s43247-024-01970-y.pdf

communications earth & environment Article https://doi.org/10.1038/s43247-024-01970-y eDNA offers opportunities for improved biodiversity monitoring within forest carbon markets Check for updates Michael C. Allen 1,2 , Julie L. Lockwood 1, Rosa Ibanez1, Josh D. Butler3,J o r d a nC .A n g l e4 & Benjamin D. Jaffe5 Forest-based carbon sequestration projects incentivize reforestation and restoration activities while offering opportunities to realize co-bene fits such as biodiversity conservation. While conservation aspects are increasingly emphasized in these projects, the rigor of biodiversity co-bene fit verification has been highly variable. Recent advances in biodiversity monitoring based on shed DNA in the environment (eDNA) offer promise for improving effectiveness, standardization, and transparency. Here we analyze 129 forest carbon projects and 396 peer-reviewed studies to identify how biodiversity co-benefits are currently veri fied within forest carbon markets, and to evaluate the potential of eDNA for tracking biodiversity change. Our analysis revealed that eDNA studies focused more on smaller organisms (microbes and invertebrates) and on temperate ecosystems compared with biodiversity- focused forest carbon projects. Efforts to align these two worlds via investments into broadening the geographic and taxonomic scope could allow greater adoption and increased accountability in biodiversity monitoring within forest carbon markets (i.e. standardized, auditable biodiversity data trails). Adapting advancements in eDNA technology to the biodiversity monitoring needs of nature- based initiatives will aid countries and organizations striving to meet global conservation commitments. Forest-based carbon (FC) sequestration projects are the primary nature- based solution (NbS) for climate change mitigation1,2 and potentially offer multiple co-bene fits, including supporting bi odiversity conservation3. However, FC projects have inconsistently documented and certified bio- diversity co-benefits4–6, resulting in calls for greater standardization and effectiveness7–10. Technological innovation offers promising new tools for scalable, standardized, and auditablebiodiversity monitoring within these projects11–13. Techniques that identify the presence of organisms based on shed genetic material— notably environmental DNA or eDNA— are espe- cially promising14,15, yet their use within FC markets remains largely unexplored. Land-use changes, particularly deforestation, are the primary drivers of terrestrial biodiversity loss16. In response, forest restoration, management, and conservation (i.e., avoided conversion) are often counted upon to help minimize these losses 7,17. Recently, privately funded projects, relying on similar approaches but focused on sequestering or conserving forest carbon stocks, have emerged 5,18. While the synergies between FC projects and biodiversity conservation are readily apparent19, there is an increased focus on validating and verifying the purported co-bene fits4,5,10,20. The recent emphasis placed on high-q uality carbon credits 18,21, and on monitoring outcomes within nature-based climate solutions7,9, suggests that expecta- tions around quantifyingbiodiversity co-benefits will continue to increase. While efforts to track and certify biodiversity benefits within FC pro- jects have improved over time, they can still lack rigor 4,5,10.T oa c h i e v e biodiversity certification within an FC scheme, a project typically must demonstrate a net positive impact on biodiversity (Table1). However, the associated criteria to support these certifications vary across programs20,22,23 and the indicators and associated monitoring programs are often viewed as inadequate4,6,9,22,24. Improving standardization,transparency, and monitor- ing rigor helps ensure positive biodiversity outcomes, and supports the long- term viability of biodiversity certification schemes4,5,21. Emerging technology for biodiversity monitoring — including advancements in the utilization of aerial imagery, LiDAR, acoustic recordings, and eDNA— appear well-suited to support biodiversity-based 1Department of Ecology, Evolution, and Natural Resources, Rutgers, The State University, New Brunswick, NJ, USA.2Tsuga Biodiversity Insights LLC, Readington, NJ, USA. 3ExxonMobil Biomedical Sciences Inc., Annandale, NJ, USA. 4ExxonMobil Biomedical Sciences Inc., Spring, TX, USA. 5ExxonMobil Research Qatar, Doha, Qatar. e-mail: [email protected] Communications Earth & Environment | (2024) 5:801 1 1234567890():,; 1234567890():,; certification within FC markets11,13,24,25. eDNA-based methods, which detect traces of genetic material left in the environment, are especially promising; they offer broad taxonomic reach, straightforward field sampling, and efficiencies at scale 14,15,26. A wide variety of taxa, from microbes to verte- brates, can be simultaneously identi fied from a single field sample (e.g., water, soil, air, or surface swabs), per mitting holistic insights into biodi- versity without the bespoke monitoring typically needed for any individual taxa 6,14. Such high-throughput species determination— often achieved using metabarcoding, the mass sequencing of short DNA fragments found within samples— c a na l s ol e a dt oc o s te fficiencies by supplanting labor-intensive morphological approaches to species identification27,28. Both metabarcoding and single species eDNA approaches (e.g., qPCR, ddPCR) have already revolutionized biodiversity moni toring and management in aquatic