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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 is returning to many conifer-dominated forests where species composition and structure have been altered by fire exclusion. Ecological effects of these fires are influenced strongly by the degree of forest change during the fire-free period. Response of fire-adapted species assemblages to extended fire-free intervals is highly variable, even in communities with similar historical fire regimes. This variability in plant community response to fire exclusion is not well understood; however, ecological mechanisms such as individual species' adaptations to disturbance or competition and underlying site characteristics that facilitate or impede establishment and growth have been proposed as potential drivers of assemblage response. We used spatially explicit dendrochronological reconstruction of tree population dynamics and fire regimes to examine the influence of historical disturbance frequency (a proxy for adaptation to disturbance or competition), and potential site productivity (a proxy for underlying site characteristics) on the stability of forest composition and structure along a continuous ecological gradient of pine, dry mixed-conifer, mesic mixed-conifer, and spruce-fir forests following fire exclusion. While average structural density increased in all forests, species composition was relatively stable in the lowest productivity pine-dominated and highest productivity spruce-fir-dominated sites immediately following fire exclusion and for the next 100 years, suggesting site productivity as a primary control on species composition and structure in forests with very different historical fire regimes. Species composition was least stable on intermediate productivity sites dominated by mixed-conifer forests, shifting from primarily fire-adapted species to competition-adapted, fire-sensitive species within 20 years of fire exclusion. Rapid changes to species composition and stand densities have been interpreted by some as evidence of high-severity fire. We demonstrate that the very different ecological process of fire exclusion can produce similar changes by shifting selective pressures from disturbance-mediated to productivity-mediated controls. Restoring disturbance-adapted species composition and structure to intermediate productivity forests may help to buffer them against projected increasing temperatures, lengthening fire seasons, and more frequent and prolonged moisture stress. Fewer management options are available to promote adaptation in forest assemblages historically constrained by underlying site productivity. [ABSTRACT FROM AUTHOR]

Interspecific variation in stomatal conductance ( GS) and transpiration ( EL) has been documented in stands of co-occurring species, and this variation has been observed to differ with tree size and canopy height increase. In this study, we present data that examine fluctuations in canopy gas exchange across co-occurring species and varying canopy heights for three montane forest chronosequences located in an inland Pacific Northwest mixed-conifer forest. With the exception of Douglas-fir ( Pseudotsuga menziesii var. glauca (Beissn.) Franco), we observed consistent declines in canopy EL and GS with increasing height for the majority of species examined in our 2-year study. Along with declines in canopy GS, we observed decreases in leaf-specific hydraulic conductance ( KL) across species as canopy height increased. Seasonally, we observed declines in canopy GS during warmer and dryer summer months of both years. These decreases in GS were significant (up to 50%) and suggest that carbon assimilation in trees was limited during dryer months due to a combination of high evaporative demand and reduced soil H2O availability. Such reductions in GS during periods of increased plant water stress suggest that forest productivity in the inland Pacific Northwest may be impacted negatively if future climate predictions of increasing growing-season water stress are realized. [ABSTRACT FROM AUTHOR]

Management efforts to promote forest resiliency as climate changes have often used historical forest structure and composition to provide general guidance for fuels reduction and forest restoration treatments. However, it has been difficult to identify what stand conditions might be fire and drought resilient because historical data and reconstruction studies are generally limited to accurate estimates only of large, live tree density and composition. Other stand features such as smaller tree densities, dead wood, understory structure, regeneration, and fuel loads have been difficult to quantify, estimate how they may vary across a landscape, or assess how they would be affected by fire under current climate conditions. We sampled old-growth, mixed-conifer forests with at least two low-intensity fires within the last 65 years in 150 plots at 48 sample sites ranging over 400 km of the Sierra Nevada. Recent fire history had the strongest influence on understory conditions with small tree density decreasing and shrub cover increasing with the increased intensity and frequency of fire associated with upper-slope and ridge-top locations. In contrast, stand structures associated with large, overstory trees such as total basal area, canopy cover, and the abundance of large snags and logs increased in topographic locations associated with more mesic, productive sites regardless of fire history. In forests with restored fire regimes, topography, fire and their interaction influence productivity and burn intensity, creating the structural heterogeneity characteristic of frequent-fire forests. [ABSTRACT FROM AUTHOR]

Abstract: Fire suppression and other past management practices in the western USA have led to dense conifer forests with high canopy cover and thick layers of surface fuels, changes likely to alter understory microclimate relative to historical conditions. Silvicultural treatments are used to restore forest resilience, but little is known about their microclimate-mediated effects on fire behavior. We measured fire-related microclimate variables for two years before and after experimental, operational-scale application of fuels-reduction thinning and group selection treatments in a Sierra Nevada mixed-conifer forest. Measurements included air speed, temperature, and relative humidity; soil temperature and moisture; and dead fuel moisture. Wind gust speed increased moderately (average 0.7ms−1 or 31% increase) in thinned forest and sharply (average 2.5ms−1 or 128% increase) in group-selection openings. Surprisingly, treatments did not affect air temperature or humidity. Soil temperatures increased by a mean of 4°C in group openings but did not increase in thinned stands. Duff moisture in group selection openings was 72% of that in the control stands, but there were no effects on moisture in other fuel particle size classes, or in thinned stands. Soil moisture increased in group-selection openings at depths down to 0.7m but did not change in thinned stands. Fire spread simulation modeling with FMAPlus indicated that elevated wind speeds could increase the fire rate of spread, but that increases are moderate and largely linear rather than exponential across the observed range of wind gust speeds. In general our results suggest that group selection openings placed in high canopy cover, Sierran mixed-conifer forests are distinct microclimatic environments that will have slightly different fire behavior than the surrounding matrix due to higher surface temperatures and faster wind speeds. Current fuels-reduction thinning practices in dry western forests, however, will have minimal microclimatic-mediated influence on wildfire behavior, and there is little cause for concern about a faster rate of fire spread or drier fuels in such stands. [Copyright &y& Elsevier]

• Understanding spatial and temporal patterns present in ectomycorrhizal fungal community structure is critical to understanding both the scale and duration of the potential impact these fungi have on the plant community. While recent studies consider the spatial structure of ectomycorrhizal communities, few studies consider how this changes over time.• Ectomycorrhizal root biomass and the similarity of community composition were measured at scales up to 20 cm replicated in nine plots and over 3 yr. Soil cores were additionally stratified into three depths.• Annual occurrence of the dominant ectomycorrhizal species was constant at larger spatial scales but varied more across years at a fine spatial scale. Turnover of ectomycorrhizal species between years was observed frequently at scales< 20 cm. The ectomycorrhizal community within a plot was more similar across years than it was to other plots sampled in the same year.• Our results demonstrate the dynamic nature of the ectomycorrhizal community even in the absence of large-scale disturbances. The potential role of root turnover and drought stress is discussed.New Phytologist(2005)doi: 10.1111/j.1469-8137.2005.01354.x© New Phytologist(2005) [ABSTRACT FROM AUTHOR]