Restricting new forests to conservation lands severely constrains carbon and biodiversity gains in New Zealand

Biological Conservation 181 (2015) 206–218

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Restricting new forests to conservation lands severely constrains carbon and biodiversity gains in New Zealand Fiona E. Carswell a,⇑, Norman W.H. Mason b, Jacob McC. Overton b, Robbie Price b, Lawrence E. Burrows a, Robert B. Allen a a b

Landcare Research, PO Box 69040, Lincoln 7640, New Zealand Landcare Research, Private Bag 3127, Hamilton 3240, New Zealand

a r t i c l e

i n f o

Article history: Received 1 July 2014 Received in revised form 27 October 2014 Accepted 3 November 2014

Keywords: Afforestation Conservation planning Ecosystem services Trade-offs Forest regeneration UN-REDD+

a b s t r a c t Increased afforestation of non-productive land could deliver win–win solutions for greenhouse gas mitigation through carbon sequestration and biodiversity gains, referred to here as increased ‘ecological integrity’. We examined the potential trade-offs when selecting non-forested lands in New Zealand for natural forest regeneration to maximise gains in either, or both, carbon and biodiversity. We also examine the effect on potential gains and trade-offs of excluding non-conservation lands from spatial planning for conservation. The most significant per-hectare gains, for both carbon and biodiversity, were those occurring on non-conservation lands because conservation lands are mainly restricted to low-productivity environments where indigenous vegetation is already well represented. By contrast, productive environments, such as alluvial plains, where almost no indigenous vegetation remains, are primarily on non-conservation lands. These lands will need to be included in any reforestation strategy or else the most degraded ecosystems will not be restored. We found that biodiversity suffers a greater trade-off when carbon gain is prioritised than carbon does when biodiversity is prioritised. Trade-offs between carbon and biodiversity were higher on non-conservation lands but decreased with increasing area regenerated. Our study shows that natural regeneration will provide substantial increases in carbon and biodiversity on non-conservation lands compared with conservation lands. This emphasised the need for improved incentives to private land owners if carbon and biodiversity gain from afforestation is to be maximised. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Protected natural areas (conservation lands) have been recognised for some time as potential carbon sinks, where carbon sequestration through afforestation could aid reductions of global carbon dioxide concentrations without displacing economic activity (Miles and Kapos, 2008). However, the primary role of conservation lands is biodiversity conservation and enhancement. The limited funds available for conservation necessitate careful consideration of the projects that can maximise biodiversity gain (Schindler and Lee, 2010). Prioritisation of carbon during reserve design for existing ecosystems can lead to lower biodiversity than if biodiversity alone is prioritised (Chan et al., 2006; Anderson et al., 2009; Naidoo et al., 2008; Moilanen et al., 2011; Thomas ⇑ Corresponding author. Tel.: +64 3 321 9631; fax: +64 3 321 9998. E-mail addresses: [email protected] (F.E. Carswell), masonn@ landcareresearch.co.nz (N.W.H. Mason), [email protected] (J.McC. Overton), [email protected] (R. Price), burrowsl@ landcareresearch.co.nz (L.E. Burrows), [email protected] (R.B. Allen). http://dx.doi.org/10.1016/j.biocon.2014.11.002 0006-3207/Ó 2014 Elsevier Ltd. All rights reserved.

et al., 2013). However, it remains unclear whether these negative trade-offs also occur during spatial allocation of natural regeneration to provide new forests. This study compares hypothetical scenarios where natural regeneration of forest is spatially allocated to maximise either or both carbon and biodiversity gain across conservation lands of New Zealand. These gains are compared with those possible when non-conservation lands are also included. The potential for carbon markets to compromise biodiversity has been known for over 10 years (e.g. Schulze et al., 2002). An international attempt to counter the potential trade-off between carbon and biodiversity has been made through the establishment of the United Nations’ REDD+ (Reducing Emissions from Deforestation and forest Degradation in developing countries) programme that specifically targets the role of conservation, sustainable management of forests and enhancement of forest carbon stocks in developing countries. One potential outcome of REDD+ is the protection of biodiversity in natural forests instead of converting natural forests to faster-growing non-native plantations. More economically developed countries, such as New Zealand, could also contribute to the twin goals of increased biodiversity and carbon

