# Priority areas for nature-based adaptation to climate change in the Alps

Titouan Dubo-Degeilh, Ignacio Palomo, Sandra Lavorel

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Titouan Dubo-Degeilh, Ignacio Palomo, Sandra Lavorel. Priority areas for nature-based adaptation to climate change in the Alps. AMBIO: A Journal of Environment and Society, 2025, 10.1007/s13280-025-02240-7. hal-05379723

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# Priority areas for nature-based adaptation to climate change in the Alps

Authors: Dubo, Titouan; Palomo, Ignacio; Lavorel, Sandra

## Abstract

Nature-based Solutions (NbS) are promising initiatives for climate change adaptation, mitigation and biodiversity conservation. Given the finite human and financial resources for NbS, identifying optimal locations is critical. Here, we identified priority areas for adaptation to drought in the European Alps using the “bright spots” approach to estimate future water deficit and surplus from groundwater and soil moisture. We identified overlaps and mismatches of these priority areas with mitigation and biodiversity conservation opportunities. Our analyses reveal that priority areas for adaptation to drought have limited overlap with regions prioritised for mitigation or biodiversity conservation. We considered potential levers to implement NbS from local socio-ecological characteristics. Notably, over 60% of croplands and wetlands identified as priority areas for adaptation to drought overlap with protected areas. In contrast, over 52% of priority areas for adaptation to drought in forests and grasslands are located in rural regions without protection status, where participatory processes with local communities will be crucial to implement NbS.

## Key-words:

Biodiversity conservation; Carbon sequestration; Drought adaptation; Nature-based Solutions; Spatial prioritisation

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# 1. Introduction

The transition towards a sustainable future is urgent to tackle the climate and biodiversity crises (Pörtner et al. 2022). Nature-based Solutions are increasingly recognised as suitable options for climate change mitigation and adaptation, biodiversity conservation, and the delivery of Nature's Contributions to People (NCP) (Cohen-Shacham et al. 2016; Seddon et al. 2020).

Research on NbS is still in its infancy (Seddon 2022), but these promising actions that protect, restore or sustainably manage ecosystems are already integrated into adaptation policies aimed at addressing the impacts of the climate and biodiversity crises. Notably, the Nature Restoration Law recently passed by the European Union incorporates objectives for NbS implementation (European Commission 2022). This law aims to restore “at least 20% of the EU’s land and sea areas by 2030 and all ecosystems in need of restoration by 2050”, and to ensure “that there is no deterioration in conservation trends or in the status of protected habitats and species” (European Commission 2022).

One of the reasons why the restoration law has been approved is the substantial NbS implementation gap, which is particularly evident in the Alps (Lückge and Simon 2022). Given the finite human and financial resources for NbS implementation, identifying priority areas is required (European Commission 2022), especially as current NbS are not always implemented where they are the most needed (Dubo et al. 2022). Although the European Alps are under multiple legislation from eight countries, they share socio-ecological features recognised by the Alpine Convention, including a common cultural heritage, rivers crossing several countries and high spatial diversity of habitats threatened by climate change (Engler et al. 2011; EEA 2020). Consequently, the prioritisation of adaptation efforts must be reasoned at the entire alpine scale. Previous prioritisation studies at the alpine scale have focused on biodiversity conservation rather than on climate adaptation and mitigation, combining protected areas and NCP supply with biodiversity indices (Ramel et al. 2020) or human pressures (Egarter Vigl et al. 2021).

Climate change will increase the frequency and intensity of drought events whatever the climate change scenario (Kotlarski et al. 2022), exacerbating impacts on biodiversity, and several NCP (Seidl et al. 2017; Egarter Vigl et al. 2021; Stephan et al. 2023). In the Alps, adaptation to drought is a frequent objective of NbS (Dubo et al. 2022), for which NbS have proven to be effective (Brang et al. 2014; Altieri et al. 2015; Pugliese et al. 2020). These encompass a range of interventions, such as agroforestry to mitigate drought impacts on agriculture; silvopastoral
practices to safeguard fodder production during extreme drought and reduce heat stress of live-
stock; natural regeneration to enhance forest resilience to drier conditions; and wetland resto-
ration to improve water connectivity within landscapes that, consequently, reduce the water
scarcity duration and protect biodiversity (Dubo et al. 2022). In these examples, NbS aim to
enhance or maintain the water retention of ecosystems, a crucial NCP for adaptation that is
expected to become increasingly valued, used and demanded in the future (Elliott et al. 2014;
Lavorel et al. 2020). To reach a just and sustainable future, NbS for adaptation to climate change
should consider where communities under larger risks are located (Birkmann et al. 2022). In
risk assessments to date, NCP supply is rarely considered (Hagenlocher et al. 2019). Commonly
used approaches separate ecological and social assessments, often excluding NCP evaluation
(Hagenlocher et al. 2018; Lazzari et al. 2021). Moreover, social vulnerability is assessed using
general economic indicators, such as health expenditure of the local government and net inflow,
that may not fully capture local well-being (Hagenlocher et al. 2019; Shah et al. 2020). Ecolog-
ical vulnerability is generally assessed with biophysical pressure indicators, like the cover of
protected areas or pollution (Hagenlocher et al. 2018; Shah et al. 2020), that can only partially
reflect the full impacts of a specific hazard on NCP. Given these limitations, practitioner-
friendly approaches that facilitate the identification of priority areas for adaptation and that
include current and future NCP for adaptation are needed (Hagenlocher et al. 2019).

