Author(s): Juul Limpens 1,*, Milena Holmgren 2, Cor M. J. Jacobs 3, Sjoerd E. A. T. M. Van der Zee 4, Edgar Karofeld 5, Frank Berendse 1
Peatlands cover less than 3% of the Earth's land surface but store almost 30% of all terrestrial soil carbon . Raised bogs are open peatlands, dominated by Sphagnum mosses, with anoxic, acidic and nutrient poor conditions that hamper the establishment and growth of vascular plants, particularly trees . Growing conditions for vascular plants could improve as the climate becomes drier and warmer. Climate change scenarios for the northern hemisphere indicate both an increase in average air temperature and more frequent drought events . Drier and warmer conditions are known to improve vascular plant growth , ,  as a result of reduced moss vitality , , as well as by increased availability of nutrients . Indeed, recent woody plant encroachment in pristine ,  and drained bogs ,  has been attributed to warmer and/or drier conditions, as well as to changes in fire frequencies associated with a drier climate.
The effects of shrub and tree encroachment on peatland functioning and, ultimately, carbon sequestration are complex. Woody-plant dominated bogs could become net sources of atmospheric carbon if net photosynthetic rates and carbon fixation are lower than the decomposition rates of plant remains and accumulated carbon in the peat , . The balance of these two processes will likely depend on the effects that trees have on their surrounding environment. Trees can potentially affect the bog environment through four main mechanisms: trees can dry soils by intercepting precipitation and transpiring water, they can increase soil nutrient availability by litter fall, they can reduce solar radiation by shading , , , and they can reduce wind influence by changing the aerodynamic properties of the bog surface . These environmental changes could further facilitate the establishment and growth of vascular plants, potentially triggering positive feedbacks that could facilitate a shift towards a woody-dominated state , . The evidence for the mechanisms described above is restricted, however, to correlative field measurements , , theoretical studies  and simulation models .
Whether trees affect the water balance towards drier conditions is one of the most important issues concerning tree encroachment in bogs. Studies on isolated trees suggest that the drying effect can be substantial , , but tree removal experiments  and modelling studies  have found inconsistent results. A major difficulty in assessing the effect of trees on the water balance is related to the different spatial scales at which trees affect the water cycle . On one hand, trees can change the land surface albedo by absorbing solar radiation ,  which can warm the air at large regional scales . Trees also intercept precipitation and lose water by transpiration, which can also contribute to drier conditions in bogs. On the other hand, trees reduce the solar radiation and the humidity gradient under their canopy. The resulting cooler and moister microclimate can translate into less...