Approaches to ecosystem restoration article summaries

Ecological restoration success is higher for natural regeneration than for active restoration in tropical forests, Crouzeilles et al. 2017

This meta-analysis comparing active restoration to natural ecosystem regeneration found the latter to be more effective. The authors conclude that “lower-cost natural regeneration surpasses active restoration in achieving tropical forest restoration success for biodiversity and vegetation structure^[7]” [Crouzeilles 2017: 4]. This conclusion runs counter to conventional wisdom that active restoration is preferable despite being more expensive.

Natural forest regeneration is the spontaneous recovery of native tree species that colonize and establish in abandoned fields or natural disturbances; this process can also be assisted through human interventions such as fencing to control livestock grazing, weed control, and fire protection. In contrast, active restoration requires planting of nursery-grown seedlings, direct seeding, and/or the manipulation of disturbance regimes (for example, thinning and burning) to speed up the recovery process, often at a high cost to establish structural features of the vegetation (hereafter termed vegetation structure), reassemble local species composition, and/or catalyze ecological succession [Crouzeilles 2017: 1].

However, “restoration success for biodiversity and vegetation structure was significantly lower in both natural regeneration and active restoration than in reference systems” [Crouzeilles 2017: 2], underscoring the importance of conserving existing intact ecosystems.  

“Restoration success for biodiversity and vegetation structure was significantly lower in both natural regeneration and active restoration than in reference systems” [Crouzeilles 2017: 2], underscoring the importance of conserving existing intact ecosystems.

Part of the explanation for the lower success of active restoration compared to natural regeneration is that the composition and/or diversity of species chosen for planting in active restoration may be inappropriate, while the species that colonize abandoned land are likely to be diverse and locally adapted.

Natural regeneration is initiated through the colonization of opportunistic and locally adapted species, resulting in a stochastic dynamic process of forest restoration that ultimately leads to higher diversity of native, locally adapted plant species than in tree planting schemes (that is, active restoration). Active restoration also can create a highly diverse habitat through human introduction of up to 6000 seedlings/ha, but tree species used in plantings often lack the full range of functional traits found in natural regrowth forests. In addition, most tropical forest plantings for restoration or forest plantations use relatively few species, that is, these plantations may not be planted primarily for biodiversity outcomes. Thus, the higher plant biodiversity in naturally regenerated systems creates more habitats and resources, which provide additional sources of food, shelter, nesting, and breeding sites, to support higher animal biodiversity [Crouzeilles 2017: 2].

Restoration and repair of Earth’s damaged ecosystems, Jones et al. 2018

This meta-analysis of 400 studies compared passive and active ecosystem repair outcomes in terms of the speed and completeness of recovery, and found little difference between the two approaches.

Active restoration did not result in faster or more complete recovery than simply ending the disturbances ecosystems face [Jones 2018: 1].

Passive recovery simply means ending the anthropogenic disturbance that was causing the degradation, while active restoration here includes anything from fertilizer application to recontouring/dredging to planting a desired species mix.

The authors speculate that the lack of different outcomes between the two approaches could be due to restoration managers correctly choosing to actively restore the ecosystems that “are not recovering on their own and require active restoration to improve recovery outcomes relative to passively recovering systems” [Jones 2018: 6]. Also, the actively managed sites in the study had, on average, less time to recover than the passively managed sites. Finally, the authors suggest there may be right and wrong ways to actively restore ecosystems, and “recommend that restoration strategies be tailored more closely to overcome the specific barriers to recovery in individual sites” [Jones 2018: 6].  

Assuming active and passive restoration achieve comparable outcomes in many cases, then passive restoration deserves serious consideration, given limited resources available for the vast amount of ecosystem repair required in the world today.

Letting ecosystems repair themselves in many cases may be the most effective restoration strategy – a counterintuitive yet critical finding that could help society allocate restoration funds more efficiently in the future [Jones 2018: 6].

The study also consistently found that across systems, ecosystems didn’t fully recover, at least not within the timeframe of the studies.

