Purification and properties of glutamine synthetase from Douglas fir roots

Glutamine synthetase (GS. EC 6.3.1.2) was purified to apparent electrophoretic homogeneity from roots of Pseudotsuga menziesii (Mirb) Franco by a three-step procedure involving diethylaminoethyl (DEAE)-Trisacryl chromatography, affinity chromatography on Matrex Gel Red A. and preparative polyacrylamide gel electrophoresis. The enzyme was purified 40-fold with a 16% recovery. The native enzyme had a molecular mass of 460 ± 5 kDa as estimated by gel filtration, interpolation of the Ferguson plots and non-denaturing gradient-PAGE. It was composed of two different subunits of 54 and 64 kDa. Affinity constants for glutamate (Glu), glutamine (Gln), ATP and ADP were 2.6, 10.5, 0.5 and 0.083 m M . respectively. The enzyme exhibited a negative cooperativity for ammonium (Hill number of 0.7) with two K_m values which were 11 and 75 μ M in the presence of ammonium concentrations lower and higher than 1.3 m M , respectively. Glycine and ADP appeared as potential inhibitors of the GS activity. The optimum pH values were 7.2 and 7.6 for the transferase and the biosynthetic assays, respectively. The enzyme lost 30% of its activity within 25 days of storage at 4°C. The optimum temperatures of activity were 40°C and 45°C for the transferase and bio-synthetic activities, respectively.     

Multiple-host fungi are the most frequent and abundant ectomycorrhizal types in a mixed stand of Douglas fir (Pseudotsuga menziesii) and bishop pine (Pinus muricata)

The ectomycorrhizal fungal associations of Douglas fir ( Pseudotsuga menziesii D. Don) and bishop pine ( Pinus muricata D. Don) were investigated in a mixed forest stand. We identified fungi directly from field-collected ectomycorrhizal (ECM) root tips using PCR-based methods. Sixteen species of fungi were found, of which twelve associated with both hosts. Rhizopogon parksii Smith was specific to Douglas fir. Three other species colonized only one of the hosts, but were too infrequent to draw conclusions about specificity. Seventy-four percent of the biomass of ECM root tips sampled in the stand were colonized by members of the Thelephoraceae and Russulaceae. All 12 species of fungi that associated with both tree species did so within a 10×40 cm soil volume, suggesting that individual fungal genotypes linked the putatively competing tree hosts.     

Ecological effects of changes in fire regimes in Pinus ponderosa ecosystems in the Colorado Front Range

Question: What is the relative importance of low‐ and high‐severity fires in shaping forest structure across the range of Pinus ponderosa in northern Colorado? Location: Colorado Front Range, USA. Methods: To assess severities of historic fires, 24 sites were sampled across an elevation range of 1800 to 2800 m for fire scars, tree establishment dates, tree mortality, and changes in tree‐ring growth. Results: Below 1950 m, the high number of fire scars, scarcity of large post‐fire cohorts, and lack of synchronous tree mortality or growth releases, indicate that historic fires were of low severity. In contrast, above 2200 m, fire severity was greater but frequency of widespread fires was substantially less. At 18 sites above 1950 m, 34 to 80% of the live trees date from establishment associated with the last moderate‐ to high‐severity fire. In these 18 sites, only 2 to 52% of the living trees pre‐date these fires suggesting that fire severities prior to any effects of fire suppression were sufficient to kill many trees. Conclusions: These findings for the P. ponderosa zone above ca. 2200 m (i.e. most of the zone) contradict the widespread perception that fire exclusion, at least at the stand scale of tens to hundreds of hectares, has resulted in unnaturally high stand densities or in an atypical abundance of shade‐tolerant species. At relatively mesic sites (e.g. higher elevation, north‐facing), the historic fire regime consisted of a variable‐severity regime, but forest structure was shaped primarily by severe fires rather than by surface fires.     

Detection and identification of gibberellins in Douglas fir (Pseudotsuga menziesii) shoots

Extracts of Douglas fir ( Pseudotsuga menziesii [Mirb.] Franco) shoots were purified by reversed and normal phase HPLC; gibberellin (GA)-like compounds detected by radioimmunoassay with antibodies against GA₄ and the Tan-ginbozu dwarf rice micro-drop biossay were analyzed by GC-MS. Three major components were identified as GA₄, GA₇, and GA₉ while smaller amounts of GA¹, GA₃ and putative GA⁹-glucosyl ester were also present.     

