Matthew F. Bekker

Aim: We tested how climate–growth relationships, temporal coherence and responses to compound climate extremes vary across a regional elevational gradient occupied by Pinus flexilis . Location: Greater Yellowstone Ecoregion, northwestern Wyoming, USA. Time Period: Dated tree‐ring material spans 514–2020 ce , with detailed analyses focused on the past millennium. Major Taxa Studied: Limber pine ( Pinus flexilis James). Methods: We developed three millennial‐length ring‐width chronologies from high‐, mid‐ and low‐elevation sites. We compared radial growth with instrumental and reconstructed climate, quantified shared and elevation‐dependent growth patterns using PCA and moving‐window analyses, and evaluated growth responses to compound hot‐dry and cool‐wet extremes. Results: Growth at all elevations was moisture sensitive and declined during warm summer conditions, indicating that heat amplified water limitation across the gradient. The chronologies shared a broad hydroclimatic signal, but the high‐ and low‐elevation sites often differed in growth magnitude and in the timing of growth responses. High‐elevation Wolf Knob temporarily decoupled from the dominant growth pattern during cool Little Ice Age conditions, whereas low‐elevation Outlaw Ridge showed the most persistent growth suppression following hot‐dry extremes. Main Conclusions: Across the sampled ecological amplitude in the GYE, P. flexilis growth reflects both shared hydroclimatic variability and elevation‐specific constraints. Future warming may reduce cold limitation near upper treeline in some years, but is likely to intensify moisture limitation and drought legacy effects at lower elevations.

The geographic distribution of forest and woodland ecosystems in the Interior West United States is strongly influenced by topographic factors that, in part, control moisture availability through their effect on insolation, precipitation capture, and retention. Through an empirical approach, we use unique, plot-level data from the Forest Inventory and Analysis Program (n = 13,437) over eight ecoregions within eight Interior West states to characterize the distribution of the 12 most abundant tree species with respect to the effects of elevation, slope aspect, and slope steepness. Across species, elevation, and aspect, most forested plots occurred on gentle slopes and the number decreased with increasing slope. Species-specific differences in microenvironmental conditions were evident in the variation between observed (plots containing a subject tree) and expected (all forest plots from the systematic sample) numbers of plots across the gradient combinations. Species groups, broadly defined as woodland, montane, and subalpine, generally exhibited similar responses and revealed more generality than hypothesized. Only Douglas-fir, white fir, subalpine fir, and Engelmann spruce exhibited significant patterns of affinity for particular aspects—most often on north and least often on south—with the relative importance of south aspects decreasing with increasing elevation. Limber pine showed unique, unimodal patterns of affinity for moderately steep slopes, with no consistent patterns by aspect or elevation. Although not significant, at high elevations, woodland species exhibited a tendency to occur more often on south aspects on gentle to intermediate slopes, and less often on north aspects. Unique microenvironments created by interactions between aspect, slope, and elevation create some predictability in the patterns of where species occur. While there were predictable and significant patterns for a few important montane and subalpine species, most species examined lacked any significant relationships to slope, slope aspect, or elevation, suggesting that patterns of occurrence in relation to physiographic factors are much broader than in common generalizations.

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.

La presencia del pino longevo de la Gran Cuenca (Pinus longaeva D.K. Bailey) en las montañas Stansbury del centro-norte de Utah fue registrada antes del año 2020. No obstante, estos reportes carecen de caracterización adecuada de la población y delineación de distintos rodales de árboles. En el verano de 2020, identificamos y documentamos la presencia de cinco rodales separados de pino longevo en las montañas Stansbury. Estos rodales se encuentran alejados de las poblaciones de pinos longevos más cercanas en otras cadenas montañosas, a una distancia de aproximadamente 120 km, y como tales, representan una población atípica única de la especie. Usamos datos de GPS para crear una base de datos del sistema de información geográfica (SIG) que delinea los cinco rodales que identificamos, y tomamos muestras de la edad y el tamaño de los árboles en uno de los rodales para compararlos con otras poblaciones de pinos longevos en la Gran Cuenca. En este trabajo presentamos dos hipótesis para explicar la presencia del pino longevo en las montañas de Stansbury: (1) esta población es un relicto de la época en que el pino longevo se encontraba ampliamente distribuido en la Gran Cuenca, y (2) la especie llegó a la cordillera a través de mecanismos de dispersión de larga distancia (LDD, por sus siglas en inglés) en algún momento durante o después de la transición Pleistoceno/Holoceno (ca. 12,000 14C YBP). Las estadísticas de los yacimientos de neotoma (rata de bosque) sugieren que el pino longevo estaba ausente o al menos no estaba muy extendido en la cuenca del norte de Bonneville durante el Pleistoceno tardío, pero los datos de los yacimientos son escasos en las montañas Stansbury y las cordilleras circundantes. Adicionalmente, presentamos posibles rutas de migración que la especie podría haber tomado para llegar a las montañas Stansbury desde el sur de la cuenca de Bonneville, donde se encontraba muy extendida durante el Pleistoceno tardío, utilizando la mayor extensión del lago Bonneville como límite. Los vectores de migración hipotéticos incluyen eventos LDD transportados por el viento o por aves granívoras. También postulamos que una pequeña población de pino longevo podría estar presente en las Montañas Oquirrh al este de las Montañas Stansbury ya que el hábitat presenta características similares, así como, nuestra identificación de un espécimen de prueba de 1964 de las Montañas Oquirrh que probablemente no se describió correctamente y que parece ser de pino longevo.

