Plant species are characterized along a spectrum of isohydry to anisohydry depending on their regulation of water potential (Ψ), but recent evidence suggests that these hydraulic strategies may largely depend on environmental conditions. Here, we evaluate the role of environmental drivers in the hydraulic behavior of Larrea tridentata, an evergreen desert shrub that withstands severe seasonal drought. Twelve individual shrubs were instrumented with automated stem psychrometers for 1.5 yrs, yielding a time-series of 2324 measurements of daily predawn and midday Ψ. A hierarchical Bayesian model of midday versus predawn Ψ was used to explore how antecedent vapor pressure deficit, soil water content, and temperature influenced hydraulic behavior. Hydraulic behavior, or the degree of isohydry, was defined as the slope (s) of the relationship between midday and predawn Ψ, where 0 < s < 1, s @ 1, and s > 1 denote partial isohydry, anisohydry, and extreme anisohydry, respectively.
Results/Conclusions
Hydraulic behavior in Larrea was highly dynamic and varied seasonally, ranging from partial isohydry to extreme anisohydry. Larrea exhibited extreme anisohydry (s ≈ 1.11) under wet soil conditions of the spring and monsoon periods, when leaf starch concentrations and photosynthetic rates were high. In contrast, partial isohydry (s ≈ 0.877) was exhibited after prolonged dry or cold conditions, which are periods of low productivity. High vapor pressure deficit and wet soils significantly increased σ (more anisohydric). Declines in σ (more isohydric) were associated with negative interaction effect of vapor pressure deficit and temperature, indicative of either hot and dry or cold and wet conditions. At relatively fast timescales, hydraulic behavior can shift to prioritize either carbon gain or hydraulic safety, depending on environmental conditions. However, the mechanisms underpinning these shifts remain unknown and can be further explored with plant hydraulics models. While Larrea is predominantly anisohydric (s ≥ 1 on 81% of the 637 study days), flexibility in Ψ regulation may be key to drought tolerance in seasonally dry climates.