The potential of peatlands in global climate change mitigation: a case study of Terceira and Flores Islands (Azores, Portugal) hydrologic services

Given the high precipitation verified in the Azores, peatlands, as water retention structures, are promoters of landscape balance, reducing the frequency of major events such as landslides. However, these ecosystems face increased land-use change disturbances that are placing the future security of these critical ecosystem services in doubt. This study aims to estimate the current and potential hydrologic services, particularly, water storage and water efflux, provided by the peatlands of Flores and Terceira Islands. Peatlands were initially identified through the distribution of Sphagnum, obtained by a spectral signature from satellite image analysis (Sentinel 2, Rapideye), completed with photointerpretation and historical data. The results show actual distributions of natural peatlands of 2766 and 2414 ha, for Terceira and Flores, respectively, which are relatively lower than the potential area estimated as 8035 and 5231 ha, correspondingly (20 and 37% of the total island area). Nowadays, these peatlands can retain 412,280,135 m3 of water. The higher altitude and more pristine areas represent the largest hydrological reservoirs and natural sources of water services. Theoretically, if all peatlands were natural, this capacity would increase to 300% of the retained water. These differences are most strongly reflected in peatland areas that are currently degraded or replaced by anthropogenic vegetation. Natural peatlands also showed greater capacity to restraint gravitational water, working as accumulation structures. Enhancing the naturalness of these areas would increase these ecosystems’ ability to act better as a buffer in extreme climate events. Actual distribution of natural peatlands is 2766 and 2414 ha, for Terceira and Flores, quite lower than the potential area. Currently retain 412,280,135 m3 of water, but this value could increase to 300% with adequate management and restoration, considering peatlands' potential area. This study follows the goals of the United Nations Sustainable Development as well as the 2030 Agenda in terms of climate change mitigation. Enhancing the naturalness would increase these ecosystems’ ability to act better as a buffer in extreme climate events. Restoration of peatlands would increase natural responses to expected climate change, protecting landscape equilibrium and population safety. Actual distribution of natural peatlands is 2766 and 2414 ha, for Terceira and Flores, quite lower than the potential area. Currently retain 412,280,135 m3 of water, but this value could increase to 300% with adequate management and restoration, considering peatlands' potential area. This study follows the goals of the United Nations Sustainable Development as well as the 2030 Agenda in terms of climate change mitigation. Enhancing the naturalness would increase these ecosystems’ ability to act better as a buffer in extreme climate events. Restoration of peatlands would increase natural responses to expected climate change, protecting landscape equilibrium and population safety.


Introduction
Peatlands are among the most valuable ecosystems on Earth since they are critical for preserving global biodiversity, providing several hydrological services [1], minimising flood risk and address to climate change [2][3][4]. Peatlands are the largest natural terrestrial carbon store, and the area covered by near-natural peatland worldwide (> 3 million km 2 ) sequesters 0.37 gigatonnes of carbon dioxide (CO 2 ) a year-storing more carbon than all other vegetation types in the world combined [2].
In the Azores archipelago, the development of wet vegetation types is favoured by the climate, which is temperate (with no dry season with a mild summer) with high precipitation and humidity levels. Wetlands are, in fact, an important element of the region's landscape due to particular clime conditions associated with geomorphologic volcanic factors. Cloudiness is more common than in any other Middle Atlantic region, and fog on the islands is a typical process [5]. Additional precipitation from fog interception is significant for many types of vegetation, including cloud forests [6]. Inland wetlands include lakes, ponds, waterlines, wet grasslands, cloud forests and peatlands [6,7]. The more extensive wetland types are peatlands, including numerous natural types as well as several anthropogenic types [7,8]. There are many natural types of peatlands in the Azores [8] and include fens, Sphagnum-dominated as well as forested. The most extensive type of natural peatlands is forestdominated [8]. A considerable area is still occupied by Sphagnum-dominated types [9]. Indeed, a considerable actual area of Sphagnum peatlands resulted from progressive degradation of forested types of peatlands [10]. Basin bogs are probably the only natural Sphagnum-dominated formations. Studies of Connor et al. published in 2012 [11] for Flores and Pico point out that Sphagnum formations (bogs) were much less frequent before human settlement in these islands. Sjörs [12] also refers that nearly all sloping peatlands (in Eurasia and North America) were formed by paludification in the post-glacial period and were once covered by woodland or in some cases grasslands.