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sequestration, using slightly different mechanisms to those posed by REDD+, namely the creation of new conservation lands through natural regeneration of indigenous forests. Existing conservation lands in more economically developed countries are generally dominated by ecosystems of little economic value for farming (e.g. Pressey, 1994; Aycrigg et al., 2013). In New Zealand, these lands tend to be in steep, cool, wet mountain environments with low soil fertility (Leathwick, 2003; Walker et al., 2006). In contrast, both in New Zealand and abroad, the areas that have been most heavily impacted by human activities (e.g. alluvial floodplains, riparian habitats and coastal ecosystems) are severely under-represented in conservation lands (Pressey, 1994; Walker et al., 2008). Current conservation lands may therefore have limited opportunities for carbon gain, since these lands are dominated by low-productivity environments with low carbon sequestration rates and low potential carbon storage. Consequently, we compare per-hectare scenarios where natural regeneration is spatially allocated across the whole of New Zealand or is restricted to existing conservation lands. There is significant potential for carbon sequestration through indigenous forest regeneration across New Zealand lands that are currently used for pastoral agriculture (Trotter et al., 2005). New Zealand was heavily deforested only recently (starting in c. CE 1200) – first by Polynesian settlers (McWethy et al., 2009) and then by Europeans, with a corresponding reduction in indigenous forest cover from approximately 85% of the total land area to less than 30% (Wilmshurst et al., 2007). Consequently, there are large areas of land that do not currently support forest but could do so, potentially. The establishment of a national plot network to measure change in carbon stocks has provided a means for objective estimation of current carbon stocks in forests and shrublands. Current stocks across this plot network also provide a means for estimating potential carbon gains on other non-forested lands assuming similar forest types can be achieved (e.g. Mason et al., 2012a). When examining potential gains we confine our investigation to the use of natural regeneration for establishing indigenous forests, as this method has been demonstrated as economically viable (Funk et al., 2014), partly because it does not require substantial capital outlay. We have assessed biodiversity gain through change in ‘ecological integrity’ during natural regeneration of indigenous forests. Ecological integrity was defined by Lee et al. (2005) as ‘the full potential of indigenous biotic and abiotic factors, and natural processes, functioning in sustainable communities, habitats, and landscapes’ and has subsequently been adopted by the New Zealand Department of Conservation (DOC) as its primary biodiversity goal (DOC, 2014a). Lee et al. (2005) suggested ecological integrity is demonstrated through long-term indigenous dominance (high influence of indigenous species on ecosystem processes compared with non-native species), occupancy by all appropriate biota, and full representation of ecosystems (environmental representation). We previously quantified gains in ecological integrity through catchment-scale natural regeneration of indigenous forests on agricultural lands (Mason et al., 2012b). Here, we extend the approach to national-scale natural regeneration with a specific focus on conservation implications. We examine scenarios where natural regeneration of indigenous forests is spatially allocated to maximise either, or both, biodiversity or carbon sequestration for the whole of New Zealand and for conservation lands only. We address two main questions: 1. Are potential carbon and biodiversity gains on conservation lands considerably lower than on non-conservation lands? 2. How big is the trade-off between carbon sequestration and biodiversity when spatially allocating natural regeneration for the mean of both values? Does the magnitude of the trade-off differ when natural regeneration is constrained to conservation lands?

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2. Material and methods 2.1. LUCAS vegetation carbon monitoring system and carbon gain estimates The Land Use and Carbon Analysis System (LUCAS) is a national plot network designed to monitor changes in forest and shrubland carbon stocks in order for New Zealand to meet its reporting obligations under the UNFCCC (United Nations Framework Convention on Climate Change). Within LUCAS, over 1250 survey plots (of 20  20 m) were established on an 8-km grid to estimate national carbon stocks in indigenous woody vegetation (Coomes et al., 2002). We estimated carbon in live- and dead-wood pools for these plots. We then modelled the sum of live and dead carbon for each plot (total current carbon, TCC) as a function of key environmental (e.g. mean annual temperature, soil nitrogen) and land-cover (e.g. forest type) variables using generalised additive modelling (GAMS). We then used the Generalised Regression and Spatial Prediction package (GRASP; Lehmann et al., 2002) to provide national maps of current carbon in woody vegetation. Current carbon stocks in non-woody vegetation types, which were not covered by the LUCAS sampling universe, were obtained from Tate et al. (1997). Details of the model used to predict current woody carbon are supplied in Mason et al. (2012a). Evidence for different types of anthropogenic disturbance (e.g. logging or clearing) was recorded in surveys of LUCAS plots. We used type of disturbance with the percentage of forest cover in the neighbourhood of the plots to construct GAM models for current carbon stocks as a function of type of disturbance. We then estimated disturbance–adjusted carbon values by comparing the predicted value (from the disturbance model) with the mean TCC values of plots with the same percentage forest cover but exhibiting no evidence of disturbance. We added the difference to the observed TCC value for the plot to give the disturbance–adjusted carbon value:

