Forest Aboveground Biomass Estimation Using Multi-Source Remote Sensing Data in Temperate Forests

Forests are a crucial part of global ecosystems. Accurately estimating aboveground biomass (AGB) is important in many applications including monitoring carbon stocks, investigating forest degradation, and designing sustainable forest management strategies. Remote sensing techniques have proved to be a cost-effective way to estimate forest AGB with timely and repeated observations. This dissertation investigated the use of multiple remotely sensed datasets for forest AGB estimation in temperate forests. We compared the performance of Landsat and lidar data—individually and fused—for estimating AGB using multiple regression models (MLR), Random Forest (RF) and Geographically Weight Regression (GWR). Our approach showed MLR performed similarly to GWR and both were better than RF. Integration of lidar and Landsat inputs outperformed either data source alone. However, although lidar provides valuable three-dimensional forest structure information, acquiring comprehensive lidar coverage is often cost prohibitive. Thus we developed a lidar sampling framework to support AGB estimation from Landsat images. We compared two sampling strategies—systematic and classification-based—and found that the systematic sampling selection method was highly dependent on site conditions and had higher model variability. The classification-based lidar sampling strategy was easy to apply and provides a framework that is readily transferable to new study sites. The performance of Sentinel-2 and Landsat 8 data for quantifying AGB in a temperate forest using RF regression was also tested. We modeled AGB using three datasets: Sentinel-2, Landsat 8, and a pseudo dataset that retained the spatial resolution of Sentinel-2 but only the spectral bands that matched those on Landsat 8. We found that while RF model parameters impact model outcomes, it is more important to focus attention on variable selection. Our results showed that the incorporation of red-edge information increased AGB estimation accuracy by approximately 6%. The additional spatial resolution improved accuracy by approximately 3%. The variable importance ranks in the RF regression model showed that in addition to the red- edge bands, the shortwave infrared bands were important either individually (in the Sentinel-2 model) or in band indices. With the growing availability of remote sensing datasets, developing tools to appropriately and efficiently apply remote sensing data is increasingly important

The global tree carrying capacity (keynote)

editorial reviewe

Alternative remote sensing materials and inventory strategies in tropical forest inventory - Case Lao PDR Materiais alternativos de sensoriamento remoto e estratégias de inventário no inventário de florestas tropicais – Caso Lao PDR

In this study, the potential of remote sensing in tropical forests is examined in relation to the diversification of sensors. We report here on the comparison of alternative methods that use multisource data from Airborne Laser Scanning (ALS), Airborne Color Infrared Photograph (CIR), Quickbird and ALOS AVNIR-2 to estimate stem volume and basal area, in Laos. The predictors of ALS metrics were calculated by means of the canopy height distribution approach, while predictors from both spectral and textual features. The correlation of remote sensing materials and field data were used to demonstrate needs for field inventory in different forest landscapes and varying tropical forest conditions. Variogram based analysis was used to derive optimal forest inventory procedure for different parts of case country. Resumo Neste estudo, o potencial do sensoriamento remoto em florestas tropicais é examinado em relação a diversidade de sensores. Registramos aqui a comparação de métodos alternativos que utilizam dados de fontes múltiplas do Airborne Laser Scanning (ALS), Airborne CIR, Quickbird e ALOS AVNIR-2 para estimar o volume do caule e a área basal em Laos. Os preditores dos dados ALS foram calculados pelo método da distribuição de altura do dossel enquanto preditores para características espectrais e textuais foram geradas, respectivamente, para os dados Airbone CIR e ALOS AVNIR-2. A correlação dos materiais de sensoriamento remoto e dados de campo foram usados para demonstrar a necessidade do inventário de campo em diferentes paisagens florestais e condições variáveis em floresta tropical. A análise baseada no variograma foi utilizada para gerar um procedimento otimizado para o inventário florestal de diferentes partes do país em estudo

Mapping and Monitoring Forest Cover

This book is a compilation of six papers that provide some valuable information about mapping and monitoring forest cover using remotely sensed imagery. Examples include mapping large areas of forest, evaluating forest change over time, combining remotely sensed imagery with ground inventory information, and mapping forest characteristics from very high spatial resolution data. Together, these results demonstrate effective techniques for effectively learning more about our very important forest resources

Methods for High-Dimensional Spatial Data: Dimension Reduction and Covariance Approximation