habitats14,15,29 and are increasingly being applied in terrestrial habitats to enhance detectability of rare species 30 and to monitor conservation outcomes31. In the context of FC markets, eDNA-based methods produce an auditable data trail that serves as a permanent record of species detected at a surveyed site 32–35. Because of these bene fits, there is growing interest for incorporating eDNA-based methods into national and global biodiversity monitoring schemes, many of which leverage local capacity for implementation 25,26,36,37. The breadth of eDNA-based methods available to monitor terrestrial above-ground biodiversity is now extensive28,30,35,38,39. As a result, there is value in considering how eDNA-based monitoring can be integrated into FC projects. In forested ecosystems, eDNA-based biodiversity surveys have been used to monitor specifics p e c i e s 30,39,40 and taxonomic assemblages such as mammals28,40–42,b i r d s34,a m p h i b i a n s27, arthropods32,35,43,44,f u n g i33,a n d plants45,46. In parallel, there is also a growing body of literature, highly relevant to FC projects, on the application of eDNA to track forest restoration progress. These studies chart the recovery of plants, fungi, bacteria, arthropods, and vertebrates post-restoration 32–35,47,48. Despite the potential bene fits, implementation of eDNA within FC projects has been limited49. Best practices are emerging for eDNA-based biodiversity monitoring within forests and other ecosystems25,30,31.A tt h e same time, guidelines for biodiversi ty monitoring within forest carbon projects have matured, with recommendations on sampling strategies, taxonomic focus, and field methods 24,50,51. However, there is a notable absence of recommended best practices for marrying eDNA and biodi- versity monitoring within FC projects. Here we conduct a systematic ana- lysis to ascertain how biodiversity is currently monitored within FC projects, focusing on those that explicitly certify biodiversity co-benefits. We conduct a parallel analysis of the literatureto identify how and where eDNA-based methods are used to monitor impacts of management or restoration on biodiversity. We use a broad de finition of eDNA that includes mixed samples of trap-collected specimens asthese studies involve similar mole- cular methods and often serve similar environmental monitoring purposes. For each pool of literature, we document which taxonomic groups are monitored, their location, and whi ch methods were used. Through data synthesis techniques, we highlight the opportunities and challenges for applying eDNA as a tool to track biodiversity within FC projects. Our analysis revealed that the use of eDNA is currently rare within FC projects; and that peer-reviewed eDNA studies occur more within temperate eco- systems and focus more on smaller organisms compared with biodiversity- focused FC projects. We provide recommendations that could allow greater adoption of eDNA and increased accountability in biodiversity monitoring within FC markets. Results We located 72 peer-reviewed studies pertaining to FC markets and biodi- versity that met our review criteria (Table2, Supplementary Fig. 1). Of those, 7 (10%) had an explicit focus on biodi versity monitoring practices and reported either the empirical findings of biodiversity monitoring (arthropods 49,52,m a m m a l s49,o rp l a n t s53,54) ,t h er e l a t i v em e r i t so fd i f f e r e n t monitoring methodologies6,24, or reviewed bigger-picture approaches 4. Most studies covered the effectiveness of FC project activities towards Table 1 | Characteristics of the four standards a used to verify biodiversity co-bene fits for the forest carbon (FC) projects considered in this analysis Standard Total number of projects Project types (no. projects) b Total area (106 ha) Total annual emissions reductions (106 Mt CO2e) Biodiversity monitoring requirements Climate, Community, and Biodiversity (CCB) Standard 80 Afforestation (50); avoided deforestation (29); improved management (1) 6.6 38.2 Requires a detailed biodiversity monitoring plan with “monitoring tasks (specific activities for measuring each indicator), staf fing, timeline, and costs associated”23 PlanVivo 26 Afforestation (12), avoided deforestation (14), improved management (8) 0.1 0.7 Requires a “Biodiversity and ecosystem service monitoring plan ” that details biodiversity indicators and monitoring approach Gold Standard 20 Afforestation (20) 0.05 0.4 Requires a “Sustainability Monitoring Plan ” with indicators and monitoring methods for each certi fied Sustainable Development Goal benefit (including biodiversity) Sustainable Development Verification Impact Standard (SD VISta) 3 Avoided deforestation (1), improved management (3) 0.2 4.4 Requires a “Natural Capital and Ecosystem Services Monitoring Plan ” that includes biodiversity indicators, monitoring methods, and monitoring frequency aPlanVivo and Gold Standard are standalone programs that verify carbon sequestration alongside biodiversity and other sustainable development benefits; CCB and SD VISta are add-on programs that allow existing forest carbon projects (generally under the Verra registries’ Verified Carbon Standard) to also verify biodiversity and sustainable development bene fits5. See Table 2 and Supplementary Note 1 for more information on the standards programs and the criteria for including projects in this analysis. bSome PlanVivo and SD VISta projects included multiple project activities. https://doi.org/10.1038/s43247-024-01970-y Article Communications Earth & Environment | (2024) 5:801 2 Table 2 | Methods used to characterize the literature on biodiversity monitoring within voluntary forest carbon (FC) markets, and on biodiversity monitoring for ecological impact using environmental DNA (eDNA) Literature pool Source a First filter Second filter Data collection Biodiversity considerations within FC markets Peer-reviewed literature ( n = 4056 total studies retrieved) Mentions forest carbon markets and biodiversity in the Title, Abstract, or Keywords ( n = 152) Both forest carbon markets and biodiversity are a substantial focus of main text ( n = 72) • Thematic focusd • Specific discussion of biodiversity monitoring practices (y/n) • Geographic location Biodiversity monitoring practices within FC projects on the voluntary market Documents for FC projects certi fied as providing sustainability or biodiversity co- benefits (n = 451 total projects) b FC projects that have progressed to the stage of reporting on biodiversity monitoring accomplishments (n = 131)c FC projects with reports made available online (n = 129) • Project typee • Taxa monitoredf • Methods usedg • Geographic location • Area of project Biodiversity and ecological impact monitoring using eDNA Peer-reviewed literature ( n = 2047 total studies retrieved) Study is based on empirical data and employs eDNA-based biodiversity monitoring ( n = 1832) Study evaluates the impact of an ecological intervention or gradient on some aspect of biodiversity (n = 324) • Taxa and habitats monitored f • Ecological intervention or gradient studiedh • Field and molecular methods usedi • Geographic location aSearch terms used to obtain peer-reviewed literature from Web of Science are in Supplementary Figs. 1 and 2. bFrom four primary co-benefit certification programs5 on the voluntary forest carbon market: Climate Community and Biodiversity (CCB) Standard [ n = 309; www.verra.org/programs/ccbs/]; Gold Standard [n = 55; www.goldstandard.org]; PlanVivo [n = 75; www.planvivo. org], and Sustainable Development Veri fied Impact Standard (SD VISta) [ n = 11; www.verra.org/programs/sd-verified-impact-standard/]. Project databases accessed March 2023. Additional inclusion details in Supplementary Note 1. cIncludes all validated CCB projects under the 3rd edition of the standard that had submitted at least provisional veri fication documents (n = 82 projects); all certi fied Gold Standard projects purporting to support Sustainable Development Goal (SDG) 15 ( “Life on Land”; n = 20); all verified forest-based PlanVivo projects (n = 26); and all forest-based SD VISta projects certified as supporting SDG 15 (n = 3). Note that two CCB projects had no reports available online and therefore did not pass the second reviewfilter. Project databases accessed March 2023. Additional inclusion details in Supplementary Note 1. dMaximizing effectiveness, optimizing location, minimizing unintended adverse effects, or monitoring biodiversity. eAfforestation, improved forest management, or avoided deforestation (i.e., Reduced Emissions from Deforestation and Forest Degradation in Devel oping Countries, or REDD +, type projects). fTaxa categories: mammals, birds, reptiles, amphibians, fish, invertebrates, plants, fungi, protists, or prokaryotes. Habitat categories: marine, freshwater, forest, grassland, or other terrestrial. gFormal visual observation methods (e.g., transect surveys), informal sightings records, camera traps, acoustic recordings, physical capture (via traps or nets), or molecular methods including eDNA. Sampling and data transparency information were also recorded, including whether numerical results, con fidence intervals, and sample size were provided; and whether external biodiversity monitoring reports were referenced that may have contained such i nformation but were not readily available online. hHabitat structure, species interactions (including invasive species), movement barriers, anthropogenic chemical pollutants, or other abiotic f actors (e.g., nutrients, temperature gradients). iSampling substrate (sediments, water, surface, whole organisms, microbiome, or diet), sample size, molecular approach (DNA or RNA, metabarcodingor qPCR-based), genomic regions targeted, and number of molecular markers used. While some substrates we included are not typically considered eDNA (e.g., a trap filled with mixed arthropods), we track them here alongside traditional eDNA approaches as they use similar laboratory methods and are used for similar environmental monitoring purposes. https://doi.org/10.1038/s43247-024-01970-y Article Communications Earth & Environment | (2024) 5:801 3 conservation (74%), while some discussed optimizing the geographic dis- tribution of FC projects for biodiversity conservation (19%) or addressed potential negative biodiversity consequences of FC projects (17%). We tallied 1323 FC projects across thefive dominant registries on the voluntary market (Verra, American Carbon Registry, Climate Action Reserve, PlanVivo, and Gold Standard) totaling ~76.3 million ha (Supple- mentary Note 1). Of these, 451 projects (33.0 million ha) were identified as explicitly verifying biodiversity co-benefits under one of four standards (listed in Table1). Of these 451 FC projects, 129 (totaling 7.0 million ha) met our review inclusion criteria (Tables1 and 2). Of the 324 peer-reviewed eDNA studies that met our review inclusion criteria (Table 2, Supplementary Fig. 2), 95% involved eDNA meta- barcoding; 5% involved single species assays (e.g., qPCR); and 4% involved eRNA metabarcoding. Forty-one percent involved