Karen Holl

University of California, Santa Cruz | UCSC · Department of Environmental Studies

Tropical forest restoration presents a potential lifeline to mitigate climate change and biodiversity crises in the Anthropocene. Yet, the extent to which human interventions, such as tree planting, accelerate the recovery of mature functioning ecosystems or redirect successional trajectories toward novel states remains uncertain due to a lack of l...

Many terrestrial plant communities, especially forests, have been shown to lag in response to rapid climate change. Grassland communities may respond more quickly to novel climates, as they consist mostly of short-lived species, which are directly exposed to macroclimate change. Here we report the rapid response of grassland communities to climate...

The effects of vertebrate seed predation on the regeneration of restored forests are not well understood because most past studies have focused on seed predation within the first few years after restoration and have measured seed removal without quantifying subsequent seedling establishment of seeds that avoid predation. Quantifying the establishme...

Transformative change is needed to align common small‐scale ecological restoration approaches with expectations to restore millions of hectares of degraded lands globally. Currently, most restoration projects target small areas using costly manual methods that cannot be scaled up to meet global commitments. We propose that a judicious integration o...

Applied nucleation and other spatially patterned restoration methods are promising approaches for scaling up projects to meet ambitious international restoration commitments in an ecologically and economically sound manner. Much of the corresponding literature to date, however, has centered around theoretical discussions and small‐scale studies tha...

Choosing effective methods to restore habitat for the diverse faunal assemblages of tropical forests is hampered by lack of long-term data comparing multiple restoration treatments. We conducted area counts of bird assemblages over 12 years (∼5–17 years since restoration) in a blocked experiment with two active planted treatments (tree plantations...

With increased interest in forest restoration comes an urgent need to provide accurate, scalable, and cost-effective monitoring tools. The ubiquity of smartphones has led to a surge in monitoring apps. We reviewed and assessed monitoring apps found through web searches and conversations with practitioners. We identified 42 apps that (1) automatical...

As global tree‐growing efforts have escalated in the past decade, copious failures and unintended consequences have prompted many reforestation best practices guidelines. The extent to which organizations have integrated these ecological and socioeconomic recommendations, however, remains uncertain. We reviewed websites of 99 intermediary organizat...

The fundamental goal of a rare plant translocation is to create self‐sustaining populations with the evolutionary resilience to persist in the long‐term. Yet most plant translocation syntheses focus on a few factors influencing short‐term benchmarks of success (e.g., survival and reproduction). Short‐term benchmarks can be misleading when trying to...

Restoring the structural complexity and functional diversity of tropical rainforest is not possible in human time scales but knowledge of the process has significantly increased over the past three decades. Strategies to restore tropical forests must build on theories of community assembly and succession, as well as understanding of both the local...

Governmental and non-governmental organizations spend considerable funding on restoring ecosystems to counter biodiversity loss, yet outcomes are often not assessed at a regional scale. Monitoring is done ≤5 years after project-implementation, if at all, and rarely assesses the effects of management practices on project success. We combined vegetat...

Livestock grazing is often used as a land management tool to maximize vegetation diversity in grassland ecosystems worldwide. Prior research has shown that cattle grazing benefits native annual forb species in California’s coastal prairies, but drought and increasing aridity may alter this relationship. In 2016 and 2017, we resurveyed the vegetatio...

Forest restoration is increasingly heralded as a global strategy to conserve biodiversity and mitigate climate change, yet long-term studies that compare the effects of different restoration strategies on tree recruit demographics are lacking. We measured tree recruit survival and growth annually in three restoration treatments—natural regeneration...

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Reversing large-scale habitat degradation and deforestation goes beyond what can be achieved by site-level ecological restoration and a landscape ecology perspective is fundamental. Here we assess the relative importance of tree cover and its configuration on forest-dependent birds and late-successional tree seedlings in restoration sites in southe...

