Makarieva A.M., Gorshkov V.G., Li B.-L. Revisiting forest impact on atmospheric water vapor transport and precipitation. Theoretical and Applied Climatology, doi: 10.1007/s00704-012-0643-9.
Water cycle on land owes itself to the atmospheric moisture transport from the ocean. Properties of the aerial rivers that ensure the “run-in” of water vapor inland to compensate for the gravitational “run-off” of liquid water from land to the ocean are of direct relevance for the regional water availability. The biotic pump concept clarifies why the moist aerial rivers flow readily from ocean to land when the latter gives home to a large forest — and why they are reluctant to do so when the forest is absent.
Compared to our previous studies, in the new paper we used a global (rather than land only) precipitation database that allows one to compare precipitation patterns on land to those over the adjacent ocean. We extended our previous approach to analyze seasonal (rather than annual only) changes in the spatial precipitation patterns in world’s major forest regions. Apart from the tropical rainforests, we analyzed precipitation distribution across world’s longest (>7,000 km) forest belt, the Eurasian boreal forest. The data describe how the active summer forest wins the water “tug-of-war” with the Atlantic Ocean. Indeed, in summer the forest steals most moisture inland and depletes the oceanic precipitation. The dormant winter forest loses this war to the ocean, such that precipitation over the Atlantic Ocean in winter, despite the oceanic evaporation is minimal, rises threefold compared to summer months. Analyzed for comparison, the unforested Australia is unable to draw moisture far inland in either wet or dry season, i.e., irrespective of moisture availability over the neighboring ocean.
While it is increasingly common to blame global change for any regional water cycle disruption, the biotic pump evidence suggests that the burden of responsibility rather rests with the regional land use practices. On large areas on both sides of the Atlantic Ocean, temperate and boreal forests are intensely harvested for timber and biofuel. These forests are artificially maintained in the early successional stages and are never allowed to recover to the natural climax state. The water regulation potential of such forests is low, while their susceptibility to fires and pests is high. The exploited forests are degrading; the relatively undisturbed old-growth forests are shrinking in size. A conflict (rarely appreciated or discussed) exists between the modern commercial value of a forest and the forest’s ability to regulate the regional water cycle and to be self-sustainable: these parameters cannot be maximized simultaneously. Therefore, the regional water safety is not about keeping the live forest biomass stationary. It is about keeping it stationary in an environmentally competent condition.
You are welcome to visit http://www.bioticregulation.ru/ab.php?id=taac to download the paper, the Electronic Appendix, as well as to have a look at some animated graphs.