Soil drought anomalies in MODIS GPP of a Mediterranean broadleaved evergreen forest

The Moderate Resolution Imaging Spectroradiometer (MODIS) yields global operational estimates of terrestrial gross primary production (GPP). In this study, we compared MOD17A2 GPP with tower eddy flux-based estimates of GPP from 2001 to 2010 over an evergreen broad-leaf Mediterranean forest in Southern France with a significant summer drought period. The MOD17A2 GPP shows seasonal variations that are inconsistent with the tower GPP, with close-to-accurate winter estimates and significant discrepancies for summer estimates which are the least accurate. The analysis indicated that the MOD17A2 GPP has high bias relative to tower GPP during severe summer drought which we hypothesized caused by soil water limitation. Our investigation showed that there was a significant correlation (R-2 = 0.77, p < 0.0001) between the relative soil water content and the relative error of MOD17A2 GPP. Therefore, the relationship between the error and the measured relative soil water content could explain anomalies in MOD17A2 GPP. The results of this study indicate that careful consideration of the water conditions input to the MOD17A2 GPP algorithm on remote sensing is required in order to provide accurate predictions of GPP. Still, continued efforts are necessary to ascertain the most appropriate index, which characterizes soil water limitation in water-limited environments using remote sensing

The utility of MODIS-sPRI for investigating the photosynthetic light-use efficiency in a Mediterranean deciduous forest

The present study investigated the utility of Moderate Resolution Imaging Spectroradiometer (MODIS)-derived sPRI (scaled photochemical reflectance index) and its relationship to photosynthetic light-use efficiency (LUE) calculated from eddy covariance tower data. The analysis was performed over two consecutive years (2003–2004) in a Mediterranean Quercus cerris L. forest site in Italy. Temperature and rainfall conditions differed markedly over the study period, with 2003 being a notable drought year and 2004 a non-drought year. MODIS ocean bands 11 (centred at 531 nm) and 12 (centred at 551 nm) were used for calculating sPRI. LUE exhibited substantial variability within 2003 and 2004, and a moderate relationship between MODIS-sPRI and LUE was observed during the wet year, and for backscattering scenes. This demonstrated the capacity of sPRI to detect xanthophyll cycle activation by vegetation during high light conditions. However, our results show that sPRI should be used with care, particularly under severe water stress conditions, when an increased influence of confounding factors, such canopy structure, illumination, and viewing angles, is observed

A remotely sensed pigment index reveals photosynthetic phenology in evergreen conifers

In evergreen conifers, where the foliage amount changes little with season, accurate detection of the underlying “photosynthetic phenology” from satellite remote sensing has been difficult, presenting challenges for global models of ecosystem carbon uptake. Here, we report a close correspondence between seasonally changing foliar pigment levels, expressed as chlorophyll/carotenoid ratios, and evergreen photosynthetic activity, leading to a “chlorophyll/carotenoid index” (CCI) that tracks evergreen photosynthesis at multiple spatial scales. When calculated from NASA’s Moderate Resolution Imaging Spectroradiometer satellite sensor, the CCI closely follows the seasonal patterns of daily gross primary productivity of evergreen conifer stands measured by eddy covariance. This discovery provides a way of monitoring evergreen photosynthetic activity from optical remote sensing, and indicates an important regulatory role for carotenoid pigments in evergreen photosynthesis. Improved methods of monitoring photosynthesis from space can improve our understanding of the global carbon budget in a warming world of changing vegetation phenology

A remotely sensed pigment index reveals photosynthetic phenology in evergreen conifers

In evergreen conifers, where the foliage amount changes little with season, accurate detection of the underlying “photosynthetic phenology” from satellite remote sensing has been difficult, presenting challenges for global models of ecosystem carbon uptake. Here, we report a close correspondence between seasonally changing foliar pigment levels, expressed as chlorophyll/carotenoid ratios, and evergreen photosynthetic activity, leading to a “chlorophyll/carotenoid index” (CCI) that tracks evergreen photosynthesis at multiple spatial scales. When calculated from NASA’s Moderate Resolution Imaging Spectroradiometer satellite sensor, the CCI closely follows the seasonal patterns of daily gross primary productivity of evergreen conifer stands measured by eddy covariance. This discovery provides a way of monitoring evergreen photosynthetic activity from optical remote sensing, and indicates an important regulatory role for carotenoid pigments in evergreen photosynthesis. Improved methods of monitoring photosynthesis from space can improve our understanding of the global carbon budget in a warming world of changing vegetation phenology

Integrated approach for monitoring the vulnerability of Mediterranean forests affected by drought-induced dieback

