Contributions to Sustainability through Young Children’s Nature Play: A Systematic Review

: Nature play with young children has been criticized for lacking the transformative power necessary for meaningfully contributing to sustainability issues. The purpose of this systematic review was to identify outcomes associated with young children’s nature play that align with Education for Sustainability outcomes, toward addressing the question of its contribution to a more sustainable future. A total of 272 citation records were screened using eligibility and quality appraisal criteria, resulting in 32 studies that were reviewed. These studies’ outcomes were coded and then mapped to an education for sustainability framework. Results suggest that nature play supports education for sustainability benchmarks of applied knowledge, dispositions, skills, and applications. The multiple and varied relevant outcomes associated with nature play suggest practitioners should not abandon nature play in the pursuit of sustainability. Implications for practice and further research are discussed.


Introduction
Early childhood is a critical period, not only in the in the context of development, but also in the context of sustainability, as values, attitudes, and foundational skills learned in early childhood extend throughout life. The importance of early childhood education for sustainability (ECEfS) is internationally recognized, yet differing approaches have been put forward to achieve its goals. One approach emphasizes time in nature, as exploratory and playful experiences in nature provide a foundation upon which children develop the attitudes and values they carry into adulthood. However, some researchers [1] have criticized this nature-oriented approach as an impediment to children's ability to work for a sustainable future.
Consequently, another overarching perspective for ECEfS advocates for a more transformative, participatory orientation through honoring young children's rights and responsibilities as agents of change and involving them in exploring worldviews, problem-posing, decision making, advocacy, and action. Davis and Elliott [1] (p. 1) urge researchers and practitioners to recognize the competences of young children as "thinkers, problem-solvers, and agents of change for sustainability." They challenge traditional environmental learning notions of young children, suggesting the need for a transformative shift toward learning that encourages young children to engage in sustainability issues in authentic and meaningful ways-locally and in broader contexts.
In the context of young children, nature play is set apart from other types of play in that it takes place in outdoor, natural settings; is unstructured or loosely structured; and involves interactions with nature (not just in nature) [27]. Nature play is also distinguished from more structured forms of nature-based learning and programming [27]. For children, playing is learning [28], and thus there are many benefits of nature play. Nature play is associated with significant benefits to children's academic and social and emotional learning, as well as health benefits [29]. In addition, nature play has been associated with environmental learning outcomes, such as exploratory behaviors [30], stewardship [31], reflective thinking [32], and curiosity [2].
In spite of these outcomes, criticisms of nature play in the context of ECEfS exist, including the fact that educators may misalign exposure to naturalized outdoor play settings with fulfillment of ECEfS expectations [13]. Nature play advocates also have been criticized for their presumed rationale for nature play: EfS is too complex for children, and age and perceived innocence necessitate the use of nature play. Additionally, the point is made that inclusion of nature play impedes the practice, examination, and acceptance of effective techniques for ECEfS [33].
In light of these criticisms, it may be helpful to be reminded of Edwards and Cutter-MacKenzie's [8] description of ECEfS as constructing understanding about environmental and sustainability content as well as developing the skills for meeting the needs of future generations. Nature play shows potential for teaching about sustainability, particularly when purposefully framed and guided toward environmental understandings. Education about sustainability, however, is not enough to foster the values, attitudes, and skills learners need to act for a sustainable future [8,34].
Nature play has the potential to build skills, attitudes, and values associated with education for sustainability, evidenced by studies mentioned prior. ECEfS in the form of unstructured nature play can support children's natural tendencies to embrace the morethan-human world [21]. While nature play sometimes has been reduced to "exposure to nature or naturalized surroundings", it is often more than that, including children having formative experiences, reflecting on their learning, and being transformed as a result of their interactions with the natural setting. Thus, the pedagogy of play is worth examining as an approach for building skills and dispositions identified as essential to EfS.
Another criticism of nature play stems from the lack of prominence of the role of the teacher. Critics point out teachers are essential to children's learning, and without making learning explicit, children would not be aware of the concepts or ECEfS or achieve the outcomes of ECEfS [17]. Fleer [35] similarly posits that children need adults to assist them in accessing the knowledge conveyed through place [8]. Thus, sustainability concepts are unlikely to be learned by children solely through nature play [8].
These criticisms of nature play further underscore varying perspectives as to how and if nature play contributes to sustainability. Thus, the context for the review at hand is this very question of nature play's contribution to sustainability that is spurring an international call for more critical and transformative approaches to ECEfS, including the adoption of pedagogies such as advocacy and action-taking in sustainability issues locally and more broadly.

