Carry-over effects of ozone on root growth and carbohydrate concentrations of ponderosa pine seedlings

Ozone exposure decreases belowground carbon allocation and root growth of plants; however, the extent to which these effects persist and the cumulative impact of ozone stress on plant growth are poorly understood. To evaluate the potential for plant compensation, we followed the progression of ozone effects, with particular emphasis on the development of new roots. Ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings were exposed to ozone for 2 years. Following removal of the seedlings from ozone, root growth was assessed to characterize the carry-over effects on new root production, and carbohydrate concentrations were measured to determine if allocation strategies differed among ozone treatments. Four months after removal from ozone, dormant seedlings had significantly lower starch concentrations in stems, coarse roots and fine roots than control seedlings. Following root flushing, starch concentrations in all seedlings decreased, with ozone-treated seedlings containing significantly less starch, sucrose, fructose, glucose and total monosaccharides than control seedlings. There was some evidence that stem starch was mobilized to compensate partially for the lower concentrations of root starch in ozone-treated seedlings; however, there was significantly less new root production in seedlings previously exposed to ozone for 2 years than in control seedlings. Early senescence of older needle age classes, perhaps resulting in inadequate available photosynthate, may be responsible for the reduction in new root production during the year following exposure to ozone. Stored carbohydrate reserves, which were depleted in seedlings previously exposed to ozone, were insufficient to compensate for the ozone-induced reduction in canopy photosynthate. We conclude that there are carry-over effects of ozone exposure on ponderosa pine seedlings, including an enhanced potential for seedling susceptibility to other stresses even in respite years when ozone concentrations are low.     

Recruitment of ponderosa pine seedlings in the Cascade Range

Abiotic stresses on seedling regeneration in xeric ecosystems are great, hence recruitment processes can be facilitated by stand factors that ameliorate the germinant-scale microenvironment. An experiment was conducted on the eastern slope of the Cascade Range to test the effects of shrub cover, simulated seed caching, and substrate on the recruitment of Pacific ponderosa pine (Pinus ponderosa var. ponderosa Dougl. ex Laws.) seedlings. Failure rates of seeds sowed in exclosures were large, with less than 30% emerging as germinants in the spring following fall sowing. Simulated seed caching improved emergence rates by more than sevenfold and was responsible for 88% of all spring germinants. Emergence rates were lowest from uncached seeds on litter. Just 16% of the crop survived the summer and fall to the month of November, or less than 5 months after emergence. Shrub cover did not affect emergence rates, but establishment rates were higher: seedlings beneath shrubs succumbed to desiccation at a slower rate than unshaded seedlings. By August there were 2.3 times more survivors at shrub-shaded sites than unshaded sites, and by the end of fall, when seedlings were considered established, more than 78% existed beneath shrubs. This study provides evidence that the natural recruitment of ponderosa pine seedlings is facilitated by the occurrence of the species’ common shrub associates.     

Auxin-impregnated hygroscopic gel: effects on ponderosa pine and common hackberry seedlings

Greenhouse and field studies were conducted to determine the effects of indole3-butyric acid (IBA) and 2,4-dichlorophenoxyacetic acid (2,4-D) on root development and survival of 3+0 bareroot Pinus ponderosa (Dougl. ex Laws.) and 2+0 bareroot Celtis occidentalis (L) seedlings. In the greenhouse, 100 and 200 ppm. IBA and 10 and 50 ppm 2,4-D were applied to seedlings through a root dip in an auxin hygroscopic gel mix. A randomized complete block analysis of variance indicated that IBA gel treatments increased root volume and root dry weight in hackberry and had no effect on ponderosa pine; 2,4-D gel treatments had detrimental effects on both species. In the field, IBA gel treated ponderosa pine seedlings exhibited improved height and survival relative to the non-treated control seedlings. There was no detectable improvement in survival in the field for hackberry. However, diameter and stem dry weight were lower for hackberry seedlings treated with the gel dip alone.     