Rising aridity, mostly driven by higher temperatures and reduced precipitation, will likely undermine the health status of forest ecosystems. Experiments and observations point to the likelihood that if climate changes proceed at its current rate, the resilience of many forests will be threated by altering their structure and functions and reducing their capability to provide ecosystem services. Such increasing drought conditions, coupled to other biotic and abiotic drivers, are synergistically leading to responses in tree morphology, physiology, growth, reproduction, and forest mortality in different areas of the Mediterranean Basin. However, our understanding of vegetation dynamics in response to climate changes is still lacking, as a robust comprehension relies on the need to obtain insights at multiple temporal and spatial scales. In this context, we sought to forecasting vegetation response to climate stressors, particularly during dieback episodes when tree vulnerability is exacerbated. The first section of this study focused on tree and shrub populations exhibiting recent dieback phenomena in Italy (Quercus pubescens, Quercus frainetto) and Spain (Pinus sylvestris, Juniperus phoenicea). The general aim was to investigate how remotely sensed measures of vegetation activity and radial growth (BAI, basal area increment) responded to climate extreme events. To this purpose, we compared trees and nearby stands showing different vigor, i.e., dieback vs non-dieback, assessed as growth decline, elevated canopy defoliation and rising tree mortality rate. To disentangle growth and NDVI responses to drought, we accounted for two components of drought, namely elevated vapor pressure deficit (VPD) and low soil moisture. As a whole, the response of the investigated species to VPD increase was characterized by growth reduction. In Scots pine, high VPD was linked to a loss of growth in declining individuals which did not respond to changes in soil moisture. Oaks responded mostly to soil moisture, whereas the juniper was the most negatively affected by higher VPD. Indeed, the different hydraulic strategies (anisohydric vs. isohydric species) could partially explain the contrasting growth responses to drought proxies. We also found that dieback stands exhibited lower NDVI values than non-dieback stands. In most cases, NDVI and BAI was positively correlated and such relation likely relied on specific time windows. In the second part of the thesis, the phenological behavior of Mediterranean oak forest stands (Quercus cerris, Quercus pubescens, and Quercus frainetto), showing evident decline symptoms, are investigated by using a satellite-based approach. We explored how a phenological (PPI, Plant Phenology Index) index would be capable to reflect the seasonal vegetative dynamics of forests affected by dieback phenomena. We found that dieback forest stands - characterized by a higher ratio of crown-defoliated trees - showed distinct phenological performance as compared to non-dieback stands. In detail, our results revealed that dieback stands lengthened the growing season by delaying autumn leaf senescence. Nevertheless, both seasonal amplitude and productivity were found to have higher values for non-dieback stands as compared to dieback stands. Furthermore, it was highlighted that non-dieback stands experienced either greening up or senescence periods more rapidly than dieback ones. Overall, our framework demonstrated that the effects of climate extremes on vegetation can be detected either in terms of canopy greenness or radial growth reductions, thus hinting at the opportunity to both employ remotely sensed data as a stand-level indicator of vegetation stress and to scaling up informations from tree to stand levels by using the maximum growing season NDVI and tree-ring width data taken at the individual scales. Our findings also highlighted how patterns of vegetation response to climate extremes may depend on both the water use strategies of trees and shrubs and site-specific climatic conditions. Hence, coupling proxies of forest productivity (NDVI, BAI) may be employed for retrospective modeling of the impact of drought stress on forest productivity and tree growth, enhancing our knowledge and forecast of drought-induced dieback phenomena in woody plant communities. Furthermore, the second part of the work revealed the phenological behaviour of Mediterranean forest populations showing clear symptoms of decline. We speculated that the lengthened growing season may be related to the dieback trees' effort to compensate for the reduction in whole-plant photosynthesis, associated to canopy decline. Increased photosynthesis during the season under higher temperatures and increased light availability, due to global warming, provided a possible explanation for the greater seasonal amplitude and productivity of healthier stands. Our findings may provide new insights on phenological response to climate change in semi-arid regions, highlighting how trees, showing clear symptoms of decline, may keep their vital activities by changing their phenological performance. What described above leads to a crucial question concerning the potential implications of observed phenological shifts on the global carbon and water balance of terrestrial ecosystems under future climate change. Hence, in the coming years, this study could provide a more comprehensive overview on climate-vegetation interactions, mainly in the Mediterranean Basin, where intensified global warming and aridification trends are expected. Nonetheless, more investigations on the interactive effects among different environmental factors, are needed to improve our understanding of the underlying mechanisms affecting vegetation response

Nutrients and water availability constrain the seasonality of vegetation activity in a Mediterranean ecosystem