Our results expand those findings to a broader range of ecosystems and geographies, and, together with previous work, suggest the majority of ecosystems have not yet recovered fully following disturbance and may not in the future. Thus, restoration should not be considered a substitute for conservation, which is a key strategy to ensure sustained support of biodiversity and delivery of ecosystem services in the future [Jones 2018: 4].

Rewilding complex ecosystems, Perino et al. 2019

A growing body of literature emphasizes the need for novel, process-oriented approaches to restoring ecosystems in our rapidly changing world. Dynamic and process-oriented approaches focus on the adaptive capacity of ecosystems and the restoration of ecosystem processes promoting biodiversity, rather than aiming to maintain or restore particular ecosystem states characterized by predefined species compositions or particular bundles of ecosystem services [Perino 2019: 1].

In contrast to other types of restoration efforts aiming to recreate the composition and appearance of an historical ecological community, rewilding focuses on ecosystem function and recognizes the dynamic and unpredictable nature of ecosystems. This article highlights three key ecological processes that rewilding aims to activate: trophic complexity, natural disturbances and dispersal.

Rewilding aims to restore these three ecological processes to foster complex and self-organizing ecosystems that require minimum human management in the long term [Perino 2019: 2].

Trophic complexity implies the presence of large vertebrates, including herbivores that modify the landscape through grazing or dam building and predators that control the herbivore populations. These keystone species can promote biodiversity in the landscapes they inhabit.

Stochastic (random) natural disturbance (such as fire or flooding) can increase ecosystem heterogeneity and complexity, allowing less competitive species to survive. Rewilding involves discontinuing both controlled anthropogenic disturbances and suppression of natural disturbances.

Rewilding actions aim to release ecosystems from continued and controlled anthropogenic disturbances to allow for natural variability and sources of stochasticity. Mowing of grassland can be reduced or replaced by natural grazing. Dams can be removed or their management modified to restore natural flood regimes. Logging can be replaced by allowing natural fire and pest regimes [Perino 2019: 4].

Dispersal – rewilding aims to remove anthropogenic barriers that limit the movement of plants and animals and thus the dispersal of their genetic material and potential for recolonization after a disturbance event. The creation of ecological corridors is an example of a rewilding activity that enhances dispersal.

The interaction of these ecological processes boosts the functioning of each. For example, the presence of larger animals facilitates seed dispersal throughout the system. High levels of dispersal, in turn, can facilitate ecosystem recovery following a disturbance.

Rewilding projects can be passive (allowing abandoned agricultural fields to recover on their own) or active (species reintroductions, for example), and are most effective when conducted in a manner that engages the local community in the process.

Rewilding: a call for boosting ecological complexity in conservation, Fernández et al. 2017

Rewilding is gaining traction as an approach to conservation. However, many different perspectives about which species and ecological processes to focus rewilding efforts on and how deeply to intervene in systems has created some confusion and contention within the field. Furthermore, the most ambitious and extreme rewilding proposals (for example, recreating communities that went extinct millennia ago) have often attracted more attention, while the more pragmatic and immediate solutions in the field are overlooked.

This article attempts to clarify the concept. The authors emphasize that rewilding is a process-oriented approach to biodiversity conservation “focused on preserving and restoring the structural and functional complexity of degraded ecosystems” [Fernandez 2017: 276].

Rewilding pursues the goal of restoring wild species interactions and their regulation of key ecosystem processes including nutrient and energy flows, vegetation succession and disturbances, drawing specific attention to the key roles of large-bodied species that are especially sensitive to the human appropriation of landscapes [Fernandez 2017: 277].

The authors suggest further research is needed. They note that while the negative effects on ecosystems of the loss of biodiversity and keystone species is well documented, ecosystem responses to species reintroduction and other rewilding efforts are not as well studied. To guide future research, the authors                                               

propose an unequivocally process-oriented formulation of the “rewilding hypothesis” as a general guidance: that the large-scale restoration of apex consumers and large herbivores promotes self-regulation in community assemblages, and increases the complexity of ecological processes in ecosystems [Fernandez 2016: 277].

Also needed is policy and management practice support, particularly in terms of protecting the areas and species in question. Proactive policies could ensure that gains made toward ecological restoration are not undermined by damaging human activity.

