Designing Restoration on Agricultural Land by Strategic Revegetation
The agriculture and conservation paradox creates a dilemma in projects that involve restoring non-agricultural habitats such as woodland on agricultural land, which can only be resolved by considering the relative values of biodiversity and ecosystem services associated with woodland vs. agricultural ecosystems (Rey Benayas et al. 2008). The reconstruction of vegetation in a landscape (“where and when to revegetate?”) is an issue that has become a research priority (Thompson et al. 2009). In the context of this article, we consider as strategic revegetation, highly specific planting (and sometimes seeding) actions that are characterized as occupying a tiny fraction of the agricultural landscape (Fig. 7.1). They are intended to enhance wildlife and particular services such as habitat provision and seed dispersal. The effects on wildlife and ecosystem services will usually depend on how much land is affected by these actions.
Strategic Revegetation in Farmed Fields
In actively farmed fields, these actions can include planting woodland islets, hedgerows and isolated trees (Fig. 7.1). They have the potential to enhance wildlife, agricultural production, and other services at the field and landscape scales since they hardly compete for farmland use (Rey Benayas and Bullock 2012), and can be considered a form of rewilding per se. Rey Benayas et al. (2008) suggested a new
Fig. 7.2 A schematic diagram of the “woodland islet and hedgerow” model proposed in this article, based on the “woodland islets” model developed by Rey Benayas et al. (2008). A denuded agricultural landscape (a) is planted with a few (here four) small (e.g. 100 m2) woodland islets and hedgerows (b) Targeted management of the islets and hedgerows allows the trees to establish, grow and reach sexual maturity rapidly. If the cropland is then abandoned the islets and hedgerows can expand and export seeds (and other organisms established in them) to the surrounding land (c) The islets and hedgerows eventually coalesce to form closed woodland (d) Alternatively the surrounding land remains in same or other uses (e) while the islets and hedgerows remain as small patches of the native woodland community as the trees grow taller. Some islets and hedgerow fragments may disappear through stochastic events (f)
concept for designing restoration of forest ecosystems on agricultural land, which uses small-scale active restoration as a driver for passive restoration over much larger areas. Establishment of “woodland islets” is an approach to designing restoration of woodlands in extensive agricultural landscapes where no remnants of native natural vegetation exist. It involves planting a number of small, densely-planted, and sparse blocks of native shrubs and trees within agricultural land that together occupy a tiny fraction of the area (< 1 %) of target land to be restored (Fig. 7.2). This approach, later called “applied nucleation” by Corbin and Holl (2012), allows direction of secondary succession by establishing small colonisation foci, while using a fraction of the resources required for large-scale reforestation. Woodland patches provide sources of seed and dispersing animals that can colonize adjacent habitats. If the surrounding land is abandoned, colonists from the islets could accelerate woodland development because dispersal of many woodland organisms will
Table 7.1 Examples of seed dispersal distances for European forest and scrub species, with information on whether dispersal is biased into open or forest habitats
continue over many years (Fig. 7.1). The landscape emphasis on a planned planting of islets maximises benefits to wildlife and the potential of allowing the islets to trigger larger-scale reforestation if the surrounding land is abandoned, which can lead to rewilding (see Chap. 1). The islets should be planted with a variety of native shrub and tree species including those identified as nurse species to take advantage of facilitation processes (Cuesta et al. 2010).
Vegetation dynamics in complex landscapes depend on interactions among environmental heterogeneity, disturbance, habitat fragmentation, and seed dispersal processes. Ozinga et al. (2009) concluded that the 'colonization deficit' of plant species due to a degraded dispersal infrastructure is as important in explaining plant diversity losses as habitat quality, and called for new measures to restore the dispersal infrastructure across entire regions. Estimates of dispersal distances for vertebratedispersed shrubs and trees (Table 7.1) suggest that the introduction of woodland islets planted about one km apart in a deforested agricultural landscape could allow colonisation over the whole landscape (Rey Benayas and Bullock 2012). Spread from these islets would be facilitated in the cases where animals disperse seeds preferentially into open habitats, whilst avoiding dense scrub or forest (Table 7.1). However, the potential for colonisation from such foci will be more limited for winddispersed trees and shrubs, which seem to disperse shorter distances, and in those cases where animals disperse seeds preferentially into wooded habitats (Table 7.1). It is possible however to direct dispersal artificially into open habitats; for example by erecting structures such as perches or hedges which attract birds and/or which act as a physical barrier to wind-dispersed seeds (Bullock and Moy 2004).
