Assessing water stress of desert vegetation using remote sensing : the case of the Tamarugo forest in the Atacama Desert (Northern Chile)

Water stress assessment of natural vegetation plays a key role in water management of desert ecosystems. It allows scientists and managers to relate water extraction rates to changes in vegetation water condition, and consequently to define safe water extraction rates for maintaining a healthy ecosystem. Previous research has shown that optical remote sensing constitutes a powerful tool for assessing vegetation water stress due to its capability of quantitatively estimating important parameters of vegetation such as leaf area index (LAI), green canopy fraction (GCF), and canopy water content (CWC). However, the estimation of these parameters using remote sensing can be challenging in the case of desert vegetation. Desert plants have to cope with high solar irradiation and limited water. In order to maintain an adequate water balance and to avoid photoinhibition, desert plants have evolved different adaptations. A common one is heliotropism or ‘solar tracking’, an ability of many desert species to move their leaves to avoid facing direct high solar irradiation levels during the day and season. This adaptation (paraheliotropism) can have an important effect on the canopy spectral reflectance measured by satellites as well as on vegetation indices such as the normalized difference vegetation index (NDVI). In this thesis, I propose a remote sensing based approach to assess water stress of desert vegetation, exemplified in the case of the Tamarugo (Prosopis tamarugo Phil) tree in the Atacama Desert (Northern Chile), a ‘solar tracker’ species, which is threatened by groundwater overexploitation. In the first chapter of this thesis (general introduction), I explained the motivation of the PhD project and elaborated four research questions, which are later discussed in chapters 2, 3, 4, and 5. The thesis concluded with chapter 6, where I provide a synthesis of the main results, general conclusions and a final reflection and outlook. In the second chapter, I studied the effects of water stress on Tamarugo plants under laboratory conditions and modelled the light-canopy interaction using the Soil-Leaf-Canopy radiative transfer model. I described for the first time pulvinar movement of Tamarugo and quantified its effects on canopy spectral reflectance with and without stress. I showed that different spectral indices have potential to assess water stress of Tamarugo by means of LAI and CWC. In the third chapter, I measured the effects of pulvinar movements on canopy reflectance for Tamarugos under field conditions and used high spatial resolution images to assess water stress at the tree level. I developed an automated process to first identify single trees and delineate their crowns, and secondly, to estimate LAI and GCF using spectral vegetation indices. These indices (NDVI and chlorophyll red-edge index) were negatively correlated to diurnal values of solar irradiation as a consequence of leaf pulvinar movements. For this reason, higher values of both vegetation indices are expected to occur in the morning and in winter (low solar radiation) than at midday or summer. In the fourth chapter I studied the effects of diurnal pulvinar movements on NDVI time series from the MODIS-Terra satellite (acquired in the morning) and the MODIS-Aqua satellite (acquired at midday) for the period 2003-2012 and the seasonal effects of pulvinar movements on NDVI time series of Landsat images for the period 1998-2012 for Tamarugo areas with and without water stress. NDVI values measured by MODIS-Terra (morning) were higher than the NDVI values measured by MODIS-Aqua (afternoon) and the difference between the two, the ΔNDVImo-mi, showed good potential as water stress indicator. In a similar way, I observed a strong seasonal effect on the Landsat NDVI signal, attributed to pulvinar movements, and the difference between winter and summer, the ΔNDVIW-S, also showed good potential for detecting and quantifying water stress. The ΔNDVImo-mi, the ΔNDVIW-S and the NDVI itself measured systematically in winter time (NDVIW) were negatively correlated with in situ groundwater depth measurements. In chapter five I used a dense NDVI time series of Landsat images for the period 1989-2013, combined with high spatial resolution satellite imagery and hydrogeological records, to provide a quantitative assessment of the water status of Tamarugo vegetation after 50 years of increasing groundwater extraction. The results showed that the NDVIW and ΔNDVIW-S of the Tamarugo vegetation declined 19% and 51%, respectively, as groundwater depleted (3 meters on average) for the period 1989-2013. Both variables were negatively correlated to groundwater depth both temporally and spatially. About 730.000 Tamarugo trees remained in the study area by 2011, from which 5.2% showed a GCF The main conclusions of this PhD thesis are summarized as follows: Heliotropism or leaf ‘solar tracking’, a common adaptation among desert plants, has an important impact on canopy spectral reflectance. As shown in the case of the Tamarugo trees, widely used vegetation indices such as the NDVI were negatively correlated to solar irradiation (the stimulus for leaf solar tracking), showing a distinct diurnal and seasonal cycle.An early symptom of water stress in paraheliotropic plants (leaves facing away the sun) is the decline of the amplitude of the diurnal and seasonal NDVI cycles. Thus, remote sensing estimations of this amplitude (e.g. the NDVI difference between winter and summer or the difference between midday and morning) can be used to detect and map early water stress of paraheliotropic vegetation.At the tree level, very high spatial resolution images combined with object based image analysis and in-situ data provided accurate estimations of the water status of small desert vegetation features, such as isolated trees. For monitoring purposes, careful consideration of the time during the day and the season at which the images are taken needs to be taken to avoid misleading interpretations.Time series analysis of historical satellite images combined with very high spatial resolution images and hydrogeological records can provide a quantitative spatio-temporal assessment of the effects of long-term groundwater extraction on desert vegetation