  ^i ; DACi ¼ TCCi þ ND  C

ð1Þ

where DACi is the disturbance–adjusted carbon value for plot i, TCCi is the total current carbon value, ND is the mean TCC value for ˆ i is the predicted carbon value from the disundisturbed plots and C turbance model. This essentially removes the human disturbance signal from carbon stock estimates in the LUCAS plots, and as such provides a measure of potential carbon storage in the absence of human disturbance. The disturbance–adjusted carbon values (DACi) were then modelled in GRASP using environmental variables to produce national maps of potential carbon storage across all lands, whether currently forested or non-forested. Potential carbon gain was estimated as the difference between potential and current carbon stocks. Details of the disturbance–adjusted carbon model and the GRASP model for Spatial Prediction of potential carbon stocks are given in Mason et al. (2012a). 2.2. Biodiversity gain through natural regeneration of indigenous forests The quantitative Vital Sites and Actions (VSA) framework was developed for assessing biodiversity benefit through management intervention (Overton et al., in press). It assesses marginal improvement in ecological integrity (sensu Lee et al., 2005) through gains in either ‘species occupancy and dominance’ or ‘environmental representation’. To assess potential gain in environmental representation through natural regeneration of indigenous forests we used a metric called ‘restored significance’,

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which indicates both intactness and irreplaceability. Restored significance was assessed at the hectare scale and has units of parts per billion (ppb), one billion representing the ecological utopia of ‘natural’ (prehuman) condition. Further details of this metric, including how prehuman condition was established, are given in Mason et al. (2012b). To model gain in restored significance from indigenous afforestation, we assumed that natural regeneration would reinstate the ecosystem functions of indigenous forests. We expect successional trajectories to follow a pattern of herbs and grasses through to forest, typical of old-field successions in other temperate countries (Hobbs and Walker, 2007). We also assumed that regeneration would eventually result in forests with a species composition that broadly reflects potential natural composition for the region and environment type. 2.3. Identifying non-forested lands that are likely to naturally regenerate tall forest Once potential national-scale carbon and biodiversity gains had been identified, gains were then constrained according to likelihood of land use for natural forest regeneration. First, we constrained our analysis to ‘Kyoto-compliant’ lands (non-forested at 1990) because these are the lands for which afforestation carries a market incentive. We used a spatial layer of Kyoto-compliant lands produced by New Zealand’s Ministry for the Environment (MfE, 2009, 2014). Second, we constrained potential gains to those sites most capable of regenerating tall forest. Natural regeneration of New Zealand forests can involve both indigenous and non-native seral species, but indigenous forests are generally achieved without management intervention given adequate seed source (Wilson, 1994). However, some agricultural landscapes are no longer capable of regenerating tall forest within a reasonable timeframe (e.g. Standish et al., 2009). We used the predicted indigenous tree occurrence probability of Mason et al. (2013) to capture variation in the likelihood of indigenous forest regeneration within a few decades (Fig. A1). Studies in post-cultural shrublands and grasslands in New Zealand indicate that once indigenous tree species colonise a site, they are very likely to achieve canopy dominance within 30–40 years (Wilson, 1994; Carswell et al., 2012). Thus, the occurrence probability of tree species is, in the New Zealand context, a reliable indicator of the likelihood that natural regeneration of tall forest will occur. Economic opportunity cost is another potential constraint on natural regeneration of indigenous forests, as land use change is extremely unlikely on land with high potential economic returns from agriculture. We accounted for economic opportunity cost through potential loss in agricultural or stock carrying capacity. Stock carrying capacity is defined as ‘‘the estimated attainable physical potential stocking rate assuming favourable socio-economic conditions and management using all appropriate technologies and techniques’’ (Newsome et al., 2008). Stock carrying capacity is expressed in units of sheep-per-hectare (breeding ewe equivalents). It is mapped spatially using an empirical model which combines expert assessments of carrying capacity with spatial data on climate, soil and topography (Fig. A2). It should be noted that mapped carrying capacity does not account for contemporary irrigation practices as it has not been updated for some years. 2.4. Optimisation and trade-offs between carbon and biodiversity gains For these analyses we considered only pixels that had non-zero values for both carbon and biodiversity (restored significance) gain in order to avoid perverse outcomes for biodiversity such as the

replacement of species-rich endemic grasslands with a monoculture of regenerating shrubland. A pixel is a hectare. Some pixels where carbon gain was predicted had zero predicted restored significance gain, since they were estimated to have 100% natural condition currently (e.g. natural sub-alpine grasslands). To incorporate likelihood of natural regeneration in comparing potential gains for pixels, we weighted carbon and biodiversity gains by tree occurrence probability. Using this approach, the highest ranking pixels will have both high potential gains and a high probability of tree occurrence. To incorporate variation in both likelihood of natural regeneration and economic opportunity cost in comparing gains for pixels we used the following expression:

