Quantifying the effect of pine mistletoe on the growth of Scots pine

Mistletoe infection results in substantial growth losses in mistletoe‐infected forests. This study reports and evaluates the results of retrospective analyses of radial growth of Scots pine (Pinus sylvestris) in relation to the level of infection of pine mistletoe (Viscum album ssp. austriacum). A total of 43 Scots pine trees were destructively sampled from different sites. Of these trees, 14 were uninfected and 29 were infected. Infection classes were determined using six‐class dwarf mistletoe rating system (DMRS). All needle and mistletoe biomass were removed completely and weighed for each sampled tree. Subsamples from needles and all mistletoe biomass were taken to the laboratory for oven‐dried weight determinations. Five‐cm‐thick wood discs were cut from the stem at the breast height (1.3 m) to determine annual basal area increment for the last 25 years. In addition to DMRS, new infection classes were created using mistletoe‐to‐needle biomass (MB/NB) ratio. The results showed that the radial growth losses could be as much as 41% to 64% at different infection levels. The rate of growth loss in relation to DMRS and MB/NB ratio was similar, but with a larger variability in DMRS values. The results showed that both DMRS rating and MB/NB ratio seem to be important for quantifying growth loss on Scots pine trees infected with mistletoe. The results of this study can also be invaluable in modelling the effects of mistletoe on the growth of Scots pine trees.     

Pine mistletoe (Viscum album ssp. austriacum) contributes to Scots pine (Pinus sylvestris) mortality in the Rhone valley of Switzerland

In recent years unusual high mortality of Scots pine (Pinus sylvestris) has been observed in the Swiss Rhone Valley. The exact causes, however, are not known. At a 2‐ha monitoring plot, tree mortality and crown condition have been monitored since 1996. Between 1996 and 2004, 59% of the Scots pines died, most of them following the drought periods 1996–1998 and 2003–2004, while only 15% of the deciduous trees died. Crown transparency, needle discolouration, dead branch percentage, mistletoe (Viscum album ssp. austriacum) rating, Tomicus sp. shoot feeding, male flowering effect, tree stem diameter, crown shading and social tree class assessed in 1998 were used in a logistic regression model to predict tree mortality. Crown transparency, mistletoe rating and percentage of dead branches were found significant in the model and the probability of tree mortality increased with increasing rankings of these parameters. Needle discolouration could be used to substitute ‘dead branch percentage’ as predictor. While crown transparency increased with mistletoe rating, for trees in the same transparency class, trees with medium and heavy mistletoe infection were two to four times more likely to die than trees with no or only low mistletoe infection. For the surviving trees we found that trees with mistletoes showed a significantly higher increase in transparency in the year following a drought than trees without, while in a drought year the opposite was true. At the beginning of the observations no significant differences in transparency had been found between the trees with and without mistletoe. However, by the end of the observation period trees with mistletoe had significantly higher crown transparency. We conclude that mistletoe infection can be considered as both a predisposing factor for tree death, by increasing needle loss following drought and a contributing factor by increasing water stress during drought.     

Needle nutrient status of Sirococcus shoot blight‐diseased and healthy Norway spruces

In this study the needle nutrient status of Sirococcus shoot blight‐diseased and healthy Norway spruces (Picea abies) was investigated. Element contents were determined in the current year and 3‐year‐old needles collected in December 2000 from 72 mature trees randomly distributed in the investigated stand. Half of the trees were severely affected by Sirococcus conigenus and had a reduced crown whereas the other trees were unaffected and vigorous. Compared with the uninfected trees, contents of Mg, Ca, P and Mn were significantly lower in the current year and 3‐year‐old needles of diseased trees. Furthermore, analyses of the nutrient contents of needles from severely affected trees revealed insufficient Mg and Ca supply and enhanced N/Mg and N/Ca ratios, when compared with threshold values. The results of this study suggest that imbalanced tree nutrition either has a significant impact on the expression of Sirococcus disease symptoms or fungal infection itself is affecting needle nutrient contents.     

