# The Role of Trees in Sand Dune Rehabilitation: Insights from Global Experiences
## 4 Discussion
### Factors Influencing the Success of Afforestation on Sandy Dunes  
Our analysis showed that numerous studies underscore the complex link between topography, soil structure, and water dynamics in the success of afforestation on sandy dunes. We started by analyzing the importance of soil structure for these ecosystems. As other authors have also observed [8; 124], measures conducted after afforestation have shown an increased improvement in nutrient contents, composition, and enzymes. The highest increase was observed in soil nutrient content and enzyme activity. We have then moved to the importance of water dynamics, with similar results obtained by other authors who focus on the changes brought by afforestation on the surrounding features. For example, the water table decreases [125] as the soil is covered with a layer of litter. The development of trees at the dunes in Israel [91] also highlights the impact of the slope in mediating water redistribution. On steep dunes, enhanced subsurface lateral flow concentrates water at the base of the dunes, facilitating rapid tree growth. This local water accumulation--amplified by litter deposition and the development of a water-repellent layer--promotes positive feedback through increased deep infiltration via finger flow. Conversely, on flatter dunes, the absence of a water-repellent layer prevents runoff generation, inhibiting the concentration of water and thus limiting tree growth. These findings illustrate that even subtle variations in topography can create starkly divergent growth trajectories within the same plantation. Soil moisture was our next reference, as afforestation projects can transform the soil-plant-water feedback in dune systems, with implications for long-term plantation resilience. For example, the long-term monitoring at Culbin Forest, Morayshire [101], demonstrates how afforestation can gradually modify dune system properties. While the mechanical composition of the sand remained largely unchanged, tree growth substantially reduced soil moisture content. However, organic matter incorporation in older stands increased the moisture-holding capacity of the upper soil layers, potentially mitigating some water limitations over time. Climate conditions were our next topic for analysis, a fact complemented by the study of Scots pine forests along the south Baltic Sea coast [100]. This case highlights the temporal and spatial variability of climate-growth relationships in dune systems. Despite differences in local microsite conditions, tree growth across sites consistently responded positively to winter-spring and summer moisture availability. However, growth was predominantly limited by winter-spring temperatures and seasonal drought, suggesting that regional climate factors may override some site-specific characteristics. This reinforces our notion that successful afforestation strategies must account for both micro- and macro-environmental conditions. We also found a connection between species composition and topography. This was also observed by some studies that have analyzed species along a topographic gradient throughout the restoration trajectory, showing that vegetation is restored on sand dunes by an upward spreading from interdune lowlands [126]. The study of _Casuarina equisefilia_ plantations in Senegal [59] further emphasizes the influence of microtopography on afforestation outcomes. Trees established in interdune depressions and flanks demonstrated significantly greater growth than those on dune summits. These microtopographic effects are likely driven by variations in water accumulation and wind exposure, echoing our findings related to all the factors that impact sand dunes. Similar patterns of growth variability were observed in coastal aforestation across the Mediterranean region. The widespread growth decline reported in Tunisian pine stands (_Pinus pinea_ and _Pinus halepensis_), originally established to stabilize dunes [100], underscores the vulnerability of aforestation projects to microsite-specific stressors.