Hydraulic stress in plants occurs under conditions of low water availability (soil moisture; θ) and/or high atmospheric-demand for water (vapor pressure deficit; D). Different species are adapted to respond to hydraulic stress by functioning along a continuum where at one end they close stomata to maintain a constant leaf water potential (ΨL) (isohydric species), and at the other they allow ΨL to decline (anisohydric species). Differences in water use along this continuum are most notable during hydrologic stress, often characterized by low θ and high D; however, θ and D are often, but not necessarily coupled at timescales of weeks or longer, and uncertainty remains about the sensitivity of different water use strategies to these variables. We quantified the effects of both θ and D on canopy conductance (Gc) among widely-distributed canopy-dominant species along the iso-anisohydric spectrum growing along a hydroclimatological gradient. Tree-level Gc was estimated using hourly sap flow observations from three sites in the eastern United States: a mesic forest in western North Carolina, and two xeric forests in southern Indiana and Missouri. Each site experienced at least one year of substantial drought conditions. Our results suggest that sensitivity of Gc to θ varies across sites and species, with Gc sensitivity being greater in dry than in wet sites, and greater for isohydric compared to anisohydric species. However, once θ limitations are accounted for, sensitivity of Gc to D remains relatively constant across sites and species. While D limitations to Gc were similar across sites and species, ranging from 16–34% reductions, θ limitations to Gc ranged from 0–40%. The similarity in species sensitivity to D is encouraging from a modeling perspective, though it implies substantial reduction to Gc will be experienced by all species in a future characterized by higher D.

The oak (Quercus) species of eastern North America are declining in abundance, threatening the many socioecological benefits they provide. We discuss the mechanisms responsible for their loss, many of which are rooted in the prevailing view that oaks are drought tolerant. We then synthesize previously published data to comprehensively review the drought response strategies of eastern US oaks, concluding that whether or not eastern oaks are drought tolerant depends firmly on the metric of success. Although the anisohydric strategy of oaks sometimes confers a gas exchange and growth advantage, it exposes oaks to damaging hydraulic failure, such that oaks are just as or more likely to perish during drought than neighboring species. Consequently, drought frequency is not a strong predictor of historic patterns of oak abundance, although long-term climate and fire frequency are strongly correlated with declines in oak dominance. The oaks’ ability to survive drought may become increasingly difficult in a drier future.

The coordination of plant leaf water potential (ΨL) regulation and xylem vulnerability to embolism is fundamental for understanding the tradeoffs between carbon uptake and risk of hydraulic damage. There is a general consensus that trees with vulnerable xylem more conservatively regulate ΨL than plants with resistant xylem. We evaluated if this paradigm applied to three important eastern US temperate tree species, Quercus alba L., Acer saccharum Marsh. and Liriodendron tulipifera L., by synthesizing 1600 ΨL observations, 122 xylem embolism curves and xylem anatomical measurements across 10 forests spanning pronounced hydroclimatological gradients and ages. We found that, unexpectedly, the species with the most vulnerable xylem (Q. alba) regulated ΨL less strictly than the other species. This relationship was found across all sites, such that coordination among traits was largely unaffected by climate and stand age. Quercus species are perceived to be among the most drought tolerant temperate US forest species; however, our results suggest their relatively loose ΨL regulation in response to hydrologic stress occurs with a substantial hydraulic cost that may expose them to novel risks in a more drought‐prone future.

Bark beetles' tree colonization success depends on their ability to overcome host 28 tree defenses, which includes resin exudation, and toxic chemicals, which deter bark beetle 29 colonization. Resin defenses during insect outbreaks are challenging to study in situ, since 30 outbreaks are stochastic events that progress quickly and thus preclude the establishment of 31 baseline observations of non-infested controls. We use synthetic aggregation pheromones to 32 demonstrate that confined Ips bark beetle herbivory can be successfully initiated to provide 33 opportunities for studying interactions between bark beetles and their hosts, including the 34 dynamics of constitutive and induced resin exudation. In Pinus taeda L. plantations between 12 35 and 19 years old in North and South Carolina, U.S., trees were affixed with pheromone lures, 36 monitored for evidence of bark beetle attacks, and resin samples were collected throughout the 37 growing season. Baiting increased beetle herbivory to an extent sufficient to produce an induced 38 resin response. Attacked trees exuded ~ 3 times more resin at some time than control trees. This 39 supports previous work that demonstrates information on constitutive resin dynamics alone 40 provide an incomplete view of a host tree's resistance to bark beetle attack. 41 42 KEY WORDS: Bark beetle outbreak, pheromones, Pinus taeda, plant defense, resin flow 43 44 45 46 47 48