Use of the near infrared similarity reflectance spectrum for the quality control of remote sensing data

ABSTRACT The shape of water-leaving reflectance spectra in the near infrared range 700-900nm is almost invariant for turbid waters and has been analysed and tabulated as a similarity spectrum by normalisation at 780nm. This similarity spectrum is used here for the quality control of seaborne reflectance measurements and for the improvement of sky glint correction. Estimates of the reflectance measurement error associated with imperfect sky glint correction from two different wavelength pairs are shown to be nearly identical. A demonstration of residual reflectance correction for data collected in cloudy, high wave conditions has shown that this correction removes a large source of variability associated with temporal variation of the wave field. The error estimate applied here to seaborne measurements has wide-ranging generality and is appropriate for any water-leaving reflectance spectra derived from seaborne, airborne or satellite borne sensors provided suitable near infrared bands are available

Senecio namibensis (Asteraceae: Senecioneae), a new species from Namibia

Senecio namibensis is described as a new species known only from the northern part of the Namib Desert in northwestern Namibia. It is a range-restricted species near-endemic to the Kaokoveld Centre of Endemism. These dwarf shrubs grow on rocky outcrops under harsh desert conditions. Diagnostic characters for Senecio namibensis include the annual or perennial habit, succulent leaves, and radiate capitula with 3–6 yellow ray florets. A comparison of some of the more prominent morphological features to differentiate between S. namibensis and its possible nearest relatives, S. englerianus and S. flavus, is provided. All three species have superficially similar looking succulent leaves, but an obvious difference is that the capitula in S. englerianus are discoid and in S. flavus disciform or obscurely radiate. Based on IUCN Red List categories and criteria, a conservation assessment of Least Concern (LC) is recommended for the new species.https://www.mapress.com/ptdm2022Plant Production and Soil Scienc

Autecological aspects of the African timber tree Pterocarpus angolensis in support of its sustainable management

Pterocarpus angolensis (Kiaat, Muninga, Mukwa) is the most widely harvested timber species within the African Miombo ecoregion. Relatively little is known about the ecology and productivity of the species that grows only in natural mixed forests. The study aims to contribute to its sustainable management by modelling the occurrence and productivity in function of environmental factors and forest structure. Autecological methods are applied on a large dataset – both recent and historical - for southern Africa and a study area in the open forests of northern Namibia and southern Angola. The dissertation demonstrates that P. angolensis is a long-living, late successional species with weak competitive abilities during the juvenile phase. Its productivity in the study area compares well to that in other parts of the distribution range because of a relative high abundance and growth rate, however the limited amount of natural regeneration and the sensitivity of the species to climate change give causes for concern. Forest managers aiming for an improved and sustainable yield need to focus on more strict fire prevention measures, on increased firefighting, and on introducing silvicultural treatments such as assisted regeneration and thinning during the juvenile phases, especially in the most productive stands.The most widely harvested timber species within the Miombo ecoregion of Africa is Pterocarpus angolensis, also known as Kiaat and Mukwa. Relatively little is known about the ecology and productivity of the species that grows in natural mixed forests where forest management interventions rarely take place. The study aims to contribute to its sustainable management by modelling the occurrence and productivity in function of environmental factors and forest structure. A biographical model for the southern African region shows how the distribution is mainly influenced by rainfall amount and seasonality and hence is sensitive to climate change. Productivity, measured both by abundance and growth, is however much less influenced by climate but rather by forest structure and anthropogenic disturbances. In a study area in the Baikiaea - Pterocarpus forests of northern Namibia and southern Angola, the species is common and characterises one of the tree communities. Stem diameter growth in the study area is about 5.5 mm per year for all diameter classes. The dissertation demonstrates that P. angolensis is a long-living, late successional species that has weak competitive abilities, especially during the juvenile phase. It therefore performs well in the open, dry Baikiaea – Pterocarpus forests. Sustainable forest management needs to focus on fire prevention measures, assisted regeneration and silvicultural treatments such as thinning during the juvenile phases in the most productive stands.nrpages: 136status: publishe

A new species of Osteospermum subgen. Tripteris (Asteraceae: Calenduleae) from the Namib Desert, Namibia

Swanepoel, Wessel, Cauwer, Vera De, Van Wyk, Abraham E. (2021): A new species of Osteospermum subgen. Tripteris (Asteraceae: Calenduleae) from the Namib Desert, Namibia. Phytotaxa 487 (3): 185-194, DOI: 10.11646/phytotaxa.487.3.1, URL: http://dx.doi.org/10.11646/phytotaxa.487.3.