Anthropogenic nitrogen (N) deposition and resulting differences in ecosystem N and phosphorus (P) ratios are expected to impact photosynthetic capacity, that is, maximum gross primary productivity (GPP). However, the interplay between N and P availability with other critical resources on seasonal dynamics of ecosystem productivity remains largely unknown. In a Mediterranean tree–grass ecosystem, we established three landscape-level (24 ha) nutrient addition treatments: N addition (NT), N and P addition (NPT), and a control site (CT). We analyzed the response of ecosystem to altered nutrient stoichiometry using eddy covariance fluxes measurements, satellite observations, and digital repeat photography. A set of metrics, including phenological transition dates (PTDs; timing of green-up and dry-down), slopes during green-up and dry-down period, and seasonal amplitude, were extracted from time series of GPP and used to represent the seasonality of vegetation activity. The seasonal amplitude of GPP was higher for NT and NPT than CT, which was attributed to changes in structure and physiology induced by fertilization. PTDs were mainly driven by rainfall and exhibited no significant differences among treatments during the green-up period. Yet, both fertilized sites senesced earlier during the dry-down period (17–19 days), which was more pronounced in the NT due to larger evapotranspiration and water usage. Fertilization also resulted in a faster increase in GPP during the green-up period and a sharper decline in GPP during the dry-down period, with less prominent decline response in NPT. Overall, we demonstrated seasonality of vegetation activity was altered after fertilization and the importance of nutrient–water interaction in such water-limited ecosystems. With the projected warming-drying trend, the positive effects of N fertilization induced by N deposition on GPP may be counteracted by an earlier and faster dry-down in particular in areas where the N:P ratio increases, with potential impact on the carbon cycle of water-limited ecosystems.The authors acknowledge the Alexander von Humboldt Foundation for supporting this research with the Max-Planck Prize to Markus Reichstein. Yunpeng Luo and Mirco Migliavacca gratefully acknowledge financial support from the China Scholarship Council. Gerardo Moreno acknowledges financial support from the grant agreement IB16185 of the Regional Government of Extremadura

Mediterranean forest resilience to drought and climate change

Enhancing resilience to climate change is a key management goal for Mediterranean ecosystems. Typically, these management plans are based on ecological knowledge of species’ tolerances derived from local studies limited in time and space. Remote sensing provides opportunities to study resilience over larger scales, but the tools needed to quantify the resilience of forests to drought and evaluate the effectiveness of management plans remain limited. This thesis examines how freely available satellite data can be used to quantify changes in forest canopies in response to climate variability. Using a combination of time-series and break-point analyses of satellite imagery I resolve limitations in forest resilience estimation and show that, for Spanish woodlands, the relative water availability during and following drought events are important in driving the canopy greenness loss and recovery. I show that despite increasing aridity, and examples of localised die-back events, Spanish forests are mostly becoming denser, with only 12% of locations analysed declining in greenness over the 18-year study period. This work demonstrates the importance of large-scale remote sensing analyses for obtaining an objective perspective on drought impacts. The thesis then explores the potential of remote sensing to map tree species in a region of regenerating woodlands near Madrid, providing the information needed for a nuanced understanding of resilience. I found that tree classification using high-resolution airborne hyperspectral imagery was highly accurate, while species maps produced using Sentinel 2 imagery (multispectral data with 10-m spatial resolution) were less successful at identifying species, with average agreement of 64% with the airborne derived map. Following on from this work, I used areas with high classification agreement between the airborne and spaceborne species information to study the effect of species composition on forest responses to droughts. I identify contrasting responses of the canopy greenness and wood production to drought. Specifically, wood production was found to be more sensitive to changes in water availability than canopy greenness. For the oak species, wood production was mirrored by changes in canopy greenness, but black pines reduced their wood production during droughts without substantial reduction in canopy greenness. I investigate the differences between the species and the mixing effects further by studying foliar compositions during a dry summer in Spain. There were strong differences between pines and oaks in the stable isotope ratios of carbon, probably driven by underlying differences in water-use efficiency, and differences in the stable isotope ratios of nitrogen, probably driven by underlying differences in species’ investments in the photosynthetic apparatus. I conclude by highlighting the implications of my research for studying the relationships between diversity and ecosystem functioning from space.PhD scholarship from Cambridge International Trus

