Climate Change Projections for Bioclimatic Distribution of Castanea sativa in Portugal

Abstract

The chestnut tree is an important forestry species worldwide, as well as a valuable food resource. Over recent years, Portugal has shown an increasing trend in chestnut tree area, as well as increases in production, hinting at the socioeconomic relevance of this agro-forestry species. In this study, bioclimatic indices are applied to analyse the spatial distribution of chestnut trees in mainland Portugal, namely growing degree days (GDD; 1900–2400◦C), annual mean temperature (AMT; 8–15◦C), summer days with maximum temperature below 32◦C (NTX), and annual precipitation (PRE; 600–1600 mm). These indices are assessed for the baseline (IBERIA01, 1989–2005) and future climates (EURO-CORDEX: 2021–2040, 2041–2060, and 2061–2080) under two forcing pathways (RCP4.5 and RCP8.5), also taking into account the chestnut tree land cover. For the baseline, the GDD showed only 10% suitability for chestnut tree cultivation in southern Portugal, whereas much higher values are found in the north of the country, and at higher altitudes (50–90%). For the AMT, higher elevation areas in northern Portugal show almost 100% suitability. Concerning NTX, the suitability reduces from the west (100–90%) to the east (40%). Regarding PRE, the suitability is heterogeneous throughout the territory, with areas under 50%. A new Chestnut Suitability Index (CSI) was then computed, which incorporates information from the four previous indices. The CSI reveals a suitability ranging from 100 to 75% in the north, while central and southern Portugal show values from 25 to 50%. For future climates, a progressive reduction in CSI was found, particularly for RCP8.5 and in the long-term period. Changes in bioclimatic conditions may restrict the 100% suitability to a narrow area in the north of the country. These reductions in chestnut bioclimatic suitability may have socio-economic and ecological implications for the management of the important agro-forestry species.