Needle Abscission Research Articles

Phenological patterns of mature longleaf pine ( Pinus palustris Miller) under two different soil moisture regimes

Significant differences in above-ground net primary productivity (ANPP) have been identified along a soil moisture gradient in mature longleaf pine-wiregrass ecosystems [Can. J. For. Res. 29 (1999) 743]. To determine if differences in ANPP were apparent at the individual tree level, patterns of mature longleaf pine ( Pinus palustris Miller) shoot and needle phenology were characterized on xeric and wet/mesic sites in southwest Georgia. Towers provided access to the crowns of the mature trees. Needle abscission patterns were monitored over one growing season (1999); shoot and needle growth patterns were monitored over three growing seasons (1999–2001). Environmental variables, including soil moisture content, predawn needle water potential and gap fraction, as well as several crown structural characteristics, were also measured. Crowns at the two sites had similar average branch characteristics, with the exception of average branch angle, which was significantly greater at the wet/mesic site. The starting dates for shoot elongation and linear shoot growth were similar at the two sites during all 3 years; cessation of shoot growth occurred significantly earlier at the wet/mesic site in 1999. Duration of shoot elongation was longer at the xeric site; however, shoot growth rate was greater at the wet/mesic site. In contrast, during all 3 years, start of needle elongation and start of linear needle growth occurred significantly earlier at the wet/mesic site; cessation of needle growth occurred significantly earlier at the wet/mesic site in 2000. Duration of needle growth was similar at both sites, but needle growth rate was greater at the wet/mesic site in 2000 and 2001. No consistent differences in shoot and needle phenology with crown position were observed, except shoots were longer in the top crown third at the xeric site during 1999 and 2001. Average needle lifespan was determined to be 15 months. Mature longleaf pine exhibited a determinant pattern (one seasonal flush) of shoot and needle growth during the first two growing seasons, and an indeterminate pattern (multiple seasonal flushes) during the third growing season. Soil moisture content was not strongly correlated to patterns of needle abscission; however, predawn needle water potential explained 17% of the variation in early growing season needle demography.

The effects of long-term, partial shading on growth and photosynthesis in Pinus radiata D. Don trees

Two 8-year-old Pinus radiata trees growing in a dry-land plantation forest were subjected to a partial shading treatment for 2 years in order to investigate the responses of photosynthesis and tree growth to a long-term reduction in illuminated leaf area. The lower 60% of the crown of shaded trees was surrounded by 50% neutral density shade cloth. This reduced radiation absorption by 13% for the trees as a whole compared to the control trees during the first year, while modelled annual whole-canopy photosynthesis was 9% lower, and was strongly regulated by seasonal soil water availability, temperature and irradiance. The shaded trees responded by increasing carbon partitioning to the branches, at the expense of the stem. As a result, basal area growth of the shaded trees was almost 50% lower than that for control trees. In the second year, the effects of the shade cloth on absorbed radiation, photosynthesis and growth were reduced. This occurred because of needle abscission below the shade cloth and rapid branch and leaf area growth above the shade cloth, while the lower foliage on the control trees became increasingly self-shaded. Allocation to stem growth increased in the shaded trees, resulting in similar relative basal area increment between shaded and control trees. The difference in modelled whole-canopy photosynthesis was smaller in the second year. This was partly due to the reduced effect of the shade cloth, but also to a significant summer drought, which severely reduced stomatal conductance, photosynthesis and stem growth in both shaded and control trees. Seasonal root-zone water deficits at this site reduce annual carbon assimilation by around 40%, and increase the proportions of carbon assimilation occurring in spring, autumn and winter.

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.

A case study of nitrogen saturation in western U.S. forests.

