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.
Forest management aims for productive and stable forests that continuously provide ecosystem goods and services, including balancing nutrient fluxes. Increasing heat and frequent droughts in ...temperate European forests make the introduction of non-native Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) an increasingly relevant climate change adaptation strategy, particularly as an admixture to native tree species, such as European beech (Fagus sylvatica L.). Douglas fir can alter biogeochemical processes in forest soils, potentially leading to an excess of nitrogen in the ecosystem, but the biotic and abiotic controls of this effect need further examination (species interactions, soil type). Here we studied nitrate leaching on plots of two contrasting textures (southern loamy and northern sandy site) planted with either pure Douglas fir, pure Norway spruce (Picea abies L. Karst.), pure European beech or a mixture of beech with either of the conifers. We used P80 suction lysimeters at 5 and 60 cm soil depth and collected soil solution over two continuous years to estimate nitrate leaching risks. We found highest concentrations of nitrate in soil solution in lower soil layers under Douglas fir (29.14 mg/L), which corroborates the findings of some studies conducted in pure stands. Comparisons of concentrations below the litter layer and below the main rooting zone implied that accumulation and microbial production of nitrate is taking place under Douglas fir. In Douglas fir-beech mixed stands, however, we found sustained significantly lower nitrate concentrations in soil solution below the main rooting zone (1.68 mg/L), implying a mixture effect. Furthermore, site played a key role in controlling nitrate concentrations in soil solution under Douglas fir on sites with finer soil texture and a lower C:N ratio. Elevated nitrate concentrations were associated with a P-limitation found in the trees, which likely caused reduced nitrate uptake. We discuss the results with regards to throughfall, litter, soil and microbial characteristics. We conclude that increased nitrate concentrations under Douglas fir stands may pose a relatively higher nitrate leaching risk than Norway spruce and a considerably higher risk compared to beech stands. However, the low susceptibility to leaching under beech stands seems to be a strong effect trait in mixtures, diminishing the high leaching potential Douglas fir induces on some sites. Low leaching potential is key to sustaining adequate nutrition in temperate forests and reducing pollution of groundwater. Our findings strongly urge forestry experts to carefully assess site conditions and foster mixtures with European beech when planting Douglas fir.
•Tree species affect nitrogen cycling; Douglas fir can increase nitrate concentrations in soil solution, posing a risk for nitrate leaching.•When mixed with European beech, nitrate concentrations remain low.•On sandy, poorer sites, nitrate leaching was much lower than on loamy sites.•P-limitation of Douglas fir caused reduced N uptake, leading to N accumulation in soil solution.•Nitrate production by microorganisms was likely stimulated under Douglas fir.
Projected climate change will affect existing forests, as substantial changes are predicted to occur during their life spans. Species that have ample intraspecific genetic differentiation, such as ...Douglas-fir (
Pseudotsuga menziesii
(Mirb.) Franco), are expected to display population-specific growth responses to climate change. Using a mixed-effects modeling approach, we describe three-year height (HT) growth response to changes in climate of interior Douglas-fir populations. We incorporate climate information at the population level, yielding a model that is specific to both species and population. We use data from provenance tests from previous studies that comprised 236 populations from Idaho, Montana, and eastern Washington, USA. The most sensitive indicator of climate was the mean temperature of the coldest month. Population maximum HT and HT growth response to changes in climate were dependent on seed source climate. All populations had optimum HT growth when transferred to climates with warmer winters; those originating in sites with the warmest winters were taller across sites and had highest HT growth at transfer distances closest to zero; those from colder climates were shortest and had optimum HT growth when transferred the farthest. Although this differential response damped the height growth differences among populations, cold-climate populations still achieved their maximum growth at lower temperatures than warm-climate populations. The results highlight the relevance of understanding climate change impacts at the population level, particularly in a species with ample genetic variation among populations.
