Forest Influences on Climate and Water Resources at the Landscape to Regional Scale - Landscape Ecology for Sustainable Environment and Culture - page 317

15.5.3 Understanding the Roles of Afforestation
in Mitigating Negative Effects of Climate Change
Forest ecosystems are large carbon sinks (Pan et al. 2011 ) and thus could play an
important role in mitigating climate change. Sustainable forest management
strategy that aims at maintaining or increasing forest carbon stocks will not only
produce sustained yield of timber or energy but also will generate the largest
sustained mitigation benefit. For example, a large afforestation effort that plans to
plant about 18 million acres of new trees to replace pasture and farming lands by
2020 are being implemented in the southeastern US, as well as in Great Lake states
and the Corn Belt states (Watson 2009 ) in the U.S. The project would be even
larger than the one carried out by the Civilian Conservation Corps during the Great
Depression, which planted 3 billion trees from 1933 to 1942.
Forests also can modulate climate by controlling energy and water transfers. If
warmer conditions increase vegetation coverage, for example, evapotranspiration
and solar radiation absorbed on the surface will increase. The change in evapo-
transpiration, which often plays a more important role, will lead to cooling. The
feedback from evapotranspiration would partially offset any greenhouse warming.
Through such feedback mechanisms, ecosystems influence their local environment
and combined with their ability to sequester atmospheric CO 2 , can act to mitigate
climate change impacts.
Small watershed studies worldwide clearly show that forests are 'biological
water pumps', (Makarieva et al. 2009 ) and they consume large amount of water to
realize other ecosystem services (e.g., carbon sequestration and moderating cli-
mate). This has been confirmed worldwide (Scott and Lesch 1997 ; Robinson et al.
2003 ; Ice and Stednick 2004 ). Thus, when other conditions are equal, compared to
other land uses, such as grasslands and urban lands, forested watersheds have
lower total water yield (Bosh and Hewlett 1982 ; Zhang et al. 2008a , b ; Wang et al.
2009 ; 2011 ) and peakflow rates/floodings (Eisenbies et al. 2007 ; Alila et al. 2009 ),
and thus reforestation can help mitigating the negative impacts of extreme storm
events (Ford et al. 2011 ; Vose et al. 2011 ) in addition to achieving carbon
sequestration benefits. Forests protect water quality (e.g., preventing soil erosion
and sediment loading in streams) under a changing climate that increases rainfall
intensity in some regions. However, these basic understanding of forest-water
relationships are based on watershed studies and data at large basin and regional
scales are still lacking. While the important role of forests in mitigating global
change through modifying the carbon cycle has been widely recognized, their
importance to mitigate extreme climate and hydrology (floods and droughts)
through the land-atmosphere interaction has yet to be fully explored and quantified
(van der Ent et al. 2010 ; Vose et al. 2012 ). This is exemplified by the recent debate
on forests influences on regional water supply (Ellison et al. 2012 ; van der Ent
et al. 2012 ) and forests' role in flood controls (Calder et al. 2007 ; Bradshaw et al.
2007 ; Laurence 2007 ; Van Dijk et al. 2009 ). Coupled climate-vegetation-hydrol-
ogy models should be useful tools for understanding the role of the vegetation in