In spatial statistics, because quantities are correlated based on their relative positions in space, data is modeled as a single realization of a multivariate stochastic process. Spatial data can be high-dimensional either through a large number of observed variables per location, or through a large number of observed locations. The two are often handled differently, with the former addressed through dimension reduction and the latter addressed through appropriate modeling of the spatial correlation between locations. The main body of this dissertation is a three-part work. Parts 2 and 3 pertain to the many variables problem, proposing novel methods of dimension reduction for spatial data. Part 4 pertains to the many locations problem, using state-of-the-art techniques to analyze a massive satellite data set, improving on the current usage of the data

Analysing mangrove forest structure and biomass in the Niger Delta

Mangrove forests are important in providing a range of ecosystem services, including food provision to local communities and carbon storage, while being globally restricted to tropical coastlines. The conservation and sustainability of mangrove forests is thus a globally important topic. Mangrove forests in the Niger Delta are known to be under high pressure from urbanisation, development, logging and oil pollution, and invasive species such as nipa palm (Nypa fruticans). These mangrove forests are poorly understood as a result of difficulty of access, social unrest and security restrictions. For example, there is no data on the relationship between disturbance and mangrove structure in the Delta, current area extent and biomass stocks of mangrove forest, its rate of loss, or the rate of nipa palm colonisation in the Niger Delta. The overall objective of this thesis is to utilise a combination of field data and earth observation to resolve these knowledge gaps. This work will estimate area and biomass of mangrove forests in the Niger Delta, and their changes over recent years through disturbance and invasive species. I used an extensive field data collection in 2016-17 to establish 25 geo-referenced 0.25-ha plots across the Niger Delta and collected 567 ground control points. I estimated aboveground biomass (AGB) from a general allometric equation based on stem surveys. Leaf area index (LAI) was recorded using hemispherical photos. I performed and evaluated a land cover classification using a combination of Advanced Land Observatory Satellite Phased Array L-band SAR (ALOS PALSAR), Landsat ETM+ and the Shuttle Radar Topography Mission Digital Elevation Model (SRTM DEM) data. I also compared two supervised classification methods: Maximum Likelihood (ML) and Support Vector machine (SVM) classifiers. I established a relationship between field estimates of AGB and Advanced Land Observatory Satellite (ALOS) L-band radar backscatter. I also estimated the area of nipa palm and mangrove forests in the Niger Delta and generated the first mangrove biomass map for the region, for 2007 and 2017 to obtain change information. Plot estimated mean AGB was 83.7 Mg ha-1 and I found significantly higher plot biomass in close proximity to protected sites and tidal influence, and the lowest in the sites where urbanisation was actively taking place. The mean LAI was 1.45 and there was a significant positive correlation between AGB and LAI (R2= 0.28). Satellite observations of NDVI for the growing season correlated positively with in-situ LAI (R2= 0.63) and AGB (R2= 0.8). Lower stem sizes (5-15cm) accounted for 70% contribution to the total biomass in disturbed plots, while undisturbed plots had a more even contribution of different size classes to AGB. Nipa palm invasion was significantly correlated to plots with larger variations in LAI (i.e. more patchy cover) and proportion of basal area removed within plots. The classification results showed SVM (overall accuracy 99.9 %) performed better than ML (98.7%) across the Niger Delta. I estimated a 2017 mangrove area of 794 561 ha and nipa extent of 11,419 ha. I discovered a 12% decrease in mangrove area and 694 % increase in nipa palm between 2007 and 2017. The highest radar-AGB relationship was from the combination of HH: HV and HV bands (R2= 0.62, p-value < 0.001). Using this relationship, I estimated a mean and total AGB of 90.5 Mg ha-1 and 82 X 106 Mg in 2007; 83.4 Mg ha-1 and 65 X 106 Mg in 2017. Local wood exploitation is removing larger stems (> 15 cm DBH) preferentially from these mangroves and creates an avenue for nipa palm colonisation. I identified opportunities to use remote sensing to estimate biomass, based on the LAI-AGB-NDVI relationship I found, and can serve as a calibration dataset for radar data to provide effective monitoring of mangrove forest degradation. It is clear from these results that remote sensing can be used to map the extent and changes in these land cover types, and thus such mapping efforts should continue for policy targeting and monitoring. I was able to show that mangroves of the Niger Delta are at risk, from rapid clearance as well as from the invasive species nipa palm. I also provide evidence of mangrove cover loss of 11 000 ha yr-1 over a decade, resulting in biomass loss rate of 100 Mg ha-1 yr-1 while mangrove degradation rate of 56 Mg ha-1 yr-1 in the Niger Delta. Assessing carbon stock of mangrove forests in the Niger Delta can create a baseline for regional conservation and regeneration plans. These plans can create opportunities for generating carbon credits under reducing emissions from deforestation and forest degradation (REDD+)

Applications of Remote Sensing Data in Mapping of Forest Growing Stock and Biomass