Object-based analysis of 8-bands Worldview2 imagery for assessing health condition of desert trees

High spatial resolution panchromatic and multispectral WorldView2 images were used to assess the health condition of Tamarugo (Prosopis tamarugo Phil.) trees in the hyperarid Atacama desert in Northern Chile. Tamarugo is a very valuable species for biodiversity conservation due to its endemic character and limited distribution range. An object-based analysis of a panchromatic WorldView2 image was carried out to identify single trees and to measure canopy area. The NIR/red-edge ratio index (NIR/Re) and Red-edge Normalized Difference Vegetation Index (ReNDVI) were calculated using the near-infrared band (772-890 nm) and the red-edge band (704-744 nm) of the 8-band multispectral WorldView2 image in order to assess health condition at the tree level. The widely used Normalized Difference Vegetation Index (NDVI) was also calculated as reference. Finally, the results were compared with field data taken over 48 trees. The developed object-based algorithm worked out satisfactory with WorldView2 panchromatic data, especially in defining canopy contours and splitting overlapping trees. Within the study area - Bellavista plantation in Pampa del Tamarugal National Reserve- a total of 90,350 trees were automatically identified with a total canopy area of 5.2 km2. The tree average canopy size was 57.6 m2. Single canopy identification allowed an accurate assignment of average spectral values from vegetation index images to the object trees. This way single values of the three vegetation indices were used to evaluate vegetation condition at tree level. Correlation coefficients between vegetation index values and green canopy percentage (GC%) measured for 48 trees in the field were significant in all cases, with the highest one for the traditional NDVI

Detecting leaf pulvinar movements on NDVI time series of desert trees: A new approach for water stress detection

Heliotropic leaf movement or leaf ‘solar tracking’ occurs for a wide variety of plants, including many desert species and some crops. This has an important effect on the canopy spectral reflectance as measured from satellites. For this reason, monitoring systems based on spectral vegetation indices, such as the normalized difference vegetation index (NDVI), should account for heliotropic movements when evaluating the health condition of such species. In the hyper-arid Atacama Desert, Northern Chile, we studied seasonal and diurnal variations of MODIS and Landsat NDVI time series of plantation stands of the endemic species Prosopis tamarugo Phil., subject to different levels of groundwater depletion. As solar irradiation increased during the day and also during the summer, the paraheliotropic leaves of Tamarugo moved to an erectophile position (parallel to the sun rays) making the NDVI signal to drop. This way, Tamarugo stands with no water stress showed a positive NDVI difference between morning and midday (¿NDVImo-mi) and between winter and summer (¿NDVIW-S). In this paper, we showed that the ¿NDVImo-mi of Tamarugo stands can be detected using MODIS Terra and Aqua images, and the ¿NDVIW-S using Landsat or MODIS Terra images. Because pulvinar movement is triggered by changes in cell turgor, the effects of water stress caused by groundwater depletion can be assessed and monitored using ¿NDVImo-mi and ¿NDVIW-S. For an 11-year time series without rainfall events, Landsat ¿NDVIW-S of Tamarugo stands showed a positive linear relationship with cumulative groundwater depletion. We conclude that both ¿NDVImo-mi and ¿NDVIW-S have potential to detect early water stress of paraheliotropic vegetation