  CCPOi Gaini jCCPO; Pregen ¼ Gaini  Pregeni  1  CCPOMax

ð2Þ

where Gaini is potential gain in either carbon or biodiversity for pixel i, CCPOi is the potential livestock carrying capacity for pixel i, CCPOMax is the maximum carrying capacity of the pixels being ranked, Pregen is the probability of regeneration. In this way pixels are ranked by potential carbon and biodiversity gain, likelihood of natural regeneration and least loss of livestock carrying capacity. Trade-offs between carbon and restored significance gain were analysed by simply ranking pixels in descending order of either carbon or restored significance gain, and selecting pixels with the highest values. Allocation of natural regeneration to pixels was also prioritised on carbon and restored significance gain simultaneously by calculating range-standardised values for each and ranking pixels according to the mean of their standardised values for carbon and restored significance gain:

Mean Gaini ¼

  1 C 0i B0i þ 2 C 0max  C 0min B0max  B0min

ð3Þ

where C 0i is carbon gain and B0i is restored significance gain for pixel I, C 0max is the maximum carbon gain, C 0min is the minimum carbon gain, B0max is the maximum restored significance gain and B0min is the minimum restored significance gain of any pixel. For restored significance we ranked the pixels in order of making the biggest difference and chose the best 10,000 then the next best 10,000 until the asymptote was reached. We acknowledge that this ranking does not take into account that restored significance would actually change with each selection as the most under-represented ecosystems become progressively better represented. However, this approach is symmetrical with that taken when selecting the best pixels to maximise carbon gain at a single point in time. We suggest that complementarity could be achieved during implementation by repeating the analysis at regular intervals as natural regeneration starts to occur at a broader scale. We also modelled cumulative trade-offs as the area of land being naturally regenerated increased (i.e. increasing area returned to indigenous forests). We did this by calculating the cumulative difference between the maximum-gain curve and the other curves as a percentage of maximum gain. For example, cumulative tradeoff to biodiversity was assessed by (a) modelling the maximum restored significance that could be achieved through prioritising for restored significance and then (b) calculating the proportion of maximum gain achieved when natural regeneration was prioritised for carbon gain, cumulatively. 3. Results Within conservation lands, just under 600,000 ha of Kyotocompliant land had non-zero values for both potential carbon gain and restored significance, our measure of biodiversity gain. By contrast, for the whole of New Zealand, over 12 million ha of

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Table 1 Summary statistics for carbon and restored significance gain on Kyoto-compliant land for the whole of New Zealand and all conservation lands for which there would be an increase both carbon and biodiversity with natural regeneration (600,000 ha in total). Also given are the mean and median values of carbon and restored significance gain for the best 600,000 ha of naturally regenerated land across all of New Zealand when optimised for mean carbon and biodiversity gain and weighted for likelihood of natural regeneration and economic opportunity cost. Statistic

25th percentile Median Mean 75th percentile Median Mean Skewnessa

All New Zealand

Conservation lands only

C (Mg ha–1)

Restored significance (ppb)

C (Mg ha–1)

Restored significance (ppb)

152 172 171 190 179 182 0.552

476 536 552 596 536 586 1.024

115 139 139 164

238 417 404 536

0.474

1.116

a

Skewness was calculated following Joanes and Gill (1998) on weighted values – i.e. post-optimisation according to likelihood of natural regeneration and economic opportunity cost. Positive skewness indicates a right-skewed distribution, with many small values and a long ‘tail’ of large values.

Fig. 1. Potential carbon gain across all of New Zealand through use of natural regeneration to afforest ‘Kyoto-compliant’ lands, weighted as a proportion of the maximum for a given hectare. Gain was predicted from the Generalised Regression and Spatial Prediction model incorporating environmental and land-cover variables across all of New Zealand. Weightings have been applied on the basis of likelihood of natural regeneration and economic opportunity cost, based on the stock carrying capacity of the land. Conservation lands of at least 10,000 ha in area have been outlined in pink. Note grey areas have been masked from the analysis as they are either not Kyoto-compliant (i.e. have pre-1990 forest) or had zero potential gain in either carbon or restored significance.

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Kyoto-compliant land satisfied these criteria. Most of the 12 million ha is currently used for agriculture. For both carbon and restored significance gain, values for all summary statistics were greater for the whole of New Zealand than for conservation lands only. This indicates that per-hectare gains were higher, on average, for New Zealand as a whole when compared with conservation lands only. The skewness statistic revealed moderate right-skew (many low and fewer high values) for restored significance for both the whole of New Zealand and conservation lands only (Table 1). By contrast, the frequency distribution for carbon gain on all New Zealand lands showed significantly less skew. Comparison of carbon gain for the whole of New Zealand and conservation lands only shows that very large areas (9.4 million ha) could potentially provide carbon gains >150 Mg per hectare nationally, but that such areas are very restricted in extent on conservation lands (220,000 ha). Similarly, areas with low restored significance gain are over-represented on conservation lands relative to the rest of New Zealand. Indeed, the 75th