Improving our understanding of the potential of forest adaptation is an urgent task in the light of predicted climate change. Long‐term alternatives for susceptible yet economically important tree ...species such as Norway spruce (Picea abies) are required, if the frequency and intensity of summer droughts will continue to increase. Although Silver fir (Abies alba) and Douglas fir (Pseudotsuga menziesii) have both been described as drought‐tolerant species, our understanding of their growth responses to drought extremes is still limited. Here, we use a dendroecological approach to assess the resistance, resilience, and recovery of these important central Europe to conifer species the exceptional droughts in 1976 and 2003. A total of 270 trees per species were sampled in 18 managed mixed‐species stands along an altitudinal gradient (400–1200 m a.s.l.) at the western slopes of the southern and central Black Forest in southwest Germany. While radial growth in all species responded similarly to the 1976 drought, Norway spruce was least resistant and resilient to the 2003 summer drought. Silver fir showed the overall highest resistance to drought, similarly to Douglas fir, which exhibited the widest growth rings. Silver fir trees from lower elevations were more drought prone than trees at higher elevations. Douglas fir and Norway spruce, however, revealed lower drought resilience at higher altitudes. Although the 1976 and 2003 drought extremes were quite different, Douglas fir maintained consistently the highest radial growth. Although our study did not examine population‐level responses, it clearly indicates that Silver fir and Douglas fir are generally more resistant and resilient to previous drought extremes and are therefore suitable alternatives to Norway spruce; Silver fir more so at higher altitudes. Cultivating these species instead of Norway spruce will contribute to maintaining a high level of productivity across many Central European mountain forests under future climate change.
The two extreme droughts in 1976 and 2003 affected negatively the radial growth response of Norway spruce, Silver and Douglas fir in the Black forest at all elevations. The 1976 drought had a less pronounced effect than the 2003 summer drought; however, firs were noticeably more resistant and resilient to extreme drought than spruce. Spruce was the most affected species when comparing performances of drought indices, and Silver fir the least affected. Douglas fir showed consistently the highest growth rates.
Standing dead trees (snags) support multiple functions within forest ecosystems by providing vertical structure, contributing to nutrient flows and carbon cycling, and serving as habitat elements for ...a diversity of organisms. In many forest landscapes, managers often use snag creation to enhance structural diversity, particularly in areas where snag loss is high and natural snag recruitment is low. Despite snag creation being used across a range of forest types on both private and public lands, a dearth of long-term studies has led to uncertainty about which techniques work best to create snags and support deadwood-dependent organisms over long (>15 y) timescales. In this study, I assessed the long-term consequences of varied snag creation treatments applied to live Douglas-fir (Pseudotsuga menziesii) trees in two study areas near Coos Bay, in southwestern Oregon, USA. Treatments included chainsaw topping, fungal inoculation, topping + inoculation, and mechanical wounding at the base. Trees were revisited 18–20 y after treatment and nearly all focal trees (97.6%, n = 809) remained standing when relocated. Markers of decay – including whether a tree was broken, cracked along the bole, had peeling bark, or harbored shelf fungi – were most pronounced on trees that had experienced one of two chainsaw topping treatments, differentiated by the number of retained branch whorls. In contrast, limited decay was observed on trees subjected to fungal inoculation and mechanical wounding treatments, likely due to the slow pace of decay processes in injured trees. The same pattern held for both bark cover and cavity cover, the latter an index of created snag use by woodpeckers, a keystone group within forest ecosystems. Finally, adding fungal inoculation to chainsaw topping of trees led to little additional decay relative to chainsaw topping alone. These findings indicate that managers should choose snag creation methods that align with the timeframe they require decaying trees to be available to deadwood-dependent organisms. They also show that combining different snag creation approaches that vary in timing of tree mortality and decay is likely to provide a longer window of use by wildlife and require lower implementation costs. Although snag creation is a widespread management tool, additional research is needed to ascertain the effectiveness of snag treatments across space and time, and to quantify the tradeoffs between the ecological benefits and financial costs that result from intentional snag creation.
•Most trees (97.6%) were standing when assessed 18-20 y post-treatment.•Fungal inoculations and mechanical wounding had limited efficacy for snag creation.•Tree decay was greatest for chainsaw-topping alone or in combination.•Chainsaw-topping was the most effective means for long-term snag creation.•Combining different creation methods may provide longer use periods to wildlife.
