New evidence for population-specific responses to drought events from tree ring chronologies of Pinus nigra ssp. laricio across the entire distribution range

Highlights

• Growth-climate relationships were analysed in black pine (ssp. laricio).

• Drought effects on stem radial growth were studied in representative populations.

• Positive impacts of precipitation in spring/summer, but negative in winter, were found.

• Corsican populations were more sensitive to drought than populations in southern Italy.

• Older trees were more resistant to drought events than younger trees.

Abstract

The increased frequency and severity of drought events due to climate warming is negatively affecting tree radial growth, particularly in drought-prone regions, such as, e.g., the Mediterranean. In this climate change hotspot, populations of the same tree species may show different growth responses to climate, due to the great variety of microclimates and environments that characterise this biogeographic region. In this study, we analysed growth-climate relationships and growth responses to drought events (i.e., resistance, recovery, and resilience) in 13 forest stands of black pine (Pinus nigra Arnold), encompassing the whole and peculiar distribution range of the ssp. laricio (Calabria, Sicily, Sardinia, and Corsica). Analysis focussed on the 1981–2010 period, which is commonly covered at all sites. Stem radial growth of trees increased under a positive spring/summer water balance. However, abundant winter precipitation had a negative impact on stem radial growth. Populations in Corsica were more sensitive to drought, showing lower resistance and resilience than those in Sicily and Sardinia. Older trees were more resistant to drought events than younger trees. Our results highlight that population-specific responses to drought events are mainly explained by tree age and local environment, suggesting geographically related patterns in tree growth and forest productivity correspond to different populations. Intraspecific variability in sensitivity to drought events should be included in species distribution models to predict the range of forest productivity responses to climate change.

Introduction

Drought periods are becoming more frequent and extended worldwide due to global warming (IPCC, 2018). Greater intensity, frequency, and duration of drought lead to an increase in evapotranspiration rates and an alteration of the start and duration of the growing season (Colangelo et al., 2018; Andersson et al., 2011), with negative impacts on tree growth and forest productivity (Allen et al., 2015). Southern Europe is predicted to experience increasing aridity in the 21st century mainly due to rising temperatures, and an increase of intensity, frequency, and duration of heatwaves and droughts (Giorgi and Lionello, 2008). Tree populations in drought-prone regions, such as the Mediterranean, are particularly vulnerable. The Mediterranean Basin is a climate change hotspot, in which the occurrence of intense and prolonged drought episodes may compromise the ecological resilience of tree species (Peñuelas et al., 2011). The great variety of microclimates, and the mutual interaction of temperature, precipitation, and evapotranspiration, determining water availability, challenge our predicting capacity of the potential of trees to cope with climate change across this biogeographic region (Pasho et al., 2012; Linares et al., 2011; Martín-Benito et al., 2010).

The Mediterranean region has a complex and heterogeneous climatic system and regime, with a northern temperate climate and a southern semi-arid climate (Sangüesa-Barreda et al., 2019). This geographic variability, accompanied by variations in regional precipitation regimes (i.e., higher precipitation in the north-east than in the south-west), may differently influence the sensitivity of tree populations of the same species to drought (Dorado-Liñán et al., 2019), entailing different growth responses to global warming across geographic regions and genetic provenances (Camarero et al., 2020; Gazol et al., 2017). Furthermore, the complexity of the Mediterranean environment, linked to changes in atmospheric circulation patterns, is not only observable on a regional scale, but also on a local scale. For example, variations in air temperatures and precipitation within a very short distance, caused by different elevations and slopes in mountainous areas, may lead to local-specific responses to climate (Römer, 2021; Doǧan and Köse, 2019).

