Climate projections predict more frequent and severe drought in coastal Pseudotsuga menziesii forests of western North America, raising concerns over how to promote drought adaptation. Thinning often ...increases drought resistance (the ability to maintain growth during a drought) and resilience (the ability to recover growth after a drought), but these effects vary with thinning intensity, shift over time, and may have tradeoffs with fiber production. We collected tree cores from a long-term thinning study with four residual density levels replicated across both uniform thinning and thinning with gaps, and used annual growth data to investigate responses to droughts occurring 8 and 21 years after thinning. For the first drought, resistance and resilience were higher in treatments with lower residual densities. For the second drought, there were no differences in drought response between the lowest and highest residual density treatments, and all treatments had lower drought resistance and resilience than for the first drought. Spatial arrangement had little impact on drought resistance or resilience and residual density level had a significant effect on the periodic annual volume increment—drought resistance tradeoff. Our results suggest that thinning can promote drought adaptation in Pseudotsuga menziesii forests, but these effects dissipate over time.
Interannual variability of wood density – an important plant functional trait and environmental proxy – in conifers is poorly understood. We therefore explored the anatomical basis of density. We ...hypothesized that earlywood density is determined by tracheid size and latewood density by wall dimensions, reflecting their different functional tasks.
To determine general patterns of variability, density parameters from 27 species and 349 sites across the Northern Hemisphere were correlated to tree-ring width parameters and local climate. We performed the same analyses with density and width derived from anatomical data comprising two species and eight sites. The contributions of tracheid size and wall dimensions to density were disentangled with sensitivity analyses.
Notably, correlations between density and width shifted from negative to positive moving from earlywood to latewood. Temperature responses of density varied intraseasonally in strength and sign. The sensitivity analyses revealed tracheid size as the main determinant of earlywood density, while wall dimensions become more influential for latewood density.
Our novel approach of integrating detailed anatomical data with large-scale tree-ring data allowed us to contribute to an improved understanding of interannual variations of conifer growth and to illustrate how conifers balance investments in the competing xylem functions of hydraulics and mechanical support.
The phenology of diameter‐growth cessation in trees will likely play a key role in mediating species and ecosystem responses to climate change. A common expectation is that warming will delay ...cessation, but the environmental and genetic influences on this process are poorly understood. We modeled the effects of temperature, photoperiod, and seed‐source climate on diameter‐growth‐cessation timing in coast Douglas‐fir (an ecologically and economically vital tree) using high‐frequency growth measurements across broad environmental gradients for a range of genotypes from different seed sources. Our model suggests that cool temperatures or short photoperiods can induce cessation in autumn. At cool locations (high latitude and elevation), cessation seems to be induced primarily by low temperatures in early autumn (under relatively long photoperiods), so warming will likely delay cessation and extend the growing season. But at warm locations (low latitude or elevation), cessation seems to be induced primarily by short photoperiods later in autumn, so warming will likely lead to only slight extensions of the growing season, reflecting photoperiod limitations on phenological shifts. Trees from seed sources experiencing frequent frosts in autumn or early winter tended to cease growth earlier in the autumn, potentially as an adaptation to avoid frost. Thus, gene flow into populations in warm locations with little frost will likely have limited potential to delay mean cessation dates because these populations already cease growth relatively late. In addition, data from an abnormal heat wave suggested that very high temperatures during long photoperiods in early summer might also induce cessation. Climate change could make these conditions more common in warm locations, leading to much earlier cessation. Thus, photoperiod cues, patterns of genetic variation, and summer heat waves could limit the capacity of coast Douglas‐fir to extend its growing season in response to climate change in the warm parts of its range.
The phenology of tree diameter‐growth cessation in autumn is an important process that strongly impacts organism and ecosystem function, but its environmental and genetic drivers are poorly understood, impeding predictions of climate change impacts. As favorable growing conditions shift later into autumn with warming, trees would likely need to delay the timing of growth cessation to track favorable climate. We studied diameter‐growth cessation in coast Douglas‐fir (a foundation species) and found that cool temperatures or short photoperiods can induce cessation, implying that in cool parts of the range low temperatures primarily trigger cessation, while in warm parts short photoperiods are the primary cue. In addition, we found that trees from seed sources with higher frost frequencies tended to cease growth earlier. Thus, climate change will likely lead to strong shifts to later cessation in cool locations, but weak shifts in warm locations, reflecting photoperiod and genetic limitations on phenological responses.