percentile value for conservation lands (536 ppb) was the same as the median value for all of New Zealand (Table 1), and only 158,000 ha of conservation land had restored significance greater or equal to this value relative to 9 million ha for all of New Zealand. Maps of total potential carbon and restored significance gain (i.e., unweighted by likelihood of land use change) are given in the Appendix (Figs. A3 and A4). Areas with greatest carbon gain once weighted for likelihood of natural regeneration and least economic opportunity cost are foothills of mountain ranges, especially in the South Island (Fig. 1). These areas are likely to be close to existing forests and are marginal for pastoral agriculture. The gains in restored significance are also relatively high in the foothills (Fig. 2). However, the areas with highest potential gains are riparian margins. Again, these are primarily in the South Island. The lower per-hectare carbon and restored significance gains on conservation lands impacted on cumulative gains with increasing area regenerated (Fig. 3). Per-hectare gains in restored significance

Fig. 2. Potential biodiversity gain (restored significance) across all of New Zealand through use of natural regeneration to afforest ‘Kyoto-compliant’ lands, weighted as a proportion of the maximum for a given hectare. Gain was predicted from the Vital Sites and Actions model (VSA) for all of New Zealand and considers potential vegetation improvement and national representation of the restored ecosystem type. Weightings have been applied on the basis of likelihood of natural regeneration and economic opportunity cost. Conservation lands of at least 10,000 ha in area have been outlined in pink. Note grey areas have been masked from the analysis as they are either not Kyotocompliant (i.e. have pre-1990 forest) or had zero potential gain in either carbon or restored significance.

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Fig. 3. Restored significance and carbon gain on Kyoto-compliant lands under different spatial allocation scenarios. During spatial allocation, gains were weighted by likelihood of natural regeneration and economic opportunity cost. Panels (a) and (b) represent change in restored significance with increasing area afforested whereas panels (c) and (d) represent carbon gain. Panels (a) and (c) represent all eligible land in New Zealand whereas panels (b) and (d) represent conservation lands only. Dotted curves indicate gains when restoration effort was allocated to optimise biodiversity (i.e. restored significance) gains. Dashed curves indicate gains when carbon gain was optimised and solid curves indicate gains when both carbon gain and restored significance were optimised.

were notably higher for the whole of New Zealand compared with conservation lands only, in spite of weighting for likelihood of natural regeneration and economic opportunity cost. Cumulative gain in carbon resulting from natural regeneration of the best 600,000 ha of non-conservation land, when prioritised on the mean of both carbon and biodiversity, was 34% higher than on the same land area of conservation land. Cumulative gain in restored significance was 46% higher on the same area of non-conservation land compared with conservation land. Note that there are only 600,000 ha of non-forested conservation land predicted to naturally regenerate with a net gain in both carbon and biodiversity. We examined cumulative trade-offs for allocation of natural regeneration across the whole of New Zealand (Fig. 4a) and conservation lands only (Fig. 4b). The y-axis represents the proportion of either biodiversity or carbon that is traded off when regeneration is prioritised for the other property. As the figure is cumulative, the trade-off appears to gradually approach an asymptote. In both panels it is clear that the trade-off to restored significance, when optimising for carbon, is higher than for carbon gain, when optimising for biodiversity. This is consistent with the expectation that susceptibility to trade-offs increases with increasing right-skewness, since the frequency distribution for restored significance was strongly right-skewed in comparison to that for carbon gain. However, optimising for the mean of both properties significantly reduced the size of the trade-off. The other point to note (from Fig. 4) is that trade-offs for the whole of New Zealand tended to be larger than for conservation lands only, especially as larger areas were selected for afforestation. This indicates that the higher gain-values for the whole of New Zealand provided greater potential for trade-offs between carbon and restored significance, although the trade-off for biodiversity is much lower when both carbon and biodiversity gain are optimised simultaneously.

Fig. 4. Comparison of cumulative trade-offs between (a) all Kyoto-compliant lands in New Zealand and (b) conservation lands only. Afforestation has been simulated using natural regeneration optimised for greatest carbon benefit (dashed curve represents the cumulative trade-off to biodiversity) or greatest biodiversity benefit (dotted curve represents the cumulative trade-off to carbon) or the rangestandardised mean of both biodiversity and carbon (solid curve represents the trade-off to biodiversity when optimised for the mean of both properties and dashdot curve represents the trade-off to carbon when optimised for the mean of both properties).

