Introduction

Global climate change is altering ecosystems’ composition and function worldwide, compromising the provision of multiple services [1]. During the last two decades, growing environmental awareness has increased efforts to reverse biodiversity loss and land degradation. Therefore, ecosystem restoration has become a global priority and numerous initiatives have been promoted to implement large-scale programs. For instance, the Bonn Challenge aims to restore 350 million hectares of degraded land by 2030 [2], and the United Nations (UN) has designated 2021–2030 the UN Decade of Ecosystem Restoration [3]. Within this framework, plant science and forestry sectors will play a fundamental role through afforestation, reforestation, and landscape restoration of degraded lands. These efforts will also contribute to climate change mitigation. In this context, the demand for forest reproductive material (i.e., seeds, seedlings, and rooted cuttings) is massive. The nursery sector has to address this ambitious target while switching toward more sustainable production systems without undermining seedling quality [4,5,6].

Bare-root and container seedlings are the main stock types used in forest restoration plantations [7]. Bare-root seedlings are cultivated directly in the field in mineral soil. In contrast, container seedlings are grown in a confined rooting volume containing a porous medium, either pure or composed of a mixture of components. These media are known as substrate, soilless medium, growth medium, or growing medium (hereafter, GM [8]). Compared with bare-root plant production, container cultivation using GM can be more cost-effective [9] and efficient depending on location and available resources. Container production shortens the production cycle, enhances plant quality, extends the outplanting period, and improves seedling field performance, especially under harsh site conditions [7, 10]. An effective GM should promote simultaneously healthy seedlings free from pests and pathogens and have appropriate physical and chemical characteristics to support optimal root and shoot development [11]. Specifically, an ideal GM should have a balanced air porosity, suitable aeration, optimal bulk density, and adequate water-holding capacity. Such characteristics ensure an efficient exchange of oxygen and carbon dioxide, enhance a proper development of fine roots and of mycorrhizae, and reduce the risk of pathogens. Achieving the right balance of these physical characteristics is important for stimulating root growth throughout the entire container, which ensures that a cohesive root plug is maintained during handling, shipping, and planting [12]. The chemical characteristics of the GM should support adequate nutrition by maintaining high cation exchange capacity, low inherent fertility to control mineral nutrient concentration through fertilization, and a slightly acidic pH to maximize the availability of mineral nutrients in solution [12] and promote the growth of specific microorganisms in the GM. All these physical and chemical properties are usually achieved with GM composed of organic materials at least by 50% in volume, frequently up to 100% [12, 13]. In addition, a good GM should be economically feasible, lightweight, and useful to promote uniform plant growth across growing seasons. Therefore, the choice of an adequate GM is a trade-off among factors such as quality requirements, plant species, costs, and the availability of raw materials [14, 15].

Sphagnum peat moss (Sphagnum spp.), generally known as peat, is commonly used worldwide in plant production as the primary component of GM due to its balanced physicochemical characteristics, mid-term stability, uniformity, availability, and competitive prices [16, 17••]. However, peat extraction is environmentally unsustainable due to slow natural peatland formation and ecological drawbacks, such as the destruction of the fragile peat bog ecosystems and the consequent loss of carbon sequestration capacity [17••, 18,19,20, 21••, 22]. On a global scale, the peatlands used as sources of GM cover approximately 2000 km2, accounting for 0.5% of the global uses of peatlands [23]. In Europe alone, over 37 million m3 of GM is produced annually [24], and the global production was calculated at 67 million m3 in 2017 with an estimated increase of 422% in 2050 [25]. In the EU, peat extraction and its use in horticulture resulted in 12 Mt of CO2 emissions in 2019, making this industry sector a significant contributor to greenhouse gases [26]. Consequently, the extraction of peat will be included in mitigation targets (LULUCF regulation EU 2018/841) under the category “wetlands” starting from 2026. In the USA, peat use in horticulture was estimated at 479,000 t in 2018, accounting for approximately 25% of the total [27]. New commercial and technological solutions for implementing sustainable processes in nurseries (i.e., water-saving techniques, plastic recycling, and the use of eco-friendly materials) have been widely investigated in recent years [28,29,30, 31••]. In this regard, using alternative GM materials is both a challenge and an opportunity for recycling organic wastes [32, 33] and reducing the use of other traditional inorganic, non-renewable materials such as sand, vermiculite, and perlite [34, 35].