This Special Issue (SI), entitled "Applications of Remote Sensing Data in Mapping of Forest Growing Stock and Biomass”, resulted from 13 peer-reviewed papers dedicated to Forestry and Biomass mapping, characterization and accounting. The papers' authors presented improvements in Remote Sensing processing techniques on satellite images, drone-acquired images and LiDAR images, both aerial and terrestrial. Regarding the images’ classification models, all authors presented supervised methods, such as Random Forest, complemented by GIS routines and biophysical variables measured on the field, which were properly georeferenced. The achieved results enable the statement that remote imagery could be successfully used as a data source for regression analysis and formulation and, in this way, used in forestry actions such as canopy structure analysis and mapping, or to estimate biomass. This collection of papers, presented in the form of a book, brings together 13 articles covering various forest issues and issues in forest biomass calculation, constituting an important work manual for those who use mixed GIS and RS techniques

Estimating tropical forest above-ground biomass at the local scale using multi-source space-borne remote sensing data

Although forest biomass estimation has attracted a great number of studies using remote sensing data, its usage still contains high uncertainties. After transitioning from deforestation to reforestation under the development of Payments for Environmental Services (PES) programmes, young forests that are dominated by numerous small regenerating understory trees are found in many areas of many developing countries. However, the lack of analysis on the effect of this understory vegetation on total AGB is one the limitations of biomass studies. Moreover, it is always challenging to estimate the biomass of tropical forest due to its complex structure, high diversity of species, and dense canopy of understory trees. Taking into account these factors, this study, therefore, aims to investigate the effect of including understory trees in accuracy of AGB estimation in complex tropical heterogeneous forest at the local scale. The research conducted three consecutive experiments, using different remote sensing data sources, being: optical data, synthetic aperture radar (SAR) data and the integration of optical and SAR data, across various forest types in different test site locations. The results provide comprehensive insights into the impact of small regenerating trees on improving AGB estimation. This major finding alone demonstrates that the role of small regenerating trees should not be automatically discounted, especially for tropical forest where a number of different tree layers is common. This is especially important in areas with a large number of small regenerating trees and where open canopy layers are young. The thesis reveals that the level of influence of small regenerating trees on each forest type is different. Therefore, the study recommends an approach to including small regenerating trees for each forest type. This thesis argues there is a need to develop local-specific allometric equations for both overstory and understory layers to improve the accuracy of biomass models. Methods required for collecting field data and calculating biomass for small regenerating trees should be considered carefully in terms of evaluating cost-effective biomass estimation for each ecological region and each species. This requirement is most critical for young forest sites where there are mixtures of mature trees and young regenerating trees

Investigating the Interactions between Fluvial Processes and Floodplain Forest Ecology in the Amazon Basin

Amazonian tropical forests account for 20-50% of global primary productivity and up to 40% of carbon stored in terrestrial biomass (Phillips et al., 1998). The Amazon is also home to the Earth’s largest river system, accounting for approximately 20% of the world’s total river discharge (Richey et al., 1989). Despite the clear global significance of the Amazon basin, substantial uncertainties remain in terms of both aboveground wood biomass and carbon storage within its extensive forests (Houghton et al., 2001), and the functioning of its river systems, particularly in terms of floodplain inundation (Wilson et al., 2007). This thesis addresses the aforementioned uncertainties through providing new insight into the interaction between fluvial processes and Amazonian floodplain varzea forests, for the Beni floodplain in north east Bolivia. Flood inundation dynamics for the Beni floodplain are quantified through application of a 1D-2D hydraulic model code, with topographical forcing provided through bare earth DEMs derived from the SRTM global elevation dataset (Farr and Kobrick, 2000). Subsequently, in the final part of the thesis, aboveground wood biomass estimates are generated for the Beni floodplain, through extrapolation of plot scale inventory measurements with respect to spatially distributed remote sensing datasets. These estimates are subsequently integrated with modelled flood inundation and maps depicting Beni river channel migration, in order to explore the influence which fluvial processes exert upon aboveground wood biomass storage in varzea forest stands. Overall, results presented within this thesis quantitatively demonstrate that fluvial processes, specifically flood inundation and lateral channel migration, exert significant impacts upon aboveground biomass storage within Beni floodplain forests. Furthermore, as a result of these influences, aboveground wood biomass storage within Beni floodplain forests is substantially lower than would be expected based upon published estimates for varzea forests across the Amazon (Baker et al., 2004; Saatchi et al., 2007). This implies that systematic overestimation of aboveground wood biomass storage for Amazonian varzea forests may constitute a significant source of uncertainty in basin scale biomass estimates