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4. Discussion Our results suggest that restricting natural regeneration of indigenous forests to conservation lands significantly reduces total and per-hectare gains in carbon and biodiversity at national scale. Our results also concur with previous work showing that biodiversity may be more susceptible to trade-offs than non-biodiversity values when prioritising ecosystem restoration projects (Maron et al., 2012; Mason et al., 2012b). Despite this, trade-offs for biodiversity gain tended to be fairly small when natural regeneration was allocated to maximise carbon and biodiversity gain simultaneously. This suggests that it may be possible for natural regeneration to contribute to national goals for both biodiversity gain and greenhouse gas mitigation, where both are considered during spatial allocation of naturally regenerated forests. Below, we discuss the implications of our results for national strategies for conservation and greenhouse gas management. We then consider what our results mean for biodiversity should natural regeneration become

spatially allocated on the basis of maximised mean values of carbon and restored significance. 4.1. Inclusion of non-conservation lands to increase national-scale carbon and biodiversity Our study strongly emphasises that for maximum increases in carbon sequestration (i.e. rapid regeneration of natural forests with high biomass) and biodiversity (i.e. increased representation of now-scarce forest types) more emphasis must be placed outside existing conservation lands. While this has been apparent for other industrialised nations for some time (e.g. Pressey, 1994; Knight, 1999), the lack of overlap between New Zealand conservation lands and areas of high potential carbon or biodiversity gain is quite clear (Fig. 1 and 2). The lack of overlap confirms that additional afforestation of conservation lands would have only a minor impact on national carbon and biodiversity gains – these lands are already ‘optimised’. This is because alluvial plains and coastal ecosystems

Fig. A1. Tree occurrence probability in non-forest vegetation in New Zealand from Mason et al. (2013). The map shows the probability of indigenous tree occurrence in nonforest vegetation predicted from mean annual temperature and distance to seed sources. Greyed areas are those which are already forested, with either indigenous or exotic tree species. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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are the locations with the greatest potential gains in both properties during natural regeneration and are not well represented within current conservation lands. Vegetation in conservation lands in New Zealand is dominated by mildly-modified to unmodified environments (Walker et al., 2006) where climatic or edaphic factors limit potential carbon and biodiversity gains resulting from natural regeneration of indigenous forests. The lands already contain a very high proportion of indigenous species and ecosystems. Spatial conservation prioritisation for all of South America produced the same result. Durán et al. (2014) found that the inclusion of agricultural lands enabled increased biodiversity representation compared with the exclusion of agricultural lands. However, our results contrast those of Durán et al. (2014) in that for New Zealand, prioritisation of biodiversity was also positively correlated with carbon gain. Given that one of the UN-CBD Aichi biodiversity targets is the restoration of at least 15% of degraded ecosystems by 2020 (CBD, 2010) New Zealand has significantly more to gain through restoration of non-conservation lands. Indeed, New Zealand has listed reforestation of non-conservation lands as the major activity to achieve this Aichi target (DOC, 2014b). In an attempt to increase the area of new forests, the New Zealand Government introduced three mechanisms targeted at private landowner participation in carbon sequestration. The New Zealand Ministry for the Environment has reported c. 30,000 ha of reforestation from natural regeneration post-1989 (MfE, 2013). Given that c. 570,000 ha of non-native forests have been planted in the same time period for reforestation purposes (MfE, 2013), it appears as if biodiversity has been a minor consideration compared with potential revenue from maximised carbon and timber. While market-based voluntary conservation has successfully increased biodiversity at more productive boreal forest sites in Finland (Mönkkönen et al., 2008), the same cannot be said for New Zealand where the market has been used to prioritise carbon value. Market-based instruments for biodiversity in central and Eastern Europe were demonstrated to be most effective when implemented in combination with traditional regulation (Chobotová, 2013). Resources for biodiversity protection/enhancement on private land in New Zealand are primarily aimed at voluntary protection and enhancement of existing forests and do not use market-based instruments. These sit alongside regulatory measures to protect existing biodiversity. Despite the central government schemes for carbon sequestration and the diverse array of support for biodiversity protection and enhancement, we are aware of only one national scheme that specifically targets carbon sequestration and biodiversity increase simultaneously. This is the Emissions-Biodiversity Exchange Programme (EBEX21Ò), where landowners earn money through carbon credits for allowing their pastoral land to revert to indigenous forest (Carswell et al., 2003). The programme measures gains in both carbon and biodiversity during the process of natural regeneration. EBEX does not currently place a strong emphasis on targeting areas where potential carbon and biodiversity gains are the greatest and would need to realise a premium price for its credits to do so. The current credit price for EBEX units (traded by carboNZero) is over 8 times that of the standard carbon units within the New Zealand ETS so we see evidence of market ‘willingness to pay’ for biodiversity benefit. However, volumes traded are typically low (7000–10,000 tonnes CO2e per annum), indicating that credit demand is from small–medium enterprises that can see market advantage in brand association with biodiverse carbon offsets. Bundling of broader ecosystem properties or ‘services’, such as clean water provision, could significantly enhance the ability to incentivise simultaneous biodiversity and carbon provision. Other authors have found that habitats with high conservation status provided greater levels of regulating and cultural ecosystem services, in general (Maes et al., 2012). Due to their protected status,