Uptake and translocation of manganese in seedlings of two varieties of Douglas fir (Pseudotsuga menziesii var. viridis and glauca)

Douglas fir (Pseudotsuga menziesii) variety glauca (DFG) but not the variety viridis (DFV) showed symptoms of manganese (Mn) toxicity in some field sites. We hypothesized that these two varieties differed in Mn metabolism. To test this hypothesis, biomass partitioning, Mn concentrations, subcellular localization and 54Mn-transport were investigated. Total Mn uptake was three-times higher in DFG than in DFV. DFV retained > 90% of 54Mn in roots, whereas > 60% was transported to the shoot in DFG. The epidermis was probably the most efficient Mn barrier since DFV contained lower Mn concentrations in cortical cells and vacuoles of roots than DFG. In both varieties, xylem loading was restricted and phloem transport was low. However, sieve cells still contained high Mn concentrations. DFV displayed higher biomass production and higher shoot : root ratios than DFG. Our results clearly show that both varieties of Douglas fir differ significantly in Mn-uptake and allocation patterns rendering DFG more vulnerable to Mn toxicity.     

Prescribed fire is associated with increased floral richness and promotes short-term increases in bee biodiversity in the ponderosa pine forest of the Southern Rocky Mountains

1. Managed low-severity surface fires are frequently implemented in efforts to restore disturbance processes to forests of North America; although the effects of managed fire on forest structure are well-studied, few studies investigate whether these disturbances cascade to impact pollinator communities.
2. We analysed bee-habitat relationships in fire-treated (1- and 3-years post-treatment) and non-treated ponderosa pine stands in Colorado to test wild bee population responses.
3. Observed bee richness and α-diversity were highest in stands 1-year post-fire and had more Anthophora, Bombus, Osmia and Lasioglossum spp. in comparison to 3-year post-fire and non-treated stands. Bee functional groups were responsive to treatments, with more below-ground nesting taxa present in stands 3 years post-fire.
4. Floral richness was the highest mid-growing season (June, July) and within 1-year post-fire stands.
5. A model analysing the effects of foraging and nesting habitat variation on bee assemblages indicated positive association between floral richness and bee α-diversity, but negative relationships with stand basal area. Nesting habitat was not associated with variation in bee assemblages.
6. We conclude that managed fire has positive short-term effects on bee biodiversity that are likely mediated by floral richness. However, these effects were not detectable by 3 years post-treatment in the southern Rocky Mountain region.

     

Changing growth response to wildfire in old‐growth ponderosa pine trees in montane forests of north central Idaho

North American fire‐adapted forests are experiencing changes in fire frequency and climate. These novel conditions may alter postwildfire responses of fire‐adapted trees that survive fires, a topic that has received little attention. Historical, frequent, low‐intensity wildfire in many fire‐adapted forests is generally thought to have a positive effect on the growth and vigor of trees that survive fires. Whether such positive effects can persist under current and future climate conditions is not known. Here, we evaluate long‐term responses to recurrent 20th‐century fires in ponderosa pine, a fire‐adapted tree species, in unlogged forests in north central Idaho. We also examine short‐term responses to individual 20th‐century fires and evaluate whether these responses have changed over time and whether potential variability relates to climate variables and time since last fire. Growth responses were assessed by comparing tree‐ring measurements from trees in stands burned repeatedly during the 20th century at roughly the historical fire frequency with trees in paired control stands that had not burned for at least 70 years. Contrary to expectations, only one site showed significant increases in long‐term growth responses in burned stands compared with control stands. Short‐term responses showed a trend of increasing negative effects of wildfire (reduced diameter growth in the burned stand compared with the control stand) in recent years that had drier winters and springs. There was no effect of time since the previous fire on growth responses to fire. The possible relationships of novel climate conditions with negative tree growth responses in trees that survive fire are discussed. A trend of negative growth responses to wildfire in old‐growth forests could have important ramifications for forest productivity and carbon balance under future climate scenarios.     

