Restoring ecological function in temperate grassy woodlands: manipulating soil nutrients, exotic annuals and native perennial grasses through carbon supplements and spring burns

Abstract

Summary1. Ecological invasions are often associated with persistent changes to underlying ecological processes. Restoration of invaded communities is dependent on manipulation of these processes to favour the target species composition and impart resistance to further invasion. We applied these principles to extensively degraded grassy woodlands in temperate agricultural regions of Australia, where widespread invasion by mediterranean annuals is related to altered ecological processes such as soil nutrient cycling. 2. We investigated carbon supplementation and spring burns, in association with reestablishment of native perennial grasses, as potential management tools for manipulating nitrogen cycling, soil seed banks and establishment conditions in degraded woodland understoreys. Through these mechanisms we aimed to enhance native cover and increase resistance to invasion by exotic annuals. 3. In two contrasting degraded remnants, repeated sucrose applications temporarily reduced soil nitrate to inferred pre-European levels, which dramatically reduced growth of exotic annuals and enhanced native perennial abundance. Repeated spring burns did not reduce soil nitrate but reduced exotic annual grasses through effects on soil seed banks and/or establishment conditions. 4. Spring burns and carbon supplements both significantly enhanced establishment of Themeda australis , a dominant tussock grass prior to European settlement. Combinations of T. australis seed addition and either spring burning or carbon supplements enhanced native plant abundance more effectively than treatments without seed addition. 5. Within 18 months of their establishment, T. australis seedlings significantly reduced soil nitrate in some treatments, providing a preliminary indication that re-establishment of a dense sward of this species may restore ecosystem function to a low-nitrate state that favours native perennials over exotic annuals. 6. Synthesis and applications . Ecological restoration can be viewed as targeted intervention in species-environment interactions, whereby ecological conditions are manipulated to enhance establishment or vigour of key species, and these species in turn help restore ecological processes that favour the target species composition. In grassy ecosystems re-establishing a perennial sward of appropriate native tussock grasses may be critical for restoring pre-disturbance nitrogen cycles and improving resistance to invasion by exotic annuals. Carbon supplements and spring burns facilitate this process through complementary mechanisms.

Turf wars: experimental tests for alternative stable states in a two‐phase coastal ecosystem

“…This result supports the suggested importance of nitrogen as a key factor in increased invasion by exotics and in changes from native-to exotic-dominated communities elsewhere (Huenneke et al 1990, Weiss 1999, Prober et al 2005. The finding that the presence of Trifolium repens, an exotic N-fixing species found in the pasture community, can facilitate this change provides additional evidence that plantavailable nitrogen is an important driver of these alternative states.…”

“…In coastal ecosystems, salt spray is an important factor in determining the distribution and growth form of plants (Sykes andWilson 1988, Yura andOgura 2006), and differences in height and physical structure of the communities may play a role in minimizing or magnifying its effects. Soil fertility can be modified through anthropogenic inputs (Weiss 1999, Prober et al 2005) and contributions by animals (Steinauer and Collins 2001). Plant feedbacks can affect soil nutrient status by influencing soil microbial processes via litter input (Buckeridge andJefferies 2007, Orwin et al 2010), and by nitrogen fixation associated with, e.g., legumes (Gylfadottir et al 2007).…”

Soil recovery after removal of the N2-fixing invasive Acacia longifolia: consequences for ecosystem restoration

“…When an invasive plant has caused impacts at the ecosystem level, for example leaving a legacy of altered soil processes and pools, removal of the species may not be sufficient to allow the ecosystem to revert to its original state for many years. This ''legacy effect'' has been referred to in the literature (Corbin and D'Antonio 2004;D'Antonio and Meyerson 2002;Hawkes et al 2005;Heneghan et al 2006), and is considered an important factor for invasion success (Blumenthal 2005;Davis and Pelsor 2001) which may hamper restoration of native ecosystems (Gordon 1998;Hobbs and Humphries 1995;Prober et al 2005). The duration of invasion may be important for the extent of invader impacts on ecosystems, and as such, a long-term perspective should be considered when studying impacts of invasive species (Strayer et al 2006).…”

The role of resource limitation in restoration of sagebrush ecosystems dominated by cheatgrass (Bromus tectorum)

“…Restoration experiments that decreased available nitrogen (N) through topsoil removal (Buisson et al 2006, Perry et al 2010, sucrose or other carbon (C) source addition (Morghan and Seastedt 1999, Blumenthal et al 2003, Suding et al 2004, Prober et al 2005, Mazzola et al 2011, and repeated burning (Ojima et al 1994, Blair 1997) reduced invasive species abundance and, in some cases, increased native species establishment , Blumenthal et al 2003, Prober et al 2005. However, reducing available soil N has not decreased the competitive advantage of invasive species in all studies (Corbin and D'Antonio 2004, Huddleston and Young 2005, James et al 2011.…”