Assessing the Area of Suitable Land for Climate Change Mitigation with Sitka Spruce (Picea sitchensis) in Scotland

Abstract

One of the key areas in which the United Kingdom government plans to achieve net zero, reduce GHG emissions and sequester carbon is through afforestation. Afforestation will also provide ecosystem services to society. The Scottish government’s target is to increase woodland cover in Scotland to 25% by 2050. In this study, a land suitability analysis was used to investigate the likelihood of achieving this target based on the biophysically suitable and available land considering the current policy constraints for planting Sitka spruce (Picea sitchensis) in Scotland. The results showed that about 19% of land area in Scotland is biophysically suitable for Sitka spruce and about 13% is biophysically suitable and available based on policy constraints. Thus, there is an opportunity for the Scottish government to increase the woodland cover in Scotland to 31.5% and exceed its 25% woodland target. However, for Scotland to achieve net zero by 2045, it will require that more trees be planted on higher-quality agricultural land, different from areas where trees are currently planted.

1. Introduction

Several scientific studies have confirmed that the Earth is getting warmer due to climate change [1,2]. Anthropogenic activities are the major drivers of climate change, particularly fossil fuel burning activities that emit greenhouse gases (GHG) into the atmosphere, followed by CO₂ emissions from land use, land-use change and forestry [3,4]. According to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report, the impacts of climate change are very likely to lead to extreme weather and climate events globally, such as heavy precipitation, increase in agricultural and ecological drought, increase in permafrost melting and loss of snow cover [5]. Some of these extreme weather events are already being experienced globally. Thus, there is a need for ambitious actions to be taken to tackle climate change and reduce GHGs emissions. The current business-as-usual trajectory would lead to an increase of 4–5 °C in global average temperature by 2100, which will have negative implications [6]. In the United Kingdom (UK), the negative implications will include extreme weather conditions, changing rainfall patterns and extreme precipitation that would lead to more population exposed to flood risks, a rise in global sea level, shrinking glaciers, loss of different plant and animal biodiversity both on land and aquatic habitats, and a reduction in crop yield [6].

The UK’s target is to reduce GHG emissions to net zero by 2050 compared to 1990 to end its contribution to global warming [6]. Although this is ambitious, the UK has agreed to this commitment as a signatory of the United Nations Convention on Climate Change (UNCCC) Paris Agreement, 2015 [7]. Between 1990 and 2019, the UK was able to reduce its GHG emissions by 40% from 608 MtCO₂e in 1990 to 365 MtCO₂e in 2019 [8]. However, between 2019 and 2020, GHG emissions stagnated in agriculture, public and residential [8]. In the UK, each devolved administration, namely Wales, Scotland, and Northern Ireland has also set its reduction targets. Scotland has committed to reducing its carbon emissions by 75% in 2035 and achieving the net-zero target by 2045 [9]. In 2018, Scotland’s land use, land-use change, and forestry (LULUCF) sector were able to offset about 5.4 MtCO₂e of emission. The LULUCF sector was also the only sector that had negative CO₂ emissions in Scotland [10]. The main sinks that make up the sector are forestry, grassland and carbon stored in wood products [10]. There has not been, however, an increase in the offsetting potential of the LULUCF sector, which has stagnated between 2015 and 2019 at around 5.7 MtCO₂e and 5.9 Mt CO₂e, respectively [8].

To meet the GHG emission targets set by the Scottish government, the forest sector has a vital role to play, given its capacity to store and sequester large amounts of carbon in trees and forest soils, and to store carbon in harvested wood products that can substitute for fossil fuels [11]. Forest covers around 13% of the total land area in the UK and it is amongst the lowest in Europe [12]. Scotland has the highest woodland cover in the UK, and this increased from 5% to 18.5% in the 20th century; however, this is well below the average woodland cover of 43% in Europe [13]. A report to the Committee on Climate Change (CCC) by Moran et al. [14] which focused on conifer forests (Sitka spruce) managed on a 49-year rotation, showed that afforestation was highly cost-effective with savings of GBP 1.82–7.12 per tonne of CO₂ sequestered. Across the UK, Scotland is considered the most cost-effective region for afforestation projects; afforestation projects can cost about GBP 26.7 per tonne of carbon sequestered [15] compared to Southeast England, where it costs about GBP 65.3 per tonne. The CCC considers any GHG abatement technology (or technique) that will cost less than GBP 100 per tonne of CO₂ absorbed as a cost-effective abatement technology [14,16]. Apart from the carbon sequestration potential of afforestation and its cost-effectiveness for climate change mitigation, forests also provide other ecosystem services. These ecosystem services include biodiversity conservation, timber and non-timber products, climate regulation, flood prevention, soil, air and water detoxification, employment opportunities, and health, social, educational, and recreational benefits [17,18]. These ecosystem services have commercial and non-commercial values such as the provision of employment, removal of carbon and air pollutants, and water cycling that contribute to making human life possible [18].

With more than 80% of Scotland’s population living in urban areas, trees can provide benefits, such as reducing air pollution and flooding, exercising and relaxation, growing local economies, improving physical and mental health, reducing the effect of urban heat island, and providing jobs for people living in urban areas [19,20]. This was especially relevant during the COVID-19 pandemic. Logan et al. [21] found that respondents visited community woodlands more often during the lockdown, developed further interest and appreciation in community woodlands, and placed significantly more value on connecting with nature and less on social and shared benefits. The Woodland in and Around Towns (WIAT) program in Scotland was launched by Forest Commission Scotland in 2005 to fund and support urban woodland [22]. According to Doick et al. [23], urban forests in Edinburgh intercepted 183,000 m³ of water per year, removed 195,000 tonnes of airborne pollutants per year, sequestered 4885 tonnes of carbon (CO₂) per year and stored 179,237 tonnes of CO₂. In Glasgow, it was estimated that urban forests intercepted 812,000 m³ of water per year, removed 283 tonnes of air pollutants per year, sequestered 9000 tonnes of CO₂ per year and stored 183,000 of carbon, with an estimated total value of GBP 4.5 million per year [22].

