Control of Nitrification by Tree Species in a Common-Garden Experiment

We studied the effect of tree species on nitrification in five young plantations and an old native beech coppice forest at the Breuil experimental site in central France. The potential net nitrification (PNN) of soil was high in beech, Corsican pine, and Douglas fir plantations (high nitrifying stands denoted H) and low in spruce and Nordmann fir plantations as well as in native forest stands (low nitrifying stands denoted L). We hypothesized that tree species would stimulate or inhibit nitrification in transplanted soil cores within a few years after the cores were transplanted between stands. We first initiated a transplant experiment where soil cores were exchanged between all stands. The PNN remained high in soil cores from H transferred to H and low in soil cores from L transferred to L. The PNN increased considerably after 16 months in soil cores transferred from L to H, whereas the transfer of soil cores from H to L decreased the PNN only slightly after 28 months. In a second transplant experiment, forest floor material was exchanged between the Douglas fir (H) and the native forest (L) stand. Six months later, the forest floor from the native forest had increased the PNN of the Douglas fir soil considerably, whereas the forest floor from Douglas fir did not affect the PNN of the soil in the native forest stand. It was concluded that beech, Corsican pine, and Douglas fir rapidly stimulate soil nitrification by either activation of suppressed nitrifier communities and/or colonization by new nitrifier communities. Conversely, the slow and irregular reduction of nitrification in spruce, Nordmann fir, and native forest was probably due to the low and heterogeneously distributed flux of inhibiting substances per volume of soil. Our experiments suggest that the inhibition of nitrification is not tightly connected to forest floor leachates, but that the forest floor both reflects and maintains the major ongoing processes. In the long term, humus build up and the production of inhibiting substances may completely block the nitrification activity.     

Tree species impact the terrestrial cycle of silicon through various uptakes

The quantification of silicon (Si) uptake by tree species is a mandatory step to study the role of forest vegetations in the global cycle of Si. Forest tree species can impact the hydrological output of dissolved Si (DSi) through root induced weathering of silicates but also through Si uptake and restitution via litterfall. Here, monospecific stands of Douglas fir, Norway spruce, Black pine, European beech and oak established in identical soil and climate conditions were used to quantify Si uptake, immobilization and restitution. We measured the Si contents in various compartments of the soil–tree system and we further studied the impact of the recycling of Si by forest trees on the DSi pool. Si is mainly accumulated in leaves and needles in comparison with other tree compartments (branches, stembark and stemwood). The immobilization of Si in tree biomass represents less than 15% of the total Si uptake. Annual Si uptake by oak and European beech stands is 18.5 and 23.3 kg ha^?1 year^?1, respectively. Black pine has a very low annual Si uptake (2.3 kg ha^?1 year^?1) in comparison with Douglas fir (30.6 kg ha^?1 year^?1) and Norway spruce (43.5 kg ha^?1 year^?1). The recycling of Si by forest trees plays a major role in the continental Si cycle since tree species greatly influence the uptake and restitution of Si. Moreover, we remark that the annual tree uptake is negatively correlated with the annual DSi output at 60 cm depth. The land–ocean fluxes of DSi are certainly influenced by geochemical processes such as weathering of primary minerals and formation of secondary minerals but also by biological processes such as root uptake.     

Elevated Cd and Zn uptake by aspen limits the phytostabilization potential compared to five other tree species

Phytostabilization of metals using trees is often promoted, although the influence of different tree species on the mobilization of metals is not yet clear. This study examined effects of six tree species on the soil characteristics pH, organic carbon (OC) content and cation exchange capacity (CEC) and on the redistribution of cadmium (Cd) and zinc (Zn) on a polluted sandy soil. Soil and biomass were sampled in 10-year-old stands growing on former agricultural land. The tree species included were silver birch (Betula pendula), oak (Quercus robur and Quercus petraea), black locust (Robinia pseudoacacia), aspen (Populus tremula), Scots pine (Pinus sylvestris) and Douglas fir (Pseudotsuga menziesii). In the short period of 10 years, only aspen caused significant changes in the soil characteristics. Due to accumulation of Cd and Zn in its leaf litter, aspen increased the total as well as the NH₄OAc-EDTA-extractable Cd and Zn concentrations in the topsoil compared to deeper soil layers and to other tree species. Also, topsoil pH, OC content and CEC were significantly higher than under most of the other species. This caused rather low ‘bioavailable’ CaCl₂-extractable concentrations under aspen. Nevertheless, given the risks of aboveground metal dispersion and topsoil accumulation, it is recommended that aspen should be avoided when afforesting Cd and Zn contaminated lands.     