The main threat faced by the peatlands in the Azores is their use as pasture for livestock [10], leading to the degradation of the peatlands. Different frequencies and intensities of cattle presence have created several 'types' of anthropogenic peatlands, corresponding to different succession stages [13,14]. There is a potential distribution of 35,000 ha of peatlands [14,15]. Less than 30% currently persists, and of these, more than 50% are under pressure [16]. The peatlands of Azores have numerous ecological [8], hydrological [16] and biochemical functions as well as social values. Of the various regulatory functions, the ones that stand out in this study are hydrological services, which are characterised by water retention structures that release water gradually (controlling peatlands efflux for surrounding areas after precipitation events) thus acting as buffers, minimising the effects of climate events, promoting landscape equilibrium and minimising the impact of extreme events, such as landslides or floods and supporting biodiversity [16]. However, due to peatland depletion and degradation, their natural functions have become narrowed. Water resource management is a global critical issue, but assumes particular relevance in small islands, as usually water resources are limited and demand is often high, thus planning is essential for water appropriation, as well as for resource conservation and protection [17]. In this study, we assume that water retention capacity is the current total water storage, which includes gravitational and interstitial water storage. The values are in a volume of water by volume of peat (or by area, considering Sphagnum peatland average depth; (m 3 (water).m −3 (or −2 ) (peat)) [14]. Studies on the determination of water volume in peatlands are scarce, (e.g. [18]), although comparative data relating to hydrological services before and after restoration are more widely studied by authors like McCarter and Price [19] and Price et al. [20]. In this study, time efflux was also estimated, assuming that it is the gravitational water temporal holding capacity (h.100 m −1 ) [14]. Information regarding this indicator is very scarce. In the Azores, these hydrological services were quantified initially for Terceira [16] and are now presented for two Azorean islands.
The need for researching this issue has been highlighted by the Food and Agriculture Organization of the United Nations [21] and the United Nations Sustainable Development Goal (SDG) number 13 (in https:// sdgs. un. org/ goals/ goal13) defined as "take urgent action to combat climate change and its impacts" as well as the goals of the 2030 Agenda (in https:// sdgs. un. org/ 2030a genda). It is currently urgent to monitor the naturalness of peatlands, as well as the respective services provided, remote sensing being the most promising method [21] which, due to the concern of the scientific community regarding this issue, has lately received much attention and resulting in the creation of advanced methodologies [22][23][24][25][26]. Plant Functional Diversity is also an important parameter to be assessed in peatlands that has consequences for services provided by ecosystems [27]. This study follows the aims of creating solutions based on remote sensing for the SDGs, a valuable resource as pointed out by PlanetLabs (https:// www. planet. com/ marke ts/ susta inabi lity/) and the European Space Agency (http:// www. esa. int/ Enabl ing_ Suppo rt/ Prepa ring_ for_ the_ Future/ Space_ for_ Earth/ ESA_ and_ the_ Susta inable_ Devel opment_ Goals). One of the main future challenges is the establishment of strategies to minimise the impacts of extreme events due to climate change, and the Azores Islands are no exception. Studies of this nature are of growing relevance due to the increasing manifestations of climate change events [28][29][30][31]. Peatlands are defined by the unique climatic conditions of each location, and their state of naturalness both conditions [32] and is conditioned by [30,33] the nature of climate change in a given region. Peatlands can be the key to limiting climate change.