Additionally, forestry contributes almost GBP 1 billion per year to Scotland’s economy and employs over 25,000 people [9]. The Scottish government expects that more job opportunities will be created over the next 10 years to meet the ambitious target of planting 15,000 trees per ha per year [9,24]. Most of the activities in the forest sectors, namely harvesting, transportation and the processing of timber and other wood products, are carried out in the rural areas and will benefit these communities by providing jobs and an attractive environment and supporting affordable rural housing [13,25]. With the Scottish government planning to achieve net zero by 2045 and increase woodland by 25% by 2050, Scotland needs large-scale forestry expansion. The government targets are to increase woodland creation by 14,000 ha per year from 2022/2023 and by 15,000 by 2024/2025 [13]. Achieving these woodland creation targets will significantly contribute to GHGs emission reduction in Scotland by locking carbon in the growing trees and at the same time delivering ecosystem services, such as reducing air and water pollution, enhancing biodiversity, and providing economic benefits from timber and wood products.

Afforestation in Scotland is not happening on good agricultural land, even though suitable land and grants are available [26]. Rather, newly created woodlands are established on less productive lands with organo-mineral soils (such as peaty gley and peaty podzol soils) that are less resilient to disturbance and less suitable for woodland [26,27]. This has been related to underlying socio-economic and cultural factors, such as farmers prioritizing food production on their land over forestry [27]. Such land between prime agricultural lowlands and environmentally constrained uncultivated uplands has been named the ‘squeezed middle’ [28].

This study will model Sitka spruce (Picea sitchensis) suitability in Scotland based on the biophysical factors and policy constraints. It will examine the feasibility of achieving the planting targets by the Scottish government based on land deemed suitable and available for planting Sitka spruce. Sitka spruce was chosen for this study because the tree species is widely planted in the UK, particularly in Scotland. It covers about 58% of Scotland’s total area occupied by coniferous trees in 2020 [29]. Sitka spruce is tolerant to wind exposure and moist soils with poor nutrients and still maintains high productivity under these conditions [30]. Additionally, Sitka spruce has been used to represent coniferous forestry based on the work by the Climate Change Committee [31] and Bell et al. [32].

The following analysis will be carried out:

i.

Determine the area of land biophysically suitable for planting Sitka spruce.

ii.

Determine the area of land biophysically suitable and not affected by policy constraints for planting Sitka spruce.

iii.

Determine the area of land that is suitable in three Land Capability for Agriculture classes.

iv.

Determine the potential for creating woodlands close to the population centers.

2. Materials and Methods

2.1. Tree Species: Sitka Spruce

Sitka spruce is a tree species native to the Pacific forests of North-West America [33]. It was imported and planted in Great Britain in the 19th century because of its suitability for climatic conditions, and it grows well on poor soils [34]. Sitka spruce is the most dominant tree species in the UK [35], covering about 507,000 hectares in Scotland [29]. It is also the most important in commercial plantations [36]. It has a relatively high yield class (YC) with an average YC of 14 m³ ha⁻¹ year⁻¹ in the UK [37]. It is fast-growing, a good source of high-quality timber [38], and can be used for a variety of purposes, such as structural poles and piles, structural timber, fencing, pallets and packaging, pulp, and energy wood [37]. It is especially preferred by the forestry industry because of the species’ ability to adapt to different site conditions, grow well on the poorest soils, and establish itself easily and with low maintenance costs [34,39].

2.2. Data Collection and Analysis

Land suitability analysis was used to determine the area of land that is suitable for planting Sitka spruce in Scotland based on biophysical factors and current policy constraints. Spatial datasets which were readily available and of the highest resolution were combined as the spatial analysis techniques using geographical information system (GIS) mapping software (ArcGIS 10.7) to identify the most suitable area for Sitka spruce in Scotland. The GIS is a type of database containing geographic data, combined with software tools for storing, manipulating, analyzing, and exploring relationships between a vast quantity of spatially referenced data [40]. Geographical information systems have been used in the environmental field to combine spatial data and carry out suitability analyses [41,42]. In this study, ArcGIS 10.7 was used to prepare the data, carry out the land suitability analysis, and produce maps and statistical outputs. The GIS analysis method used for this project is similar to the approach by Bell et al. [32] to evaluate the biophysical and policy constraints suitability of two tree species: sessile oak and Sitka spruce in Wales. All datasets used for the suitability analysis were in raster format (50 m pixel size) and British National Grid projections. The data that were collected in vector formats were transformed into raster layers. The total administrative area of Scotland (7,890,000 ha) was used as a boundary for the input data to have maps of the same extent. The datasets were combined using the Raster math tool on ArcGIS 10.7 such that the output is based on the pixel values of the input raster layer (Figure 1).

2.3. Assessing Biophysically Suitable Areas for Planting Sitka Spruce in Scotland

The first analysis that was carried out was on the biophysical suitability of Sitka spruce in Scotland using the Sitka spruce climatic timber suitability map from Ecological Site Classification (ESC). The climate data were combined with land capability for agriculture (LCA), land capability for forestry (LCF), land cover map (LCM), and existing woodland datasets to carry out the biophysical suitability analysis.