What explains landscape patterns of tree mortality caused by bark beetle outbreaks in Greater Yellowstone?

Aim Bark beetle outbreaks have recently affected extensive areas of western North American forests, and factors explaining landscape patterns of tree mortality are poorly understood. The objective of this study was to determine the relative importance of stand structure, topography, soil characteristics, landscape context (the characteristics of the landscape surrounding the focal stand) and beetle pressure (the abundance of local beetle population eruptions around the focal stand a few years before the outbreak) to explain landscape patterns of tree mortality during outbreaks of three species: the mountain pine beetle, which attacks lodgepole pine and whitebark pine; the spruce beetle, which feeds on Engelmann spruce; and the Douglas‐fir beetle, which attacks Douglas‐fir. A second objective was to identify common variables that explain tree mortality among beetle–tree host pairings during outbreaks. Location Greater Yellowstone ecosystem, Wyoming, USA. Methods We used field surveys to quantify stand structure, soil characteristics and topography at the plot level in susceptible stands of each forest type showing different severities of infestation (0–98% mortality; n= 129 plots). We then used forest cover and beetle infestation maps derived from remote sensing to develop landscape context and beetle pressure metrics at different spatial scales. Plot‐level and landscape‐level variables were used to explain outbreak severity. Results Engelmann spruce and Douglas‐fir mortality were best predicted using landscape‐level variables alone. Lodgepole pine mortality was best predicted by both landscape‐level and plot‐level variables. Whitebark pine mortality was best – although poorly – predicted by plot‐level variables. Models including landscape context and beetle pressure were much better at predicting outbreak severity than models that only included plot‐level measures, except for whitebark pine. Main conclusions Landscape‐level variables, particularly beetle pressure, were the most consistent predictors of subsequent outbreak severity within susceptible stands of all four host species. These results may help forest managers identify vulnerable locations during ongoing outbreaks.     

Soil and tree chemistry reflected the cumulative impact of acid deposition in Pinus banksiana and Populus tremuloides stands in the Athabasca oil sands region in western Canada

The spatial variability of soil chemistry and Ca/Al ratios of soil solution and fine roots were investigated in jack pine (Pinus banksiana) and trembling aspen (Populus tremuloides, aspen) stands to assess the impact of chronic acid deposition on boreal forest ecosystems in the Athabasca oil sands region (AOSR) in Alberta, Canada. Available SO₄²⁻ (as the sum of soluble and adsorbed SO₄²⁻) accumulated in the soil near tree boles of both species, reflecting the influence of canopy intercepted SO₄²⁻. In jack pine stands, pH and soluble base cation concentrations decreased towards tree boles due to increased SO₄²⁻ leaching; the reverse was found in aspen stands due to deposition of base cations leached from the canopy. As a result, Ca/Al ratios in the soluble fraction in soils near jack pine boles were 5–20 times lower than that near aspen boles. The Ca/Al ratio did not reach the critical limits of 1.0 for soil solution (ranged from 1.0 to 4.1) or 0.5 for fine roots (0.7–7.9) in the studied watersheds. However, Alⁿ⁺ concentrations in the soil solution ranged from 0.2 to 4.1 mg L⁻¹ in NE7 and from 0.1 to 8.5 mg L⁻¹ in SM8 that can inhibit the growth of white spruce (Picea glauca) seedlings that commonly succeed aspen in upland sites in the AOSR. We suggest that the spatial variation caused by tree canopies/stems will affect forest regeneration and the effect of acid deposition on forest succession in the AOSR should be further studied.     