The impact of Sirococcus shoot blight on radial and height growth of Norway spruce (Picea abies) in young plantations

The impact of Sirococcus shoot blight on the radial and height growth of Norway spruce in a young plantation (approximately 20 years old) was investigated by examining the increment losses for four infection intensities (classes). The average diameter at breast height of trees in the lightly damaged class was 72% when compared with the average diameter of the healthy trees, whereas moderately and severely damaged tree classes were 67 and 57%, respectively. Using tree ring analysis, the development of radial growth over time due to intensity of infection was studied. Height growth of affected trees was also significantly reduced (up to 43%) compared with the healthy trees, thus indicating a dramatic impact of Sirococcus conigenus on the growth of young Norway spruce.     

Environmental Effects on Norway Spruce Needle Wax

In spring, tubular wax structures were observed on the needle surfaces of new Norway spruce needles and also on previous-year needles. In young plants, as in mature trees, the fusing and eroding of the wax tubes on needles of increasing age had taken place in relation to their exposure, particularly to wind and precipitation. The quantity of needle wax and certain fractions of the extracted wax differed in mature trees at two field sites and in young clonal plants. No difference in wax quantity and only small differences in wax composition occurred among young plants which had undergone different treatments (fumigation with O³, SO², ambient air, filtered air, or planting at the sites). Environmental effects on wax structure, wax quantity and the presence of certain compounds in the wax were identified.     

Vitality rating of picea abies by defoliation class and other vigour indicators

The study was made to assess the relationship between visually estimated defoliation class and a number of other variables depicting the vitality of Norway spruce (n=50). Variables characterizing crown size and condition were determined on the standing trees. Electrical impedance (an indication of the physiological status of a tree) was measured in the inner bark tissue of the living trees. Shoot growth and needle variables were determined from the detached sample branches. Negative correlations were found between defoliation class and the growth parameters (5‐year height, radial and volume growth of the trunk). Positive correlations were observed between the needle loss class and the abundance of branch damage, secondary shoots and impedance values. 18 variables (defoliation excluded) describing tree size and vitality were summarized in a factor analysis incorporating 5 factors. These were interpreted as 1) vitality, 2) foliage discoloration, 3) tree size, 4) needle number and 5) needle size factors. The tree‐specific score values for the first factor were interpreted as “vitality indices”;. The rank correlation between these score values and defoliation degree was 0.835. This result suggests that the defoliation class and vitality parameters contributing to the first factor give a consistent estimate of the tree condition.     

Controlling and predicting the spread of heterobasidion annosum from infected stumps and trees of picea abies

Two Norway spruce stands with heavy infections of Heterobasidion annosum were clear‐cut in 1957 and 1959 in Sweden. The stumps were extracted, the soil sifted to remove most of the roots, and young Norway spruce were planted. After 25 and 28 years, H. annosum had infected 1 % and 2% of trees on plots where stumps had been removed and 17 % and 12% of the trees on control plots, respectively. Several of the H. annosum clones fruiting on old‐growth stumps were also detected in decayed, standing trees. The same fungal clone was found to be infecting adjacent trees from several old‐growth stumps. In addition to old stumps, stumps from recent thinnings and diseased living trees were traced as infection sources. Their relative importance in spreading disease was estimated. Disease risk predictions based on the distance of a tree from various infection sources correlated well with observed frequencies of rot.     

Effects of Mexican dwarf mistletoe (Arceuthobium vaginatum subsp. vaginatum) on the growth of Pinus cooperi in Durango,Mexico—A case study

Mexican dwarf mistletoe (Arceuthobium vaginatum subsp. vaginatum, Viscaceae) is the most widespread and damaging parasitic plant in Mexico. It parasitizes 10 species of pines (Pinus spp., Pinaceae) as principal hosts, including Pinus cooperi, one of the economically most important pines in the state of Durango. As a case study, we used stem analysis to estimate the effects of Mexican dwarf mistletoe on volume and height growth of dwarf mistletoe‐infected P. cooperi in western Durango (Ejido El Brillante, Municipality Pueblo Nuevo). This case study sampled a total of 48 trees, 12 in each of four infection classes estimated using the 6‐class dwarf mistletoe rating system (DMR): uninfected (DMR 0), lightly infected (DMR 1–2), moderately infected (DMR 3–4) and severely infected (DMR 5–6). Significant reductions in both volume and height growth were found for moderately and severely infected trees when compared to uninfected trees. On average, reductions in volume growth and height growth were as high as 50% and 17%, respectively. The largest growth reductions were for moderately infected trees, but large growth reductions also occurred for severely infected trees. Because of the reduced growth associated with moderate to severe infection, Mexican dwarf mistletoe‐infested pine forests in Durango, where timber production is a high priority, should be managed using harvesting practices that reduce dwarf mistletoe infection, and thereby, increase forest productivity.     

