International Journal of Applied Earth Observation and Geoinformation
Using multi-date high spectral resolution data to assess the physiological status of macroscopically undamaged foliage on a regional scale
Graphical abstract
Introduction
Forests play an important role in regulation of the global climate via the global carbon cycle, evapotranspiration, and earth surface albedo (Bonan, 2008, Jackson et al., 2008). Moreover, forests provide humans with a whole range of ecosystem services including provision of food and forest products, regulation of the hydrological cycle, protection of soil resources, etc. Forest health and ecosystem functioning have recently been influenced by anthropogenic activities and their consequences, such as air pollution, surface mining, heavy metal contamination (Aznar et al., 2009), and other biotic and abiotic stress factors such as pest invasions and soil acidification (Sebesta et al., 2011), which had an especially high effect on central Europe. Therefore, large-scale monitoring of forest health and its methodologies are in the forefront of interest for scientists as well as forest managers.
Many aspects of the physiological state of trees are more or less connected with the concentrations of two main groups of leaf photosynthetic pigments: chlorophylls and carotenoids (Ustin et al., 2009). Vegetation with a high concentration of chlorophyll is considered to be healthy, as the chlorophyll content is linked to greater light-use efficiency, photosynthetic activity and carbon dioxide uptake (Blackburn, 2007, Kramer, 1981, Wu et al., 2008). Chlorophyll generally decreases under stress and during senescence (Blackburn, 2007). Carotenoids play the main role in the process of incident light absorption, transportation of energy to the reaction center of the photosystems, and heat dissipation of energy in case of high irradiation (Demmig-Adams and Adams, 1996). Combination of the influences of chlorophylls and carotenoids is thus connected with light-use efficiency (Landsberg et al., 1997). However, higher carotenoid to chlorophyll ratios indicate vegetation stress and senescence (Demmig-Adams and Adams, 1996, Young and Britton, 1990).
Therefore, the content of biochemical compounds such as photosynthetic pigments can be used as non-specific indicators of the actual tree physiological status, stress and the pre-visible tree damage. Moreover, the contents of photosynthetic pigments are closely related to photosynthetic performance and can serve as non-specific stress indicators in a very early stage, when the needles do not yet show any microscopic or macroscopic damage symptoms (Lepedus et al., 2005, Soukupova et al., 2000, Tzvetkova and Hadjiivanova, 2006). When dealing with photosynthetic pigments as vegetation stress indicators, seasonal dynamics in pigment contents in evergreen conifers must be taken into account. Changes in pigment levels reflect the normal physiological responses in the plant as well as the responses to environmental stress (Gamon and Surfus, 1999, Gitelson et al., 2001, Gitelson et al., 2002, Grisham et al., 2010). The chlorophyll content in needles increases in the spring (from May/June) and during the summer (July–September) and then remains relatively constant until October, when it again decreases during the frost hardening process (Oquist and Huner, 2003). Some of the carotenoids (e.g., lutein, β-carotene) exhibit stable contents during the seasons in contrast to several groups of xanthopylls (e.g., antheraxanthin, zeaxanthin; Yatsko et al., 2011), which increase significantly during the winter and serve as effective protection of the photosynthetic apparatus under conditions of high irradiance and low temperature during the winter and spring (Maslova et al., 2009). The total carotenoid to total chlorophyll ratio decreases in parallel with the chlorophyll increase in the spring (Martz et al., 2007).
Conventional laboratory analyses of leaf biochemical parameters can be very precise, although they have a number of disadvantages (e.g., limited number of samples, high labor and cost demands). It has been demonstrated that variations in leaf biochemical composition affect foliar optical properties (Carter and Knapp, 2001, Clevers and Kooistra, 2012, Kokaly et al., 2009, Schlerf et al., 2010, Sims and Gamon, 2002, Ustin et al., 2009, Zhang et al., 2012). The spectral reflectance characteristics of plant canopies are influenced by the chemical composition, internal leaf structure and spatial distribution of the leaves (Asner, 1998, Ollinger, 2011, Zwiggelaar, 1998). Leaf pigments are well positioned to absorb incident light and can be assessed with spectral reflectance. The more important absorption pigments and their characteristic absorption wavelengths/wavebands were reviewed by Zwiggelaar (1998).
Modern remote sensing has become a novel tool not only for detecting target materials but also for monitoring dynamic processes and physical-property induced changes. The use of multispectral imagery has been demonstrated to effectively map the distribution of ecosystem types and vegetation systems (Everitt et al., 2002, Gould, 2000, Knorn et al., 2009, Lamb and Brown, 2001, Vogelmann et al., 2012); however, the low spectral resolution of multispectral imagery is a major limitation. On the other hand, imagery with higher spectral resolution (e.g., hyperspectral) provides sufficient spectral resolution to describe diagnostic absorption signatures and allows sufficiently detailed species discrimination and biochemical differentiation (Aspinall, 2002, Feret and Asner, 2013, Kokaly et al., 2009, Lass and Prather, 2004, Majeke et al., 2008, Odagawa and Okada, 2009, Underwood et al., 2003, Ustin et al., 2004, Zhao et al., 2013).