Spatio-temporal assessment of beech growth in relation to climate extremes in Slovenia – An integrated approach using remote sensing and tree-ring data

Climate change is predicted to affect tree growth due to increased frequency and intensity of extreme events such as ice storms, droughts and heatwaves. Yet, there is still a lot of uncertainty on how trees respond to an increase in frequency of extreme events. Use of both ground-based wood increment (i.e. ring width) and remotely sensed data (i.e. vegetation indices) can be used to scale-up ground measurements, where there is a link between the two, but this has only been demonstrated in a few studies. We used tree-ring data together with crown features derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) to assess the effect of extreme climate events on the growth of beech (Fagus sylvatica L.) in Slovenia. We found evidence that years with climate extremes during the growing season (drought, high temperatures) had a lower ring width index (RWI) but we could not find such evidence for the remotely sensed EVI (Enhanced Vegetation Index). However, when assessing specific events where leaf burning or wilting has been reported (e.g. August 2011) we did see large EVI anomalies. This implies that the impact of drought or heatwave events cannot be captured by EVI anomalies until physical damage on the canopy is caused. This also means that upscaling the effect of climate extremes on RWI by using EVI anomalies is not straightforward. An exception is the 2014 ice storm that caused a large decline in both RWI and EVI. Extreme climatic parameters explained just a small part of the variation in both RWI and EVI by, which could indicate an effect of other climate variables (e.g. late frost) or biotic stressors such as insect outbreaks. Furthermore, we found that RWI was lower in the year after a climate extreme occurred in the late summer. Most likely due to the gradual increase in temperature and more frequent drought we found negative trends in RWI and EVI. EVI maps could indicate where beech is sensitive to climate changes and could be used for planning mitigation interventions. Logical next steps should focus on a tree-based understanding of the short -and long-term effects of climate extremes on tree growth and survival, taking into account differential carbon allocation to the crown (EVI) and to wood-based variables. This research highlights the value of an integrated approach for upscaling tree-based knowledge to the forest level

A multi-scale approach to assess the effect of groundwater extraction on Prosopis tamarugo in the Atacama Desert

Groundwater-dependent ecosystems occur in arid and semi-arid areas worldwide and are sensitive to changes in groundwater availability. Prosopis tamarugo Phil, endemic to the Atacama Desert, is threatened by groundwater overexploitation due to mining and urban consumption. The effect of groundwater depletion on two representative sites (low -and high-depletion) was studied using a multi-scale approach, combining remote sensing based estimations of canopy growth and water condition, and tree-ring based analysis of stem growth. On the stand level two NDVI (Normalized Difference Vegetation Index) -derived parameters: NDVI in winter and the difference between NDVI in summer and winter showed significant negative trends in the high-depletion site, indicating drought stress. Radial stem growth of viable P. tamarugo trees was 48% lower in the high-depletion site. At the tree level, the Green Canopy Fraction (GCF) also indicated drought stress since a larger percentage of trees fell within lower GCF classes. Groundwater depletion of 3 m, reaching a groundwater depth of >10 m, increased drought stress, and led to reduced growth in viable trees. Viable trees may be able to adapt to the drop in groundwater levels by increasing root growth, whereas for non-viable trees, the effects might be detrimental. © 2016 Elsevier Ltd. All rights reserved