Changes in shoot properties in relation to vertical positions within the crown of mature canopy trees of Abies mariesii and Abies veitchii

We investigated current shoot properties in two contrasting vertical positions (leader crown; LC, and lower branch; LB) within the crowns of mature trees of two subalpine conifer species, Abies mariesii and A. veitchii. For both LCs and LBs, shoot length decreased with increasing branching order. However, shoot properties were different between LCs and LBs. Shoots in LCs had more needle biomass per unit of shoot length. Shoots in sunny conditions pack needles closer along the shoot and intercept incoming light more completely. This causes the shoots in the LCs to have more needles. In contrast, less needle packing per unit shoot length in LBs results in the avoidance of mutual shading among needles in order to intercept limited light more effectively. Because branch systems in lower layers tend to be more shaded, the quantity of irradiance received by the shoots in LBs is smaller. Thus, reduced needle amounts on the shoots in LBs reflect the needle arrangement acclimating to the lower light availability. This study suggests the importance of changes in the properties of individual shoot as a component of a branch system and accordingly a whole-crown system in mature canopy trees of A. mariesii and A. veitchii.     

Tree recovery during the aftermath of an outbreak episode of the Hungarian spruce scale in southern Sweden

In 2010, the first, and so far only, infestation of the Hungarian spruce scale (Physokermes inopinatus) and accompanying sooty mould occurred in Scania, southernmost Sweden. About 1000?ha of Norway spruce (Picea abies) were affected, and the trees suffered from the sucking of the insects as well as from the dense sooty mould that covered the needles. Salvage cuttings were carried out in many of the massively attacked forest stands, both in response to the fear that the trees otherwise would die, e.g. from secondary bark beetle attacks and to prevent spreading of the infestation. The aim of this study was to provide basic, quantitative knowledge on the aftermath response of trees that were heavily infested, but not exposed to salvage cutting. Growth characteristics, in terms of needle weight, shoot length and tree-ring size were measured on infested and uninfested trees to compare and contrast the spruce growth before, during and after the scale outbreak. The infestation resulted in dwarf annual shoots, stunted needles and thin tree rings. The needle weight returned to normal the following year, whereas shoot length and tree rings required one growing season before full recovery.     

Impact of eastern dwarf mistletoe (Arceuthobium pusillum) infection on the needles of red spruce (Picea rubens) and white spruce (Picea glauca): oxygen exchange, morphology and composition

Eastern dwarf mistletoe (Arceuthobium pusillum Peck) is a hemiparasitic angiosperm that infects white spruce (Picea glauca (Moench) Voss) and red spruce (P. rubens Sarg.) in northeastern North America. The effects of mistletoe infection differ substantially between white and red spruce, with white spruce suffering greater infection-induced mortality. In the present study, we sought to determine the role that species-specific differences in needle-scale responses to parasitism may play in the observed differences in the effect of infection on host tree health. Based on the measurements made, the most apparent effect of parasitism was a reduction in needle size distal to infections. The magnitude of this reduction was greater in white spruce than in red spruce. Eastern dwarf mistletoe was a sink for host photosynthate in red spruce and white spruce; however, there were no adjustments in needle photosynthetic capacities in either host to accommodate the added sink demands of the parasite. Needle total nonstructural carbohydrate concentrations (TNC) were also unaltered by infection. Red spruce needles had higher TNC concentrations despite having lower overall photosynthetic capacities, suggesting that red spruce may be more sink limited and therefore better able to satisfy the added sink demands of parasitic infection. However, if carbon availability limits the growth of the mistletoe, one may expect that the extent of the parasitic infection would be greater in red spruce. Yet in the field, the extent of infection is generally greater in white spruce. Taken together, these results suggest that dwarf mistletoe may not substantially perturb the carbon balance of either host spruce species and that species-specific differences in needle-scale responses to the parasite cannot explain the contrasting effects of infection on white spruce and red spruce.     

