Balancing the risks of hydraulic failure and carbon starvation: a twig scale analysis in declining Scots pine.

Yann Salmon,Jordi Martinez‐Vilalta,Maurizio Mencuccini,José M Torres‐Ruiz,Hervé Cochard,Rafael Poyatos,Patrick Meir

doi:10.1111/pce.12572

Abstract

Understanding physiological processes involved in drought‐induced mortality is important for predicting the future of forests and for modelling the carbon and water cycles. Recent research has highlighted the variable risks of carbon starvation and hydraulic failure in drought‐exposed trees. However, little is known about the specific responses of leaves and supporting twigs, despite their critical role in balancing carbon acquisition and water loss. Comparing healthy (non‐defoliated) and unhealthy (defoliated) Scots pine at the same site, we measured the physiological variables involved in regulating carbon and water resources. Defoliated trees showed different responses to summer drought compared with non‐defoliated trees. Defoliated trees maintained gas exchange while non‐defoliated trees reduced photosynthesis and transpiration during the drought period. At the branch scale, very few differences were observed in non‐structural carbohydrate concentrations between health classes. However, defoliated trees tended to have lower water potentials and smaller hydraulic safety margins. While non‐defoliated trees showed a typical response to drought for an isohydric species, the physiology appears to be driven in defoliated trees by the need to maintain carbon resources in twigs. These responses put defoliated trees at higher risk of branch hydraulic failure and help explain the interaction between carbon starvation and hydraulic failure in dying trees.

Highlights

Episodes of tree mortality in response to high temperature and extreme drought have been reported worldwide in the last few decades (e.g., Breshears et al, 2005, Allen et al, 2010, Peng et al, 2011, IPCC, 2014)
Defoliated trees had a lower number of needles per year, with this difference being mostly due to the year 2010 (17±10 vs. 43±8, respectively, Fig. 2B)
Our results offer new insights regarding the ongoing interactions between physiological processes leading to death in a Mediterranean Scots pine population

Summary

Introduction

Episodes of tree mortality in response to high temperature and extreme drought have been reported worldwide in the last few decades (e.g., Breshears et al, 2005, Allen et al, 2010, Peng et al, 2011, IPCC, 2014). As future climate scenarios under on-going climate change predict that drought frequencies and intensities will increase in several regions worldwide (Collins et al, 2013), the ability to understand, predict and model future tree response and survival to water deficit has become increasingly important. Based on multiple observational and experimental studies, a theoretical framework has been developed in recent years proposing several mechanisms responsible for drought-induced mortality (cf., recent review by McDowell et al, 2011): 1) C starvation (e.g., Galiano et al, 2011, Adams et al, 2013, Mitchell et al, 2013); 2) hydraulic failure of the plant water transport system (e.g., Anderegg & Anderegg, 2013, Mitchell et al, 2013); and 3) phloem transport failure (e.g., Sala, 2009, Adams et al, 2013, Hartmann et al, 2013, McDowell et al, 2013). The relative importance and interactions among these processes remain the source of on-going debates (e.g., McDowell et al, 2008, McDowell & Sevanto, 2010, Sala et al, 2010)

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Discussion

Conclusion