Large mulches and manual release enhance growth of ponderosa pine seedlings

Woody and herbaceous vegetation that captures scarce soil moisture often kills or stunts the growth of conifer seedlings in young plantations in the Sierra Nevada of California. Two methods for excluding this vegetation are to apply large (3-m×3-m), long-lasting (at least 5 years) mulches around the seedlings soon after planting, or to repeatedly grub seedlings of competing vegetation in study plots. Both techniques were tested and proved to be effective, significantly increasing ponderosa pine diameter and height relative to the control and to seedlings surrounded by small paper mulches. The effective treatments were large enough to enable conifer seedlings to capture site resources unimpeded by competing plants. Eight species of shrubs, seventeen forbs, four grasses, and one fern grew naturally on the study site.Results suggest that the forester now has two biologically effective means for controlling unwanted vegetation and for attaining rapid early pine growth in environments similar to the study area. Although more expensive, the large heavy polyester mulches ($9.90 per seedling over 5 years) give the forester an alternative to manual release ($2.05 per seedling over 5 years).     

Induced competition among scots pine seedlings and its effect on future timber quality

Height and diameter increment and timber quality in Scots pine (Pinus sylvestris L.) planted as seedlings of different sizes were studied in two experiments. When compared 32–37 years after planting, seedlings that were smaller than neighbouring seedlings at the time of planting, had generally attained lower height and diameter, and had higher mortality compared to seedlings that were of equal size or larger than their neighbours. Tree size was strongly related to seedling size two years after planting. Trees originating from small seedlings had thinner annual rings and branches than trees originating from large seedlings. These characters are of importance for timber quality. It seems that the competitive ability of a tree is to a large extent established early in stand development.     

Performance of ponderosa pine seedlings under four herbicide regimes in northeast Oregon

The herbicide, hexazinone, was applied four ways over ponderosa pine, 2–0 seedlings planted in northeast Oregon. The four treatments were two broadcast applications, a single broadcast application, a large spot application, and a small spot application. Seedling survival and growth were monitored for five growing seasons. Results indicate that survival more than doubled with either large or small spot applications compared to no application, and either one or two broadcast applications can increase survival an additional 30% over spot applications. Differences in stem volume were substantial, with two broadcast applications yielding more than twice the volume of a single broadcast treatment and more than five times the volume of seedlings treated with spot applications. Trees in small spots were still three times bigger than surviving seedlings in the control. These results are consistent with the concept of competition threshold. Management implications were considered in terms of cost of established seedlings. Although broadcast applications cost more per acre than spot applications, gains in seedling survival, growth and quality offset the additional cost and translate into lower established seedling costs. The cost effectiveness of broadcast applications also may be seen in the elimination of replanting or in-planting requirements and increases in long-term growth potential of the established trees.     

Accentuation of gas exchange gradients in flushes of ponderosa pine exposed to ozone

Two-year-old ponderosa pine seedlings (Pinus ponderosa Laws.) were exposed to episodic O(3) concentrations in open-top chambers for two consecutive growing seasons (June through September of 1990 and 1991). Near the end of the second season of O(3) exposure, gas exchange was measured on needles of surviving flushes at saturating CO(2) and photosynthetic photon flux density (PPFD). Both photosynthetic capacity (A(sat)) and stomatal conductance to water vapor (g(wv)) declined linearly with needle age but differences within a flush were also found. Gas exchange rates of needles from the base of the current-year flush were significantly lower than those of needles from the top of the flush, even though age differences between needles were negligible. Although most measurements were conducted at saturating CO(2), similar patterns of gas exchange were also found at 350 micro mol mol(-1) CO(2), indicating that photosynthesis of needles at the bottom of the flush was more strongly affected by O(3) than that of needles at the top of the flush, even though the potential for O(3) uptake was probably less in needles at the bottom of the flush because of reduced stomatal conductance. Carboxylation efficiency (deltaA/deltaC(i)), the linear slope of the A/C(i) response, was highly correlated with A(sat), varying with needle age, needle position in the flush and O(3) exposure, but the magnitude of the reductions was greater than for A(sat). We conclude that susceptibility to O(3) damage among needles of an individual seedling varies not only with needle age but also with needle position, and that reductions in photosynthetic capacity may not be directly attributable to increased uptake of the pollutant. The data also indicate the need to consider within-flush variation when estimating whole-plant carbon gain and O(3) uptake.     

