DOI: 10.5061/dryad.1ns1rn918
title: Data from: Interactions between beech and oak seedlings can modify the effects of hotter droughts and the onset of hydraulic failure

description: Mixing species with contrasting resource use strategies could reduce
 forest vulnerability to extreme events. Yet, how species diversity affects
 seedling hydraulic responses to heat and drought, including mortality
 risk, is largely unknown. Using open-top chambers, we assessed how, over
 several years, species interactions (monocultures vs. mixtures) modulate
 heat and drought impacts on the hydraulic traits of juvenile European
 beech and pubescent oak. Using modelling, we estimated species interaction
 effects on timing to drought-induced mortality and the underlying
 mechanisms driving these impacts. We show that mixtures mitigate adverse
 heat and drought impacts for oak (less negative leaf water potential,
 higher stomatal conductance, and delayed stomatal closure) but enhance
 them for beech (lower water potential and stomatal conductance, narrower
 leaf safety margins, faster tree mortality). Potential underlying
 mechanisms include oak’s larger canopy and higher transpiration, allowing
 for quicker exhaustion of soil water in mixtures. Our findings highlight
 that diversity has the potential to alter the effects of extreme events,
 which would ensure that some species persist even if others remain
 sensitive. Among the many processes driving diversity effects, differences
 in canopy size and transpiration associated to the stomatal regulation
 strategy seem the primary mechanisms driving mortality vulnerability in
 mixed seedling plantations.
description: Water-related physiological traits,
 including light‐saturated assimilation (A<sub>max</sub>),
 stomatal conductance (g<sub>s</sub>), water potential at
 predawn (Ψ<sub>predawn</sub>)
 and midday (Ψ<sub>midday</sub>),
 water potential at turgor loss point (Ψ<sub>TLP</sub>),
 minimum stomatal conductance (g<sub>min</sub>), individual
 leaf area (LA<sub>leaf</sub>) and specific leaf area (SLA)
 were measured on 96 seedlings of European beech and pubescent oak grown in
 monoculture and in mixture. These physiological traits were used to
 calculate the stomatal safety margin (Ψ<sub>TLP </sub>-
 P50;
 SM<sub>P50</sub>) using the P50 extracted from a
 database, and the leaf safety margin (Ψ<sub>midday </sub>- Ψ<sub>TLP</sub>; SM<sub>leaf</sub>).
 Measurements were done at the late growing season (i.e.,
 September) during three years (from 2020 to 2022). The trees were under a
 fully crossed combination of two air temperature and soil moisture
 regimes. The gas exchange measurements were carried out between 9 am and 3
 pm when the trees were the most active. Further, the model SurEAU simulated the time to
 stomatal closure (TSC) and to hydraulic failure (THF) using the
 physiological traits collected in 2022.
description: # Interactions between beech and oak seedlings can modify the effects of
 hotter droughts and the onset of hydraulic failure
 [https://doi.org/10.5061/dryad.1ns1rn918](https://doi.org/10.5061/dryad.1ns1rn918) Water-related physiological traits, including light‐saturated assimilation (A<sub>max</sub>), stomatal conductance (g<sub>s</sub>), water potential at predawn (Ψ<sub>predawn</sub>) and midday (Ψ<sub>midday</sub>), water potential at turgor loss point (Ψ<sub>TLP</sub>), minimum stomatal conductance (g<sub>min</sub>), individual leaf area (LA<sub>leaf</sub>) and specific leaf area (SLA) were measured on 96 seedlings of European beech and pubescent oak grown in monoculture and in mixture. These physiological traits were used to calculate the stomatal safety margin (Ψ<sub>TLP </sub>\- P50; SM<sub>P50</sub>) using the P50 extracted from a database, and the leaf safety margin (Ψ<sub>midday </sub>\- Ψ<sub>TLP</sub>; SM<sub>leaf</sub>). Measurements were done at the late growing season (i.e., September) during three years (from 2020 to 2022). The trees were under a fully crossed combination of two air temperature and soil moisture regimes. Further, the model SurEAU simulated the time to stomatal closure (TSC) and to hydraulic failure (THF) using the physiological traits collected in 2022. Physiological data (Amax, gs, Ψ<sub>predawn</sub>, Ψ<sub>midday</sub>, Ψ<sub>TLP</sub>, g<sub>min</sub>, LA<sub>leaf</sub>, SLA, SM<sub>P50,</sub> and SM<sub>leaf</sub>) for some years and trees could be missing, indicated as "n/a" in the file "Physiological\_data\_file" due to technical issues (e.g., leaf damaged, leaf too small etc). ## Description of the data and file structure Metadata in file "Physiological\_data\_file" | Column | Entry | Value | Unit | Explanation | Remark | | ------ | ----- | ----- | ---- | ----------- | ------ | | A | Year | 3 | | Year (2020, 2021, 2022) | | | B | Chamber | 16 | | Chamber number | | | C | Treatment | 4 | | Treatment type (C: control, H: heating, D: Drought, HD: heating + drought) | | | D | Heating | 2 | | If there is a heating treatment (YES or NO) | | | E | Drought | 2 | | If there is a drought treatment (YES or NO) | | | F | Tree | 96 | | Tree number | | | G | Compartment | 8 | | Compartment number (from 1 to 8) | | | H | Species.combination | 2 | | Type of species combination (monoculture; mixture) | | | I | Species | 2 | | Type of species (B: beech, O: oak) | | | J | WPpd | Numerical | MPa | Leaf water potential at Predawn | | | K | WPmid | Numerical | MPa | Leaf water potential at midday | | | L | gmin | Numerical | mmol.m<sup>-2</sup>.s<sup>-1</sup> | Minimum stomatal conductance | | | M | WPtlp | Numerical | MPa | Water potential at turgor loss point | | | N | Amax | Numerical | µmol m⁻² s⁻¹ | Light-saturated maximal carbon assimilation at 400ppm CO2 | | | o | gs | Numerical | mol m⁻² s⁻¹ | Light-saturated stomatal conductance at 400ppm CO2 | | | P | LAleaf | Numerical | cm² | Average leaf area | | | Q | SLA | Numerical | cm².g<sup>-1</sup> | Specific leaf area | | | R | P50 | Numerical | MPa | Water potential at 50% cavitation in the stem (data from Choat et al, 2012) | | | S | SMP50 | Numerical | MPa | Stomatal safety margin (WPtlp - P50) | | | T | SMleaf | Numerical | MPa | Leaf safety margin (WPmid - TLP) | | | U | VPD | Numerical | kPa | Average Vapor Pressure Deficit in each chamber during the field campaign | | | V | SoilWC | Numerical | % | Average Soil Water Content in each chamber during the field campaign | | Metadata in file "Simulation\_data\_file" | Column | Entry | Value | Unit | Explanation | Remark | | ------ | ----- | ----- | ---- | ----------- | ------ | | A | Simulation | 6 | | Simulation number (model was run 6 times) | | | B | Species | 2 | | Type of species (B: beech, O: oak) | | | C | Species combination | 2 | | Type of species combination (monoculture; mixture) | | | D | Treatment | 4 | | Treatment type (C: control, H: heating, D: Drought, HD: heating + drought) | | | E | THF | Numerical | Days | Time to hydraulic failure (PLC at 100%) | THF results from a simulation so the unit is still days even if the value contained decimals. | | F | TSC | Numerical | Days | Time to stomatal closure | TSC results from simulation so the unit is still days even if the value contained decimals. |