Purpose
The purpose of this study was to identify outcomes of young children's nature play that can contribute to a more sustainable future. The aim was not to further categorize or emphasize differences across different types of nature play approaches and programs, nor was it to evaluate the effectiveness of nature play programs or link specific program characteristics to outcomes. Instead, the review sought to identify outcomes that are associated with nature play with young children that align with EfS outcomes, toward addressing the question of its contribution in the context of EfS. Thus, the overarching was it to evaluate the effectiveness of nature play programs or link specific program ch acteristics to outcomes. Instead, the review sought to identify outcomes that are associa with nature play with young children that align with EfS outcomes, toward address the question of its contribution in the context of EfS. Thus, the overarching methodolo can be thought of in terms of two general phases: (1) the systematic review, which yield outcomes associated with young children's nature play, and (2) the mapping of these o comes to an established framework of EfS outcomes.

Systematic Review Design and Search Process
In general, the methodology for the systematic review followed the process used Ardoin and Bowers [36] in their review of outcomes of early childhood environmen education. Their methodology was modeled after PRISMA and its criteria for conduct and reporting systematic reviews [37]. This process entails the general steps of identify records using search terms, initial screening of records, reviewing records for eligibil and reviewing and synthesizing the resulting studies that were eligible for inclusion the review. See Figure 1 for a summary of this process.  [37] and exclusion criteria guided by Ardoin and Bowers [36].
The review of Dankiw et al. (2020) included studies published through July 2018, a Ardoin and Bowers [27] included studies published through December 2018. Databa used in Ardoin and Bowers's [36] review of early childhood environmental education p gram outcomes and Dankiw et al.'s [27] review of unstructured nature play outcom  [37] and exclusion criteria guided by Ardoin and Bowers [36].
The review of Dankiw et al. (2020) included studies published through July 2018, and Ardoin and Bowers [27] included studies published through December 2018. Databases used in Ardoin and Bowers's [36] review of early childhood environmental education program outcomes and Dankiw et al.'s [27] review of unstructured nature play outcomes were used to identify studies published since the timeframe of their reviews, which was the time period from July 2018 through to the end of March 2021.
Across their two reviews, the following academic databases had been used: Academic Search Premier, Africa-Wide Information, British Education Index, Education Full Text, Embase, Emcare, Environment Index, ERIC, GreenFILE, MEDLINE, PsycINFO, The Cochrane Library, and The Joanna Briggs Institute. However, the research team for this systematic review at hand did not have institutional library access to several of the databases Ardoin and Bowers [36] also used manual searching of Children, Youth and Environments and the International Journal of Early Childhood Environmental Education, as articles in these two publications did not consistently appear in database searches. Thus, these two journals were also included in this step. The journal Sustainability was also manually searched to identify studies related to our search terms published between July 2018 and March 2021, in light of their inclusion of EfS and ECEfS articles.
To identify records, we used the following search terms, which were drawn from the two aforementioned systematic reviews: preschool, kindergarten, free play, forest school, childcare, day care, early childhood, early elementary, early primary, nursery school, primary grade, toddler, young child, young children, education for sustainability, education for sustainable development, environmental education, forest kindergarten, nature preschool, nature-based preschool, and sustainability education. The terms used by Dankiw et al. [27] and Ardoin and Bowers [36] that were less specific (forest, nature, natural, outside, outdoors, play, green school, green space, childcare, outdoor classroom) were removed from the search terms list, as preliminary searches yield hundreds of thousands of articles meeting the initial search criteria. The following additional specific search terms were included in this review to further focus and yield relevant articles for the specific purpose at hand: early childhood education, early years, nature play, nature kindergarten, forest preschool, outdoor play, and early childhood education for sustainability.
The database searches identified a total of 238 citation records after duplicates were removed, and the manual searches of the three journals identified an additional 34 unique citation records. The combined results from both of these search strategies yielded a total of 272 citation records for this study identification step.