Data with very high spectral resolution – also referred to as imaging spectroscopy (IS) data, which is also known in the remote sensing community as hyperspectral data – has been successfully used in earlier studies to detect vegetation stress and damage (Campbell et al., 2004, Campbell et al., 2007, Hamzeh et al., 2013, Hernandez-Clemente et al., 2011, Pu et al., 2008, Rathod et al., 2013, Romer et al., 2012, Sanches et al., 2013a, van der Meer et al., 2002, van der Werff et al., 2008). In the forestry context, most published precedents used IS data as a basis for identification of stress-sensitive wavelengths (e.g., Ahern, 1988, Masaitis et al., 2013, Sanches et al., 2013a), for development of stress-sensitive vegetation indices (e.g., Carter and Miller, 1994, Carter and Knapp, 2001, de Jong et al., 2012, Lausch et al., 2013) and for integrating stress-sensitive indices in more complex models (e.g., Fassnacht et al., 2012, Pontius et al., 2008, Shafri et al., 2012, Zarco-Tejada et al., 2004, Zhao et al., 2013).
In contrast, only a few approaches (Asner and Martin, 2009, Kampe et al., 2010) have dealt with the use of hyperspectral remote sensing (image spectroscopy) to assess the physiological status of vegetation on a regional scale, the term “regional” is understood to mean related or limited to a particular region, area or part, as of a country or the body. Moreover, little or no research has been done on assessing vegetation health utilizing multi-date hyperspectral image data, as a time-series of hyperspectral data and reliable methods to extract change/stress information for remotely sensed data analysis are still lacking. We recently proposed a new method for assessing forest health condition via normalization and further statistical integration of optical indices retrieved from HS image data (Misurec et al., 2012). To assess subtle changes in the physiological status of macroscopically undamaged foliage of Norway spruce, this method integrated the following HyMap derived parameters: quantitative retrieval of chlorophyll concentrations (Cab); Red-Edge Position (REP) (Curran et al., 1995) – the inflection point of the spectral curve in the red-edge region, which is shifted to shorter wavelengths under vegetation stress (e.g., the presence of heavy metals in the soil) (Chang and Collins, 1983, Clevers et al., 2002, Curran et al., 1995, Horler et al., 1983, Cho et al., 2012, Rock et al., 1988, Sanches et al., 2013b); and the Structure Insensitive Pigment Index (SIPI) (Penuelas et al., 1995) which is sensitive to the ratio of bulk carotenoids to chlorophyll. This method is easily applicable and, after further testing and confirming its general applicability, it has a potential to be adopted for other coniferous species or as a monitoring tool if using multi-temporal HS data.
Two temporal HS image data sets (HyMap 2009 and 2010 image data) were processed in order to answer whether the suggested method (Misurec et al., 2012) would be valid if using other temporal HS image data and sensitive enough to detect changes between two subsequent growing seasons. To justify this point, the following issues needed to be studied:
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What are variations in biochemical parameters between two subsequent growing seasons of a Norway spruce forest? Accordingly, the foliar pigment content from the samples collected in two subsequent growing seasons 2009 (28–29 July) and 2010 (2–3 September) were compared and statistically assessed.
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Whether the detected stress can be linked with or explained by the geochemical conditions of the forest soils? To answer this question vegetation stress has been assessed within the selected Norway spruce sites while putting together information on the forest stand's geochemical conditions, foliar biochemistry (pigment contents) and the temporal differences detected by classifying the two HS image datasets acquired one year apart.
Section snippets
Test site
The study was performed in the Sokolov basin in the Western part of the Czech Republic, in a region affected by long-term extensive lignite mining (Fig. 1). Due to the mining activities and coal burning power plants that were built in the immediate vicinity of the mined area, this region is one of the most contaminated areas of the Czech Republic where high abundances of trace elements have been detected (Suchara et al., 2011). The average altitude of the study region is about 470 m. Because the
Site soil characteristics
The soil chemistry at all the sites was characterized by low exchangeable pH in both organic (Ol + Of, Oh) and mineral soils (A0–10, A10–20) (Fig. 3). However, the mineral soil chemistry reflected the composition of the parent material. Erika, the most acidic site, is underlain by sandstone and quartzite characterized by extremely low base cation contents. As a consequence, the lowest base saturation (2–2.5%) and Bc/Al ratio (0.04–0.06) were measured in the top mineral soil (0–20 cm). Slightly
Discussion
All the sites are located in the vicinity of the Sokolov basin, an area with historically high emissions of acidifying compounds (SO2, NOx). Unfavorable low Bc/Al ratios across all the sites might be connected to the high deposition of S and N in the past and subsequently to the anthropogenic acidification of forest soils in this region (Kram et al., 1997, Oulehle et al., 2006). Soil acidification promotes leaching of base cations and mobilization of aluminum (Kram et al., 2009). Thus,
Conclusions
In this study, the recently proposed method for assessing forest health conditions using optical indices retrieved from HS data (Misurec et al., 2012) was applied to the two temporal HyMap date sets acquired in 07/2009 and 08/2010 for detecting stress in Norway spruce forests on a regional scale. The classification results were validated by ground truth data (foliar biochemistry: Cab, Car and Car/Cab) and were associated with the geochemical conditions of the forest stands (pH, macronutrient
Acknowledgements
The present research is being undertaken mainly within the framework of a grant No. 205/09/1989 (HypSo: grant funded by the Czech Science Foundation). Additionally, the research was also supported by grants No. 244242 (EO-MINERS: FP7 grant funded by the EC) and No. LM2010007 (CzeCOS). Many thanks are due to Dr. Petr Rojík (Sokolovská uhelná a.s.) for his substantial assistance with the field campaign. The colleagues from DLR are thanked for the atmospheric correction of the HyMap 2010 data set.
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