Effectiveness of freezer storage in fulfilling the chilling requirement of fall-lifted ponderosa pine seedlings

The degree to which freezer storage fulfilled the chilling requirement of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings of two sources was determined by monitoring their development after potting or planting. The seedlings were lifted in September, October, November, or March and subjected to storage before outplanting. The fulfillment of chilling was assessed by measuring days to budbreak, cumulative percentage of seedlings flushing, foliated shoot length, and rate of bud abortion. The effect of freezer storage depended on stage of seedling development at lifting, length of storage, and seed source. Storage did not totally replace winter conditions, especially for seedlings lifted in September and October. Those from a high-elevation seed source flushed sooner than those from a low-elevation source. Delayed budbreak after planting of early lifted seedlings from the high-elevation source disappeared in the second year, but those from the low-elevation source continued to show effects. Seedlings lifted and stored in November had patterns of budbreak that were similar to those of seedlings that had overwintered in beds.     

Temporal allocation of (14)C to extramatrical hyphae of ectomycorrhizal ponderosa pine seedlings

Ponderosa pine seedlings were inoculated with Hebeloma crustuliniforme either in growth pouches before they were transplanted to root-mycocosms (P seedlings), or at the time of transfer to root-mycocosms (V seedlings). Uninoculated seedlings served as controls (U seedlings). The use of root-mycocosms allowed examination of portions of hyphae separate from roots and rooting substrate but still in symbiosis with the host. The results thus provided a quantitative basis for estimating hyphal mass and carbon allocation to extramatrical hyphae. The amount of (14)CO(2) fixed after a 2-h exposure was greatest for P seedlings and least for uninoculated seedlings. Four and nine days after exposure, (14)C content was greatest in uninoculated seedlings and least in inoculated seedlings. In isotope distribution and dry mass accumulation, V seedlings were more similar to U than to P seedlings. Calculated on a dry weight basis, the allocation of isotope to mycelium suggested that extramatrical hyphae of P seedlings were a stronger sink for carbon than extramatrical hyphae of V seedlings. Differences in inoculation methods resulted in differences in carbon allocation and physiology of extramatrical hyphae that could affect seedling establishment and survival. Seedlings inoculated by one method cannot serve as surrogates for mycorrhizal seedlings produced by other inoculation techniques.     

Predicting survival of planted Douglas-fir and ponderosa pine seedlings on dry,low-elevation sites in southwestern Oregon

Four equations were developed for predicting the probability of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] and ponderosa pine (Pinus ponderosa Dougl. ex Laws.) survival for the first (0–1) and first to third (1–3) growing seasons after applying mulching, scalping, or artificial shading (shade cards) treatments in plantations in southwestern Oregon, U.S.A. Variables describing conifer size, levels of competing vegetation, presence of silvicultural treatments, site factors, and climate factors were collected from 13 sites ranging from 0 to 6 years after planting and examined as potential predictors of survival. Age, stem diameter, a competition index for shrubs, severity of growing season at time of treatment, average annual precipitation, aspect, and slope angle were predictors of Douglas-fir survival during 0–1 and 1–3 growing seasons after treatment; the presence of silvicultural treatments was also a predictor only during the first growing season after treatment. Age, aspect, and slope angle were predictors of ponderosa pine survival over both 0–1 and 1–3 growing seasons after treatment; height-diameter ratio, competition indices for herbs, shrubs, and hardwoods, silvicultural treatment, severity of growing season at time of treatment, and average annual precipitation were also predictors only during the first growing season after treatment; crown width was a predictor of survival only during 1–3 growing seasons after treatment. When significant in the models, predicted probability of survival increases with treatments, less severe weather conditions, diameter, crown width, age, and precipitation; probability decreases with increasing height-diameter ratio and competition indices for herbs, shrubs, and hardwoods.     

Response of stomatal conductance to drought in ponderosa pine: implications for carbon and ozone uptake

