a. Ecosystem services and biodiversity before and after NbS implementation
The overall supply of ES increases after the implementation of NbS (refer to Supplementary Information II for details of the LULC changes analysis and detailed results). For heatwave mitigation (Fig. 1A), urban NbS show the highest median increase (53%), followed by forest (39%), wetland (9%), and river (-2%). In terms of flood regulation (Fig. 1B), urban NbS again show the highest median increase (89%), followed by forest (68%), river (52%), and wetland (-32%). Similarly, for climate regulation (Fig. 1C), urban NbS demonstrate the highest median increase (1,658%), followed by wetland (132%), forest (105%), and river (10%). Landslide protection (Fig. 1D) is exclusively provided by forest ecosystems with slopes exceeding 15%, resulting in a substantial median decrease in landslide probability (-52%). Regarding biodiversity (Supplementary Information II-g), wetland NbS target the restoration of an average of 7.3 ± 2.6 priority habitats according to the EU Habitats Directive per project, followed by river (5.5 ± 2 habitats), forest (1.3 ± 2.9 habitats), and urban (0.2 ± 1 habitats).
b. Beneficiaries of NbS outcomes
Different types of NbS have varying impacts on the number of beneficiaries, with urban NbS benefiting considerably more people than river, forest or wetland NbS (Fig. 2; Supplementary information III Tables S2-S4 for details). For the mitigation of heatwaves, urban NbS demonstrate the highest benefits (Fig. 2A), benefitting 1,492 ± 1,605 to 3,567 ± 3,479 inhabitants within 240 to 450 meters on average per NbS (21,436 to 53,510 inhabitants in total). For flood regulation (Fig. 2B), urban NbS demonstrate the highest benefits, with 598 ± 1,239 to 1,370 ± 1,782 inhabitants within 100 to 200 meters (13,954 to 20,553 inhabitants in total). Regarding forest landslide protection, an average of 39 ± 117 inhabitants benefit within 200 meters (1,010 inhabitants in total). This indicates that forest NbS are often located far from potential beneficiaries. In terms of GDP per km2 (Fig. 2D), urban NbS are situated in areas with the highest median GDP per km2 (20 million Eur), followed by river (1.5 million Eur), wetland (172,575 Eur) and forest (53,970 Eur).
c. Economic value of ecosystem services delivered by NbS and cost-effectiveness
The economic value of the the four assessed ES per hectare is 424,662 Eur with a total value of 676 million Eur (Table 1). Urban NbS have the highest median value per hectare (2.5 million Eur/ha), followed by forest (34,413 Eur/ha), wetland (29,213 Eur/ha) and river (4,638 Eur/ha). The flood regulation ES constitute 87% of the total ES value provided by all NbS (590 million Eur), followed by heatwave mitigation (8%), climate regulation (4%) and landslide protection (1%). In terms of value per hectare of individual ES, the most valuable ES delivered by NbS are flood mitigation (370,318 Eur/ha), followed by heatwave mitigation (33,143 Eur/ha), climate regulation (17,286 Eur/ha) and landslide protection (12,296 Eur/ha). The high standard deviation in economic value across NbS is linked to numerous zero values for certain ES (Fig. 3A-D). For instance, heatwave mitigation is only observed in areas affected by the heat island effect, primarily in urban settings, with rural areas showing no impact (64 NbS present zero values). Similarly, flood regulation economic value depends on the presence of infrastructure and the NbS's ability to retain water, resulting in 38 instances with zero values (see Supplementary Information IV Table S6 for details).
The total cost of all actions is 217 million Eur, with a median cost of 20,094 Eur/ha. River NbS show the highest median costs (265,847 Eur/ha), followed by urban (89,752 Eur/ha), wetland (20,094 Eur/ha) and forest (3,232 Eur/ha). Regarding the median cost of the individual actions (Fig. 3E), the highest values are for river NbS (592,839 Eur), followed by wetland (40,188 Eur), urban (21,000 Eur) and forest (19,348 Eur).
Considering the economic value of ES and the costs per ecosystem, the cost-effectiveness ratio is notably higher for urban, at 4.2 times, river at 3.1 times, forest at 1.6 times and wetland NbS at 0.6 times (excluding biodiversity considerations) (Fig. 3F). Considering all ecosystem types, the cumulative value of NbS surpasses the total cost by a factor of 3 to 1. For additional details on the economic assessment and cost-effectiveness, refer to Supplementary Information IV.
