What drives growth of Scots pine in continental Mediterranean climates: Drought, low temperatures or both?
Introduction
Plant growth seasonality is characterized by favorable and adverse climatic conditions for tree growth, such as those faced by forests subjected to continental Mediterranean climates, where trees endure the double climatic stress of cold winters and summer droughts (Mitrakos, 1980). As a result, optimal growth conditions occur during the two milder periods with increased rainfall (i.e., spring and autumn) which usually produce a bimodal pattern of cambial activity (Camarero et al., 2010), so the question arises: which climatic factor limits tree growth to a greater extent, the winter cold constraints or drought during the main growing season (spring to summer)? The roles of the two stressors on growth processes such as wood formation are clearly different. For instance, the winter cold affects prior carbohydrate use by reducing photosynthesis and respiration rates (Gimeno et al., 2012), and may determine how well dry soils are recharged with water before spring cambial resumption takes place in spring (Pasho et al., 2011); whereas, drought directly constrains growth rates during the growing season (Camarero et al., 2014, Gutiérrez et al., 2011).
Understanding the effects of climate on tree growth is particularly relevant in continental areas of the Mediterranean Basin, which are considered major climate-change hot-spots where both warming and aridification trends have been observed (Giorgi, 2006). The Mediterranean region is exposed to transitional climatic conditions between the temperate and continental climate in the North and the subtropical climates further South (Köppen, 1936). Recent trends toward a warmer and drier climate have had negative effects on tree growth (Sánchez-Salguero et al., 2012) and forest productivity (Madrigal-González and Zavala, 2014). Thus, it would be of interest to assess how cold and droughts are related to tree growth in such area, particularly over the Western Mediterranean Basin (Xoplaki et al., 2012).
In connection to recent climate trends, in cold-limited Circum-Mediterranean areas such as the high-elevation ranges (e.g., Pyrenees, Iberian mountains, Balkans, Apennines, etc.) or plateaus (e.g., Iberian and Anatolian peninsulas) warmer winter temperatures could facilitate subsequent tree growth by extending the length of the growing season (Vaganov et al., 2006). In contrast, more frequent summer heat waves (Pichler and Oberhuber, 2007) and/or related droughts could limit tree growth by shortening the growing season (Eilmann et al., 2011, Galván et al., 2014). Lastly, the responsiveness of trees to climate is; however, highly dependent on topographical features such as elevation or aspect (Büntgen et al., 2012, Rigling et al., 2002), which moderate the water availability in Mediterranean mountain sites (Camarero et al., 2013, Candel-Pérez et al., 2012).
A comparison of long-term climate and tree growth data usually reveals that the climatic conditions prior to the growing season play a prominent role in wood formation (Fritts, 2001), and Circum-Mediterranean forests are no exception (e.g., Büntgen et al., 2010, Camarero et al., 2013). The dendrochronological approach allows a treés growth responses to climate to be inferred on a monthly scale, albeit such inferences should be complemented with short-term data (e.g., by using climate data compiled on a weekly or biweekly basis) and would be validated through xylogenesis studies (Camarero et al., 2010, Vaganov et al., 2006).
We used this multi-scalar approach to investigate how low winter temperatures and drought during the growing season in a continental Mediterranean climate drive the seasonal radial growth of Scots pine (Pinus sylvestris L.). This is the most widespread conifer species in the world, but it reaches its Southern and dry limit in the Iberian Peninsula (Mirov, 1967). First, we relate growth to changing climate conditions since 1900 at local scales by sampling three sites in the Spanish Central System at different elevations, and therefore varying climatic conditions. We expected lower winter temperatures would be the main climatic constraint on tree growth at high-elevation sites and spring-to-summer water deficits at low-elevation sites. Second, we extrapolated this local analysis to a regional scale along a latitudinal gradient using existing Scots pine tree-ring growth series form continental Mediterranean sites in Spain. We hypothesize that at the global limit to Scots pine distribution, the lowest sites and Southernmost sites will be responsive to drought; whereas, the highest and Northernmost sites will be most sensitive to winter temperature conditions.
Section snippets
Local study area
The local study area is located in the Valsaín forests in “Sierra de Guadarrama” National park, a mountainous area situated on the North-facing slopes of the Spanish Central System near Madrid (40°49′N, 4°1′W, elevation range 1100–2125 m a.s.l.). The forests studied are dominated by managed Scots pine stands and mixed with other less abundant tree species such as Quercus pyrenaica Willd. The geological substrates are granite and gneiss. Soils are usually acid, with humic cambisols or leptosols
Climate-growth associations along the local elevational gradient
Trees from the high-elevation site were older and larger than those from the other sites, which may explain why they formed wider tree-rings (Table 1). Trees from the middle- and low-elevation sites were shorter than those from the high-elevation, respectively. The year-to-year persistence (AC) and inter-annual variability (MS) of growth were also greater at higher elevation which suggests that climatic constraints on growth are more important for high-elevation site (Table 1). The remaining
Discussion
Dry conditions during the growing season constitute the main constraint on Scots pine growth under continental Mediterranean climates, although previous low winter temperatures also play an important role as a growth limiting factor. Furthermore, recent climate warming is exacerbating the negative impact of summer drought on radial growth of Southern European Scots pine populations, presumably due to increased evapotranspiration and reduced soil water availability (Gea-Izquierdo et al., 2014,
Acknowledgements
We were glad to be able to use the E-OBS dataset (EU- project ENSEMBLES) and the data providers in the ECA&D project (http://www.ecad.eu), and thank Dr. Geert Jan van Oldenborgh for his assistance with these data. We are grateful to the researchers who provided data on the International Tree-Ring Data Bank. This study was funded by the projects CoMo-ReAdapt (CGL2013-48843-C2-1-R) and FORRISK (Interreg IV B SUDOE 2007-2013). The authors thank CENEAM, Montes de Valsaín and OAPN and all
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