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A special issue of Remote Sensing (ISSN 2072-4292). This special issue belongs to the section "Forest Remote Sensing".

Deadline for manuscript submissions: closed (31 October 2019) | Viewed by 39910

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Prof. Dr. Heiko Balzter

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1. Centre for Landscape and Climate Research (CLCR), University of Leicester, & National Centre for Earth Observation (NCEO), Leicester, UK
2. School of Geography, Geology and the Environment, University of Leicester, Leicester, UK
Interests: quantitative understanding of climate change; land use change impacts on ecosystem services; spatial-temporal patterns and processes; forest aboveground biomass; logging detection; fire monitoring; mapping tree disease outbreaks
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Special Issue Information

Dear Colleagues,

Active remote sensing enables the acquisition of data independent of indirect illumination. The wide availability of Synthetic Aperture Radar (SAR), Light Detection and Ranging (LiDAR) and other active remote sensing techniques has led to a phenomenal growth in active remote sensing applications. One of the dominant research applications is in remote sensing of forests, because of their global significance for the carbon cycle, mitigation of climate change and biodiversity (gaining renewed global attention through the Paris Climate Agreement), as well as a wide range of essential ecosystem services for people. This technology has also enabled a much more detailed understanding of the 3D forest structure and its dynamics over time, including effects of tree diseases on forests, impacts of storm damage, forest fires and selective logging.

This Special Issue invites research papers describing cutting-edge research on active remote sensing of forests using any active remote sensing technology from any platform—be it tripod, drone, aircraft, satellite or any other. I wish to put together a journal issue that describes out-of-the-box approaches to the sensing of forest canopies at any scale, from leaves or needles to forest stands or national, continental or global scales. Research that integrates active with passive remote sensing approaches is also relevant. This Special Issue will be open access and will provide a compendium of novel and significant active remote sensing methods and applications.

Prof. Heiko Balzter
Guest Editor

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Keywords

Synthetic Aperture Radar (SAR)
LiDAR
active remote sensing
forest biomass
tree height
vegetation structure

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Research

19 pages, 18663 KiB

Open AccessArticle

Multi-Temporal and Multi-Frequency SAR Analysis for Forest Land Cover Mapping of the Mai-Ndombe District (Democratic Republic of Congo)

by Jörg Haarpaintner and Heidi Hindberg

Remote Sens. 2019, 11(24), 2999; https://doi.org/10.3390/rs11242999 - 13 Dec 2019

Cited by 9 | Viewed by 3831

Abstract

The European Space Agency’s (ESA) “SAR for REDD” project aims to support complementing optical remote sensing capacities in Africa with synthetic aperture radar (SAR) for Reducing Emissions from Deforestation and Forest Degradation (REDD). The aim of this study is to assess and compare Sentinel-1 C-band, ALOS-2 PALSAR-2 L-band and combined C/L-band SAR-based land cover mapping over a large tropical area in the Democratic Republic of Congo (DRC). The overall approach is to benefit from multi-temporal observations acquired from 2015 to 2017 to extract statistical parameters and seasonality of backscatters to improve forest land cover (FLC) classification. We investigate whether and to what extent the denser time series of C- band SAR can compensate for the L-band’s deeper vegetation penetration depth and known better FLC mapping performance. The supervised classification differentiates into forest, inundated forest, woody savannah, dry and wet grassland, and river swamps. Several feature combinations of statistical parameters from both, single and multi-frequency observations in a multivariate maximum-likelihood classification are compared. The FLC maps are reclassified into forest, savannah, and grassland (FSG) and validated with a systematic sampling grid of manual interpretations of very-high-resolution optical satellite data. Using the temporal variability of the dual-polarized backscatters, in the form of either wet/dry seasonal averages or using the statistical variance, in addition to the average backscatter, increased the classification accuracies by 4–5 percent points and 1–2 percent points for C- and L-band, respectively. For the FSG validation overall accuracies of 84.4%, 89.1%, and 90.0% were achieved for single frequency C- and L-band, and C/L-band combined, respectively. The resulting forest/non-forest (FNF) maps with accuracies of 90.3%, 92.2%, and 93.3%, respectively, are then compared to the Landsat-based Global Forest Change program’s and JAXA’s ALOS-1/2 based global FNF maps. Full article

(This article belongs to the Special Issue Advances in Active Remote Sensing of Forests)