In this context, the goal of this work is: (1) to calculate current volumetric water retention capacity of peatlands from the current distribution of peatlands in Flores and Terceira Islands and compare with equal parameters estimation, considering the potential distribution of peatlands in those islands; (2) calculate water time efflux, comparing natural and degraded peatlands. These two parameters, water storage and time efflux can be related to other services like erosion protection (by regulating the efflux of the water) and local scale climate change mitigation (by retaining water) [10,14,16].
This manuscript is organised as follows: in Sect. 1 we describe the current knowledge of the Azorean peatlands, from the typologies, main threats and services such as water retention capacity and time efflux, analysed in this study and the importance of peatlands hydrological services in a context of climate change; Sect. 2, corresponds to the methodological description and is divided in three parts, it begins with an explanation of how the actual and potential peatland areas were obtained, on which an analysis of the services considered in this research was carried out; Our results (Sect. 3) are presented, also in 3 parts, quantifying the actual peatland areas, as well as the potential areas and calculating the hydrological services (water retention and water efflux) in both scenarios, presenting the data in cartographic and numerical form; The following section (number 4) is the discussion, where the results obtained in this study are compared with other realities, showing an identical range of values, including also the comparison of the actual and potential hydrological services of Flores and Terceira Islands; The final section, the conclusion, points out how a possible peatland recovery could mitigate potential effects of climate change in the region.

Study area
The Azores archipelago is located in the North Atlantic Ocean, between latitudes 36° 55′ N and 37° 31′ N and longitudes 25° 00′ W and 31° 16′ W, and is about 1360 km distant from the Portuguese mainland (Fig. 1). The archipelago comprises nine volcanic islands with an area of 2322 km 2 . The annual average temperature of the archipelago varies between 14 and 18 °C in coastal areas and between 6 and 12 °C in higher altitude areas, and precipitation increases from the east (e.g. Santa Maria) to west (e.g. Flores). According to the "Atlas Climático dos Arquipélagos das Canárias, da Madeira e dos Açores" [34], using Köppen classification, the Azores' climate is mainly Temperate (temperate without dry season with summer temperate). Much of the original vegetation of the Azores was dense evergreen forest (including Laurissilva, Tertiary remnants of European forests), heathlands in naturally disturbed habitats and peatlands on high plateaus [6]. In continental Europe, the landscape is the result of a long interaction process between men and the environment. Such interaction has occurred only rather recently in the Azores, which were uninhabited until the middle of the fifteenth century AD [35].
The islands selected for this study are Flores (142 km 2 , 914 m a.s.l max. at Morro Alto) in the western group and Terceira (402 km 2 , 1023 m a.s.l at Santa Barbara Mountain), in the central group (Figs. 1 and 2), as these have the largest areas of peatlands [36]. The soil of the peatland study areas is classified predominantly as Histosols, giving way at its margins to andosols with placic horizons. Andosols are modern soils with high organic matter content, developed from volcanic pyroclastic material in a wet temperate Atlantic climate [37,38]. The presence of a placic horizon (Bsm horizon) characterised by the accumulation of iron and magnesium, also known as an 'iron pan' or 'iron band' , is an important ecological factor because it restricts soil drainage, increasing sub-superficial water quantity. The placic horizon is, in Terceira, continuous above 500 m a.s.l [37], and, as far as we know, no information exists for Flores.
In the study islands, precipitation (including horizontal precipitation) varies between 10,690 mm/year in Terceira and 17 708 mm/year in Flores, with humidity of 100%  Table 1). The average annual temperature is 18.2 °C in Terceira, and in Flores, it is slightly lower; however, the wind velocity has an opposite trend, with higher values in Flores Island (Table 1). In both islands, the lowest altitude peatlands are located at an altitude range of 400-500 m a.s.l. The most extensive type of natural peatlands in both islands is forested, dominated by the shrub/tree Juniperus brevifolia [6,8,9]. Considering degraded types, Sphagnum landscapes prevail, mainly in Terceira [9, 10].