Virtually complete nitrification of the available ammonium in soil and nitrification activity in the forest floor are important factors predisposing forests in the San Bernardino Mountains of southern California to nitrogen (N) saturation. As a result, inorganic N in the soil solution is dominated by nitrate. High nitrification rates also generate elevated nitric oxide (NO) emissions from soil. High-base cation saturation of these soils means that soil calcium depletion or effects associated with soil acidification are not an immediate risk for forest health as has been postulated for mesic forests in the eastern U.S. Physiological disturbance (e.g., altered carbon [C] cycling, reduced fine root biomass, premature needle abscission) of ozone-sensitive ponderosa pine trees exposed to high N deposition and high ozone levels appear to be the greater threat to forest sustainability. However, N deposition appears to offset the aboveground growth depression effects of ozone exposure. High nitrification activity reported for many western ecosystems suggests that with chronic N inputs these systems are prone to N saturation and hydrologic and gaseous losses of N. High runoff during the winter wet season in California forests under a Mediterranean climate may further predispose these watersheds to high nitrate leachate losses. After 4 years of N fertilization at a severely N saturated site in the San Bernardino Mountains, bole growth unexpectedly increased. Reduced C allocation below- ground at this site, presumably in response to ozone or N or both pollutants, may enhance the bole growth response to added N.

Incidence and Impact of Swiss Needle Cast in Forest Plantations of Douglas-fir in Coastal Oregon.

An epidemic of Swiss needle cast, caused by the ascomycete Phaeocryptopus gaeumannii, is causing defoliation and growth reductions in Douglas-fir forest plantations along the Oregon Coast. The area of symptomatic plantations has been monitored annually since 1996 by aerial survey; in spring 1999, 119,500 ha were affected. Pathogen and symptom development have also been monitored on nine permanent plots in stands of differing disease severity. Infection levels and symptom severity are greatest in low elevation plantations close to the coast. In areas of severe disease, trees retain only current year needles. Defoliation is proportional to the number of stomata occluded by pseudothecia of the fungus, with needles being shed when about 50% of stomata are occupied, regardless of needle age. Fungus sporulation and premature needle abscission are greatest on the upper branches of trees. Annual application of fungicides increases needle retention significantly. Tree height and diameter growth and total tree volume are reduced by disease, and tree volume is significantly correlated with needle retention on our plot trees. The epidemic continues to be most severe in Douglas-fir plantations established on sites where Sitka spruce and western hemlock or red alder predominated in earlier times.

Poor stomatal control of water balance and the abscission of green needles from a declining stand of spruce trees [Picea abies (L.) Karst.] from the northern Alps

On a site on the west slope of the Wank in the northern Alps changes in water potential, osmotic potential and transpiration rate were measured in spruce trees during the dry summer months of 1991. The pattern of decrease in water potential and osmotic potential on days of varying evaporative demand from trees of widely different decline conditions was used to describe the relative ability of the trees to withstand drought stress. Stress diagrams served as a tool for interpreting the state of health of each tree. The criterion is independent of the water situation of the tree and the other external conditions of the respective experiment. These diagrams clearly show that the foliage of spruces with high needle losses reaches the limit of endurance relatively early. For equal evaporative demand much lower turgor levels were observed in spruces with high needle loss compared to undamaged trees. Associated with the occurrence of low turgor values was the shedding of green needles. The abscission zone was shown in sections. The accumulation of highly fluorescent substances in the bundle sheath cells of the same material was described. Incomplete to non-existent stomatal control over water loss was attributed to modifications in the cell walls of the stomata which appear to alter the ability of the guard cells to sense changes in either atmospheric or cellular hydration. Our studies point to the following situation: air pollution directly affects stomatal control, the loss of stomatal control changes the drought avoidance abilities of the foliage and, as a consequence, low turgor levels occur and premature needle abscission is induced. As site water balance decreases, either due to a dry year or to poor moisture holding abilities of the soil, these conditions become apparent.

