Introduction

Eucalyptus forest has existed in Portugal and Europe since the second mid-19th century and has shown a systematic increase in Continental Portugal over the last 50 years (^{CELPA 2016}; ^{ICNF 2020} a). According to National Forest Inventory (^{ICNF 2020} b) in 20015 eucalyptus globulus was the specie with the highest forest land occupation (845,000 hectares) representing 26.2% of the total Portuguese forest area. Although between 2005 and 2015 the occupation of the soil by eucalyptus globulus had grown about 7.5%, the existing volume increased slightly 0.2% mainly due wildfires. During this period, an average of more than 44.7 thousand hectares (Kha) per year were burnt because of an average of 20753 forest fires per year (^{Pordata 2020}). Furthermore, the severity of wildfires in Portugal in 2017 with a total burnt area of more 539 Kha, equivalent to 500 Kha in forest space, comprising 329 Kha in forest stands and 170 Kha in scrublands (^{ICNF 2019}), certainly caused an important degradation on the existing volume of eucalyptus. Forest fires like land conversion, tillage, overgrazing, and soil erosion are anthropogenic causes of soil organic matter (SOM) loss (^{Brandão and Milà i Canals 2013}).

Eucalyptus forest are multifunction systems that provide forest products (e.g., wood), non-forestry (e.g., essential oils) and environmental (e.g., carbon sequestration). Portuguese eucalyptus globulus is a leader in exports of goods with high national added value, contributes to the generation of thousands of jobs and creates value for forest owners and for the economic agents involved (^{CELPA 2016}).

Life cycle assessment (LCA) is one of the standardized (^{ISO 2006} a,^b) techniques being developed to better understand and address the environmental aspects and potential environmental impacts throughout a product´s life cycle from raw material acquisition through production, use, end-of-life treatment, recycling, and final disposal (i.e., cradle-to-gate).

To assess the environmental impacts associated with the production of eucalyptus wood in Portugal the LCA (ISO, 2006 a,b) have been applied to some products of eucalyptus (^{Lopes et al. 2003}; ^{Vieira et al. 2010}; ^{Dias & Arroja 2012}). The products contribution for a series of impact categories were used in these studies like global warming, acidification, eutrophication, human toxicity, abiotic depletion, etc., but no one assessed the impacts caused by land use (land occupation and land transformation) that is essential for the full assessment of forest products (^{Doka et al 2002}; ^{Perminova et al 2016}).

Impacts of land occupation and land transformation on biodiversity, biotic production, and soil quality have been studied by some authors (^{Hauschild et al 2011}; Mattila et al 2012; ^{Faragò et al 2019}; ^{Perminova et al 2016}). Hauschild et al (2011) evaluated various methods of land use impact in the context of LCA and considered that the most appropriate among the existing approaches in the European context is the method by Milà i Canal at the midpoint level. This method has a focus on soil quality, and its indicator describes the changes in soil organic matter (SOM) associated with land interventions (^{Milà ì Canals et al. 2007}).

keeping in mind the economic, social, and environmental relevance of eucalyptus globulus, the aim of this study is to assess the evolution on land use impact category of Portuguese eucalyptus between 2005 and 2015. The results of this study can be important to support future decision-making regarding the best management options for Portuguese forest planning.

1. Methods

The Life Cycle Assessment method recommended in the ISO 14040/44 (^{ISO 2006}a; ^{ISO 2006}b) standards was adopted in this study. It includes four phases: goal and scope of the study where the functional unit and the system boundary take place; inventory analysis that includes data collection and building the inventory table; impact assessment that transforms the data in the inventory table into impact categories indicators using the characterization factors; and interpretation of the results.

1.1 Goal and scope of the study

Functional unit and system boundary

The functional unit (FU) is given as 1 m³ of eucalyptus globulus, standing, in forest and the function of the system being studied is to produce eucalyptus trees for different uses.

The system boundary for the product system being studied is represented in Figure 1. The activities/substances included in the boundary are related with regeneration of eucalyptus trees in the forest. The output is eucalyptus trees standing in forest and the inputs are those related with the occupation and transformation of land.

Figure 1 Eucalyptus system boundary (gate-to-gate) 

3. Results and discussion

With the help of SimaPro 9.1 PhD software (^{PRé Consultant 2020}), data from Table 1 were used to build the inventory table (Table 2). In this table, the result of transformation, from forest, natural is equal to the result of transformation, to forest, intensive plus the result of transformation, to traffic area, rail/road embankment.

Using ILCD 2011 Midpoint+ V1.11 method the land use impact per functional unit for the years 2005 and 2015 was assessed. The comparative results by substance are presented in Tab. 3 and illustrated in Fig. 2.

From the Table 3, the total carbon deficit attributed to FU in 2005 was 21832 kg C deficit and 23430 kg C deficit in 2015. It means that, in this period, the deficit in carbon increase of approximately 7.3%. Transformation, to traffic area, rail/road embankment with 17015 Kg C deficit in 2005 and 18260 Kg C deficit in 2015 was the activity/substance that contributed with the largest value for the total. Occupation, forest, intensive was the second most significant activity with a land use impact of 4255 Kg C deficit in 2005 and 4566 Kg C deficit in 2015. Occupation traffic area was the activity that presented the lower impact on land use with 653 Kg C deficit and 701 Kg C deficit in 2005 and 2015, respectively.

It should be noted that the impact on land use by the land transformation from forest natural into forest intensive is null. It means that from the -3639 Kg C deficit (credit) in 2005 and -3902 Kg C deficit in 2015 of the activity transformation, from forest, natural, -3544 Kg C deficit in 2005 and -3902 Kg C deficit in 2015 are due to the transformation, to forest, intensive. The remaining - 91 Kg C deficit in 2005 and -97 Kg C deficit in 2015 are attributed to the land that was transformed from forest natural into traffic area.

Figure 2 Comparative land use environmental profile of eucalyptus, standing, in forest in 2005 and 2015 using ILCD 2011 Midpoint+ method. Acronyms: OFI (Occupation, forest, intensive); OTA (Occupation, traffic area, rail/road embankment); TFFN (Transformation, from forest, natural); TTFI (Transformation, to forest, intensive); TTTA (Transformation, to traffic area, rail/road embankment) 

As illustrated in Fig. 2, transformation, to traffic area, rail/road embankment (TTTA) was the activity with the largest contribution to the total amount representing 78 %. Occupation, forest, intensive (OFI) was the second most representative activity with approximately 19.5% of the total value followed by occupation, traffic area, rail/road embankment (OTA) that accounted for 3%. Transformation, from forest, natural (TFFN) presented a positive contribution (minus sign) to the land use impact of eucalyptus.

Conclusion

This paper presents the land use impact evolution expressed as kilogram C deficit per m³ of Portuguese eucalyptus between 2005 and 2015 by using the life cycle assessment methodology and the SimaPro 9.1.1 PhD software.

Land use impact from land transformation into forest roads are much higher than impacts from land occupation which agrees with the results of the study carried out by ^{Sandin et al. (2013}) in cotton and wood-based fibers.

The land use impact of eucalyptus increases of approximately 7.3% from 2005 to 2015. Land transformation into forest roads is the most relevant activity representing approximately 78 % of the total impact while forest land occupation represents only 19.5%.

The 6.8% reduction in eucalyptus productivity in the period under study, as a result of forest fires and other anthropogenic causes, plays an important role in the evolution of eucalyptus land use impact.

Acknowledgements

This work was financially supported by the Portuguese Foundation for Science and Technology (FCT) through Polytechnic Institute of Viseu by the CERNAS (Research Centre for Natural Resources, Environment and Society) project UIDB/00681/2020.