Despite growing in wet lowland and riparian settings, Taxodium distichum (L.) Rich. (bald cypress) has a strong response to hydroclimate variability, and tree ring chronologies derived from bald cypress have been used extensively to reconstruct drought, precipitation and streamflow. Previous studies have also demonstrated that false rings in bald cypress appear to be the result of variations in water availability during the growing season. In this study 28 trees from two sites located adjacent to the Choctawhatchee River in Northwestern Florida, USA were used to develop a false ring record extending from 1881 to 2014. Twenty false ring events were recorded during the available instrumental era (1931–2014). This record was compared with daily and monthly streamflow data from a nearby gage. All 20 of the false-ring events recorded during the instrumental period occurred during years in which greatly increased streamflow occurred late in the growing season. Many of these wet events appear to be the result of rainfall resulting from landfalling tropical cyclones. We also found that the intra-annual position of false rings within growth rings reflects streamflow variability and combining the false-ring record with tree ring width chronologies improves the estimation of overall summer streamflow by 14%. Future work using these and other quantitative approaches for the identification and measurement of false ring variables in tree rings may improve tree-ring reconstructions of streamflow and potentially the record of tropical cyclone rainfall events.

The tight coupling between drought and insect attack threatens trees in a warmer world. Trees with differing levels of hydraulic safety and efficiency likely have different susceptibilities to insect attack, but how the hydraulic architecture of woody tissues moderates the relationship between drought and insects remains unknown at the species scale. Here, we test how hydraulic safety and efficiency, derived from anatomical and isotopic measurements of the stem xylem, coordinate to regulate growth and survival of whitebark pines in the Rocky Mountains, USA. We assess intraspecific and ontogenetic variability in xylem hydraulic traits by comparing trees that survived or were killed during a mountain pine beetle outbreak. At the individual tree scale, we found synergies between safety and efficiency: at the time of the outbreak, trees with higher cell-wall reinforcement and wood density (safety) also had higher average theoretical hydraulic conductivity and carbon-use efficiency (efficiency), and this was associated with higher growth, carbon storage and survival. At the annual timescale, however, we found tradeoffs between safety and efficiency: higher safety was associated with lower efficiency for seven of nine pairwise comparisons. Yet, years that optimized both safety and efficiency showed the most tree growth and carbon storage. The xylem hydraulic traits underlying tree growth and survival were not stationary over time, but prominent differences at the time of the outbreak suggest that both higher hydraulic safety and efficiency were positively associated with resistance against bark beetles. The importance of hydraulic traits to tree survival suggests that tree responses to increasing herbivory will depend on tree responses to climate warming and hydrological changes.

Streamflow variability is a critical component of water availability across the South Atlantic-Gulf (SAG) water resource region of the United States, yet long-term coherence among basins remains poorly understood. We developed independent May–July streamflow reconstructions for the Roanoke River (South Atlantic), Pascagoula River (southern Mississippi Basin), and St. Johns River (northern and central Florida) spanning 1100–2015 CE. Each reconstruction is highly skillful (RE = 0.39–0.62; CE = 0.39–0.62) and explains 51–63% of observed variance. Across the 916-year record, only five droughts affected all three basins simultaneously, yet three occurred since 2000 (2006, 2007, 2011). These 21st-century droughts were broader and more spatially coherent than comparable events in 1491 and 1587. Basin-to-basin comparisons reveal shared low-flow years were most frequent between the Pascagoula and St. Johns Rivers (15 events), followed by Roanoke–Pascagoula (12) and Roanoke–St. Johns (9). When the St. Johns River experienced low [high] flows, the Pascagoula River had a 40% [48%] likelihood of concurrent extremes—the highest regional coherence observed. Low-flow events for individual basins lasted 2–3 years on average, with the longest drought persisting 26 years on the St. Johns (1459 CE). For both the Pascagoula and St. Johns River, return intervals based on observational streamflow records underestimated the recurrence frequency of extreme events like the early-2000s low flow event by 100–500 years. In contrast, observationally based return intervals on the Roanoke overestimate the length of time between the driest events, indicating severe droughts are more likely to occur than previously thought. These findings illustrate the risk of multi-basin droughts in the SAG region, particularly for the closely linked Pascagoula and St. Johns basins. Future drought planning and water-resource management must account not only for drought in individual basins, but the complex effects of synchronous hydrologic drought across the region.