Satellite-Image Based Analysis of Vegetation Affected by Volcanic Ash Deposits by Co. SEP

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Satellite-Image Based Analysis of Vegetation Affected by Volcanic Ash Deposits – Case Study at Cordón Caulle (Chile) Guido Staub*1, Irlanda Mora2, Henry Montecino3, Juan Carlos Báez4 Departamento de Ciencias Geodésicas y Geomática, Universidad de Concepción – Campus Los Ángeles, J. A. Coloma 0201, Los Ángeles, Chile 1,2,3

Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Blanco Encalada 2002, Santiago, Chile

gstaub@udec.cl; 2irlanda.mora.rosales@gmail.com; 3henrymontecino@gmail.com; 4jcgeodesy@gmail.com

Abstract In recent years, a series of volcanic eruptions occurred in Chile, affecting flora, fauna and human beings. Space borne monitoring of the environment affected by volcanic ash deposits is a major task in order to determine recuperation processes. The objective of this paper is to study vegetation affected by volcanic ash deposits using a combined multi-temporal – multispectral image analysis approach applied to the volcanic complex Puyehue - Cordón Caulle. An annual survey based on Landsat 7 ETM+ satellite images was carried out. Special focus was given on three specific test sites where ground-truthing was possible and reference data was accessible. Our results indicate that not only ash covered vegetation reduced, but also its photosynthetic activity started to develop when ash emission decreased. In this particular case we conclude that the possibility for vegetation recovery depends mostly on winds, precipitation and landform of the study sites. Keywords Remote Sensing; Vegetation; NDVI; Volcanic Eruption

Introduction Located 83 kilometers northeast of the city of Osorno, the so-called Volcanic Complex Puyehue - Cordón Caulle VCPCC, see figure 1, which corresponds to a volcano-tectonic depression (that is about 13 km long and 4 km wide), has a number of monogenetic volcanic centers, including pumice cones, lava domes, volcanic fissures and the largest geothermal center of southern volcanic zone of Chile (Sepúlveda et al. [24]). The Caldera Cordillera is a collapsed stratovolcano of the Pleistocene and is located at the northwest boundary of the volcanic complex, while the Puyehue stratovolcano is located in the southwest (as shown by Katsui and Katz [11], Campos et al. [2], Lara et al. [15], Lara et al. [16] and Lara et al. [17]).

40° 34' 57''S –––

72° 06' 42''W

FIG. 1 VCPCC SHOWN AS ALI HIGH-RESOLUTION IMAGE; OBSERVED OCTOBER 22, 2011 (IMAGE COURTESY BY EARTHOBSERVATORY.NASA.GOV)

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Historic eruptions of the volcanic fissure Cordón Caulle, classified with a Volcanic Explosivity Index (VEI) of 3, occurred in 1921 and 1960. Both were very explosive and produced heavy ash fall. For the 1960 eruption, it is supposed that it was a direct result of the largest earthquake ever recorded in Chile (Valdivia 1960), which has hit the city of Valdivia with a magnitude of M = 9.5 Richter and its epicenter was located only 240 km from VCPCC. In 2011, the VCPCC erupted again. Its explosiveness was categorized 3 on the logarithmic VEI scale. The emission of gas and ash produced severe damage in the surrounding area of the volcano, which is a typical consequence (Kirianov [12]). An eruptive column with a height of approximately 9km emerged and persisted for 10 days and affected the Nahuel Huapi area with intense ash fall, at a distance of 100 km northeast of the crater (Hernández et al. [9]). In particular, ash deposits affected flora and fauna in the southeast of the crater during this event (Seguel [22]) for a prolonged time period. Photosynthetic activity of vegetation decreased significantly as volcanic ash deposits affected old growth forest, predominant in the National Park Puyehue. 72° 06'

40° 34' 57''S –––

2 3 1

FIG. 2 MAP OF PUYEHUE NATIONAL PARK (COURTESY BY CONAF.CL). BLACK SQUARES INDICATE LOCATION OF SPECIFIC STUDY SITES SELECTED FOR THIS INVESTIGATION. (DETAILS FOR THESE ARE SHOWN IN FIGURE 3)

