reach of the Heihe River to the water yield of the upper catchment was analyzed by time series analysis of monthly observations of precipitation in the upper and lower catchment, river streamflow downstream of the modern irrigation schemes and satellite observations of vegetation index. Firstly, remotely sensed NDVI data acquired by Terra-MODIS are used to monitor the vegetation dynamic for a seven years period between 2000 and 2006. Due to cloud-contamination, atmospheric influence and different solar and viewing angles, however, the quality and consistence of time series of remotely sensed NDVI data are degraded. A Fourier Transform method – the Harmonic Analysis of Time Series (HANTS) algorithm – is used to reconstruct cloud- and noise-free NDVI time series data from the Terra-MODIS NDVI dataset. Modification is made on HANTS by adding additional parameters to deal with large data gaps in yearly time series in combination with a Temporal-Similarity-Statistics (TSS) method developed in this study to seek for initial values for the large gap periods. Secondly, the same Fourier Transform method is used to model time series of the vegetation phenology. The reconstructed cloud-free NDVI time series data are used to study the relationship between the water availability (i.e. the local precipitation and upstream water yield) and the evolution of vegetation conditions in Ejina Oasis from 2000 to 2006. Anomalies in precipitation, streamflow, and vegetation index are detected by comparing each year with the average year. The results showed that: the previous year total runoff had a significant relationship with the vegetation growth in Ejina Oasis and that anomalies in the spring monthly runoff of the Heihe River influenced the phenology of vegetation in the entire oasis. Warmer climate expressed by the degree-days showed positive influence on the vegetation phenology in particular during drier years. The time of maximum green-up is uniform throughout the oasis during wetter years, but showed a clear S-N gradient (downstream) during drier years.
Comparing observations and process-based simulationsof biosphere-atmosphere exchanges on multiple timescales
Moors, E.J. ; Mahecha, M.D. ; Reichstein, M. ; Jung, M. ; Seneviratne, S.I. ; Zaehle, S. ; Beer, C. ; Braakhekke, M.C. ; Carvalhais, N. ; Lange, H. ; Maire, G. Le - \ 2010
Journal of Geophysical Research: Biogeosciences 115 (2010). - ISSN 2169-8953 - 21 p.
net ecosystem exchange - interannual time scales - global vegetation model - energy-balance closure - eddy covariance - stomatal conductance - climate-change - water-vapor - pine forest - long-term
Terrestrial biosphere models are indispensable tools for analyzing the biosphere-atmosphere exchange of carbon and water. Evaluation of these models using site level observations scrutinizes our current understanding of biospheric responses to meteorological variables. Here we propose a novel model-data comparison strategy considering that CO2 and H2O exchanges fluctuate on a wide range of timescales. Decomposing simulated and observed time series into subsignals allows to quantify model performance as a function of frequency, and to localize model-data disagreement in time. This approach is illustrated using site level predictions from two models of different complexity, Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) and Lund-Potsdam-Jena (LPJ), at four eddy covariance towers in different climates. Frequency-dependent errors reveal substantial model-data disagreement in seasonal-annual and high-frequency net CO2 fluxes. By localizing these errors in time we can trace these back, for example, to overestimations of seasonal-annual periodicities of ecosystem respiration during spring greenup and autumn in both models. In the same frequencies, systematic misrepresentations of CO2 uptake severely affect the performance of LPJ, which is a consequence of the parsimonious representation of phenology. ORCHIDEE shows pronounced model-data disagreements in the high-frequency fluctuations of evapotranspiration across the four sites. We highlight the advantages that our novel methodology offers for a rigorous model evaluation compared to classical model evaluation approaches. We propose that ongoing model development will benefit from considering model-data (dis)agreements in the time-frequency domain
Effects of elevated CO2 and N deposition on CH4 emissions from European mires
Silvola, J. ; Saarnio, S. ; Foot, J. ; Sundh, I. ; Greenup, A. ; Heijmans, M.M.P.D. ; Ekberg, A. ; Mitchell, E.P. ; Breemen, N. van - \ 2003
Global Biogeochemical Cycles 17 (2003)2 - 1068. - ISSN 0886-6236 - p. 37 - 1-37-12.
atmospheric carbon-dioxide - methane emissions - boreal mire - raised co2 - northern peatlands - water-table - nitrogen deposition - bog vegetation - forest soils - temperature
 Methane fluxes were measured at five sites representing oligotrophic peatlands along a European transect. Five study plots were subjected to elevated CO2 concentration (560 ppm), and five plots to NH4NO3 (3 or 5 g N yr(-1)). The CH4 emissions from the control plots correlated in most cases with the soil temperatures. The depth of the water table, the pH, and the DOC, N and SO4 concentrations were only weakly correlated with the CH4 emissions. The elevated CO2 treatment gave nonsignificantly higher CH4 emissions at three sites and lower at two sites. The N treatment resulted in higher methane emissions at three sites (nonsignificant). At one site, the CH4 fluxes of the N-treatment plots were significantly lower than those of the control plots. These results were not in agreement with our hypotheses, nor with the results obtained in some earlier studies. However, the results are consistent with the results of the vegetation analyses, which showed no significant treatment effects on species relationships or biomass production.