Policies and practices should be developed in order to enforce the idea that rewilding is about reducing the human control on ecosystem processes. It must begin to include varied objectives to alleviate pressures on wildlife populations such as a full legal protection of large predators based on their unique ecological roles and not just depending on their conservation status; the eradication of predator control programs; or the elimination of game management practices such as wildlife fencing, introduction of alien game populations, supplementary feeding and others that profoundly alter the natural regulation and the genetic structure of large herbivore populations [Fernandez 2016: 278].

The differences between rewilding and restoring an ecologically degraded landscape, du Toit & Pettorelli 2019

This commentary distinguishes between restoration and rewilding of ecosystems, explaining that the latter aims at ecological adaptation to novel local environmental conditions wrought by global climate change. By contrast, restoration, as defined here, aims to recreate and maintain an historical state or condition of an ecosystem, regardless of current environmental conditions.

Although the two words are often conflated,

Rewilding is thus conceptually different from restoring. It is an adaptive approach to conserving ecological functionality under changing environmental conditions, to which historical benchmarks are less relevant than to restoring. It inherently acknowledges and promotes unpredictability, while placing the emphasis on function over species composition [du Toit & Pettorelli 2019: 2].

The authors assert that rewilding is better suited to preserving biodiversity and ecosystem function under present and future conditions.

It is difficult to imagine how conserving biodiversity and ecosystem services could be possible in predicted future scenarios without rewilding. Simply stated, anthropogenic environmental forcing makes ecosystem restoration a diminishing option [du Toit & Pettorelli 2019: 4].

Reintroducing rewilding to restoration – rejecting the search for novelty, Hayward et al. 2019

This perspective piece argues against scientific or public adoption of the term “rewilding,” which the authors view as being generally synonymous with the classical and better-understood concept of ecological restoration. Definitions of restoration are sufficient to encompass practices espoused in rewilding.

Early definitions of restoration describe the practice as “the process of repairing damage caused by humans to the diversity and dynamics of indigenous ecosystems” (Jackson et al., 1995). Although at the time of this definition, restoration science was still developing, it was clear that it had established itself under the broad banner of repairing damaged ecosystems. … More recently, restoration has been defined as “any activity whose aim it is to ultimately achieve ecosystem recovery, insofar as possible and relative to an appropriate local native model (termed here a reference ecosystem), regardless of the period of time required to achieve the recovery outcome” (McDonald et al., 2016) [Hayward 2019: 257].

The term rewilding, which has evolved over time, “was arguably conceived to promote the original authors’ view of conservation via cores [habitats], corridors, and carnivores” [Hayward 2019: 256]. In this early context,

‘rewilding’ referred to conservation and management interventions that focused on reintroducing keystone predators and ensuring that they had sufficient interconnected space to live. The authors emphasized within their original work that rewilding was “one essential element in most efforts to restore fully functioning ecosystems” (Soulé and Noss, 1998). As such, it is clear that rewilding was originally aimed to be a term that referred to one component of ecological restoration [Hayward 2019: 256].

Since then, rewilding has come to refer to practices involving “translocating substitute species to fill vacant ecological niches left by extinct species” [Hayward 2019: 257] or reintroducing locally extinct species, or simply allowing natural succession to occur on abandoned land. Given multiple definitions, all of which relate to the idea of restoration, the term rewilding is seen here as superfluous and confusing.

Given the lack of clear differences between rewilding and restoration in both definition and practice, we see little need for these competing terms within scientific discourse [Hayward 2019: 257].

However, the authors suggest two positive contributions from the rewilding discourse. Because of its overall focus on large fauna, rewilding has captured public imagination and interest in conservation, while also helping to shift a potential vegetation bias among restoration practitioners/scientists toward equal emphasis on the ecosystem role of animals.

Therefore, rather than adopting a new term with copious definitions that lack clarity, this debate can be used as an opportunity to adaptively improve current restoration practice by incorporating a more equal focus between flora and fauna [Hayward 2019: 258].