In general, Remote Sensing is of great importance for vegetation monitoring and landscape change detection after any volcanic activity (Clarkson and Clarkson [3]). Based on Landsat TM observations, Kuzera et al. [13] proved for the Mt. St. Helens volcano that within ten years after the 1980 eruption predominant vegetation regenerated significantly. Vegetation health monitoring based on the Normalized Difference Vegetation Index (NDVI), plant water content studies and photosynthetic activity research is already well understood (as shown in Gamon et al. [7], Pelkey et al. [19], Sims and Gamos [26], Xiao and McPerson [30] and Pettorelli et al. [20]). The NDVI is calculated as (NIR - RED)/(NIR + RED), where NIR is the reflectance radiated in the near-infrared waveband and RED is the reflectance radiated in the visible red waveband (Justice et al. [10]). Higher NDVI values, between 0.4 and 1, indicate a higher photosynthetic activity potential (Sellers [23] and Tucker et al. [28]). It has widely been demonstrated that multi-temporal NDVI studies based on Landsat satellite imagery are useful for monitoring vegetation dynamics (Altorre et al., 2011, Fraser et al., [6], Sever et al. [25] and Raynolds et al. [21]). Due to the possibility that Remote Sensing allows almost near real time monitoring of volcanic eruptions, e.g. in Hawaii, Galapagos Islands or the Philippines (Mouginis-Mark [18]), it has to be considered an important tool for the study 2

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of natural disasters. Nevertheless, quantification of ash deposits and its impact on vegetation is poorly studied so far (Sobrino and Julien [27]); whereas Costantini [4] studied the impact of disturbances on landscape in general. Within the 26,000 hectares wide area of Puyehue National Park three specific sectors (see figure 2) were identified. Dominating forests are evergreen and different landforms can be observed. The first corresponds to a small area near Aguas Calientes (southwest of the crater) in hilly country; the second area encompasses the border crossing complex Cardinal A. Samoré (south of the crater) in the Gol-Gol valley; the third sector includes an area near Route 215 next to the side of Laguna El Pato (southeast of the crater) in the high mountains of the Andes. All these are of almost equal size and cover an area of approximately 43 hectares. Experts of CONAF (Corporación Nacional Forestal – National Forest Cooperation) had already carried out intensive fieldwork and studies in 2011. These three specific areas of interest were chosen because on the one hand they were accessible during our research and on the other hand, because reference data for comparison was facilitated by CONAF. In consequence we were able to carry out ground-truthing and to check our results obtained from image analysis. In particular, data provided by CONAF, which already had established test areas of 25 square kilometers for vegetation monitoring earlier, was very valuable. Their data allowed us to confirm tendencies in vegetation recovery that we observed by satellite image analysis.

FIG. 3 FALSE COLOR LANDSAT 7ETM+ IMAGE (DARK LINES DUE TO LINE DROP-OUTS) OF STUDY AREA (LEFT: SEPTEMBER 2011; RIGHT: SEPTEMBER 2012). WHITE RECTANGLE INDICATES AREA FOR WHICH NDVI WAS CALCULATED. YELLOW POINT INDICATES AREA FOR WHICH A 6X6 MATRIX WAS EXTRACTED TO CARRY OUT A DETAILED STUDY OF NDVI.

Data and Methodology Satellite Images The selected images have to comply with two fundamental characteristics. First, they have to cover the most affected area of the Puyehue National Park and second, they should show those areas that contain the highest agglomeration of native, perennial trees; because contamination is easily detectable when ash deposits are

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accumulated on leaves during the entire year and do not disappear in autumn when affected trees drop their leaves. For this purpose, we used a multispectral Landsat 7 ETM+ satellite image time series during the months of September 2010 (as a reference before eruption of VCPCC), September 2011 (to study contamination due to ash deposits during VCPCC eruption) and September 2012 (post-eruption analysis to detect regeneration processes). Methodology We carried out a study that is based on a three-step methodology, which couples satellite imagery and in-situ data collection. At first, a visual image interpretation is carried out. It is based on a visual analysis of the following Landsat 7 ETM+ typical band combinations for true and false color visualization: 3,2,1; 4,3,2; 5,4,2. The natural color band combination (3,2,1), is used in order to visualize the landscape as it appears in reality (Geospatial Data Service Center [8] and USGS [29]). Band 3 discriminates vegetation slopes; Band 2 helps to assess vegetation vigor; Band 1 allows distinguishing soil from vegetation (USGS [29]). Whereas false color combinations, such as 4,3,2 and 5,4,2, are generally used to highlight biomass content in plants (Band 4) and moisture content in vegetation and soils (Band 5). Subsequently, an analytical image analysis, which is based on NDVI calculation that is carried out after vegetation and photosynthetic activity potential, is approximated by visual image interpretation. As mentioned earlier, numeric determination of biomass content allows discriminating live green plants from less vital plants. Furthermore, to be able to perform this study, the good first-hand knowledge of the areas of interest has to be acquired about the flora and fauna. Therefore, between the months of January and February 2012 (i), and later on during December 7 and 8 in 2012 (ii), we realized field work. For each of the sector under investigation we determined the percentage of vegetation covered by ash, the percentage of dead vegetation, the percentage of new vegetation and wrote down any other observation of interest. What is more important was the fact that we were able to gather relevant information by inquiries and interviews of locals, reports written by personal of CONAF and OVDAS (Observatorio Vulcanológico de los Andes del Sur – Southern Andes Vulcanologic Observatory) and articles published by the local press agencies (Hernández et al. [9], La Nacion [14], Seguel [22] and Benaprés [1]). Results Sector 1: Route 215 For this sector, the calculated NDVI average is 0.68, indicating high photosynthetic activity potential prior to VCPCC eruptions for September 2010. This value will be used as a reference for the results of NDVI in later years.

