Prediction scenarios of past, present, and future environmental suitability for the Mediterranean species Arbutus unedo L

Abstract

Climate change is a challenge for forests in the coming decades, with a major impact on species adaptation and distribution. The Mediterranean Basin is one of the most vulnerable hotspots for biodiversity conservation under climate change in the world. This research aimed at studying a Mediterranean species well adapted to the region: the Arbutus unedo L. (strawberry tree). The MaxEnt, a presence-only species-distribution software, was used to model A. unedo's environmental suitability. The current species potential distribution was accessed based on actual occurrences and selected environmental variables and subsequently projected for the Last Glacial Maximum (LGM), the Mid-Holocene (MH), and the years 2050 and 2070, considering the two Representative Concentration Pathways: RCP4.5 and RCP8.5. Results from the LGM projection suggest the presence of refugia in the core of the Mediterranean Basin, in particular the Iberian Peninsula (IP). The projections for the MH indicate increasing climatic suitability for the species and an eastward expansion, relatively to LGM. The predicted future environmental changes will most likely act as a catalyst for suitable habitat loss and a range shift towards the North is likely to occur.

Figure 1

The response curves of predicted suitability values for the selected environmental variables. BIO1 (annual mean temperature, ºC), BIO3 (isothermality, %), BIO7 (temperature annual range, ºC), BIO9 (mean temperature of driest quarter, ºC), BIO15 (precipitation seasonality, %), BIO16 (precipitation of wettest quarter, mm), BIO18 (precipitation of warmest quarter, mm) and Slope (degrees).

Figure 2

Arbutus unedo suitability areas. (a) LGM. (b) MH. (c) Current. (d) Evolution of the suitability areas from the LGM to the RCP 8.5 2070. (e) RCP 4.5 2050. (f) RCP 4.5 2070. (g) RCP 8.5 2050. (h) RCP 8.5 2070. Suitability class range: 0.0–0.2, non-suitable area; 0.2–0.4, low-suitability area; 0.4–0.6, general-suitability area; 0.6–0.8, medium-suitability area and 0.8–1.0, high-suitability area. Equal-area projection EPSG:3035. The map was made using the sf 1.0–3 and the terra 1.4–14 (https://rspatial.org/terra/) packages in R 4.1.0^,.

Figure 3

Changes in A. unedo distribution area between scenarios. Areas of expansion in green, and areas of contraction in red. The background grey intensity ranges from low current suitability (white) up to high current suitability (dark grey). Therefore, the four extreme cases are bright green (maximum expansion from low suitability); bright red (maximum contraction from high suitability); dark grey (suitability is and remains high) and light grey (suitability is and remains low). Equal-area projection EPSG:3035. The map was made using the sf 1.0–3 and the terra 1.4–14 (https://rspatial.org/terra/) packages in R 4.1.0^,.

Figure 4

Differences between current and projected past and future scenarios suitability histograms. The x-axis is suitability_current – suitability_past for past scenarios and suitability_future – suitability_current for future scenarios. Each coloured band refers to a class of current suitability and its area in the histogram is proportional to the geographical area of the region with current suitability within this class. Areas with increasing/decreasing suitability (“expansion”/”contraction”) correspond to positive/negative differences and lies on the right/left hand side of the vertical red line. The area of each coloured band is the same across all histograms; the relation between the areas on each side of the vertical red line gives information about the extent of contraction relative to expansion in each projected scenario.

Figure 5

Map of the pollen sites in the Mediterranean region with Arbutus/Arbutus unedo records. Data retrieved from the Neotoma Paleoecology Database. The site number 2 is a charcoal remain. According to the oldest age of the record, the following site labeling was used: Late Pleistocene (from 34–11.7 ka), black-filled circles, Early-Middle Holocene (11.7–4.2 ka), green-filled circles, and Late Holocene-Present (4.2–0 ka), orange-filled triangles. The sites were numbered according to Supplementary Table S3, and the records age above 10 ka, for clarity, were displayed in deep blue. Equal-area projection EPSG:3035. The map was made using the sf 1.0–3 and the terra 1.4–14 (https://rspatial.org/terra/) packages in R 4.1.0^,.

Figure 6

Arbutus unedo L. distribution area and occurrences. Black dots represent the species’ occurrences (Supplementary Table S1), and the green area represents the species native distribution range for A. unedo downloaded from the data sets in. Equal-area projection EPSG:3035. The map was made using the sf 1.0–3 and the terra 1.4–14 (https://rspatial.org/terra/) packages in R 4.1.0^,.