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Climate change is expected to lead to higher temperatures in the Mediterranean region of northern California in the Sierra Nevada. Dendroclimatic studies typically focus on large, old trees, but there relatively limited understanding on how climatic sensitivity can vary with trees of different size classes. We collected tree increment cores and compared radial growth responses of small (20.32 to ≤40.64 cm), medium (40.64 to ≤60.96 cm), and large (>60.96 cm) diameter mixed conifer species in the Sierra Nevada to different climate variables (temperature, precipitation, and climate moisture index (CMI)). The most common tree species encountered were white fir (Abies concolor (Gord. & Glend.) Lindl.), followed by sugar pine (Pinus lambertiana Dougl.), ponderosa pine (Pinus ponderosa Dougl. Ex P. & C. Laws), and incense cedar (Calocedrus decurrens (Torr.) Florin). One of the most coherent responses from all diameter groups and across all species was the positive response to increasing minimum winter temperatures. All diameter groups and species also responded positively to precipitation and CMI at some point in the analysis period, which is the seasonal window of April of the prior year to October of the current year of ring formation. Perhaps the most notable difference when comparing the three diameter groups to climate was the higher occurrence of negative responses to the temperature of the previous year from the largest diameter group, as well as the higher number of negative responses to temperature in general. These results suggest that larger trees may be more sensitive to future climate projections compared with smaller trees and they may carry those effects into the next year. The use of dendroclimatology to assess how mixed conifer species in the Sierra Nevada responded to past climate is a key resource that can be used to infer how trees may respond to a future changing climate. [ABSTRACT FROM AUTHOR]

As fire regimes shift in western North America, patches of stand-replacing fire are becoming larger. Forest succession in these patches is not well understood. There is concern that competition with rapidly reestablishing shrubs, combined with dispersal limitation, may delay or impede conifer recovery and/or shift tree composition toward shade-tolerant species. However, tree–shrub interactions and shrub neighborhood dynamics have not been closely examined. To investigate the patterns and processes determining forest recovery after severe wildfire, we developed a data-driven simulation model that we use to predict conifer emergence above the shrub canopy. Our model results showed that ponderosa pine (Pinus ponderosa) emerged at a faster rate than white fir (Abies concolor) under whitethorn ceanothus (Ceanothus cordulatus) and deerbrush (Ceanothus integerrimus), and at a similar rate under greenleaf manzanita (Arctostaphylos patula). Across all shrub species, ponderosa pine had a relative advantage over fir in the period between conifer establishment and peak shrub competition, requiring a mean of 16 years for 50% of individuals to emerge compared to 22 years for fir. Fir emergence rates then surpassed those of pine, leading to similar overall emergence by simulation end: 83% ± 7% for pine and 82% ± 7% for fir. These results show that, on balance, shrub neighborhood dynamics do not produce an ecological filter favoring firs for the initial cohort of established seedlings, but emergence patterns are sensitive to shrub species. Further applications of this data-driven simulation framework could improve understanding of other important components of post-fire succession. [ABSTRACT FROM AUTHOR]

In mixed stands, species complementarity (e.g. facilitation and competition reduction) may enhance forest tree productivity. Although positive mixture effects have been identified in forests world‐wide, the majority of studies have focused on two‐species interactions in managed systems with high functional diversity. We extended this line of research to examine mixture effects on tree productivity across landscape‐scale compositional and environmental gradients in the low functional diversity, fire‐suppressed, mixed‐conifer forests of the U.S. Interior West.We investigated mixture effects on the productivity of Pinus ponderosa, Pseudotsuga menziesii and Abies concolor. Using region‐wide forest inventory data, we created individual‐tree generalized linear mixed models and examined the growth of these species across community gradients. We compared the relative influences of stand structure, age, competition and environmental stress on mixture effects using multi‐model inference. We analysed growth of neighbouring tree species to infer whether a mixture effect in a single species translated to the stand‐level.We found support for a positive mixture effect in P. menziesii, although our results were equivocal in light of a weaker but still plausible alternative model. Growth of P. menziesii neighbouring species in mixed stands declined or held constant depending on aridity, suggesting that a positive mixture effect in P. menziesii does not necessarily extend to the stand level. We found no evidence for mixture effects in P. ponderosa, A. concolor or their neighbouring species.Complementarity appears to have a limited influence on tree growth in the mixed‐conifer systems of the U.S. Interior West, reflecting limited functional diversity. Historical changes in stand structure following fire exclusion, particularly high stand densities, may limit the potential for positive species mixture effects. The limited species pool of Interior West forests increases the risk that, without careful management, what functional diversity exists could be lost to compositional changes resulting from stand dynamics or disturbance. [ABSTRACT FROM AUTHOR]

