Introduction
Ecosystem services refer to the benefits people obtain, either directly or indirectly, from ecosystems (Nahlik et al.
2012). Production forests provide a diverse range of ecosystem services beneficial to societal wellbeing, including for example the storage and sequestration of atmospheric carbon, wood for building and energy, and environments for recreation. Despite the breadth of this capacity, forest management models are often adopted which enhance the delivery of single services, such as timber, to the detriment of other services, such as regulatory or cultural services (Bennett et al.
2009; Raudsepp-Hearne et al.
2010). A key challenge this century is to identify production forest alternatives better suited to the sustainable provision of a breadth of such services for a growing human populace (Gustafsson et al.
2012).
Whereas monocultures have excelled at providing large quantities of wood per unit area, this has often come at the expense of biodiversity (Lindenmayer and Franklin
2002), with resultant implications for additional ecosystem services (Jactel et al.
2009; Griess and Knoke
2011). In contrast, mixed-species approaches to production forestry, in which stands are designed around the targeted production of two or more tree species, may be less prone to such stark tradeoffs, and may even provide increased production and economic outcomes relative to monocultures (Griess and Knoke
2011; Paquette and Messier
2011; Gamfeldt et al.
2013; Bielak et al.
2014). Furthermore, the risks, uncertainties and increasingly observed damage inflicted on production forests by climate change (Seidl et al.
2014), may favor the increased use of mixed-species stands, as they provide managers with alternative directions for future stand development (Millar et al.
2007).
Whereas there is evidence that tree species mixtures in general provide a breadth of potential benefits relative to monocultures (Gamfeldt et al.
2013), the extent to which multiple ecosystem services can be simultaneously derived from specific mixed-species alternatives is less clear. For many regions, it remains to be determined how well individual mixed-species alternatives can balance the net tradeoffs and synergies among ecosystem services and adaptive capacity. Providing relevant insights in this regard requires scientific evaluations of an encompassing suite of ecosystem services, biodiversity, and other considerations derived from specific mixture versus monoculture forestry alternatives, within a given biogeographical context. From such studies, insights can be gained regarding the collective benefits and tradeoffs of a given mixed-forest alternative, with outcomes of relevance to forest owners, managers, and policy makers. Such studies should provide a more justified basis for motivating the adoption of mixed-species approaches, or alternatively, a better understanding of the reasons behind the continued widespread reliance on monocultures (Kelty
2006).
Here we conduct such an assessment in Sweden, where current policies and environmental goals are actively supporting the adoption of mixtures (SOU
2013). Our reference condition consists of a subset of Sweden’s even-aged Norway spruce (
Picea
abies; hereafter spruce)-dominated stands. We contrast this reference condition with two mixed-species production forest alternatives which dominate scientific consideration and the public discourse in Sweden: mixtures of spruce with either birch (
Betula pendula or
B. pubescens, hereafter birch) or Scots pine (
Pinus
sylvestris; hereafter pine). We evaluate the incentives, obstacles, and implications from the combined perspectives of biodiversity conservation, silviculture, production, economics, recreation, esthetics, ecological risks, water quality, and adaptive capacity. Our primary aim is to provide an overview of a broad range of relevant considerations, rather than a comprehensive review of each topic assessed.
Discussion
Relative to spruce monocultures, the adoption of spruce–birch or spruce–pine mixtures in Sweden can be expected to produce positive outcomes for forest biodiversity, water quality, and esthetic and recreational values, as well as likely reducing stand vulnerability to pest and pathogen damage (Fig.
1). These results support the contention that specific tree species mixtures can in fact achieve many of the broad categories of benefits commonly associated with mixtures in general. If any of these specific outcomes are prioritized over other considerations, then spruce–birch and spruce–pine mixtures appear to be clearly preferable production forest alternatives to spruce monocultures. In general, however, such results must be considered as part of the complex suite of incentives and disincentives for adopting mixtures, for which each decision maker will likely vary in how they prioritize any single concern, uncertainty, or benefit (Puettmann et al.
2015). Our results also highlight that even within targeted categories of concern, such as provisioning or regulatory ecosystem services, the emergent picture was complex. Whereas the two mixtures considered did reduce some stand vulnerabilities, other risks were projected to increase. Likewise, though some production and economic outcomes were likely to improve, other costs would be incurred. Overall, both mixtures considered were deemed to result in positive outcomes for the majority of issues assessed, but the conclusions reached from our assessment will nevertheless be dependent on the values that stakeholders place on the different ecosystem goods and services.
With respect to the potential wood production capacity of these mixtures, there are too few experimental studies to draw definitive conclusions for the variety of Swedish conditions. It is important to note however that even in those circumstances where equal or higher production capacity could safely be projected for particular site conditions, this may not result in mixture adoption. Previous studies emphasize that owners and managers frequently lack the necessary confidence and knowledge to switch to mixtures, despite proven production benefits. This reluctance is often linked to the associated increase in management complexity and related uncertainties regarding outcomes (Knoke et al.
