Detecting the fingerprint of drought across Europe’s forests: do carbon isotope ratios and stem growth rates tell similar stories?

Abstract

Background: Understanding how trees respond to drought is critical to forecasting both short and long-term impacts of climate change on forests. The isotopic ratio of 13 C to 12 C stored in wood -referred to as δ

Spatio‐temporal patterns of tree growth as related to carbon isotope fractionation in European forests under changing climate

“…Conversely, the extent of common summer climate signals present in carbon isotopes suggests a tight stomatal control of water losses and, indirectly, photosynthetic activity at peak season across most of Europe (Cullen, Adams, Anderson, & Grierson, ). These results suggest a partial decoupling between leaf‐ and stem‐level processes (Jucker et al, ) as the result of different potentially contributory processes: (a) environmental constraints governing sink activity (i.e. meristematic growth) before affecting source activity (i.e., carbon uptake), particularly during drought or low temperatures (Körner et al ); (b) temporal shifts between foliar dynamics and xylogenesis (Seftigen et al, ); and (c) changes in carbon allocation patterns with increasing temperature, which may vary between different biomes and functional groups (Way & Oren, ).…”

“…These lines of evidence support (direct or indirect) effects of carbon uptake processes on above‐ground growth. However, carbohydrates are used for various processes other than growth (e.g., maintenance, respiration, reproduction), and carbon availability might seldom limit tree growth (Palacio, Hoch, Sala, Körner, & Millard, ; but see Wiley & Helliker, ), which suggests that the relationship between productivity and stable isotopes might not be straightforward (Jucker et al, ). Alternative physiological mechanisms related to above‐ground growth might interact with photosynthetic processes; for example, a critical turgor disrupting cell growth or the appearance of hydraulic constraints under drought (Sperry, ), or the weakening of meristematic growth under low temperatures (Rossi et al, ).…”

Having the right neighbors: how tree species diversity modulates drought impacts on forests

“…Regarding inconsistencies among studies that impact their comparison, the most notable is how drought impacts were characterized and measured: most studies used changes in annual growth rates, but some used more complex functional traits such as transpiration and water‐use efficiency (Table ). A reduction in transpiration, however, or an increase in stomatal closure does not necessarily translate into a reduction in growth (Jucker et al ., ), leading to potentially contrasting conclusions from studies looking at different traits. Other noteworthy differences include the scale (tree‐level vs ecosystem‐level assessments) and approach (experimental vs observational studies).…”

Cumulative growth and stress responses to the 2018–2019 drought in a European floodplain forest

“…This growth response to drought can be quantified using the growth resistance index of Lloret et al (2011), which may be an especially suitable approach when rapid impact assessments are needed and no data is available on the post disturbance period. Next to growth, the carbon isotope ratio of 13 C to 12 C in wood – called δ 13 C – is a widely-used physiological indicator of a treès water status and drought stress (Farquhar, Ehleringer, & Hubick, 1989; Grossiord et al, 2014; Jucker et al, 2017). The δ 13 C composition in tissues of C3 plants like most trees is a record of the ratio between intercellular and ambient CO2 concentration during the time of carbon fixation that is modulated by both, CO2 assimilation and stomatal aperture (Farquhar et al, 1989).…”

“…This growth response to drought can be quantified using the growth resistance index of Lloret et al (2011), which may be an especially suitable approach when rapid impact assessments are needed and no data is available on the post disturbance period. Next to growth, the carbon isotope ratio of 13 C to 12 C in woodcalled δ 13 Cis a widely-used physiological indicator of a tree`s water status and drought stress (Farquhar, Ehleringer, & Hubick, 1989;Grossiord et al, 2014;Jucker et al, 2017). Under ample water supply and fully open stomata, trees discriminate against the heavier 13 C in favor of the lighter 12 C. However, under water shortage, stomatal conductance is more strongly down-regulated than CO2 assimilation, which induces an increase of δ 13 C in the wood formed during drought (Farquhar et al, 1989;Grossiord et al, 2014).…”