Study Screening and Eligibility
The inclusion criteria for the systematic literature review at hand were as follows: studies that focused on young children aged from birth through to age eight, reported outcomes for a program that was described as and/or used nature play, and those that were designed as empirical research or evaluation. Exclusion criteria were drawn from Ardoin and Bowers [36]: studies that did not meet the inclusion criteria, articles that were in practitioner-oriented journals (that tend to describe classroom activities or include lesson plans), studies that were in the form of dissertations or conference papers/abstracts, and textbooks or book chapters.
The studies from the two completed systematic reviews (n = 66 and n = 16), along with the 272 eligible studies identified from the database searches from July 2018 to March 2021, resulted in a sample of 354 studies that moved forward into the next step of the systematic review process. Patterning after the approach used by Ardoin and Bowers [36], the abstracts of the 272 records (from July 2018 to March 2021) were screened using the above exclusion criteria. The abstracts of the 66 studies from Ardoin and Bowers [36] and the 16 studies from Dankiw et al. [27] were also reviewed for eligibility against the inclusion criteria and exclusion criteria. This was necessary, as Ardoin and Bowers [36] focused on environmental education programs in the context of early childhood, which includes but is not limited to nature play. Similarly, while Dankiw et al. [27] focused on Sustainability 2021, 13, 7443 7 of 36 nature play, their review included studies of children ranging in ages up to age 12, which exceeded the early childhood period of birth through age eight.
A total of 301 records were excluded through the eligibility screening process. The full-text articles for the remaining 56 records were then located and further reviewed to ensure they met the inclusion criteria.

Quality Appraisal
The criteria used for the quality appraisal were drawn from Ardoin and Bowers [36]. The criteria were as follows: peer-reviewed research, including a program description, including information on the research methods and data, and the findings needed to be sufficiently detailed. Fifty-six studies were reviewed using these criteria, with 24 studies not meeting these criteria. Thus, there were 32 studies that moved forward in the process for inclusion in the review.

Data Analysis
The data analysis step was undertaken using the final set of 32 studies, which were published during the time period of January 1995 to March 2021 and met the criteria from the preceding steps. A spreadsheet was created and used to record the following information for each of the 32 studies included in the review: authors and publication date, location of study (country), age of program participants, program description, research methodology, and summary of study outcomes. In the initial review, the reported outcomes from each study were recorded but not coded. Additionally, if findings were null or negative, this was noted on the spreadsheet.
Then, each study's reported outcomes were coded on the spreadsheet using categories adapted from Ardoin and Bowers [36], as well as from the North American Association for Environmental Education Framework for Assessing Environmental Literacy [38], early childhood learning domain descriptions from the Minnesota Early Childhood Indicators of Progress [39], and the Center on the Developing Child at Harvard University [40]. See Table A1 (Appendix A) for a listing of the outcome categories, a description of each, and their source.
These outcome categories were used again in the next step of the coding process, where all of the specific outcomes within any one outcome category were reviewed to check for internal consistency and conceptual coherence as a category of outcomes. Two researchers were involved in this process of reviewing studies and recording information on the spreadsheet, coding articles according to outcome categories, and reviewing the coding for consistency and coherence. Researchers first worked independently and then reviewed the resulting coding and category sets. Inconsistencies or discrepancies were discussed, and the study was re-reviewed, toward agreement on finalized coding. The next step of the data analysis process involved aligning the study outcomes with the ECEfS outcomes by mapping them to ECEfS outcomes. This alignment/mapping was conducted using two guiding documents for EfS, the Cloud Institute's Education for a Sustainable Future Benchmarks for Individual and Social Learning [41], which is for all ages, and secondly, a related document specifically designed for ECEfS, the Cloud Institute's Education for a Sustainable Future Standards and Performance Indicators PreK-2 Edition [42], which is a more narrow set of content standards/applied knowledge standards than in Cloud Institute's Education for a Sustainable Future Benchmarks for Individual and Social Learning The outcome categories and corresponding specific outcomes from the 32 studies were mapped against the benchmarks and standards in these two guiding documents by one of the researchers, with a second researcher reviewing and confirming the mapped outcomes, and a final review of the coding and mapping by two additional researchers. Discrepancies were discussed toward reaching agreement on this mapping process. While the performance indicators were helpful in the mapping process in terms of conveying the meaning of the specific standard at hand, the nature play outcomes were mapped to the standards, not the performance indicators.