2.3.1. Ecological Site Classification—Climate Data

Ecological Site Classification (ESC) is a web-based decision support system to help forest managers and planners select tree species that are ecologically suited to sites, instead of selecting a species and trying to modify the site to suit [43]. The ESC methodology generates suitability maps at a resolution of 250 m². The ESC matches four climatic (accumulated temperature, moisture deficit, windiness, and continentally) and two edaphic variables (soil moisture and soil nutrients) with the ecological requirements of tree species using a knowledge-based model. The suitability classes were based on information from the ESC website [44]. ESC climate data were used by Matthew et al. [45] and Thomas et al. [46] to model suitable land for planting trees in Scotland. The description of the climate data and suitability classes used is shown in

Table 1. Description of the climate data classification from ESC [44].

Dataset	Source	Data Description	Suitability Classes
Climate data	Forest Research from the ESC website [44]	The climate suitability map of Sitka Spruce from ESC is based only on climatic variables; AT5, CT, DAMS and MD 1. The climate map was calculated from the mean temperature and rainfall from 1961–1990.	0–0.29—Unsuitable 0.3–0.5—Limited 0.5–1—Suitable

.

2.3.2. Land Capability for Agriculture (LCA)

The LCA classification is used to rank land based on its potential productivity and cropping flexibility. This is determined by the extent to which the physical characteristics of the land (soil, climate, and relief) impose long-term restrictions on its use. The LCA is a seven-class system. Four of the classes are further subdivided into divisions. Class 1 represents land with the highest potential flexibility of use whereas Class 7 is of limited agricultural value. Biophysical factors, such as soil texture, soil wetness, climate, topography and erosion, and the interactions between them are important for tree growth [47]. Sing and Aitkenhead [48] included all the land capability classes as potentially available for afforestation. However, in this study, the focus is on land suitable within LCA classes 1–5.3. The LCA classes 6.1–6.3 were classified as having limited suitability because of their low-quality agricultural value with very severe soil and climate limitations [49]; planting trees on such land might limit the climate change mitigation potential [45]. A more detailed description of the LCA classifications is outlined by Brown et al. [27]. The suitability classes used for the LCA classification are provided in

Table 2. Description of the LCA classification [49].

Dataset	Source	Data Description	Suitability Classes
Land Capability for Agriculture (LCA)	Bibby et al. [49]	LCA is based on biophysical factors such as climate, soil texture and structure, wetness, drought, stoniness, shallowness, topography, and erosion. The two available LCA data (1:50,000 and 1:250,000) were combined. The LCA classified land into 13 classes and subclasses 2.	Classes 1–5.3—Suitable Classes 6.1–6.3—Limited Class 7— Unsuitable

.

2.3.3. Land Capability for Forestry (LCF)

The LCF data were also used to model the biophysical suitability. The LCF aims to present detailed information on soil, climate, windthrow, and relief in a form which will be of value to land-use planners, foresters, consultants, and others involved in optimizing the use of land resources. The LCF is based on seven limitations: climate, wind, nutrients, topography, drought, wetness, and soil. The LCF data were previously used by Brown [26] to model tree suitability in Scotland. The LCF suitability classes used for this study were based on Bibby et al. [50] and are shown in

Table 3. Description of the LCF classification [50].

Dataset	Source	Data Description	Suitability Classes
Land Capability for Forestry (LCF)	Bibby et al. [50]	The LCF data classified land suitable for planting trees in Scotland based on climate, soil, topography, wind, soil nutrient, drought, and wetness. The LCF data resolution is 1:250,000. It classified land into 7 summary classes 3.	F1–F4—Suitable F5–F6—Limited for commercial forestry F7—Unsuitable for commercial forestry but might

.

2.3.4. Areas Excluded from Potential Afforestation

Excluded areas are where afforestation, i.e., new plantations, cannot happen because these areas are existing woodland, built-up areas, and mountain tops. Data on existing woodland in Scotland were derived from Scotland’s National Forest Inventory (NFI) woodland data. The land cover map of Scotland provided mountainous and built-up areas and was derived from Ordinance Survey [51]. Datasets on existing woodland and land cover map (LCM) and the suitability classes used are described in

Table 4. Description of the NFI and LCM data [51,52].

Dataset	Source	Data Description	Suitability Classes
National Forest Inventory (NFI)	Forestry Commission Open Data [52]	The NFI woodland data for Scotland provides information on the location and extent of all forests and woodland over 0.5 ha. The data was derived from an aerial photograph, satellite images and administrative records on newly planted woodland areas.	Non-existing woodland— Suitable Existing woodland—Unsuitable
Land Cover Map (LCM)	EDINA Environment Digimap Service [51]	LCM of Scotland 2019 was used to identify the land use and land cover. It was further reclassified into arable, grassland, mountain, heath and bog, woodland, and others.	Arable, grassland, heath, and bog–Suitable Mountain, woodland and Others 4—Unsuitable

.

2.3.5. Creation of Suitability Classes for Sitka Spruce

The pixel values of the input biophysical datasets were used to divide the land in Scotland into three major categories, suitable, limited suitability, and unsuitable, based on the Food and Agriculture Organization (FAO) land suitability guidelines [53]. The FAO suitability guidelines were redefined as follows:

-

Suitable: Land that has no significant limitations based on biophysical factors (such as urban, mountain, coastal), and biological factors (such as soil, climate, and wind) and is not an existing woodland.

