Mapping of wind-thrown forests using satellite SAR images

The study focuses on investigation and evaluation of wind- thrown forest mapping using satellite remotely sensed data from three synthetic aperture radar (SAR) sensors. The study is carried out at Remningstorp, a test site in the south of Sweden dominated by coniferous forest, where trees were manual felled to simulate wind-thrown forest. The satellite data consisted of time series of HH polarized SAR images acquired by the Advanced Land Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR), Radarsat-2 (C-band) and TerraSAR-X (X- band). The results from visual interpretation of SAR images acquired before and after the simulated wind-throw together with corresponding ratio images show that ALOS PALSAR HH polarized intensity images are not able to detect wind- thrown forest, probably due to too coarse spatial resolution. In contrast, the wind-thrown forest is clearly visible in the Radarsat-2 and TerraSAR-X HH polarized images, implying that it may be possible to develop a new application using these SAR data for mapping of wind-thrown forests

Hemiboreaalsete metsade kaardistamine interferomeetrilise tehisava-radari andmetelt

Väitekirja elektrooniline versioon ei sisalda publikatsioone.Käesolev doktoritöö uurib tehisavaradari (SAR) kasutusvõimalusi metsa kõrguse hindamiseks hemiboreaalsete metsade vööndis. Uurimistöö viidi läbi Tartu Üli¬kooli, Tartu Observatooriumi, Aalto Ülikooli, Euroopa Kosmoseagentuuri (ESA) kaugseire keskuse ESRIN ja Reach-U koostöös. Uurimistöös kasutatud satelliidi¬andmed on pärit Saksa Kosmosekeskuse (DLR) kõrglahutusega bistaatilise X-laineala tehisavaradari TanDEM-X satelliidipaarilt. Sagedasti uuenevad satelliidiandmed, nende globaalne katvus ja kõrge ruumi¬line lahutus võimaldavad tehisavaradari abil kaardistada metsi ning nendes toimu¬vaid muutusi suurtel maa-aladel. Radari abil on võimalik saada kõrge lahutusvõimega pilte, mis on tundlikud taimestikule, maapinna karedusele ja dielektrilistele omadustele. Sünkroonis lendava radaripaari samaaegselt tehtud pildid elimineerivad võimalikud ajalised muutused taimestikus ning tänu sellele on radariandmetest võimalik tuletada metsade vertikaalset struktuuri ja kõrgust. Uurimistöös käsitletakse tehisavaradari interferomeetrilise koherentsuse tund¬likkust metsa kõrguse suhtes ning analüüsitakse, millised keskkonna ja klimaati¬lised tingimused ning satelliidi orbiidiga seotud parameetrid mõjutavad radari¬piltidelt erinevate puuliikide kõrguse hindamise täpsust. Lisaks keskendub väitekiri interferomeetrilisele koherentsusele tuginevate mudelite analüüsi¬misele ning nende täpsuse hindamisele operatiivse metsa kõrguse kaardistamise raken-duseks. Vaatluse alla on võetud kolm testala, mis asuvad Soomaa rahvuspargis, Võrtsjärve idakaldal Rannus ja Peipsiveere looduskaitsealal ning katavad kokku 2291 hektarit metsa. 23 TanDEM-X satelliidipildi koherentsuspilte võrreldakse samadel testaladel aerolaserskaneerimise (LiDAR) abil mõõdetud puistute kõrgu¬sega, mis on omakorda jagatud kolme rühma (kuused, männid ja laia¬lehised segametsad). RVoG (Random Volume over Ground) taimekatte mudel ning sellest tule¬tatud lihtsamad pooleempiirilised mudelid sobituvad olemasolevate TanDEM-X koherentsuse ning LiDARi metsa puistute kõrgusandmetega hästi. Töö tule¬mused kinnitavad, et tulevikus on suurte ja erinevatest metsatüüpidest koosne¬vate metsade kõrguse kosmosest kaardistamisel otstarbekas kasutusele võtta esmalt just soovitatud lihtsamad ja universaalsemad mudelid.This thesis presents research in the field of radar remote sensing and contributes to the forest monitoring application development using space-borne synthetic aperture radar (SAR). Satellite data is particularly useful for large-scale forestry applications making high revisit monitoring of the state of forests worldwide possible. The sensitivity of SAR to the dielectric and geometrical properties of the targets, penetration capacity and coherent imaging properties make it a unique tool for mapping and monitoring forest biomes. SAR satellites are also capable of retrieving additional information about the structure of the forest, tree height and biomass estimates as an essential input for monitoring the changes in the carbon stocks. Interferometric SAR (InSAR) is an advanced SAR imaging technique that allows the retrieval of forest parameters while working in nearly all weather conditions, independently of daylight and cloud cover. This research concen¬trates on assessing the impact of different variables affecting hemiboreal forest height estimation from space-borne X-band interferometric SAR coherence data. In particular, the research analyses the changes in coherence dynamics related to seasonal conditions, tree species and imaging properties using a large collection of interferometric SAR images from different seasons over a four-year period. The study is carried out over three test sites in Estonia using the extensive multi-temporal dataset of 23 TanDEM-X images, covering 2291 hectares of forests to describe the relation between the interferometric SAR coherence mag¬nitude and forest parameters. The work demonstrates how the correlation of interferometric coherence and Airborne LiDAR Scanning (ALS)-derived forest height varies for pine and deciduous tree species, for summer (leaf-on) and winter (leaf-off) conditions and for flooded forest floor. A simple semi-empirical modelling approach is proposed as being suitable for wide area forest mapping with limited a priori information under a range of seasonal and environ¬¬mental conditions. A Random Volume over Ground (RVoG) model and three semi-empirical models are compared and validated against a large dataset of coherence magnitude and ALS-measured data over hemiboreal forests in Estonia. The results show that all proposed models perform well in describing the relationship between hemiboreal forest height and interferometric coherence, allowing in future to derive forest stand height with an accuracy suitable for a wide range of applications