Results
Of the 32 studies reviewed, 15 were studies from the United States, six were from Canada, four were from Turkey, three were from the United Kingdom, two were from Australia, one was from Greece, and one study was from Hong Kong. Most of the studies (22 of 32) were conducted with preschool-and/or kindergarten-aged children (typically ages three to five years old), with five studies that included participants younger than three years old, and two studies that included children up to age eight. Many of the studies (14 of 32) used qualitative methods, 12 used quantitative methods, and 6 used mixed methods in their research methodology. While all of the programs studied included and emphasized nature play, the range of contexts and settings varied. Many of the studies were of nature preschools or forest kindergartens. Others were nature play integrated within early childhood education centers/program through the help of a community partner such a nature center, park, or wildlife sanctuary. Some of these programs were traditional childcare, preschool, or kindergarten classes (including several located at universities or in urban areas) that were incorporating trips to local natural areas into their curriculum toward the provision of nature play opportunities. Others were adapting their onsite facilities to include naturalized spaces. The studies collectively displayed a wide array of positive outcomes across many domains. Table A2 summarizes the 32 studies reviewed, including each study's location, program description, age of participants, research methodology, and reported outcomes.
There were 98 total outcomes of nature play reported by the 32 studies. The most frequently reported outcomes across all the studies were connection to nature; stewardship of plants, wildlife, living things, and nature/compassionate care for nature; self-confidence; and self-regulation/self-management/self-control, each with six separate studies reporting these as outcomes. Five different studies reported prosocial skills and behaviors, and exploratory skills was also reported in five studies as an outcome of nature play. Fourteen other outcomes of nature play were each reported four or more times across the 32 studies, and 27 outcomes had three or more mentions across the studies. Table A3 displays  The second step of the analysis involved mapping the outcomes from the 32 studies to the ECEfS guiding documents [41,42] (see Table A4). Regarding the benchmark applied knowledge, there was evidence from the reviewed nature play studies for most of the standards, including the following: cultural preservation and transformation, responsible local and global citizenship, the dynamics of systems and change, inventing and affecting the future, multiple perspectives, and strong sense of place. Of these, responsible local and global citizenship, inventing and affecting the future, and strong sense of place had the greatest breadth and quantity of supporting studies. Three of the standards within the applied knowledge benchmark did not have evidence from nature play studies reviewed: sustainable economics, healthy commons, and natural laws and ecological principles.
Regarding the benchmark dispositions, both standards (being and relating) had supporting evidence from nature play studies, and both were mapped to a wide range of nature play outcomes and associated studies (see Table A4). The same was true for the benchmark skills, with the standards of thinking skills and hands-on skills both having a breadth and quantity of evidence from nature play studies, as shown in Table A4. Finally, the applications and actions benchmark had supporting evidence from nature play studies for all if its standards, including the following: build capacity, design and create, lead and govern, be just and fair, and participate and collaborate (see Table A4). Table A5 provides an overall summary representing the ECEfS benchmarks and standards and the respective supporting evidence of outcomes attained through nature play. While many outcomes from the 32 nature play studies included in this systematic review did align with the ECEfS outcomes, there were some outcomes that did not. These include the outcomes in the categories of physical development outcomes and mental well-being. Moreover, the specific outcomes of self-expression, self-care, self-regulation, management, and control; early literacy and early numeracy; and skills for being in, moving in, and interacting with nature; confidence in nature; trust in and of nature; and autonomy did not directly align with ECEfS benchmarks, standards, or performance indicators.

