Drought responses and their effects on radial stem growth of two co-occurring conifer species in the Mediterranean mountain range

Abstract

Key message Patterns of stem radial variations showed thatCedrus libaniA. Rich. was less limited by summer drought than co-occurringJuniperus excelsaM. Bieb.Cedrus libanirecovered faster from tree water deficit and showed significantly higher radial growth rates and annual stem increments thanJ. excelsa. However, the ability ofJ. excelsato grow more hours per year may indicate a potential benefit in more extreme conditions. Context Knowledge about species-specific drought responses is needed to manage productive forests in drought prone areas. Under water shortage, trees commonly show stem shrinkage, which is assumed to inhibit growth. Aims We investigated whether the two co-existing conifers Juniperus excelsa M. Bieb. and Cedrus libani A. Rich. (growing at the Taurus Mountains, SW-Turkey) show differences in water relations and stem growth in order to evaluate their respective drought tolerance. Methods Stem radius changes were hourly monitored over 2 years using high-resolution point dendrometers. Radial stem growth, tree water deficit-induced stem shrinkage, and maximum daily shrinkage were extracted from stem radius change measurements, investigated for their patterns, and related to environmental conditions. Results Cedrus libani recovered from tree water deficit under higher temperature and vapor pressure deficit than J. excelsa. The number of hours during which stem growth occurred was higher for J. excelsa; however, growth rates and annual increments were significantly lower than in C. libani. Both species showed highest maximum daily shrinkage during the driest months indicating the ability to maintain gas exchange all year round. Conclusion Juniperus excelsa showed a more conservative growth strategy while C. libani was less limited by summer drought and showed more annual stem increment under the conditions investigated.

Why trees grow at night

“…Such data are often coarse and are obtained typically with an annual resolution of the underlying tree ring samples (Schweingruber, 1996 ; Babst et␣al ., 2018 ) or, in less frequent cases, small wood samples obtained at (bi‐)weekly intervals (Zweifel et␣al ., 2006 ; Delpierre et␣al ., 2016 ; Huang et␣al ., 2020 ; Peters et␣al ., 2021 ). Daily or even hourly‐resolved growth responses of mature forest trees have been rarely recorded (Schurr et␣al ., 2006 ), most data covers only short periods and were monitored in single stands (Ziaco & Biondi, 2018 ; Knüsel et␣al ., 2019 ; Güney et␣al ., 2020 ).…”

“…There remain uncertainties with potential hygroscopic effects of the bark of some tree species when the stem surface is moistened by rain (Oberhuber et␣al ., 2020 ). Further, the zero‐growth approach does not consider the still largely unknown bark degradation processes (Gricar et␣al ., 2015 ; Güney et␣al ., 2020 ), especially after frost in the winter period (Zweifel & Häsler, 2000 ; Charrier et␣al ., 2017 ). Uncertainties also remain due to technical issues, for example, the electronic and mechanical temperature behaviour of dendrometers, the way the sensor is anchored in the tree, or the way the raw data is cleaned from outliers and shifts (Haeni et␣al ., 2020 ).…”

“…Meanwhile, the zero‐growth approach has become widely accepted and applied (Dietrich & Kahmen, 2019 ; Schafer et␣al ., 2019 ; Eitel et␣al ., 2020 ; Güney et␣al ., 2020 ; Lamacque et␣al ., 2020 ; Pappas et␣al ., 2020 ; Sellier & Segura, 2020 ). Accordingly, growth is defined as the radial stem increase above a previously reached stem radial maximum (Fig.…”

TreeNet–The Biological Drought and Growth Indicator Network

“…In addition, there is uncertainty about the largely unknown processes of bark degradation or phloem collapse (Gricar et al, 2015;Güney et al, 2020), especially after frost in winter, which induces large changes in stem size (Zweifel and Häsler, 2000;Charrier et al, 2017). Bark degradation may falsely delay the determined growth initiation in spring obtained with the zerogrowth concept, as the missing (degraded) tissue is interpreted as a reversible water deficit of the tree and not as an irreversible structural volume loss.…”

“…Uncertainty also remains with the assumption of the zero-growth concept that no growth takes place in the case of stem shrinkage, which means that the turgor necessary for growth is not achieved (Zweifel et al, 2016) and no cell expansion is possible under such conditions. Even though the underlying concept of Lockhart (1965) is theoretically sound and many recent papers have provided evidence for its usefulness (Dietrich and Kahmen, 2019;Schafer et al, 2019;Aryal et al, 2020;Eitel et al, 2020;Güney et al, 2020;Lamacque et al, 2020;Pappas et al, 2020;Sellier and Segura, 2020;Dukat et al, 2021;Meng et al, 2021;Nehemy et al, 2021;Pierrat et al, 2021;Zweifel et al, 2021), the separation of dendrometer data into GRO and TWD has not yet been tested with an independent method. The main reason for this lack of rigorous testing is that there is no alternative method capable of cross-checking hourly resolved dendrometer data in mature forest trees over a reasonable period of time (i.e., without excessive destructive sampling).…”