PS 43-171 - Temperature dependence of leaf minimum conductance in 24 Quercus species

Abstract: A rise in global temperatures and vapor pressure deficits (VPD) is increasing plant water demand, and is seen as a major driver of large scale plant mortality. Plants typically close their stomata when moisture is limited. Despite this mechanism for preserving internal water supplies, plants still experience transient water loss to the atmosphere, across leaf surfaces even when stomata are closed. Recently, the implications of controlling transient water loss, known as the leaf minimum conductance (g_min; mmol m^-2 s^-1), has been recognized as a key trait that determines how long a plant can survive during drought. g_min is typically measured under laboratory conditions using a benchtop dehydration technique. Though this method allows for interspecific comparisons under standard conditions, it captures a small snapshot of potential g_min behavior . Indeed, little is known about the mechanisms controlling g_min under unfavorable environments and particularly during elevated temperatures and high VPD conditions typical of droughts. For example, vegetation models assuming a positive temperature dependence of g_min resulted in ~ 60% greater plant mortality globally than models assuming fixed g_min under projected climate scenarios. Yet, whether g_min exhibits a temperature response remains unclear, and the degree of variability across species is not well understood. Conflicting evidence regarding g_min temperature dependence from empirical studies provide no clear patterns, primarily due to the few species studied, and lack of consistent methodology. The goal of this study is to investigate the temperature response of g_min­ in 24 species of oaks (genus Quercus) from two common gardens in California (CA) and Massachusetts (MA). We hypothesized that species would show a positive temperature dependence of g_min and that species growing in MA would show a stronger response of g_min to elevated temperatures than those growing in CA. However, we found that all species exhibit a non-linear negative temperature dependence of g_min with no differences observed between species from CA or MA or between deciduous and evergreen species. At 25°C, g_min varied 6.8-fold and ranged from 1.19 to 8.09 mmol m^-2 s^-1 across species while all species converged to a g_min of 0.57 ± 0.06 mmol m^-2 s^-1 SE at 45°C. Our results suggest that g_min in oak species is temperature dependent and this trait is highly conserved across the genus. We then explore the underlying anatomical and physiological mechanisms for reductions in g_min at high temperature.