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18 pages, 3384 KiB

Open AccessArticle

Sensitivity of Bistatic TanDEM-X Data to Stand Structural Parameters in Temperate Forests

by Stefan Erasmi, Malte Semmler, Peter Schall and Michael Schlund

Remote Sens. 2019, 11(24), 2966; https://doi.org/10.3390/rs11242966 - 11 Dec 2019

Cited by 8 | Viewed by 2734

Abstract

Synthetic aperture radar (SAR) satellite data provide a valuable means for the large-scale and long-term monitoring of structural components of forest stands. The potential of TanDEM-X interferometric SAR (InSAR) for the assessment of forest structural properties has been widely verified. However, present studies are mostly restricted to homogeneous forests and do not account for stratification in assessing model performance. A systematic sensitivity analysis of the TanDEM-X SAR signal to forest structural parameters was carried out with emphasis on different strata of forest stands (location of the study site, forest type, and development stage). Forest structure was parameterized by forest height metrics and stem volume. Results show that X-band volume coherence is highly sensitive to the forest canopy. Volume scattering within the canopy is dependent on the vertical heterogeneity of the forest stand. In general, TanDEM-X coherence is more sensitive to forest vertical structure compared to backscatter. The relations between TanDEM-X volume coherence and forest structural properties were significant at the level of a single test site as well as across sites in temperate forests in Germany. Forest type does not affect the overall relationship between the SAR signal and the forests’ vertical structure. The prediction of forest structural parameters based on the outcome of the sensitivity analysis yielded model accuracies between 15% (relative root mean square error) for Lorey’s height and 32% for stem volume. The global database of single-polarized bistatic TanDEM-X data provides an important source for mapping structural parameters in temperate forests at large scale, irrespective of forest type. Full article

(This article belongs to the Special Issue Advances in Active Remote Sensing of Forests)

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18 pages, 4794 KiB

Open AccessArticle

A Weighted SVM-Based Approach to Tree Species Classification at Individual Tree Crown Level Using LiDAR Data

by Hoang Minh Nguyen, Begüm Demir and Michele Dalponte

Remote Sens. 2019, 11(24), 2948; https://doi.org/10.3390/rs11242948 - 9 Dec 2019

Cited by 16 | Viewed by 3712

Abstract

Tree species classification at individual tree crowns (ITCs) level, using remote-sensing data, requires the availability of a sufficient number of reliable reference samples (i.e., training samples) to be used in the learning phase of the classifier. The classification performance of the tree species is mainly affected by two main issues: (i) an imbalanced distribution of the tree species classes, and (ii) the presence of unreliable samples due to field collection errors, coordinate misalignments, and ITCs delineation errors. To address these problems, in this paper, we present a weighted Support Vector Machine (wSVM)-based approach for the detection of tree species at ITC level. The proposed approach initially extracts (i) different weights associated to different classes of tree species, to mitigate the effect of the imbalanced distribution of the classes; and (ii) different weights associated to different training samples according to their importance for the classification problem, to reduce the effect of unreliable samples. Then, in order to exploit different weights in the learning phase of the classifier a wSVM algorithm is used. The features to characterize the tree species at ITC level are extracted from both the elevation and intensity of airborne light detection and ranging (LiDAR) data. Experimental results obtained on two study areas located in the Italian Alps show the effectiveness of the proposed approach. Full article

(This article belongs to the Special Issue Advances in Active Remote Sensing of Forests)

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15 pages, 1191 KiB

Open AccessArticle

Classifications of Forest Change by Using Bitemporal Airborne Laser Scanner Data

by Lennart Noordermeer, Roar Økseter, Hans Ole Ørka, Terje Gobakken, Erik Næsset and Ole Martin Bollandsås

Remote Sens. 2019, 11(18), 2145; https://doi.org/10.3390/rs11182145 - 14 Sep 2019

Cited by 21 | Viewed by 3707

Abstract

Changes in forest areas have great impact on a range of ecosystem functions, and monitoring forest change across different spatial and temporal resolutions is a central task in forestry. At the spatial scales of municipalities, forest properties and stands, local inventories are carried out periodically to inform forest management, in which airborne laser scanner (ALS) data are often used to estimate forest attributes. As local forest inventories are repeated, the availability of bitemporal field and ALS data is increasing. The aim of this study was to assess the utility of bitemporal ALS data for classification of dominant height change, aboveground biomass change, forest disturbances, and forestry activities. We used data obtained from 558 field plots and four repeated ALS-based forest inventories in southeastern Norway, with temporal resolutions ranging from 11 to 15 years. We applied the k-nearest neighbor method for classification of: (i) increasing versus decreasing dominant height, (ii) increasing versus decreasing aboveground biomass, (iii) undisturbed versus disturbed forest, and (iv) forestry activities, namely untouched, partial harvest, and clearcut. Leave-one-out cross-validation revealed overall accuracies of 96%, 95%, 89%, and 88% across districts for the four change classifications, respectively. Thus, our results demonstrate that various changes in forest structure can be classified with high accuracy at plot level using data from repeated ALS-based forest inventories. Full article