FIG. 4 a) NDVI (AREA MARKED BY YELLOW DOT IN FIG. 2) AND b) HISTOGRAM (AREA MARKED BY WHITE RECTANGULAR IN FIG. 2) FOR SEPTEMBER 2011 AND (UPPER) AND 2012 (LOWER)

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Reviews of the histograms (figure 4b) indicate a notable increase in the vigor of vegetation. NDVI values in 2011 are lower when compared with those obtained for 2012. For 2011 NDVI values are in the range between -0.03 and +0.4, indicating that photosynthetic activity potential is low. In contrast, NDVI values obtained for 2012 are higher and extend from 0.2 to a maximum value of 0.83 indicating that the vegetation in this particular sector already started its recovery process. A quite similar result can be found, reviewing the extracted 6x6 matrix. Grey scale reflects NDVI values calculated for each cell within the area under investigation. All these values are about 0.3 higher in September 2012 than that in September 2011. Sector 2: Border Crossing Cardinal A. SamorĂŠ The calculated NDVI average prior to VCPCC eruption (September 2010) is 0.5. A review of NDVI values for 2011 indicates that the vigour, and therefore photosynthetic activity potential of the studied vegetation are low; and NDVI values between -0.02 and 0.3 confirm this. The histogram of NDVI for September 2011 shows a negative trend towards zero, indicating low photosynthetic activity due to the large amount of ash deposits accumulated on vegetation and foliage. In addition we extracted all the pixels that have significant higher NDVI values (NDVI values from 0.4 to 0.7) for September 2011. As a result, we estimate that only an area of approximately 4.2 hectares containing healthy vegetation exists. Therefore we conclude that a huge amount of the predominant vegetation in the study area is affected (compared to the total 43.1 hectares that comprise the study area).

FIG. 5 a) NDVI (AREA MARKED BY YELLOW DOT IN FIG. 2) AND b) HISTOGRAM (AREA MARKED BY WHITE RECTANGULAR IN FIG. 2) FOR SEPTEMBER 2011 AND (UPPER) AND 2012 (LOWER)

For September 2011 we obtained a mean NDVI of 0.04, while for 2012 a 0.38 was calculated. NDVI values are more than 5 times higher in 2012 than that in 2011. Although the 2012 values are not that high to consider existing vegetation as healthy (between 0.2 and 0.5), vegetation regeneration is observable as photosynthetic activity potential increases. By field trips and ground-truthing it was possible to verify these advances. Sector 3: Laguna El Pato In this sector, a particular climate situation has to be factored in; during the winter months of 2011 and 2012, heavy snowfall was recorded. Precipitation deviation from normal during August and September 2011 and June 2012 was positive, 30mm, 10mm and 30mm respectively (Deutscher Wetterdienst [5]). Therefore, besides volcanic ash deposit, there was also snow cover that affected photosynthetic activity.

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For September 2010 the calculated NDVI average recorded a value of 0.6. Figure 6 indicates that NDVI values for September 2011 are in the range of -0.05 and -0.042. For 2012, and NDVI values show that photosynthetic activity potential remains low. This can be explained by the already mentioned huge snow cover in this particular study site. There are only slight changes observable in the 6x6 matrices shown in figure 5. A fact that confirms that photosynthetic activity potential has not advanced as in case of sector 2.

FIG. 6 a) NDVI (AREA MARKED BY YELLOW DOT IN FIG. 2) AND b) HISTOGRAM (AREA MARKED BY WHITE RECTANGULAR IN FIG. 2) FOR SEPTEMBER 2011 AND (UPPER) AND 2012 (LOWER)

