State of research 

Introduction

The climate is becoming less favorable for forests. Rising temperatures and reduced precipitation are leading to more intense and frequent heat waves and droughts, with consequences for the composition, structure and functioning of forest ecosystems. Studies carried out on a global scale, in which scientists from the University of Basilicata have participated, have shown that: drought-induced tree mortality can cause significant changes in forest communities: e.g. the entry of species from drier environments into temperate forests, or the transition of forests to mixed tree/shrub open-canopy communities [1]; drought-induced tree mortality and forest decline are widespread throughout the world, in all biomes and with many species affected [2].

Tree mortality and forest decline are increasingly common in Italy. The most affected forests are deciduous oak forests (Turkey oak, Quercus cerris L.; downy oak, Quercus pubescens Wild.; Hungarian oak, Quercus frainetto Ten.; pedunculate oak, Quercus robus L.). A few years ago, our research group reported cases of declining oak forests throughout the Italian peninsula [3]. More recently, we have shown that oak forests are highly vulnerable to severe heat waves and droughts [4]. These issues have been the subject of functional studies for some time in marginal areas of the southern Apennines (Basilicata) and in floodplain forests within protected areas in southern and northern Italy.

In this section we summarize the results of the research carried out so far at the University of Basilicata. These studies, which are now being continued within the Agritech project, are useful for interpreting the eco-physiological behavior of oak species under drought conditions, for monitoring and forecasting the temporal dynamics of forests under climatic stress, and for defining adaptive management aimed at improving the resilience of deciduous oak forests.

Tree phenotype and genotype

Changes in wood anatomical traits were investigated as a tool to predict drought-induced dieback in Mediterranean ring-porous oak species. Over a 38-year period (1980-2017), different growth patterns and xylem traits were observed in decaying (D) trees compared to non-decaying (ND) oak trees. This helps to identify which climatic factors are most important in causing hydraulic failure and tree mortality [17].

This issue also merits consideration in light of atmospheric nitrogen deposition and increased nitrogen availability.  A meta-analysis suggested that increased nitrogen availability may favor the formation of xylem conduits with improved resistance to drought-induced cavitation [8]. It has also been observed that increased nitrogen deposition and availability may have beneficial effects on oak physiology, both in terms of water use efficiency and carbon uptake [9]. 

Intra-specific differences in resistance to extreme drought underlie the adaptive potential of tree populations. The Agritech project addresses this issue by combining ecophysiological, dendro-ecological and dendro-genomic analyses.

The studies carried out so far warn of the need to properly deal with confounding factors under field conditions and stress the need for appropriate sampling protocols to investigate the relationships between genetic and phenotypic traits. For example, it was found that the presence of pathogens (e.g. Phytophthora quercina) may be a predisposing factor for tree decline [10], and that small-scale environmental variability may modify the effects of drought on the forest community [4].

Remote sensing 

The possibility of combining ground-based and remotely sensed (satellite RS) observations has been explored to develop methods and metrics that may be useful for interpreting and predicting oak forest decline. The well-known NDVI index was found to correlate well with ground-based variables, such as forest stand basal area and tree growth [11]. A novel metric for quantifying water deficit was proposed and found to be effective in predicting atmospheric conditions that favor drought [12]. Overall, remote sensing provides effective tools for real-time and large-scale monitoring of the effects of drought stress on oak forest growth and productivity [13, 14].

This issue has been further addressed in a review aimed at discussing critical issues, needs and solutions when combining remote sensing and dendroecological methods to assess forest vulnerability and decline [19].

Floodplain forests

In temperate regions, floodplain forests are relict forest ecosystems. Changes in the rainfall regime caused by climate change may jeopardize these valuable and now rare ecosystems, which retain a high level of biodiversity. In the Bosco Pantano of Policoro (Matera, Basilicata, southern Italy), research and management efforts are aimed at saving the relict floodplain forest and its most important species (Quercus robur L., pedunculate oak) [15]. Genetic studies have shown that the oak population in Bosco Pantano is genetically distinct from other Italian oak populations and genetically depleted. It may be the last population of a southern genetic cluster, with adaptations to the Mediterranean climate not present in other populations. Even interspecific differences are important for the conservation of these ecosystems. Recently [16], we highlighted a different response to climatic stress among pedunculate oak and its associated species (ash (Fraxinus angustifolia Vahl.), alder (Alnus glutinosa L.), elm (Ulmus minor Mill.)). In a study carried out in Bosco Pantano and in the floodplain forest of Ticino (Milan, Lombardy, Northern Italy), ash was found to be the most sensitive in the Mediterranean environment (Bosco Pantano), where oak and elm were also negatively affected by water shortage. Alder, on the other hand, appeared to be the least sensitive to drought in both environments.