Monoterpene emission from coniferous trees in response to elevated CO2 concentration and climate warming

It was hypothesized that high CO₂ availability would increase monoterpene emission to the atmosphere. This hypothesis was based on resource allocation theory which predicts increased production of plant secondary compounds when carbon is in excess of that required for growth. Monoterpene emission rates were measured from needles of (a) Ponderosa pine grown at different CO₂ concentrations and soil nitrogen levels, and (b) Douglas fir grown at different CO₂ concentrations. Ponderosa pine grown at 700 μmol mol^–1 CO₂ exhibited increased photosynthetic rates and needle starch to nitrogen (N) ratios when compared to trees grown at 350 μmol mol^–1 CO₂. Nitrogen availability had no consistent effect on photosynthesis. Douglas fir grown at 550 μmol mol^–1 CO₂ exhibited increased photosynthetic rates as compared to growth at 350 μmol mol^–1 CO₂ in old, but not young needles, and there was no influence on the starch/N ratio. In neither species was there a significant effect of elevated growth CO₂ on needle monoterpene concentration or emission rate. The influence of climate warming and leaf area index (LAI) on monoterpene emission were also investigated. Douglas fir grown at elevated CO₂ plus a 4 °C increase in growth temperature exhibited no change in needle monoterpene concentration, despite a predicted 50% increase in emission rate. At elevated CO₂ concentration the LAI increased in Ponderosa pine, but not Douglas fir. The combination of increased LAI and climate warming are predicted to cause an 80% increase in monoterpene emissions from Ponderosa pine forests and a 50% increase in emissions from Douglas fir forests. This study demonstrates that although growth at elevated CO₂ may not affect the rate of monoterpene emission per unit biomass, the effect of elevated CO₂ on LAI, and the effect of climate warming on monoterpene biosynthesis and volatilization, could increase canopy monoterpene emission rate.     

Establishment of

We compared establishment of Douglas fir (Pseudotsuga menziesii) and Corsican pine (Pinus nigra) seedlings in kanuka (Kunzea ericoides) and manuka (Leptospermum scoparium) shrubland to test the hypothesis that Douglas fir, because of its greater shade tolerance, is better able to establish in woody communities than pine species. Seed of the conifer species was sown under a range of canopy covers at six sites, the cover being low-statured vegetation in openings between stands, stand edges, and moderate and dense canopies. After three growing seasons, survival of Corsican pine seedlings was greatest in the open and declined progressively as canopy cover increased. This contrasted with Douglas fir, where survival was greatest at the canopy edge. Survival of Douglas fir seedlings significantly exceeded that of Corscican pine seedlings under dense canopy positions. Seedling numbers of both species declined significantly with increasing leaf area index of manuka, but not kanuka stands, where seedling numbers were lower. Leaf area index of manuka stands accounted for substantially greater variation in number and survival of Corsican pine than Douglas fir seedlings. It is concluded that Douglas fir is better able to establish in shaded environments in woody communities than Corsican pine; however, further monitoring is required to confirm the long-term survival of both species under the moderate and dense canopy positions in this trial.     

Temperature and vegetation effects on soil organic carbon quality along a forested mean annual temperature gradient in North America

Both climate and plant species are hypothesized to influence soil organic carbon (SOC) quality, but accurate prediction of how SOC process rates respond to global change will require an improved understanding of how SOC quality varies with mean annual temperature (MAT) and forest type. We investigated SOC quality in paired hardwood and pine stands growing in coarse textured soils located along a 22 °C gradient in MAT. To do this, we conducted 80‐day incubation experiments at 10 and 30 °C to quantify SOC decomposition rates, which we used to kinetically define SOC quality. We used these experiments to test the hypotheses that SOC quality decreases with MAT, and that SOC quality is higher under pine than hardwood tree species. We found that both SOC quantity and quality decreased with increasing MAT. During the 30 °C incubation, temperature sensitivity (Q₁₀) values were strongly and positively related to SOC decomposition rates, indicating that substrate supply can influence temperature responsiveness of SOC decomposition rates. For a limited number of dates, Q₁₀ was negatively related to MAT. Soil chemical properties could not explain observed patterns in soil quality. Soil pH and cation exchange capacity (CEC) both declined with increasing MAT, and soil C quality was positively related to pH but negatively related to CEC. Clay mineralogy of soils also could not explain patterns of SOC quality as complex (2 : 1), high CEC clay minerals occurred in cold climate soils while warm climate soils were dominated by simpler (1 : 1), low CEC clay minerals. While hardwood sites contained more SOC than pine sites, with differences declining with MAT, clay content was also higher in hardwood soils. In contrast, there was no difference in SOC quality between pine and hardwood soils. Overall, these findings indicate that SOC quantity and quality may both decrease in response to global warming, despite long‐term changes in soil chemistry and mineralogy that favor decomposition.     