Discussion
The backdrop for this study was the question that emerged from diverging viewpoints regarding ECEfS pedagogies: What contribution to sustainability is made through the pedagogical practice of nature play, or is a reorientation of the nature play movement needed toward more critical and transformative pedagogies [2]? Thus, the systematic review at hand sought to identify outcomes of young children's nature play that further the aims of EfS.
First, though, it is important to acknowledge limitations to this review so that the results can be interpreted in the context of these limitations. While the intent was to use the same databases as used in the two prior systematic reviews of nature play and early childhood EE, not all of the databases were accessible through our associated universities, and thus substitutions were made. Moreover, researchers of nature play studies may have published in other journals that weren't among those searched in this review. Consequently, there may be more evidence (more alignment) than what the results of this study suggest.
Alternatively, there may be less evidence (less alignment) than what the results of this study suggest, due to the criteria used in the quality appraisal step, which had been drawn from Ardoin and Bowers [36]. To be included in the review, the study had to be peerreviewed research and include sufficiently detailed information on the research methods and data, and the findings needed to be sufficiently detailed. Applying these criteria, however, was more challenging than anticipated. Additionally, varying levels of rigor and internal and external validity were not fully accounted for through this appraisal, and thus from one study to another, the evidence may not be equally strong. Also challenging was the inclusion criteria that specified the study needed to be of nature play. Some of the studies were of programs that had nature play but had additional components as well. The extent to which programs had components beyond nature play was not accounted for in the review and analysis. Moreover, the mapping of nature play outcomes to the ECEfS framework was somewhat subjective, particularly as to the degree of relevance needed in order for the outcome to be mapped. Other researchers may have mapped them differently or more/less extensively. Finally, it is important to note that this review process was not to designed to enable identifying which standards were most or least supported through nature play, as the number of supporting studies mapped to any one standard in the framework does not necessarily suggest quantity of evidence, but potentially instead the breadth of the standard and/or the breadth (level of specificity) of the nature play outcome itself.
However, in spite of these limitations and on the basis of the results of the review, the answer to the question of nature play's contribution to sustainability is both extensive and rich. In addition to evidence suggesting nature play is supporting the development of children across domains as well as the development of environmental literacy, the results of this review illustrate the many ways in which outcomes associated with nature play are relevant to EfS and ultimately a more sustainable future. Nature play appears to be contributing to applied knowledge in the context of sustainability, specifically cultural preservation and transformation, responsible local and global citizenship, the dynamics of systems and change, inventing and affecting the future, multiple perspectives, and strong sense of place. Nature play also appears to be contributing to sustainability aims by furthering the dispositions of being and relating. In addition, nature play is contributing thinking skills and hands-on skills, as well as contributing to the applications and actions of building capacity, designing and creating, leading and governing, being just and fair, and participating and collaborating. Not only was there alignment, there also was more alignment than what was anticipated, as well as unexpected alignment. There also were many interconnections, as opposed to one-to-one mapping, with multiple studies and associated outcomes mapping to more than one standard in the ECEfS framework.
While the question of nature play's contribution to sustainability was the emphasis of the review, the results prompt another relevant question: Is nature play sufficient as a pedagogy for EfS with young children? While most of the ECEfS standards were mapped to nature play studies, and thus could be considered supported through nature play, several within the applied knowledge benchmark were not: sustainable economics, healthy commons, natural laws, and ecological principles. Additionally, while most standards had associated outcomes from nature play studies, for manageability and feasibility, mapping was done at the level of the standards as opposed to the level of the performance indicators. Thus, within any one standard, it is likely that the complete set of performance indicators did not have accompanying evidence of being achieved through nature play, and thus the standard would likely not be met in its entirety solely through nature play. That is not to say it could not be, but that there is not existing research evidence from the review at hand (it may exist but did not surface in this review, or it perhaps could be happening through nature play, but not yet assessed through research). Thus, it would be appropriate to suggest that nature play may not be (or is not yet known to be) sufficient toward meeting all of the desired outcomes of ECEfS.
Additionally, the response to these questions of nature play's contribution to sustainability and the sufficiency of nature play as a pedagogical approach for ECEfS depends on the framework used. The ECEfS outcomes used in this review, drawn from Cloud [41,42], somewhat mirrored the outcome domains of environmental literacy (knowledge, attitudes, skills, actions/behaviors): applied knowledge, dispositions, skills, applications, and actions. However, just as there is not yet a universally shared understanding of "sustainability", [41], there also is not a universally shared understanding of the aims of EfS.
Thus, whether nature play contributes to sustainability, and the extent to which it does so, depends on the framing of sustainability and on the EfS framework used. If, for example, Australia's Belonging, Being and Becoming: The Early Years Learning Framework had been used, the mapping (and thus level of sufficiency) would differ from what surfaced through the review and mapping at hand. As described by Elliott [43], children are not only belonging, being, and becoming in their sociocultural systems, but with respect to the earth's systems. Thus, their framework allows for a biocentric rather than a solely humancentered interpretation of what it means to belong, be, and become (and consequently, their early childhood benchmarks in essence are one in the same as the ECEfS benchmarks). In Elliott's [43] description of ECEfS outcomes, it is clear how relevant nature play can be as a strategy for EfS, especially in light of the outcomes that surfaced through this review of nature play research: . . . children need opportunities to experience relationships of belonging with nature and construct understandings about the complex dynamic interdependencies between humans and the Earth. Being is fully experiencing the here and now and natural elements offer children sensory-rich opportunities for being in the moment, while Becoming is about a process of change, children becoming active and empowered participants for sustainability in a rapidly changing climate.
[33] (para 7) The question of the extent to which nature play contributes, or nature play's sufficiency as a pedagogical approach in the context of ECEfS, also leads to the question of whether or not nature play needs to be sufficient. Does nature play need to be the sole pedagogy in order to be considered valid in the context of society's quest for sustainability? A somewhat parallel question is raised in science education and in the context of young children. This introductory text precedes the early learning standards in the domain of scientific thinking for Minnesota's (United States) Early Indicators of Child Progress [39]: The indicators in the Scientific Thinking domain . . . reflect the new thinking in the science education field: that for young learners, scientific inquiry is more beneficial than occasional and unconnected science activities. Therefore, the focus for this domain is on scientific processes more than specific science content with the idea that this approach will lay the foundation for developing ways of thinking that support more rigorous academic study in the Scientific Thinking domain in the elementary school years.
[44] (p. 1) Accordingly, it could be argued that the inquiry and exploration arising naturally in the context of nature play might be more beneficial than unconnected, content-focused activities that are aimed to toward building sustainability-related knowledge, or actionoriented activities that lack a meaningful connection to children's spheres of experiences. Conceivably, nature play is laying the foundation for developing ways of thinking (and ways of being and relating) that support more rigorous learning in the school years and beyond. This should not be seen as problematic, nor as a fault or as a source of criticism, as seldom if ever is there one sole pedagogy, practice, or program that supports learners toward competency across an entire set of benchmarks and standards that does not take away from nature play's value. Nature play can be used alongside other pedagogies, as well as serve as the foundation for the scaffolding of other sustainability experiences as children grow.
This sentiment is consistent with Green [45] (p. 317), who writes, "a child does not learn how to run before discovering how to coordinate his feet to take his first steps". Even Elliot [33], who has advocated for ECEfS pedagogies that are more transformative than nature play, states children need opportunities to experience relationships of belonging with nature, which this review of nature play outcomes certainly demonstrates. Elliot also notes the importance of children "being", where they are able to be fully present as they experience nature through sensory-rich opportunities, which also happens in nature play.
It seems through being in nature and developing a sense of belonging, the becoming becomes possible. Elliot [33] describes becoming as a process of change, where children becoming active and empowered participants for sustainability in the midst of our rapidly changing climate. The results of this review suggest nature play is facilitating action, as children were actively participating in ways such as demonstrating respectful interactions with and compassionate and care toward other living creatures, teaching others about plants and animals, and encouraging their families to adopt environmental behaviors modeled in their nature preschools/play environments. While these actions are likely not the transformative actions for which Elliot and others advocate [1], Green's [45] indication of the meaningfulness of these smaller-scale actions within children's immediate sphere seems quite relevant: "It is important to consider the significance of children's initial efforts as these provide stepping-stones to bigger and wider-reaching initiatives . . . making a difference at a smaller level will have an impact on how a child develops his or her sense of self in relation to the living world." Thus, not only does this underscore the relevance of these actions that may not seem very transformative, perhaps this also suggests that the belonging and being are not only steps toward becoming, but also that the becoming might further the sense of belonging in nature.