Foliar ozone injury and radial growth of ponderosa pine

Foliar injury symptoms and stem diameter growth were measured on well-watered and drought-stressed ponderosa pine (Pinusponderosa Dougl. ex Laws.) seedlings at the end of each of 3 years of exposure to three levels of ozone: charcoal-filtered air, nonfiltered air, and nonfiltered air plus 1.5 times ambient ozone. Ozone-injury indices were constructed by adding percent chlorotic mottle and percent necrosis for each needle age-class. Percent necrosis was weighted from 1 to 5 times to construct different indices. Seedlings grown in nonfiltered air plus 1.5 times ambient ozone developed severe foliar injury after 2 years of exposure and were the only seedlings with significant reductions in radial growth after three seasons of exposure to a mean seasonal ozone concentration of 88 ppb. Foliage that had developed >30% chlorotic mottle by September of the 2nd year had abscised by September of the following year. Reduction in radial growth was significantly correlated with amount of foliar injury in well-watered trees, and the best-fit regression equation occurred when percent necrosis was weighted by a factor of 4. Regressions between foliar-injury indices and radial growth in drought-stressed trees were not significant, nor were regressions between radial growth and foliar injury among well-watered trees with only 1 year of premature needle abscission. The low R2 (0.30) between foliar injury and radial growth in well-watered trees and the lack of a significant regression in drought-stressed trees suggest that detection of reductions in stem diameter growth of ponderosa pine in the field in response to ozone injury could be difficult, except for severely injured trees with fewer than 2 years of foliar retention.

Physiological response to controlled freezing of attached red spruce branches

Previous studies of foliar cold tolerance in red spruce ( Picea rubens Sarg.) have been conducted on isolated shoots or needles. These studies therefore could not show either the type or amount of visible injury to attached foliage in a natural environment after controlled freezing stress, or the physiological response, if any, of visibly undamaged foliage. It was hypothesized that even when visible injury did not develop, foliage on frozen, attached branches would show long-term alterations in electrolyte leakage, water balance, and gas exchange capacity as a result of severe freezing stress. In mid-December 1990, intact, attached branches of red spruce saplings which had been exposed to controlled ozone levels (1 × or 2 × ambient) and precipitation acidity (pH 3.1 or 5.1) were frozen to −48 or −54°C. Neither ozone nor precipitation acidity affected any measure of freezing injury at P ⩽ 0.1. During the 3 months following freezing, current-year shoots developed either a brown needle discoloration, needle abscission without visible discoloration, or no obvious needle discoloration or abscission. This is the first experiment to show that severe freezing stress can lead to abscission of visibly undamaged needles from an attached branch. All frozen current-year needles had elevated relative electrolyte loss ( REL) with 24 hr after freezing, but REL subsequently declined for needles without visible discoloration. One day after freezing, average relative water content ( RWC) of needles on frozen and unfrozen shoots was about 85%, and RWC remained above 80% throughout the winter for unfrozen shoots. By mid-January, needle RWC on visibly injured shoots averaged about 50%. RWC of experimentally frozen needles without visible injury feel more slowly to between 65 and 76% by late February. Across all shoot types, REL just after freezing was negatively correlated with needle RWC 1 month later, and also with maximum dark respiration ( R), conductance to water vapor ( g), and light-saturated net photosynthesis ( A) measured on detached shoots at 12–20°C in late January to February. In unfrozen shoots, net photosynthesis reached a maximum of about 5 μmol m −2 s −1. Shoots with visible needle injury and shoots from branches which later showed needle abs cission had very large reductions in mean maximum gas exchange (> 50% for R, > 60% for g, and > 90% for A) compared to unfrozen shoots. On frozen branches which retained needles, non-visibly injured shoots had declining dark respiration rates and reductions of about 40% in maximum conductance and net photosynthesis, suggesting physiological damage from the freezing stress despite a lack of visible needle injury. Net photosynthesis and conductance remained lower in non-visibly injured frozen shoots compared to unfrozen shoots in late March and early April, more than 3 months after freezing. Initial net photosynthesis of frozen shoots placed in the growth chamber in late January was strongly correlated with maximum net photosynthesis. Initial net photosynthesis divided by initial conductance ( A i/ g i) was even more strongly correlated with maximum net photosynthesis ( r =0.95. A i/ g i was also negatively correlated with visible needle injury estimated in late March. A i/ g i measured on detached shoots, under favorable conditions for photosynthesis, may therefore be an early indicator of either latent visible needle injury or reduced photosynthetic capacity for freeze-damaged trees.

