Browsing by Author "Correia, Alexandra C."
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- Carbon stock estimation in a Mediterranean riparian forest: a case study combining field data and UAV imageryPublication . Fernandes, Maria Rosário; Aguiar, Francisca C.; Martins, Maria João; Rico, Nuno; Ferreira, Maria Teresa; Correia, Alexandra C.This study aims to estimate the total biomass aboveground and soil carbon stocks in a Mediterranean riparian forest and identify the contribution of the different species and ecosystem compartments to the overall riparian carbon reservoir. We used a combined field and object-based image analysis (OBIA) approach, based on unmanned aerial vehicle (UAV) multispectral imagery, to assess C stock of three dominant riparian species. A linear discriminator was designed, based on a set of spectral variables previously selected in an optimal way, permitting the classification of the species corresponding to every object in the study area. This made it possible to estimate the area occupied by each species and its contribution to the tree aboveground biomass (AGB). Three uncertainty levels were considered, related to the trade-o between the number of unclassified and misclassified objects, leading to an error control associated with the estimated tree AGB.We found that riparian woodlands dominated by Acacia dealbata Link showed the highest average carbon stock per unit area (251 90 tC ha1) followed by Alnus glutinosa (L.) Gaertner (162 12 tC ha1) and by Salix salviifolia Brot. (73 17 tC ha1), which are mainly related to the stem density, vegetation development and successional stage of the different stands. The woody tree compartment showed the highest inputs (79%), followed by the understory vegetation (12%) and lastly by the soil mineral layer (9%). Spectral vegetation indices developed to suppress saturation effects were consistently selected as important variables for species classification. The total tree AGB in the study area varies from 734 to 1053 tC according to the distinct levels of uncertainty. This study provided the foundations for the assessment of the riparian carbon sequestration and the economic value of the carbon stocks provided by similar Mediterranean riparian forests, a highly relevant ecosystem service for the regulation of climate change effects
- Effects of an extremely dry winter on net ecosystem carbon exchange and tree phenology at cork oak woodlandPublication . Costa-e-Silva, Filipe; Correia, Alexandra C.; Piayda, Arndt; Dubbert, Maren; Rebmann, Corinna; Cuntz, Matthias; Werner, Christiane; David, Jorge Soares; Pereira, João SantosIn seasonally dry climates, such as the Mediterranean, lack of rainfall in the usually wet winter may originate severe droughts which are a main cause of inter-annual variation in carbon sequestration. Leaf phenology variability may alter the seasonal pattern of photosynthetic uptake, which in turn is determined by leaf gas exchange limitations. The current study is based on the monitoring of an extremely dry winter in an evergreen cork oak woodland under the Mediterranean climate of central Portugal. Results are focused on net ecosystem CO2 exchange (NEE), phenology and tree growth measurements during two contrasting years: 2011, a wet year with a typical summer drought pattern and 2012, with an extremely unusual dry winter (only 10mmof total rainfall) that exacerbated the following summer drought effects. Main aims of this study were to assess the effects of an extreme dry winter in (1) annual and seasonal net ecosystem CO2 exchange, and in (2) cork oak phenology. The dry year 2012 was marked by a 45% lower carbon sequestration (−214 vs. −388gCm−2 year−1) and a 63% lower annual tree diameter growth but only a 9% lower leaf area index compared to the wet year 2011. A significant reduction of 15% in yearly carbon sequestration was associated with leaf phenological events of canopy renewal in the early spring. In contrast to male flower production, fruit setting was severely depressed by water stress with a 54% decrease during the dry year. Our results suggest that leaf growth and leaf area maintenance are resilient ecophysiological processes under winter drought and are a priority carbon sink for photoassimilates in contrast to tree diameter growth. Thus, carbon sequestration reductions under low water availabilities in cork oak woodland should be ascribed to stomatal regulation or photosynthetic limitations and to a lesser extent to leaf area reductions
- Florestas mistas de Sobreiro e Pinheiro-Manso: Conhecimento, desafios e recomendações de gestãoPublication . Correia, Alexandra C.; Barcik, Pedro; Lloberas Lafuente, David; Ramires, Inês; Pinto, Clara; Kurz-Besson, Cathy; Marcelo, Maria da Encarnação; Branco, Manuela; Barreiro, Susana; Botequim, Brigite; Nunes, Leónia; Tomé, Margarida; Costa-e-Silva, Filipe
- Using high resolution UAV imagery to estimate tree variables in Pinus pinea plantation in PortugalPublication . Guerra-Hernández, Juan; González-Ferreiro, Eduardo; Sarmento, Alexandre; Silva, João; Nunes, Alexandra; Correia, Alexandra C.; Fontes, Luís; Tomé, Margarida; Díaz-Varela, RamónAim of study: The study aims to analyse the potential use of lowcost unmanned aerial vehicle (UAV) imagery for the estimation of Pinus pinea L. variables at the individual tree level (position, tree height and crown diameter). Area of study: This study was conducted under the PINEA project focused on 16 ha of umbrella pine afforestation (Portugal) subjected to different treatments. Material and methods: The workflow involved: a) image acquisition with consumer-grade cameras on board an UAV; b) orthomosaic and digital surface model (DSM) generation using structure-from-motion (SfM) image reconstruction; and c) automatic individual tree segmentation by using a mixed pixel‑and region‑based algorithm. Main results: The results of individual tree segmentation (position, height and crown diameter) were validated using field measurements from 3 inventory plots in the study area. All the trees of the plots were correctly detected. The RMSE values for the predicted heights and crown widths were 0.45 m and 0.63 m, respectively. Research highlights: The results demonstrate that tree variables can be automatically extracted from high resolution imagery. We highlight the use of UAV as a fast, reliable and cost-effective technique for small scale applications