Ethanol synthesis, nitrogen, carbohydrates, and growth in tissues from nitrogen fertilized Pseudotsuga menziesii (Mirb.) Franco and Pinus ponderosa Dougl. ex Laws. seedlings

 Seedlings of Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, and ponderosa pine, Pinus ponderosa Dougl. ex Laws., were grown in a controlled environment and fertilized with nutrient solutions containing 150 ppm (+N), or 0 ppm nitrogen (−N). These treatments greatly altered seedling growth, and the concentrations of N and carbohydrates in their tissues. Metabolically active tissues, such as roots, incubated with a limited supply of O₂ became hypoxic faster and synthesized more ethanol than less active tissues, such as needles. All tissues that were incubated for 4 h in N₂ synthesized ethanol. Needles incubated in N₂ and light had much lower quantities of ethanol than needles in N₂ and dark, suggesting that O₂ from photosynthetsis limited internal anoxia. Most tissues from +N seedlings synthesized greater quantities of ethanol in N₂ anoxia than tissues from −N seedlings, probably because they were able to produce more enzymes with a greater availability of N. However, this increase in ethanol synthesis between N treatments was most pronounced in the phloem. Ethanol and soluble sugar concentrations were negatively related in needles and positively related in roots of N+ seedlings, but not −N seedlings. Starch concentrations had no effect on the amount of ethanol produced by any tissue. Regardless of N treatments, all tissues from ponderosa pine produced more N₂-induced ethanol than Douglas-fir, in part because its tissues contained different concentrations of soluble sugars and N as a consequence of phenological differences between the species. However, ponderosa pine tissues may also maintain greater quantities of anaerobic enzymes, or their isozymes than Douglas-fir. Received: 22 February 1998 / Accepted 23 June 1998     

Hydraulic redistribution of soil water in two old-growth coniferous forests: quantifying patterns and controls

Although hydraulic redistribution of soil water (HR) by roots is a widespread phenomenon, the processes governing spatial and temporal patterns of HR are not well understood. We incorporated soil/plant biophysical properties into a simple model based on Darcy's law to predict seasonal trajectories of HR. We investigated the spatial and temporal variability of HR across multiple years in two old-growth coniferous forest ecosystems with contrasting species and moisture regimes by measurement of soil water content (theta) and water potential (Psi) throughout the upper soil profile, root distribution and conductivity, and relevant climate variables. Large HR variability within sites (0-0.5 mm d(-1)) was attributed to spatial patterns of roots, soil moisture and depletion. HR accounted for 3-9% of estimated total site water depletion seasonally, peaking at 0.16 mm d(-1) (ponderosa pine; Pinus ponderosa) or 0.30 mm d(-1) (Douglas-fir; Pseudotsuga menziesii), then declining as modeled pathway conductance dropped with increasing root cavitation. While HR can vary tremendously within a site, among years and among ecosystems, this variability can be explained by natural variability in Psi gradients and seasonal courses of root conductivity.     

Spatial and temporal variation in historic fire regimes in subalpine forests across the Colorado Front Range in Rocky Mountain National Park, Colorado, USA

Aim The historical variability of fire regimes must be understood in the context of drivers of the occurrence of fire operating at a range of spatial scales from local site conditions to broad‐scale climatic variation. In the present study we examine fire history and variations in the fire regime at multiple spatial and temporal scales for subalpine forests of Engelmann spruce–subalpine fir (Picea engelmannii, Abies lasiocarpa) and lodgepole pine (Pinus contorta) of the southern Rocky Mountains. Location The study area is the subalpine zone of spruce–fir and lodgepole pine forests in the southern sector of Rocky Mountain National Park (ROMO), Colorado, USA, which straddles the continental divide of the northern Colorado Front Range (40°20′ N and 105°40′ W). Methods We used a combination of dendroecological and Geographic Information System methods to reconstruct fire history, including fire year, severity and extent at the forest patch level, for c. 30,000 ha of subalpine forest. We aggregated fire history information at appropriate spatial scales to test for drivers of the fire regime at local, meso, and regional scales. Results The fire histories covered c. 30,000 ha of forest and were based on a total of 676 partial cross‐sections of fire‐scarred trees and 6152 tree‐core age samples. The subalpine forest fire regime of ROMO is dominated by infrequent, extensive, stand‐replacing fire events, whereas surface fires affected only 1–3% of the forested area. Main conclusions Local‐scale influences on fire regimes are reflected by differences in the relative proportions of stands of different ages between the lodgepole pine and spruce–fir forest types. Lodgepole pine stands all originated following fires in the last 400 years; in contrast, large areas of spruce–fir forests consisted of stands not affected by fire in the past 400 years. Meso‐scale influences on fire regimes are reflected by fewer but larger fires on the west vs. east side of the continental divide. These differences appear to be explained by less frequent and severe drought on the west side, and by the spread of fires from lower‐elevation mixed‐conifer montane forests on the east side. Regional‐scale climatic variation is the primary driver of infrequent, large fire events, but its effects are modulated by local‐ and meso‐scale abiotic and biotic factors. The low incidence of fire during the period of fire‐suppression policy in the twentieth century is not unique in comparison with the previous 300 years of fire history. There is no evidence that fire suppression has resulted in either the fire regime or current forest conditions being outside their historic ranges of variability during the past 400 years. Furthermore, in the context of fuel treatments to reduce fire hazard, regardless of restoration goals, the association of extremely large and severe fires with infrequent and exceptional drought calls into question the future effectiveness of tree thinning to mitigate fire hazard in the subalpine zone.     

