CAS Workshop on Ecosystem Succession Theory and Practice of Ecological Restoration 

Restoration and management activities in California watershed result from interdisciplinary efforts from CALFED Bay-Delta (CalFed), other multiagency, and public groups.  The CALFED Bay-Delta Program established in May 1995 consists of several key program elements that will help ecosystem restoration and species recovery.  One of these elements, the Ecosystem Restoration Program (ERP), was developed to guide restoration actions and ensure attainment of ecosystem health (also called ecological integrity).  The strategy described in the ERP to restore ecological integrity is based on the restoration of ecological processes that are associated with stream flow, stream channels, watersheds, and flood plains that in turn, support habitats and associated species.  In addition, the CALFED Program established Multi-species Conservation Strategy (MSCS), and Science programs, designed to work in conjunction with the ERP to increase protection of listed species, and ensure the application of sound scientific principles in ecosystem restoration actions. From 1997 to 2001, the CALFED effort has led to ecosystem restoration grants for 326 projects, totaling $336 million. 

Historically, aquatic habitat in California Sacramento River tributaries was degraded by a wide variety of land and water development activities. The current restoration activities are focused on identified priority tributaries, which are physical attributes capable of supporting populations of sensitive and endangered species such as steelhead and Chinook salmon.  The range of restoration actions includes:1) identify priority streams to focus on restoration actions; 2) facilitate and participate in collaborative processes among interested and affected parties that are directed at gaining community acceptance and funding for restoration actions; 3) increase the quality and quantity of water flows below dams; 4) provide fish passage, restore flood plain and gravel bed processes; 5) and maintain healthy ecosystem processes in the watershed. 

Plant systems are important elements in watershed ecosystem restoration, which include the following actions: 1) restoring woody riparian vegetation; 2) revegetating the banks of creeks; 3) shallow water and wetland restoration; 4) improving water quality and reducing sediment of watershed; 5) dust control.  Particularly, plant nitrogen fixation systems can provide effective and cost efficient tools, which are suited to the ecosystem restoration. Legume-Rhizobium and actinorhizal plants-Frankia are two major plant symbiotic nitrogen fixation systems, which offer a low-input and cost effective tools in ecosystem restoration.  The biology of these symbiotic nitrogen fixation systems has been well studied, including their mechanisms of Legume or actinorhizal plant infection by Rhizobium or Frankia strains, interactions of plants and microbes, and plant nodule development. Plant nitrogen fixation systems have significant impacts on the ecosystems nitrogen input, and their role in ecosystem redevelopment through ecological succession has been studied.  For example, herbaceous actinorhizal N-fixing shrubs (such as Ceanothus spp.) are common pioneer species following fire in prairie and forested ecosystems in northwest region of U.S. These plants provide an important source of plant available N in ecosystems.  There are more than 50 Ceanothus species adapted in different habitats of California such as serpentine volcanic substrate condition in chaparral, foothill woodland, closed-cone pine forest, high elevation under dry conditions, desert, and costal conditions.  Alnus is another N-fixing woody plant species widely distributed in the watershed of California. 

Reviewing the recent ecological applications of plant nitrogen fixation in ecosystem restorations (such as subtropical savanna and tropic conditions) is a key step to understand its role and implications in ecological conservation, and the future research direction. The variability in the potential for symbiotic N-fixing woody plant systems (Legume-Rhizobium and actinorhizal plants-Frankia) has been evaluated in a subtropical savanna ecosystem.  Nodulation and N₂ fixation among woody legume can occur across a broad range of soil conditions and depths with significant impacts on local and regional N-cycles. The ability of  N₂ fixation may contribute to their successful colonization of grassland and savanna ecosystems during the shifts from grass to woody plant dominated condition. Effects of N-fixing trees on soil nitrogen and organic matter during forest restoration have been studied in low-montane ecosystems of Ecuador by comparing nitrogen-fixing and non- nitrogen-fixing trees.  N-fixing trees improved significantly soil and abiotic conditions for forest regeneration.  Long-term effects and dynamics of woody plant N₂ fixation under different conditions and its limitation have also been explored in forest restoration of altered montane cattle pastures in Hawaii. The N-fixing trees play a relatively more important role in restoration of more degraded sites and in their soil and ecosystem development. Estimated input of N via fixation declined between 6 and 20 years indicates decrease in N₂ fixation overtime. A decreased P availability over time may be one of the limiting factors related to this decline. We need to determine if P addition could be one of the long-term management strategies.  

In summary, applications of plant nitrogen fixation in restorations of ecosystems are still in the early stage. We need to know how plant nitrogen fixation systems could be effectively used to manage natural ecosystems and to restore the degraded environment. Further assessment of these systems related to ecosystem restorations will improve our understanding of their global importance, benefits, limitations, and future challenge regarding our actions to protect ecological integrity of environments.

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