Investigations on the content indole-3-acetic spruce needles of healthy and damaged trees of various sites

Summary Surveys were made on the concentration of indole-3-acetic acid (IAA) in spruce trees of various sites. The concentrations of the hormone were specified with a radioimmunoassay after the needle extract had been purified in several steps. The content of IAA in needles of damaged trees was distinctly lower than the content in needles of healthy trees. Another feature was the annual rythm of the IAA content with the maximum of concentration in June. A distinct diurnal course with highest level of concentration during the early afternoon hours was also observed. The diurnal rhythm of the damaged trees was clearly lower. The supposition is that the lower IAA content is responsible for a premature needle abscission.

Implications of physiological responses to chronic air pollution for forest decline in the southeastern United States

The effects of acidic precipitation and ozone on physiological processes in loblolly pine (Pinus taeda L.) were assessed over time to determine dose-response relationships of cumulative exposure to varying levels of ozone and acid rain in combination. Seedlings of known provenance were grown in open-top field chambers in a clear-cut forest site in Duke Forest in the Piedmont of North Carolina and were exposed to pH 5.2 or 3.5 precipitation and several proportional ozone treatments (0.5-3.0x ambient ozone). No direct effects of acid precipitation on gas exchange and carbon fixation were found. Elevated ozone exposures had strong negative effects on needle abscission and net photosynthesis. Exposure-response relationships for photosynthesis indicated that a threshold level of approximately 100 ppm·h of cumulative ozone exposure (12-h summation) had to be reached before significant inhibition occurred. Results suggest that chronic pollutant exposure over the growing season has direct negative effects on carbon fixation in loblolly pine that can significantly reduce long-term growth potential. Current ambient ozone levels in the southeastern United States could be responsible for an approximate 19% reduction in instantaneous carbon fixation rates in loblolly pine by the end of one growing season. An 80 to 90% decrease in net photosynthesis was found at 2.25 and 3.0x ambient ozone concentrations, levels often found in major cities and surrounding areas in the South.

Effects of radiational heating at low air temperature on water balance, cold tolerance, and visible injury of red spruce foliage

Recent studies have shown that winter needle mortality in red spruce (Picea rubens Sarg.) is increased by exposure to direct solar radiation, possibly as a result of photo-oxidative damage, accelerated winter desiccation, or reduced cold tolerance due to heating of sun-exposed needles. In an experiment at controlled subfreezing air temperatures of -10 to -20 degrees C, visible radiation was less effective than infrared radiation in producing needle desiccation and visible injury during freeze-thaw cycles. However, visible radiation produced a red-brown color in injured needles, similar to natural winter injury, whereas injured needles exposed to infrared radiation were yellow and injured needles kept in darkness were dark brown. Thus, visible radiation was necessary to produce the red-brown color of damaged needles, but not the injury itself. Needle desiccation was not strongly correlated with visible injury, but the pattern of variation in visible injury among trees and the positive correlation between electrolyte leakage and visible injury suggested that freezing damage following freeze-thaw cycles might cause the visible injury. This was confirmed by a second experiment that showed loss of cold hardiness in needles thawed by radiational heating for six consecutive days. Even with a constant nighttime temperature of -10 degrees C, six days of radiational heating of needles to above freezing caused a small (2.8 degrees C) mean decrease in needle cold tolerance, as measured by electrolyte leakage. Continuous darkness at -10 degrees C for six days resulted in an estimated 5.6 degrees C mean increase in needle cold tolerance. Freezing injury stimulated desiccation: cooling at 4 degrees C h(-1) to -43 or -48 degrees C increased the dehydration rate of isolated shoots by a factor of two to three during the first day after thawing. Within three days at 15 to 22 degrees C and 50% relative humidity, the mean water content of these shoots fell to 60% or lower, compared to 90% or greater for unfrozen controls or shoots subject to less severe freezing stress. In some but not all severely freeze-stressed shoots, accelerated needle desiccation and abscission were accompanied by a red-brown color typical of red spruce winter needle injury. We conclude that severe winter desiccation in red spruce may often be due to prior freezing injury, increased as a result of exposure to direct solar radiation. Furthermore, freezing injury in red spruce may sometimes cause desiccation and abscission of green needles.

