Abstract
Reforestation has been assumed as a natural solution to recover soil water content, thereby increasing freshwater supply. Mono-plantation of fast-growing species is the first step for performing reforestation to prevent frequent and heavy rain-induced landslide in tropics. However, fast-growing species may have negative hydraulic response to seasonal drought to maintain high growth rate and, thus, may make it difficult for reforestation in tropics to recover soil water content. We tested this hypothesis in a setting involving (a) a reforestation project, which mono-planted eight fast-growing tree species to successfully restore a 0.2-km2 extremely degraded tropical rainforest, and (b) its adjacent undisturbed tropical rainforest in Sanya City, Hainan, China. We found that, for maintaining invariably high growth rates across wet to dry seasons, the eight mono-planted fast-growing tree species had comparable transpiration rates and very high soil water uptake, which in turn led to a large (3 times) reduction in soil water content from the wet to dry seasons in this reforested area. Moreover, soil water content for the adjacent undisturbed tropical rainforest was much higher (1.5 to 5 times) than that for the reforested area in both wet and dry seasons. Thus, the invariably very high water demand from the wet to dry seasons for the mono-planted fast-growing species possesses difficulty in the recovery of soil water content. We suggest, in the next step, to mix many native-species along with the currently planted fast-growing nonnative species in this reforestation project to recover soil water content.
Highlights
Human beings face a freshwater scarcity problem on account of the steadily increasing freshwater demand [1]
For adapting to the limited water supply in the dry season, native species were inclined towards having low photosynthesis water need and high drought stress tolerance, which in turn resulted in lower soil water uptake but higher soil infiltration, thereby maintaining a very high soil water content
We found that the eight nonnative species had much higher transpiration, photosynthesis rate, stomatal conductance, and leaf hydraulic conductivity than those for native species in both seasons
Summary
Human beings face a freshwater scarcity problem on account of the steadily increasing freshwater demand [1]. Soil water content is one of the main freshwater resources [2], and globally, forests play a key role in maintaining them [3]. Historic human disturbance (e.g., ore mining and unreasonable agricultural use) have resulted in very high deforestation and degradation in tropical rainforest worldwide, which in turn has led to a large amount of global freshwater loss [4,5,6,7]. Relatively few studies have evaluated the influence of large spatial or temporal reforestation projects on soil water content, especially in the humid subtropics and the tropics [12]. Nearly no study has evaluated the influence of large spatial or temporal reforestation projects on soil water content in humid tropics [8]
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