Previous studies identified trade-offs and synergies between biodiversity and climate change mitigation (Jung et al. 2021), climate change adaptation and mitigation (Villarreal-Rosas et al. 2023), or NCP and biodiversity (Felix et al. 2022). However, no previous study has integrated biodiversity conservation, climate change adaptation and mitigation and NCP in the prioritisation of NbS. Furthermore, most prioritisation studies do not address the socio-ecological context that influences the implementation of NbS despite its fundamental importance (Crouzat et al. 2015)

In this study, we: (1) identify priority areas for NbS implementation to adapt to droughts in the Alps and characterise their NCP; (2) evaluate the mismatches and overlaps of these areas for climate change mitigation and biodiversity conservation; and (3) analyse the socio-ecological contexts of these areas that relate to the future implementation of NbS.
## 2. Material & Methods:

### 2.1. Theoretical Framework

This study is framed by the Social-Ecological System Framework (Ostrom 2009), implemented to analyse NbS for addressing drought risk. We followed the definition of “risk” by IPCC as “the potential for adverse consequences for human or ecological systems”, resulting from a combination of (i) hazards, i.e. “the potential occurrence of a natural or human-induced physical event or trend” that may cause damages, such as droughts; (ii) exposure, i.e. “people, livelihoods, environmental services and resources, infrastructure, or economic, social, or cultural assets in places that could be adversely affected by physical event” including ecosystem services; and (iii) vulnerability, i.e. “the propensity or predisposition” of the exposed element to be adversely affected by a physical event (Lavell et al. 2012). Despite the interdisciplinary nature of the risk assessment literature, only a few studies adapted this framework to socio-ecological systems, including the Global Delta Risk Index (Hagenlocher et al. 2018) and the Vulnerability and Risk Assessment of Social-ecological Systems in NbS (Shah et al. 2020).

Many spatial prioritisation approaches focus on hotspots of a target variable to inform protection priorities, such as biodiversity richness, sequestrated carbon or NCP (Cimatti et al. 2022). In contrast, other spatial approaches identified as priority areas the coldspots of a target variable, assuming those would be the places with the largest potential to increase this value through ecosystem restoration actions (Pan et al. 2023). However, both approaches need to be considered when prioritising such actions as NbS that can enhance or maintain NCP for adaptation. Focusing on droughts, land-use and land-cover (LULC), the supply, flow and the demand of NCP as well as the ecological and social context need to be considered in the implementation of NbS (Fig. 1) (Shah et al. 2020). Drought hazard is often related to soil moisture, i.e. the volume of water within the unsaturated zone of the topsoil. Drought is commonly declared when soil moisture decreases under a threshold (Dilawar et al. 2022). The adaptation options to address drought include watering, e.g. with irrigation systems, and increasing soil water retention, e.g. through agroecological practices (Altieri et al. 2015). Here, we defined NCP for adaptation to drought as the available volume of water contained in both topsoil and aquifers. These volumes of water are connected to rivers from which the water flows (Haas and Birk 2017). The volume of water contained in topsoil (hereafter “soil moisture”) and in aquifers (i.e. the percolating water through the unsaturated zone, hereafter “groundwater”) are positively
correlated through their shared dependence on precipitation, canopy interception, snow accumulation and melting, infiltration, evapotranspiration and runoff generation (Haas and Birk 2017; Samaniego et al. 2018).

However, groundwater and soil moisture may respond differently to drought. Therefore, within areas experiencing the most severe drought, we applied the “bright spots” approach (Cinner et al. 2016) to identify places where groundwater volume is higher (bright spots) or lower (dark spots) than expected compared to soil moisture value, i.e. areas presenting a surplus and a deficit of groundwater, respectively.

<figure><img src="image_4.png" /><figcaption>**Fig. 1:** Conceptual framework for the characterisation of social-ecological systems in the context of Nature-based Solutions spatial prioritisation for adaptation to drought in Alpine landscapes, adapted from Ostrom (2009); Shah et al. (2020).</figcaption></figure>

## 2.1. NbS options for drought adaptation in alpine lands

To determine which NbS options can be implemented for adaptation to drought and where, we identified the most frequently implemented NbS that aim to adapt to drought using the POR-TAL database (Table S1; Dubo et al., 2022). For each of the four main LULC of the database these are agroecological practices, such as agroforestry practices and soil conservation approaches (croplands), silvopastoral practices (grasslands), protection or restoration (wetlands) and sustainable management which encompasses the natural regeneration and the promotion of mixed forests (forests). We extracted the areas corresponding to these four selected LULC from
the CORINE (Coordination of Information on the Environment) Land-Cover dataset (Copernicus Land Monitoring Service 2021; Table S1).