terrestrial habitats, such as forests (21%), grasslands (10%), and others such as caves or disturbed lands (10%). The most common biodiversity impacts assessed using eDNA in forested habitats were changes in habitat structure (78%), species inter- actions (10%), pollutants (6%), and other abiotic factors (e.g., salinity, temperature; 6%). Below, we compare the current state of eDNA-based biodiversity monitoring with the monitoring activities occurring within FC projects that verify biodiversity benefits. Taxonomic scope Of the 129 biodiversity-certify ing FC projects analyzed (Table 1), 75% reported some on-the-ground monitoring of animal populations; the others relied on forest cover measurements, community education, or surveillance against illegal activities to support claims of animal biodiversity enhance- ment. Among the 97 projects that monitored animal populations, most covered birds (84%), mammals (70%), reptiles (23%), amphibians (18%), invertebrates (18%), andfish (7%) (Figs.1 and 2). One FC project monitored fungi, while none monitored protists or bacteria. While all FC projects assessed trees for carbon, 74% of projects discussed native forest commu- nities (beyond carbon value), including only a few that discussed understory species (e.g., shrubs, herbs). The number of taxonomic groups studied per project ranged from 1 to 6 (median: 3; Fig.3). In contrast to taxa monitored wit hin FC projects, eDNA studies in terrestrial habitats focused on fungi (51% of 134 studies), prokaryotes (40%), invertebrates (30%), protists (9%), and plants (7%), with fewer studies involving mammals (4%), reptiles (2%), and birds (1%; Figs.1 and 2). Most studies monitored one taxonomic group (median: 1), while the maximum number monitored was 7 (Fig. 3). The fraction of eDNA studies that monitored vertebrates increased from ~5% in 2013 to over 20% in 2023 (binomial GLM;p = 0.03; Fig. 4). Geographic distribution Most (86%) of the 129 biodiversity-certifying FC projects (Table1) were in tropical or sub-tropical regions, pa rticularly Central and South America (34%), Asia (31%), and Africa (17%; Fig. 1). The remainder were in tem- perate areas of Asia (11%), North America (2%), Europe (1%), or Oceania (1%; Fig. 1). Most of the peer-reviewed studies on biodiversity monitoring within FC projects (83% of 41) were also in tropical or sub-tropical locations (Africa, the Americas, Asia, and Oceania); the remainder were in temperate Europe, Oceania, and North America. In contrast, only 32% of peer-reviewed eDNA biodiversity monitoring studies we analyzed were conducted i n tropical and sub-tropical areas, mainly in Asia (14%), Oceania (7%), or Central and South America (6%; Fig. 1 | Characteristics of biodiversity monitoring. Comparison of taxonomic scope, geographic dis- tribution, and methodological rigor of biodiversity monitoring within forest carbon (FC) projects (n = 129) and terrestrial eDNA studies ( n = 134). In the top row, icons represent taxonomic groups monitored: plants, mammals, birds, amphibians, reptiles, fish, invertebrates, fungi, prokaryotes, and protists (icon size is proportional to the % of FC projects or eDNA studies). Fish were detected inci- dentally in some terrestrial eDNA studies. The maps in the middle row show locations of FC projects (left) and eDNA studies (right) included in this analysis. In the bottom row, the % of FC projects or eDNA studies meeting four different criteria of methodological rigor and transparency are shown. The icons in the bottom row represent the different biodiversity survey methods used, shown in decreasing order of commonness from top to bot- tom: formal visual observation surveys, incidental observations, camera traps, conventional traps, bioacoustics, and eDNA-based methods. Organism images from www.phylopic.org are used under Creative Commons licenses (see section “Acknowledgements”). https://doi.org/10.1038/s43247-024-01970-y Article Communications Earth & Environment | (2024) 5:801 4 Fig. 1). The majority (68%) were in temperate or arctic areas, notably in Europe (42%), North America (12%), Oceania (6%), and Asia (5%; Fig.1). Methodological rigor and transparency FC project reporting varied in methodological rigor and transparency across the biodiversity certification programs we include di no u ra n a l y s i s( T a b l e1, Fig. 5). Of the 97 FC projects that monitored animal populations, 72% employed a replicated sample-based survey design, 22% relied only on unstandardized observations (mostly community-derived), while 6% did not provide suf ficient information for us to assess their methodology (Fig. 5). The most common forms of standardized animal monitoring were visual plot or transect surveys (49% ), followed by camera traps (15%), physical traps or mist nets (7%), bioacoustics (2%), or eDNA-based mark- recapture (two studies that focused on elephants). Most projects used two survey methods (median: 2), but the maximum number of survey methods employed in a single FC project was six (Fig.3). Of the 70 FC projects that performed standardized animal surveys, only 69% reported sample sizes (37% of all projects; Figs. 1 and 5). Mean sample size was 21 for visual observations, 27 for camera traps, and 34 for conventional traps; no sample sizes were provided for the few acoustic or DNA-based monitoring efforts. Only one project reported estimates of uncertainty (e.g., error bars) around a measure of population abundance, while none reported uncertainty surrounding community metrics (e.g., species richness or diversity). Information