2008; Pawson et al.
2013; Puettmann et al.
2015).
Economic outcomes are also context dependent, varying for example with harvesting costs, species-specific timber price lists, and the extent to which the natural regeneration of birch or pine can be exploited. However, an additional issue of importance is how economic considerations are evaluated. For example, greater or lesser emphasis may be placed on the importance of achieving greater yields, versus the importance of minimizing economic or ecological risks (Knoke et al.
2008). Depending on the disturbance of primary concern, mixtures may have a distinct advantage when evaluated from a risk minimization perspective and thus be favored even if the yield is equivalent or even less than monocultures. With respect to such economic risks, the two mixtures considered should also provide owners with increased management flexibility relative to monocultures.
Production forest alternatives must also be evaluated with respect to their capacity to address two problematic challenges posed by anthropogenic climate change: increased uncertainty and risk. Climate change is already affecting the capacity of production forests to deliver ecosystem services, due to altered environmental conditions and increased frequency and the extent of disturbances (Seidl et al.
2014). Over the coming century, the uncertainties inherent to climate change projection and long-term forest management (Millar et al.
2007) will likely be compounded by uncertainties from, for example, the establishment of new pests and pathogens, and the altered behavior and physiology of pest and pathogen species already present within a system (Pautasso et al.
2010). One of the principal recommended strategies for addressing such uncertainties is to “spread the risk” by diversifying tree species composition at stand and landscape scales (Felton et al.
2010a; Pawson et al.
2013). The inclusion of birch or pine in an otherwise spruce-dominated stand can thus be seen as an effective risk-spreading strategy, as it provides owners and managers with alternative directions for stand development when unforeseen disturbance events occur (Millar et al.
2007).
Targeted efforts are also required to reduce the vulnerability of stands to the specific disturbances projected to increase within a region due to anthropogenic climate change. Of direct concern with respect to spruce monocultures is the potential increased risk of pest and pathogen outbreaks, and climatic conditions more conducive to storm damage (Grundmann et al.
2011). Both spruce–birch and spruce–pine mixtures appear to reduce stand vulnerability to such risks. This can be considered to be a win–win adaptation strategy, as the use of these mixtures simultaneously diversifies stand conditions, to address the risks and uncertainties of climate change, while concurrently retaining the tree species for which the most extensive ecological and silvicultural knowledge base exists in Sweden (i.e., spruce).
There is an important and necessary caveat, however, with respect to linking mixture adoption with risk reduction, as spruce–pine mixtures may in fact increase stand vulnerability to some climate-related disturbances. Climate change could bring drier summer climates to southern Sweden and, if coupled with prolonged periods without precipitation, may increase the risk of forest fires (Kjellström et al.
2014). In such cases, the addition of pine to spruce production forests may in fact increase the risk of fire-related production losses (Fig.
1b). This highlights the importance of not conflating the adoption of mixtures with a generic capacity to reduce stand vulnerability to disturbance. Any resultant reductions in risk will be individual to the specific mixture’s tree species composition, regional context, and disturbance type (e.g., wind, fire, pest, and pathogen species) considered. The response of forest owners to recent storm damage in Sweden helps illustrate this point.
Concerns regarding the vulnerability of Sweden’s production forests to climate change rose after a storm hit southern Sweden in 2005 and damaged 75 million m
3 of wood within what was primarily spruce-dominated forests (Svensson et al.
2011). As a result, compensatory governmental funding was specifically targeted to encourage forest owners to regenerate with broadleaf tree species and associated mixtures. However, due in part to forest owners’ concerns regarding the susceptibility of such stands to damage by browsing ungulates, the majority of this funding went unused (Ulmanen et al.
2012). In this case, both financial incentives and the potential to reduce one long-term risk (windthrow) proved insufficient to overcome the other perceived risks (browsing damage) and uncertainties of adopting mixtures (Lidskog and Sjödin
2014). Whereas financial incentives are often a proposed means of encouraging the adoption of production forest alternatives (Puettmann et al.
2015), the outcomes observed in Sweden indicate how such efforts may readily be derailed if they are inadequate in relation to the perceived risks and uncertainties of the proposed alternative.
Conclusion
Relative to spruce monocultures, spruce–birch and spruce–pine mixtures appear to provide better outcomes in terms of biodiversity, recreational and esthetic values, water quality, and economic flexibility, as well as addressing some of the growing risks and uncertainties caused by anthropogenic climate change. Despite such benefits, several obstacles to the uptake of these tree species mixtures appear to remain, including browsing pressure, increased management complexity, and a continued uncertainty regarding their economic and production outcomes. On the basis of this study, we hope that research can be targeted toward resolving remaining obstacles and uncertainties, and increased opportunities may be identified for the adoption of mixtures.