## 2.2. Identification of priority areas

Our prioritisation approach followed six methodological steps presented in Fig. 2. The three first steps describe the process for calculating the priority index. The next steps analyse the location of identified priority areas for the four LULC considered here, namely croplands, grasslands, forests and wetlands (fourth step), the potential co-benefits of implementing NbS in priority areas for carbon sequestration and biodiversity conservation (fifth step), and the landscapes under which priority areas are embedded (sixth step). Each step is detailed in the Appendix 1, and the datasets detailed in the Table S1. All data analyses and plots were performed with R (version 4.1.0; R Core Team, 2024) and QGIS (version 3.16.5; QGIS Development Team, 2022).

Given a drought event, with a decrease in soil moisture and groundwater, the impacts are different across regions, reflecting site-specific adapted ecosystems across the range of climates in the Alps (Stephan et al. 2021). The “bright spot” approach enables to identify areas where values deviate the most from what is statistically expected from explanatory variables. This approach, previously applied in conservation science (Cinner et al. 2016) and to assess landscape multifunctionality (Frei et al. 2018), is suited to regional analysis across climates and ecosystems. We built this analysis on the strong linear correlation which we found between soil moisture and groundwater volume.

We downloaded the seasonal groundwater and soil moisture raster data for the reference period (average of historical value from 1970 to 2000, hereafter referred to as 2000) and the future period (average of predicted values under the highest emissions scenario, RCP 8.5, from 2085 and 2115, hereafter referred to as 2100), assessed at the pan-European scale by the EDgE project (End-to-end Demonstrator for improved decision-making in the water sector in Europe) using a high-resolution multimodel ensemble (Samaniego et al. 2019; Table S1). Then, we computed the median values of the rasters at the lowest watershed level from the HydroSHEDS database, which is adequate for large scale hydrological analysis (Villarreal-Rosas et al. 2023; Table S1).
We computed one statistical linear model per season using soil moisture as an explanatory variable and the groundwater as a response variable (absolute value or log-transformed). We selected the most significant model, i.e. the one using log-transformed groundwater values from the summer season (r2 = 0.7; p-value < 0.001; Fig. S2), from which we calculated the deviation between the response and the fitted values, hereafter referred to as groundwater anomalies (Fig. 2, step 1, Eq. 1).

$$ \text{Eq. 1} \qquad d_i = gw_i - f(sm_i) $$

with $d_i$ the groundwater anomaly resulting from the deviation value in the watershed $i$, $gw_i$ the future groundwater value in the watershed $i$; $sm_i$ the future soil moisture value in the watershed $i$, and $f(sm_i)$ the fitted groundwater value in the watershed $i$ with the selected linear model. A positive groundwater anomaly ($d_i > 0$) indicates a higher future groundwater value than expected from future soil moisture with the model. In contrast, a negative groundwater anomaly ($d_i < 0$) indicates a lower future groundwater value than expected.

We standardized the groundwater anomalies using the min-max normalisation formula (Eq. 2), and we computed the priority index by multiplying the standardised groundwater anomalies to the standardised future drought intensity, derived from the soil moisture relative change from the reference and the future period (Fig. 2, step 2; Eq. 3).

$$ \text{Eq. 2} \qquad un_i = \frac{max-u_i}{max-min} $$

with $u_i$ the value of the watershed $i$, max and min the maximum and the minimum value across all the watershed, and $un_i$ the normalised value of the watershed $i$.

$$ \text{Eq. 3} \qquad P_i = DDn_i * dn_i $$

with $P_i$ the rescaled priority index of the watershed $i$; $DDn_i$ the normalised and rescaled relative change of drought duration index; and $dn_i$ the normalised and rescaled groundwater anomaly of the watershed $i$.
We further defined exclusion criteria reflecting the biophysical feasibility of NbS implementa-
tion, based on grey and academic literature (Fig. 2, step 3). For silvopastoral practices and sus-
tainable forest management, we excluded grasslands and forests that exceed the elevation of
the future treeline distribution (Mauri et al. 2022 ; Table S1). Given the difficulty of planting
trees in steep areas, croplands and grasslands located where slopes exceed 35° were excluded
for implementation of agroecological and silvopastoral practices (Table S1). We did not iden-
tify exclusion criteria for wetlands, as we considered they can all be either protected or restored
depending on their conditions (Odgaard et al. 2017). Finally, for each LULC we selected the
10% of the resulting areas with the highest positive priority index, hereafter referred to as pri-
ority areas with water surplus, and the 10% of those with the lowest negative priority index,
hereafter referred to as priority areas with water deficit (Fig. 2, step 4).
<figure><img src="image_5.png" /><figcaption>**Fig. 2:** Methodological steps to identify the priority areas to implement Nature-based Solutions (NbS) to adapt to drought in the Alps. Steps 1 and 2 describe the computation of the priority level based on soil moisture, groundwater and drought intensity. Step 3 describes the filtering step applied for each LULC considering biophysical constraints for NbS implementation. Step 4 describes the selection of priority areas considering the 10% of the highest priority index for both positive and negative values. Step 5 describes the analysis of potential co-benefits within priority areas. Step 6 describes the characterisation of the landscapes surrounding priority areas based on Nature's Contributions to People (NCP) values.</figcaption></figure>
## 2.3. Potential co-benefits for climate mitigation and biodiversity