Physiological effects of five months exposure to low concentrations of O3 and NH3 on Douglas fir (Pseudotsuga menziesii)

Three years old seedlings of Douglas fir (Pseudotsuga menziesii) were exposed lo filtered air, O₃ (day and night concentrations of 78 and 30 μgm^?3: respectively). NH₃ (54 μg m^?3) and to a mixture of NH₃+O₃ (day and night concentrations of 49 + 83 and 49 + 44 μg m^?3 respectively), for 5 months in fumigation chambers. Both gas exchange and chlorophyll fluorescence were measured on shoots which had sprouted at the beginning of the exposure period. After 4. 8, 10 and 20 weeks of exposure, light response curves of electron transport rate (J) were determined, in which J was deduced from chlorophyll fluorescence. Net CO₂ assimiialion was measured at maximum light intensity of 560) μmol m^?2 S^?1 (P_n.560). After 8 and 10 weeks of exposure also light response curves of CO₂ assimilation were assessed. Shoots exposed to O₃ showed a reduction in net CO₂ assimilation as compared to the control shoots during the entire exposure period. The reduction was related lo a lower chlorophyll content and a lower electron transport rate, whereas no effect on quantum yield efficiency (qy) was observed. In contrast, shoots exposed to NH₃ showed a positive effect on photosynthesis. Shoots exposed to NH₃. + O₃ showed a rapid increase in P_n.560, in the period between 4 and 8 weeks to a level equal of that of the NH₃-treatment. After this period a decline in P_n.560 was observed. After 10 weeks of exposure shoots exposed to O₃ showed an increased transpiration rate in the dark as compared to the control shoots. In addition, water use efficiency (WUE) declined as a result of an increase in leaf conductance. Both observations indicate that the stomatal apparatus was affected by O₃. A high transpiration rate in the dark was also found for shoots esposed to NHX. However, shoots exposed to NH₃+ O₃ showed neither an effect on WUE, nor an effect on transpiration rate in the dark. The possibility that NH₃ delayed the O₃ induced effects on photosynthesis and stomatal conductance is discussed.     

Associations between carbon isotope ratios of ecosystem respiration, water availability and canopy conductance

We tested the hypothesis that the stable carbon isotope signature of ecosystem respiration (δ¹³C_R) was regulated by canopy conductance (G_c) using weekly Keeling plots (n=51) from a semiarid old‐growth ponderosa pine (Pinus ponderosa) forest in Oregon, USA. For a comparison of forests in two contrasting climates we also evaluated trends in δ¹³C_R from a wet 20‐year‐old Douglas‐fir (Pseudotsuga menziesii) plantation located near the Pacific Ocean. Intraannual variability in δ¹³C_R was greater than 8.0‰ at both sites, was highest during autumn, winter, and spring when rainfall was abundant, and lowest during summer drought. The δ¹³C_R of the dry pine forest was consistently more positive than the wetter Douglas‐fir forest (mean annual δ¹³C_R: ?25.41‰ vs. ?26.23‰, respectively, P=0.07). At the Douglas‐fir forest, δ¹³C_R–climate relationships were consistent with predictions based on stomatal regulation of carbon isotope discrimination (Δ). Soil water content (SWC) and vapor pressure deficit (vpd) were the most important factors governing δ¹³C_R in this forest throughout the year. In contrast, δ¹³C_R at the pine forest was relatively insensitive to SWC or vpd, and exhibited a smaller drought‐related enrichment (～2‰) than the enrichment observed during drought at the Douglas‐fir forest (～5‰). Groundwater access at the pine forest may buffer canopy–gas exchange from drought. Despite this potential buffering, δ¹³C_R at the pine forest was significantly but weakly related to canopy conductance (G_c), suggesting that δ¹³C_R remains coupled to canopy–gas exchange despite groundwater access. During drought, δ¹³C_R was strongly correlated with soil temperature at both forests. The hypothesis that canopy‐level physiology is a critical regulator of δ¹³C_R was supported; however, belowground respiration may become more important during rain‐free periods.     

