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1. Many plant species experience large differences in soil moisture availability within a season, potentially leading to a wide range of leaf water potentials (Psi(LEAF)). In order to decrease the risk of leaf dehydration, among species, there is a continuum ranging from strict control (isohydry) to little control (anisohydry) of minimum Psi(LEAF). 2. In central Texas USA, species are exposed to a range of soil moisture from wet springs to hot, dry summers. There are diverging water management strategies among the four dominant woody species in this system; two of these species are more isohydric (Prosopis glandulosa, Quercus fusiformis) while two others are more anisohydric (Diospyros texana, Juniperus asheii). 3. To maintain leaf turgor and photosynthesis during periods of limited soil moisture, anisohydric species may adjust leaf hydraulic parameters more than isohydric species. To test this hypothesis, we quantified iso/anisohydry from 3 years of Psi(LEAF) predawn and midday measurements, and we measured the changes in turgor loss points (Psi(TLP)), osmotic potential at full hydration (Psi(pi 100)), and resistance to leaf hydraulic dysfunction (leaf P-50) throughout the spring and summer of 2016. 4. Diospyros and Juniperus experienced more negative Psi(LEAF) and adjusted Psi(TLP) and Psi(pi 100) in response to both drying soils during the summer also in response to rainfall events during September. In contrast, the more isohydric species (Quercus and Prosopis) did not appear to adjust Psi(TLP) or Psi(pi 100) in response to soil moisture. The more anisohydric species also adjusted leaf P-50 during periods of reduced soil moisture. 5. Our results suggest that species that experience wider ranges of Psi(LEAF) have a greater ability to alter leaf hydraulic properties. This provides insight on how species with different strategies for water potential regulation may modify properties to mitigate drought effects in the future.

Background The eastern Edwards Plateau supports a mosaic of woodlands, savannas, and shrubland in which native plant and animal species are often still dominant. Some woodlands are dominated by a mix of native woody species, including Ashe juniper (Juniperus ashei ), oak species ( Quercus spp.), and other hardwoods. Other woodlands are nearly pure Ashe juniper; these are particularly susceptible to crown fires. The savannas were once, and still can be, maintained by surface fires. Results We hypothesize that frequent surface fires once kept some of the mixed woodlands more open and more diverse (a “lost community”) and that these fires would have reduced the abundance of Ashe juniper, which does not resprout from the base, and allowed oak regeneration, which is currently failing. The absence of fire, the current failure of oak regeneration, and high white-tailed deer densities together favor the “juniperization” of woodlands, that is, the conversion of mixed woodlands into nearly pure stands of Ashe juniper. Surface fires in savannas can sometimes control woody encroachment and the non-native grass King Ranch bluestem ( Bothriochloa ischaemum ), although the particular fire characteristics required are not yet clear. The current lack of fire in savannas favors their conversion to woodlands. Since under present conditions Ashe juniper is the primary encroacher, without fire or mechanical clearing these savannas are also on trajectories towards nearly pure stands of Ashe juniper. Conclusions Prescribed fire, sometimes paired with mechanical thinning, offers land managers in this region a tool for achieving many goals, including increasing native biodiversity and reducing wildfire danger. However, more study of the effects of fires of different intensities and frequencies in these woodlands, savannas, and shrublands is needed to better inform the use of prescribed fire in this region.