Overview of ocean colour: theoretical background, sensors and applicability to detection and monitoring of harmful algal blooms (capabilities and limitations)

Anecdotal and journalistic references to harmful algal blooms (HABs) often report the impact on water colour, using terms such as “red tide”. Indeed probably the earliest recording (Exodus 7:20-21) of a harmful algal bloom (HAB) remarks that the waters “turned to blood”. A few thousand years later photoelectric devices have replaced the human eye as detector, allowing a transition from subjective and descriptive observations to detailed and quantitative information on water constituents. The objective of this chapter is to summarise the status of optical remote sensing of HABs by reviewing ocean colour theory and giving an overview of available systems. Applications of optical remote sensing to the monitoring of HABs are reviewed, noting capabilities and limitations, and future perspectives are discussed. The scope of this chapter, as given by the title “ocean colour”, covers passive systems for optical remote sensing using the sun as direct light source (Mobley, 1994). While the term optical remote sensing could be considered to cover a wide variety of sensors, including the human eye, airborne photographic cameras, shipborne above-water radiometers, etc. the scope here will be limited to sensors providing digital image-based information, thus covering essentially satellite and airborne radiometers. The focus is on reflectance-based techniques because of their overwhelming importance in the available data stream. However, it is noted that some information on phytoplankton (without specific relevance to HABs) can be retrieved from remote sensing of sun-stimulated fluorescence (Babin, 2005; Gower et al. 1999). Active remote sensing systems based on, for example, laser-induced fluorescence (Hoge and Swift, 1981) are excluded. Applications are drawn mainly from marine waters because of the user interest and experience there though the validity of the theory and systems to estuarine and inland waters is also considered. The applications described in this chapter are drawn essentially from HAB studies. However, the theory described as well as many of the remarks made in this chapter are relevant both to HABs and to the much wider field of ecosystem dynamics. In fact, as will be discussed in detail later, ocean colour is used primarily for algae bloom (AB) detection and generally requires supplementary information from seaborne measurements or expert knowledge for determination of the harmfulness of the event. The question “Is the AB a HAB?” is a major challenge for the ocean colour technique. The definition of “harmful” in this context is left for others (Cullen, 2005; Geohab, 2001; Graneli et al. 1998). The present chapter thus covers a wide range of algal species with very different severities of impact. The content of this chapter has been selected according to the spirit of the workshop to provide readers with the theory relevant to understanding the basic principles of ocean colour observations and with an appreciation of the current capabilities and limits of these tools for HAB detection. The target readership is students and potential end-users of coastal observation systems who should thus acquire sufficient knowledge for the initiation and use of such observing tools. Thus, the focus is on general principles of ocean colour remote sensing and a broad understanding of processes with emphasis on their spectral reflectance, loosely termed as “colour”. Optical remote sensing specialists who require a more detailed description of radiative transfer theory and especially the complications of interacting processes, such as multiple scattering, and second order optical effects, should consult the references in the appropriate sections. Readers with no interest in the details of ocean colour theory and data processing may skip sections 2, 3.1 and 3.2 but are strongly advised to be aware of the variety of methods presented in sections 3.3, 3.4 and 3.5 and especially of their limitations. It is common for products from the main ocean colour sensors to give very different results in coastal waters even for data acquired almost simultaneously (De Cauwer et al. 2004). Key questions which are to be answered in this chapter include: • What is ocean colour? • What sensors are available and suitable for HAB detection? • What information can be obtained from such sensors? • What are the limitations of the ocean colour technique for HAB detection? • What developments can be expected in the future?info:eu-repo/semantics/publishe

Jaarlijkse groei van Pterocarpus angolensis (Kiaat) en andere boomsoorten in Kavango, Namibië

De meest geprefereerde houtsoort voor zagerij in Namibië en het grootste deel van zuidelijk Afrika, is Pterocarpus angolensis (Kiaat). Er is echter niet genoeg informatie over de jaarlijkse groei van de soort om te bepalen of de oogst duurzaam is. De soort bereikt in Namibië de zuidelijke grens van zijn verspreidingsgebied en men zou kunnen verwachten dat de jaarlijkse groei lager is dan in andere landen waarvoor meer gegevens beschikbaar zijn binnen de literatuur. Deze studie wil bijdragen aan de bepaling van de jaarlijkse diametergroei van Kiaat in Namibië door het meten van de jaarlijkse groei in de regio Kavango. De diametergroei van Kiaat zal worden vergeleken met een paar andere soorten waarvoor boorkernen werden verzameld. Hierbij maakt men gebruik een presslerboor voor het winnen van boorspanen die worden geëgaliseerd met de Core-microtome om ze daaropvolgend met LintabTM software te analyseren. Er werd ook informatie gewonnen uit stamschijven die gedurende het onderzoek te beschikking werden gesteld voor manuele tellingen. Naast Kiaat werd een soortgelijke analyse uitgevoerd op Burkea africana, Baikiaea plurijuga, Terminalia sericea en Schinziophyton rautanenii om na te gaan of ook deze soorten geschikt zijn voor jaarringanalyse op basis van boorspanen. Het onderzoek centraliseert zich binnen de regio Kavango maar beoogt tevens vergelijkingen met naburige regionen binnen en buiten Namibië. Gekeken naar de kwaliteit van boorspanen en de vraag of andere houtsoorten bruikbaar zijn voor jaarringanalyse, merkt men vooral technische problemen door intern rot of holle stammen bij Baikiaea plurijuga en Burkea africana. Voor Terminalia sericea zijn goede boorspanen verzameld maar is het vooral moeilijk om daadwerkelijk tot de kern van de stam te geraken vanwege excentriciteit. Voor Schinziophyton rautanenii vermoedt men geen jaarringen te zien, aangezien het gaat om een stamsucculent. Uit de jaarringanalyse haalt men groeicurven die de relatie tussen diameter en leeftijd weergeven voor stamschijven. Voor boorspanen wordt met LintabTM software de effectieve ringbreedte bepaald en de daaruit volgende groeicurve. Uit een eerste reeks resultaten blijkt de groei in Kavango sneller te verlopen dan drogere gebieden maar trager dan in gebieden met meer neerslag. Terminalia sericea heeft als pioniersoort een snellere groei dan Pterocarpus angolensis, Baikiaea plurijuga en Burkea africana. Analyse toont aan dat de groei van bomen in Kavango onder andere wordt beïnvloed door neerslag en dat de groeipatronen per soort sterkt verschillen