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natural indigenous forests do not currently provide much timber as a provisioning service and their rate of carbon sequestration is less than in pre-harvest exotic forests (sensu Yao et al., 2013). However, indigenous forests provide more wild food, pollinator habitat and recreational activities than exotic forests (King et al., 2013; Newstrom-Lloyd, 2013; Clough, 2013). Most significantly, afforestation of fertile riparian areas within intensively-farmed land is expected to maximise improvements in water quality (DaviesColley et al., 2009), a much-needed service in a country currently experiencing increasing agricultural intensification. Once again, the service provision is greater under indigenous forests than exotic mostly because of the periodic disturbance associated with exotic harvest and removal of the canopy cover (Davies-Colley, 2013). However, without current local markets for clean water provision, a stronger international price signal for carbon or increased evidence that purchasers of biodiverse carbon offsets enjoy significantly improved access to international markets, we do not anticipate a marked increase in sales of such offsets. The asymmetry between income realisation for carbon versus biodiversity has been observed in other REDD-like projects (Phelps et al., 2012). We note, with interest, that New Zealand’s highly deregulated economy is able to avoid some of the inequities associated with developingnation participation in REDD-like activities as individual landowners control land-use change and also directly reap the benefits from carbon sequestration, ecotourism, etc. In contrast, in some REDDlike projects, neighbouring communities are excluded from the local forests where they make their livelihoods while tourism operators from outside the local area are those that benefit most from the conservation status (e.g. Kari and Korhonen-Kurki, 2013). However, because New Zealand is dependent on private landowners taking action themselves, the price realised for combined carbon and biodiversity gain is one of the critical instruments for maximisation. In addition, market and policy uncertainty are thought to be highly influential in landowner decision-making. For example, a recent study showed that natural regeneration for carbon would be more profitable than grazing on 27% of agricultural lands in the area studied (Funk et al., 2014). Despite the favourable economics, actual participation was minimal (MfE, 2013). 4.2. Trade-offs between carbon and biodiversity gain We predict gains in mean carbon and biodiversity to be within 10–20% of the maximum potential gains for either variable (i.e. if prioritised independently) when natural regeneration is spatially allocated to simultaneously maximise carbon and biodiversity gains. This suggests that natural regeneration can be allocated to target the mean of biodiversity and carbon gain without strongly compromising either objective. A similar result was found in a recent study also conducted at national and continental scales, but for biodiversity and carbon in existing ecosystems, rather than new forests (Thomas et al., 2013). That study demonstrated how a combined carbon–biodiversity strategy could achieve at least 90% of the benefit of prioritisation for either property individually. In contrast, Mason et al. (2012b) demonstrated that variables become more susceptible to trade-offs as their frequency distributions become more right-skewed (i.e. when they have many low and few high values). Our metric for biodiversity gain, restored significance, was moderately right-skewed, whether we considered the whole of New Zealand or conservation lands only, and it is therefore more susceptible to the trade-off than carbon gain, which does not show the same degree of skew at national scale. Our analyses only deal with one aspect of ecological integrity: environmental representation. Another primary goal of conservation is to avoid species extinction through increasing the populations and geographic distribution of threatened species. For estimates of potential biodiversity gain to best serve national

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conservation aims, they need to reflect the current and potential distributions of threatened species, as well as indicate representation of ecosystems. Therefore, restored significance, and its parent measure, environmental representation, would need to be a useful surrogate for the occurrence of threatened species at the national scale. Unfortunately, existing work suggests that environmental representation and the occurrence of threatened species are only weakly related (Monks et al., in press). The species-occupancy strand of the VSA framework does explicitly incorporate current and potential distributions of threatened species, but it was not used here as we cannot assume that afforestation on its own will adequately increase the occurrence of rare and threatened species. For example, de Lange et al. (2009) show that of the 180 mostthreatened New Zealand plants only about 10% are tree species, the taxa most likely to benefit from afforestation – especially where carbon gain is maximised. However, forest loss has been responsible for forest bird decline or extinction in some regions or localities where no or little forest is left, and habitat restoration is a necessary precursor to forest bird re-establishment there (Innes et al., 2010). Because of the severity

of mammalian predation on defenceless birds it has been stressed that additional interventions such as pest control and translocation will be required to increase populations of threatened species in restored ecosystems (Innes et al., 2010; Maron et al., 2012). Therefore, while our mapping of trade-offs is useful for planning increased representation of target ecosystems, it does not provide guidance for targeting restoration to protect threatened species per se. Thomas et al. (2013) advocate the use of complementarity (site selection that complements other sites selected) to preserve threatened species in low-carbon environments. Our analysis uses complementarity to choose the most under-represented ecosystems at a single point in time. Whilst our analysis cannot explicitly predict threatened species within those ecosystems, it does exclude areas with no net gain in biodiversity during regeneration. Areas with no net gain include ecosystems that are already well-represented at national scale and are likely to have experienced little anthropogenic change. Therefore, areas with low-carbon but a lot of indigenous vegetation will not have been spatially allocated to natural regeneration during our optimisation as there would be no net biodiversity gain. A necessary next step in implementation