An important component of the hydrological niche involves the partitioning of water sources, but in landscapes characterized by shallow soils over fractured bedrock, root growth is highly constrained. We conducted a study to determine how physical constraints in the root zone affected the water use of three tree species that commonly coexist on the Edwards Plateau of central Texas; cedar elm (Ulmus crassifolia), live oak (Quercus fusiformis), and Ashe juniper (Juniperus ashei). The year of the study was unusually dry; minimum predawn water potentials measured in August were -8 MPa in juniper, less than -8 MPa in elm, and -5 MPa in oak. All year long, species used nearly identical water sources, based on stable isotope analysis of stem water. Sap flow velocities began to decline simultaneously in May, but the rate of decline was fastest for oak and slowest for juniper. Thus, species partitioned water by time when they could not partition water by source. Juniper lost 15—30 % of its stem hydraulic conductivity, while percent loss for oak was 70—75 %, and 90 % for elm. There was no tree mortality in the year of the study, but 2 years later, after an even more severe drought in 2011, we recorded 34, 14, 6, and 1 % mortality among oak, elm, juniper, and Texas persimmon (Diospyros texana), respectively. Among the study species, mortality rates ranked in the same order as the rate of sap flow decline in 2009. Among the angiosperms, mortality rates correlated with wood density, lending further support to the hypothesis that species with more cavitation-resistant xylem are more susceptible to catastrophic hydraulic failure under acute drought.

• To assess hydraulic architecture and limitations to water transport across whole trees, we compared xylem anatomy, vulnerability to cavitation (Ψ50) and specific hydraulic conductivity (Ks) of stems, shallow roots and deep roots (from caves to 20 m depth) for four species: Juniperus ashei, Bumelia lanuginosa, Quercus fusiformis and Quercus sinuata. • Mean, maximum and hydraulically weighted (Dh) conduit diameters and Kswere largest in deep roots, intermediate in shallow roots, and smallest in stems (P < 0.05 for each). Mean vessel diameters of deep roots were 2.1-4.2-fold greater than in stems, and Kswas seven to 38 times larger in the deep roots. • Ψ50also increased from stems to roots with depth, as much as 24-fold from stems to deep roots in B. lanuginosa. For all species together, Ψ50was positively correlated with both Dhand Ks, suggesting a potential trade-off exists between conducting efficiency and safety. • The anatomical and hydraulic differences documented here suggest that the structure of deep roots minimizes flow resistance and maximizes deep water uptake.

Background The southeastern United States consists of diverse ecosystems, many of which are fire-dependent. Fires were common during pre-European times, and many were anthropogenic in origin. Understanding how prescribed burning practices in use today compare to historic fire regimes can provide perspective and context on the role of fire in critical ecosystems. On the Aransas National Wildlife Refuge (ANWR), prescribed burning is conducted to prevent live oak ( Quercus fusiformis ) encroachment and preserve the openness of the herbaceous wetlands and grasslands for endangered whooping cranes ( Grus americana ) and Aplomado falcons ( Falco femoralis ). This field note builds a digital fire atlas of recent prescribed burning on the refuge and compares it to the historical fire ecology of ANWR. Results Findings indicate that the refuge is maintaining fire-dependent ecosystems with an extensive burn program that includes a fire return interval between 2 and 10 years on a majority of the refuge, with some locations experiencing much longer intervals. These fire return intervals are much shorter than the historic burn regime according to LANDFIRE. Conclusions Following the fire return intervals projected by LANDFIRE, which project longer intervals than the prescribed fire program, would likely be detrimental to endangered species management by allowing increased woody plant encroachment and loss of open habitat important to whooping cranes and Aplomado falcons. Since prescribed fire is part of the management objectives on many national wildlife refuges in the United States, quantifying recent and historical fire ecology can provide useful insights into future management efforts, particularly in cases where endangered species are of special concern and management efforts may be counter to historical disturbance regimes.