In the last two decades, a vast body of scientific literature assessing the performance of peat alternatives for producing forest nursery seedlings has been published [31••, 32,33,34,35,36,37, 38••, 39, 40•]. Treated and untreated waste and renewable raw materials show great potential as GM constituents and standalone substrates [17••]. However, the dispersal of studies in different plant species under different local conditions and with different measured GM and plant characteristics limits generalizations. Additionally, studied alternative materials are frequently related to local recycling chains [41, 42]. Therefore, a global overview of available information about alternative GM materials is needed.

To the best of our knowledge, no systematic review of scientific literature on the effectiveness of alternative-to-peat materials for enhancing plant growth and GM characteristics has been conducted in the forest nursery sector. In this study, we aimed to identify the most promising alternative GM materials based on their effectiveness, sustainability, and innovation. This information can provide the knowledge background for promoting new policies and practices to achieve higher sustainability in the forest nursery sector. We performed a broad-scale review of peer-reviewed and grey literature about a variety of GM materials used to grow forest plants. We collected information about (i) the most studied alternative materials to peat; (ii) the proportion of these materials in GM composition and whether there was eventual use of traditional materials such as peat, soil, or other inorganic traditional materials; and (iii) the effective alternative materials on GM physicochemical characteristics and seedling growth compared with peat both in the nursery and in the field. Collected information was then evaluated across different biomes and climates, including whether plant responses vary between angiosperms and gymnosperms due to their functional differences that modulate plant response to nursery cultivation [43]. Moreover, we also summarized the origin of prevalent alternative GM materials and the world distribution of case studies.

Materials and Methods

Literature Collection

We performed a systematic review of the literature on non-peat materials used in GM for growing forest seedlings in container nurseries following the guidelines of the Collaboration for Environmental Evidence (2010) (e.g., [44]).

Selected papers met the following criteria:

  • Studied forest plant species;

  • Tested alternative (i.e., sustainable, innovative, and non-peat) GM materials;

  • Included peat or other traditional non-renewable or non-sustainable materials in the GM to compare with the alternative materials.

Additionally, we selected articles that contained information about one or more of the following groups of variables:

  • Physical and chemical characteristics of the GM such as porosity, pH, or cation exchange capacity;

  • Morpho-physiological attributes of nursery seedlings such as height, root collar diameter, biomass, and organ and/or plant nutrient concentration;

  • Seedling field survival and /or growth.

The primary search included peer-reviewed papers and was conducted in the following databases:

  • Web of Science (WoS — https://www.webofknowledge.com) owned by Clarivate is a paid-access platform providing access to multiple databases that provide reference and citation data from academic journals, conference proceedings, and other documents in various academic disciplines;

  • Scopus (https://www.scopus.com), Elsevier’s abstract and citation database, covers three types of sources: book series, journals, and trade journals in top-level subject fields of life sciences, social sciences, physical sciences, and health sciences;

  • Directory of Open Access Journals (DOAJ — https://doaj.org/) is a website that hosts a community-curated list of open access journals, maintained by Infrastructure Services for Open Access (IS4OA);

  • Reforestation, Nurseries, & Genetic Resources (RNGR — https://rngr.net/), sponsored by the United States Department of Agriculture (USDA), a collaborative effort between Forest Service and Southern Regional Extension Forestry agencies, supply people who grow forest and conservation seedlings with the very latest technical information providing a searchable database of over 11,000 technical articles;

  • Canadian Forest Service Publications (https://cfs.nrcan.gc.ca/publications) hosted by the Government of Canada—Canadian Forest Service (CFS) and including articles, books, reports, and CFS-prepared series and leaflets;

  • Scientific Electronic Library Online (SciELO — https://www.scielo.org/), managed by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil, in collaboration with the Latin American and Caribbean Center for Health Science Information (BIREME), is a digital library that provides a software platform for the publication of scientific journals in electronic format and in the context of unabridged editions;

  • Google Scholar (https://scholar.google.com/), owned by Google, is a freely accessible web search engine that indexes the full text or metadata of scholarly literature across an array of publishing formats and disciplines.