Fig. A2. Stock carrying capacity for all of New Zealand, except Stewart Island in units of sheep-per-hectare (breeding ewe equivalents). Stock carrying capacity is defined as ‘‘the estimated attainable physical potential stocking rate assuming favourable socio-economic conditions and management using all appropriate technologies and techniques’’. Data reproduced with the permission of Landcare Research New Zealand Limited. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) This spatial layer of Land Use Capability has been obtained from the NZ Land Resource Inventory: https://lris.scinfo.org.nz/layer/ 76-nzlri-land-use-capability/.

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Fig. A3. Potential carbon gain in live and dead vegetation through use of natural regeneration to afforest ‘Kyoto-compliant’ lands, as predicted from the Generalised Regression and Spatial Prediction model incorporating environmental and land-cover variables across all of New Zealand. Conservation lands of at least 10,000 ha in area have been outlined in pink. Note grey areas have been masked from the analysis as they are either not Kyoto-compliant (i.e. have pre-1990 forest) or had zero potential gain in either carbon or restored significance. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

would be consideration of the actual spatial arrangement of regenerated forests such as many fragmented pieces versus fewer larger parcels (sensu Gilroy et al., 2014; Ekroos et al., 2014). In a pan-tropical analysis Jantz et al. (2014) found that corridors between protected areas had significant carbon benefits in addition to improving connectivity to the benefit of biodiversity. Given that explicit incorporation of threatened species is more likely to increase the right-skew in the distribution of biodiversity gain when optimised for carbon gain, caution is required in asserting that incorporating non-biodiversity objectives, like carbon gain, in natural regeneration for indigenous forests will not greatly compromise conservation objectives. Other authors have found that deliberate inclusion of some single-objective projects is necessary in restoration planning where thresholds exist in the tradeoffs between one or more restoration objectives (Maron and Cockfield, 2008). Bradshaw et al. (2013) suggest that if biodiversity persistence is prioritised during planning and implementation stages for Australian projects, carbon sequestration goals need not compromise biodiversity. In summary, we modelled national-scale natural regeneration of indigenous forests and predicted significantly greater

per-hectare gains in both carbon and biodiversity on nonconservation lands in New Zealand. Conservation lands can be considered as largely optimised already for both carbon and biodiversity. Optimising the spatial allocation of natural regeneration for carbon and biodiversity together does not significantly compromise biodiversity gains but additional incentives will be required to effect this land-use change on the lands that could provide the biggest gains.

Acknowledgements This work was supported by the Department of Conservation’s Wild Animal Control for Emissions Management (WACEM) programme (Science Investigation No. 4072), and Ministry of Business, Innovation and Employment (MBIE) core funding to Landcare Research. We acknowledge permission from the New Zealand Ministry for the Environment to use the Land Use and Carbon Analysis System (LUCAS) plot data. We thank S. Walker, P. Bellingham and two anonymous reviewers for their comments on the manuscript.

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Fig. A4. Potential biodiversity gain (restored significance) through use of natural regeneration to afforest ‘Kyoto-compliant’ lands, as predicted from the Vital Sites and Actions model (VSA) for all of New Zealand. Gain considers potential vegetation improvement and national representation of the restored ecosystem type. Conservation lands of at least 10,000 ha in area have been outlined in pink. Note grey areas have been masked from the analysis as they are either not Kyoto-compliant (i.e. have pre-1990 forest) or had zero potential gain in either carbon or restored significance. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Glossary Afforestation: Changing land use from non-forest to forest lands. Carbon sink: An area of forest sequestering more carbon from the atmosphere through growth and recruitment than it is losing through mortality. Conservation lands: protected natural areas; in New Zealand the land administered by the Department of Conservation (DOC). Greenhouse gas: An atmospheric gas that absorbs and emits radiation within the infrared range, e.g. water vapour, carbon dioxide, methane, nitrous oxide and ozone. Kyoto-compliant land: We use Kyoto-compliant land to refer to lands eligible for afforestation under Article 3.3 of the Kyoto Protocol, namely lands

without forest at 31st December 1989. Here, they are primarily agricultural. LUCAS: The Land Use and Carbon Analysis System is the New Zealand plot network for monitoring changes in forest and shrubland carbon stocks to meet reporting obligations under the UNFCCC REDD+ United Nations’ Reducing Emissions from Deforestation and forest Degradation in developing countries program. Reforestation: Planting trees in human-cleared treeless areas. UNFCCC: United Nations Framework Convention on Climate Change. Vital Sites & Actions: The VSA framework was developed in New Zealand for assessing biodiversity benefit resulting from management action.