Land surface models (LSMs) typically use empirical functions to represent vegetation responses to soil drought. These functions largely neglect recent advances in plant ecophysiology that link xylem hydraulic functioning with stomatal responses to climate.We developed an analytical stomatal optimization model based on xylem hydraulics (SOX) to predict plant responses to drought. Coupling SOX to the Joint UK Land Environment Simulator (JULES) LSM, we conducted a global evaluation of SOX against leaf-and ecosystemlevel observations. SOX simulates leaf stomatal conductance responses to climate for woody plants more accurately and parsimoniously than the existing JULES stomatal conductance model. An ecosystem-level evaluation at 70 eddy flux sites shows that SOX decreases the sensitivity of gross primary productivity (GPP) to soil moisture, which improves the model agreement with observations and increases the predicted annual GPP by 30% in relation to JULES. SOX decreases JULES root-mean-square error in GPP by up to 45% in evergreen tropical forests, and can simulate realistic patterns of canopy water potential and soil water dynamics at the studied sites.SOX provides a parsimonious way to incorporate recent advances in plant hydraulics and optimality theory into LSMs, and an alternative to empirical stress factors.

The effect of tree diversity on forest productivity and resilience has been the subject of numerous research programs in the past decade. Large research projects like the BEF-China experiment, and networks like TreeDivNet and EuMIXFOR are evidence for the large investments into deciphering diversity-productivity relationships (DPR) in mixed forests around the globe (Zhang et al., 2012). For example, EuMIXFOR established a network of hundreds of research plots with a triplet design of a mixed Fagus sylvatica -Pinus sylvestris stands compared to pure stands of the two species (Ruiz-Peinado et al., 2018). A global meta-analysis showed that forest productivity increases with species richness and trait variation (Zhang et al., 2012). Mixed forests are, on average, 24% more productive than monoculture forests, with large variability among studies. Indeed, cases where mixtures are less productive than monocultures also exist (Forrester, 2014). In the BEF-China tree diversity experiment in a subtropical forest, tree growth increased with neighborhood species richness, leading to a positive DPR at the community scale (Fichtner et al., 2018). In a tropical

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Summary Lycophytes are the earliest diverging extant lineage of vascular plants, sister to all other vascular plants. Given that most species are adapted to ever‐wet environments, it has been hypothesized that lycophytes, and by extension the common ancestor of all vascular plants, have few adaptations to drought. We investigated the responses to drought of key fitness‐related traits such as stomatal regulation, shoot hydraulic conductance (Kshoot) and stem xylem embolism resistance in Selaginella haematodes and S. pulcherrima, both native to tropical understory. During drought stomata in both species were found to close before declines in Kshoot, with a 50% loss of Kshoot occurring at −1.7 and −2.5 MPa in S. haematodes and S. pulcherrima, respectively. Direct observational methods revealed that the xylem of both species was resistant to embolism formation, with 50% of embolized xylem area occurring at −3.0 and −4.6 MPa in S. haematodes and S. pulcherrima, respectively. X‐ray microcomputed tomography images of stems revealed that the decline in Kshoot occurred with the formation of an air‐filled lacuna, disconnecting the central vascular cylinder from the cortex. We propose that embolism‐resistant xylem and large capacitance, provided by collapsing inner cortical cells, is essential for Selaginella survival during water deficit.

Summary Xylem embolism impairs hydraulic conductivity in trees and drives drought‐induced mortality. While embolism has been monitored in vivo in potted plants, and research has revealed evidence of embolism in field‐grown trees, continuous in situ monitoring of cavitation in forests is lacking. Seasonal patterns of embolism were monitored in branchlets of Aleppo pine (Pinus halepensis) trees growing in a dry Mediterranean forest. Optical visualization (OV) sensors were installed on terminal branches, in addition to monthly sampling for micro computed tomography scans. We detected 208 cavitation events among four trees, which represented an embolism increase from zero to c. 12% along the dry season. Virtually all the cavitation events occurred during daytime hours, with 77% occurring between 10:00 and 17:00 h. The probability for cavitation in a given hour increased as vapor pressure deficit (VPD) increased, up to a probability of 42% for cavitation when VPD > 5 kPa. The findings uniquely reveal the instantaneous environmental conditions that lead to cavitation. The increased likelihood of cavitation events under high VPD in water‐stressed pines is the first empirical support for this long hypothesized relationship. Our observations suggest that low levels of embolism are common in Aleppo pine trees at the dry edge of their distribution.