ORIGINAL ARTICLEScots pine plantations growth adaptation to climate warming in locations at the southernmost distribution limit of the species
Introduction
Pine plantations cover wide extensions across drought-prone areas like the Mediterranean Basin, where most afforestation efforts were done during the 20th century (FAO, 2006). These plantations provide a set of services such as regulation of the water flow, protection against erosion, timber production, CO2 uptake and storage, and conservation of biodiversity (Brockerhoff et al., 2008; Gómez-Aparicio et al., 2009; Ruiz-Peinado et al., 2017; Valbuena-Carabaña et al., 2010). However, certain characteristics of some plantations make them more vulnerable to climatic (e.g., drought) and biotic stressors (e.g., pests) such as high stand density due to a lack of management, low structural and genetic diversity, and a defective adaptation derived from an unsuitable seed origin (Alia et al., 1995; Benito Garzón et al., 2011; Gil et al., 2009). Abrupt climate extremes such as droughts may have profound consequences on the performance of maladapted forests and non-acclimated afforestations, which may lead to growth decline, dieback and mortality, and also to a loss of ecosystem integrity and function (Sánchez-Salguero et al., 2012).
Spain is one of the countries with the largest area of pine plantations in the world (FAO, 2006; Payn et al., 2015). In the mid-19th century Spanish woodlands covered approximately six million hectares, representing 12.5 % of the national territory (Armenteras, 1903), an extension resulting from logging and intensive agricultural use during previous centuries (Valbuena-Carabaña et al., 2010). Nowadays woodlands cover more than sixteen million hectares (Vadell et al., 2016). In total, more than five million hectares were planted since 1877, approximately ten percent of the entire country area (Vadell et al., 2016). However, the seed origin and quality were not considered in reforestations carried out in Spain until the second half of the 20th century, and many plantations were carried out with non-local provenances (Gil et al., 2009).
Due to a shift towards warmer climate conditions, the capacity of adaptation may be reduced in homogeneous and poorly managed pine plantations (Sánchez-Salguero et al., 2012). The last 30 years constituted the warmest period at the Earth’s surface since 1850 (IPCC, 2014). Ongoing climate change will increase the recurrence and severity of adverse climate extremes such as severe droughts and heat waves, especially in Southern Europe, a climate change hotspot (Jacob et al., 2014). This scenario affects specially those tree populations located at the southernmost, xeric limit of the tree species distribution area (Sánchez-Salguero et al., 2017, 2012). In relation with the climate change, several cases of growth decline and dieback have been documented in drought-prone conifer forests (Camarero et al., 2015; Macias et al., 2006; Sarris et al., 2011, 2007).
The study of sensitive forests, due to their location or origin, can help to understanding the impact that climate change is having and will have on these ecosystems throughout the 21st century. Here we study the growth dynamics and its relationship with climate in two Scots pine (Pinus sylvestris L.) plantations located in the center (Sierra de Guadarrama) and South (Sierra Nevada) of Spain. The latter site is located at the southernmost distribution limit of the species. Scots pine is widely distributed across Eurasia, and it has been profusely planted worldwide. In Spain it is the second tree species most used in plantations (Valbuena-Carabaña et al., 2010), creating forests with productive, ecological and/or soil protection purposes. The plantations in Sierra de Guadarrama were established in the second half of the 20th century, whereas most of Sierra Nevada plantations were established in the middle of that century. Both sites are currently protected areas. The seed source is the same in both study sites, central Spain (Mesa Garrido, 2016), a provenance (hereafter CS provenance) encompassing Sierra de Guadarrama. This implies that Sierra de Guadarrama plantations are better adapted to local conditions, whereas Sierra Nevada plantations may be maladapted (Rubio-Cuadrado et al., 2018). Consequently, we hypothesize that plantations are less adapted to local climate and to the forecasted warmer and increasingly arid climate in Sierra Nevada than in Sierra de Guadarrama. Our specific objectives are: (i) to compare the growth trends of Scots pine plantations in a wet (Sierra de Guadarrama) vs. a dry site (Sierra Nevada) with common seed source from CS provenance; (ii) to analyze the influence of climatic factors on the radial growth in these two climatically contrasting sites; (iii) to analyze the effect of droughts on radial growth in both sites; (iv) and to project their growth over the course of the 21st century. In this work we consider radial growth as a proxy of sensitivity to climate warming and drought stress.
Section snippets
Study area
We compared Scots pine plantations located in Sierra de Guadarrama (Segovia province, central Spain) and Sierra Nevada (Granada province, southeastern Spain) located 400 km southwards (Fig. 1). Both areas are currently included in the Spanish network of National Parks. In Sierra de Guadarrama the climate is continental Mediterranean with average temperature of 9.9 °C and annual precipitation of 975 mm (estimated data for the study area, see Section 2.4). In Sierra Nevada the climate is
Climate and drought trends
In both sites there has been a significant increase in temperature since the 1970s, while rainfall did not show any trend (Fig. 2). Although wet periods were frequent between 1950 and 1980, with positive SPEI values, the aridity has increased since 1980 in both sites, as shown by the frequency of negative SPEI values (Fig. 3).
Growth rates and trends
Planted Scots pine individuals in Sierra de Guadarrama showed significantly higher growth rates (BAI) than Sierra Nevada pines from 1990 to 2014 (Fig. 4). In Sierra de
Growth trends
Scots pine plantations located in Sierra de Guadarrama and Sierra Nevada sites have similar competition characteristics, with basal area equal to 32.5 and 30.4 m2 ha−1 respectively (Table 1). However, the different climatic conditions, in particular the higher precipitation in Sierra de Guadarrama site (Figs. 2 and S1 in Appendix A of the Supplementary material), enable maintaining higher radial-growth rates (Fig. 4). During the initial years of growth, BAI follows a logistic function (Fig. 4
Conclusions
Analyzing growth rates and trends of Scots pine plantations in two sites under different climate conditions has demonstrated that plantations in the drier site (Sierra Nevada), located at the species’ southernmost distribution limit, showed a stable and lower growth than plantations in Sierra de Guadarrama, located in wetter sites. In addition, if the current climate-growth relationships hold, Sierra Nevada plantations will maintain the current limited growth over the 21st century. Furthermore,
Declaration of Competing Interest
None.
Acknowledgments
This project has been funded by the National Parks Autonomous Agency of the Spanish Ministry for Ecological Transition through the project 979S/2013, by the Autonomous Community of Madrid through the project P2013/MAE-2760, and through the project AGL2016-76769-C2-1-R of the Spanish Ministry of Science, Innovation and Universities. ARC is grateful for the support of a PhD fellowship (FPU15/03533) from the Spanish Ministry of Science, Innovation and Universities.
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