To gain insight into the limitations imposed by a typical Mediterranean-climate summer drought on the uptake of carbon and ozone in the ponderosa pine (Pinus ponderosa Dougl. ex Laws.) ecosystem, we compared diurnal trends in leaf physiology of young trees in a watered and a control plot located in the Sierra Nevada Mountains, CA, USA (Blodgett Forest, 38 degrees 53' N, 120 degrees 37' W, 1315 m elevation). Predawn water potential of trees in the watered plot remained above -0.3 MPa throughout the growing season, whereas it dropped in the control plot from -0.24 to -0.52 MPa between late May and mid-August. Photosynthesis and stomatal conductance of trees in the watered plot were relatively insensitive to atmospheric vapor pressure deficit (VPD), whereas gas exchange of trees in the control plot varied with changes in soil water, VPD and temperature. Although the 1998 growing season was abnormally wet, we saw a pronounced drought effect at the control site. Over the 2 months following the onset of watering, carbon and ozone uptake were measured on three days at widely spaced intervals. Carbon uptake per unit leaf area by 1-year-old foliage of trees in the control plot was 39, 35 and 30% less, respectively, than in the watered plot, and estimated ozone deposition per unit leaf area (ozone concentration times stomatal conductance) was 36, 46 and 41% less.     

Nitrogen availability modifies the ozone responses of Scots pine seedlings exposed in an open-field system

Three-year-old Scots pine (Pinus sylvestris L.) seedlings were exposed to either ambient or elevated (1.5-1.6 x ambient) ozone concentration ([O3]) for three growing seasons in an open-field fumigation facility where they were irrigated during the growing season with a nutrient solution providing nitrogen (N) at 70 (LN treatment), 100 (control) or 150% (HN treatment) of the optimum supply rate. Treatment effects were most evident during the third year of exposure, when the ambient [O3] + HN treatment enhanced whole-plant biomass, root/shoot dry weight ratio, needle pigment concentrations and the number of chloroplast plastoglobuli in the mesophyll cells in current-year (C) needles, whereas it reduced starch accumulation in C needles and abscission of 2-year-old (C+2) needles. In the control fertilization, 3 years of exposure to elevated [O3] decreased stem-base diameter and increased K concentration and electron density of chloroplast stroma in C needles. Plants in the HN treatment exposed for 3 years to elevated [O3] had significantly lower heights, current-year main shoot length and root/shoot dry mass ratio than control plants, and increased abscission of C+2 needles. In contrast, O3-induced changes in the ultrastructure of mesophyll cells were most evident in seedlings grown for 3 years in the LN treatment. We conclude that, in Scots pine, a relatively O3-tolerant species, chronic O3 exposure leads to cumulative growth reduction, increased needle abscission and changes in carbon allocation that are strongly influenced by plant N availability.     

Long-term prescribed burning increases nutrient uptake and growth of loblolly pine seedlings

Loblolly pine (Pinus taeda L.) seedlings were grown from seed in a greenhouse on A1 horizon soil collected from field plots that have been burned each winter or maintained in an unburned condition for 33 years. Soils from burned and unburned plots were treated with phosphorus (P), P and calcium (Ca), or left untreated. After 32 weeks, height, biomass, and nitrogen (N) and P uptake were greater on soil from burned versus unburned plots, although application of P masked these effects. Addition of P increased plant biomass, seedling height, and uptake of N and P, but depressed levels of soil NH₄N. These results suggest that long-term prescribed burning may have a positive effect on nutrient availability that will benefit seedlings of the next stand.     

Seed rain of ponderosa pine beneath partial overstories

An analysis was conducted of the 1999 ponderosa pine (Pinus ponderosa var. ponderosa Dougl. ex Laws.) seed crop at two stands of differing overstory density in each of two sites that differed in productivity (forest community series) on the east slope of the Cascade Range in central Oregon, USA. A total 2,166 viable seeds were collected between 2 September 1999 and 6 June 2000. Total seed quantities varied more by site than by tree density, with the xeric site producing six-fold greater seed yields. Within each site, the stand of higher density produced more viable seed. Per-tree cone yields were also greater at the site of lesser productivity. At three of the four stands, tree fecundity (as measured by cone counts) was positively related to diameter at breast height and height, but not to live crown ratio. A difference in temporal seedfall patterns among the two sites occurred late and was relatively minor: most seedfall occurred within the first month after it started regardless of absolute seed production. Seedfall distribution varied spatially within stands, but no seed trap (of 55) received less than an equivalent 28 thousand seeds per hectare. Seedfall was not perceived to be a limiting factor on natural regeneration at any of the four partial-overstory stands in this study.     