We also collected grey literature defined as works that have not been peer-reviewed (e.g., scientific reports, non-ISI papers, national reports, theses, conference proceedings, conference abstracts, posters, conference presentations, unpublished work, books, literature reviews) [45]. The database search was initially made on 5 March 2021 and updated on 2 July 2021. We did not explicitly impose geographic, year of publication, or language restrictions but we were interested in published studies related to forest nurseries, reforestation, and/or restoration, growing medium and seedling quality, and field performance. Thus, the terms were searched in titles, abstracts, and keywords and were based on the following string: (nursery OR reforest* OR restor* OR reveget* OR afforest*) AND (medi* OR growing medi* OR soilless medi* OR substrat*) AND (seedling quality OR morphology OR field performance OR survival). Translated terms to other languages were employed to conduct searches in the native languages of the authors.

Article Screening

To identify studies relevant to the objectives of our review, collected articles were screened by following steps:

  1. 1.

    Papers were discarded if the title indicated that the study was out of scope.

  2. 2.

    For papers with acceptable titles, the abstracts were reviewed, and the papers were discarded if they did not satisfy the inclusion criteria.

  3. 3.

    Articles that passed the two steps above were fully read, and only those including the required information related to GM characteristics, nursery seedlings, or field performance were included in the study.

The above screening resulted in 866 articles: 542 in WoS, Scopus, and Google Scholar; 198 in SciELO; 97 in the Directory of Open Access Journals; 27 in the USDA database; two in the Canadian Forest Service databases; and 28 in grey literature (four from Finland and 24 from Spain).

Data Extraction and Analysis

From each article, we extracted the following information:

  1. 1)

    Study location (country and coordinates);

  2. 2)

    Biome where the experiment was carried out using the classification in Whittaker [46];

  3. 3)

    Plant growth form (herb, shrub, tree);

  4. 4)

    Plant species.

Some of the selected articles described several case studies, i.e., testing different GM mixtures. Thus, we identified case studies based on materials used as the unique alternative component of the GM or the material that was present in the highest proportion (hereafter referred to as “prevalent”) for mixtures of alternative materials in a given GM. As a result, we identified 11 widely used alternative materials: tree bark, biochar, chips, coir, compost, manure, rice, sawdust, sludge, wood fiber, and other organic wastes (Tables 1, S1).

Table 1 A brief description of the 11 main alternative materials composing the growing media (GM), and their classification as sustainable (S), innovative (I), and innovative and sustainable (IS)
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For each case study, we extracted the following information:

  • The proportion in the volume of the alternative material in relation to other components of the GM according to five classes: 1 = 1–20%, 2 = 21–40%, 3 = 41–60%, 4 = 61–80%, 5 = 81–100%; when the GM included more than one alternative material, the proportion of each material was recorded;

  • The origin of each alternative material in the GM mixture according to the related productive sector: agriculture (A), animal husbandry (An), green wastes (Gr, plant waste coming from non-agricultural sectors such as forests, gardens, and parks), industry (I), and solid municipal wastes (Mu). Manure (An) was further classified according to the originating livestock (bovine, goat, horse, pig, poultry, and quail).

For each case study, peat, soil, or inorganic materials (i.e., perlite and vermiculite) were considered traditional GM components. GM with alternative materials were classified in three ways: innovative (I) when commercially uncommon and/or scarcely used in the forestry nursery sector; environmentally sustainable (S) when its production and use involve the conservation of peatlands and other ecosystems, environment preservation, and renewal of natural resources and/or waste material; or both innovative and sustainable (IS). According to these criteria bark, manure, sawdust, and wood fiber were classified as sustainable (S) and the rest of the materials as innovative and sustainable (IS); none of the defined alternative materials was classified as innovative and not sustainable (Table 1).

