Drought-induced tree mortality: from discrete observations to comprehensive research

There is an increasing need for a global monitoring network to assess rates of tree mortality. In spite of numerous reports of tree death, mostly linked to climate change and related droughts, quantitative assessments of tree mortality rates and the magnitude of tree mortality events are largely missing ( Hartmann et al. 2015). Except for human-caused deforestation, the largest forest losses in the past decade are likely those in central Russia following the 2010 wildfires and those in NW North America due to the mountain pine beetle epidemic. Although tree mortality was not the direct result of drought in either case, the 2010 wildfires were related to an exceptional heat wave ( de Groot et al. 2013) and beetle infestation dynamics are closely linked with a warming trend ( Bentz et al. 2010). Identifying the underlying causes of tree mortality events requires deciphering the eco-physiological pathways leading to tree death using field measurements. Three major mechanisms have been proposed to explain how trees die during drought: (i) hydraulic failure; (ii) carbon starvation as a result of prolonged stomatal closure; and (iii) increased attacks by biotic agents, promoted by reduced plant defence capabilities ( McDowell et al. 2008, Martínez-Vilalta 2014). The investigation of each potential mechanism has exposed important knowledge gaps and technical difficulties. These are exemplified by recent discussions about the definition and scope of carbon starvation, such as whether trees might lose all their carbon reserves before death ( McDowell 2011) or whether they may lose the ability to allocate carbon to certain tissues ( Sala et al. 2010). Other discussions have debated whether xylem cavitation and its reversal are uncommon ( Cochard and Delzon 2013), or rather routine in certain species ( Klein et al. 2014). On top of these debates, methodological issues have been raised, regarding the measurement both of hydraulic traits ( Jansen et al. 2015) and of non-structural carbohydrates (NSCs; Quentin 2014). In light of these hardships, studies investigating any of the three mortality mechanisms in a coherent way have already made key contributions to our understanding of the physiology underpinning tree mortality. However, an investigation of more than one mechanism in a forest die-off study would require an extensive, fully integrated research project, or even several complementary projects. The paper by Aguade et al. (2015) in this issue of Tree Physiology is among the first studies investigating all three mechanisms simultaneously. Studying drought-stressed Scots pine (Pinus sylvestris L.) growing near the edge of its distribution in a forest in northern Spain, Aguade et al. skilfully combined water relations measurements with an assessment of the tree carbon economy and a study of root rot infection by a fungal pathogen. This comprehensive research approach complements that of another recent Tree Physiology paper about the relationship between seasonal carbohydrate dynamics and the degree of fungal infection on Douglas fir needles ( Saffell et al. 2014). To these two components, Aguade et al. (2015) added the essential hydraulic aspect, studying all three mortality mechanisms in a drought context. Both these papers highlight the intertwined nature of the physiological mechanisms leading to tree death. In order to partition the specific contribution of each mechanism in the field, interactions between the mechanisms must be carefully studied. A realistic research approach to disentangling potential mortality mechanisms must consider two different facets of carbon and water in plants: they are necessary for plant life both at the whole-tree scale (i.e., as fluxes) and for specific functional tissues. Drought can impact whole-tree hydraulic conductivity, via xylem cavitation, and can affect leaf hydration, arresting meristematic growth through the loss of Commentary