The success of conifers over much of the world's terrestrial surface is largely attributable to their tolerance to cold stress (i.e., cold hardiness). Due to an increase in climate variability, ...climate change may reduce conifer cold hardiness, which in turn could impact ecosystem functioning and productivity in conifer‐dominated forests. The expression of cold hardiness is a product of environmental cues (E), genetic differentiation (G), and their interaction (G × E), although few studies have considered all components together. To better understand and manage for the impacts of climate change on conifer cold hardiness, we conducted a common garden experiment replicated in three test environments (cool, moderate, and warm) using 35 populations of coast Douglas‐fir (Pseudotsuga menziesii var. menziesii) to test the hypotheses: (i) cool‐temperature cues in fall are necessary to trigger cold hardening, (ii) there is large genetic variation among populations in cold hardiness that can be predicted from seed‐source climate variables, (iii) observed differences among populations in cold hardiness in situ are dependent on effective environmental cues, and (iv) movement of seed sources from warmer to cooler climates will increase risk to cold injury. During fall 2012, we visually assessed cold damage of bud, needle, and stem tissues following artificial freeze tests. Cool‐temperature cues (e.g., degree hours below 2 °C) at the test sites were associated with cold hardening, which were minimal at the moderate test site owing to mild fall temperatures. Populations differed 3‐fold in cold hardiness, with winter minimum temperatures and fall frost dates as strong seed‐source climate predictors of cold hardiness, and with summer temperatures and aridity as secondary predictors. Seed‐source movement resulted in only modest increases in cold damage. Our findings indicate that increased fall temperatures delay cold hardening, warmer/drier summers confer a degree of cold hardiness, and seed‐source movement from warmer to cooler climates may be a viable option for adapting coniferous forest to future climate.
•We studied pure and mixed stands of European beech, Douglas fir and Norway spruce.•Tree species effects were mainly found in the forest floor and in the mineral topsoil.•Forest floor OC and N stocks ...are highest in conifer stands.•Total soil OC stocks are smaller under beech compared to Douglas fir and spruce.•Total soil OC and N stocks in mixed stands are between those of the respective pure stands.
Numerous studies have addressed tree species effects on forest soil carbon (C) and nitrogen (N); however, knowledge of how and to what extent specific tree species and species mixtures impact forest soil C and N stocks is scarce and inconsistent across soil types. Therefore, we studied three forest sites in Southern Germany differing in parent material, soil properties as well as nutrient and water supply. Each site comprises adjacent groups of pure mature European beech (Fagus sylvatica), Douglas fir (Pseudotsuga menziesii) and Norway spruce (Picea abies) as well as single-tree mixtures of beech with Douglas fir or Norway spruce. To account for tree-species-specific spatial heterogeneity, we sampled the forest floor and mineral soil to a depth of 60cm at different distances from the trees.
Significant tree species and species mixing effects on soil organic carbon (OC) and N concentrations, C/N ratios and soil OC and N stocks were mainly found in the forest floor and in the uppermost (0–15cm) mineral soil. Forest floor OC and N stocks and total soil OC stocks were higher under Douglas fir and Norway spruce compared with beech. While tree species effects on soil OC and N were present across sites, the influence of soil type induced variations in their magnitude. The forest floor C/N ratio under Douglas fir was low and comparable with beech in soils developed from nutrient-rich parent material, whereas it was higher and similar to spruce in the soil formed from sandstone. Tree species-specific differences in foliar nutrient concentrations between beech and conifer stands might influence litter decomposition rates among the species and thus modify soil OC and N stocks.