For each case study, the effectiveness of alternative materials was established in comparison with peat for selected physicochemical characteristics of the GM, specific morphological traits of nursery seedlings, and field performance. We discarded case studies that did not statistically compare alternative materials with peat. The effectiveness of an alternative material was defined at three levels as follows:

  1. (a)

    Promising, when the GM’s pH ranged from 4.0 to 7.0 and total porosity ranged from 60 to 90% according to Landis et al. [12]; otherwise, material was considered to have a negative effect on physicochemical characteristics of the growing media;

  2. (b)

    Effective, when the nursery growth of forest seedlings cultivated with the alternative material was significantly higher or not different than the growth of those cultivated with peat; otherwise, the material was considered to have a negative effect on nursery growth. As several parameters were used to assess growth, we selected parameters in order of importance following Andivia et al. [43, 47]: total biomass, shoot biomass, root collar diameter, or shoot height;

  3. (c)

    Most effective, when the field performance of seedlings cultivated with the alternative material was significantly higher or not different from the performance of seedlings cultivated with peat in the nursery; otherwise, material was considered to have a negative effect on-field performance. Field performance parameters were also selected in order of importance: survival, root collar diameter, and shoot height [43].

We performed a chi-square test to assess differences in the number of positive (promising, effective, and most effective) and negative results between angiosperms and gymnosperms for each alternative material. For every alternative material, we also explored its global effectiveness by tallying the proportion of positive cases for each of the effectiveness categories.

Results

Characteristics of Case Studies

After abstract screening and paper carefully reading, we retained 191 references that resulted in 1671 case studies (i.e., different GM mixtures). Studies spanned from 1974 to 2021 and were globally distributed (Tables S1, S2). Brazil accounted for the highest number of publications (44), which included 449 case studies, followed by the USA (25 publications and 256 study cases), Spain, Greece, and Mexico (Table S3).

We compiled information on 181 forest plant species (79.7% trees, 15.7% shrubs, and 4.7% herbs). Most species were angiosperms (73.1%), which were the dominant taxa in most biomes except in the temperate grassland/desert and boreal forest biomes where gymnosperms were dominant (Fig. 1). Most case studies (> 65%) were of species from the tropical seasonal forest/savanna and woodland/shrubland while the less represented biomes were the temperate grassland/desert, tropical rainforest and boreal forest (Fig. 1).

Fig. 1
figure 1

Distribution of case studies (i.e., GM) across terrestrial biomes (Whittaker., 1975) and species group (angiosperm and gymnosperm). Case studies were distributed across biomes as follows: tropical seasonal forest/savanna 36.5%, woodland/shrubland 31.6%, temperate forest 15.0%, subtropical desert 8.5%, temperate grassland/desert 4.1%, tropical rainforest 3.8%, and boreal forest 0.4%

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Origin of Alternative Materials

All alternative GM materials originated from recycled materials or by-products (Table 1). Bark, coir, chips, sawdust, and wood fiber resulted from activities or processes related to vegetation management or vegetal production, even when coir required additional chemical treatment. Biochar originated mainly from green and agricultural wastes (Fig. 2). Green and municipality wastes together provided > 60% of the compost, while > 90% of other organic wastes derived from agriculture and green wastes (Fig. 2). Finally, the municipality waste was the source of half of the sludge produced (Fig. 2). Moreover, bovine and poultry farming was the source of ca. 90% of the manure (data not shown).

Fig. 2
figure 2

Origin of the five prevalent alternative materials composing the GM. Values are expressed as the percentage of case studies that reported the origin of the alternative material

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The prevalence of tested alternative materials differed among continents (Fig. 3). Manure was the most frequent material in Africa and South America; compost was the material most used in Europe, Asia, and Oceania; bark was the most tested alternative material in North America; and sawdust was mainly used in Africa (Fig. 3).