The ecohydrology of karst has not received much attention, despite the disproportionally large contribution of karst aquifers to freshwater supplies. Karst savannas, like many savannas elsewhere, are encroached by woody plants, with possibly negative consequences on aquifer recharge. However, the role of savanna tree species in hydrological processes remains unclear, not least because the location and water absorption zones of tree roots in the spatially complex subsurface strata are unknown. This study examined the water sources and water relations of two savanna trees, Quercus fusiformis (Small) and Juniperus ashei (Buchholz) in the karst region of the eastern Edwards Plateau, Texas (USA). Stable isotope analysis of stem water revealed that both species took up evaporatively enriched water during the warm season, suggesting a relatively shallow water source in the epikarst, the transition zone between soil and bedrock. Q. fusiformis had consistently higher predawn water potentials than J. ashei during drought, and thus was probably deeper-rooted and less capable of maintaining gas exchange at low water potentials. Although the water potential of both species recovered after drought-breaking spring and summer rain events, associated shifts in stem water isotope ratios did not indicate significant uptake of rainwater from the shallow soil. A hypothesis is developed to explain this phenomenon invoking a piston-flow mechanism that pushes water stored in macropores into the active root zones of the trees. Epikarst structure varied greatly with parent material and topography, and had strong effects on seasonal fluctuations in plant water status. The study suggests that tree species of the Edwards Plateau do not commonly reduce aquifer recharge by tapping directly into perched water tables, but more likely by reducing water storage in the epikarst. A more general conclusion is that models of savanna water relations based on Walter's two-layer model may not apply unequivocally to karst savannas.

Background and AimsDespite the importance of vessels in angiosperm roots for plant water transport, there is little research on the microanatomy of woody plant roots. Vessels in roots can be interconnected networks or nearly solitary, with few vessel–vessel connections. Species with few connections are common in arid habitats, presumably to isolate embolisms. In this study, measurements were made of root vessel pit sizes, vessel air-seeding pressures, pit membrane thicknesses and the degree of vessel interconnectedness in deep (approx. 20 m) and shallow (<10 cm) roots of two co-occurring species, Sideroxylon lanuginosum and Quercus fusiformis.MethodsScanning electron microscopy was used to image pit dimensions and to measure the distance between connected vessels. The number of connected vessels in larger samples was determined by using high-resolution computed tomography and three-dimensional (3-D) image analysis. Individual vessel air-seeding pressures were measured using a microcapillary method. The thickness of pit membranes was measured using transmission electron microscopy.Key ResultsVessel pit size varied across both species and rooting depths. Deep Q. fusiformis roots had the largest pits overall (>500 µm) and more large pits than either shallow Q. fusiformis roots or S. lanuginosum roots. Vessel air-seeding pressures were approximately four times greater in Q. fusiformis than in S. lanuginosum and 1·3–1·9 times greater in shallow roots than in deep roots. Sideroxylon lanuginosum had 34–44 % of its vessels interconnected, whereas Q. fusiformis only had 1–6 % of its vessels connected. Vessel air-seeding pressures were unrelated to pit membrane thickness but showed a positive relationship with vessel interconnectedness.ConclusionsThese data support the hypothesis that species with more vessel–vessel integration are often less resistant to embolism than species with isolated vessels. This study also highlights the usefulness of tomography for vessel network analysis and the important role of 3-D xylem organization in plant hydraulic function.

Herein we report results of transplant experiments that link variation in host plant quality to herbivore fitness at the local scale (among adjacent plants) with the process of local (demic) adaptation at the landscape scale to explain the observed distribution of the specialist gall former Belonocnema treatae (Hymenoptera: Cynipidae) within populations of its host plant, Quercus fusiformis. Field surveys show that leaf gall densities vary by orders of magnitude among adjacent trees and that high-gall-density trees are both rare (<5%) and patchily distributed. B. treatae from each of five high-gall-density trees were reared on (1) the four nearest low-gall-density trees, (2) the four alternative high-gall-density trees, and (3) their natal trees (control). Each treatment (source × rearing site) was replicated three times. Nine components of performance that sequentially contribute to fitness were evaluated with over 21 000 galls censused across the 25 experimental trees. When reared on their natal trees and compared with low-gall-density neighbors, transplanted gall formers had higher gall initiation success (P < 0.05), produced more (P < 0.001) and larger galls (P < 0.001), and produced a higher proportion of galls that exceeded the threshold size for natural enemy avoidance (P < 0.05). Comparison of gall-former performance on natal vs. alternative high-gall-density trees demonstrated significant (P < 0.001) differences in six performance measures with five differing in the direction predicted by the hypothesis of local adaptation. Overall, these linked experiments document direct and indirect effects of host plant variation on gall-former performance and demonstrate convincingly that (1) high-gall-density trees equate to high-quality trees that are surrounded by trees of relatively lower quality to the herbivore and (2) gall-former populations have become locally adapted to individual trees.