Because of its overall focus on large fauna, rewilding has captured public imagination and interest in conservation, while also helping to shift a potential vegetation bias among restoration practitioners/scientists toward equal emphasis on the ecosystem role of animals.

Intact forests in the United States: proforestation mitigates climate change and serves the greatest good, Moomaw 2019

The concept of “proforestation” presented here means letting existing forests continue to grow and reach their full ecological potential. Due to intensive management practices, most existing forests sequester carbon at only half (or less) of their potential rate. In addition to storing (embodying) more carbon than their smaller counterparts, large trees also sequester carbon at a faster rate. For example, “Each year a single tree that is 100 cm in diameter adds the equivalent biomass of an entire 10-20 cm diameter tree, further underscoring the role of large trees” [Moomaw 2019: 4]. Imagine, reader, that every year you planted a whole new medium-sized tree – that’s essentially what large trees are doing.

“Each year a single tree that is 100 cm in diameter adds the equivalent biomass of an entire 10-20 cm diameter tree, further underscoring the role of large trees” [Moomaw 2019: 4].

Much of Maine’s forests have been harvested continuously for 200 years and have a carbon density less than one-third of the forests of Southern Vermont and New Hampshire, Northwestern Connecticut and Western Massachusetts – a region that has not been significantly harvested over the past 75-150 years. …

Ecosystem services accrue as forests age for centuries. Far from plateauing in terms of carbon sequestration (or added wood) at a relatively young age as was long believed, older forests (e.g., >200 years of age without intervention) contain a variety of habitats, typically continue to sequester additional carbon for many decades or even centuries, and sequester significantly more carbon than younger and managed stands [Moomaw 2019: 5].

Because existing forests are already sequestering carbon, and will continue at an increasing rate as tree size grows, the author argues that proforestation is a more effective immediate solution than either reforestation (planting new trees where they had been cleared for agriculture, etc.) or afforestation (planting trees in new places), though these other two approaches are important for longer term ecosystem function. Moomaw et al. argue that the urgency of removing CO2 makes it imperative to keep existing forests growing.

Globally, existing forests only store approximately half of their potential due to past and present management, and many existing forests are capable of immediate and even more extensive growth for many decades. During the timeframe while seedlings planted for afforestation and reforestation are growing (yet will never achieve the carbon density of an intact forest), proforestation is a safe, highly effective, immediate natural solution that does not rely on uncertain discounted future benefits inherent in other options [Moomaw 2019: 7].

Furthermore, existing, older forests are critical habitats for threatened wildlife, even small intact woods.

Forest bird guilds also benefit from small intact forests in urban landscapes relative to unprotected matrix forests. Several bird species in the U.S. that are globally threatened – including the wood thrush, cerulean warbler, marbled murrelet, and spotted owl are, in part, dependent on intact, older forests with large trees [Moomaw 2019: 5].

In sum, proforestation provides the most effective solution to dual global crises – climate change and biodiversity loss. It is the only practical, rapid, economical, and effective means for atmospheric CDR [carbon dioxide removal] among the multiple options that have been proposed because it removes more atmospheric carbon dioxide in the immediate future and continues to sequester it long-term. Proforestation will increase the diversity of many groups of organisms and provide numerous additional and important ecosystem services. While multiple strategies will be needed to address global environmental crises, proforestation is a very low-cost option for increasing carbon sequestration that does not require additional land beyond what is already forested and provides new forest related jobs and opportunities along with a wide array of quantifiable ecosystem services, including human health [Moomaw 2019: 8].

Plant diversity enhances the reclamation of degraded lands by stimulating plant-soil feedbacks, Jia et al. 2020

This study tested biodiversity effects on ecosystem function in the process of reviving severely degraded and contaminated land, and found that “increasing plant diversity greatly enhanced the reclamation of these lands” [Jia 2020: 1].  

Prior to implementing the reclamation experiment, the degraded mine wasteland investigated in this study was heavily impacted by past mining activities and was devoid of vegetation for more than a decade and the soil lacked structure, contained high levels of toxic metals and low levels of nutrients. … our results showed that higher plant species richness enhanced land reclamation across all standard measures of reclamation success and specifically resulted in higher vegetation coverage, biomass yield and stability for all 3 years [of the experiment] [Jia 2020: 6].