To quantify climate mitigation potential, we quantified the amount of current soil organic carbon (SOC) and the maximum carbon that can be potentially sequestered in these soils across priority areas, within each LULC category (Fig. 2, step 5). These variables were assessed and mapped at the European Union level (Lugato et al. 2014a; Lugato et al. 2014b; Table S1; Appendix 1), considering realistic alternative management practices to evaluate the potential carbon sequestration, including agroecological practices such as straw incorporation and reduced tillage (Lugato et al. 2014b). Because the NbS we considered here do not intend to change the land-cover, we considered the SOC as the main carbon sink for potential carbon sequestration from future NbS implementation. We conducted a first analysis of variance (ANOVA) to detect whether current SOC and its sequestration potential differ between areas with water deficit and those with water surplus.

Regarding biodiversity potential, the European Environment Agency assessed at the European Union level the status of habitats for biodiversity conservation according to habitat range, structure, functions and population dynamics of its typical species. Habitats are categorised under three statuses: “favourable”, “inadequate” and “bad” status for biodiversity conservation (EEA 2020). We quantified the proportion of each status within priority areas, and performed $\chi^2$ tests, for each LULC category, to detect whether the distribution of habitat status differs between areas with water deficit and those with water surplus (Fig. 2, step 5).

Finally, differences in current SOC or potential carbon sequestration for each LULC across the two types of priority areas (i.e. water deficit or surplus) and across conservation status of habitats were tested using ANOVA.

The available datasets at the European scale do not cover Switzerland, and due to the limited number of priority areas found within this country, the co-benefits analyses excluded this country.
## 2.4. Characterisation of socio-ecological contexts surrounding priority areas

We characterised priority areas by analysing, for each LULC, their distribution across elevation and across countries, based on the Worldclim database (Fick and Hijmans 2017; Table S1). Moreover, we assessed the proportion of priority areas within and outside protected areas, considering the IUCN categories of protected areas (UNEP-WCMC 2023; Table S1).

We characterised the socio-ecological systems surrounding priority areas, hereafter “landscapes”, by their values of material, non-material and regulating NCP as indicators of socio-ecological conditions (Takahashi et al. 2022). We extracted the amount of NCP supply, flow and demand assessed at the municipality scale in the Alps by the AlpES project for eight NCP: drinkable freshwater, fodder from grasslands, timber from forests, water filtration, protective function of forests against landslides, carbon sequestration, outdoor recreation, habitat for symbolic biodiversity (Schirpke et al. 2019, Table S1). Then, to characterise the variety of landscapes surrounding priority areas, we performed a clustering analysis for each LULC based on NCP values of municipalities containing the priority areas and extracted the NCP variables that discriminated each cluster. We grouped the clusters from different LULC that share more than half of their discriminating NCP variables and validated the group with the observation of the areas from satellite images. Finally, we compared the proportion of these clusters across priority areas with a water surplus or a water deficit using $\chi^2$ tests (Fig. 2, step 6).

# 3. Results:

## 3.1. Distribution of priority areas across the Alps

Priority areas to implement NbS for adaptation to drought are unevenly distributed along elevation (Fig. S3) and across Alpine countries (Fig. S4). Regions with water deficit are mainly located in lowlands, close to mountainous areas, in the southwestern Alps (in France and Italy) and in the northeastern Alps (in Austria). Regions with water surplus are in two main locations: the western Alps (in France and Switzerland, Fig. S4) and southern hillsides (in Italy, Fig. S4).

Priority areas within croplands and forests are located consistently with the elevation distribution of their respective LULC, i.e. croplands located in lowland valleys, and forests at mid-elevation (Fig. S3). Priority areas within grasslands and wetlands are evenly distributed along elevation, with areas with water deficit at lower elevations than those with water surplus. The number of priority areas within wetlands is limited compared to priority areas in other LULC (1% of all priority areas) and concentrated in two main zones. Those with water surplus are
mostly located in the western Alps, corresponding to the Dombes region (France, Fig. S4), and those with water deficit are mostly located in the eastern Alps, corresponding to the Neusiedl Lake region (Austria, Fig. S4).