Iberian peninsula ecosystem carbon fluxes: a model-data integration study

Dissertação apresentada para obtenção do Grau de Doutor em Engenharia do Ambiente pela Universidade Nova de Lisboa,Faculdade de Ciências e TecnologiaTerrestrial ecosystems play a key role within the context of the global carbon cycle. Characterizing and understanding ecosystem level responses and feedbacks to climate drivers is essential for diagnostic purposes as well as climate modelling projections. Consequently,numerous modelling and data driven approaches emerge, aiming the appraisal of biosphereatmosphere carbon fluxes. The combination of biogeochemical models with observations of ecosystem carbon fluxes in a model-data integration framework enables the recognition of potential limitations of modelling approaches. In this regard, the steady-state assumption represents a general approach in the initialization routines of biogeochemical models that entails limitations in the ability to simulate net ecosystem fluxes and in model development exercises. The present research addresses the generalized assumption of initial steady-state conditions in ecosystem carbon pools for modelling carbon fluxes of terrestrial ecosystems, from local to regional scales. At local scale, this study aims to evaluate the implications of equilibrium assumptions on modelling performance and on optimized parameters and uncertainty estimates based on a model-data integration approach. These results further aim to support the estimates of regional net ecosystem fluxes, following a bottom-up approach, by focusing on parameters governing net primary production (NPP) and heterotrophic respiration (RH)processes, which determine the simulation of the net ecosystem production fluxes in the CASA model. An underlying goal of the current research is addressed by focusing on Mediterranean ecosystem types, or ecosystems potentially present in Iberia, and evaluate the general ability of terrestrial biogeochemical models in estimating net ecosystem fluxes for the Iberian Peninsula region. At regional scales, and given the limited information available, the main objective is to minimize the implications of the initial conditions in the evaluation of the temporal dynamics of net ecosystem fluxes. Inverse model parameter optimizations at site level are constrained by eddy-covariance measurements of net ecosystem fluxes and driven by local observations of meteorological variables and vegetation biophysical variables from remote sensing products. Optimizations under steady-state conditions show significantly poorer model performance and higher parameter uncertainties when compared to optimizations under relaxed initial conditions. In addition, assuming initial steady-state conditions tend to bias parameter retrievals – reducing NPP sensitivity to water availability and RH responses to temperature – in order to prescribe sink conditions. But nonequilibrium conditions can be experienced in soil and/or vegetation carbon pools under alternative underlying dynamics, which are solely discernible through the integration of additional information sources, circumventing equifinality issues.Portuguese Foundation for Science and Technology (FCT),the European Union under Operational Program “Science and Innovation” (POCI 2010), PhD grant ref. SFRH/BD/6517/2001, co-sponsored by the European Social Fund. Further support,concerning the final months of the PhD, was provided by a Max Planck Society research fellowship

Diurnal and Seasonal Proximally Sensed Photochemical Reflectance Index (PRI) in a High-Stress Semi-Arid Mixed Conifer Forest

A lack of accurate, reliable data on coupled carbon and water fluxes for Earth’s expansive ecosystems remains a major barrier to a complete understanding of the terrestrial carbon cycle. The remotely sensed Photochemical Reflectance Index (PRI) measures deepoxidation of the xanthophyll cycle at wavelength 531nm and is one of the few pigment-based vegetation indices sensitive to rapid plant physiological responses. PRI presents new opportunities to study ecosystems on a diurnal time scale, as well as seasonal processes in evergreen systems where complex vegetation dynamics are not reflected by small annual changes in chlorophyll content or leaf structure. However, systematic PRI acquisition in conjunction with leaf and ecosystem flux measurements are needed in natural, diverse ecosystems. The growing field of proximal remote sensing affords the opportunity to bridge leaf, canopy and ecosystem scales, for a physiological inspection of whole ecosystem dynamics. The Southwest U.S. provides a natural setting for examining the influence of environmental drivers on the productivity of drought-sensitive forests, as well as for evaluating our ability to track these relationships using optical methods. We studied PRI in a semi-arid, sub-alpine mixed conifer forest, in order to assess its ability to serve as a proxy for dynamic photoprotection. We combined canopy spectral measurements with eddy covariance flux and sap flow methods to determine the sensitivity of PRI to seasonal changes in gross primary productivity (GPP) and the ecohydrological variability of a high stress environment. In addition, we combined top-of-canopy leaf-level gas exchange, chlorophyll fluorescence, and hyperspectral measurements to determine the sensitivity of PRI to diurnal changes in needle photosynthetic function, and confirm the extent to which canopy diurnal patterns reflect leaf physiology. At the canopy scale we found that the relationship between PRI and GPP was inconsistent over the course of the monsoon season, shifting from a negative relationship in July and August (R2=.62), to a positive relationship in September (R2=.48). Multiple linear regression with soil moisture and air temperature showed that PRI responded to dynamic water and energy limitations of this system (R2=.41). We report for the first time a relationship between seasonal PRI and sap flow in a natural forest (R2=.55). These results suggest that on a seasonal scale PRI is an effective indicator of photosynthetic response to ecohydrological constraints. On a diurnal scale we found that PRI remained constant throughout the day at both leaf and canopy scales, and we suggest that saturated light conditions drive retention of xanthophylls in a de-epoxidized state. We contribute evidence that remotely sensed PRI has potential to fill a major gap in our ability to distinguish how water availability influences forest productivity and associated carbon dynamics