noted as missing may exist in unpublished reports, which were mentioned in the documentation reports for several projects (n = 28), but that we found were not accessible online. Aside from aggregate species lists, onlyfive projects made raw abundance or diversity data available (one foranimals, four for plants; Figs.1 and 5). Nearly all the eDNA studies that we analyzed (323 of 324) employed a replicated survey design, with a median sample size of 50 (mean: 97; range: 2–1724). The most common environmental substrate sampled in terrestrial eDNA studies was soil (62% of 134 studies), followed by vegetation surfaces (3%), water (1%), and air (1%). Metabarcoding of trap-collected arthropods was also common (18%), as were studies of organisms ’ diet (10%) or microbiome (10%). Thirty percent of studies targeted more than one genomic region, which generally increased the taxonomic diversity of organisms documented (Fig. 3). Most eDNA studies (72%) made raw or processed data available via public repositories, mainly those hosted by the National Center for Biotechnology Information, European Bioinformatics Institute, or Dryad digital repository (Fig.1). Discussion Our analysis illuminates the current state and a potential future for eDNA in biodiversity monitoringwithin the voluntary forest carbon (FC) market. We Fig. 3 | The number of taxonomic groups mon- itored. The number of taxonomic groups monitored by environmental DNA (eDNA) studies (red sym- bols) or forest carbon (FC) projects (green symbols) in relation to the number of genomic regions used (eDNA studies only; left plot) or the number of field methods employed (right plot). For eDNA studies, different eDNA collection substrates were con- sidered different field methods. Field methods for FC projects and taxonomic categories are listed in Fig. 1. Six FC projects that did not specify whichfield methods were used are excluded from the plot (three each from PlanVivo and Gold Standard). Individual data points are shown as open circles; boxes show the median, upper, and lower quartiles; whiskers represent the range of data not considered to include outliers (i.e., within 1.5× the interquartile range). Fig. 2 | Taxonomic focus of eDNA studies and forest carbon projects. Taxonomic focus of biodi- versity monitoring within forest carbon (FC) pro- jects and peer-reviewed studies employing eDNA- based methods to study ecological impact in ter- restrial environments. The bars show the percent of FC projects that verify biodiversity benefits (n = 129; left plot) or peer-reviewed eDNA studies ( n = 134; right plot) that reported monitoring each taxonomic group listed on the y-axis. https://doi.org/10.1038/s43247-024-01970-y Article Communications Earth & Environment | (2024) 5:801 5 confirmed that biodiversity monitoring and reporting are highly variable across projects and certifying programs, and often lack rigor and trans- parency. The detailed picture we prov ide complements previous efforts, which were mainly higher-level overviews 20,22 or limited to single species and certifying programs6; no other studies have considered the potential role of eDNA within FC markets. Our analy sis revealed stark taxonomic and geographic differences between bio diversity monitoring within peer- reviewed eDNA studies and FC projects. While the use of eDNA is currently rare within FC projects, the literature compiled for our analysis collectively suggests considerable potential for its inclusion, notably the many examples we found of successful forest biodiversity monitoring using eDNA. Below, we synthesize this potential, highlighting gaps that must be addressed to facilitate broad-scale use of eDNA monitoring in FC projects. The ability to monitor a wide rang e of taxonomic groups simulta- neously using a singlefield method is one of the most attractive features of eDNA-based monitoring 14,15. Our analysis revealed that numerous studies already employ eDNA metabarcodingapproaches to document success of forest restoration32–35,47,48. These studies could directly inform the approa- ches taken in FC projects in terms of documenting biodiversity changes in response to reforestation or carbon-focused forest management. Below we explore this in greater detail for major taxonomic groups. Native plant diversity provides insights into forest health and is a common indicator of restoration success55,56. While FC projects do include plant surveys, most are focused on carbon accounting and do not quantify diversity of non-tree species. Takin g a more holistic approach to plant community monitoring could be bene ficial for biodiversity certi fication within FC projects. eDNA-based monitoring of plant diversity is still rela- tively uncommon 45 (Fig. 2), however, the broad-spectrum monitoring it facilitates (e.g., via herbivore scat, pollen, water, or soil samples) shows great promise35,45,46. Incorporating eDNA-based monitoring into vegetation sur- veys may prove particularly useful within species-rich tropical forests where morphological identification is difficult. eDNA-based monitoring could also be beneficial within temperate forests if time or cost savings can be realized relative to visual vegetation sampling45. However, seed and pollen dispersal strategies could result in DNA transport from distant locations, potentially confounding datasets and interpretation; and the development of DNA reference databases for plant sh a sl a g g e dt h a to fo t h e rg r o u p s46.T h e s e aspects limit the broader utility andadoption of eDNA. Thus, fundamental research