Nutrient dynamics in singleleaf pinyon (Pinus monophylla Torr & Frem.) needles.

Nutrient retranslocation from existing foliage of singleleaf pinyon pine (Pinus monophylla Torr. & Frem.) did not meet the nutrient demands of new needles on the same branch. Singleleaf pinyon pines, with an average age of 177 years, located at an elevation of 2300 m on a ridge in The Sweetwater Range in Nevada, USA retained needles for 18 or more years and increased needle number per branch by 17-18% annually. With age, needles gained weight and accumulated Ca, Mg, Fe, Zn, P, and Mn. The amount of N per needle showed no consistent trend with needle age. Only Cu and K declined in concentrations and amounts with needle age and these differences were not statistically significant. Retranslocation of nutrients at needle abscission could provide only 2 to 13% of the N, P, K, Fe, Cu, and Mg required by new needles. We conclude that nutrient capital in existing foliage constitutes a long-term nutrient reserve for the tree rather than a mobile nutrient pool.

IMPLICATIONS OF PHYSIOLOGICAL RESPONSES TO CHRONIC AIR POLLUTION FOR FOREST DECLINE IN THE SOUTHEASTERN UNITED STATES

The effects of acidic precipitation and ozone on physiological processes in loblolly pine (Pinus taeda L.) were assessed over time to determine dose-response relationships of cumulative exposure to varying levels of ozone and acid rain in combination. Seedlings of known provenance were grown in open-top field chambers in a clear-cut forest site in Duke Forest in the Piedmont of North Carolina and were exposed to pH 5.2 or 3.5 precipitation and several proportional ozone treatments (0.5-3.0x ambient ozone). No direct effects of acid precipitation on gas exchange and carbon fixation were found. Elevated ozone exposures had strong negative effects on needle abscission and net photosynthesis. Exposure-response relationships for photosynthesis indicated that a threshold level of approximately 100 ppm·h of cumulative ozone exposure (12-h summation) had to be reached before significant inhibition occurred. Results suggest that chronic pollutant exposure over the growing season has direct negative effects on carbon fixation in loblolly pine that can significantly reduce long-term growth potential. Current ambient ozone levels in the southeastern United States could be responsible for an approximate 19% reduction in instantaneous carbon fixation rates in loblolly pine by the end of one growing season. An 80 to 90% decrease in net photosynthesis was found at 2.25 and 3.0x ambient ozone concentrations, levels often found in major cities and surrounding areas in the South.

Increased Site Fertility and Litter Decomposition Rate in High-Pollution Sites in the San Bernardino Mountains

Abstract Some possible factors causing enhanced litter decomposition in high-pollution sites in the San Bernardino Mountains of southern California were investigated. Nitrogen concentration of soil, as well as foliage and litter of ponderosa pine (Pinus ponderosa Laws.) and Jeffrey pine (Pinus jeffreyi Grev. & Balf.) were greater in high-pollution plots than in moderate- or low-pollution plots. Nitrogen concentration of soil, foliage, and litter of ponderosa pine (ozone-sensitive), and of the ozone-tolerant spedes, sugar pine (Pinus lambertiana Dougl.) and incense cedar (Calocedrus decurrens [Torr.] Florin.), were all higher at a higher pollution site than at a moderate-pollution site. The rate of litter decomposition for all three species was also greater at the high-pollution site. Results suggest that the primary factor causing enhanced decomposition of L-layer litter in high-pollution plots is greater site fertility, leading to the production of foliage and litter that is higher in N than litter from moderate- or low-pollution plots. Although foliage of ponderosa pine trees exhibiting symptoms of severe ozone damage in high-pollution plots abscissed 2-4 years earlier than foliage of trees in low-pollution plots, retranslocation of N and P prior to needle abscission was not inhibited in high-pollution plots. Higher N levels in litter in the high-pollution plots does not appear to be due to reduced retranslocation of N from senescing foliage. Litter from ponderosa pine trees with severe symptoms of ozone damage (chlorotic mottle) decomposed at the same rate as litter from adjacent trees with no visible chlorotic mottle. Treating ponderosa and Jeffrey pine litter from four plots with 2, 20, or 200 mM NH4NO3 significantly increased the rate of litter decomposition at one or more N concentrations in three of the four plots. Total CO2 generated by the decomposing litter during a 15 or 17 day period tended to increase with N amendment, but was significantly increased only with litter from the dogwood plot treated with 200 mM NH4NO3. In some areas in the San Bernardino Mountains, chronic N deposition may stimulate litter decomposition by: (1) increasing site fertility, leading to higher concentrations of N in foliage and litter, and (2) accumulation of N onto litter and plant canopies, which stimulates litter decomposition upon wetting. For. Sci. 37(4):1163-1181.