Susceptibility of red fir and white fir to fir dwarf mistletoe (Arceuthobium abietinum) in California

Fir dwarf mistletoe (Arceuthobium abietinum, Viscaceae) is a damaging parasite of true firs (Abies spp.) in California. In the Sierra Nevada Mountains, fir dwarf mistletoe consists of two special forms (formae speciales, f. sp.) which are morphologically similar, but exhibit a high degree of host specificity: A. abietinum f. sp. concoloris whose principal host is Sierra white fir (Abies lowiana) and A. abietinum f. sp. magnificae which only parasitizes red fir (Abies magnifica). Although field observations and cross‐inoculation studies have consistently supported the extreme host specialization demonstrated by these special forms of fir dwarf mistletoe, little quantitative data from mixed red fir/white fir stands infested with fir dwarf mistletoe have been available. Therefore, I sampled 20 mixed conifer stands dominated by red and white fir and infested with f. sp. concoloris (8 stands) or f. sp. magnificae (12 stands) from near Mount Shasta to the southern Sierra Nevada Mountains near Sherman Pass. Infection of red and white fir was evaluated using temporary circular plots (0.012 ha) established around large, severely infected trees and with rectangular transects placed within fir dwarf mistletoe‐infested stands. In each plot and transect, live trees >5 cm in diameter at breast height were assigned a dwarf mistletoe rating (6‐class system). A total of 55 plots and 51 transects were established in the 20 stands, and over 1,900 red firs and 1,700 white firs were sampled. Ninety‐nine per cent of the red firs and 98% of the white firs were infected in stands infested with f. sp. magnificae or f. sp. concoloris, respectively. In contrast, no infection of red fir was observed in stands infested with f. sp. concoloris and no infected white firs were observed in stands infested with f. sp. magnificae. These data provide further support that the special forms of fir dwarf mistletoe are extremely host specific in California.     

Effect of dothistroma needle blight on needle and shoot lengths

The needles of pine trees are indicative of the overall health of the tree, and their length is affected by many factors. This study describes the effect of high levels of pathogen infection on lengths of both needles and shoots. Dothistroma septosporum is an important foliar pathogen of pines causing necrosis and premature defoliation with successive years of high infection leading to growth reduction and in extreme cases tree death. Corsican pine trees with all foliage infected by D. septosporum had needles of primary, secondary and tertiary shoots reduced by 31.8%, 44.0% and 64.2%, respectively, compared to non‐infected trees. Needle lengths were reduced both in the upper and lower portions of the canopy, with a greater reduction lower in the canopy. Trees with high levels of infection had shorter shoots in the upper canopy with fewer, shorter needles on these shoots compared to trees with low levels of infection. The results demonstrate the substantial negative effect on needle and shoot lengths of trees with high levels of D. septosporum infection, comparable to factors such as water and nutrient availability known to have a strong influence on these parameters. The reductions in length reduce the photosynthetic capacity of the tree and compound the immediate reductions caused by necrosis and premature defoliation. These effects on needle and shoot lengths contribute to the reductions in volume growth of affected trees and, moreover, are longer lasting than the immediate effects of necrosis and premature defoliation.     

Transformation of western hemlock (Tsuga heterophylla) tree crowns by dwarf mistletoe (Arceuthobium tsugense,Viscaceae)