Furthermore, higher biodiversity plots had higher levels of organic carbon in the soil, higher soil microorganism abundance, lower fungal pathogens, and lower heavy metal concentrations in plant tissue.

The most striking impact of plant diversity on soil was on the microbial communities. Both soil fungal and bacterial OTUs [operational taxonomic units^[8]] increased significantly with plant species richness. More importantly, we found that higher plant species richness significantly increased the relative abundance of soil cellulolytic bacteria that degrade cellulose and are thus essential components of nutrient cycling [Jia 2020: 7].

High ecosystem service delivery potential of small woodlands in agricultural landscapes, Valdes 2020

This article assesses the ecological value of small woodlands relative to larger ones. The authors conclude that:

…smaller woodlands potentially deliver multiple services at higher performance levels on a per area basis than larger woodlands of a similar age, even if the larger woodlands harbor a higher biodiversity [Valdes 2020: 12].

Because of their high edge-to-core ratio, smaller woodlots get more sunlight and more nutrient input from surrounding farmland, resulting in denser vegetation cover and higher biomass production at edges.

This altered functioning in turn increases the delivery potential of some services, such as game production potential, due to an increased quantity of food available for game, and topsoil carbon storage, due to the faster incorporation of organic matter in the soil. Tick-borne disease risk is, however, lower, likely due to decreased larval densities in the unfavorable (e.g. hotter and drier) microclimatic conditions at the edge [Valdes 2020: 12].

While smaller woodlands were more apt to deliver “multiple services at higher levels of performance per area than larger woodlands of a similar age,” the greater biodiversity of larger woodlands increased certain individual ecosystem services.

The supply potential of several individual ecosystem services indirectly increased in larger and more ancient woodlands because it was dependent on higher levels of biodiversity. For example, abundance of usable plants and game production potential might have increased due to a positive correlation with vascular plant diversity, while pest control potential probably increased due to bottom-up effects through the trophic chain. On the contrary, tick-borne disease risk, topsoil carbon storage and stemwood volume were unrelated to multidiversity, probably because they depended on particular environmental conditions or on the presence and abundance of specific species rather than on species richness per se.

Finally, it should be noted that we focused on the service delivery potential on a per area basis and that the total amount of services provided by large patches might still be larger than that of small patches. Our findings should therefore not be interpreted as a trade-off between large, biodiverse patches versus small patches that have a higher potential to deliver services, but rather as an observation that small woodlands in agricultural landscapes have the potential to deliver a high flow of services relative to their size [Valdes 2020: 12].

Effectiveness of the Miyawaki method in Mediterranean forest restoration programs, Shirone, Salis & Vessela 2011

This study tested the Miyawaki method of rapid natural forest regeneration (which has been shown to work in Japan and elsewhere) in the arid Mediterranean. In this area, millennia of human civilization have resulted in degraded soils and reduced and changed forest cover, traditional reforestation efforts have often failed, and desertification is a looming threat. The Miyawaki method speeds up the process of ecological succession by densely planting a multilayer forest made up of a wide diversity of indigenous species.

In a natural forest cycle, as Clements (1916) described, annual plants on barren land are succeeded by perennial grass, sun-tolerant shrubs, light-demanding, fast-growing trees, and finally natural forests; each step may require decades, and the climax vegetation could be formed after two centuries or more. Currently, most forest reforestation programs adopt a scheme of planting one or more early successional species; after successful establishment, they are gradually replaced by intermediate species (either naturally or by planting), until late successional species arise. This pattern tries to simulate natural processes of ecological succession, from pioneer species to climax vegetation. However, it requires several silvicultural practices and normally takes a long time [Shirone 2011: 82].

In the Miyawaki method, by contrast, one plants all at once the many plant species normally present in a native forest community, thus bypassing the earliest stages of ecological succession. Other tree-planting methods favor fast-growing non-native tree species, while omitting understory species – in other words, creating a simple plantation rather than a forest community that functions ecologically and can evolve and sustain itself.