Ethylene Production and Influence of Silver Thiosulfate on Ethylene Sensitivity of Cut Evergreens

A survey of 16 cut evergreen species found six clustered groupings of species based on ethylene production at 2 and 21C. Ethylene production (in nanoliters per kilogram of fresh weight per hour) at 21C ranged from 26 for Juniperus virginiana to 2800 for Sequoia sempervirens. Exposure to 0.1 or 1.0 ppm ethylene for 72 hours at 2C resulted in minor effects on two species, while significantly delaying senescence of Sequoia sempervirens. Silver thiosulfate (STS) pretreatment decreased or increased longevity of six species, but all effects were minor. Longevity of cut evergreens when held in preservative solution ranged from 14 days for Pinus sylvestris to 56 days for Chamaecyparis lawsoniana. Senescence symptoms observed were needle abscission, desiccation, and/or chlorosis.

Foliar injury and growth response of red spruce to sulfate and nitrate acidic mist

Seedlings of red spruce (Picearubens Sarg.) were exposed repeatedly, either in the field or in the greenhouse, to sulfuric acid mist, nitric acid mist (with or without ammonium ions), or a combination of sulfuric and nitric acid mist (with or without ammonium ions) for 10 to 39% of the time over periods of 6 to 19 weeks. Sulfuric acid mist produced visible foliar injury after repeated overnight exposures to pH 3 or less. Foliar symptoms were significantly less or absent after exposures to nitric acid or combined sulphuric–nitric acid mist at the same pH values. Abscission of needles during the subsequent winter also was increased by sulfuric acid mist. Intermittent exposures that contained fewer hours of treatment with acidic mist and longer drying periods than continuous exposures caused greater amounts of foliar injury. Wind during the drying periods enhanced foliar injury, especially with sulfuric acid mist. Significant reductions in shoot elongation occurred with sulfuric acid mist in those experiments where seedlings continued to grow throughout the treatment period. Different provenances responded similarly to acidic mist. The results indicate that red spruce is particularly vulnerable to injury from acidic mist when sulfate concentrations are high and when mist exposures are intermittent with repeated periods of drying of liquid on foliage.

Litter Decomposition Across an Air‐Pollution Gradient in the San Bernardino Mountains

AbstractAir pollution may affect forest ecosystems by altering nutrient cycling rates. The objective of this study was to compare decomposition rates of L‐layer litter of ponderosa pine (Pinus ponderosa Laws.) and Jeffrey pine (Pinus jeffreyi Grev. & Balf.) collected from across an air‐pollution gradient in the San Bernardino Mountains in southern California. Litter from the more polluted western areas decomposed at a significantly (P = 0.01) faster rate than litter from plots in areas exposed to moderate or low pollution levels. The Shannon diversity index values for litter fungi were 1.41, 1.84, and 2.25 for plots in areas exposed to low, moderate, and high pollution levels. Nitrogen content of litter was greatest in the high pollution plots and was positively correlated with litter decomposition rate. Higher N and lower Ca content of litter at the more polluted western sites suggest that litter in the western plots originates from younger foliage than litter at the less polluted sites, possibly because of ozone (O3)‐induced needle abscission. Possible factors contributing to increased decomposition and fungal diversity in the western plots include: higher litter nutrient content because litter originates from younger foliage, higher soil N levels which may result in higher foliar N content, greater input of N from air pollution, and O3 damage to needles creating favorable sites for microbial decomposers.

