The role of deep vadose zone water in tree transpiration during drought periods in karst settings – Insights from isotopic tracing and leaf water potential

Abstract

Karst environments are unusual because their dry, stony and shallow soils seem to be unfavorable to vegetation, and yet they are often covered with forests. How can trees survive in these environments ? Where do they find the water that allows them to survive? This study uses midday and predawn water potentials and xylem water isotopes of branches to assess tree water status and the origin of transpired water. Monitoring was conducted during the summers of 2014 and 2015 in two dissimilar plots of Mediterranean forest located in karst environments. The results show that the three monitored tree species (Abies alba Mill, Fagus sylvatica L, and Quercus ilex L.) use deep water resources present in the karst vadose zone (unsaturated zone) more intensively during drier years. Quercus ilex, a species welladapted to water stress, which grows at the drier site, uses the deep water resource very early in the summer season. Conversely, the two other species exploit the deep water resource only during severe drought. These results open up new perspectives to a better understanding of ecohydrological equilibrium and to improved water balance modeling in karst forest settings. 1-Introduction: Changes in temperature and rainfall patterns across many forest ecosystems are expected to increase the risk of drought-induced tree mortality (Allen et al. 2010). Tree resistance to drought is therefore of concern to scientists (Chaves et al. 2002; Breda et al. 2006) and stakeholders (Spies et al. 2010; Keenan 2015). To cope with drought, trees have developed various strategies at different time scales. Stomata closure is a universal process designed to limit transpiration (Limousin et al. 2010), avoid water potential drop, and prevent irreversible damage caused by embolism to the plant hydraulic system (Martin-StPaul et al. 2017). Other mechanisms that take place at longer time scales involve changes in leaf area to decrease water consumption (Limousin et al. 2009, van Hees 1997, Martin-StPaul et al. 2013). Instead of regulating water loss, trees can also take up water from groundwater when the water table is accessible (e.g.

Tree water deficit and dynamic source water partitioning

“…While the steady‐state assumption is common in many ecohydrological investigations (Penna et al, 2018), the patterns we observed in the xylem along with soil water δ 2 H and δ 18 O indicate changes in source water partitioning. Plasticity in root water uptake at seasonal scales have also been observed previously (Antunes et al, 2018; S. D. Carrière, Martin‐StPaul, et al, 2020; del Castillo et al, 2016; Ellsworth & Sternberg, 2015; P. Martín‐Gómez, Aguilera, et al, 2017). However, the mechanisms that explain changes in depth of uptake (and hence presumably water age in the transport volume) relative to water availability are difficult to quantify at finer temporal scales.…”

“…Plasticity in root water uptake at seasonal scales have also been observed previously (Antunes et al, 2018;S. D. Carrière, Martin-StPaul, et al, 2020;del Castillo et al, 2016;Ellsworth & Sternberg, 2015;P.…”

Water uptake depth is coordinated with leaf water potential, water‐use efficiency and drought vulnerability in karst vegetation

“…We found evidence of distinct ecohydrological niche segregation among coexisting species, which has also been documented previously in karst (Querejeta et al ., 2007; Nie et al ., 2012; Kukowski et al ., 2013; Guo et al ., 2015; Nardini et al ., 2016; Ding et al ., 2018; Carrière et al ., 2019) and nonkarst ecosystems (Ehleringer et al ., 1991; West et al ., 2007; Silvertown et al ., 2015; Beyer et al ., 2016). We inferred the water uptake depths of the species from the isotopic composition of their xylem water, based on well established knowledge of the steep vertical gradients in evaporative isotopic enrichment of soil water that develop in drying soils during prolonged rainless periods (with strong isotopic enrichment near the surface that exponentially decreases with depth; Allison et al ., 1983).…”

Vertical decoupling of soil nutrients and water under climate warming reduces plant cumulative nutrient uptake, water‐use efficiency and productivity

“…The depths at which plants extract soil water are dependent on soil moisture conditions and on plants having active roots in the different soil/bedrock layers (Jackson et al, 2000;Ryel et al, 2002;Ding et al, 2021). In dryland ecosystems, plants with a dimorphic root system are able to use water from both topsoil and subsoil/bedrock layers that have been recharged during previous seasons (Ehleringer et al, 1991;Pierret et al, 2016;Rempe & Dietrich, 2018;Carri ere et al, 2020;Dawson et al, 2020;Germon et al, 2020;Schwinning, 2020;Nardini et al, 2021). Ryel et al (2010) postulated that dryland plants have evolved to rapidly utilize the soil moisture available in shallow depths to maximize nutrient capture.…”