# The Role of Trees in Sand Dune Rehabilitation: Insights from Global Experiences
## 3 Results
### Effects of Afforestation on Sand Dunes  
Afforestation and reforestation work from sand dunes located in arid regions raise important questions about their impact on groundwater. This is related more specifically to the fact that increased tree transpiration can balance groundwater recharge. Sand dunes are already very effective in groundwater recharge as rainfall infiltrates into dunes, allowing for a considerable amount of percolated water that replenishes the groundwater reservoir [90]. However, the balance between positive and negative effects is noted in most of the studies we have identified. From an historic perspective, sand dunes were considered dynamic and hazardous systems that needed stabilization. This led to planting exotic trees that could stabilize them, leading at the same time to an increase in tourism and urban development. However, this entire process also led to erosion processes [102]. A good example comes from the northern Negev desert, Israel, where the afforestation with _Tamarix aphylla_ was examined by some authors. The afforestation in this area showed a hierarchical impact on both vegetation and biodiversity. For example, the change in land-use led to a 30-50% decline in species and litter production. Furthermore, the _T. aphylla_ leaves and litter showed an accumulation of salts that also impacted the soil salinity by a factor of 4-5 beneath tree canopies. Overall, the plant cover and aboveground biomass were reduced by 30%. The positive effects of afforestation (the shade and wind-blockage provided by trees) were mainly observed outside the canopy, while the plant cover was slightly impacted. This proves that afforestation can enhance plant diversity locally, but also that the negative impacts are harsh on both ecosystem functions and the local microhabitats [103]. Another example comes from China, where afforestation works led to improvements in soil properties. More importantly, soil organic carbon and total nitrogen concentrations increased between 3.2-6.5 times. Total phosphorus also rose by 15.1% in _Pinus sylvestris_ plantations and 24.3% in _Caragana microphylla_ plantations. This proves that afforestation improves plant diversity, productivity, and the soil's physical condition [55]. The same positive-negative balance can also be observed in coastal dune afforestation. These have often been praised for stabilizing dunes and shielding nearby urban places from strong winds, sand, and salt spray. However, these interventions and the predominant use of exotic species have negative impacts on the recovery of dunes and can sometimes lead to coastline retreats. For example, the 2010 typhoon from the Korean Peninsula impacted sand dunes in different ways: some retreated further, while others recovered through the natural winter aeolian transport. Furthermore, pine tree afforested dunes showed less natural recovery than grass-covered ones. Although pine forests have reduced wind speeds, they also lead to shifts in flora as native species have declined [104]. The dunes from northeast Buenos Aires were impacted by a high paced urbanization from the early 20 th century. This has altered both dune landforms and vegetation. The dunes were stabilized by the introduction of exotic shrubs and trees, but this also impacted the wind transportation of sediments toward the beach. Over time, this disrupted the sand supply, accelerating beach erosion. In addition, the native herbaceous species have been replaced by foreign woody plants, affecting the plant community as well as dune dynamics and mobility [105]. Another example comes from South Africa, where the _Leptospermum laavigatum_ Australian shrub was introduced in the 1800s in order to stabilize dunes. Over time, this species has become a weed, especially for the endangered fynbos ecosystems from the Western Cape Province. In some cases, it even replaced _Acacia saligna_, another invasive species introduced in this area [70]. Another invasive species was noted in Portugal.