In general, the following results are noteworthy: In September 2011, 62 percent of the predominant vegetation was under a thick layer of volcanic material, whereas in September 2012 only 38 percent of the total vegetation was covered by volcanic material. This also provided a positive effect on photosynthetic activity. This effect was higher in case of vegetation that was exposed to volcanic contamination only for a short period of time (in the southwest of the crater) during the eruption of Cordรณn Caulle. In particular, study areas 2 and 3 were more affected by ash deposit than sector 1. NDVI (calculated for a 6x6 matrix marked by a yellow dot) of these areas are significantly higher in 2012 (1: 0,547; 2: 0,117) than that in 2011 (1: 0,045; 2: 0,056) due to recuperation processes. Nevertheless, all study areas (1, 2 and 3), which were analysed in detail, show a positive NDVI trend. This was confirmed by the calculation of some basic statistics such as those shown in table 1. The mean value of the NDVI in all 3 study sites was lower in 2011 compared to that in 2012. The same applies to the variance of the NDVI. At study sites 1 and 3 it is 5 times higher in 2012 than it was in 2011. At study site 2 it is 3 times higher. TABLE 1 STATISTICAL NDVI ANALYSIS OF THE THREE STUDY SITES

Mean Sector 1 Sector 2 Sector 3

Variance

Year

0.236

0.002

2011

0.486

0.011

2012

0.042

0.001

2011

0.383

0.003

2012

-0.039

0.002

2011

0.041

0.010

2012

By fieldwork it was possible to generate the following table 2. As in case of the results already mentioned, potential of photosynthetic activity has increased. This basically relies on effects like wind and rain that cleaned foliage of ash covered trees. Nevertheless, new vegetation that started to grow has to be considered as well. Especially in

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areas where only little ash deposit still remains on the ground, growth of for example Chusquea quila (Desvaux) and perennial herbaceous plants were observed. TABLE 2 RESULT OF FIELDWORK DURING JANUARY, FEBRUARY AND DECEMBER 2012

% of vegetation covered by ash (i)

Sector 1

Sector 2

Sector 3

% of dead vegetation (i)

% of new vegetation (ii)

Presence of bamboo, herbaceous plants and Filicopsida. Buds developed during spring 2012.

Presence of Nothofagus dombeyi Mirb. (Oerst.), Filicopsida, N. pumilio (POEPP & ENDL) KRASSER 1896 and Chusquea quila. Perennial species able to start recovery.

Presence of N. Betuloides (MIRB.) OBERST. Perennial trees affected not only by ash deposit but also by snow fall.

Other significant observations (i or ii)

Vegetation found during fieldwork (ii)

Discussion and Conclusion Although the eruption of Cordรณn Caulle in 2011 did severe damage to Flora and Fauna, a slow recuperation of the vegetation affected by ash deposits can be observed. The decrease in contamination could primarily be related to the cessation of volcanic activity and secondly to natural climatic phenomena such as rainfall and prevailing winds in the study area. Recovery of photosynthetic activity with the affected vegetation started almost immediately after volcanic ash deposits were washed away by rain and wind. A quantitative analysis of NDVI-related statistical measures (table 1) indicates that potential of photosynthetic activity increased in all three sectors. Nevertheless only for sector 2 a significant change in vegetation recovery can be detected. This is mainly due to wind-terrain interaction. As sector 2 is in a valley, strong winds that are typical in this area help to clean ash covered foliage. For sectors 1 and 3 such recovery cannot be detected yet due to external factors like prolonged snow cover during observation period. Furthermore steepness of the terrain is also of great importance. Vegetation is not exposed to strong winds that might help to reduce ash cover. Only rainfall might help to do so. The calculated NDVI values were compared with data collected during field trips and it was possible to prove that ash fall in vegetation caused dryness of the leaves in evergreen trees. This ultimately led to a decrease in the potential of photosynthetic activity, lower NDVI values. Several reports published by CONAF allowed us to double check our results and to identify the most affected vegetation during and after the eruption event. That sector 3 showed that low recovery of photosynthetic activity has two causes. First, heavy snow fall was followed by prolonged snow cover and secondly, the ash plume moved steadily to the southeast for a period of 4 months. Whether potential for photosynthetic activity after a volcanic eruption and continuous ash deposit can be recovered or not, it depends on several factors such as climate, geomorphology and plant characteristics. Therefore not only magnitude of volcanic eruption and ash deposit has to be analyzed, but also fieldwork has to be realized in order to understand study site properties. This study shows that it is critical to assess the effects of volcanic eruptions on the ecosystem by modern techniques such as Remote Sensing. Not only because it allows quantification of damage on vegetation in a wide study area after a volcanic eruption has happened, but also because it allows long-term monitoring of regeneration

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processes. In particular, for a country like Chile that confronts natural disasters quite frequently, it is crucial to develop long-term strategies in order to be prepared and to diminish possible damages. ACKNOWLEDGEMENT

The authors would like to thank CONAF and SERNAGEOMIN for having facilitated rich information on the volcanic complex Puyehue - Cordón Caulle and vegetation affected by the 2011 eruption. REFERENCES

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