Implications for Practice
While this systematic review was useful toward recognizing the many and varied contributions of nature play in the context of sustainability, the review and process of mapping nature play outcomes to the ECEfS framework also provides guidance for practice and research. Regarding implications for practice, Spearman and Eckhoff [46] suggest considering sustainability at a scale that is accessible to children, framed as "little 's' sustainability", in a place-based and local, community and systems context. It seems evident from the results of this review that nature play provides a context that makes sustainability not only accessible to young children, but also meaningful in terms of contributing to "big 'S' Sustainability". Consequently, early childhood practitioners should not abandon nature play in the pursuit of sustainability.
With that said, an awareness of the range of performance indicators embedded with the ECEfS benchmarks and standards may open up new opportunities for practitioners to further deepen or broaden nature play and the conversations and explorations it naturally sparks. Edwards and Cutter-Mackenzie [8] refer to purposefully framed play that stems from observations of children's play; conceivably, it could also stem from particular indicators or standards from the ECEfS framework. They also propose teacher-enhanced approaches to nature play, whereby teachers augment nature play with more direct instruction on sustainability concepts, toward explicit connections between experience and content [8]. Ultimately, however, sustainability is, in principle, inclusive of multiple ways of knowing and encouraging of multiple pathways to achieve environmental, societal, and economic prosperity. As Wilson [21] suggests, there does not need to be a choice between "saving" and "savoring" nature. Pearson and Degotardi [47] promote inclusive ECEfS, which embraces broad and diverse values and practices across global, sociocultural context as an embodiment of the best practices of EfS [21].