Control of Nitrous Oxide Emissions in European Beech,Norway Spruce and Scots Pine Forests

Elevated nitrogen deposition has increased tree growth, the storage of soil organic matter, and nitrate leaching in many European forests, but little is known about the effect of tree species and nitrogen deposition on nitrous oxide emission. Here we report soil N₂O emission from European beech, Scots pine and Norway spruce forests in two study areas of Germany with distinct climate, N deposition and soils. N₂O emissions and throughfall input of nitrate and ammonium were measured biweekly during growing season and monthly during dormant season over a 28 months period. Annual N₂O emission rates ranged between 0.4 and 1.3 kg N ha^?1 year^?1 among the stands and were higher in 1998 than in 1999 due to higher precipitation during the growing season of 1998. A 2-way-ANOVA revealed that N₂O fluxes were significantly higher (p<0.001) at Solling than at Unterlüß while tree species had no effect on N₂O emissions. Soil texture and the amount of throughfall explained together 94% of the variance among the stands, indicating that increasing portions of silt and clay may promote the formation of N₂O in wet forest soils. Moreover, cumulative N₂O fluxes were significantly correlated (r² = 0.60, p<0.001) with cumulative NO ₃ ^? fluxes at 10 cm depth as an indicator of N saturation, however, the slope of the regression curve indicates a rather weak effect of NO ₃ ^? fluxes on N₂O emissions. N input by throughfall was not correlated with N₂O emissions and only 1.6–3.2% of N input was released as N₂O to the atmosphere. Our results suggest that elevated N inputs have little effect on N₂O emissions in beech, spruce and pine forests.     

Comparative Study of Carbon Storage in Different Forest Stands in Subtropical China

Selection and development of tree species with high fixing CO₂ capacity is an increasing problem worldwide. A comparative study on carbon fixation ability of three forest stands was conducted at Linlong Mountain, Li’nan County, Zhejiang Province, China. The results showed that total carbon storage in the ecosystems of Moso bamboo, Chinese fir, and Masson pine stands were 104.83, 95.66, and 96.49 t C/ha, respectively. The spatial distribution of carbon storage in the three ecosystems decreased in the order: soil > tree story > the vegetation under the forests. Carbon storage in the soils under Moso bamboo, Chinese fir, and Masson pine stands accounted for 65.3, 61.4, and 55.6% of the total CSs, respectively. The Moso bamboo forest ecosystem fixed 1.69 and 1.63 times as much C (9.64 t C/ha/year) as the Chinese fir and Masson pine forest ecosystems, respectively.     

The fate of15N enriched throughfall in two coniferous forest stands at different nitrogen deposition levels