## 3.2. Co-benefits of priority areas

### 3.2.1. Climate change mitigation

Among priority areas within croplands, grasslands and wetlands, those with water deficit currently store significantly more SOC than those with water surplus (Fig. 3). This suggests that preserving high amount of groundwater cannot be synergistic with high SOC. However, priority areas with water deficit have the largest potential to increase SOC, highlighting their potential to simultaneously increase SOC and water retention.

Regarding priority areas within forests, those with water surplus currently store more SOC, suggesting a potential synergy in preserving water regulation and SOC. Similarly to priority areas in other LULC, those with water deficit have a significantly greater potential to increase SOC, suggesting their potential to simultaneously increase water regulation and SOC (Fig. 3).

<figure><img src="image_6.png" /><figcaption>**Fig. 3:** Distribution of current (darker colour) and potential (lighter colour) soil carbon content (tC/ha, log-transformed) in priority areas to implement NbS for adaptation to drought, considering areas with water surplus (in blue) and with water deficit (in brown) for four LULC: croplands (A), grasslands (B), forests (C), and wetlands (D). The crosses represent the means of the distributions, and the letters indicate whether the distribution of values is significantly different (ANOVA; p < 0.05), for each LULC. The alphabetical order indicates the increasing sense of the values (a<b<c<d). The distributions between LULC are not compared here. The values "n" below the distributions represent the rounded number of areas for each category.</figcaption></figure>
### 3.2.2. Habitat conservation status for biodiversity

Overall, most of the priority areas for adapting to drought are classified as “inadequate” or “bad” for biodiversity conservation (96%), representing a higher proportion than across the entire Alps (83%) whatever the LULC (Fig. 4). This suggests that wherever NbS are implemented they have a large potential to benefit biodiversity. However, an in-depth analysis within these areas showed limited synergies between priorities for adaptation and for biodiversity conservation. Within croplands, forests and wetlands, priority areas with water deficit are within habitats with a better conservation status for biodiversity than those with water surplus, of which more than half are in habitats with “bad” status. Priority areas within grasslands present the opposite pattern, with a larger proportion of habitats with “bad” status within areas with water surplus compared to those with water deficit, suggesting potential synergy to preserve both water volume and biodiversity.

<figure><img src="image_7.png" /><figcaption>**Fig. 4:** Barplots of the number of priority areas to implement NbS for adaptation to drought depending on the status of the ecosystems for biodiversity conservation, considering areas with water surplus and with water deficit for four LULC: croplands (A), grasslands (B), forests (C), and wetlands (D). The bar at the right side displays the distribution of the status of the ecosystems for biodiversity conservation across the Alpine Space. The stars indicate for each LULC and for each status of ecosystem that the distributions of areas with water surplus and with water deficit are significantly different (χ² test; p < 0.05). The values "n" below the bars represent the rounded number of areas for each category.</figcaption></figure>

### 3.2.3. Co-occurrence of benefits for climate mitigation and biodiversity conservation

Our analyses revealed many mismatches and few overlaps of priority areas for adaptation and the potential co-benefits regarding biodiversity conservation and carbon sequestration. Areas with water surplus do not consistently coincide with large soil carbon stocks (along with a low
potential for increasing sequestration) or with “favourable” status for biodiversity. Areas with water deficit coincide partially with ecosystems with a “bad” status and a substantial potential for increasing SOC, offering optimal locations to benefit from the multiple promises of NbS implementation.

* Associations between current soil organic carbon and biodiversity

For croplands and forests, priority areas with water surplus and with a better status for biodiversity conservation store more SOC than those with lower status (Fig. S5). This suggests potential synergies in protecting biodiversity and preserving SOC from future deterioration in priority areas, especially for forests where those priority areas store higher SOC than priority areas with water deficit, strengthening synergy with drought adaptation as well.

For priority areas within wetlands and grasslands, we found no synergies across objectives, suggesting trade-offs in NbS planning.

* Associations between potential carbon sequestration and biodiversity

For grasslands and forests, among priority areas with water deficit, those with “favourable” status for biodiversity conservation have the least potential for carbon sequestration (Fig. S5), suggesting synergies in restoring areas to benefit simultaneously biodiversity and carbon sequestration. For wetlands, priority areas with water deficit present greater sequestration potential than those with water surplus, strengthening the synergy with drought adaptation as well. No synergies between potential carbon sequestration and other objectives can be identified for priority areas within croplands.
### 3.3. Landscapes comprising priority areas

#### 3.3.1. Bundles of Nature's Contributions to People

<figure><img src="image_8.png" /></figure>

**Fig. 5:** Barplots of the proportions of the clusters identified for the priority areas for adaptation to drought considering areas with water deficit and areas with water surplus for four land-cover: croplands (A), grasslands (B), forests (C), and wetlands (D). The clusters are identified based on the values of the supply, the flow and the demand of eight Nature's Contributions to People assessed and mapped at the alpine scale. The clusters with similar Nature's Contributions to People values were grouped, resulting in six landscapes for which names were given accordingly to their main characteristics. The stars indicate for each LULC and for each cluster that the distributions of areas with water surplus and with water deficit are significantly different (χ² test; p < 0.05). The values "n" below the bars represent the rounded number of areas for each category.