-

Limited Suitability: Land that can support the plantation of the species but have limitations due to soil and climate conditions and will lead to a reduction in productivity or benefits, except if the crop is managed more intensively.

-

Unsuitable: Land that is biophysically and/or biologically unsuitable for planting Sitka spruce.

The land is assigned to a category based on the following rules (Figure 2):

-

If one or more of the input layers have an “Unsuitable” value, the overall suitability is classified as “Unsuitable”.

-

If one or more of any input layers has a “Limited” value, and none of the input layers is “Unsuitable”, the overall suitability is classified as “Limited suitability”.

-

If all the input layers have a “Suitable” value, the overall suitability is classified as “Suitable”.

2.4. Policy Constraints and Reduced Area Suitable for Afforestation with Sitka Spruce

The second suitability analysis was carried out on land areas affected by policy constraints. Apart from the land being biophysically suitable for planting trees, during tree planting projects, it is important to consider the policy constraints that may restrict afforestation even in suitable areas [54]. This analysis is significant to avoid overestimation of the area of land that is effectively available for planting trees. For example, peatland and peaty soils store the greatest amount of carbon compared to other soil types and are not suitable for afforestation [55]. Land with designations such as special areas of conservation (SAC), site of special scientific interest (SSSI) and special protection areas (SPA) sites are also not available for planting trees.

The policy constraint data were also classified into three major categories:

-

Suitable: Land that is not under any policy constraints or restrictions.

-

Limited Suitability: Land where planting may be possible but restricted due to policy constraints.

-

Unsuitable: Land that is completely unsuitable for planting trees under the current policies due to national or EU designations and policies.

The land is assigned to a category using the same rules as above (Figure 2). The land policy constraints datasets used for this project and the reason for restrictions are explained in

Table 5. Data, source, reason, and suitability classes used for policy constraint suitability.

Policy Constraint (Datasets Used)	Source	Reason	Suitability Classes
LCA (Class 1–3.1)	Bibby et al. [49]	Based on the land-use strategy published by the Scottish government in 2011, woodland creation should be targeted away from prime agricultural lands (Classes 1–3.1) [56].	Classes 1–3.1— Unsuitable Classes 3.2–7— Suitable
Onshore Wind farms	Scottish Natural Heritage (SNH) [57]	Trees are cut down rather than planted during the planning and construction of onshore wind farms.	Approved/Installed wind farms— Unsuitable Application/Scoping stage— Limited Area not onshore wind farms— Suitable
Greenspaces	EDINA Environment Digimap Service [51]	Planting trees in greenspaces in Scotland may reduce the benefits they provide to their community, especially in terms of openness.	Greenspaces— Limited Areas not greenspaces— Suitable
Historic Environmental Sites	Historic Environment Scotland [58]	Scheduled Ancient Monuments (SAMs) are legally protected in Scotland. Trees can be planted on Battlefields, Gardens and Landscapes, however, there may be some restrictions due to their historic and cultural importance [59].	SAMs—Unsuitable Historic sites but SAMs—Limited Non-Historic sites—Suitable
World Heritage Site (WHS)	Historic Environment Scotland [58]	WHS are exceptional places and are of international importance. During planting operations, WHS should be protected and preserved [60].	WHS—Limited Non-WHS—Suitable
Conservation designations sites	SNH [57]	Designated sites such as SSSI, SAC, SPA, Ramsar Sites, and NNR in Scotland are protected by National and European legislation [60].	Designated areas—Unsuitable Non-designated areas—Suitable
Acid Sensitive Catchments	Forest Research [12]	Planting coniferous trees such as Sitka spruce over 30% of Acid sensitive catchment area can increase water acidity and reduce water quality [61].	Acid Sensitive area—Limited Area not acid sensitive—Suitable
Peatland	SNH [57]	Deep peats are soils with a surface peat layer greater than 0.5 m [57]. Current government policy does not approve or fund tree planting on deep peat because of the large amount of carbon stored in them [62].	Area of deep peats—Unsuitable Area with some deep peats—Limited Area with no deep peats—Suitable

below.

2.5. Overall Suitability for Afforestation with Sitka Spruce

The overall suitability of Sitka Spruce in Scotland was determined by combining the raster layers of biophysical data with policy constraint data. The method used to combine the data is the same as previous methods (Figure 2).

2.6. Calculation of Suitable Land Using LCA Classes

Land that is suitable considering biophysical factors and policy constrained was further categorized into three LCA classes based on The James Hutton Institute [63]: arable agriculture (1–3.1), mixed agriculture (3.2–4.2) and improved grassland (5.1–5.3). This is to determine the area of land that is potentially available for planting Sitka spruce for each type of agricultural land. Rough grazing (6.1–7) was already classified as limited/unsuitable in the biophysical suitability analysis, and thus not included. However, Classes 1–3.1 were removed as a policy constrained to do this analysis.

2.7. Calculation of Suitable Areas for Afforestation with Sitka Spruce within WIAT Buffer

After considering the biophysical and policy suitability for Sitka spruce planting, the suitable area in and around towns was estimated. The woodland in and around towns (WIAT) are those within 1 km of over 2000 inhabitants. The overall suitable land based on biophysical and policy constraints for Sitka spruce was calculated in Section 2.6. and clipped into the WIAT buffer to determine the land area that falls within the 1 km of towns. The WIAT dataset was downloaded from Scottish Forestry Open Data [64].