Constraints on tree growth, impacts of tropical cyclones and outcomes of community management in the Miombo woodlands

The Miombo woodlands of southern Africa are a globally significant store of carbon (C) and biodiversity. They also provide services for more than 150M people across several of the world’s most economically impoverished countries. The Miombo woodlands are dynamic, with extensive resource loss accompanied by areas of regrowth and increase. Disturbance processes, both from natural processes and widespread anthropogenic activities, are critical in maintaining woody biomass in these ecosystems, although intensity of disturbance varies widely. Increase in woody biomass has been observed in the Miombo, though the drivers of this trend are uncertain and the fundamental constraints on trees and woodlands not well understood. Ultimately, both losses and gains can be difficult to detect and hard to attribute to a particular cause. The aim of this thesis is to use field data and remote sensing to add to understanding of the constraints on tree populations in the Miombo and the impacts of severe environmental disturbances and management interventions on woodland structure. Tree growth is a crucial demographic rate in African woodlands and plays a key role in shaping woodland structure and C cycling. However, observations of tree growth are relatively lacking in the Miombo and the determinants of tree growth rates are poorly known. In Chapter 2 I use data collected from long-term monitoring of permanent sample plots in Mozambique and Tanzania and linear mixed modelling to estimate tree growth increments and assess the relative importance of different determinants of tree growth. The estimated growth (diameter increment) in these plots was 1.8 ± 0.17 mm/yr. Climate and edaphic factors explained little variation in tree growth. Tree-tree competition was found to be a significant constraint on growth (trees in experiencing competition levels in the top 5% of values grew 1.24 ± 0.08 mm/yr slower on average than those in the bottom 5%) as was stem wounding (wounded trees grew 0.84 ± 0.04 mm/yr slower). Root symbioses (both fungal and bacterial symbionts) which aid in the uptake of nutrients were found to have a strong positive impact on growth, particularly ectomycorrhizal associations which are common to dominant species in the Miombo. The impacts of tree-tree competition and nutrient symbioses are poorly represented in biogeochemical models in these ecosystems but this analysis suggests they are critical, whilst the subtle impacts of human interaction with trees (through wounding) are also possibly underappreciated. Tropical Cyclones can have substantial long-term impacts on woodland structure in affected areas and projections indicate that the impacts of Cyclones will increase in southeastern Africa over the coming century. There are few studies which have documented the immediate impacts or long-term responses of woodland ecosystems to this damage. In Chapter 3 I analyse data from a survey of eight permanent sample plots setup explicitly to assess the damage caused by Cyclone Idai to in woodlands in Gorongosa National Park, central Mozambique. It is found that Cyclone Idai caused damage primarily to large trees, thus whilst only 2% of trees were felled these individuals represented 8.5% of overall basal area. The implications of this damage are discussed in context of the constraints on trees in these woodlands, and whilst the damage is severe it is concluded that the outcomes are highly uncertain. Whilst damage from the cyclone is substantial, detecting change in woodland structure is challenging in these ecosystems. In Chapter 4 I explore the possibility of upscaling field observations of treefall occurrence using data from a small unmanned aerial vehicle (drone) and satellite radar. Drone survey produced comparable estimates of treefall intensity to the PSP observations (in terms of fallen number of stems, fallen basal area and carbon) and allowed survey of 155 ha, capturing widespread damage across the study area. In the study area radar backscattering intensity in C-Band radar reduced in the two years after the cyclone relative to the two years before whilst interferometric coherence increased - both in agreement with radar theory - although backscattering intensity in L-Band radar increased. Whilst significant relationships were identified between change in radar data and the intensity of damage in drone surveys, there appears to be limited ability to map variations in treefall intensity across the wider landscape using this method, or to determine areal impacts on above ground C thereafter. It is concluded that repeat analysis may yield better results however. In Tanzania, Village Land Forest Reserves (VLFRs, a form of participatory forest management) aim to promote sustainable profit from woodland resources, although the impact of VLFRs on land cover change rates is uncertain. In Chapter 5 I use satellite radar to map deforestation and a degradation across an area of southern Tanzania from 2010-2018 and statistical matching to compare rates of land cover change within a sample of VLFRs to woodlands under comparable resource pressure outside VLFRs or other protection status. It is found that VLFRs in the majority of cases were very effective in reducing deforestation (with five of seven having rates close to zero) and also reduced degradation rates (though to a lesser degree). Increasing density of woody biomass in forested areas was observed in all VLFRs, but varied widely across the sample (from 0.2 - 1.5 tC ha yr-1) and was in five of seven VLFRs below that observed in woodland areas with no protection status in this region (+0.7 tC ha yr-1). Whilst it appears that VLFR establishment achieved its intended goal of sustainable profit from woodlands resources from 2010-2018, further work is required to understand variation in outcome across the observed sample. This methodology however shows promise in continued assessment of VLFR performance for this purpose