Tree growth response to drought and temperature in a mountain landscape in northern Arizona, USA

Aim To understand how tree growth response to regional drought and temperature varies between tree species, elevations and forest types in a mountain landscape. Location Twenty‐one sites on an elevation gradient of 1500 m on the San Francisco Peaks, northern Arizona, USA. Methods Tree‐ring data for the years 1950–2000 for eight tree species (Abies lasiocarpa var. arizonica (Merriam) Lemm., Picea engelmannii Parry ex Engelm., Pinus aristata Engelm., Pinus edulis Engelm., Pinus flexilis James, Pinus ponderosa Dougl. ex Laws., Pseudotsuga menziesii var. glauca (Beissn.) Franco and Quercus gambelii Nutt.) were used to compare sensitivity of radial growth to regional drought and temperature among co‐occurring species at the same site, and between sites that differed in elevation and species composition. Results For Picea engelmannii, Pinus flexilis, Pinus ponderosa and Pseudotsuga menziesii, trees in drier, low‐elevation stands generally had greater sensitivity of radial growth to regional drought than trees of the same species in wetter, high‐elevation stands. Species low in their elevational range had greater drought sensitivity than co‐occurring species high in their elevational range at the pinyon‐juniper/ponderosa pine forest ecotone, ponderosa pine/mixed conifer forest ecotone and high‐elevation invaded meadows, but not at the mixed conifer/subalpine forest ecotone. Sensitivity of radial growth to regional drought was greater at drier, low‐elevation compared with wetter, high‐elevation forests. Yearly growth was positively correlated with measures of regional water availability at all sites, except high‐elevation invaded meadows where growth was weakly correlated with all climatic factors. Yearly growth in high‐elevation forests up to 3300 m a.s.l. was more strongly correlated with water availability than temperature. Main conclusions Severe regional drought reduced growth of all dominant tree species over a gradient of precipitation and temperature represented by a 1500‐m change in elevation, but response to drought varied between species and stands. Growth was reduced the most in drier, low‐elevation forests and in species growing low in their elevational range in ecotones, and the least for trees that had recently invaded high‐elevation meadows. Constraints on tree growth from drought and high temperature are important for high‐elevation subalpine forests located near the southern‐most range of the dominant species.     

Soil functional responses to ecological restoration treatments in frequent‐fire forests of the western United States: a systematic review

We investigated general effects of ecological restoration treatments on soil function in frequent‐fire forests of the western United States using a systematic review methodology. We searched numerous publication databases for original research papers and used well‐defined criteria developed a priori to select papers for review. We used meta‐analysis and qualitative summaries to compare reported responses of macronutrients, nitrogen cycling, and soil respiration among tree thinning (thin), prescribed fire (burn), and thinning plus prescribed fire treatments (composite). Results of meta‐analysis showed that mean differences in macronutrients were consistently higher in composite treatments (standardized using controls) when compared to thin‐only and burn‐only treatments. Mean responses related to nitrogen cycling showed similar patterns, with significant increases detected in composite treatments for all nitrogen cycling variables (mineralization, ammonification, and nitrification) and insignificant responses for the majority of the burn‐only and thin‐only treatments. Mean difference in response for soil respiration following composite treatments showed increases as compared to the controls, and no significant differences were detected in response to burn‐ and thin‐only treatments. While soil function, nutrient cycling, and soil respiration differed among treatments, the most significant effects were observed for nitrogen and carbon responses, net mineralization and nitrification, ammonium availability, and soil respiration rate, which experienced the greatest increase following treatments that were both thinned and burned.