Fig. 3
figure 3

Geographical distribution (%) of the prevalent alternative materials

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Use of Alternative Materials

Biochar, compost, manure, and wood fiber were mainly used as single alternative materials in GM and they made up 41–60% of the GM mixture in more than 30% of the case studies (Table 2). The remaining materials were equally used alone or mixed with other alternative materials. Chips constituted 81–100% of the GM mixture in 48% of the case studies, whereas coir and manure made up 21–40% of the GM mixture in 31.6% and 50.7% of the case studies, respectively (Table 2).

Table 2 Classes of proportion (%) for each alternative material used for composing the GM
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In most case studies, bark, chips, coir, compost, other organic material, sawdust, and sludge were tested without mixing with other materials (Table 3). The remaining alternative materials (biochar, manure, rice, sawdust, and wood fiber) were mainly mixed in various proportions with soil and peat (Table 3). Manure and bark were the most and the least mixed material with soil, respectively. The soil was also mixed with other materials such as other organic materials, rice, and wood fiber. Peat was mixed with all alternative GM, although its proportion varied largely. Inorganic traditional materials such as perlite and vermiculite were mixed with bark and chips in 31.7 and 26.7% of the case studies, respectively. Biochar and compost were mixed in the same proportion as peat and other traditional inorganic materials in 7.4% and 14.5% of case studies, respectively (Table 3).

Table 3 The percentage (%) of case studies using traditional base-material mixed with alternative material in the GM (total case studies 1366)
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GMs were also mixed with more than one alternative material. Bark, compost, other organic materials, rice, and sludge were the most frequently mixed materials in those cases (> 7%), while biochar, chips, manure, and wood fiber were the least frequently mixed (Table S4). The wood fiber was only combined with bark (Table S4). Finally, the most frequently tested combinations were bark (as the main alternative material) mixed with compost or sludge (secondary alternative materials), rice with other organic waste, and sawdust with bark (Table S4).

Effectiveness of Alternative Materials

Alternative materials were “promising” according to their effect on GM physicochemical characteristics in 61.7% of all case studies, half of which included bark, compost, other organic material, and rice. Except for sludge in the temperate biomes and coir, compost, and manure in the tropical biomes, the majority of alternative materials were promising in more than 50% of the case studies (Table 4). Coir, compost, and manure performed better in temperate than in tropical biomes, while bark, chips, other organic material, rice, sawdust, and sludge performed better in tropical than in temperate biomes. Wood fiber showed remarkable “promising” results in both temperate and tropical biomes although the number of case studies was low (Table 4).

Table 4 The percentage (%) of total case studies with promising alternative material (i.e., positive effect on growing media characteristics) within each alternative material distinguishing between biomes according to Whittaker (1975)
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Alternative materials were found to be “effective” in promoting seedling growth at the nursery stage in 62.1% of the case studies. When each material was considered individually, more than 50% of the cases showed positive results, except for coir in angiosperms, which success rate was 43.5%. Additionally, organic materials and wood fiber in gymnosperms had effective rates of 35.3% and 8.3% respectively. Compost, manure, other organic material, and sludge showed the highest effectiveness, between 21.9 and 11.2% of the total “effective” case studies. We only found significant differences in effectiveness between angiosperms and gymnosperms for other organic materials (Table 5). The effectiveness of alternative materials in angiosperms was observed in > 50% of cases except for coir (43.5%), whereas there were not enough case studies to define chips and wood fiber as effective materials (Table 5). The effectiveness of alternative materials was higher in tropical than in temperate biomes, except for bark and biochar (Table 5). Manure and sludge were generally “effective” despite the limited number of available case studies. However, their effectiveness was comparatively lower in temperate and boreal biomes than in other biomes. Rice, biochar, and bark were only tested in temperate biomes and resulted in “effective” in > 50% of the case studies.