Kaugseire Eestis 2014 : artiklikogumik

TäistekstHoiate käes artiklikogumikku, millesse autorid on koondanud oma parimad tulemused ja näited, kuidas kaugseiremeetod võimaldab jälgida keskkonda Maal. Eesti teadlaste, üliõpilasteja spetsialistide huvi leida rakendusi, millega saab efektiivselt ja mitmekülgselt uurida vee-kogusid, maapinda, metsa ja atmosfääri, on leidnud tunnustust paljudes rahvusvahelistes publikatsioonides, kuid eestikeelsena ilmub selline kogumik teist korda pärast 2008. aasta esimest populaarset trükist. Vahepeal on võimalused jõudsalt edasi arenenud. Euroopa Liidu programm Copernicus (varasema nimetusega GMES), mis mõeldud globaalseks Maa keskkonna ja turvalisuse seireks, on teadusuuringute etapist väljunud ja esimesed operatiivsed teenused on käivitunud. Eesti on liikmesriigina nii panustanud selle ettevalmistusse kui ka kannab ühiselt teiste riikidega vastutust, et see programm ellu viia, ehitada koostöös Euroopa Kosmoseagentuuriga Sentinel-seeria satelliidid ning nende tule-mitele põhinedes luua kvaliteetseid teenuseid. See ei ole olnud odav, see ei ole olnud lihtne. Miks me siis seda ikkagi teinud oleme? Hea ülevaate Eesti looduskeskkonna oludest annab värske, kuues Eesti kliimaaruanne. Eesti koos oma saarestiku ja rohkete üksikute saartega laiub raskesti piiritletava poolsaarena Soome ja Riia lahe vahel. Loodusgeograafiliselt iseloomustab meid pikk rannajoon (ligikaudu 3700 km), mille moodustavad nii mere-, järve- kui ka jõgede rannad. Meie arvukad väikesed saared, mida on üle 1620, ja umbes 1450 järve vajavad jälgimist ja hoolt. Aeglaselt muutuvad, kuid väga tundlikud sooalad, mis katavad ligikaudu 15% territooriumist, on sageli ka raskesti ligipääsetavad. Ligikaudu pool maismaast on kaetud metsadega, kolmandik on põllumaad ja tilluke 2% on asulad ja linnad. Meie vahelduvad klimaatilised tingimused, mere ja sisevee-kogude rohkus ning aluskivimite varieeruvus loovad eelduse mitmepalgeliste ökosüsteemidearenguks. Me kaitseme oma territooriumil üle 570 taime- ja loomaliigi. Samas oleme üksväiksema inimasustuse tihedusega riike Euroopas – keskmiselt 30 inimest ruutkilomeetri kohta, kuid ainult kolmandik neist elab väljapool linnu. Kuidas siis ikkagi kõigest sellest ülevaadet saada? Ega üksi ei saagi! Kolm naaberriiki, Eesti, Soome ja Venemaa, on kuulutanud käesoleva, 2014. aasta Soome lahe aastaks, et ühiselt välja selgitada Soome lahe ökosüsteemi seisund. See on võimalik tänu riikide tihedale koostööle, kaasaegsetele vaatlusmeetoditele ja arenenud mudelitele. Keskendu-takse viiele võtmeprobleemile: kalandus, ökosüsteemi tervis, mitmekesisus, meresõiduohutus ja merealade ruumiline planeerimine. Kaugseire on tõestanud end väärtusliku vahendina meid ümbritseva maailma mõistmisel ja pakub ka mitmes nimetatud valdkonnas lahendusi. Siiski on selleks vaja teadmisi – nii selle kohta, mida on võimalik teha, kui ka selle kohta, mida on vaja teha. Loodame, et siia kogutud artiklid aitavad kaasa vastastikuse mõistmise ja mõtestamise protsessile, et eestikeelsed terminid muutuvad lähedasemaks neile kasutajatele, kes alles asuvad tutvuma kosmosevaldkonna tehnoloogiatega, ning et kogenud spetsialistid leiavad oma töödele uudsed väljundid. Käesolev kogumik on saanud teoks Tartu Observatooriumi eestvedamisel ja Keskkonnaagentuuri kaasabil, kuid sisukuse eest väärivad tunnustust artiklite autorid. Täname EASi kosmosevaldkonna teadlikkuse tõstmise ja koolituse programmi. Anu ReinartTartu Observatooriumi direktorKäesoleva väljaande andmete kasutamisel või tsiteerimisel palume viidata allikale