A Stepwise Approach to Increasing Ecological Complexity in Forest Landscape Restoration

Continuing deforestation (Keenan et al. 2015) and forest degradation (Sloan and Sayer 2015) reinforce the need for restoration (Aronson and Alexander 2013, Hanberry et al. 2015). Restoration will in turn be profoundly influenced by climate change (Harris et al. 2006). Even restoration projects with narrow objectives, such as fuelwood production (Montagnini and Nair 2004) or biodiversity conservation (Bullock et al. 2011), will be expected to contribute to climate mitigation (carbon sequestration and storage) and adaptation (ecosystem services). Restored forests, if they are to deliver effective ecosystem services, need to be resilient to changing conditions, which may require purposeful decisions about the direction of restoration. Furthermore, the long time-scale involved (Maginnis and Jackson 2012) means that management aims and societal needs are likely to change during the life of a restoration program. Forest Landscape Restoration initiatives, from the outset, need to be adaptable and capable of modification (Mansourian et al. 2017).

Resilience is defined as: “the amount of change a system can undergo without changing state” (IPCC TAR 2001, 993). Ecologists distinguish between engineering resilience, the ability of an ecosystem to return to its pre-disturbance state, and ecological resilience or equilibrium dynamics, the ability of an ecosystem to absorb impacts and remain below a threshold of change into a different state (Walker et al. 2004). Engineering resilience—the ability to “bounce back” becomes rarer under climate change, while ecological resilience will likely result in modifications to an ecosystem’s taxonomic composition and even its structure, albeit changes that retain many of the original ecosystem’s functional characteristics (Stanturf et al. 2014). Ecosystems may exist in more than one stable state (Schroder et al. 2005), suggesting that there is no simple “right” and “wrong” restoration pathway, and that restoration goals must remain open-ended (Heller and Hobbs 2014) and, if necessary, adapted over time.

Forest restoration projects need to incorporate resilience into their objectives if they are to contribute to an ecosystem that functions in the long term. High diversity ecosystems are increasingly considered more resilient and provide a wider range and better-quality ecosystem services (Loreau et al. 2001), although simplistic, linear links between biodiversity and ecosystem function are to be treated with caution (Lasky et al. 2014). One hypothesis is that species richness increases ecosystem resilience by increasing the interdependencies and robustness of the system (the stability-diversity hypothesis, Doak et al. 1998). Others suggest that functional diversity plays the pivotal role (Díaz and Cabido 2001); including species that maintain biological functions (e.g., seed dispersers). Richness tends to increase redundancy and therefore buffers ecosystems against loss of individual species. Restoration projects are encouraged to increase diversity to increase resilience (Harris et al. 2006). This coincides well with the objectives of restoration projects aimed at biodiversity conservation, in which diversity also generally has a high value, particularly as an adaptation response to climate change (Foden et al. 2008).

The twin incentives of climate resilience and biodiversity conservation should encourage the modification of many existing forest restoration projects. The pursuit of these objectives can be enhanced through a stepwise approach to forest restoration, which aims to gradually increase ecological complexity. This could include, for example, increasing the mix of tree species (and hence associated species), encouraging a mixed age stand, planning to retain old-growth fragments in restored forests, and potentially reintroducing expected species that disappeared prior to restoration. We suggest that stepwise forest landscape restoration could be defined as: a process of deliberately pursuing incremental gains that supplement or speed up natural succession towards increased ecological complexity, increased resilience, and increased diversity of benefits.

A stepwise approach could be part of the original restoration plans or emerge over time, or be a reaction to a problem, possibly in an existing forest (Table 1). Complexity can be factored into restoration at any stage although the earlier it is anticipated, the easier it will be achieved. The principle of non-regression is important; responsible restoration projects should maintain complexity and avoid loss of diversity, such as might occur through replacing mixed woodland with a monoculture plantation.

The following examples are illustrative:

1. 1. Pre...