Table 5 The percentage (%) of total case studies where the alternative material was effective (i.e., positive effect on nursery seedling growth compared to peat or other traditional materials), distinguishing by species type and the biome according to Whittaker (1975)
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A limited number of case studies (n = 82) have evaluated the effectiveness of alternative materials on seedling field performance, particularly for gymnosperms (Table 6). We found positive results in 57.3% of the case studies, and in most of them, the GM was composed of bark, compost, other organic material, rice, or sludge (10.5%, 17.5%, 10.5%, 26.3%, and 15.8% of the total “most promising” case studies, respectively). We found significant differences between angiosperms and gymnosperms in effectiveness when other organic materials, rice, and sludge were used (Table 6). However, other organic materials and rice hulls were found “most effective” only in angiosperms (75.0 and 77.3% of the case studies, respectively). Sludge was “most effective” in 80% of case studies in gymnosperms, while in angiosperms only 20% of case studies resulted in “most effective.” In angiosperms and in all alternative materials, more of the 50% of the case studies resulted the most promising, with the only exception of bark (42.9%) and sludge (only one case study).

Table 6 The percentage (%) of total case studies where the alternative material was classified as most effective (i.e., positive effect on seedling field performance compared to peat or other traditional materials) distinguishing by species type and the biome according to Whittaker (1975)
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Discussion

This is, to the best of our knowledge, the first study that globally reviews alternative-to-peat materials in GM composition for the forest nursery sector. Our study contributes to increasing knowledge and systematization of GM alternative materials at a global scale, which can facilitate informed decision-making for researchers, practitioners, and policymakers. This knowledge can help improve the productivity and environmental sustainability of the forest nursery sector in a world of increasing resource scarcity and climatic uncertainty.

We classified alternative materials according to their proportion in the GM composition, the biome of origin (i.e., climatic conditions), and species groups (i.e., angiosperm and gymnosperm). Such a high diversity of alternative GM underscores the interest in this topic worldwide and precludes defining the “silver bullet” alternative material or GM mixture for growing forest seedlings at the global scale. On the contrary, our analysis indicates that available alternative GM materials are contingent on local ecological and economical particularities and most of them can be integrated into the local circular bio-economy scheme. Thus, the use of in situ products or by-products would efficiently contribute to global sustainability.

Several Alternative Materials Are as Effective as Peat for Producing High-Quality Forest Container Seedlings

The production of containerized seedlings in nurseries must be sustainable and ensure stable yield and plant quality [48]. While productivity and seedling quality have traditionally been the primary focuses of nursery cultivation, the environmental impact of nursery production has become a main concern. Moreover, with the increasing demand for forest seedlings due to global restoration/reforestation initiatives, it will be necessary to replace peat with sustainable materials [49]. The selection of alternative materials should consider local availability and the environmental impacts of material processing. Our review shows that the physicochemical properties and their subsequent effectivity on seedling nursery growth and field performance vary among alternative GM materials. Bark and rice had the best GM physicochemical characteristics, while compost and manure promote greater seedling growth compared to other components, likely due to their fertilization capacity [50, 51]. Seedlings cultivated in compost and rice hull-derived materials showed the highest field performance.

There are alternative materials with suitable physicochemical properties that promote the growth and quality of both angiosperm and gymnosperm seedlings, or sometimes even better than peat [37, 52]. For example, sewage sludge induces a significant increase in seed germination, seedling above- and below-ground biomass, and morphology of Pinus pinaster [53]. Green compost, produced from shredded branches of three fast-growing species (Acacia cyanophylla, Acacia cyclops, and Eucalyptus gomphocephala), showed seed germination rates similar to peat: vermiculite substrate and high-quality seedlings of Ceratonia siliqua L., although these seedlings had significantly smaller shoots and root systems than those produced in peat substrate [54]. Seedlings of three Quercus species (Q. robur, Q. pubescens, and Q. ilex) grown with coir, although smaller in size when compared with those grown in compost, were compatible with standard Quercus forest stocktype size and showed a proportionally higher root system development and fibrosity [36]. Biochar from orchard pruning and/or compost from olive mill residues showed an improvement in the growing substrate properties leading to accelerated growth and development of Populus euroamericana [40•]. Raw biochar and acidified biochar improved the rooting and growth of Rosmarinus officinalis cuttings, and the raw biochar gave satisfactory results for both shoot and root growth of R. officinalis and Phyllirea angustifolia seedlings [55•]. Willow biochar amendment increased the aboveground growth of Picea abies seedlings and root biomass, as well as the root collar diameter of Betula pendula seedlings and boreal conifers [56•]. The addition of raw pine sawdust to a substrate composed of 25% peat moss and 50% composted bark improved Pinus cembroides growth, which indicates that sawdust addition may be a suitable alternative especially if the cost–benefit analysis is a concern to the nursery manager [57].