3. Results

3.1. Biophysical Suitability for Sitka Spruce

The analysis showed that 6,183,300 ha of land is climatically suitable for planting Sitka spruce in Scotland. An area summary of the biophysical suitability analysis is provided in

Table 6. Unsuitable/limited/suitable land areas based on each biophysical dataset.

	Area in ha of Suitability Classes
Data Set	Unsuitable	Limited Suitability	Suitable
Climate	775,472	929,594	6,183,300
LCA	578,909	3,854,070	3,439,509
LCF	2,461,955	3,281,425	2,133,269
LCM	2,019,463	0	5,871,109
Existing Woodland	1,446,053	0	6,426,800

and Figure 3a below. The table and figure show the area of land that is suitable, limited, or unsuitable for each data used for the biophysical suitability analysis.

The total area of land that is biophysically suitable for planting Sitka spruce when the overlapping climate data with LCA with LCF with LCM and existing woodland are accounted for is shown in

Table 7. Area of land unsuitable/limited/suitable for Sitka spruce based on biophysical factors.

Class	Area (In Hectares)
Unsuitable	4,148,438
Limited suitability	2,172,764
Suitable	1,468,549

and the percentage is represented graphically in Figure 3b. More than half of the biophysically unsuitable area was due to LCF class 7; representing land incapable of producing timber crops due to extreme climatic and topographic limitations. They are commonly found in the Grampian and Northern Highlands [50].

The spatial distribution of land area biophysically unsuitable/limited/suitable for planting Sitka spruce in Scotland is shown on a map in Figure 4. Land area classified as biophysically suitable has the potential for supporting Sitka spruce based on climate, soil, and topography and is not existing woodland or developed area. The suitable land area is represented as green on the map and occupies 1,468,549 ha, about 19% of the land area in Scotland. Land with limited suitability is shown on the map as orange and covers 2,172,764 ha, approximately 28% of Scotland. The remaining land area is biophysically unsuitable and occupies 4,148,438 ha, around 53% of Scotland and is displayed as red on the map.

3.2. Area Suitable for Afforestation with Sitka Spruce Considering Policy Constraints

Table 8. Unsuitable/limited/suitable areas based on current policy constraints in Scotland.

Data	Unsuitable (ha)	Limited (ha)	Suitable (ha)
LCA-(Class 1–3.1 excluded)	455,416	0	7,417,073
Windfarms	146,393	167,982	7,576,198
Greenspaces	0	480	7,842,600
Historic Sites	17,437	99,359	7,773,776
World Heritage Sites	46,979	0	7,843,593
Conservation designations	1,431,472	0	6,459,101
Acid Sensitive Catchments	184,059	0	7,706,514
Peatland	2,854,845	312,582	4,704,193

and Figure 5a show detailed results from the suitability analysis for Sitka spruce based on the policy constraints in Scotland. The table shows policy constraints in terms of land area that different policies impose on tree planting. However, there is some overlapping between the land area affected by policy constraints, such as between peatland and conservation areas. About 70% of the locations deemed unsuitable due to policy constraints are areas with deep peat soils and occupy a land area of 2,854,845 ha (Figure 5b). The total land area affected by policy constraints accounting for overlapping is around 52% of Scotland, which is equal to 4,072,739 ha.

3.3. Overall Suitability for Afforestation with Sitka Spruce

The land area that is both biophysically suitable and is not constrained by policy constraints for planting Sitka spruce reaches 999,234 ha, which represents about 13% of the land territory in Scotland (

Table 9. Overall land area suitable for Sitka spruce (biophysical and policy constraints).

Class	Area (In Hectares)
Unsuitable	5,808,436
Limited suitability	978,418
Suitable	999,234

, Figure 6a). The map showing the overall suitability map of Sitka spruce in Scotland is presented in Figure 6b below. The suitable and available land for planting Sitka spruce based on results from this study is clustered in the northeast, central and southern parts of Scotland with small areas of land suitable in the highlands.

3.4. Suitable Areas for Afforestation with Sitka Spruce within LCA Classes

The results from the assessment carried out on land suitable for planting Sitka spruce and land area that is within each LCA class considering both biophysical factors and policy constraints (excluding prime agriculture land from policy constraint) showed that most of the suitable land is in the LCA classes 3.2–4.2 (Mixed agriculture) with 830,227 ha, approximately 61% of the overall suitable land (Figure 7a,b). Arable land (Classes 1–3.1) that were classified as suitable covered 351,918 ha while suitable improved grassland (Classes 5.1–5.3) covered 169,007 ha (

Table 10. Area and percentage of suitable for afforestation with Sitka spruce in each LCA class.

LCA Class	Area (ha)	Percentage of Suitable Afforestation Areas
Arable	351,918	26%
Mixed Agriculture	830,227	61%
Improved grassland	169,007	13%

).

3.5. Suitable Area for Afforestation with Sitka Spruce within the WIAT Buffer

The data presented in

Table 11. Area of suitable land that is within and outside WIAT 1 km buffer.

Suitability	Within WIAT (ha)	Outside WIAT (ha)	% Suitable Land
Biophysically suitable	169,530	1,299,020	12%
Overall Suitability	102,031	897,203	10%

and Figure 8a are the results from the clipping of biophysically suitable and overall suitable land within a 1 km buffer around towns with more than 2000 populations used as the criteria by WIAT to create woodland close to population centers in Scotland. The results showed that about 12% and 169,530 ha of the biophysically suitable land for Sitka spruce is within the 1 km buffer. Considering the overall suitability (biophysical and policy constraints), 102,031 ha of land, which is about 10% of the potentially suitable and available land, is within the buffer. Figure 8b is a map of suitable and available land for Sitka spruce in Scotland that is within the WIAT buffer.