Patterns of forest composition and their long term environmental drivers in the tropical dry forest transition zone of southern Africa

Background Tropical dry forests cover less than 13 % of the world’s tropical forests and their area and biodiversity are declining. In southern Africa, the major threat is increasing population pressure, while drought caused by climate change is a potential threat in the drier transition zones to shrub land. Monitoring climate change impacts in these transition zones is difficult as there is inadequate information on forest composition to allow disentanglement from other environmental drivers. Methods This study combined historical and modern forest inventories covering an area of 21,000 km2 in a transition zone in Namibia and Angola to distinguish late succession tree communities, to understand their dependence on site factors, and to detect trends in the forest composition over the last 40 years. Results The woodlands were dominated by six tree species that represented 84 % of the total basal area and can be referred to as Baikiaea - Pterocarpus woodlands. A boosted regression tree analysis revealed that late succession tree communities are primarily determined by climate and topography. The Schinziophyton rautanenii and Baikiaea plurijuga communities are common on slightly inclined dune or valley slopes and had the highest basal area (5.5 – 6.2 m2 ha−1). The Burkea africana - Guibourtia coleosperma and Pterocarpus angolensis – Dialium englerianum communities are typical for the sandy plateaux and have a higher proportion of smaller stems caused by a higher fire frequency. A decrease in overall basal area or a trend of increasing domination by the more drought and cold resilient B. africana community was not confirmed by the historical data, but there were significant decreases in basal area for Ochna pulchra and the valuable fruit tree D. englerianum. Conclusions The slope communities are more sheltered from fire, frost and drought but are more susceptible to human expansion. The community with the important timber tree P. angolensis can best withstand high fire frequency but shows signs of a higher vulnerability to climate change. Conservation and climate adaptation strategies should include protection of the slope communities through refuges. Follow-up studies are needed on short term dynamics, especially near the edges of the transition zone towards shrub land

Patterns of forest composition and their long term environmental drivers in the tropical dry forest transition zone of southern Africa

Background: Tropical dry forests cover less than 13 % of the world’s tropical forests and their area and biodiversity are declining. In southern Africa, the major threat is increasing population pressure, while drought caused by climate change is a potential threat in the drier transition zones to shrub land. Monitoring climate change impacts in these transition zones is difficult as there is inadequate information on forest composition to allow disentanglement from other environmental drivers. Methods: This study combined historical and modern forest inventories covering an area of 21,000 km2 in a transition zone in Namibia and Angola to distinguish late succession tree communities, to understand their dependence on site factors, and to detect trends in the forest composition over the last 40 years. Results: The woodlands were dominated by six tree species that represented 84 % of the total basal area and can be referred to as Baikiaea - Pterocarpus woodlands. A boosted regression tree analysis revealed that late succession tree communities are primarily determined by climate and topography. The Schinziophyton rautanenii and Baikiaea plurijuga communities are common on slightly inclined dune or valley slopes and had the highest basal area (5.5 – 6.2 m2 ha−1). The Burkea africana - Guibourtia coleosperma and Pterocarpus angolensis – Dialium englerianum communities are typical for the sandy plateaux and have a higher proportion of smaller stems caused by a higher fire frequency. A decrease in overall basal area or a trend of increasing domination by the more drought and cold resilient B. africana community was not confirmed by the historical data, but there were significant decreases in basal area for Ochna pulchra and the valuable fruit tree D. englerianum. Conclusions: The slope communities are more sheltered from fire, frost and drought but are more susceptible to human expansion. The community with the important timber tree P. angolensis can best withstand high fire frequency but shows signs of a higher vulnerability to climate change. Conservation and climate adaptation strategies should include protection of the slope communities through refuges. Follow-up studies are needed on short term dynamics, especially near the edges of the transition zone towards shrub land.status: publishe