The clustering analysis of NCP supply, demand and flow identified six typical landscapes surrounding priority areas (Fig. 5, Fig. S6).

The *hillsides and elevated natural areas* landscape provides a large amount of non-market NCP, including freshwater from ecosystems, forest protection against landslides, and areas for outdoor recreation and biodiversity (Fig. S6). These areas correspond, among others, to French and Italian National parks (Fig. S4). When comparing the distribution of this landscape across priority areas, we found that this landscape is more frequent in regions with water surplus than those with water deficit, for each related LULC: forests, croplands and grasslands (Fig. 5). This landscape is prevalent among priority areas selected within grasslands (63%) and forests (25%) but represents only 2% of the priority areas selected within croplands.

The *productive lowlands* landscape provides significant marketable production, such as fodder and wood production, and outdoor recreation (Fig. S6). This landscape comprises a quarter of priority areas selected within croplands (25%), whether for areas with water deficit or surplus.
In contrast, for priority areas selected within wetlands, this landscape comprises a quarter of those with water surplus (25%), but a limited part of those with water deficit (2%).

The *populated lowlands* landscape delivers high flow and demand for freshwater and timber. This landscape also supplies significant outdoor recreation opportunities. For croplands, this landscape comprises similar proportion of areas with water deficit (69%) and with water surplus (68%). In contrast, for wetlands, almost all priority areas with water deficit are located within the populated lowlands (98%), although this landscape comprises a significant but lower proportion of priority areas with water surplus (64%) (Fig. 5).

The *rural silvopastoral regions* are characterised by substantial water filtration capacity and timber production, as well as high fodder production, flow and demand (Fig. S6). This landscape comprises most of the priority areas selected within forests (80% and 70% for forests with water deficit and surplus, respectively), located among others in the eastern Alps close to the Neusiedl Lake (Austria, Fig. S4), and in a large western Alps region around Lyon (France, Fig. S4). In contrast, for grasslands, a limited proportion of priority areas (27% and 14% for areas with water deficit and surplus, respectively) belongs to this landscape, including regions such as Augsburg (Germany, Fig. S4) or Lyon (France) surroundings.

The *recreative low mountains* provide high flow and demand for freshwater and outdoor recreation, as well as high wood flow (and its demand) and symbolic biodiversity (Fig. S6). This landscape mostly comprises priority areas selected within grasslands, with similar proportions for areas with water deficit (19%) and those with water surplus (14%). These areas include, among others, grasslands within the Jura massif (France, Fig. S4), and the Ardèche mountains (France, Fig. S4). A limited number of priority areas selected within forests (2%) belongs to this landscape, only those with water surplus.

The last landscape comprises the *few large wetlands* corresponding to 7% of the priority areas with water surplus and discriminating by their significant value for forest protection against landslides and the high supply and demand for water filtration (Fig. S6).

### 3.3.2. Overlaps with protected areas

Overall, 40% of priority areas for adaptation to drought are located within protected areas, which is a higher proportion than the protected areas cover across the Alps (29%, Fig. 6). However, the distributions differ across LULC. Almost all priority areas with water deficit selected within croplands and wetlands are located in protected areas (84% and 94%, respectively), as well as most of those with water surplus, albeit to a slightly lesser extent (60% for those within
croplands and 74% for those within wetlands) (Fig. 6). In contrast, less than half of priority areas selected within grasslands and forests overlap protected areas, for both areas with water deficit (48% and 38%, respectively) and those with water surplus (34% and 31%, respectively, Fig. 6).

Only a few priority areas overlap with protected areas with a high protection level when considering all LULC, although it accounts for 56% of the priority areas with water deficit selected within wetlands (Fig. 6).

<figure><img src="image_9.png" /><figcaption>**Fig. 6:** Barplots of the number of priority areas to implement NbS for adaptation to drought located outside protected areas, within specific IUCN category, or within protected areas without a specific category (Not categorised), considering areas with both water surplus and deficit, for four LULC: croplands (A), grasslands (B), forests (C), and wetlands (D). The bar on the right side displays the same distribution across the Alpine Space. The stars indicate for each LULC and for each IUCN category that the distributions of areas with water surplus and with water deficit are significantly different (χ² test; p < 0.05). The values "n" below the bars represent the rounded number of areas for each category.</figcaption></figure>