Dwarf mistletoes (Arceuthobium species) are arboreal, hemiparasitic plants of conifers that can change the structure and function of the tree crown. Hemlock dwarf mistletoe (Arceuthobium tsugense subsp. tsugense) principally parasitizes western hemlock (Tsuga heterophylla) and effects 10.8% of all western hemlock trees in Oregon, USA. In this study, we climbed 16 western hemlock trees (age 97–321 years, height 33–54.7 m) across a gradient of infection (0%–100% of branches infected) and measured occurrence of all dwarf mistletoe infections, dwarf mistletoe caused deformities, foliage, branch and crown metrics, and sapwood area. We then modelled over 25 different response variables using linear and generalized linear models with three metrics of severity as explanatory variables: total infection incidence, proportion of all live branches infected, and proportion of all live, infected branches with 33 per cent or more foliage distal to infection. A strong effect of dwarf mistletoe intensification was the reduction of branch foliage and an increase in the proportional amount of foliage distal to infections, with severely infected trees having the majority of foliage distal to infections. Increasing severity led to an apparent crown compaction as crown volumes decreased and became increasingly comprised of deformities. Sapwood area was unrelated to infection severity. Branch length and diameters were unrelated to increasing infection severity despite severely infected branches supporting 1–70 infections. The most severely infected tree had 3,615 individual plants in the crown. Our results suggested that shifts in crown structure and branch deformation, foliage amount, and foliage distal to infection, reflected a likely reduction of capacity for tree growth that coincided with a hypothesized increase in resource demand by dwarf mistletoe plants as infection severity intensified.     

Mortality and growth of dwarf mistletoe‐infected red and white fir and the efficacy of thinning for reducing associated losses

In managed forests dominated by true fir (Abies) species, stands are often restocked using understory trees retained during timber harvest, making the effects of dwarf mistletoe (Arceuthobium spp.) infestation on small true fir a concern. This study examined the response of small red (A. magnifica) and white (A. concolor) fir and their dwarf mistletoes (A. abietinum f.sp. magnificae and A. abietinum f.sp. concoloris, respectively) to precommercial thinning treatments in fir‐dominated stands in the Sierra Nevada Mountains of California. Tree diameters and dwarf mistletoe ratings were monitored from 1981 to 2001, and mortality was observed from 1981 to 2006. Red and white fir survival and radial growth decreased significantly with greater mistletoe ratings and increased with larger diameters and live crown ratios. Thinning significantly increased survival and growth of white, but not red fir. Over the course of the study, mistletoe ratings for both fir species did not change significantly in unthinned stands, but increased in thinned stands. However, while statistically significant, increases in mistletoe ratings in thinned stands were gradual and mistletoe ratings did not statistically differ between treatments 20 years post‐thinning. Additionally, thinning did not significantly influence mistletoe spread to uninfected trees, indicating that precommercial thinning in lightly infested red and white fir stands will not likely result in substantial increases in dwarf mistletoe abundance over typical harvesting intervals. Overall, while severe dwarf mistletoe infection ratings reduced tree survival and growth, because ratings remained low, actual losses resulting from mistletoes during this study were minor and will not likely result in substantial economic losses over normal harvesting intervals. This finding indicates that precommercial thinning treatments designed specifically to compensate for mistletoe‐associated losses may not be necessary when managing red and white fir for timber production.     

Infection of Norway spruce container seedlings by Gremmeniella abietina

First‐ and second‐year containerized Norway spruce seedlings were inoculated with conidia of type A (large tree type) and type B (small tree type) of Gremmeniella abietina var. abietina at different times during the summer. The appearance of symptoms after artificial inoculation and natural infection on spruce seedlings were recorded the following spring and compared with the disease symptoms on Scots pine seedlings. The proportion of diseased seedlings after inoculation reached as high as 80%. The susceptible period during the summer began later on the first‐year seedlings than on the second‐year seedlings, and was similar for the pine seedlings. Susceptibility of first‐year seedlings was highest in August and on second‐year seedlings in July. The accumulated temperature sum, relative humidity and height growth for first‐ and second‐year seedlings was assessed. Natural infection in 2002 caused more disease on pine than on spruce seedlings. Experimental thinning of seedlings had no effect on disease incidence. In a preliminary comparison between the ability of A and B types to cause disease in Norway spruce seedlings, type B caused more damage than type A after inoculation. However, type A caused a high disease frequency in other experiments in this study. Symptoms on Norway spruce seedlings often first occurred in the mid‐section of the shoot, and were similar to those observed on pine seedlings: needles turned brown, starting at the needle base, in the spring following inoculation. On first‐year spruce, diseased needles were shed rapidly, in contrast to a slower rate of shedding on first‐year pine seedlings. Pycnidia developed about 2 years after inoculation (on pine 1 year after inoculation). On Norway spruce seedlings the lower part of the shoot, including the lateral shoots, often remained alive. The experiments show that G. abietina can cause disease on containerized Norway spruce seedlings under nursery conditions in Finland. The coincidence of spore dispersal, seedling susceptibility and predisposing factors are important in disease development.     