Introduction of ponderosa pine and Douglas-fir to Argentina

Patterns of shoot elongation of 2-yr seedlings from native North American populations of ponderosa pine and Douglas-fir were compared to those of Argentine land races originating from unknown provenances. The comparisons were conducted in Moscow, Idaho (USA), and suggested that the ponderosa pine land race was descended from a California provenance at low or middle elevations but that the growth potential of the land race was only mediocre in comparison to eight native populations. The Douglas-fir land race exhibited a relatively high growth potential in comparison to 19 native provenances and undoubtedly originated from a mild coastal environment. The results provide concrete recommendations for upgrading the growth potential of the Argentine land races by importing germ plasm of specific provenances.     

Influence of moisture stress and induced resistance in ponderosa pine,Pinus ponderosa Dougl. ex. Laws,on the pine sawfly,Neodiprion autumnalis Smith

Plant moisture stress is artificially maintained for ponderosa pine, Pinus ponderosa Dougl. ex Laws, seedlings using polyethylene glycol. Stressed seedlings produce significantly lower amounts of needle nitrogen, phenols, and tannins than non-stressed seedlings. Field tests reveal that previously damaged foliage is a significantly poorer food source for the pine sawfly, Neodiprion autumnalis Smith (Hymenoptera: Diprionidae), than non-damaged foliage. Field studies further indicate that the effect of induction by defoliation is more important than the effect of stress level. These results are discussed in terms of their potential impact on future studies on preferential selection by herbivores of stressed host plants.     

Nutrient availability alters belowground respiration of ozone-exposed ponderosa pine

Exposure to ozone (O(3)) and changes in soil fertility influence both the metabolism of plant roots and their interaction with rhizosphere organisms. Because one indication of altered root metabolism is a change in belowground respiratory activity, we used specially designed measurement chambers to assess the effects of O(3) and nutrient availability on belowground respiratory activity of potted three-year-old ponderosa pine (Pinus ponderosa Dougl. ex Laws.). Seedlings were exposed to a factorial combination of three O(3) treatments and three fertilization treatments in open-top O(3) exposure chambers. Ozone exposure decreased and high nutrient supply increased total plant dry weight, but root/shoot ratios were not affected. In general, exposure to O(3) increased rates of belowground O(2) uptake and CO(2) release and the respiratory quotient (RQ, CO(2)/O(2)), although seasonal differences were detected. In October, following the second season of O(3) exposure, rates of belowground O(2) uptake and CO(2) release and RQ were increased in trees in the high-O(3) exposure treatment by 22, 73 and 32%, respectively, over values in control trees in charcoal-filtered air. Increasing nutrient supply resulted in decreasing rates of belowground O(2) uptake and CO(2) release but it had little effect on RQ. In the high-nutrient supply treatment, rates of belowground O(2) uptake and CO(2) release were decreased by 38 and 39%, respectively, compared with rates in the low-nutrient supply treatment. At the end of the second growing season, the high-nutrient supply treatment had decreased lateral root total nonstructural carbohydrates by 22% compared with the low-nutrient supply treatment. Nutrient availability altered the belowground respiratory response to O(3), such that the response to O(3) was greatest in the low-nutrient supply treatment. Significant O(3) effects on belowground respiratory activity were apparent before any reduction in total plant growth was found, suggesting that roots and rhizosphere organisms may be early indicators of physiological dysfunction in stressed seedlings.     

The effect of late summer fertilization on the frost hardening of second-year Scots pine seedlings

In this study the effect of summer fertilization on the initiation of frost hardening of containerized second-year Scots pine (Pinus sylvestris L.) seedlings is studied. During the second growing season three different fertilization programs (water soluble NPK with micronutrients) determined by electrical conductivity of peat water extract (0.2, 0.5 and 1.2 mS cm^-1) were initiated. The growth and nutrient concentrations of needles were monitored during the fertilization period. The frost hardiness of seedlings was assessed on four separate occasions at two week intervals from August 7 to September 18. This assessment was based on artificial freezing tests and visual damage scoring of tissue browning on current-year needles. Clear differences in foliar N, P and K concentrations were observed between the fertilization treatments. Fertilization prolonged the growing period of needles and increased root collar diameter. In all the tests, the highest fertilization level resulted in the highest level of frost hardiness. The difference between the fertilization treatments ranged from 1 °C to 2.2 °C. Frost hardiness increased with an increase in foliar nitrogen concentration and slightly less consistently with increases in foliar phosphorus and potassium concentrations.