4. Discussion

Scotland has the legal obligation to reach net-zero GHG emissions by 2045, starting with a reduction of 75% by 2030 compared to 1990 [9]. One of the key measures by which the UK government plans to achieve net zero and reduce carbon emissions is through afforestation [31]. Forests and woodlands have been recognized as important tools by the Scottish government in reducing GHG emissions and at the same time providing other ecosystem services [65]. The Scottish government has promised to support the afforestation of the right trees, both productive forests and native woodlands, in the right place so forests can deliver a range of environmental, social, and economic benefits [24,66]. However, the policies to achieve the woodland targets do not usually take into consideration the most suitable locations to plant different tree species considering biophysical factors and policy constraints. The land area that is biophysically and socio-economically suitable for afforestation in Scotland is not so abundant. This study produced maps that indicate the potentially biophysically and policy-suitable locations for afforestation with Sitka spruce in Scotland.

4.1. Land Suitable for Afforestation with Sitka Spruce in Scotland

The result of the study showed that 1,468,549 ha of land is biophysically suitable to support Sitka spruce in Scotland. However, when policy constraints in Scotland are considered, especially those on deep peats, conservation designated areas and prime agricultural land, these put significant restrictions on suitable land, making such land unsuitable for afforestation. Taking into consideration the policy constraints, only about 68% (999,234 ha) of the biophysically suitable land is available for planting Sitka spruce. In Wales, the policy constraints to afforestation projects are similar to Scotland. A study by Bell et al. [32] showed that about 1,410,290 ha are currently unsuitable for planting Sitka spruce in Wales due to biophysical and policy constraints. There might be some opportunities for planting trees in the areas classified as limited on the map (Figure 6b), such as acid catchment areas or prime agricultural land through proper planning and considerations, for example, by practicing agroforestry. On the other hand, in areas of land with peaty gley soils, which are usually classified as suitable for afforestation, there are carbon (C) loss emissions in the peat layer over the first rotation of 30 years [67] and should therefore be considered unsuitable for afforestation from the climate change policy point of view [26].

The results from this study on the overall spatial distribution of land potentially suitable for Sitka spruce are similar to the land pattern considered available for afforestation in Scotland shown on the map produced by Sing et al. [65]. The results from Sing et al. [65] showed that about 2.68 million ha of land is suitable and available for afforestation, more than the 999,234 ha calculated for this study. A more updated analysis of the study by Sing and Aitkenhead [48], which included the planting of trees on higher-quality agricultural land and excluded the deep peat soil (>0.5 m depth) resulted in an increase of about 270,000 ha of potential land available for woodland expansion in Scotland, from 2.68 million ha to 2.96 million ha. Planting on land classified as low-quality agricultural land (LCA Classes 6.1–7) will lead to net negative emissions, and planting on higher-quality agricultural land (LCA Classes 1–3.1) will lead to significant competition with other land uses particularly agriculture. This is the reason low-quality agricultural land was classified as limited suitability and unsuitable throughout the analysis undertaken in this study.

The 13% of the land area considered to be potentially suitable for Sitka spruce resulting from this study, plus the current 18.5% woodland area in Scotland could increase the total woodland cover to 31.5%. This is well above the Scottish government’s target of 25% of woodland cover by 2050 and closer to the 40% average in Europe. However, the proportion of agricultural areas that will be used to plant trees is limited by socio-economic and cultural factors [27,68].

It should be noted, however, that due to the large scale of the datasets used, there will inevitably be some misclassification, for example, where an area broadly classified as unsuitable may be locally suitable. The suitability maps produced were at a national scale and thus, are indicators of areas that are most likely to be suitable. For regional or local scale afforestation projects, individual site assessments such as using the ESC website and inputting soil and vegetation information are suggested. It should also be noted that under future climate scenarios, the land area currently suitable for planting Sitka spruce in Scotland may decline significantly in the future. In Wales, Bell et al. [32] predicted a reduction in the area currently considered suitable for planting Sitka spruce from 3306.2 km² of the land area to 2184.9 km² and 1919.1 km² by 2050, under high and medium GHG emissions, respectively. The rise in the average temperature in the UK by 0.8 °C in the past 30 years [69] is most likely to influence the distribution, growth rate, and ecosystem services provided by tree species [70]. This will affect the land suitability for tree species, such as Sitka spruce, used for softwood production in the UK, which is predicted to decline under future climate [71]. Therefore, the maps of suitable areas for Sitka spruce produced in this study are only an indication of where suitable areas for afforestation are currently located and aim at contributing to the general debate about the implementation of afforestation measures to mitigate climate change.

4.2. Policy Targets for Afforestation in Scotland

The afforestation target in Scotland is to increase forest area by 15,000 ha per year so that by 2032 the woodland cover will increase from the current 18.5% of Scotland’s land area to 21% [13,24]. Despite this target, newly planted woodlands have exceeded 10,000 ha only in 2019 and 2020 since 2002 [29], and this is below the average planting rate of over 20,000 ha per year in the 1980s [29]. This means that the afforestation planting targets are not being achieved. Additionally, due to the low planting rate in most years and the ageing of existing forests in the UK, the rate of carbon sequestration of the forests has been predicted to decline in the future [69]. The reduction in the tree planting rate in recent decades in Scotland has been associated with the multi-policy aspiration for land [26]. Analysis from this study showed that about 999,234 ha of land area within LCA Classes 3.2–5.3 inclusive, which make up the “squeezed middle” zone defined by Slee et al. [28] (Class 6.1 was excluded) are suitable for Sitka spruce. However, new woodland in Scotland has not been significantly created on these improved or higher-quality agricultural lands [26].