The stable isotope¹⁵N was added as (¹⁵NH₄)₂SO₄ to throughfall water for one year, to study the fate of the deposited nitrogen at different levels of N deposition in two N saturated coniferous forests ecosystems in the Netherlands. The fate of the¹⁵N was followed at high-N (44–55 kg N ha^–1 yr^–1) 1) and low-N (4–6 kg N ha^–1 yr^–1) deposition in plots established under transparent roofs build under the canopy in a Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) and Scots pine (Pinus sylvestris L.) forest.The applied¹⁵N was detectable in needles and twigs, the soil and soil water leaching below the rooting zone (90 cm depth). Total¹⁵N recovery in major ecosystem compartments was 71–100% during two successive growing seasons after the start of a year-round¹⁵N application to throughfall-N. Nine months after the year-round¹⁵N application, the¹⁵N assimilated into tree biomass was 29–33% of the¹⁵N added in the Douglas fir stand and less than 17% in the Scots pine stand. At the same time total¹⁵N retention in the soil (down to 70 cm) of the high-N plots was about 37% of the deposited¹⁵NH₄-N, whereas 46% and 65% of the¹⁵N was found in the soil of the low-N deposition plots at the Douglas fir and Scots pine stand, respectively. The organic layers accounted for 60% of the¹⁵N retained in the soil. The total N deposition exceeded the demand of the vegetation and microbial immobilization. Total¹⁵N leaching losses within a year (below 90 cm) were 10–20% in the high-N deposition plots in comparison to 2–6% in the lowered nitrogen input plots. Relative retention in the soil and vegetation increased at lower N-input levels.Species differences in uptake and tree health seem to contribute to lower¹⁵N recoveries in the Scots pine trees compared to the Douglas fir trees. The excessive N deposition and resulting N saturation lead to conditions were the health and functioning of biota were negatively influenced. At decreased N deposition, lower leaching losses together with increased soil and plant retention indicated a change in the fate of the¹⁵N deposited. This may have resulted from changes in ecosystem processes, and thus a shift along the continuum of N saturation to N limitation.     

Differential selection of North American and Scandinavian conifer browse by northwestern moose (Alces alces andersoni) in winter

Scandinavian moose (Alces alces) eat Scots pine (Pinus sylvestris) in winter. Although North American moose are known to eat conifers such as true firs (Abies spp.) in winter, substantial consumption of pine by moose in North America has not been documented. Here, we document short-term winter preferences of human-habituated northwestern moose (Alces alces andersoni) for branches of mature North American and European conifer species as determined by a cafeteria-style feeding trial. Moose selected for species such as Douglas fir (Pseudotsuga menziesii; from which they took the smallest bite diameters) while avoiding species such as lodgepole pine (Pinus contorta; from which they took the largest bites) and hybrid white spruce (Picea glauca × engelmanii). The amount of species-specific biomass consumed by moose was negatively correlated with bite diameters taken from branches of those species and did not appear to be significantly influenced by differences in twig morphology between species. Our trial suggests that northwestern moose readily consume conifers in winter and, from the species we tested, prefer Douglas fir. While no clear preference existed between Scots pine and lodgepole pine, moose avoided lodgepole pine, but not Scots pine, relative to Douglas fir. Our trial suggests that northwestern moose are more likely to feed on the branches of Douglas fir than pine, which may be of interest to foresters managing conifers within the North American range of moose, particularly where Scots pine are being considered for planting.     