Our review identifies compost as the most studied alternative material globally and the best-performing GM component in terms of physicochemical characteristics, seedling nursery growth, and field performance. Manure, other organic material, bark, rice hulls, and sludge also showed effectiveness albeit to a lesser extent compared to compost.

The number of studies reporting the effect of alternative materials on field performance between species groups was, unfortunately, small, and therefore, we could not evaluate the effectiveness of all materials. It is important to highlight that rice hulls, which are generally associated with other alternative materials (mostly coir, manure, and other organic material) and traditional materials (soil or peat), were very effective in increasing field performance in angiosperms but had negative effects in gymnosperms. Sludge, associated mainly with bark and chips, showed the opposite trend affecting positively the growth performance of gymnosperms in both the nursery and the field.

We finally underscore that further experiments at the local scale are necessary for testing the proportions of novel and sustainable materials composing GM to define the reliability of alternative and innovative GMs for forest nurseries.

The Effectiveness of Alternative Materials Should Be Tested According to Regional Availability

Compost, other organic materials, and bark represent almost 50% of the alternative materials studied at the global scale. Our results indicate that about 70% of compost comes from organic material generated from agriculture, forestry, and animal breeding, which reinforces the importance of such sectors in the circular economy [58,59,60]. The prevalence of alternative materials in GM showed remarkable differences among world regions reflecting different local socio-economic and legislation contexts, available materials, and environmental specificities. Compost and sludge obtained from waste, the by-product of urban activities, are widely used in North America and Europe representing more than 50% of the case studies while no record was found in Africa and Oceania. These regional differences likely reflect different levels of urbanization, technology, and environmental legislation among regions [61]. The importance of the local context is particularly evident in South America. The use of alternative materials originated from municipalities in Brazil, where nursery activities are thriving due to vast plantation activities [62], which is similar to North America due to the development of large metropolitan areas. In contrast, such materials are rarely considered in other South American countries.

When considering alternatives to peat, it is important to carefully assess both sustainability and cost factors. A recent review [63] found that the cost of the container (i.e., Deepot™ D40) and growing media per single cavity in a forest nursery case study in Lebanon were three times lower when using coir compared to peat. However, a life cycle assessment (LCA) of coir compared to peat and perlite for rooftop farming in Spain revealed that while coir was more environmentally friendly, it did not perform well from a social perspective in key indicators such as child labor, fair pay, gender inequality, and impact on community infrastructure [64]. Additionally, a study in Latvia raised concerns about the sustainability of coir, particularly in relation to its impact on global warming [65], and other studies have suggested that the cost of refining coir to be used as growing media and the transportation costs from tropical areas of origin could pose significant constraints [17••, 49, 66]. It is worth mentioning that the abundant unused coir by-products in many coconut-producing countries can cause serious environmental problems [22]. In this context, transportation can play an important role in accessing alternative material [22]. However, sustainability concerns related to long-distance transportation of alternative materials emphasize the importance of developing a local circular economy supply chain and promoting local infrastructure development for processing available resources. Moreover, although there are several underutilized alternative materials such as green waste, bark, and wood fibers, their other possible uses may limit the expansion of the supply of growing media, especially in some areas of the world like the EU, where the growing demand for alternative materials could lead to an increase in the prices [22].

All considered, our evidence-based review could help to develop (i) a consistent approach for the identification, selection, and characterization of alternative material and (ii) a rational understanding of the practical and economic situation involved in the use of GM aimed at choosing the most appropriate material for the cultivation of forest seedlings.

