Abstract
Anthropogenic stress and disturbance of forest ecosystems (FES) has been increasing at all scales from local to global. In rapidly changing environments, in-situ terrestrial FES monitoring approaches have made tremendous progress but they are intensive and often integrate subjective indicators for forest health (FH). Remote sensing (RS) bridges the gaps of these limitations, by monitoring indicators of FH on different spatio-temporal scales, and in a cost-effective, rapid, repetitive and objective manner. In this paper, we provide an overview of the definitions of FH, discussing the drivers, processes, stress and adaptation mechanisms of forest plants, and how we can observe FH with RS. We introduce the concept of spectral traits (ST) and spectral trait variations (STV) in the context of FH monitoring and discuss the prospects, limitations and constraints. Stress, disturbances and resource limitations can cause changes in FES taxonomic, structural and functional diversity; we provide examples how the ST/STV approach can be used for monitoring these FES characteristics. We show that RS based assessments of FH indicators using the ST/STV approach is a competent, affordable, repetitive and objective technique for monitoring. Even though the possibilities for observing the taxonomic diversity of animal species is limited with RS, the taxonomy of forest tree species can be recorded with RS, even though its accuracy is subject to certain constraints. RS has proved successful for monitoring the impacts from stress on structural and functional diversity. In particular, it has proven to be very suitable for recording the short-term dynamics of stress on FH, which cannot be cost-effectively recorded using in-situ methods. This paper gives an overview of the ST/STV approach, whereas the second paper of this series concentrates on discussing in-situ terrestrial monitoring, in-situ RS approaches and RS sensors and techniques for measuring ST/STV for FH.
Highlights
About one third of the Earth’s land surface is covered by forests [1]
We consider the following research questions: (i) How can forest health be defined? What are its mechanisms, drivers and processes? On which spatial and temporal scales should indicators of forest health be measured? (ii) Why and under which conditions are remote-sensing approaches suitable for observing indicators of forest health? (iii) What is the concept of spectral traits (ST) and spectral trait variations (STV) for quantifying, monitoring and assessing forest health using remote sensing? Through addressing these questions, we discuss the usability of the ST/STV approach to measure stress and disturbances in taxonomic, structural and functional diversity of forest ecosystems (FES)
forest health (FH) is characterized by a high degree of stability, elasticity and resistance to stress, disturbances and resource limitations, when a scale- and FES-dependent taxonomic, phylogenetic, structural as well as functional diversity is maintained or at least restored after the impacts of stress
Summary
About one third of the Earth’s land surface is covered by forests [1]. They provide important ecosystem services, such as carbon sequestration, habitat for species that contribute to maintaining biodiversity, mitigating climate change and the ability to filter and maintain water resources [2,3]. It follows that forest ecosystem health has gained popular attention over recent years with growing concerns about air pollution, acid rain, global climate change, population growth, and long-term resource management. In response to these environmental concerns and to current legislative and policy directions, federal and state agencies worldwide have been working together to develop programs for monitoring and reporting on the status and trends of forest ecosystem health. Local to regional FH assessments can use sensors with small area coverage and high spatial resolution, while global assessments are conducted at coarse scales Developed sensors such as Worldview-3 provide more than 10 spectral bands over the visible-near infrared-short wave infrared regions of the spectrum with very high spatial resolution (0.3–2.5 m pixel size). We consider the following research questions: (i) How can forest health be defined? What are its mechanisms, drivers and processes? On which spatial and temporal scales should indicators of forest health be measured? (ii) Why and under which conditions are remote-sensing approaches suitable for observing indicators of forest health? (iii) What is the concept of spectral traits (ST) and spectral trait variations (STV) for quantifying, monitoring and assessing forest health using remote sensing? Through addressing these questions, we discuss the usability of the ST/STV approach to measure stress and disturbances in taxonomic, structural and functional diversity of FES
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