Table 1
Results of economic value of Ecosystem Services (ES) by Nature-based Solution (NbS) type in Eur. This table provides insights into the total value, value per action (excluding area considerations), and value per hectare (total ES value divided by the NbS type's total area). (*) Landslide protection value corresponds to the 20 m buffer. The total value for all ecosystems is the sum of the total values across ES. The total value per hectare for all ES is calculated by dividing the total value of all ES by the total number of hectares per ecosystem type. The average value per hectare is determined by summing the values of all ES per NbS, dividing by each NbS's area, and calculating the average of these values (± Standard Deviation).
|
Heatwave mitigation
|
Flood regulation
|
Climate regulation
|
Landslide protection*
|
All ecosystem services
|
|
Total ES value
|
Avg. ES value per action
|
ES value per ha
|
Total ES value
|
ES value per action
|
ES value per ha
|
Total value
|
ES value per action
|
ES value per ha
|
Total ES value
|
ES value per action
|
ES value per ha
|
Total ES value
|
ES value per hectare
|
Avrg. ES value per ha
|
FOR
|
2,095,345
|
682 ± 2,887
|
2,030
|
1,078
|
41 ± 166
|
1
|
22,398,683
|
861,488 ± 1,727,840
|
21,696
|
6,240,739
|
520,062 ± 1,057,604
|
12,296
|
30,735,845
|
60,557
|
70,286 ± 122,811
|
RIV
|
23,747,246
|
11,833 ± 33,300
|
81,015
|
513,362,401
|
30,197,788 ± 124,517,788
|
1,751,373
|
1,152,433
|
67,790 ± 213,819
|
3,932
|
0
|
0
|
0
|
538,262,080
|
1,836,319
|
3,722,388 ± 10,789,163
|
URB
|
26,988,046
|
15,241 ± 21,342
|
370,511
|
77,204,124
|
5,146,942 ± 19,375,248
|
1,059,914
|
1,006,560
|
67,104 ± 138,561
|
13,819
|
0
|
0
|
0
|
105,198,731
|
1,444,243
|
15,016,442 ± 28,511,703
|
WET
|
0
|
0
|
0
|
-265,391
|
-9,829 ± 43,712
|
-1,356
|
2,996,630
|
110,986 ± 126,407
|
15,315
|
0
|
0
|
0
|
2,731,239
|
244
|
25,416 ± 11,696
|
Total
|
52,830,637
|
|
33,143
|
590,302,213
|
|
370,318
|
27,554,308
|
|
17,286
|
6,240,739
|
|
12,296
|
676,927,895
|
424,662
|
3,424,011 ± 13,743,091
|
d. Integrated analysis of NbS
A comparative assessment of NbS per ecosystem type considering their benefits, beneficiaries and costs-effectiveness shows clear differences among them. Concerning ES supply and biodiversity (Fig. 4A), urban NbS supply substantial flood regulation and heatwave mitigation. However, their weakness lies in their limited biodiversity co-benefits. Despite this, urban actions have the highest cost-effectiveness (Fig. 4B) reflecting their placement in valuable infrastructure areas. For the same reason, these projects benefit a larger number of beneficiaries in flood regulation and heatwave mitigation than wetlands (Fig. 4C).
River NbS excel in their primary purpose of flood regulation, while their location on flat terrain explains low effectiveness in landslide protection. Importantly, they play a significant role in enhancing priority habitats for biodiversity (Fig. 4A). Their economic value is remarkable, with the highest value for flood regulation, and high values for heatwave mitigation and climate regulation (Fig. 4B). River NbS actions score second in terms of number of beneficiaries due to the presence of settlements close to rivers (Fig. 4C).
Forest NbS demonstrate the highest values for landslide protection and heatwave mitigation. However, as they primarily consist of native species plantations their biodiversity co-benefits are limited. Remarkably, among 26 forest actions, only seven incorporate species linked with priority habitats outlined in the EU Habitats Directive. The cost-effectiveness reflects a moderated economic value for climate regulation and landslide protection (Fig. 4B). However, forest NbS actions are usually located far from beneficiaries (Fig. 4C).
Wetland NbS demonstrate high values for various ES (Fig. 4A), except for landslide protection, due to their location in low-slope areas and flood regulation. However, they are the most relevant in providing biodiversity co-benefits. In spite of their good performance for carbon sequestration (Fig. 4B), wetlands achieve a lesser economic value due to their lesser value for other ES given their location away from densely populated areas (Fig. 4C), and even though they impact more people than forest NbS for flood regulation and heatwave mitigation.