Global mapping efforts to date have relied on vague and oversimplified definitions of “abandoned” agricultural land which results in overestimates of the land area that is likely to support persistent increases in forest cover and associated carbon sequestration. We propose a new conceptualization of abandoned agricultural land that incorporates ch...

Choosing appropriate forest restoration interventions is challenging. Natural regeneration can rapidly facilitate forest recovery in many situations. However, barriers such as dispersal limitation and competition with non-native species can require assisted restoration approaches to facilitate plant community recovery. We used a study that has dire...

The central motivation to restore ecosystems at a planetary scale has been to reverse degradation and provide multiple environmental benefits, but key global players like governments may be more interested in social outcomes from undertaking restoration, such as job creation. Assessing the job opportunities stemming from ongoing restoration program...

Understanding tropical biology is important for solving complex problems such as climate change, biodiversity loss, and zoonotic pandemics, but biology curricula view research mostly via a temperate-zone lens. Integrating tropical research into biology education is urgently needed to tackle these issues. Tropical biology is currently largely absent...

Extensive evidence shows that regional (gamma) diversity is often lower across restored landscapes than in reference landscapes, in part due to common restoration practices that favor widespread species through selection of easily-grown species with high survival and propagation practices that reduce genetic diversity. We discuss approaches to coun...

The vast sums of money being spent on planting trees have the potential to transform landscapes and slow global warming but will accomplish little if trees do not survive and grow. We discuss ten key questions to decide which of the numerous tree-growing projects are most likely to succeed.

In a recently published paper, Silva et al. conclude that placing fruit feeders (induced seed dispersal) in restored tropical forests serves to enrich the diversity of animal species and seeds in recovering forests and therefore is a cost‐effective tropical forest restoration strategy. We argue that due to poor experimental design and choice of res...

Large‐seeded, animal‐dispersed (LSAD) trees include some of the most valuable and threatened species in the tropics, but they are chronically underrepresented in regenerating forests. Toucans disperse many LSAD species, so attracting toucans to regenerating forests should help re‐establish more diverse tree communities. We ask: (1) What constitutes...

Remnant trees and forest fragments in agricultural landscapes can be important sources of propagules to facilitate forest recovery. However, many studies simply quantify forest cover in the surrounding landscape as a percentage, with little attention given to species composition, and subsequently fail to detect an effect on recruitment patterns. We...

Insect herbivory is one of the major drivers of seedling mortality in the tropics and influences plant abundances and community composition. Anthropogenic disturbance can alter patterns of insect herbivory with potential consequences on plant communities in restored forests. We planted seedlings of early‐ and later‐stage successional tree species i...

Climate change will increase uncertainty in restoration outcomes due to greater water stress and other abiotic filters that limit plant survival. Drought‐related plant functional traits can help species withstand filters in a semi‐arid environment. Our objective was to provide guidance for selecting species to improve restoration success in a chang...

In degraded tropical landscapes, lack of seed dispersal can strongly limit recovery, and restoration interventions can overcome this barrier by attracting dispersers. However, seed dispersal patterns are typically studied over short time periods, thus the influences of temporal and spatial variability on seed arrival cannot be teased apart. The cho...

Applied nucleation (i.e., planting vegetation patches) is a restoration strategy that better recreates natural ecosystem heterogeneity and requires fewer resources compared to planting the entire area. Whereas applied nucleation shows promise as a forest restoration strategy, this approach has received little study in grassland restoration, where t...

Reintroduction is an increasingly common practice to conserve and recover threatened and endangered plant species, so understanding how practitioners view their work and identifying persistent resource mismatches are key to the long-term viability of these listed species. We interviewed practitioners involved in reintroduction projects for 14 speci...

Successful restoration of California terrestrial ecosystems requires knowledge of seed germination rates for a diversity of native species. We quantified greenhouse germination of 25 native perennial forbs, shrubs, and grasses collected coastally near Santa Cruz, CA for restoration at the Younger Lagoon Reserve and nine annual forb species from int...