Yield reduction in spruce infected with Armillaria solidipes in the southern interior of British Columbia

The effect of non‐lethal root infection on the yield of planted spruce (Picea engelmannii x glauca hybrid spruce) in southern British Columbia was determined in a 34‐year‐old, mixed‐species stand infected with Armillaria solidipes Peck. Yield reduction correlated with trees’ infection duration. Individual trees infected for 24 years had about 42% less volume than their healthy equivalents. Using the incidence of infection in stands in this area and the individual losses per tree, reduction at the stand level was approximated at 11% and increasing.     

Revealing past needle density in Pinus spp.

The number of short shoots per shoot length, or needle density, is species typical, and it shows year‐to‐year variation within species. By modification of the needle trace method, long‐term needle density chronology was produced in a Scots pine (Pinus sylvestris L.) stand, located at the northern timberline in Finland. Treewise, needle density varied between 9 and 14 short shoots per long shoot (stem internode) centimetre, the annual minimum and maximum values being 5 and 37 short shoots cm^?1. The stand‐specific long‐term average was 10.5 short shoots cm^?1, and the mean annual value varied between 17 and 8 short shoots cm^?1 in 1951 and 1984, respectively. The long‐term pattern in needle density was one of decline with time between 1950 and the mid‐1970s, then to slightly increase on entering the 1990s. The years when the density was relatively high were 1957, 1968 and 1981, indicative of some climatic extremes.     

Pathogenicity of Ceratocystis resinifera to Norway spruce

The blue‐stain fungus Ceratocystis resinifera colonizes wounds on living Picea spp. and other conifers in Europe and North America. Little is known regarding the pathogenicity of this fungus and consequently, four Norwegian C. resinifera isolates were inoculated on to Norway spruce (Picea abies) using two different techniques. These included single‐point inoculations on young trees (two inoculations per tree on 14‐year‐old trees) and mass‐inoculations on older trees (～200 inoculations per tree on 34‐year‐old trees). In both experiments, C. resinifera induced minor symptoms that in most cases did not differ significantly from inoculation with sterile agar. The virulent blue‐stain fungus C. polonica, which was inoculated for comparative purposes, induced extensive symptoms, causing 83% dead cambium circumference and 82% blue‐stained sapwood, and long necrotic lesions in the phloem. The results suggest that C. resinifera is non‐pathogenic or only mildly pathogenic to Norway spruce and does not present a threat to these trees.     

Epicuticular waxes,peroxidase activity and contents of chlorophyll,phaeophytin and carotenoids of needles from Pinus syivestris and picea abies as indicators of air pollution in the region of Stockholm

To trace any possible effects of air pollution stress and injuries on trees around a city centre, needles of Scots pine (Pinus Syivestris L.) and Norway spruce (Picea abies (L.) Karst.) from sites of different distances and directions from Stockholm were analysed. Comparisons were made between needles of different ages, a half to three and a half years old (C ‐ C + 3) for pine up to six and a half years old (C ‐ C + 6) for spruce. Epicuticular waxes, studied by scanning electron microscopy (SEM), showed age‐dependent but site‐independent changes. Different age‐patterns were found in the pine and spruce needles but no dust coverings or lesions were detected, irrespective of the sites. A proportionally increasing peroxidase activity (EC 1.11.1.7) with increasing needle‐age up to the third year was observed in both species. No certain indication of site‐effects, expressed by an unproportional increase of the enzyme activity with increasing needle‐age could be found. Chlorophyll a and b, phaeophytin a and b and carotenoid analyses on the corresponding materials only indicated disturbances in trees from some scattered sampling plots. Thus, neither could the pigment analyses reveal any signs of a general pollution stress in pine and spruce trees from the area in question.