Changes in tree growth rates can affect tree mortality and forest feedbacks to the global carbon cycle. As air temperature increases, evaporative demand also increases, increasing effective drought ...in forest ecosystems. Using a spatially comprehensive network of Douglas fir (Pseudotsuga menziesii) chronologies from 122 locations that represent distinct climate environments in the western United States, we show that increased temperature decreases growth via vapor pressure deficit (VPD) across all latitudes. Using an ensemble of global circulation models, we project an increase in both the mean VPD associated with the lowest growth extremes and the probability of exceeding these VPD values. As temperature continues to increase in future decades, we can expect deficit-related stress to increase and consequently Douglas fir growth to decrease throughout its US range.
Most temperate woody plants have a winter chilling requirement to prevent budburst during mid-winter periods of warm weather. The date of spring budburst is dependent on both chilling and forcing; ...modeling this date is an important part of predicting potential effects of global warming on trees. There is no clear evidence from the literature that the curves of chilling or forcing effectiveness differ by species so we combined our data and published information to develop new curves on the effectiveness of temperature for chilling and forcing. The new curves predict effectiveness over a wide range of temperatures and we suggest both functions may be operating at the same time. We present experimental data from 13 winter environments for 5 genotypes of Douglas-fir (
Pseudotsuga menziesii var.
menziesii) and use them to test various assumptions of starting and stopping dates for accumulating chilling and forcing units and the relationship between budburst and the accumulation of chilling and forcing units. Chilling started too early to be effective in one treatment but the other 12 environments resulted in budburst from many combinations of chilling and forcing. Previous reports have suggested benefits or cancellations of effects from alternating day/night or periodic temperatures. Our simple models do not include these effects but nevertheless were effective in predicting relationships between chilling and forcing for treatments with a wide range of conditions. Overall, the date of budburst changed only slightly (+1 to −11 days) across a wide range of treatments in our colder test environment (Olympia, WA, USA) but was substantially later (+29 days) in the warmest treatment in our warmer environment (Corvallis, OR, USA). An analysis of historical climate data for both environments predicted a wide range in date to budburst could result from the same mean temperature due to the relative weightings of specific temperatures in the chilling and forcing functions. In the absence of improved understanding of the basic physiological mechanisms involved in dormancy induction and release, we suggest that simple, universal functions be considered for modeling the effectiveness of temperature for chilling and forcing. Future research should be designed to determine the exact shape of the curves; data are particularly lacking at the temperature extremes. We discuss the implications of our data and proposed functions for predicting effects of climate change. Both suggest that the trend toward earlier budburst will be reversed if winter temperatures rise substantially.
•Douglas fir cannot compete with broadleaves in productive European beech forests.•Abundant Douglas fir regeneration was found on dry and less productive forest sites.•Monitoring of Douglas fir ...regeneration is recommended on dry, nutrient-poor sites.
Douglas fir (Pseudotsuga menziesii) has regenerated naturally in European forests since the middle of the 20th century. Some cases of an invasive character of the species have been reported under specific site conditions, but systematic data on the extent of natural regeneration and spread of Douglas fir across different forest communities are largely lacking. Due to its potential tolerance to increasing summer droughts, Douglas fir has been suggested as a sustainable future tree species for Central European forests. In this study, we investigated natural regeneration of Douglas fir in comparison to native tree species in 39 forest stands in Switzerland belonging to different forest communities. We analyzed the regeneration success of Douglas fir, Norway spruce (Picea abies), silver fir (Abies alba), and European beech (Fagus sylvatica) with respect to ecological site conditions. The proportion of Douglas fir seedlings (<130 cm height) was <5% in the majority of stands; but in four stands, Douglas fir was the most abundant species among seedlings. In most other stands, Norway spruce (occurring in 37 stands/dominant in 10 stands), silver fir (38/2 stands) and beech seedlings (35/10 stands) were more abundant than Douglas fir seedlings. Saplings (≥130 cm height but <12 cm diameter at breast height) of Douglas fir were observed in five stands with proportions between 10% and 23% and in eight stands with proportions of <10%, in particular in stands with a high proportion of Douglas fir seed trees. Beech saplings occurred in 28 stands and were most abundant in eleven stands. Saplings of silver fir (24/3 stands) and Norway spruce (19/6 stands) were less frequent. The abundance of Douglas fir seedlings correlated positively with the proximity to seed trees and light transmission of the canopy, but negatively with understory vegetation cover and litter abundance. Ungulate browsing did not significantly affect the regeneration of any tree species in the study stands. On the Central Plateau, Douglas fir was mainly planted in productive beech forest communities where it is strongly limited due to its low competitiveness compared to beech and other fast-growing deciduous tree species. On dry and less productive sites, where the canopy is not closed, Douglas fir is able to establish successfully. On such sites, a close monitoring of Douglas fir regeneration and the potential implementation of control measures is recommended.