To constrain global warming, we must strongly curtail greenhouse gas emissions and capture excess atmospheric carbon dioxide1,2. Regrowing natural forests is a prominent strategy for capturing additional carbon³, but accurate assessments of its potential are limited by uncertainty and variability in carbon accumulation rates2,3. To assess why and w...

A growing number of initiatives at global, regional and national scales propose to plant millions, billions or even trillions of trees as a simple solution to resolve complex environmental problems. However, tree planting is much more complicated than it seems. We summarize the multifaceted decision‐making process needed and offer guidelines to inc...

Applied nucleation, mostly based upon planting tree islands, has been proposed as a cost‐effective strategy to meet ambitious global forest and landscape restoration targets. We review results from a 15‐year study, replicated at 15 sites in southern Costa Rica, that compares applied nucleation to natural regeneration and mixed‐species tree plantati...

Both dispersal‐ and niche‐based factors can impose major barriers on tree establishment. Our understanding of how these factors interact to determine recruitment rates is based primarily on findings from mature tropical forests, despite the fact that a majority of tropical forests are now secondary. Consequently, factors influencing seed limitation...

Tree planting must be carefully planned and implemented to achieve desired outcomes

Plant traits—the morphological, anatomical, physiological, biochemical and phenological characteristics of plants—determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research sp...

Limited funding is a major barrier to implementing ambitious global restoration commitments, so reducing restoration costs is essential to upscale restoration. The lack of rigorous analyses about the major components and drivers of restoration costs limit the development of alternatives to reduce costs and the selection of the most cost-effective m...

To meet their large-scale forest and landscape restoration targets, countries must find ways to accommodate areas for conservation alongside agricultural production. In some pasture-dominated regions of Latin America, intensive silvopastoral systems (SPS) are being promoted to increase cattle productivity on certain lands while facilitating the rem...

International forest landscape restoration commitments have promoted the restoration of millions of hectares of degraded and deforested lands globally, but few forest restoration approaches provide both ecologically‐sound and financially‐viable solutions for achieving the spatial scale proposed. One potential revenue source for restoration is selec...

Conservation and restoration of riparian vegetation in agricultural landscapes has had mixed success at protecting in‐stream habitat, potentially due to the mismatch between watershed‐scale impacts and reach‐scale restoration. Prioritizing contiguous placement of small‐scale restoration interventions may effectively create larger‐scale restoration...

Tropical grassland-savanna mosaics are threatened globally, but they are challenging to restore because highly competitive pasture grasses inhibit recovery and are not shaded out by the patchy tree cover. We analyzed the outcomes of restoration projects and experiments established over four years in 55 ha of abandoned pastures dominated by invasive...

We welcome the opportunity to further discuss our analysis and conclusions [1] that Larkin et al.'s [2] (hereafter LEA) comment provides. In this response, we first discuss mischaracterizations and criticisms of our analyses, then highlight how the main conclusions from both LEA's and our analyses are similar, and end with further discussion of wha...

Defining the reference system for restoration projects in regions characterized by complex vegetation mosaics is challenging. Misidentification of anthropogenically‐degraded grasslands or savannas as natural vegetation can lead to inappropriate restoration efforts and legislation. Here we use the Cerrado region of Brazil as an example of the import...

Bangladesh provides a significant global public good by hosting nearly one million Rohingya refugees. Most are living in camps in Cox’s Bazar District, where resources and livelihoods are strained. The refugee situation is likely to be protracted, and medium-term planning is critical. CGD has been working with local and international partners to un...

In tropical areas with high levels of fragmentation due to agricultural use, forest fragments play an important role for biodiversity conservation at the landscape scale. But these fragments are subject to recurrent disturbances, which lead to arrested succession and loss of functional groups. In such cases, active restoration, such as enrichment p...

Restoration frequently aims to improve native species biodiversity at a site, but practitioners have limited resources. In diverse ecosystems, the selective use of certain guilds or species can come at the cost of species that are more challenging to incorporate, resulting in the overall homogenization of the ecosystem and a relative loss of biodiv...