# 4. Discussion:

## 4.1. Priority areas for adaptation to drought

This study identified priority areas for adaptation to future drought based on future groundwater and drought intensity. The priority areas with water deficit, which would be suitable for NbS that increase water retention such as wetland restoration or agroforestry practices, are found in similar locations to areas previously identified as being sensitive to drought in the Alps, mostly located in the south-west and at lower elevations (Kotlarski et al. 2022). These locations are already experiencing increased mean annual temperature and a change in precipitation distribution, with increased rainfall during the winter season and a decrease during the summer season. The low groundwater values are not surprising in the Mediterranean region which has a
low infiltration rate (Martinsen et al. 2022). In contrast, priority areas with water surplus, which would be suitable for protection measures or sustainably managed ecosystems, are located in the western part of the Alps (in France) and the southern hillsides (in Italy), and do not correspond to regions highlighted by previous drought assessments (Tramblay et al. 2020). Despite greater drought intensity, these areas benefit from large groundwater accumulation downstream of mountain catchments (Schirpke et al. 2019).

Due to the higher priority index at low elevation, alpine meadows were not in priority areas within grasslands for adaptation to drought. However, crucial fodder production provided in these areas is recognized as being increasingly vulnerable to climate change (Deléglise et al. 2022). The few priority grasslands are located in the Southern Alps, including the Lubéron Regional Park (France, Fig. S4) as one of the areas with highest future drought intensity, and in Lessinia (Italy, Fig. S4), corresponding to areas with high-temperature increase but which benefit from plentiful groundwater downstream of mountain catchments (Kotlarski et al. 2022).

## 4.2. Multiple co-benefits potential of priority areas for NbS

Although NbS are considered as adaptation options to simultaneously address several challenges, a limited number of studies have addressed the spatial overlap between priority areas to maintain or enhance NCP for adaptation, carbon sequestration and biodiversity. We found that potential synergies exist to simultaneously address two of these three objectives, within specific priority areas. Specifically, we found that all LULC show overlap of priority areas for increasing water regulation and SOC, consistent with a previous study on NbS for climate adaptation and mitigation (Villarreal-Rosas et al. 2023). Priority areas within forests also have the potential to maintain high water provision and SOC simultaneously. For biodiversity, most priority areas are within habitats without “favourable” status for biodiversity conservation, offering the opportunity to improve biodiversity conservation alongside with water regulation, particularly for priority areas within grasslands. Consistent with previous spatial assessments, the implementation of NbS should consider these areas to maximise their potential for maintaining or simultaneously enhancing water regulation and habitat biodiversity with a limited number of interventions (Ou et al. 2022). Nevertheless, when considering current water-related NCP (i.e. prior to climate change impacts), mismatches exist between priority areas for maintaining these NCP and those focused on biodiversity conservation (Ramel et al. 2020). Future spatial planning research should, therefore, consider other anthropogenic pressures, especially to prevent areas less sensitive to climate change from future degradation (Egarter Vigl et al. 2021).
We found that synergies to simultaneously address priorities for adaptation to drought, carbon sequestration and biodiversity conservation are limited to some priority areas with water deficit within wetlands and some priority areas with water surplus within forests. These findings contrast with a global assessment that found substantial potential of NbS to jointly conserve carbon, water and threatened species (Jung et al. 2021) at a given place.

Additionally, although it was not the aim of this study, we found substantial SOC in habitats with a “favourable” status for biodiversity conservation, especially within forests and croplands with water surplus. Moreover, all the priority areas with a high potential for carbon sequestration overlap with habitats with a “bad” conservation status, confirming the win-win potential of restoration actions for these two objectives (Jung et al. 2021).

### 4.3. Levers and barriers to implement NbS

The levers and barriers to the implementation of NbS depend on the context and landscape characteristics (Topp et al., 2021, Dubo et al, 2023) and stakeholders’ visions (Lavorel et al. 2019; Locatelli et al. 2022). Protected areas, within which 40% of priority areas for adaptation are located, offer optimal areas to implement NbS, due to their available resources dedicated to conserve biodiversity (Watson et al. 2014). Biodiversity conservation plans of protected areas offer an existing instrument for the implementation of NbS. Furthermore, protected areas offer legal and governance clarity and capacity, that would support the co-design of NbS initiatives through participatory approaches with local stakeholders (Ruangpan et al. 2021). Priority areas within *hillsides and elevated natural areas* may benefit from this influence to implement NbS because most of them overlap protected areas. Priority areas within wetlands with water deficit are mostly located in protected areas, in particular those with high level protection, reflecting the strong representation of wetlands as conservation targets for provisioning biodiversity habitat and regulating NCP (Kingsford et al. 2021). Yet, given differences in human pressures across various levels of protection among protected areas (Egarter Vigl et al. 2021), conservationists and planners need to consider these pressures as criteria for prioritising different NbS.