is still needed to improve robustness and build out reference Fig. 5 | Methodological rigor and transparency. Comparison of methodological and reporting aspects of animal biodiversity monitoring within forest carbon (FC) projects from three biodiversity co-bene fitv e r ification standards: the Climate, Community, and Biodiversity (CCB) Standard, PlanVivo, and Gold Standard. A fourth, the Sustainable Development Veri fied Impact Standard (SD VISta), is not shown as it was represented by only three projects, and none reported performing animal monitoring. The bars show the % of projects meeting the methodological or reporting criteria listed on the y-axis. Sample size for all bars is the total number of projects: CCB, n = 80; PlanVivo, n = 26; Gold Standard, n = 20. Fig. 4 | Evidence for increasing prevalence of eDNA studies monitoring vertebrates over time. The x-axis in both graphs represents the year pub- lished, while the y-axis is the percent of eDNA stu- dies analyzed. The left panel shows the empirical % of eDNA studies involving vertebrates in different year intervals (sample size shown in gray at the base of each bar). The right panel shows the model- predicted values along with 95% con fidence inter- vals (error bars) from a binomial (logistic) GLM model describing the increase. https://doi.org/10.1038/s43247-024-01970-y Article Communications Earth & Environment | (2024) 5:801 6 databases to realize the full potential of eDNA to monitor trends in plant biodiversity. Most forest-based eDNA studies focused on fungi, protists, and bac- teria (Figs. 1 and 2). This focus is likely because eDNA overcomes the challenges of conventional sampling for these groups47,48. While microbial monitoring is rare within FC projects, they may be beneficial to monitor as these taxa are sensitive to environmen tal changes and provide essential ecosystem services47. As the use of microbial ecological indicators continues to develop57, biodiversity reporting within FC projects could be adapted to include these aspects, resulting in a more nuanced understanding of eco- system functioning currently absent from FC biodiversity co-bene fit schemes (e.g., nutrient cycling, forest pathogens). Terrestrial invertebrates are common monitoring targets within FC projects 49,52 (Fig. 2), as they are useful bioindicators58. However, monitoring using morphology-based identification methods can be challenging due to the need for diverse trapping methods, laborious specimen sorting, and considerable taxo nomic expertise 35,44,52. eDNA-based approaches for arthropod monitoring that span soil44,49,w a t e r59, and plant surfaces43,60 may efficiently overcome these limitations. These approaches reducefield effort as they require no trap set-up or ch ecking, specimen sorting, or morpho- logical identification. At least one FC projecthas adopted such an approach (TerraBio initiative49), demonstrating via soil eDNA metabarcoding that invertebrate communities of carbon-rich agroforestry areas resemble those of native forest. This study joins others in demonstrating the potentially practical application of non-lethal invertebrate eDNA sampling for biodi- versity monitoring 44. Vertebrates are by far the dominantgroup monitored within existing FC projects that verify biodiversity co-bene fits (Fig. 1). While terrestrial vertebrates are underrepresented w ithin eDNA biodiversity monitoring studies, field and lab eDNA methods for these groups are developing rapidly34,40,42,61,62. For example, eDNA sampling performed similarly to camera traps and visual transect surveys for some mammals, and showed superior performance for small and nocturnal species 41,63,64.As u i t eo fe D N A collection substrates has been successfully used to monitor vertebrates, including soil 41,w a t e r27,63,64, plant surfaces 40,42,61, coverboards39,a i r38,a n d invertebrate parasites62. Despite the growing interest in new eDNA-based vertebrate monitoring tools, and the in creasing prevalence of vertebrates within eDNA studies (Fig. 4), we identi fied only three FC projects employing eDNA-based methods. The TerraBio forest carbon project 49 used eDNA metabarcoding to detect Amazonian mammals, using the same soil samples to also detect arthropods. Two FC projects in Cambodia used elephant scat eDNA to estimate popul ation sizes via mark-recapture. As with aquatic systems, more research into the dynamics of DNA deposition, persistence, and transport within terrestrial systems 65 is needed to facilitate more widespread use for biodiversitychange detection and other applica- tions. Nevertheless, the existence of early adopters, and the wave of recent terrestrial vertebrate eDNA research, suggests these approaches to verte- brate biodiversity monitoring should become more commonplace within FC markets. W ef o u n dt h a tm o s tF Cp r o j e c t sc e r t ified as providing biodiversity benefits occurred in tropical or sub-tro pical regions (86%). This pattern likely reflects recognition that these regions contain high-biodiversity value, and that FC projects can be mechan isms to support biodiversity con- servation objectives 19,66. The pattern may also result from regional pre- ference for project developers to certify via certain registries (e.g., ACR and CAR projects are prevalent in North America and do not verify biodiversity co-benefits). In contrast to the FC projects, only 32% of eDNA studies were con- ducted in tropical or sub-tropical biomes. While the high species richness found in the tropics complicates morphological taxonomic