Comparing Successional Models to Restore Riparian Forest Understory along the Sacramento River, CA


 

Over the last 150 years, 98% of the riparian forest along the Sacramento River have been converted to agricultural and developed land.  Concerns about loss of habitat and ecosystem services led to large-scale restoration efforts. 

Initial efforts (1989-1999) were directed towards planting trees and shrubs.  A 2001 survey of 15 sites indicated the forest understory was dominiated by non-native invasive plants (Holl and Crone 2004). 

I resurveyed these sites in 2007 and found the recovery of native understory may be happening at a slow rate, however results varied by site.  Native understory cover was still much lower than found in 10 reference sites.  The composition of understory plants shifted from light-demanding towards shade-adapted native and exotic species. 

More recent efforts (1999-present) were directed towards planting trees, shrubs, and understory species.  I surveyed 20 of these sites in 2007 and found mixed success.  Sites planted with 4-13 understory species at high densities had greater cover than sites that were planted with few or no understory species.  Restorers had the most success planting California blackberry (Rubus ursinus) and marginal success planting mugwort (Artemisia douglasiana) and native grasses.     

Take home message:

Many native species do not colonize restored sites on their own in <20 yrs and should be planted.  Shade-adapted native species do not tolerate conditions early on, and may need to be planted after the forest canopy has developed (~10 years following restoration). 

Restoration Sites can be viewed here

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