Actual peatland distribution
For the establishment of Sphagnum distribution, two sources were used: Sentinel-2 images (2018-12-07 for Terceira and 2019-01-22 for Flores) and Rapideye (2017-12-21 for Terceira and 2012-04-16 for Flores), with 2800 Terceira and 7500 Flores ground-truths. The modelling was done in ArcGIS 10.6.1 with a supervised classification with maximum likelihood using 25 and 50% Reject Fraction (RF). For ground-truthing, ecological fieldwork was done. For the cartographic definition of typologies with low coverage of Sphagnum or even in its absence other sources of information were integrated: ATLANTIDA©GEVA database [6,8,10] and photointerpretation of Planet Scope Images.
Variables for the definition of the potential distribution were various and generated in a raster of 100 × 100 m. Each sequential variable act as an overlay layer that cuts the previous one. The variables were E and W aspect orientation, Slope (> 9º), Curvature (< −0.55) and TOPEX [40]. Considering our field knowledge as well as actual peatlands distribution, Sphagnum peatlands distribution are more oriented to W and forested peatlands are more oriented to E aspect orientation; secondly slope was included in this analysis, considering that when below < 9º gave potential distribution of hillside peatlands (includes blanket  , when slope is above > 9º, gave potential distribution of forested peatlands. The application of curvature and TOPEX allowed the identification of severely exposed to wind (one of the TOPEX Classes) areas, limitation of the model and to the presence of peatlands, according to our field knowledge. After the application of these general landscape variables, endorheic basins were generated in GIS, identifying the potential areas of Basin Sphagnum peatlands, here designated as Natural Mixed Sphagnum. The last variable used was geomorphology used as a possible limitation factor of peatland distribution. Here, recent lava fields were excluded as potential area of peatlands as it highly permeable geological material. The combination of these thematic layers, of exclusive character, allowed to obtain the potential peatland map for the two islands considered in this study.

Assessment of hydrological services
We highlight that Sphagnum and forested peatland water services values were taken from a previous study on Terceira Island [16] and applied to the extension of the peatland's typology. For the actual and potential quantification of hydrological services of water retention and time efflux, reference values were established. To define these reference values (considering peatland type and naturalness degree), we studied eight representative peatlands in Terceira Island [16,26,41]. These were surveyed with Ground Penetrating Radar for the tri-dimensional modelling of peatlands' internal deep and layer structures. Field coring was done with peat collected layer by layer for bulk density, water retention capacity and water efflux determination. The obtained reference values were estimated by Pereira [16] and applied to the different typologies defined in actual and potential peatlands, for the two islands under analysis, and the services were quantified.

Actual distribution of peatlands
The Sphagnum distribution (Fig. 3)

Potential distribution of peatlands
The potential distribution of the peatlands (assumes all formations as natural) is expressed in Fig. 5 and

Hydrological services
Considering the study of the eight reference peatlands, the results showed that hydrological services, such as water retention and water efflux, varied with peat type. At the same time, peatlands with lower naturalness degrees reveal different water retention curves along with the depth profile of the peat from the natural ones (Fig. 6). However, we found a more relevant relation of these services associated with peat depth (Figs. 7 and 8). In terms of disturbances, it was possible to establish a tendency in which more degraded peatlands tend to have lower peat depth, diminishing peatland water services provided.
Considering the values obtained for the hydrological services of all types of peatlands (Table 3), mixed peatlands show a greater capacity to contain gravitational water, being more important for buffering torrential regimes after large rainfall events. The same type also reveals the The results show an actual distribution of natural peatlands of 2766 ha and 2414 ha, for Terceira and Flores, respectively, which is quite lower than the potential area estimated as 8079 ha and 5268 ha, correspondingly. These peatlands currently can retain 412,280,135 m 3 of water (Table 2). Theoretically, if all peatlands were in a natural state, this capacity would increase to 300% of the retained water.