Questions Natural regeneration is increasingly recognized as a potentially cost‐effective strategy to reach ambitious forest landscape restoration targets, but rates of recovery are notoriously variable. We asked how well initial habitat conditions after cessation of agriculture predict forest recovery after nearly a decade. We aimed to provide lan...

We agree with Ramírez-Soto and colleagues that applied nucleation can be an effective approach for tropical forest restoration both in lowland and higher elevation tropical forests. We also contend that it is cheaper than standard plantation-style plantings and is straightforward to train personnel to implement this approach.

Given that few ecosystems on the Earth have been unaffected by humans, restoring them holds great promise for stemming the biodiversity crisis and ensuring ecosystem services are provided to humanity. Nonetheless, few studies have documented the recovery of ecosystems globally or the rates at which ecosystems recover. Even fewer have addressed the...

Multi-year studies comparing changes in litterfall biomass and nutrient inputs in sites under different restoration practices are lacking. We evaluated litterfall dynamics and nutrient inputs at 5 yr and after a decade of recovery in four treatments (natural regeneration—no planting, plantation—entire area planted, tree islands—planting in patches,...

In the past few years, numerous global, national, and regional targets have been set to restore millions of hectares of tropical forest to achieve multiple goals, including carbon sequestration, biodiversity conservation, improvements in the quality and supply of water, and support of human livelihoods. To achieve these ambitious goals, restoration...

Why do some restored ecosystems persist for centuries while others are quickly converted to alternative land uses or land covers? We propose that restored ecosystems have a temporal dimension that is variable, often finite, and likely predictable to some extent based on attributes of stakeholders, environment, and governance. The longevity of a res...

Monitoring protocols are needed to evaluate the millions of hectares of land that are being proposed for forest restoration in the coming decades. Standardized proposals are critical to evaluate efficacy of restoration strategies, identify triggers for corrective actions, compare results across projects, and generally learn from past projects to in...

Recent initiatives at regional, national, and global scales have called for unprecedented levels of forest restoration to counteract decades of rapid deforestation ( 1 , 2 ). Thus far, 30 countries have committed to restore 91 million hectares (ha) of deforested landscapes, an area the size of Venezuela, by 2020; at the 2014 United Nations Climate...

Global forest restoration targets have been set, yet policy makers and land managers lack guiding principles on how to invest limited resources to achieve them. We conducted a meta-analysis of 166 studies in naturally regenerating and actively restored forests worldwide to answer: (1) To what extent do floral and faunal abundance and diversity and...

PRISMA 2009 flow diagram. (DOC)

(a) Relationship between the original response ratio (RR) and transformed RR; (b) Normal quantile plot of RR raw data. (BMP)

Details on database compilation and variables. (DOCX)

PRISMA 2009 checklist. (DOC)

Relative importance of factors estimated by model averaging. (DOCX)

Degradation level. (a) After all previous land-use types and (b) in actively and passively restored sites after agriculture only. (BMP)

Distribution of number of variables and studies in each category. (DOCX)

Results of model comparisons. (DOCX)

Developing restoration strategies that accelerate natural successional processes and are resource‐efficient is critical to facilitating tropical forest recovery across millions of hectares of deforested lands in the tropics. We compared tree recruitment after a decade in three restoration treatments (natural regeneration, applied nucleation/island...

A substantial share of the remaining tropical forest cover is represented by historically degraded fragments exposed to severe edge effects, where ruderal plants proliferate vigorously and may arrest succession. We tested climber plant cutting as strategy to restore a semideciduous tropical forest remnant that is dominated by ruderal climbers. We c...

Establishing diverse mycorrhizal fungal communities is considered important for forest recovery, yet mycorrhizae may have complex effects on tree growth depending on the composition of fungal species present. In an effort to understand the role of mycorrhizal fungi community in forest restoration in southern Costa Rica, we sampled the arbuscular my...