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Extensive high-severity fires are creating large shrubfields in many dry conifer forests of the interior western USA, raising concerns about forest-to-shrub conversion. This study evaluates the role of disturbance in shrubfield formation, maintenance and succession in the Jemez Mountains, New Mexico. We compared the environmental conditions of extant Gambel oak (Quercus gambelii) shrubfields with adjoining dry conifer forests and used dendroecological methods to determine the multi-century fire history and successional dynamics of five of the largest shrubfields (76–340 ha). Across the study area, 349 shrubfields (5–368 ha) occur in similar topographic and climate settings as dry conifer forests. This suggests disturbance, rather than other biophysical factors, may explain their origins and persistence. Gambel oak ages and tree-ring fire scars in our sampled shrub-fields indicate they historically (1664–1899) burned concurrently with adjoining conifer forests and have persisted for over 115 years in the absence of fire. Aerial imagery from 1935 confirmed almost no change in sampled shrubfield patch sizes or boundaries over the twentieth century. The largest shrubfield we identified is less than 4 % the size of the largest conifer-depleted and substantially shrub-dominated area recently formed in the Jemez following extensive high-severity wildfires, indicating considerable departure from historical patterns and processes. Projected hotter droughts and increasingly large high-severity fires could trigger more forest-to-shrub transitions and maintain existing shrubfields, inhibiting conifer forest recovery. Restoration of surface fire regimes and associated historical forest structures likely could reduce the rate and patch size of dry conifer forests being converted to shrubfields.

The rare pit hypothesis predicts that the extensive inter-vessel pitting in large early-wood vessels of ring-porous trees should render many of these vessels extremely vulnerable to cavitation by air-seeding. This prediction was tested in Quercus gambelii. Cavitation was assessed from native hydraulic conductivity at field sap tension and in dehydrated branches. Single-vessel air injections gave air-seeding pressures through vessel files; these data were used to estimate air-seeding pressures for inter-vessel walls and pits. Extensive cavitation occurred at xylem sap tensions below 1 MPa. Refilling occurred below 0.5 MPa and was inhibited by phloem girdling. Remaining vessels cavitated over a wide range to above 4 MPa. Similarly, 40% of injected vessel files air-seeded below 1.0 MPa, whereas the remainder seeded over a wide range exceeding 5 MPa. Inter-vessel walls averaged 1.02 MPa air-seeding pressure, similar and opposite to the mean cavitation tension of 1.22 MPa. Consistent with the rare pit hypothesis, only 7% of inter-vessel pits were estimated to air-seed by 1.22 MPa. The results confirm the rare pit prediction that a significant fraction of large vessels in Q. gambelii experience high probability of failure by air-seeding.

Key messageEmbryonic axes of acorns have greater variability of mass than cotyledons or pericarp. Above/below ground seedling mass depends on oak species.Species of the genus oak are of great importance for forest ecosystems in almost all of the Holarctic. Knowledge of the diversity of the size of acorns and their parts would allow better use of the opportunity of genetic resources storage, both for the protection of endangered oak species and for forestry practices. The initial development of seedlings in reference to the size of the acorn, its parts and acorn maturation time was also investigated. The fresh and dry mass of individual acorns, embryonic axes and cotyledons of 15 species (Quercus agrifolia, Q. gambelli, Q. kelloggii, Q. alba, Q. bicolor, Q. coccinea, Q. falcata, Q. lyrata, Q. nigra, Q. prinus, Q. rubra, Q. cerris, Q. ilex, Q. petraea, and Q. robur) were determined. In the second part of the study, the mass of the ca. 3-month-old seedlings that had completed the first stage of growth, with a division on the leaves, the shoot and the root, was measured. The greatest variation in mass both between and within the species was demonstrated for the embryonic axis. The mass of seedlings depends on the size of the acorn and the cotyledons contained therein; whereas, the mass of the embryonic axis has no significant influence on the mass of seedlings.

Southwestern ponderosa pine forests are vulnerable to fire-driven conversion in a warming and drying climate, yet little is known about what kinds of ecological communities may replace them. To characterize postfire vegetation trajectories and their environmental determinants, plant assemblages (361 sample plots including 229 vascular plant species, surveyed in 2017) were sampled within eight burns that occurred between 2000 and 2003. I used nonmetric multidimensional scaling, k-means clustering, principal component analysis, and random forest models to assess relationships between vegetation pattern, topographic and landscape factors, and gridded climate data. I describe seven postfire community types, including regenerating forests of ponderosa pine, aspen, and mixed conifers, shrub-dominated communities of Gambel oak and mixed species, and herb-dominated communities of native bunchgrasses and mixtures of ruderal, native, and nonnative species. Forest recovery was generally associated with cooler, mesic sites in proximity to forested refugia; shifts toward scrub and grassland types were most common in warmer, dryer locations distant from forested refugia. Under future climate scenarios, models project decreases in postfire forest recovery and increases in nonforest vegetation. However, forest to nonforest conversion was partially offset under a scenario of reduced burn severity and increased retention of forested refugia, highlighting important management opportunities. Burning trends in the southwestern United States suggest that postfire vegetation will occupy a growing landscape fraction, compelling renewed management focus on these areas and paradigm shifts that accommodate ecological change. I illustrate how management decisions around resisting, accepting, or directing change could be informed by an understanding of processes and patterns of postfire community variation and likely future trajectories.