The phenotypic plasticity of tree species determines their capacity to tolerate and recover from stress, influencing their resilience, although still difficult to be well-defined (Schwarz et al., 2020), and survival to climate change and disturbance events (Sánchez-Salguero et al., 2018; Gazol et al., 2017). Regardless of the ecophysiological and growth strategies adopted by tree species to withstand xeric conditions in summer, they may have limited capacity to tolerate severe drought events. For instance, black pine (Pinus nigra Arnold) has higher frost tolerance than other pines occurring in the Mediterranean region (Fernández-Pérez et al., 2018), but it is considered less able to cope with drought (e.g., González de Andrés and Camarero, 2020; Serra‑Maluquer et al., 2018). Several studies revealed that black pine is highly sensitive to drought (Janssen et al., 2018; Köse et al., 2017; Granda et al., 2013; Linares and Tíscar, 2010; Martín-Benito et al., 2008). In Spain and France, tree-ring studies have shown that the radial growth of black pine is positively influenced by prior wet and cold autumns, warm winters, and cool and wet springs (Camarero et al., 2015, 2013; Amodei et al., 2013; Andreu et al., 2007; Lebourgeois, 2000). In Turkey, spring/summer precipitation was shown to be the main driver of radial growth of black pine (e.g., Köse et al., 2017, 2012). Yet, in temperate regions, black pine is considered a relatively drought-tolerant species and, as such, a possible candidate for assisted migration in dry forest sites of Central Europe and a potential substitute for indigenous conifers under future climate scenarios, being able to adapt to various soil types and climate conditions (Quézel and Barbéro, 1988). These studies suggest that drought tolerance may differ between seed sources, determining the capacity of black pine to acclimate to local environmental conditions.

Black pine has a wide and fragmented distribution range, extending throughout the Mediterranean basin and central-eastern Europe, because of several glacial and interglacial climatic oscillations and geological events (Thompson, 2005). Probably because of its fragmented distribution range, the diversity in black pine's morphological, physiological, and ecological traits has led to the classification of this species into six different subspecies (Quézel and Médail, 2003). According to karyological (Cesca and Peruzzi, 2002) and biochemical studies (Afzal-Rafii et al., 1996; Scaltsoyiannes et al., 1994), black pine ssp. laricio has been regarded both as a very divergent subspecies and a taxonomically independent species. Black pine ssp. laricio has a unique distribution range, encompassing two big islands and an isolated peninsula (i.e., Sicily, Corsica, and Calabria). It has been regarded both as a very divergent black pine subspecies and a taxonomically independent species, and in Corsica shows a strong genetic distance from the subspecies nigra and salzmannii (Aguinagalde et al., 1997) and a distinct genetic variation even within the same subspecies, in terms of absolute growth (i.e., lower total height increment in Corsican populations than in those of southern Italy; Thiel et al., 2012). Black pine ssp. laricio has a conservative water-use strategy (Cinnirella et al., 2002) and a durable wood, which is rich in resin and easy to process (best log quality, in terms of stem form, branch quality, and timber volume). Due to its central position within the distribution range of black pine ssp. laricio can be considered an excellent model to study intraspecific growth-climate relationships and cope with climate-smart forestry requirements (Tognetti et al., 2022). Although populations of black pine ssp. laricio from their main range areas have been studied at molecular (Bonavita et al., 2016) and genetic levels (Scaltsoyiannes et al., 2009; Afzal-Rafii and Dodd, 2007), a comprehensive analysis of climate-growth relationships for this subspecies is currently lacking. Indeed, dendrochronological studies conducted on this subspecies have focused on certain localities (e.g., Mazza et al., 2018; Szymczak et al., 2014), though they have never considered climate-growth relationships and drought responses across the entire distribution range of black pine ssp. laricio. Therefore, insights on climate-growth relationships and drought responses in black pine ssp. laricio across its entire distribution range may become crucial to assess and implement adaptive management strategies aimed at increasing forest resilience to climate change in a peculiar environmental setting, like the one in central Mediterranean.

In this study, we analyse climate-growth relationships and tree growth responses to drought, in black pine ssp. laricio. We focus on 13 forest stands located in four regions (Calabria, Sicily, Sardinia, and Corsica), embracing the whole distribution range of this black pine ssp., in the southern Mediterranean Basin. We hypothesize that i) mild winters and relatively moist spring/summer months increase stem radial growth; and ii) responses to drought in Corsican populations are different from those in southern Italy, with a higher sensitivity and a lower resilience to drought events. To describe tree growth responses to drought episodes, we use three different indices: resistance (Rt), recovery (Rc), and resilience (Rs; Lloret et al., 2011). Resistance is the capacity to reduce the impact of the disturbance (e.g., drought event). Recovery is the ability to return to pre-disturbance growth levels after the drought event. Resilience is the capacity of trees to maintain their functions after the impact of a disturbance. Growth-based resilience indices for drought responses of trees provide useful information in the analysis of disturbance events and the application of silvicultural treatments (e.g., selective thinning).