The tridimensional spatial representation (software: ArcScene, with extrusion of the values for each raster cell of 100 × 100 m) of the hydrological services of the two study islands allows the identification of the most relevant areas for hydrological services, currently and in potential terms as well as the major differences in these two situations. The higher altitude and more pristine areas are the largest reservoirs and natural sources of water services on these islands in the present day. On both islands, the current predominant retention value is 500 m 3 /100 m 2 peat (Figs. 9 and 10). However, in the potential analysis scenario, the overall holding capacity presents much higher values, with a significant increase of water retention values and change in the location of more relevant areas for middle altitude with hillside and mixed Sphagnum peatlands. These differences are reflected most strongly in peatland areas currently degraded or replaced by anthropogenic vegetation typologies.
In both islands, the prevailing peatland efflux value in the landscape is currently 7.8 h/100 m (Figs. 11 and 12). Potential values increase this water movement to values between 9 and 67 h/100 m. In addition, some areas of the landscape that currently have no significant peatland water efflux would have this capacity, if we considered the potential areas of peatland distribution.

Discussion
The highlands of most islands in the Azores are dominated by wetlands. This study mapped the distribution of natural peatlands of 2766 and 2414 ha for Terceira and Flores, respectively, which can currently retain 412,280,135 m 3 of water. The actual area of peatlands corresponds to the remnants of about five centuries of land transformation, which have been subject to human action and transformed into agricultural areas. Indeed, the potential area estimated in these study points to 8079 ha and 5268 ha Fig. 6 Trends of gravitational and interstitial water retention at the midpoint of each layer, considering the maximum depth of each kind of Sphagnum peatland (I-degraded, II-natural). Interstitial water is associated with retention capacity; gravitational water is associated with efflux delay Fig. 7 Relation between Sphagnum peatland depth and water retention capacity. Peat depth obtained by GPR and modulated in GIS. Water retention capacity values obtained in the lab as described by Pereira (2015) [16] Fig. 8 Relation between Sphagnum peatland's depth and water efflux. Peat depth obtained by GPR and modulated in GIS. Time efflux values obtained in the lab as described by Pereira (2015) [16] for Terceira and Flores, respectively. If all peatlands were in a natural state, this capacity to retain would increase by 300%. In both islands, the peatland efflux value is currently 7.8 h/100 m, which could increase to 67 h/100 m. The peatlands of the Flores Island are less disturbed [8,14], which is why the ratio between potential and actual water retention as well as efflux values are lower.
The study results showed that hydrological services of water retention follow the trend of those presented by [42] mentioning for USA values between 80 and 90% of water by volume of peat. Similar values are found in other northern hemisphere peatlands [32,43,44]. Campos [14,   Although the water retention values are globally identical, considering average values, it is recognised that, even intrinsically, water volume changes within a peatland according to the nature of its peat [42] or between peatlands according to their vegetation, typology or degree of naturalness [16]. Our study achieved volume values of as diverse as 167% for natural Sphagnum dominated peatlands (much higher than the mentioned studies), 79% for Sphagnum degraded peatlands, 23% for natural forested and 5% for peat soil pasture. These data point to a decrease of relevance in this service with the increase of disturbance.
In both islands, the peatland efflux class value is 7.8 h/100 m, which could increase to a maximum value of 67 h/100 m, considering the potential area. Water efflux services are related to the porosity of the peat, with the upper, newer, more fibrous layers having a higher porosity than the lower, more compact layers [42,46,47]. The reduction of the peatland area has been decreasing the efflux, since the water retention time in the peat has been decreasing and there has been an increase in the torrential regime of the islands' creeks with serious erosion problems and with frequent flooding of houses built on their margins.