Fire exclusion in historically frequent-fire forests of the southwestern United States has altered forest structure and increased the probability of highseverity fire. Warmer and drier conditions, coupled with dispersal distance limitations, are impeding tree seedling establishment and survival following high-severity fire. High-severity patches are commonly dominated by non-forest vegetation, a state that can be reinforced by subsequent fire events. We sought to determine the influence of fire probability on post-fire vegetation development in a severely burned landscape in New Mexico, USA. We used LANDIS-II to simulate three fire probability scenarios—historical fire probability, contemporary fire probability, and the mean of the two—with contemporary climate. As fire probability increased, the mean size of the largest fires and the mean landscape fire severity increased. These changes in fire characteristics resulted in decreased total aboveground biomass and photosynthetic capacity on the landscape after 50 years. Additionally, the distribution of individual species biomass shifted, with early successional species, especially those that resprout after fire, increasing as a fraction of total biomass with increasing fire occurrence. Counter to empirical data, our simulations did not show a conifer establishment limitation, suggesting a source of uncertainty that will need to be addressed to improve projections of forest dynamics under future climate. Even without limited conifer regeneration, continued increases in fire frequency are likely to favor resprouting species and result in a loss of forest biomass and ecosystem productivity in this southwestern forest landscape.

Many endophytic fungi have the potential to function as saprotrophs when living host tissues senesce and enter the litter pool. The consumption of plant litter by fungi obviously requires moisture but, in the arid, western USA, the native range of Quercus gambelii Nutt., most of the precipitation occurs during the coldest months of the year. Therefore, we hypothesized that the endophytic fungi of Q . gambelii have the potential to function as psychrotolerant saprotrophs, which we defined in this study as an organism capable of significant growth on leaf litter at 5°C. We further hypothesized that a tradeoff exists between growth of endophytic fungi at 5°C and at 17°C such that fungal isolates are either cold- or warm-temperature specialists. Consistent with our first hypothesis, we found that 36 of our 40 isolates consumed leaf litter at 5°C, but there was a surprisingly high degree of variability among isolates in this ability, even among isolates of a given species. Contrary to our second hypothesis, there was no tradeoff between saprotrophic growth at 5°C and saprotrophic growth at 17°C. Indeed, the isolates that grew poorly as saprotrophs at 5°C were generally those that grew poorly as saprotrophs at 17°C. By virtue of being endophytic, endophytic fungi have priority in litter over decomposer fungi that colonize plant tissues only after they enter the litter pool. Moreover, by virtue of being psychrotolerant, some endophytic fungi may function as saprotrophs during the cold months of the year when moisture is temporarily available. Therefore, we suggest that some endophytic fungi of Q . gambelii could play significant ecosystem roles in litter decomposition and nutrient cycling.

Understanding processes leading to disease emergence is important for effective disease management and prevention of future epidemics. Utilizing whole genome sequencing, we studied the phylogenetic relationship and diversity of two populations of the bacterial oak pathogen Lonsdalea quercina from western North America (Colorado and California) and compared these populations to other Lonsdalea species found worldwide. Phylogenetic analysis separated Colorado and California populations into two Lonsdalea clades, with genetic divergence near species boundaries, suggesting long isolation and populations that differ in genetic structure and distribution and possibly their polyphyletic origin. Genotypes collected from different host species and habitats were randomly distributed within the California cluster. Most Colorado isolates from introduced planted trees, however, were distinct from three isolates collected from a natural stand of Colorado native Quercus gambelii , indicating cryptic population structure. The California identical core genotypes distribution varied, while Colorado identical core genotypes were always collected from neighboring trees. Despite its recent emergence, the Colorado population had higher nucleotide diversity, possibly due to its long presence in Colorado or due to migrants moving with nursery stock. Overall, results suggest independent pathogen emergence in two states likely driven by changes in host-microbe interactions due to ecosystems changes. Further studies are warranted to understand evolutionary relationships among L. quercina from different areas, including the red oak native habitat in northeastern USA.