Implications for Research
The range of performance indicators, standards, and benchmarks within the ECEfS framework used in this systematic review also opens up possibilities for new research directions. While many of these already have supporting evidence, some have not yet been studied directly. Knowing what evidence exists provides an opportunity to further consider that evidence toward determining if additional or more rigorous evidence may be needed, and gaps in the framework signal where research is needed to determine nature play's impact. Collectively, this can aid in our understanding of what nature play can and cannot offer in the context of sustainability so that other pedagogies can be drawn from and used as children grow toward an enduring kinship with nature, as well as toward an ever-deepening and ongoing participation in visioning, creating, and engaging in a healthy and just present and future.
A specific area for further research is regarding several interwoven criticisms of nature play (see [21] for an extended discussion). One is that nature play is not as relationshiporiented as EfS. Yet, evidence from this review suggests nature play is quite relationshiporiented, as there were many nature play studies mapped to the ECEfS standard of relating. Another criticism is that nature play promotes a view of nature as something separate from humans-something to be studied, experienced, and cared for, as opposed to a post-humanism orientation that aims for seeing "nature as extended self", co-habiting a shared planet and being entangled with and of nature, not just in nature (as in [13][14][15]). While a number of nature play studies yielded outcomes suggesting that nature play was associated with actions of caring for nature, there also were many studies suggesting it was not only furthering children's connection to nature, but also their environmental identities, feelings of belonging, and even a sense of intimacy and at home-ness in nature.
As noted prior, Green [45] suggests caretaking and stewardship actions that make a difference at a smaller level will have an impact on how a child develops their sense of self in relation to the living world. Thus, potentially rather than stewardship furthering the divide between nature and humans and prompting the need for pedagogies other than nature play to support a "beyond-stewardship relationship with nature" [12], perhaps stewardship is actually furthering children's feelings of belonging in and to nature, as well as their understandings of being part of it. If this is translated into human contexts, this possibility seems plausible; deep and extended care for a family member, for example, does not necessarily lead to a widening gap between the caregiver and family member but can and often does lead to feelings of deepened connection and an extension of self. Thus further research is needed to better understand the possibility of stewardship coexisting with a cognitive and affective sense of oneness within nature and a recognition of the interdependence between people and the more-than-human world, or if instead stewardship and nature play more broadly contribute to a dualism that places "humans strictly outside the natural world of which they are a part of, and may thereby inadvertently perpetuate the very alienation it seeks to overcome" [48] (p. 109).
Related to this, further research might also investigate the suggestion that in addition to unstructured play in nature, children need opportunities to participate in re-making the world, on the basis of the understanding that they are a part of-rather than separate fromthat world [16], as well as experiences that extend their caring beyond their individual interests and concerns [4], which is where social justice begins [16]. The review at hand, on the contrary, suggests nature play was affording opportunities for children to demonstrate caring and collaborative helping behaviors; children's display of empathy and a sense of compassion, concern, and responsibility for others suggests they already were participating in re-making the world through interactions that furthered a sense of community and belonging that extended beyond their individual interests. Thus, further research might seek to understand the role of nature play in not only fostering caring and participation and as a mechanism toward social justice, but also if those behaviors are rooted in and/or furthering feelings of being part of versus separate from the world. Additionally, research might also investigate the transferability of outcomes such as empathy, compassion, and care across human and more-than-human nature contexts, or the potential for these dispositions and behaviors to be mutually reinforcing of a blurred boundary between humans and nature, where compassion and empathy, for example, toward other humans actually entails compassion and empathy with nature and vice versa.