Natural 15N abundance in two nitrogen saturated forest ecosystems

Natural ¹⁵N abundance values were measured in needles, twigs, wood, soil, bulk precipitation, throughfall and soil water in a Douglas fir (Pseudotsuga menziesii (Mirb.) and a Scots pine (Pinus sylvestris L.) stand receiving high loads of nitrogen in throughfall (>50 kg N ha⁻¹ year⁻¹). In the Douglas fir stand δ¹⁵N values of the vegetation ranged between −5.7 and −4.2‰ with little variation between different compartments. The vegetation of the Scots pine stand was less depleted in ¹⁵N and varied from −3.3 to −1.2‰δ¹⁵N. At both sites δ¹⁵N values increased with soil depth, from −5.7‰ and −1.2‰ in the organic layer to +4.1‰ and +4.7‰ at 70 cm soil depth in the Douglas fir and Scots pine stand, respectively. The δ¹⁵N values of inorganic nitrogen in bulk precipitation showed a seasonal variation with a mean in NH₄ ⁺-N of −0.6‰ at the Douglas fir stand and +10.8‰ at the Scots pine stand. In soil water below the organic layer NH₄ ⁺-N was enriched and NO₃ ⁻-N depleted in ¹⁵N, which was interpreted as being caused by isotope fractionation accompanying high nitrification rates in the organic layers. Mean δ¹⁵N values of NH₄ ⁺ and NO₃ ⁻ were very similar in the drainage water at 90 cm soil depth at both sites (−7.1 to −3.8‰). A dynamic N cycling model was used to test the sensitivity of the natural abundance values for the amount of N deposition, the ¹⁵N ratio of atmospheric N deposited and for the intrinsic isotope discrimination factors associated with N transformation processes. Simulated δ¹⁵N values for the N saturated ecosystems appeared particularly sensitive to the ¹⁵N ratio of atmospheric N inputs and discrimination factors during nitrification and mineralization. The N-saturated coniferous forest ecosystems studied were not characterized by elevated natural ¹⁵N abundance values. The results indicated that the natural ¹⁵N abundance values can only be used as indicators for the stage of nitrogen saturation of an ecosystem if the δ¹⁵N values of the deposited N and isotope fractionation factors are taken into consideration. Combining dynamic isotope models and natural ¹⁵N abundance values seems a promising technique for interpreting natural ¹⁵N abundance values found in these forest ecosystems. Received: 5 May 1996 / Accepted: 10 April 1997     

Tree‐ring stable isotopes record the impact of a foliar fungal pathogen on CO2 assimilation and growth in Douglas‐fir

Swiss needle cast (SNC) is a fungal disease of Douglas‐fir (Pseudotsuga menziesii) that has recently become prevalent in coastal areas of the Pacific Northwest. We used growth measurements and stable isotopes of carbon and oxygen in tree‐rings of Douglas‐fir and a non‐susceptible reference species (western hemlock, Tsuga heterophylla) to evaluate their use as proxies for variation in past SNC infection, particularly in relation to potential explanatory climate factors. We sampled trees from an Oregon site where a fungicide trial took place from 1996 to 2000, which enabled the comparison of stable isotope values between trees with and without disease. Carbon stable isotope discrimination (Δ¹³C) of treated Douglas‐fir tree‐rings was greater than that of untreated Douglas‐fir tree‐rings during the fungicide treatment period. Both annual growth and tree‐ring Δ¹³C increased with treatment such that treated Douglas‐fir had values similar to co‐occurring western hemlock during the treatment period. There was no difference in the tree‐ring oxygen stable isotope ratio between treated and untreated Douglas‐fir. Tree‐ring Δ¹³C of diseased Douglas‐fir was negatively correlated with relative humidity during the two previous summers, consistent with increased leaf colonization by SNC under high humidity conditions that leads to greater disease severity in following years.     

Resin and the Resistance of Conifers to Fomes annosus

Roots of young trees of different conifer species, Scots pine,Corsican pine, Sitka spruce, and Douglas fir were inoculatedwith Fomes annosus. In the pines, infection was negligible butin the other genera the central wood was extensively invaded.The process of infection was studied further in Sitka spruceby means of a root-severing technique in which an inoculum blockwas forced between the cut ends of a root. In most trees a centralrot developed which in some extended up into the butt: the meanextent of growth recorded was 30 cm in 14 months. In the severedroot portions infection also occurred but F. annosus was subsequentlyreplaced to a considerable extent by other micro-organisms,particularly in trees growing on a peat soil. A similar centralrot developed after inoculations of Norway spruce. The resistance of pine sapwood and the outer wood of spruceand Douglas fir can be explained at least partially by the structureand viability of the resin duct system. Larch, Douglas fir,spruce, and pine form a series showing increasing complexityand activity of the resin system. In addition, studies carriedout on the resin yields of pines growing under different conditionsshowed that the resistance of a tree to F. annosus is correlatedwith its ability to mobilize resin. Site factors, forest management,host age vigour, genetic constitution, and past history mayall influence resin mobilization.     