Increases in global temperature, sea-level rise, ocean acidification and other climate change impacts are seriously affecting coastal areas and low-lying coastal countries as well as small islands such as the Azores [36]. The survival of many societies and the biological support systems of the planet are at risk. The Regional Programme for Climate Change in the Azores (PRAC [48]), presented several possible future scenarios for Azores' climate. Regarding precipitation, the projections show a slight upward trend in winter, which may reach 10% and a decrease in summer. Extreme wind and storm events can occur with higher frequency and intensity. Floods accompanied by landslides are quite frequent in the Azores. One of the   [50], the intrinsic physical characteristics of the 727 hydrographic basins of the Azores, generally torrential, of small size and steep slope and characterised by a reduced concentration-time, are aspects that contribute to aggravate the hazard of the events. This could become more common and intense, associated with climate change [48]). As shown in this study, peatlands are extremely important landscape regulators, which are far below their potential capacity due to high disturbance. The potential restoration of peatlands or their status amelioration represents an average increase value of 300% in water retention that could theoretically be achieved. Additionally, we must highlight that these two islands are the ones characterised by holding larger areas of peatlands and those in a better state of conservation. This is the case of the pasture-dominated mosaic between Serra Santa Barbara and Pico Alto (Terceira), which has an enormous potential increase of 1 418% in water retention. The pristine north-central plateau of Flores shows a potential increase of 132%, contrasting with the 432% potential increase of the southern part of the island, which is more disturbed. Another environmental imbalance is the irregularity in the flow of Azorean waterlines, causing serious ecological problems, besides affecting the human quality of life. In our study areas, there is a clear relation between the existence of headwaters and the presence of areas dominated by peatlands (Fig. 4). In their natural state, peatlands regulate water flows and help minimise the risk of flooding. The application of differences between actual and potential water services to the Ribeira Grande waterline catchment area (in Flores), showed a potential increase of 313% in retained water.
Urgent action is required to protect and sustainably manage and restore peatlands for global biodiversity protection, which can also play an important role in reducing GHG emissions. This involves protection from degrading activities such as agricultural conversion and restoration of waterlogged conditions required for peat formation, to prevent the release of carbon stored in peat soil and restore hydrological services.

Conclusion
There is more than 5100 ha of natural peatlands on Terceira and Flores. These regulate water infiltration through gradual and continuous transfer to lower groundwater levels. Control surface and subsurface runoff, reducing the torrential regime after heavy rains, diminish soil erosion and regulate the microclimate on islands through evapotranspiration. The stored and gradually released water mitigates the summer effects by being available to recharge aquifers and springs. Peatlands also stabilize the soil and retain large amounts of water, buffering the torrential effect resulting from storms, then releasing the water gradually, minimising downstream torrential effects. Peatland vegetation also delays surface water runoff after major storms, characteristic of the Azorean archipelago, reducing soil erosion. However, disturbance diminishes their intervention in the hydrological cycle control of the landscape. Considering that more than 8000 ha of these islands are degraded, the negative impact of disturbance is quite visible. Nowadays, peatlands can retain 15,412,280 135 m 3 of water, but theoretically, this capacity could increase to 300%. This gradual loss of peatland areas may be associated with a loss of capacity of the subtract to retain water and thus promote more frequent occurrences of extreme events such as landslides, in a region where sometimes the rainfall is very high (the highest value recorded is 276 mm for Furnas S. Miguel in 1974) [51].
The implementation of restoration measures would significantly increase the buffering capacities of peatlands in a scenario of climate change.
Remote Sensing of the peatlands, using satellite and GPR data can be a Sustainable Development Solution to the United Nations Sustainable Development Goal number 13, to detect changes in the naturalness of the peatlands, minimising the risks of degradation (landslides, floods, changes in the regime of streams to torrential), as well as promote knowledge regarding the services provided by them. Urgent action is required to protect, sustainably manage and restore peatlands. This involves protecting them from degrading activities such as agricultural conversion and drainage, as well as restoring the waterlogged conditions. It is essential that future land use and planning of peatland incorporate the principles and practices of wise use to promote sustainable management, especially concerning biodiversity, carbon and hydrology.