Initiation and development of latent infections by Rhabdocline parkeri on Douglas-fir

Quantitative estimates of frequencies of latent infections by Rhabdocline parkeri Sherw. on Douglas-fir were obtained by direct microscopy of cleared needles from three half-sib trees. The infections are intracellular and are confined to a single epidermal cell until the onset of needle senescence, a period of 2 – 5 years. Infection frequencies varied widely among trees but increase logarithmically with needle age on all three trees. Differences in infection frequencies among trees were not demonstrably related to levels of infestation by Contarinia spp., a needle-galling dipteran. Active colonization of the needle resumes at the onset of needle senescence, beginning with the production of haustoria in cells adjacent to the original infection sites. Rapid colonization of the needle and sporulation of R. parkeri coincide with needle abscission, occurring before substantial colonization of the needles by saprophytic fungi. Saprophytic fungi colonized needles rapidly following abscission, but R. parkeri was recoverable in culture from needles up to 35 days after abscission.

Soluble Carbohydrate Concentration of Fraser Fir Foliage and Its Relationship to Postharvest Needle Retention

Abstract Seasonal changes in soluble carbohydrates of Fraser fir [Abies fraseri (Pursh) Poir.] needles were monitored in Fall 1984, Spring 1985, and Fall 1985 through Spring 1986. Raffinose concentration increased in the fall and decreased in the spring. There was a 23-fold increase in raffinose concentration from Aug. 1985 to Jan. 1986. Sucrose concentration varied from fall to spring with the lowest concentration occurring in February. Postharvest needle abscission from harvested branches held 6 weeks without water was inversely correlated with raffinose concentration at the time of harvest. Diurnal fluctuations in soluble carbohydrates were monitored on 12 July and 26 Oct. 1985. Raffinose concentration fluctuated slightly on both dates with a decrease during the dark period. On 12 July, sucrose increased during the day and decreased at night, whereas hexoses decreased in the day and increased at night. No significant diurnal changes in sucrose or hexose were evident on 26 Oct. Controlled-environment studies at 24° (day)/18°C (night), 18°/12°, and 12°/6° showed that most of the raffinose accumulation was due to low temperature; the remainder to short days. Postharvest needle loss was lowest in plants with high needle raffinose concentrations resulting from the 12°/6° temperature. Storage without water resulted in significant postharvest needle loss for shoots from plants preconditioned with 24°/18° and 18°/12°, but not for those exposed to 12°/76°. Compared to long days, plants preconditioned with short days lost fewer needles following harvest.

Litter-Fall In a Pinus radiata Forest: The Effect of Irrigation and Fertilizer Treatments

(1) A 15 year-old Pinus radiata plantation near Sale, Victoria, Australia, was irrigated with waste-water for 3 years. A litter-fall of 400 g m-2 year-1 was recorded in irrigated plots compared with 320 g m-2 year-1 in non-irrigated plots. The increase in litter-fall resulted from an increase in the production of new needles and greater shedding of older needles. Periods of soil water deficit caused increased shedding of needles in both irrigated and non-irrigated plots. (2) Irrigation with waste-water increased the concentration and amounts of most elements in the litter-fall which were present in the waste-water. Whilst the effect of irrigation on tree growth and the rate of turnover in organic matter was beneficial during the period of study, increases in pH and alkalinity in the standing litter and soil could eventually reduce the availability of iron and other trace elements. Superphosphate fertilizer decreased litter-fall slightly and the concentration of most elements except phosphorus. (3) Distinct seasonal patterns were observed in the concentrations of nutrients in litterfall and the following hypothesis has been proposed to explain these seasonal changes: increasing stress during spring and summer caused abscission of mature needles to occur more rapidly and higher in the crown, resulting in higher concentrations of N, P and K in falling litter. Male strobili contained very low concentrations of Ca and a sharp reduction in the concentration of this element occurred in litter-fall when strobili were shed in spring. Seasonal patterns observed in the concentrations of N, P and K in litter-fall were the reverse of those found in living foliage. The efficiency of nutrient withdrawal thus appeared to reach maximum values in winter and minimum values in summer.