(This article belongs to the Special Issue Advances in Active Remote Sensing of Forests)

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20 pages, 8110 KiB

Open AccessArticle

Airborne Lidar Sampling Strategies to Enhance Forest Aboveground Biomass Estimation from Landsat Imagery

by Siqi Li, Lindi J. Quackenbush and Jungho Im

Remote Sens. 2019, 11(16), 1906; https://doi.org/10.3390/rs11161906 - 15 Aug 2019

Cited by 12 | Viewed by 3358

Abstract

Accurately estimating aboveground biomass (AGB) is important in many applications, including monitoring carbon stocks, investigating deforestation and forest degradation, and designing sustainable forest management strategies. Although lidar provides critical three-dimensional forest structure information for estimating AGB, acquiring comprehensive lidar coverage is often cost prohibitive. This research focused on developing a lidar sampling framework to support AGB estimation from Landsat images. Two sampling strategies, systematic and classification-based, were tested and compared. The proposed strategies were implemented over a temperate forest study site in northern New York State and the processes were then validated at a similar site located in central New York State. Our results demonstrated that while the inclusion of lidar data using systematic or classification-based sampling supports AGB estimation, the systematic sampling selection method was highly dependent on site conditions and had higher accuracy variability. Of the 12 systematic sampling plans, R² values ranged from 0.14 to 0.41 and plot root mean square error (RMSE) ranged from 84.2 to 93.9 Mg ha⁻¹. The classification-based sampling outperformed 75% of the systematic sampling strategies at the primary site with R² of 0.26 and RMSE of 70.1 Mg ha⁻¹. The classification-based lidar sampling strategy was relatively easy to apply and was readily transferable to a new study site. Adopting this method at the validation site, the classification-based sampling also worked effectively, with an R² of 0.40 and an RMSE of 108.2 Mg ha⁻¹ compared to the full lidar coverage model with an R² of 0.58 and an RMSE of 96.0 Mg ha⁻¹. This study evaluated different lidar sample selection methods to identify an efficient and effective approach to reduce the volume and cost of lidar acquisitions. The forest type classification-based sampling method described in this study could facilitate cost-effective lidar data collection in future studies. Full article

(This article belongs to the Special Issue Advances in Active Remote Sensing of Forests)

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24 pages, 28174 KiB

Open AccessArticle

Progressive Cascaded Convolutional Neural Networks for Single Tree Detection with Google Earth Imagery

by Tianyang Dong, Yuqi Shen, Jian Zhang, Yang Ye and Jing Fan

Remote Sens. 2019, 11(15), 1786; https://doi.org/10.3390/rs11151786 - 30 Jul 2019

Cited by 16 | Viewed by 3618

Abstract

High-resolution remote sensing images can not only help forestry administrative departments achieve high-precision forest resource surveys, wood yield estimations and forest mapping but also provide decision-making support for urban greening projects. Many scholars have studied ways to detect single trees from remote sensing images and proposed many detection methods. However, the existing single tree detection methods have many errors of commission and omission in complex scenes, close values on the digital data of the image for background and trees, unclear canopy contour and abnormal shape caused by illumination shadows. To solve these problems, this paper presents progressive cascaded convolutional neural networks for single tree detection with Google Earth imagery and adopts three progressive classification branches to train and detect tree samples with different classification difficulties. In this method, the feature extraction modules of three CNN networks are progressively cascaded, and the network layer in the branches determined whether to filter the samples and feed back to the feature extraction module to improve the precision of single tree detection. In addition, the mechanism of two-phase training is used to improve the efficiency of model training. To verify the validity and practicability of our method, three forest plots located in Hangzhou City, China, Phang Nga Province, Thailand and Florida, USA were selected as test areas, and the tree detection results of different methods, including the region-growing, template-matching, convolutional neural network and our progressive cascaded convolutional neural network, are presented. The results indicate that our method has the best detection performance. Our method not only has higher precision and recall but also has good robustness to forest scenes with different complexity levels. The F1 measure analysis in the three plots was 81.0%, which is improved by 14.5%, 18.9% and 5.0%, respectively, compared with other existing methods. Full article