Multilayer forests, and natural biocoenosis [ecological community] is possible, and well-developed ecosystems can be quickly established because of the simultaneous use of intermediate and late successional species in plantations. The Miyawaki method involves surveying the potential natural vegetation of the area to be reforested and recovering topsoil to a depth of 20– 30 cm by mixing the soil and a compost from organic materials, such as fallen leaves, mowed grass, etc. In this way, the time of the natural process of soil evolution, established by the vegetational succession itself, is reduced [Shirone 2011: 82].

The authors of this study found that compared to traditional reforestation, there was “a more rapid development of trees on the Miyawaki plots, in particular, early-successional species [especially maritime pine]. The benefits over previous methods are remarkable and comparable with those obtained by Miyawaki in Asia and South America.” The Miyawaki method favoring denser plantings works even in arid climates, in spite of traditional views favoring sparse plantings in arid places, although the optimal density for the Mediterranean still needs testing, according to the authors.

In fact, low plant density has been traditionally retained as appropriate in arid and semiarid environments in order to avoid competition for water resources between plants, but it is now evident that cooperative processes, e.g., mutual shading, prevail over competitive processes. High plant density also reduces the impact of acorn predators, thus encouraging oak regeneration, i.e., the main late-successional forest species in Mediterranean environments. In addition, excellent plant stock remains fundamental for planting success in harsh environments [Shirone 2011: 91].

When is a forest a forest? Forest concepts and definitions in the era of forest and landscape restoration, Chazdon et al. 2016

This article analyzes the policy context for forest ecosystem restoration, arguing that it is heavily shaped by the way we define a forest. The use of a forest definition lacking ecological considerations severely undermines conservation and restoration initiatives.  

We live in an era of unprecedented environmental change, motivating equally unprecedented global actions to protect and restore forest ecosystems. These efforts could fail to achieve their ambitious goals if they are not informed by clear and appropriate concepts and definitions of forests [Chazdon 2016: 1].

There are multiple definitions of a forest. Early European and internationally adopted definitions tended to define forests according to their usage for timber. FAO’s 1948 definition created for assessing wood harvesting potential of the world’s forests is still in use today. Yet new definitions have since been created that emphasize conservation, carbon sequestration and biodiversity values of forests.

However, national and global forest assessments tend to use narrow technical definitions that ignore ecological values of forested land.

In many cases, forest assessments do not distinguish between land covered by natural and planted forests. Thus, if natural forests are cleared and replaced with plantations, no net loss of forest cover is reported [Chazdon 2016: 6].

In other words, areas that should not be considered forest in ecological terms are counted as forest – an obfuscation with disastrous environmental outcomes. Similarly, ecologically important yet small patches of trees that are not counted in forest inventories and lack legal protection are at risk of being lost.

Areas classified as ‘‘non-forests’’ are as important to forest definitions as are forests. More than 43 % of agricultural land globally is in agroforestry systems with 10 % tree cover. In Rwanda and Brazil, forest inventories using a 0.5-ha threshold ignore substantial areas of small forest fragments, agroforests, and woodlots, leading to underestimates of actual tree cover. Small patches of trees and even isolated remnant trees can hold high ecological and conservation value, and can play an important role in enhancing landscape connectivity, local biodiversity, and local livelihoods [Chazdon 2016: 7].

Information from participatory local monitoring and remote sensing technology that distinguishes “among successional stages of forests, selectively logged forests, and single-species plantations” [Chazdon 2016: 10] is needed.

Access to this information will allow countries and international agencies to track changes in natural forest cover, and to monitor processes of restoration, rehabilitation, and afforestation within a landscape context and, consequently, make informed policy decisions. We are on the frontier of developing new ways of monitoring and assessing land cover that will provide robust indicators of the quality and origins of tree cover and enable new ways of viewing and defining forests and reforests. To see beyond the overly simplified categories of forest loss, forest degradation, and forest gain, we need to develop and apply more adapted and nuanced definitions that will deepen our understanding of the drivers and outcomes of land-use change and forest dynamics within landscapes [Chazdon 2016: 10].