Forest floor OC stocks were significantly higher in mixed beech–conifer stands compared with pure beech, and most often smaller than or similar to pure conifer stands. Forest floor N stocks showed the same tendency, but differences were inconsistent and not always significant across sites. Admixture of beech with Douglas fir or Norway spruce reduced the share of OC and N stored in the forest floor compared with the pure conifer stands and significantly increased mineral topsoil (0–15cm) OC stocks compared with pure beech stands. Hence, the vertical distribution of OC and N in the soil profile varied depending on the tree species composition. Total soil (forest floor+mineral soil) OC and N stocks of mixtures were similar to pure beech, pure conifers or intermediate depending on site and soil type.
One of the main ideas of non-native tree species introduction into forest stands is to replace declining native species. The same is also valid for industry; the wood of native species should be ...replaced by a wood of the same or even better quality. Douglas-fir is often compared to other coniferous tree species based on its production. This study compared Douglas-fir wood properties with European commercial species, namely Norway spruce, Scots pine, and European larch. Trees representing different sites and ages were tested for wood density, shrinkage, and compression strength. In all cases, Douglas-fir outclassed spruce and pine in density and strength. The difference was striking, especially for spruce, where the density was surpassed by Douglas-fir by more than 100 kg.m−3 (above 25%). In the case of compression, the strength of Douglas-fir was up to 12.3 MPa higher (above 33%) compared to spruce. The only species that obtained higher figures was larch. Wood shrinkage was comparable to European softwoods. Therefore, Douglas-fir wood can be regarded as an excellent and promising substitute for the European processing industry.
•Douglas-fir represents promising non-native species for Europe.•It should be a substitute, especially for Norway spruce in industry.•We tested some of the properties important for applications.•In contrast to previous studies, tested species grew together in the same stand.•Douglas-fir exceeded not only spruce but also Scots pine.
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•Pseudotsuga menziesii outerbark has a high proportion of cork between phloem layers.•Milling and fractionation are selective: coarse particle fractions are cork-enriched.•Content of ...potential bioactive compounds is high specially in phloem-rich fractions.•Lipophilic extracts and suberin are preferentially present in cork-rich fractions.•Douglas-fir bark valorization should apply separately to cork and phloem fractions.
The outerbark of Pseudotsuga menziesii was chemically analyzed after selective fractioning into different particles sizes. Fractionation was selective: after grinding, the coarse fraction (>2mm) was enriched in cork and was obtained in higher yield (53.9%).
The mean bark chemical composition was, as % o.d. mass: ash 5.9%; total extractives 26.7%; lignin 29.5% and suberin 22.0%. The fine fraction of bark was richer in extractives (31.7%) namely polar compounds soluble in ethanol and water (29.2%). The content in lipophilic extractives was higher in the coarse fraction (5.5%). The polysaccharides contained glucose (65.7% of total neutral monossacharides), mannose (11.8%), xylose (9.0%), arabinose (7.1%) and galactose (6.5%). The ethanol and water extracts contained phenolics, flavonoids, condensed and hydrolysable tannins. The lipophilic extracts were analysed by GC–MS, directly and after saponification, revealing two major compounds: β-sitosterol and tetracosanoic acid. The fine fraction was enriched in extractives and polysaccharides as well as in inorganics, and impoverished in suberin in comparison with coarse fractions.
Milling and fractionation may be applied as pre-treatment for Douglas-fir bark utilization, by separation of cork-rich fractions for cork-based products e.g., composites, and phloem-rich fractions for extraction of polar extractives and further processing of the polysaccharide-lignin matrix under a biorefinery approach.
Macroclimate warming is affecting ecosystems world‐wide. Tree canopies, however, can significantly buffer temperature fluctuations, giving rise to lower maximum temperatures, higher minimum ...temperatures and enhanced water availability at the forest floor. Forests, therefore, can act as refugia for heat‐sensitive species with important implications for climate‐change responses in the understorey of forests. These forest microclimate offsets have been well‐studied, but far less is known about how microclimate offsets are modulated by tree species mixing.
We installed temperature and air humidity loggers in a multisite tree diversity experiment to quantify the role of tree species richness and composition on below‐canopy offsets and investigate mechanisms underlying these effects.