Paleoclimate reconstructions for the western US show spatial variability in the timing, duration, and magnitude of climate changes within the Medieval Climate Anomaly (MCA, ca. 900–1350 CE) and Little Ice Age (LIA, ca. 1350–1850 CE), indicating that additional data are needed to more completely characterize late-Holocene climate change in the region. Here, we use dendrochronology to investigate how climate changes during the MCA and LIA affected a treeline, whitebark pine (Pinus albicaulis Engelm.) ecosystem in the Greater Yellowstone Ecoregion (GYE). We present two new millennial-length tree-ring chronologies and multiple lines of tree-ring evidence from living and remnant whitebark pine and Engelmann spruce (Picea engelmannii Parry ex. Engelm.) trees, including patterns of establishment and mortality; changes in tree growth; frost rings; and blue-intensity-based, reconstructed summer temperatures, to highlight the terminus of the LIA as one of the coldest periods of the last millennium for the GYE. Patterns of tree establishment and mortality indicate conditions favorable to recruitment during the latter half of the MCA and climate-induced mortality of trees during the middle-to-late LIA. These patterns correspond with decreased growth, frost damage, and reconstructed cooler temperature anomalies for the 1800–1850 CE period. Results provide important insight into how past climate change affected important GYE ecosystems and highlight the value of using multiple lines of proxy evidence, along with climate reconstructions of high spatial resolution, to better describe spatial and temporal variability in MCA and LIA climate and the ecological influence of climate change.

Background The geographic distribution of forest and woodland ecosystems in the Interior West United States is strongly influenced by topographic gradients that, in part, control moisture availability through their effect on insolation, and precipitation capture and retention. Through an empirical approach, we use unique, plot-level data from the Forest Inventory and Analysis Program ( n = 13,437) over eight ecoregions within eight Interior West states to characterize the distribution of the 12 most abundant tree species with respect to the effects of elevation, slope aspect, and slope steepness. Results Across species, elevation, and aspect, most plots occurred on gentle slopes and the number decreased with increasing slope. Species-specific differences to microenvironmental conditions were evident in the variation between observed (plots containing a subject tree) and expected (all forest plots from the systematic sample) numbers of plots across the gradient combinations. Species groups, broadly defined as woodland, montane, and subalpine, generally exhibited similar responses and revealed more generality than hypothesized. Only Douglas-fir, white fir, subalpine fir, and Engelmann spruce exhibited significant patterns of affinity for particular aspects—most often on north and least often on south—with the relative importance of south aspects decreasing with increasing elevation. Limber pine showed unique, unimodal patterns of affinity for moderately steep slopes, with no consistent patterns by aspect or elevation. Although not significant, at high elevations woodland species exhibited a tendency to occur more often on south aspects on gentle to intermediate slopes, and less often on north aspects. Conclusions Unique microenvironments created by interactions between aspect, slope, and elevation create some predictability in patterns of geographic distribution. However, the general lack of species-specific response suggests that patterns of occurrence in relation to physiographic gradients is much broader than in common generalizations.

Human activities are changing patterns of ecological disturbance globally. In North American deserts, wildfire is increasing in size and frequency due to fuel characteristics of invasive annual grasses. Fire reduces the abundance and cover of native vegetation in desert ecosystems. In this study, we sought to characterize stem growth and reproductive output of a dominant native shrub in the Mojave Desert, creosote bush (Larrea tridentata (DC.) Coville) following wildfires that occurred in 2005. We sampled 55 shrubs along burned and unburned transects 12 years after the fires (2017) and quantified age, stem diameter, stem number, radial and vertical growth rates, and fruit production for each shrub. The shrubs on the burn transects were most likely postfire resprouts based on stem age while stems from unburn transects dated from before the fire. Stem and vertical growth rates for shrubs on burned transects were 2.6 and 1.7 times higher than that observed for shrubs on unburned transects. Fruit production of shrubs along burned transects was 4.7‐fold more than shrubs along paired unburned transects. Growth rates and fruit production of shrubs in burned areas did not differ with increasing distance from the burn perimeter. Positive growth and reproduction responses of creosote following wildfires could be critical for soil stabilization and re‐establishment of native plant communities in this desert system. Additional research is needed to assess if repeat fires that are characteristic of invasive grass‐fire cycles may limit these benefits.