Research on sustainability leadership in SMEs is important as there is a growing awareness within the SME literature that traditional operating procedures incurring high energy and water consumption costs, minimal recycling, and generation of considerable waste (among others) are no longer sustainable ^[1][2][1,2]. More than 70 years ago, Stogdill [3] noted that the study of leadership is an analysis of relationships and that all organizations operate within a large cultural and environmental framework. The objectives of organizations should be to maximise social values relative to the limited resources available ^[3][4][3,4]. For contemporary scholars, there is increasing interest and strategic concern to reduce and recycle waste, use less energy and water, develop new products or services, and to create efficiencies and increase innovation ^[5][6][5,6]. SME leaders are in a powerful position to promote positive environmental sustainability and eco-efficient outcomes in their firms. Indeed, leaders and leadership are a ‘key interpreter’ of how organizations respond to environmental challenges [7]. Although there is an emerging leadership literature describing sustainable, environmental and sustainability leadership in the education sector ^[8][9][8,9], large companies ^[10][11][12][10,11,12], and government [13], many extant studies only take the narrative related to SME sustainability leadership so far. Whilst leadership is considered an important capability to respond and adapt to external environmental and economic fluctuations [14], there is a lack of agreement and understanding about the types of leadership necessary for positive environmental sustainability ^[7][15][16][7,15,16]. What is missing in leadership literature is a theoretical framework that provides a new way of thinking about existing sustainability leadership dimensions. This is important in the study of sustainability leadership since scholars and practitioners are often no wiser about which leadership behaviours best suit eco-efficient outcomes and sustainable practices. It is particularly important for SME leaders who lack the necessary knowledge, capital, and resources to address and implement sustainability initiatives ^[17][18][17,18]. Thus, the aim of this paper is to classify the emerging leadership skills and knowledge necessary to achieve improved SME environmental performance.

While the sustainability literature is one of the most interesting fields of scholarly inquiry related to achieving eco-efficient outcomes, specific knowledge about sustainability leadership practices and skills is not well defined across industry settings and within knowledge domains ^[12][19][12,19]. General descriptions about sustainable, environmental, and sustainability leadership are increasingly being used to imply what sustainability-focused leaders do, their interactions, their relationships, and how they address complex sustainable challenges, even while the contexts in which leaders operate are difficult and complex ^[20][21][20,21]. While leadership is considered an important capability to respond and adapt to external environmental and economic fluctuations ^[14][22][14,22], agreement about the types of leadership practices necessary to achieve positive environmental sustainability is less transparent ^[7][12][16][7,12,16]. Scholars note that sustainability challenges require new leadership skills and practices that ‘help support managers to become effective sustainability leaders in their organizations’ [12]. Accordingly, this review seeks to expand relative scientific contribution by drawing upon a range of theoretical ideas, and the most pervasive leadership skills and practices relevant within a leadership domain. Here, the review is designed to provide more revelatory value by improving the classification and compilation of leadership skills and behaviours by elevating their utility within an SME context.

While extant studies examining leader practices have been linked to sustainability ^[7][23][7,23], environmental leadership practice [24], and sustainable productivity outcomes ^[10][22][24][10,22,24], few clear links exist between leadership practice cause and effect in respect of sustainable outcomes more generally. Thus far, scholars use sustainable, environmental, and sustainability terminologies interchangeably, yet this often creates confusion in the literature, making it more difficult for scholars and practitioners to identify which sustainability leadership practices are more germane within a given context ^[20][25][26][20,25,26]. In order to make sense of the literature and to clarify often contradictory terms, this paper offers a framework that provides a nuanced understanding of the practices relating to sustainability leadership.

Different leadership practices do not only reflect individual-level leadership attributes, but also organization-level practices embedded as structures, systems, and routines as mechanisms to create change. A typical example of structures and systems is sustainable production processes to reduce waste or technological structures to reduce emissions. These structures potentially create innovation ^[27][28][27,28]. Organizations design structures and systems to create institutional knowledge that increases the organizational capacity ^[29][30][29,30] of leaders to respond to environmental challenges necessary to guide how the organization itself learns ^[31][32][31,32], including knowledge about how different aspects of organization design have an important mediating influence on environmental management [33]. As an organization-level attribute, sustainable structures and systems will need to reflect what leaders pay attention to [34], including the routines that are created that lead to sustainable knowledge creation ^[35][36][35,36]. Within the existing literature on sustainability leadership, it is less clear how these structures and systems are created to support sustainable innovations in general.

Similarly, how SMEs introduce sustainability measures by creating strong internal and external network capital linkages needs to be better understood [37]. Social capital buttresses the development of social relationships [38], where social relationships comprise of both internal and external social capital that provide linkages between individuals and external firms [39]. Social capital more generally refers to the ‘ability of actors to secure benefits by virtue of membership in social networks or other social structures’ [40]. External relationships refer to networks established among external actors ^[38][41][42][38,41,42], that together lead to network capital benefits such as co-created knowledge routines through leadership experience ties ^[43][44][43,44]. This involves an affinity between individual and organization-level leadership attributes with a priority to establish social, as well as organization capital ^[39][45][39,45]. Since sustainability is concerned about the impact of decisions on the natural environment, leaders need to work more closely within and across networks.