Changes to the N cycle following bark beetle outbreaks in two contrasting conifer forest types

Outbreaks of Dendroctonus beetles are causing extensive mortality in conifer forests throughout North America. However, nitrogen (N) cycling impacts among forest types are not well known. We quantified beetle-induced changes in forest structure, soil temperature, and N cycling in Douglas-fir (Pseudotsuga menziesii) forests of Greater Yellowstone (WY, USA), and compared them to published lodgepole pine (Pinus contorta var. latifolia) data. Five undisturbed stands were compared to five beetle-killed stands (4–5 years post-outbreak). We hypothesized greater N cycling responses in Douglas-fir due to higher overall N stocks. Undisturbed Douglas-fir stands had greater litter N pools, soil N, and net N mineralization than lodgepole pine. Several responses to disturbance were similar between forest types, including a pulse of N-enriched litter, doubling of soil N availability, 30–50 % increase in understory cover, and 20 % increase in foliar N concentration of unattacked trees. However, the response of some ecosystem properties notably varied by host forest type. Soil temperature was unaffected in Douglas-fir, but lowered in lodgepole pine. Fresh foliar %N was uncorrelated with net N mineralization in Douglas-fir, but positively correlated in lodgepole pine. Though soil ammonium and nitrate, net N mineralization, and net nitrification all doubled, they remained low in both forest types (<6 μg N g soil^?1 NH₄ ⁺or NO₃ ^?; <25 μg N g soil^?1 year^?1 net N mineralization; <8 μg N g soil^?1 year^?1 net nitrification). Results suggest that beetle disturbance affected litter and soil N cycling similarly in each forest type, despite substantial differences in pre-disturbance biogeochemistry. In contrast, soil temperature and soil N–foliar N linkages differed between host forest types. This result suggests that disturbance type may be a better predictor of litter and soil N responses than forest type due to similar disturbance mechanisms and disturbance legacies across both host–beetle systems.     

Rapid Increases in Forest Understory Diversity and Productivity following a Mountain Pine Beetle (Dendroctonus ponderosae) Outbreak in Pine Forests

The current unprecedented outbreak of mountain pine beetle (Dendroctonus ponderosae) in lodgepole pine (Pinus contorta) forests of western Canada has resulted in a landscape consisting of a mosaic of forest stands at different stages of mortality. Within forest stands, understory communities are the reservoir of the majority of plant species diversity and influence the composition of future forests in response to disturbance. Although changes to stand composition following beetle outbreaks are well documented, information on immediate responses of forest understory plant communities is limited. The objective of this study was to examine the effects of D. ponderosae-induced tree mortality on initial changes in diversity and productivity of understory plant communities. We established a total of 110 1-m² plots across eleven mature lodgepole pine forests to measure changes in understory diversity and productivity as a function of tree mortality and below ground resource availability across multiple years. Overall, understory community diversity and productivity increased across the gradient of increased tree mortality. Richness of herbaceous perennials increased with tree mortality as well as soil moisture and nutrient levels. In contrast, the diversity of woody perennials did not change across the gradient of tree mortality. Understory vegetation, namely herbaceous perennials, showed an immediate response to improved growing conditions caused by increases in tree mortality. How this increased pulse in understory richness and productivity affects future forest trajectories in a novel system is unknown.     

Nitrogen metabolism of Douglas fir and Scots pine as affected by optimal nutrition and water supply under conditions of relatively high atmospheric nitrogen deposition

Nitrogen metabolism of the needles of 40-year-old Douglas fir and Scots pine trees, growing in two forest stands on cation-poor and acidic sandy soil with a relatively high atmospheric nitrogen deposition was studied. The composition of the free amino acid (FAA) pool, the concentrations of total nitrogen and soluble protein and the activities of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) were determined in the needles. An excessive nitrogen supply by a high atmospheric nitrogen deposition in both forest stands was indicated by the high concentrations of total nitrogen and the amino acids arginine, glutamic acid, glutamine and aspartic acid in control trees. In addition the effect of optimal nutrition and water supply (fertigation) on the needle nitrogen metabolism was evaluated. The total concentration of the FAA pool in needles of both tree species was lower in the fertigated than in the non-fertigated (control) trees, except for 1-year-old needles of Scots pine, in which the concentration after fertigation did not differ from the control. The lower total FAA concentration in the fertigated trees could be attributed to arginine, the concentration of which was on average 60% lower than in the control. Neither the concentration of soluble protein nor the activity of GS were influenced by fertigation. The activity of GDH in fertigated trees only differed significantly from the control in October. Scots pine needles had higher concentrations of protein (50%) and higher activities of GS (44%) and GDH (25%) than Douglas fir needles. Possible explanations for the lower vitality of Douglas fir compared to Scots pine are given.     