The ability of songbirds to survive and reproduce depends on many factors, one of which is the ability to acquire enough food. We quantified foraging behavior, nestinghabitat vegetation composition, and available arthropod prey of the Vireo atricapilla (Black-capped Vireo) in Texas during 2010 and 2011. We used observational surveys of foraging behavior and vegetation time-use to quantify the Black-capped Vireos' foraging behavior and vegetative use versus availability (i.e., mean proportion of use vs. vegetative species availability). We collected descriptive data on the Black-capped Vireos' foraging use of available vegetative species and compared among vegetative species, year, and within-season sampling periods. In 2010 and 2011, we identified and mapped 49 and 63 breeding territories and repeatedly surveyed 30 and 58 territories for foraging activity, respectively. Data analysis focused on the foraging use of the 3 most commonly used and available tree species: Juniperus ashei (Ashe Juniper), Quercus sinuata (Shin Oak), and Q. fusiformis (Live Oak). Ashe Juniper, Shin Oak, and Live Oak together made up 78.8% and 83.6% of total proportion of substrate for foraging efforts in 2010 and 2011, respectively. Ashe Juniper had the highest proportion (~28–50%) of foraging effort in 2010, 2011, and all but 1 sampling period for both years. We also repeatedly collected branch clippings from within a random subset of surveyed Black-capped Vireo territories to identify potentially available arthropod foods. We evaluated by order richness, total abundance, and dry biomass to make comparisons among vegetative species, year, and within-season sampling periods. We found significant differences in the biomass of arthropod orders Acari and Thysanoptera in 2010 and between orders Acari and Hymenoptera in 2011 among the 3 focal vegetative species. Examination of additional descriptive data suggests seasonal changes in potentially available arthropod foods. Our research underscores the importance of vegetation composition to Black-capped Vireos that may help habitat managers select for potential vegetative species distributions to optimize food resources throughout the breeding season for this species.

Host-specific phytophagous insects that are short lived and reliant on ephemeral plant tissues provide an excellent system in which to investigate the consequences of disruption in the timing of resource availability on consumer populations and their subsequent interactions with higher tropic levels. The specialist herbivore, Belonocnema treatae (Hymenoptera: Cynipidae) induces galls on only newly flushed leaves of live oak, Quercus fusiformis. In central Texas (USA) episodic defoliation of the host creates variation in the timing of resource availability and results in heterogeneous populations of B. treatae that initiate development at different times. We manipulated the timing of leaf flush in live oak via artificial defoliation to test the hypothesis that a 6- to 8-week delay in the availability of resources alters the timing of this gall former's life cycle events, performance and survivorship on its host, and susceptibility to natural enemies. B. treatae exhibits plasticity in development time, as the interval from egg to emergence was significantly reduced when gallers oviposited into the delayed leaf flush. As a consequence, the phenologies of gall maturation and adult emergence remain synchronized in spite of variation in the timing of resource availability. Per capita gall production and gall-former performance are not significantly affected by the timing of resource availability. The timing of resource availability and natural enemies interact, however, to produce strong effects on survivorship: when exposed to natural enemies, B. treatae developing in galls initiated by delayed oviposition exhibited an order-of-magnitude increase in survivorship. Developmental plasticity allows this gall former to circumvent disruptions in resource availability, maintain synchrony of life cycle events, and results in reduced vulnerability to natural enemies following defoliation of the host plant.