The woodland islets approach maintains flexibility of land use, which is critical in agricultural landscapes where land use is subject to a number of fluctuating social, environmental, policy and economic drivers (e.g. Romero-Calcerrada and Perry 2004; Rounsevell et al 2005). It provides a means of reconciling competition for land use among agriculture, conservation and woodland restoration at the landscape scale. This could increase the economic feasibility of large-scale restoration projects and facilitate the involvement of local human communities in the restoration process. The woodland islets idea has similarities to other approaches involving planting small areas of trees on farms, such as tree clumps, woodlots, hedges, living fences, or shelterbelts and agro-forestry systems. Particularly, the revegetation of property boundaries, field margins and track edges in farmland to create living fences (Barnes and Williamson 2006; see Chap. 6) has the same function in triggering passive revegetation as woodland islets (Forget et al. 2013); thus, the “woodland islets” concept could be expanded to the “woodland islets and hedgerows” concept (Fig. 7.2). Planting isolated trees may also provide a disproportionate positive value for wildlife and potential for seed dispersal (DeMars et al. 2010; Fischer et al. 2010). Besides providing a dispersal infrastructure, woodland islets and hedgerows provide habitat or enhance the farmed environment for wildlife. These benefits have been well documented for plant species (e.g. Poggio et al. 2010) and small animals such as insects (e.g. Noordijk et al. 2010), but they are also critical for medium-sized and even large mammals. For instance, Pereira and Rodríguez (2010) documented the value of hedgerows and narrow strips of riparian forest for the Egyptian mongoose Herpestes ichneumon and the common genet Genetta genetta. They found that mongooses and genets strongly selected linear woody vegetation, and that open fields, dehesa and olive groves were avoided. Similarly, Blanco and Cortés (2007) demonstrated that hedgerows and small woodland patches were important for wolf Canis lupus –an iconic species for the rewilding conceptliving in
agricultural habitats in northern Spain.
Other Options for Strategic Revegetation in Agricultural Landscapes
While we concentrate on woodlands here, the islets approach to restoring a dispersal infrastructure could be used for other (semi-)natural habitats such as species-rich grasslands, scrub, or heathland (e.g. Hooftman and Bullock 2012). Other strategic revegetation actions in agricultural landscapes but unrelated to the farmed environment itself could, for example, target road verges and roundabouts, and riparian systems (Fig. 7.1). These would provide similar benefits in terms of dispersal infrastructure as those explained above for woodland islets and hedgerows. The revegetation of roadsides and roundabouts may bring additional benefits such as slope stabilization and aesthetic value. However, these revegated artificial elements may also function as “ecological traps” that put at risk the wildlife attracted by them through increasing traffic collisions (Fahrig and Rytwinski 2009). Additionally, they may also decrease visibility for drivers and be dangerous obstacles in case of crashes, so safety considerations should also be considered before revegetation is decided. Thus, they should be carefully planned and supported by the planners and contractors.
Riparian systems often support the only natural or semi-natural communities at the local level in agricultural landscapes, but frequently this vegetation has been extirpated or highly degraded, and the riverside has been ploughed. It has been shown that riparian vegetation is critical for wildlife conservation (Forget et al. 2013) and provision of ecosystem services such as water regulation and purification; thus, we suggest that strategic revegetation of creeks and rivers with native species should be considered a priority in agricultural landscapes and enforced by competent public administrations.