Fire mosses, including Ceratodon purpureus , Funaria hygrometrica and Bryum argenteum , can achieve high cover within months to years after high-severity fire, but do so heterogeneously across space and time. We conducted a survey of moss cover and erosion-related functions after 10 wildfires in Pinus ponderosa and mixed-conifer forests of the southwestern USA. We sampled 65 plots in high-severity patches, stratifying by elevation and insolation over each fire. Using three landscape-scale predictor variables and one temporal predictor, we explained 37% of the variance in fire moss cover using a random forest model. The predictors in order of importance were: equinox insolation (sunlight/day), pre-fire vegetation type, pre-fire soil organic carbon and time since fire. Within each plot we examined differences between bare and moss-covered soil surface microsites and found moss-covered microsites had a mean increase of 55% water infiltration, 106% shear strength, 162% compressive strength and 195% aggregate stability. We tested a suite of nutrients, finding 35% less manganese in the moss-covered soil. This research demonstrated that post-fire colonisation by moss is predictable and that colonisation improves soil surface erosion resistance and hydrological function, with implications for managing severely burned landscapes. Fire mosses are early successional species that can colonise severely burned landscapes. We conducted a survey of 10 wildfires in the southwestern USA. Fire moss cover increased in shaded north-facing hillslopes in relatively wet locations. When compared with the bare soil surface, fire mosses reduced erosion and enhanced water infiltration. [ABSTRACT FROM AUTHOR]

Context: Fire regimes in many dry forests of western North America are substantially different from historical conditions, and there is concern about the ability of these forests to recover following severe wildfire. Fire refugia, unburned or low-severity burned patches where trees survived fire, may serve as essential propagule sources that enable forest regeneration. Objectives: To quantify the influence of fire refugia spatial pattern and other biophysical factors on the process of post-fire tree regeneration; in particular examining both the proximity and density of surrounding refugia to characterize the landscape of refugial seed sources. Methods: We surveyed regeneration at 135 sites in stand-replacement patches across a gradient of fire refugia density in eastern Oregon, USA. We characterized the influence of refugial seed source pattern and other environmental factors on the abundance of regenerating seedlings, and examined the relationship between post-fire climate and the temporal pattern of ponderosa pine seedling establishment. Results: Tree seedlings were present in 83% of plots 12–17 years post-fire, and densities varied substantially (0–67800 stems ha−1, median = 1100). Variation in seedling abundance was driven by the spatial patterns of refugial seed sources. Despite widespread post-fire shrub cover, high-severity burned forests have not undergone a persistent type conversion to shrublands. Ponderosa pine seedling establishment peaked 5–11 years after fire and was not closely associated with post-fire climate. Conclusions: Fire refugia and the seed sources they contain fostered tree regeneration in severely burned patches. Management practices that reduce refugia within post-fire landscapes may negatively influence essential forest recovery processes. [ABSTRACT FROM AUTHOR]

Twentieth-century land management has altered the structure and composition of mixed-conifer forests and decreased their resilience to fire, drought, and insects in many parts of the Interior West. These forests occur across a wide range of environmental settings and historical disturbance regimes, so their response to land management is likely to vary across landscapes and among ecoregions. However, this variation has not been well characterized and hampers the development of appropriate management and restoration plans. We identified mixed-conifer types in central Oregon based on historical structure and composition, and successional trajectories following recent changes in land use, and evaluated how these types were distributed across environmental gradients. We used field data from 171 sites sampled across a range of environmental settings in two subregions: the eastern Cascades and the Ochoco Mountains. We identified four forest types in the eastern Cascades and four analogous types with lower densities in the Ochoco Mountains. All types historically contained ponderosa pine, but differed in the historical and modern proportions of shade-tolerant vs. shade-intolerant tree species. The Persistent Ponderosa Pine and Recent Douglas-fir types occupied relatively hot- dry environments compared to Recent Grand Fir and Persistent Shade Tolerant sites, which occupied warm-moist and cold-wet environments, respectively. Twentieth-century selective harvesting halved the density of large trees, with some variation among forest types. In contrast, the density of small trees doubled or tripled early in the 20th century, probably due to land-use change and a relatively cool, wet climate. Contrary to the common perception that dry ponderosa pine forests are the most highly departed from historical conditions, we found a greater departure in the modern composition of small trees in warm-moist environments than in either hot-dry or cold-wet environments. Furthermore, shade-tolerant trees began infilling earlier in cold-wet than in hot-dry environments and also in topographically shaded sites in the Ochoco Mountains. Our new classification could be used to prioritize management that seeks to restore structure and composition or create resilience in mixed-conifer forests of the region. [ABSTRACT FROM AUTHOR]

The article presents a study which looks into the impact of the fire suppression, grazing and timber harvesting of the dry, ponderosa pine forests in the west part of the U.S. These forests, which have severely been experiencing a buildup of fuels as a result of the said activities, are suggested to be provided with an important ecological backdrop, such as the Black-backed Woodpecker, in order to prevent the same things from happening. On account of this, researchers of the study highlights several essential details gathered upon the research execution.