identification, making eDNA approaches attractive, the genetic libraries (i.e., reference libraries) used to generate species lists from eDNA are still sparse 45,67. Nevertheless, there has been rapid growth in the number of published studies that have implemented eDNA - b a s e dt o o l st os u r v e yi nt r o p i c a l rainforests 27,28,49,63,67. We suggest that this growth in using eDNA surveys in the tropics would accelerate with the adoption of eDNA methods for cer- tifying biodiversity co-benefits within FC markets. Temperate FC projects also offer a pathway to demonstrate the value of eDNA to support certifi- cation of biodiversity benefits, as eDNA application is more established in these systems and biodiversity certi fication for temperate FC projects is relatively rare. The decreasing costs of sequencing68 and the proliferation of eDNA sample processing companies 28,36 also promise to make eDNA applications more globally accessible. The methodological rigor and transparency of biodiversity monitoring efforts varied widely among FC projects. Only 54% of projects conducted structured, replicated animal monitoring, and only 69% of those reported sample sizes. Only one project reported uncertainty (i.e., confidence inter- vals) while only three corrected for detection biases (e.g., distance sampling, occupancy modeling 69). These design and reporting characteristics are crucial for assessing progress in biodiversity indicators 69,70 as they help distinguish true change in species richness or diversity from statistical artifact (e.g., change from observer skill level or device sensitivity). Indeed, such change detection is a key motivation for the biodiversity monitoring plans required by the four FC programs analyzed 23,50 (Table1). Historically, eDNA studies cannot report on changes in species abundance14, however, advanced sampling design (e.g., oc cupancy modeling) may be able to overcome these limitations70. Occupancy models can integrate eDNA-based presence–absence data to generate estimates of the extent of an organism within an area, along with uncertainty in those estimates 40,70.T h i si n f o r - mation can be correlated with abundance, tracked over time, used to gen- erate robust data on species ’ populations and community diversity metrics40,69,70, and can be integrated with other data commonly collected for FC projects (e.g., camera traps and bioacoustics40,69–71). Regardless of whe- ther eDNA-based methods are used, biodiversity monitoring within FC projects would benefit from more careful consideration of study design— including replication, uncertainty, and detection bias — to support more robust long-term assessment of biodiversity change. Like other remote monitoring approaches (e.g., camera traps, bioa- coustics) eDNA methods leave an auditable data trail 13.T h i so u t c o m ea l l o w s for transparent accounting, a core principle of carbon offsets, but which has not been applied as rigorously to biodiversity monitoring and reporting within FC projects 4,9,10. Such accessibility would allow the data to be repurposed for improved monitoring over time, and could be leveraged more broadly to support research, biodiversity conservation, and progress towards national biodiversity targets 36,37. Very few FC projects openly shared biodiversity data (only 4% of projects analyzed). In contrast, the data from eDNA studies are commonly depositedin public repositories (72% of stu- dies analyzed). Further, standardizedformats for archiving and reporting eDNA-derived biodiversity data are rapidly emerging at the local, national25, and international36,37 levels, including in relation to FC projects49. Given that the biodiversity benefits of FC projects may be realized more gradually, the ability to store, re-use, and interpret data over long time periods— ideally, via openly accessible repositories— is key to increasing the integrity of FC projects50. Conclusions The world is increasingly looking to NbS for sustainable climate change mitigation strategies2,7, and the large-scale implementation of NbS is likely to be necessary to achieve global climate goals1. The rapid emergence of these projects, particularly those involvingFC sequestration, has been associated with increased scrutiny20. Despite the continued technical challenges72,t h e r e have been improvements in the measuring, reporting, and verification of carbon in natural systems5,20,21. At the same time, the biodiversity co-benefits of FC projects are increasingly recognized and valued8,10. With less than half of FC projects we discovered (451 of 1323 projects) currently implementing formal biodiversity co-benefitv e r ification, there is room for much wider adoption. While the magnitude of co-bene fits will vary from project to project, the need to accurately assess reference biodiversity levels and change is critical to their ultimate viability. This need highlights an opportunity to integrate emerging technologies that improve transparency and https://doi.org/10.1038/s43247-024-01970-y Article Communications Earth & Environment | (2024) 5:801 7 accountability in measuring these co-benefits5,10,13.A se D N A - b a s e dt e c h - nology matures, there is an opportunity to develop operationalized sam- pling protocols and provide robust, standardized, and auditable biodiversity co-benefit data, and to contribute to broader biodiversity objectives across geographic scales4,7,11,13,25. In FC markets and other NbS, a powerful approach could include joining eDNA sampling with robust modeling frameworks such as occupancy