In this paper, a Gambel oak / Douglas-fir community on the Manti–La Sal National Forest in central Utah, USA, is described. The site experienced a stand-replacing disturbance event circa 1856 and was subsequently partially burned by the Seeley Fire of 2012. These 2 events, one that likely reset the forested system and another that split the site into 2 conditions—burned and unburned—provided the opportunity to learn more about forest succession, fire regimes, and species interactions in this vegetation type. The data collected suggest that this vegetation type may experience a high-severity, low-frequency fire regime in which the sprouting species, primarily oak but also aspen and maple, occupy the site immediately after a disturbance. Over time, Douglas-fir can germinate and become established in the semishade of these deciduous species, eventually becoming the dominant tree/shrub species until the next event resets the area's successional trajectory. En este artículo se describe una comunidad de robles de Gambel y abetos de Douglas en el bosque nacional Manti–La Sal, en el centro de Utah (Estados Unidos). El sitio experimentó un evento de alteración que reemplazó a la masa forestal alrededor de 1856 y posteriormente fue quemado parcialmente por el incendio Seeley de 2012. Estos dos eventos, uno que probablemente restableció el sistema forestal y otro que dividió el sitio en dos condiciones (quemado y no quemado) brindaron la oportunidad de aprender más sobre la sucesión forestal, los regímenes de incendios y las interacciones de las especies en este tipo de vegetación. Los datos recopilados sugieren que este tipo de vegetación puede experimentar un régimen de incendios de alta severidad y baja frecuencia en el que las especies que brotan, principalmente robles, pero también álamos y arces, ocupan el sitio inmediatamente después de una perturbación. Con el tiempo, los abetos de Douglas pueden germinar y establecerse en la semisombra de estas especies caducifolias, y eventualmente convertirse en la especie de árbol/arbusto dominante, hasta que el próximo evento restablezca la trayectoria sucesional del área.

Background and AimsQuercus species are often considered ‘foundation’ components of several temperate and/or subtropical forest ecosystems. However, the populations of some species are declining and there is considerable urgency to develop ex situ conservation strategies. In this study, the storage physiology of seeds within Quercus was explored in order to determine factors that affect survival during cryopreservation and to provide a quantitative assessment of seed recalcitrance to support future studies of this complex trait.MethodsWater relations and survival of excised axes in response to water loss and cryo-exposure were compared for four Quercus species from subtropical China (Q. franchetii, Q. schottkyana) and temperate USA (Q. gambelii, Q. rubra).Key ResultsSeed tissues initially had high water contents and water potentials. Desiccation tolerance of the embryonic axis was not correlated with the post-shedding rainfall patterns where the samples originated. Instead, higher desiccation tolerance was observed in samples growing in areas with colder winters. Survival following cryo-exposure correlated with desiccation tolerance. Among species, plumule tissues were more sensitive than radicles to excision, desiccation and cryo-exposure, and this led to a higher proportion of abnormally developing embryos during recovery following stress.ConclusionsQuercus species adapted to arid and semi-humid climates still produce recalcitrant seeds. The ability to avoid freezing rather than drought may be a more important selection factor to increase desiccation tolerance. Cryopreservation of recalcitrant germplasm from temperate species is currently feasible, whilst additional protective treatments are needed for ex situ conservation of Quercus from tropical and subtropical areas.

Aim The Madrean Sky Island Archipelago is a North American biodiversity hotspot composed of ~60 isolated mountains that span the Cordilleran Gap between the Rocky Mountains and the Sierra Madre Occidental. Characterized by discrete patches of high‐elevation montane habitat, these “sky islands” serve as stepping stones across a “sea” of desert scrub/grassland. Over this coming century, the region is expected to shift towards a warmer and drier climate. We used species distribution modelling to predict how the spatial distribution of montane habitat will be affected by climate change. Location Madrean Sky Island Archipelago, south‐west United States and north‐west Mexico (latitude, 29–34°N; longitude, 107–112°W). Methods To approximate the current distribution of montane habitat, we built species distribution models for five high‐elevation species (Ceanothus fendleri, Pinus strobiformis, Quercus gambelii, Sciurus aberti, and Synuchus dubius). The resulting models were projected under multiple climate change scenarios—four greenhouse gas concentration trajectories (RCP 2.6, 4.5, 6.0, and 8.5) for each of three climate models (CCSM4, MPI‐ESM‐LR, and NorESM1‐M)—to generate predicted distributions for the years 2050 and 2070. We performed chi‐squared tests to detect any future changes to total montane habitat area, and Conover–Iman tests to evaluate isolation among the discrete montane habitat patches. Results While the climate models differ with respect to their predictions as to how severe the effects of future climate change will be, they all agree that by as early as year 2050, there will be significant montane habitat loss and increased montane habitat patch isolation across the Madrean Archipelago region under a worst‐case climate change scenario (RCP 8.5). Main conclusions Our results suggest that under 21st‐century climate change, the Madrean Sky Islands will become increasingly isolated due to montane habitat loss. This may affect their ability to serve as stepping stones and have negative implications for the region's biodiversity.