In 2019/2020, most of the new tree planting in Scotland (~10,780 ha) occurred on privately owned land, representing approximately 98% of the total new planting [29]. Thus, policies on and planning of afforestation projects will hugely depend on the objectives private landowners have for their land. The Scottish government offers grants to landowners as incentives to encourage them to plant more trees in the right place to provide the economic and environmental benefits of afforestation [72]. However, even with such grants, landowners often are not willing to convert their land to forests [27,73]. There is not much consideration or understanding by landowners of the carbon sequestration or substitution potentials of trees and wood products [28,73]. Brown et al. [27] showed that new and existing woodlands in Scotland have been mostly established on low-quality agricultural land, on LCA class 6.3 land and often in soils with high carbon content. Planting on this type of agricultural land with high carbon content is likely to cause soil disturbance and significant carbon loss during afforestation practices and thus low carbon sequestration potential [27,74].

Afforestation competes with other land use objectives, particularly from agriculture with farmers prioritizing food production on their land over forestry. This leads to new woodland being created on marginal to less productive land that is not necessarily suitable for that woodland type and with significant climate change implications [27]. Nijnik et al. [15] consider that land initially used for livestock grazing as the cheapest place for afforestation in Scotland, and Brown [26] also pointed out that the more realistic approach to carbon gains is woodland replacing grassland. Matthews et al. [45] also stated that in Scotland in 2011, 49,683 ha of grassland had no livestock and 230,577 ha of grassland had less than 0.25 livestock per ha. This current study identified about 169,007 ha of improved grassland suitable for planting Sitka spruce. Planting on grasslands is crucial to achieving the afforestation targets, as food production will be less affected [45]. Barriers to tree planting on improved grassland in Scotland include land tenure issues, agricultural subsidies, and high initial costs [26].

4.3. Afforestation as an Opportunity to Reduce GHG Emissions from Agriculture

In the UK, there has been a significant reduction in emissions from the agricultural sector compared to 1990 and this has been associated with changes in agricultural practices such as efficiency improvements in farming, reduction in the amount of nitrogen fertilizer and tillage practices in grassland [75]. In 2019, GHG emissions from agricultural activities account for approximately 16% of the total estimated GHG emissions in Scotland [76], and unlike other major sources of GHG emissions, the main GHGs emitted from agriculture are methane (CH₄) and nitrous oxide (N₂O) rather than CO₂ [77], with CH₄ and N₂O having a higher global warming impact than CO₂ [75]. The Scottish Government plans to reduce up to 24% of the total agricultural emission by 2032 using 2019 emission as a baseline [77]. The results from this study showed that there is available land to create woodland alongside agriculture, which could sequester carbon and contribute to achieving the agricultural emission reduction target by offsetting some of the GHG emissions from agriculture.

About 26% representing 351,918 ha of the overall suitable area calculated for each LCA class in this study is arable land (see Table 10, Figure 7). The productive agricultural land is important for maintaining food security, especially under the changing climate [78] and given the more recent food shortages related to the Russian-Ukrainian war, and thus hardly available for afforestation. A similar situation was established by the study of Farrelly and Gallagher [41] in the Republic of Ireland where 65% of land deemed suitable for afforestation was classified as productive agricultural land and thus is expected to be devoted for agricultural purposes. Nevertheless, through strategic planning and after ensuring good consideration for the agricultural sector and food security in Scotland, small woodland and hedgerows can be created on suitable farmlands. This can benefit farmers by controlling soil erosion and reducing pollution from agriculture, producing wood fuels, improving farm biodiversity, and providing shade and shelter [69,79]. This will also increase the carbon that would be locked into trees and soil, and in wood biomass when the timber products from the trees are used to replace fossil fuel-intensive products [74].

The Scottish government is working with farmers to increase woodland plantation on appropriate land on farms [24]. Lampkin et al. [79] stated that planting 10% of farmland with trees could sequester by 2045 about 0.57 MtCO₂e of the 7.6 MtCO₂e emitted by Scottish agriculture in 2017. Additionally, in England and Wales, the National Farmers’ Union (NFU) plans to increase woodland and hedgerows to deliver GHG savings of between 0.5 MtCO₂e and 0.7 MtCO₂e per year, respectively [80]. Initiatives such as a carbon trading system or a tax on carbon emissions can be used by the government to trigger the creation of small woodlands on agricultural land. Slee et al. [73] consider that involving farmers in Scotland in buying carbon credits can be a cost-effective strategy for the farmers to increase afforestation on woodland, particularly those responsible for high GHG emissions on their farms. This could also increase tree planting on higher-quality land, which has the potential to store more carbon than those planted on lesser-quality land.

4.4. Opportunities and Risks of Carbon Offsetting

The 999,234 ha of land in Scotland identified in this study as suitable and available for afforestation could be an opportunity for interested individuals or companies to carry out tree planting for carbon offsetting and receive compensation for it. Carbon offsetting is becoming popular as a means for companies, individuals and even countries to mitigate GHG emissions. However, afforestation for carbon offsetting purposes can be highly controversial [81]. This is because offsetting using nature-based techniques, such as afforestation, might not significantly lead to emission reduction (which is the sole aim of carbon offsetting) or sometimes, might even cause net positive emissions due to the occurrence of diseases, wildfire, saturation, emissions from before and during tree planting, and lack of proper planning [82,83]. Another major concern raised about carbon offsetting is that it allows companies and individuals to continue to release GHG emissions. Carbon offsetting through afforestation alone might also distract the attention from the urgent need to reduce GHG emissions by companies and individuals [84]. Carbon offsetting also can cause a potential rise in land prices and rents, and rural inequalities in Scotland [84,85].