modeling, allowing cost-effective, scientifically-sound monitoring of virtually any taxa 14,27,28,70.T h i sa p p r o a c h would leverage the strengths of eDNA technology to provide an auditable data trail while also facilitating th e involvement of local and indigenous community members, for example, in study design and data collection 23,26. Regardless of the exact approach, achieving standardized and robust bio- diversity monitoring at scale within forest carbon markets will require creative solutions. Theflexible and powerful suite of eDNA-based methods appears well-suited to meaningfully contribute to this effort. Methods We explored the potential role of eDNAfor biodiversity monitoring within voluntary FC projects. To do so, we conducted a three-phase systematic review of the peer-reviewed and gray literature. Following standard prac- tices for systematic reviews 73,w efirst used Web of Science to search the peer- reviewed literature for studies a bout biodiversity within FC markets (Table 2, Supplementary Fig. 1). Second, we obtained project reports and documentation from registries associated with the four main sustainability and biodiversity co-bene fit certifying programs on the voluntary FC market5,20 (Tables1 and 2). As part of this effort, we also assessed the to tal size of the FC market, including projects that do not currently verify biodiversity co-benefits. We did so by accessing three other project databases (see Supplementary Note 1): Verified Carbon Standard, includingonly projects not co-certified by the standards in Table 1; American Carbon Registry; and Climate Action Reserve (see Data Availability section). Together,the FC projects we considered repre- sent >97% of the carbon credits issued on the voluntary FC market20. Finally, we used Web of Science to systematically search for peer- reviewed literature on eDNA-based biodiversity monitoring programs. This search was limited to studies of ecological impacts, including those resulting from interventions such as restoration efforts, or from anthropogenic or natural drivers such as pollutiono rl a n d - u s eg r a d i e n t s( T a b l e2, Supple- mentary Fig. 2). This three-phase process resulted in three pools of literature. Thefirst captured the scholarly discussion su rrounding biodiversity monitoring within FC projects. The second highlighted voluntary FC projects with on- the-ground biodiversity monitoringin place. The third characterized the current state of eDNA-based monitoring of biodiversity change. To con- textualize the potential role for eDNA-based biodiversity monitoring within FC markets, we collected data on each study or project report in the three final literature pools. Data on empirical studies and projects included taxa, monitoring method, question addressed,site characteristics, and geographic location (Table2). Data collected on peer-reviewed literature on biodiversity monitoring within FC markets also included thematic focus and whether biodiversity monitoring practices we re explicitly discussed. Additional methodological details, including review inclusion and exclusion criteria, are outlined in Table 2 and Supplementary Material (Supplementary Figs. 1 and 2, Supplementary Note 1). Data availability The annotated literature and forest carbon project databases are publicly available as tab-separated text files at: https://doi.org/10.5281/zenodo. 13830752. Data and reports for forest carbon projects were retrieved from the respective registry websites (see Supplementary Note 1 for links). Code availability R code and data to reproduce the analyses andfigures are publicly available via GitHub (https://github.com/mikeallen-eco/eDNA_forest_carbon)a n d Zenodo (https://doi.org/10.5281/zenodo.13830752). Received: 22 May 2024; Accepted: 16 December 2024; References

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73. Page, M. J. et al. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. Int. J. Surg. 88, 105906 (2021). Acknowledgements We thank R. Almeida, S. Dickey, C. Eddy, K. Fitz, A. Kisurin, K. Kyle, J. Ramirez-Garofalo, L. Tkacenko, A. Vastano, and E. Waltman for helpful feedback and J. Vastano for assistance with data collection. Icons in Fig. 1 were created by M. Michaud, Ghedo, T. M. Keesey, C. De Rito, F. Sayol, L. Simons, M. Tan, G. Palomo Munoz, and E. J. McTavish. Funding for this study was provided by ExxonMobil. Author contributions B.D.J., J.C.A., J.D.B., J.L.L., and M.C.A. collectively conceived the idea. M.C.A. and R.I. assembled the literature and collected data. M.C.A. assembled the final databases and wrote the code to create figures and analyze the data. M.C.A. led the writing of the manuscript. All authors contributed meaningfully to shaping the manuscript during revisions. Competing interests The authors declare no competing interests. Additional information Supplementary informationThe online version contains supplementary material available at https://doi.org/10.1038/s43247-024-01970-y . Correspondenceand requests for materials should be addressed to Michael C. Allen. Peer review informationCommunications Earth & Environmentthanks and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Martina Grecequet. A peer reviewfile is available. Reprints and permissions informationis available at http://www.nature.com/reprints Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article ’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ . © The Author(s) 2024 https://doi.org/10.1038/s43247-024-01970-y Article Communications Earth & Environment | (2024) 5:801 10