(This article belongs to the Special Issue Advances in Active Remote Sensing of Forests)

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28 pages, 1870 KiB

Open AccessArticle

Comparative Study on Variable Selection Approaches in Establishment of Remote Sensing Model for Forest Biomass Estimation

by Xiaohui Yu, Hongli Ge, Dengsheng Lu, Maozhen Zhang, Zhouxiang Lai and Rentu Yao

Remote Sens. 2019, 11(12), 1437; https://doi.org/10.3390/rs11121437 - 17 Jun 2019

Cited by 23 | Viewed by 4032

Abstract

In the field of quantitative remote sensing of forest biomass, a prominent phenomenon is the increasing number of explanatory variables. Then how to effectively select explanatory variables has become an important issue. Linear regression model is one of the commonly used remote sensing models. In the process of establishing the linear regression model, a vital step is to select explanatory variables. Focusing on variable selection and model stability, this paper conducts a comparative study on the performance of eight linear regression parameter estimation methods (Stepwise Regression Method (SR), Criterions Based on The Bayes Method (BIC), Criterions Based on The Bayes Method (AIC), Criterions Based on Prediction Error (Cp), Least Absolute Shrinkage and Selection Operator (Lasso), Adaptive Lasso, Smoothly Clipped Absolute Deviation (SCAD), Non-negative garrote (NNG)) in the subtropical forest biomass remote sensing model development. For the purpose of comparison, OLS and RR, are commonly used as methods with no variable selection ability, and are also compared and discussed. The performance of five aspects are evaluated in this paper: (i) Determination coefficient, prediction error, model error, etc., (ii) significance test about the difference between determination coefficients, (iii) parameter stability, (iv) variable selection stability and (v) variable selection ability of the methods. All the results are obtained through a five ten-fold CV. Some evaluation indexes are calculated with or without degrees of freedom. The results show that BIC performs best in comprehensive evaluation, while NNG, Cp and AIC perform poorly as a whole. Other methods show a great difference in the performance on each index. SR has a strong capability in variable selection, although it is poor in commonly used indexes. The short-wave infrared band and the texture features derived from it are selected most frequently by various methods, indicating that these variables play an important role in forest biomass estimation. Some of the conclusions in this paper are likely to change as the study object changes. The ultimate goal of this paper is to introduce various model establishment methods with variable selection capability, so that we can have more choices when establishing similar models, and we can know how to select the most appropriate and effective method for specific problems. Full article

(This article belongs to the Special Issue Advances in Active Remote Sensing of Forests)

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16 pages, 2765 KiB

Open AccessArticle

Development and Validation of a Photo-Based Measurement System to Calculate the Debarking Percentages of Processed Logs

by Joachim B. Heppelmann, Eric R. Labelle, Thomas Seifert, Stefan Seifert and Stefan Wittkopf

Remote Sens. 2019, 11(9), 1133; https://doi.org/10.3390/rs11091133 - 12 May 2019

Cited by 2 | Viewed by 3870

Abstract

Within a research project investigating the applicability and performance of modified harvesting heads used during the debarking of coniferous tree species, the actual debarking percentage of processed logs needed to be evaluated. Therefore, a computer-based photo-optical measurement system (Stemsurf) designed to assess the debarking percentage recorded in the field was developed, tested under laboratory conditions, and applied in live field operations. In total, 1720 processed logs of coniferous species from modified harvesting heads were recorded and analyzed within Stemsurf. With a single log image as the input, the overall debarking percentage was calculated by further estimating the un-displayed part of the log surface by defining polygons representing the differently debarked areas of the log surface. To assess the precision and bias of the developed measurement system, 480 images were captured under laboratory conditions on an artificial log with defined surface polygons. Within the laboratory test, the standard deviation of average debarking percentages remained within a 4% variation. A positive bias of 6.7% was caused by distortion and perspective effects. This resulted in an average underestimation of 1.1% for the summer debarking percentages gathered from field operations. The software generally performed as anticipated through field and lab testing and offered a suitable alternative of assessing stem debarking percentage, a task that should increase in importance as more operations are targeting debarked products. Full article

(This article belongs to the Special Issue Advances in Active Remote Sensing of Forests)