A tabular pruning rule in tree-based fast nearest neighbor search algorithms

Some fast nearest neighbor search (NNS) algorithms using metric properties have appeared in the last years for reducing computational cost. Depending on the structure used to store the training set, different strategies to speed up the search have been defined. For instance, pruning rules avoid the search of some branches of a tree in a tree-based search algorithm. In this paper, we propose a new and simple pruning rule that can be used in most of the tree-based search algorithms. All the information needed by the rule can be stored in a table (at preprocessing time). Moreover, the rule can be computed in constant time. This approach is evaluated through real and artificial data experiments. In order to test its performance, the rule is compared to and combined with other previously defined rules.Spanish CICIyT for partial support of this work through projects DPI2006-15542-C04-1, TIN2006-14932-C02, GV06/166, the IST Programme of the European Community, under the PASCAL Network of Excellence, IST 2002-506778

Spatiotemporal Peatland Productivity and Climate Relationships Across the Western South American Altiplano

The South American Altiplano is one of the largest semiarid high-altitude plateaus in the world. Within the Altiplano, peatlands known as “bofedales” are important components of regional hydrology and provide key water resources and ecosystem services to Andean communities. Warming temperatures, changes in hydroclimate, and shifting atmospheric circulation patterns all affect peatland dynamics and hydrology. It is therefore urgent to better understand the relationships between climate variability and the spatiotemporal variations in peatland productivity across the Altiplano. Here, we explore climate influences on peatland vegetation using 31 years of Landsat data. We focus specifically on the bofedal network in the western Altiplano, the driest sector of the plateau, and use the satellite-derived Normalized Difference Vegetation Index (NDVI) as an indicator of productivity. We develop temporally and spatially continuous NDVI products at multiple scales in order to evaluate relationships with climate variables over the past three decades. We demonstrate that cumulative precipitation and snow persistence over the prior 2 years are strongly associated with growing season productivity. A step change in peatland productivity between 2013–2015 drives an increasing trend in NDVI and is likely a response to consecutive years of anomalously high snow accumulation and rainfall. Early summer minimum temperatures emerge as a secondary influence on productivity. Understanding large-scale productivity dynamics and characterizing the response of bofedales to climate variability over the last three decades provides a baseline to monitor the responses of Andean peatlands to climate change. © 2021. American Geophysical Union. All Rights Reserved.6 month embargo; first published: 24 May 2021This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

50 years of water extraction in the Pampa del Tamarugal basin: Can Prosopis tamarugo trees survive in the hyper-arid Atacama Desert (Northern Chile)?

Groundwater-dependent ecosystems are threatened worldwide by unsustainable groundwater (GW) extraction. This is the case of the Prosopis tamarugo Phil forest in the hyper-arid Atacama Desert (Northern Chile), one of the most extreme ecosystems on Earth. Despite concerns about the conservation of this ecosystem, little research has been done to quantify the effects of the increasing GW depth (GWD) on the Tamarugo population. Here we provide a spatio-temporal assessment of the water condition of Tamarugo trees and propose GWD thresholds for their conservation. We studied spatio-temporal changes of GWD and the water status of the forest using Landsat images and hydrogeological records (1988–2013). This was complemented with a digital inventory and estimation of the green canopy fraction (GCF) of all trees using fine resolution satellite images. Since Tamarugos are solar trackers, their canopy spectral reflectance changes on a diurnal and seasonal basis. Thus, novel remote sensing drought stress indicators were defined: the mean NDVI in winter (NDVIW) accounting for foliage loss and the NDVI difference between mean winter and summer (¿NDVIW-S) accounting for canopy water loss. NDVIW and ¿NDVIW-S of the Tamarugo forest declined on average 19% and 51%, respectively, while GW depleted on average 3 m over the period 1988–2013. About 730,000 trees were identified in the study area, from which 5.2% showed a GCF 12 m increasingly limited the paraheliotropic leaf movement, leading to dehydration and foliage loss. Tamarugos at 12–16 m GWD suffered moderate drought stress while GWD of 16–20 m implied severe drought stress. We suggest 20 m GWD as a critical threshold for the survival of Tamarugo trees