Grasslands and forests are often considered in management plans of protected areas, due to the value of their habitat for many endemic species (Kingsford et al. 2021). However, within grasslands and forests, we found less than half of priority areas within protected areas. Substantial effort in communication and awareness raising could therefore be needed to successfully implement NbS outside protected areas in places where their acceptance remains limited (Dai et al. 2021). Within *recreative low mountain* areas, mostly represented by priority areas within
grasslands, this effort can be achieved through the existing communication network developed for tourism (Stubelj Ars and Bohanec 2010). However, co-production with local stakeholders is required to implement NbS within *rural silvopastoral regions*, dominated by priority areas within grasslands and forests, because most of the existing adaptation initiatives are reactive and not proactive (van Loon-Steensma and Goldsworthy 2022; Dubo et al. 2023).

Additionally, addressing strong barriers, such as societal values, should also be prioritised because of their influence on the decision-making process (Brondizio et al. 2019). In the identified *productive lowlands*, comprising some of the priority areas within croplands and wetlands with local presence of fodder and wood production, stakeholders may express strong resistance to change due to the inherent uncertainties in changing practices (Nicholls and Altieri 2018). In contrast, priority areas in *populated lowlands* dominated by croplands, offer greater opportunities to implement new uses (e.g. to shift to a recreation-based economy) than the ones in *productive lowlands*, given the demand for environmentally friendly products and recreation from the urban population (Stubelj Ars and Bohanec 2010). The expressed priorities from stakeholders can differ between *productive lowlands* and *recreative low mountains*, strengthening the role of stakeholder engagement in NbS planning to consider and balance local needs (Ruangpan et al. 2021).

These levers can be adapted beyond the Alps, as landscapes we identified can be found in other regions. Conversely, lessons from other regions illustrate how strong ecological stewardship of local communities (Maseyk et al. 2018; Locatelli et al. 2022) and robust financial instruments (Zandersen et al. 2021) can enhance the protection of local resources.

### 4.4. Limitations and perspectives

We applied the “bright spots” approach to determine the priority areas to implement NbS with two biophysical variables, namely groundwater and soil moisture. In contrast with previous risk assessments, we did not include social data in our assessment, such as income, age and gender because of the limited representativeness of the local context in these indicators at a regional scale (Preston et al. 2011). In addition, we did not integrate into the spatial prioritisation quantitative assets that are expected to be impacted, such as crop or timber production. Instead, we identified areas where the impact of drought is expected to be higher whatever the number of affected assets. Due to the inherent people-dependent nature of vulnerability (Stephan et al. 2023), participatory approaches are best suited to define prioritisation rules considering local contexts and visions (Ruangpan et al. 2021).
This study focuses on the drought hazard, whereas the Alps and other mountains experience multiple impacts from heatwaves, floods and landslides. We suggest that future adaptation studies apply our methodology to multiple hazards.

The analyses reveal spatial overlaps and mismatches in relation to spatial planning strategies at a large scale. However, NbS implementation at individual sites should be planned and assessed with consideration for local benefits and synergies, guided, for example, by the IUCN Global Standard for NbS (IUCN 2020).

## 5. Conclusion

Optimal areas to implement NbS need to be identified considering the limited resources to address the interlinked crises of climate change and biodiversity loss. NbS can foster the adaptation of alpine socio-ecological systems to drought by enhancing soil water retention and increasing rainwater infiltration. Our proposed prioritisation methodology to implement NbS for adaptation to drought relied on two indicators: soil moisture and groundwater. This approach highlighted areas of water deficit that should be prioritised for water regulation enhancement (i.e. restoration or agroforestry NbS) and areas of water surplus where safeguarding this resource is crucial to address increasing drought intensity (i.e. protection or sustainable management NbS). Our analyses identified southwestern and northeastern lowlands in the Alps as priority areas with water deficit. In contrast, the southern hillsides and the western Alps emerged as priority areas with water surplus.

We identified limited potential to simultaneously address priorities for adaptation to drought, biodiversity conservation and carbon sequestration, highlighting the urgent need to optimize NbS implementation in areas where synergies exist, but also to consider implementing NbS to address a given challenge where needed.

Implementing NbS in priority areas within croplands and wetlands could be supported by decision processes in existing protected areas. However, for other priority areas located in productive, densely populated or tourism regions where multiple pressures may deepen the adaptation gap, co-construction processes with local stakeholders are required to develop a regional adaptation strategy. Decision-makers can help implement future NbS by fostering institutional practitioners towards alternative practices for adaptation, e.g. in protected areas and national water agencies; creating funding opportunities to implement NbS for private practitioners willing to
change practices, e.g. towards nature-based economy; fostering co-creation process and
knowledge hubs to co-produce NbS in shared landscapes.

Future research needs to explore strategies to foster NbS implementation in these regions, un-
derstand the potential outcomes of NbS implementation in priority areas, while accounting for
other existing hazards, and evaluating the distribution of benefits among beneficiaries.

Data availability

The data supporting the findings of this study are available within the paper and its Supplemen-
tary Information - Table S1. The authors-generated code is made available on GITHUB:
https://github.com/Titouan-Dubo/NBS_priority.

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