GM Sustainability Can Be Increased by Removing and Reducing the Proportion of Peat in Mixtures

GMs can be pure or can be a mixture of materials [33, 67]. Mixtures are designed to enhance the physicochemical characteristics of GM (e.g., [53, 68, 69], according to the specific physiological requirements of plants [70]. Moreover, GM mixtures should also provide economic and environmental benefits (e.g., [71, 72]), although the value and the feasibility of alternative materials in practical nursery activities need to be further demonstrated. Traditional unsustainable materials, such as peat, vermiculite, and natural soil, are commonly mixed with alternative sustainable ones, which were part of the GM in 74% of the case studies (54% only for peat or soil). However, this aspect is also influenced by local contexts. For instance, in Europe, America, and Oceania, GMs constituted only by alternative materials are about one-third of the case studies in this review, while in Asia and Africa, GMs made of alternative materials account for less than 8%.

In many tropical areas or less developed countries, many nurseries lack the resources to obtain commercial nursery substrates and often resort to using local and sustainable materials, such as natural or forest soil. However, the use of natural forest soil as a substrate is an environmentally unsustainable practice. Harvesting topsoil for nursery purposes can be likened to mining operations, depleting a resource that has taken years to form which can cause erosion and site degradation [11]. In addition, the soil has been found to be an inadequate growing medium for containerized forest nursery production in Africa, Asia, and Latin America, producing poor-quality nursery seedlings [12]. The use of soil should be avoided for several technic reasons, including the considerable variability in its physical and chemical properties; the limited availability of nutrients; the potential presence of harmful soil microorganisms, weed seeds, insects, and other diseases; and the presence of clay and silt that can impede water drainage and soil aeration [11, 12].

Conclusions and Future Directions

The extensive literature produced worldwide demonstrates the widespread interest in the GM sector. However, similar to meta-analytical approaches in other fields [73], one of the main challenges in conducting a study analyzing this literature is the high variability among published experiments, significantly limiting the number of studies that can be effectively included. In our case, the variability was mainly due to the type of alternative materials used, the potential inclusion of traditional materials, and the varying proportions of each material in the GM. Furthermore, inconsistencies in reporting study methodology and statistics across articles hindered the inclusion of a significant number of studies. As a result, the establishment of criteria for analyzing available information inevitably affected the evaluation and the analytical comparison of the categories of alternative materials included in our study. Nevertheless, this work represents the first significant effort to quantify and analytically describe the utilization and effectiveness of materials other than peat.

Forest researchers and practitioners should keep exploring suitable combinations of renewable primary materials with valuable waste compounds for forest nursery activities, which would minimize environmental impacts and costs. Our review shows that most of the tested alternative materials were effective for growing nursery forest seedlings. However, few studies evaluated the field outplanting performance of container seedlings raised in GM composed of alternative materials, despite that field performance represents the acid test for the effectiveness of nursery cultivation treatments. Nonetheless, bark and sludge showed the best results in field outplanting performance for angiosperms and gymnosperms, respectively. The present study represents a stepping stone toward implementing knowledge-based practices for the sustainable growth of forest seedlings in nurseries. However, detailed, repeated, and statistically consistent studies should be performed in the future to allow more quantitative analyses. Moreover, to increase the number of alternative materials that otherwise would not be suitable as GM components, it is necessary to develop knowledge of specific chemical, thermochemical, and physical treatments to optimize their physicochemical properties for seedling growth [67]. However, these treatments might have a high environmental impact, turning an innovative and sustainable material into an unsustainable one. Future research should move from just testing the effect of GM on seedling morphological traits to a more comprehensive overview of GM effectiveness by including field tests of seedling performance. In a world of increasing natural resources scarcity and climatic crisis, the use of sustainable alternative GM has much to offer in a truly green industry, utilizing sustainable resources, and minimizing waste materials while improving the productivity and efficiency of container seedling production and forest restoration. Thus, it is urgent to move toward (i) a standardization on a local scale of the alternative material production process, (ii) the development of protocols for the use of the alternative material in forest nursery and field outplanting, and (iii) the identification of the best-performing mixtures based on local materials.