Can testate amoebae (protozoa) and other micro-organisms help to overcome biogeographic bias in large scale global change research?
Mitchell, E.A.D. ; Gilbert, D. ; Butler, A. ; Grosvernier, P. ; Albinsson, C. ; Rydin, H. ; Heijmans, M.M.P.D. ; Hoosbeek, M.R. ; Greenup, A. ; Foot, J. ; Saarinen, T. ; Vasander, H. - \ 2001
In: Global Change and Protected Areas / Visconti, G., - p. 301 - 310.
klimaatverandering - kooldioxide - vegetatie - micro-organismen - climatic change - carbon dioxide - vegetation - microorganisms
|Effects on elevated CO2 and N deposition on CH4 emissions from European bogs
Saarnio, S. ; Silvola, J. ; Foot, J.P. ; Sundh, I. ; Greenup, A. ; Heijmans, M. ; Joabsson, A. ; Mitchell, E. ; Breemen, N. van - \ 2000
In: Sustaining Our Peatlands : 11th International Peat Congress, Canada 2000 / Rochefort, L., Daigle, J.Y., Canada : Gerry Hood - ISBN 9789519774442 - p. 1088 - 1088.
|Testate amoebae (protozoa) and other micro-organisms in sphagnum peatlands : biogeography, ecology and effect of elevated CO2
Mitchell, E.A.D. ; Gilbert, D. ; Buttler, A. ; Grosvernier, P. ; Albinsson, C. ; Rydin, H. ; Heijmans, M.M.P.D. ; Hoosbeek, M.R. ; Greenup, A. ; Foot, J.P. ; Saarinen, T. ; Vasander, H. ; Gobat, J.M. - \ 2000
In: Sustaining Our Peatlands : 11th International Peat Congress, Canada 2000 / Rochefort, L., Daigle, J.Y., Canada : Gerry Hood - ISBN 9789519774442 - p. 1087 - 1087.
|Modeling the effects of elevated CO2 and N on the biogeochemistry of European bogs
Hoosbeek, M.R. ; Berendse, F. ; Foot, J.P. ; Greenup, A. ; Grosvernier, P. ; Mitchell, E.A.D. ; Rydin, H. ; Saarinen, T. ; Saarnio, S. ; Wallen, B. - \ 2000
In: Sustaining Our Peatlands : 11th International Peat Congress, Canada 2000 / Rochefort, L., Daigle, J.Y., Canada : Gerry Hood - ISBN 9789519774442 - p. 1086 - 1086.
Relationships among testate amoebae (Protozoa), vegetation and water chemistry in five Sphagnum-dominated peatlands in Europe
Mitchell, E.A.D. ; Buttler, A. ; Grosvernier, P. ; Rydin, H. ; Albinsson, C. ; Greenup, A.L. ; Heijmans, M.M.P.D. ; Hoosbeek, M.R. ; Saarinen, T. - \ 2000
New Phytologist 145 (2000). - ISSN 0028-646X - p. 95 - 106.
turf - hoogveengronden - veengronden - grondanalyse - vegetatie - amoeba - grondwater - peat - bog soils - peat soils - soil analysis - vegetation - amoeba - groundwater
To study the relationships between groups of organisms and the degree to which these relationships are consistent across major climatic gradients, we analysed the testate amoeba (Protozoa) communities, vegetation and water chemistry of one peatland in five countries: Switzerland, The Netherlands, Great Britain, Sweden and Finland, as part of the BERI (Bog Ecosystem Research Initiative) project. The relationships between the different data sets and subsets were investigated by means of detrended correspondence analysis, canonical correspondence analysis and Mantel permutation tests. The comparison of data on vegetation and testate amoebae showed that inter-site differences are more pronounced for the vegetation than for the testate amoebae species assemblage. Testate amoebae are a useful tool in multi-site studies and in environmental monitoring of peatlands because: (1) the number of species in Sphagnum-dominated peatlands is much higher than for mosses or vascular plants; (2) most peatland species are cosmopolitan in their distributions and therefore less affected than plants by biogeographical distribution patterns, thus differences in testate amoeba assemblages can be interpreted primarily in terms of ecology; (3) they are closely related to the ecological characteristics of the exact spot where they live, therefore they can be used to analyse small-scale gradients that play a major role in the functioning of peatland ecosystems. This study revealed the existence of small-scale vertical gradients within the vegetation and life-form niche separation in response to water chemistry. The deep-rooted plants such as Carex spp. and Eriophorum spp. are related to the chemistry of water sampled at or near the ground water table, whereas the mosses are not. Testate amoebae were shown to be ecologically more closely related to the chemistry of water sampled at or near the water table level and to the mosses than to the deep-rooted plants.