Conclusions
The results of this review suggest nature play is a valid contributor to sustainability outcomes, and thus it should be acknowledged and embraced as an effective EfS approach with numerous, wide ranging benefits to young children. The use of a variety of pedagogies across the span of a child's educational years can provide the experiences, content, and applications that can be drawn upon, individually and collectively, to create a sustainable future. Nature play is a good match for the early childhood years when children are developing skills and abilities across multiple domains. In addition to the strong physical, social, and cognitive outcomes afforded though nature play, children are gaining knowledge, skills, dispositions, and actions that are foundational to sustainability, and in many cases, these outcomes overlap. Thus, integrating sustainability and early childhood education represents what Wilson [21,49] describes as a "goodness of fit". One does not have to choose between education for sustainability and early childhood education, underscoring the description of early childhood education as having "all the possibilities in the world" [50] (p. 369) to further human-nature flourishing and a more just, sustainable world.
Author Contributions: All authors were involved in the conceptualization of the study. J.E. served as the lead investigator and author and was responsible for developing the methodology. K.M. led the search process, including the identification of records, accessing articles, and initial screening and coding of articles. Both were involved in the review of articles against eligibility criteria, as well as in the quality appraisal, coding, and mapping processes. K.M. led the data curation, organization, and visualizations, including the creation of tables and compilation of references. P.S. and R.S. served as the external reviewers, confirming the coding and mapping, in the context of establishing dependability of the study's findings. They also contributed to the review of and revisions to the manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.        Table A3. Results of the coding of study outcomes into the outcome categories.

Environmental Literacy Development: Knowledge
Knowledge about nature [51] Knowledge of local plants and animals [63,64,70] Knowledge of cause-and-effect relationships [59] Knowledge of growing/harvesting local food [63] Environmental awareness and knowledge [66] Use of plant/nature terminology [56] Table A3. Cont.

Skills and Competencies
Skills for being in/moving about in/interacting with natural environment [70] Spatial autonomy (sense of comfort, independence, and self-confidence) in nature [64,71,76] Trust in interactions in/with nature [63,71] Environmental Literacy Development:

Other: Changes in Play Behavior
Increased play with natural materials; increased independent play [54] Higher levels of pretend play [58] More expansive forms of play [32] Frequency and depth of dramatic play [57,74]