From pattern to process: linking intrinsic water‐use efficiency to drought‐induced forest decline

Abstract

The rise in atmospheric CO 2 concentrations (Ca) has been related to tree growth enhancement and increasing intrinsic water-use efficiency (iWUE). However, the extent that rising Ca has led to increased long-term iWUE and whether climate could explain deviations from expected Ca-induced growth enhancement are still poorly understood. The aim of this research was to use Ca and local climatic variability to explain changes during the 20th century in growth and tree ring and needle d 13 C in declining and nondeclining Abies alba stands from the Spanish Pyrenees, near the southern distribution limit of this species. The temporal trends of iWUE were calculated under three theoretical scenarios for the regulation of plant-gas exchange at increasing Ca. We tested different linear mixed-effects models by multimodel selection criteria to predict basal area increment (BAI), a proxy of tree radial growth, using these scenarios and local temperature together with precipitation data as predictors. The theoretical scenario assuming the strongest response to Ca explained 66-81% of the iWUE variance and 28-56% of the observed BAI variance, whereas local climatic variables together explained less than 11-21% of the BAI variance. Our results are consistent with a droughtinduced limitation of the tree growth response to rising CO 2 and a decreasing rate of iWUE improvement from the 1980s onward in declining A. alba stands subjected to lower water availability.

The Multiple Factors Explaining Decline in Mountain Forests: Historical Logging and Warming-Related Drought Stress is Causing Silver-Fir Dieback in the Aragón Pyrenees

“…Macias et al (2006) also suggested that silver fir is experiencing a greater late-summer drought stress in the Spanish Pyrenees and that the effects of water shortage on growth were more intense in low-elevation stands subjected to higher water deficit than elsewhere. The detected increase in drought stress was not only due to a decrease in precipitation, since similar dry periods occurred in the 1940s, and suggest a link with the 1980s warming when the water-use efficiency of declining trees also increased sharply (Linares and Camarero 2012). The most pronounced warming in the western than in the eastern Aragón Pyrenees indicates that warming-induced drought stress triggered forest dieback westwards (Fig.…”

“…The 1980s releases found only in the declining sites were caused by the felling of dying trees, but many of the surviving trees did not improve their growth in the long term and, in many cases, died. These facts suggest that their performance was permanently affected by drought stress leading to an irreversible reduction in growth and suggesting a loss in stomatal regulation of declining trees despite the recent rise in atmospheric CO 2 concentrations (Linares and Camarero 2012). Additional research is required to establish the links between soil-water availability, growth trends and dieback if these processes are to be used as monitors of the effects of climate warming on mountain forests.…”

Process models and model-data fusion in dendroecology

“…Even when an increased WUE is observed, it is not necessarily related to a tree-growth enhancement (Andreu-Hayles et al, 2011;Peñuelas et al, 2011). Nevertheless some studies have acknowledged that WUE enables a better resistance to drought (Linares and Camarero, 2012;Girardin et al, 2012) and thus a strong fertilization effect (Keenan et al, 2013).…”

Regulation of Water Use in the Southernmost European Fir (Abies pinsapo Boiss.): Drought Avoidance Matters

“…Drought-induced tree decline has been reported for several ecosystems and is usually focused on isohydric tree species, such as relict Mediterranean mountain conifers [4]. This tree die-off is mainly characterized by declining growth patterns, increases in defoliation and extensive death [1][2][3].…”

“…Considerable literature on recent forest decline draws attention to a direct link between climate change and the water relations of trees. Extreme climatic events, such as drought, are among the main drivers of forest dynamics and are usually related to forest decline of drought-sensitive tree species [1][2][3][4]. Understanding contrasting drought sensitivity among tree species and how these differences will affect their water and carbon balances is therefore an important topic to forecast likely responses to global warming and increasing drought stress in forests [5].…”