Aim Climate warming and increased wildfire activity are hypothesized to catalyse biogeographical shifts, reducing the resilience of fire‐prone forests world‐wide. Two key mechanisms underpinning ...hypotheses are: (1) reduced seed availability in large stand‐replacing burn patches, and (2) reduced seedling establishment/survival after post‐fire drought. We tested for regional evidence consistent with these mechanisms in an extensive fire‐prone forest biome by assessing post‐fire tree seedling establishment, a key indicator of forest resilience. Location Subalpine forests, US Rocky Mountains. Methods We analysed post‐fire tree seedling establishment from 184 field plots where stand‐replacing forest fires were followed by varying post‐fire climate conditions. Generalized linear mixed models tested how establishment rates varied with post‐fire drought severity and distance to seed source (among other relevant factors) for tree species with contrasting post‐fire regeneration adaptations. Results Total post‐fire tree seedling establishment (all species combined) declined sharply with greater post‐fire drought severity and with greater distance to seed sources (i.e. the interior of burn patches). Effects varied among key species groups. For conifers that dominate present‐day subalpine forests (Picea engelmannii, Abies lasiocarpa), post‐fire seedling establishment declined sharply with both factors. One exception was serotinous Pinus contorta, which did not vary with either factor. For montane species expected to move upslope under future climate change (Larix occidentalis, Pseudotsuga menziesii, Populus tremuloides) and upper treeline species (Pinus albicaulis), establishment was unrelated to either factor. Greater post‐fire tree seedling establishment on cooler/wetter aspects suggested local topographic refugia during post‐fire droughts. Main conclusions If future drought and wildfire patterns manifest as expected, post‐fire tree seedling establishment of species that currently characterize subalpine forests could be substantially reduced. Compensatory increases from lower montane and upper treeline species may partially offset these reductions, but our data suggest important near‐ to mid‐term shifts in the composition and structure of high‐elevation forests under continued climate warming and increased wildfire activity.
Mixed-species plantations have been suggested as ecologically and economically viable alternatives to monocultures. We examined the growth response of coastal Douglas-fir (Pseudotsuga menziesii var. ...menziesii (Mirb.) Franco) and western redcedar (Thuja plicata Donn ex D. Don in Lamb.) to different species mixtures (Douglas-fir : western redcedar mixtures of 1:0, 1:1, 1:3, and 0:1) and planting densities (500, 1000, and 1500 stems·ha
−1
) in a dry maritime forest on eastern Vancouver Island, Canada. Twenty-two years postplanting, species mixture significantly affected diameter and height growth (p < 0.001), with stand diameter and height generally decreasing with increasing redcedar composition. Inherent variation in soil productivity across the plantation (carbon:nitrogen ratio) equally constrained stand growth. The widest spacing had larger diameter than the closest spacing (p = 0.025) but the least stand basal area compared with the other spacing treatments (p = 0.003–0.031). Stand volume was significantly affected by mixture × density interaction (p = 0.024) and generally declined with increasing proportion of redcedar and decreasing stand density. In the first decades after plantation establishment, inherent species growth traits and soil fertility were most important in dictating stand productivity. Because of the differences in resource utilization of both species, stand dynamics may change as competition for light and soil resources increases. Evaluation of silvicultural recommendations regarding mixtures of both species will continue with plantation development.
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