Early colonization of red alder and Douglas fir by ectomycorrhizal fungi and Frankia in soils from the Oregon coast range

The potential for mycorrhizal formation and Frankia nodulation were studied in soils from six sites in the Pacific Northwest. The sites included young and old alder stands, a 1-year-old conifer clear-cut, a young conifer plantation, and rotation-aged and old-growth conifer stands. A bioassay procedure was used with both red alder and Douglas fir seedlings as hosts. After 6 weeks growth, seedlings of both hosts were harvested every 3 weeks for 21 weeks and numbers of nodules and ectomycorrhizal types estimated. Nodules formed on red alder and ectomycorrhizae formed on both alder and Douglas fir in soil from all sites. Nodulation potential was highest in soil from the alder stands and the conifer plantation. Seven morphologically distinct ectomycorrhizal types were recovered on Douglas fir and five on alder. Only Thelephora terrestris, a broad-host-range mycobiont, formed mycorrhizae on both hosts. New ectomycorrhizal types formed on both hosts throughout the bioassay. Ectomycorrhizal colonization of alder was greatest in the alder and clear-cut soils. Low ectomycorrhizal colonization on alder was found in soils from sites where conifers were actively growing. Ectomycorrhizal colonization of Douglas fir was highest in the young alder and conifer plantation soils and was low in the rotation-aged conifer soil. The highest diversity of ectomycorrhizal types was found on alder in the conifer clear-cut soil and on Douglas fir in the rotation-aged conifer soil. Effects of host specificity, nodulation and mycorrhiza-forming potential and nodule-mycorrhiza interactions on seedling establishment are discussed in relation to seral stage dynamics and attributes of pioneer ectomycorrhizal fungal species.     

Assessing forest vulnerability to climate warming using a process‐based model of tree growth: bad prospects for rear‐edges

Growth models can be used to assess forest vulnerability to climate warming. If global warming amplifies water deficit in drought‐prone areas, tree populations located at the driest and southernmost distribution limits (rear‐edges) should be particularly threatened. Here, we address these statements by analyzing and projecting growth responses to climate of three major tree species (silver fir, Abies alba; Scots pine, Pinus sylvestris; and mountain pine, Pinus uncinata) in mountainous areas of NE Spain. This region is subjected to Mediterranean continental conditions, it encompasses wide climatic, topographic and environmental gradients, and, more importantly, it includes rear‐edges of the continuous distributions of these tree species. We used tree‐ring width data from a network of 110 forests in combination with the process‐based Vaganov–Shashkin‐Lite growth model and climate–growth analyses to forecast changes in tree growth during the 21st century. Climatic projections were based on four ensembles CO₂ emission scenarios. Warm and dry conditions during the growing season constrain silver fir and Scots pine growth, particularly at the species rear‐edge. By contrast, growth of high‐elevation mountain pine forests is enhanced by climate warming. The emission scenario (RCP 8.5) corresponding to the most pronounced warming (+1.4 to 4.8 °C) forecasted mean growth reductions of ?10.7% and ?16.4% in silver fir and Scots pine, respectively, after 2050. This indicates that rising temperatures could amplify drought stress and thus constrain the growth of silver fir and Scots pine rear‐edge populations growing at xeric sites. Contrastingly, mountain pine growth is expected to increase by +12.5% due to a longer and warmer growing season. The projections of growth reduction in silver fir and Scots pine portend dieback and a contraction of their species distribution areas through potential local extinctions of the most vulnerable driest rear‐edge stands. Our modeling approach provides accessible tools to evaluate forest vulnerability to warmer conditions.