Even though the fastest approach to reduce GHG emissions is obviously by significantly reducing or eliminating human activities that are responsible for GHG emissions, it might be currently impossible to completely avoid activities such as transport and agriculture that emits GHGs, especially with the available technology. Therefore, carbon offsetting subjected to Woodland Carbon Code that was created in 2011 in the UK could be a good option to remove GHGs from the atmosphere. The Woodland Carbon Code aims at enabling companies, organizations, or individuals to reduce their carbon footprint by creating woodland [86,87]. Through the Woodland Carbon Code, there has been an increase in afforestation projects in the UK by business organizations such as Allstar, Bilfinger GVA, and BWOC, which have purchased carbon units to increase carbon capture [86]. For example, Shell is working with the Forestry and Land Scotland (FLS) to generate carbon credits through the planting or regeneration of 1 million trees and invested about £5 million to offset vehicle fuel emissions [88]. Additionally, Scottish-owned companies like the Scottish National Investment Bank have invested GBP 50 million to sequester 1.2 million tCO₂e over 20 years [89]. Three hundred and thirty-five projects covering 25,186 ha were already registered in Scotland by March 2021, to enable buyers (companies, organizations, or individuals) to invest in carbon sequestration projects [29]. The quest of companies for available land for carbon offsetting is potentially contributing, however, to tree planting on low-quality agricultural and consequently to net carbon loss, as most of the existing and newly planted trees in Scotland are on this type of land [26,27] rather on the available suitable land, namely, mixed agriculture, and improved grassland, identified by this study.

4.5. Potential of Creating Woodland in and around Town (WIAT)

The Forestry Commission Scotland is committed to creating new urban woodland in Scotland to improve the quality of life, health and wellbeing of the people living in towns and cities [22]. The results from this study show that 102,031 ha of land are suitable and available for planting Sitka spruce within 1 km of WIAT buffer; a buffer used by the Forestry Commission to identify areas for the creation of urban woodland. A similar result was obtained by Sing et al. [65] who estimated an area of 145,000 ha of land suitable for the creation of a new WIAT in Scotland. Planting trees on land surrounding Scottish towns and cities can provide a wide range of ecosystem services to urban residents [90].

Even though planting urban trees will have a potentially small impact on the reduction of high GHGs emitted by cities and towns, the positive effects on the environment, health and pollution absorption are highly beneficial for residents [91]. The capacity of the newly established WIAT to provide a wide range of ecosystem services to urban residents would, however, limit the area of Sitka spruce that could be planted in the 102,000 hectares available for WIAT estimated by this study. As Santamour [92] recommended, no more than 10% of any species should be planted in a particular urban area to be resilient to pests and diseases. In addition, competition for development is higher in cities than in rural areas, and small-scale and temporary tree plantings might be preferred by councils [91].

5. Conclusions

The Scottish government’s objectives to achieve net zero by 2045 and increase woodland by 25% by 2050 require a large-scale expansion in forest areas in Scotland. The government targets are to increase woodland creation by 14,000 ha per year from 2022/2023 and by 15,000 per year by 2024/2025 [13]. These targets are set to reduce significantly GHGs emissions reduction to contribute to Scotland’s net-zero agenda. However, depending on how afforestation practices are undertaken, this target could also have a wide range of co-benefits, namely ecosystem services such as health and well-being. This study assessed the amount of land that is biophysically and politically suitable for planting trees and more specifically Sitka spruce. Results from this study showed that the 25% woodland cover target by the Scottish government could be achievable, as 999,234 ha representing 13% of the land area in Scotland is suitable and available for planting trees, such as Sitka spruce. This will increase the woodland cover in Scotland to 31.5%. Analysis of the potential of planting Sitka spruce in towns and cities using the woodland in and around town (WIAT) buffer also showed that trees can be planted on 102,031 ha of land within the WIAT buffer and Sitka spruce in at least 10% of this land.

To achieve net-zero carbon emissions with the contribution from afforestation, more woodland should be created on higher-quality agricultural land and where the competition with food production is low, for example, improved grassland areas. Agroforestry and woodlands on farms are some of the practices that could be implemented to simultaneously manage livestock and trees. The obligation of companies and farmers to follow the woodland carbon code could potentially mitigate the perverse effects of carbon offsetting and increase the area of forest on higher-quality agricultural land and increase the amount of carbon stored in trees. For example, lowland farmers with higher GHG emissions could pay farmers in the upland areas, who farm on wetter and less productive land, to plant trees that could offset their emissions. Data collection and monitoring of carbon sequestration should also be reported continuously during the rotation period on newly established forests, so companies, farmers and the government could accurately assess the contribution of afforestation to climate change mitigation and the net-zero agenda.

We recommend further studies in understanding where new planting of Sitka spruce is occurring in Scotland, in which type of soils, and which soil management practices are implemented during afforestation. This is to better understand soil carbon emissions during afforestation and to be able to estimate a more accurate mitigation potential of afforestation for a tree rotation. In addition, is also important to target the most biophysically and socio-economic suitable areas for afforestation with incentives for land owners and also to assess the potential co-benefits of afforestation to local communities.