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20 pages, 7230 KiB

Open AccessArticle

Particle Swarm Optimization-Based Noise Filtering Algorithm for Photon Cloud Data in Forest Area

by Jiapeng Huang, Yanqiu Xing, Haotian You, Lei Qin, Jing Tian and Jianming Ma

Remote Sens. 2019, 11(8), 980; https://doi.org/10.3390/rs11080980 - 24 Apr 2019

Cited by 37 | Viewed by 4627

Abstract

The Ice, Cloud, and land Elevation Satellite-2 (ICESat-2), which is equipped with the Advanced Topographic Laser Altimeter System (ATLAS), was launched successfully in 15 September 2018. The ATLAS represents a micro-pulse photon-counting laser system, which is expected to provide more comprehensive and scientific data for carbon storage. However, the ATLAS system is sensitive to the background noise, which poses a tremendous challenge to the photon cloud noise filtering. Moreover, the Density Based Spatial Clustering of Applications with Noise (DBSCAN) is a commonly used algorithm for noise removal from the photon cloud but there has not been an in-depth study on its parameter selection yet. This paper presents an automatic photon cloud filtering algorithm based on the Particle Swarm Optimization (PSO) algorithm, which can be used to optimize the two key parameters of the DBSCAN algorithm instead of using the manual parameter adjustment. The Particle Swarm Optimization Density Based Spatial Clustering of Applications with Noise (PSODBSCAN) algorithm was tested at different laser intensities and laser pointing types using the MATLAS dataset of the forests located in Virginia, East Coast, and the West Coast, USA. The results showed that the PSODBSCAN algorithm and the localized statistical algorithm were effective in identifying the background noise and preserving the signal photons in the raw MATLAS data. Namely, the PSODBSCAN achieved the mean F value of 0.9759, and the localized statistical algorithm achieved the mean F value of 0.6978. For both laser pointing types and laser intensities, the proposed algorithm achieved better results than the localized statistical algorithm. Therefore, the PSODBSCAN algorithm could support the MATLAS photon cloud data noise filtering applicably without manually selecting parameters. Full article

(This article belongs to the Special Issue Advances in Active Remote Sensing of Forests)

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15 pages, 26490 KiB

Open AccessArticle

Rubber Tree Crown Segmentation and Property Retrieval Using Ground-Based Mobile LiDAR after Natural Disturbances

by Ting Yun, Kang Jiang, Hu Hou, Feng An, Bangqian Chen, Anna Jiang, Weizheng Li and Lianfeng Xue

Remote Sens. 2019, 11(8), 903; https://doi.org/10.3390/rs11080903 - 13 Apr 2019

Cited by 49 | Viewed by 5464

Abstract

Rubber trees in southern China are often impacted by natural disturbances, and accurate rubber tree crown segmentation and property retrieval are of great significance for forest cultivation treatments and silvicultural risk management. Here, three plots of different rubber tree clones, PR107, CATAS 7-20-59 and CATAS 8-7-9, that were recently impacted by hurricanes and chilling injury were taken as the study targets. Through data collection using ground-based mobile light detection and ranging (LiDAR) technology, a weighted Rayleigh entropy method based on the scanned branch data obtained from the region growing algorithm was proposed to calculate the trunk inclination angle and crown centre of each tree. A watershed algorithm based on the extracted crown centres was then adopted for tree crown segmentation, and a variety of tree properties were successfully extracted to evaluate the susceptibility of different rubber tree clones facing natural disturbances. The results show that the angles between the first-order branches and trunk ranged from 35.1–67.7° for rubber tree clone PR107, which is larger than the angles for clone CATAS 7-20-59, which ranged from 20.2–43.2°. Clone PR107 had the maximum number of scanned leaf points, lowest tree height and a crown volume that was larger than that of CATAS 7-20-59, which generates more frontal leaf area to oppose wind flow and reduces the gaps among tree crowns, inducing strong wind loading on the tree body. These factors result in more severe hurricane damage, resulting in trunk inclination angles that are larger for PR107 than CATAS 7-20-59. In addition, the rubber tree clone CATAS 8-7-9 had the minimal number of scanned leaf points and the smallest tree crown volume, reflecting its vulnerability to both hurricanes and chilling injury. The results are verified by field measurements. The work quantitatively assesses the susceptibility of different rubber tree clones under the impacts of natural disturbances using ground-based mobile LiDAR. Full article

(This article belongs to the Special Issue Advances in Active Remote Sensing of Forests)

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