2. Sustainable, Environmental and Sustainability Leadership

There is a significant body of literature related to the functions of sustainable, environmental, and sustainability leadership over the last 30 years. Extant research treats these as separate bodies of literature, yet while the embodiment of functions and processes could be deemed as dissimilar, fine nuances and complementary characteristics exist with respect to leadership practices. Sustainable leadership (SL) is often described in terms of being socially and environmentally responsible, preserving and sustaining the environment, and as a shared responsibility between both internal and external players as they plan for long-term sustainable goals ^[4][10][23][4,10,23]. Similarly, environmental leadership (EL) relies on practices that care for and protect the natural environment such as the deep-seated values and beliefs of organizations and the moral commitment of its members towards achieving a sustainable outcome ^[22][46][22,47]. Slightly different is sustainability leadership (ST-L). Here, it is about anyone who takes responsibility for acting on sustainable challenges including but not limited to initiating and influencing change towards some sustainable future ^[16][47][16,48]. Similarly, for ST-L, setting direction while simultaneously creating organizational alignment underpins a commitment towards employee and stakeholder buy-in ^[48][49]. Interestingly, leadership practice becomes critical for SL, EL, and ST-L practices, such as establishing a creative culture that enables the organization to learn new things and increase its innovative processes ^[7][25][7,25]. Leadership practices more generally underpin the nuanced differences between the three definitions. At their most basic, leadership practices involve key knowledge, skills, and abilities that comprise the individual-level attributes of individual leaders ^[49][50][51][50,51,52]. First, practices related to developing SL skills pertains to the cognitive antecedents of leaders, namely establishing a mindset for innovation, creating shared responsibility and passion between stakeholders, and developing the motivation towards achieving sustainable goals ^{[8][9][10][52]}[8,9,10,53]. RWesearchers call these innovation and relational practices. Second, practices related to EL pertains to an embedded personal moral imperative towards protecting the environment, identifying individual and organizational levels of influence, and establishing a set of sustainable personal values and norms ^{[24][46][52][53]}[24,47,53,54]. ThWey call these ideological and moral practices. Lastly, practices related to ST-L pertain to what leaders do in relation to direction-setting and top management commitment, relational-oriented practices, and task-oriented practices towards developing action plans, aligning goals, and dealing with complex internal and external environmental adaptation ^[51][54][55][52,55,56]. ThWey call these strategic adaptation practices. In sum, these leadership practices underpin what leaders do, the tasks and functions that guide action to build environmentally sustainable outcomes (ESO) within the SME context and form the basis of the theoretical framework (Figure 1). What is illustrated in Figure 1 is that leadership capital does not stand in isolation of other sustainable knowledge practices, which are explained in more detail later in this review paper. TheiOur research suggests that leadership will occur not only at the individual level but will necessitate integration with sustainable practices at the organization and network capital level. For instance, sharing positive change and building environmental norms will almost certainly be fashioned at the organizational level ^[56][57], while mobilizing employees is not only an individual skill but also an organizational process that helps establish long-term ecological goals ^[57][58]. Similarly, the idea of co-creation within the sustainability literature involves moving beyond individual social relations to forging value-chain based production processes [35], as well as the self-development of actors (SME Leaders) networking, collaborating, and forming ecosystems that support sustainable productivity goals ^[58][59]. Co-creation itself appears to invoke a set of leadership practices focused on the creation of an integrated network of suppliers, vendors, and customers ^[48][49]. In sum, the theoretical framework is a new approach aimed at bringing to the forefront many leadership skills and practices in SMEs. For the first of the sustainable knowledge practices presented in Figure 1, the separate but related bodies of sustainable and environmental leadership practice are transitioned into one generic term, that is, sustainability leadership (ST-L hereafter). This review thus seeks to address what studies appear to describe as separate leadership practices by clarifying that, collectively, such practices are complementary and integrated individual-level leadership attributes. This is similar to how complementarity is described within the human capital resource (HCR) literature, where an ‘employee and the firm possess distinct HCR that, when combined, have the potential to create an amount of value that is greater than the sum of the individual parts’ ^[59][60]. While nuanced differences can be observed in the three approaches, it makes little sense to separate the practices of leaders across similar sustainability tasks. Thus, rwesearchers provide greater clarity of the relationship between ST-L and how to achieve environmentally sustainable and eco-efficient outcomes based on what leaders do by exploring which leadership practices are more germane within a given ST-L context. Accordingly, ST-L is the basis of leadership practices (see Figure 1) and is discussed in more detail next. An analysis and summary of extant research that informs the relationships in Figure 1 is outlined in Table 1 as follows.