Microclimate offsets highly depended on tree species identity and mixing. Not only monocultures of larch Larix × eurolepis and Douglas fir Pseudotsuga menziesii had a high performance for thermal buffering, but also mixtures with species such as birch Betula pendula and pine Pinus sylvestris. Indirect effects brought about by tree species mixing were found to amplify microclimate offsets, mostly via increased canopy cover.
Synthesis and applications. Our findings show that microclimate offsetting highly depends on tree species identity and diversity, and that buffered forest microclimates can be achieved rapidly in young plantations, depending on the species being planted. Based on our findings, we recommend planting mixtures instead of monocultures. Not only because species mixing was found to amplify microclimate offsets in some of the investigated stands, but also because tree species mixing is likely to increase the resilience of forests to climate change. Hence, we expect that also on the long‐term species mixing might be beneficial for microclimate buffering. Forest managers and policy makers can use our finding to optimise the design of tree plantations to minimise climate‐change impacts on below‐canopy biodiversity and functioning.
Our findings show that microclimate offsetting highly depends on tree species identity and diversity, and that buffered forest microclimates can be achieved rapidly in young plantations, depending on the species being planted. Based on our findings, we recommend planting mixtures instead of monocultures. Not only because species mixing was found to amplify microclimate offsets in some of the investigated stands, but also because tree species mixing is likely to increase the resilience of forests to climate change. Hence, we expect that also on the long‐term species mixing might be beneficial for microclimate buffering. Forest managers and policy makers can use our finding to optimise the design of tree plantations to minimise climate‐change impacts on below‐canopy biodiversity and functioning.
Climate change and disturbance are altering forests and the rates and locations of tree regeneration. In semi-arid forests of the southwestern USA, limitations imposed by hot and dry conditions are ...likely to influence seedling survival. We examined how the survival of 1-year seedlings of five southwestern US conifer species whose southwestern distributions range from warmer and drier woodlands and forests (Pinus edulis Engelm., Pinus ponderosa Douglas ex C. Lawson) to cooler and wetter subalpine forests (Pseudotsuga menziesii (Mirb.) Franco, Abies concolor (Gord. & Glend.) Lindl. Ex Hildebr. and Picea engelmannii Parry ex Engelm.) changed in response to low moisture availability, high temperatures and high vapor pressure deficit in incubators. We used a Bayesian framework to construct discrete-time proportional hazard models that explained 55-75% of the species-specific survival variability. We applied these to the recent climate (1980-2019) of the southwestern USA as well as 1980-2099 CMIP5 climate projections with the RCP8.5 emissions pathway. We found that the more mesic species (i.e., P. menziesii, A. concolor and P. engelmannii) were more susceptible to the effects of hot and dry periods. However, their existing ranges are not projected to experience the conditions we tested as early in the 21st century as the more xeric P. edulis and P. ponderosa, leading to lower percentages of their existing ranges predicted to experience seedling-killing conditions. By late-century, extensive areas of each species southwestern range could experience climate conditions that increase the likelihood of seedling mortality. These results demonstrate that empirically derived physiological limitations can be used to inform where species composition or vegetation type change are likely to occur in the southwestern USA.
Climatic adaptations are the foundation of conifer genecology, but populations also display variation in traits for nitrogen (N) utilization, along with some heritable specificity for ectomycorrhizal ...fungi (EMF). We examined soil and EMF influences on assisted migration of Douglas‐fir (Pseudotsuga menziesii var. menziesii) by comparing two contrasting maritime populations planted up to 400 km northward in southwestern British Columbia. Soil N availability and host N status (via δ¹⁵N) were assessed across 12 maritime test sites, whereas EMF on local and introduced hosts were quantified by morphotyping with molecular analysis. Climatic transfer effects were only significant with soil N concentrations of test sites as a covariate, and illustrated how height growth was compromised for populations originating from relatively dry or cool maritime environments. We also found evidence for EMF maladaptation, where height declined by up to 15% with the extent of dissimilarity in EMF communities of southern populations relative to local hosts. The results demonstrate how geographic structure in belowground environments can contribute to conifer genecology. Differences in the inherent growth potential of conifers may be partly related to nutritional adaptations arising under native soil fertility, and optimization of this growth potential likely requires close affiliation with local EMF communities.
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