Frequencies and patterns of browsing by large herbivores on conifer seedlings

Browsing by large herbivores on planted and naturally regenerated conifer seedlings (Picea abies and Pinus sylvestris) was recorded in 104 clear‐cuts in east‐central Sweden during 1990 and in 47 clear‐cuts in 1991. The number of seedlings browsed and the browsing patterns were analysed in relation to seedling type. Browsing frequencies were also compared between forest stands with different site productivities and subjected to different management practices. The variation in the number of seedlings browsed in 1990 was explained mainly by seedling category. Among both planted and naturally regenerated seedlings, pine was browsed more than spruce. Two‐year‐old containerized seedlings of spruce was browsed more than 4‐yr‐old bare‐rooted spruce. In 1991, browsing was more equal among species and seedling types. Number of seedlings with their leader browsed and the amount of biomass left after browsing differed significantly between seedling types. Seedlings that had been browsed in 1990 experienced significantly higher browsing frequencies in 1991 when compared with unbrowsed seedlings. The effects of stand characteristics were not found to be significant.     

Effects of thinning on nutrient content pools in two Pinus sylvestris forests in the western Pyrenees

Abstract

Nutrient content [nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg)] in needles, branches and stems before (P0, not thinned) and after thinning (P20 and P30, 20% and 30% of basal area removed, respectively) was studied in 37- and 32-year-old Pinus sylvestris L. forests in the western Pyrenees: Garde and Aspurz. Thinning significantly decreased all nutrient pools in P20 and P30 relative to P0 on both sites, but no significant differences were found between P20 and P30 owing to low statistical power. Thinning increased the differences between the two forests for total above-ground content of P and Mg, and for the content of N, P and K in the 1999 needles cohort. The former result was due in part to the higher concentration of P (needles and branches) and Mg (needles) in Garde. Therefore, the importance of needles relative to the other nutrient pools increased for N and P after thinning. This may have resulted from the fact that the removal of dead trees with low needle biomass was greater in Garde than in Aspurz. Based on the percentage of basal area removed, nutrient removal in Garde for P30 was higher than expected, apparently owing to an increase in branch removal relative to P20. These results indicate that effects of thinning on nutrient pools were influenced by differences in stand structure and nutrient tissue concentrations between sites.     

Browsing by large herbivores on Scots pine (Pinus sylvestris) seedlings in mixture with ash (Fraxinus excelsior) or silver birch (Betula pendula)

Abstract

Food selection by large herbivores occurs at a hierarchy of scales, for example landscape, patch or plant. Several hypotheses regarding food plant selection on patch or plant level have been developed. In this cafeteria-type design field experiment, conducted during one winter immediately after planting, we tested the effect of species mixture on browsing by large herbivores (mainly roe deer) on Scots pine seedlings in mixture with seedlings of ash (highly preferred) or silver birch (less preferred). Browsing on Scots pine was not affected by species mixture, neither in terms of the number of browsed pines nor browsing intensity. Instead, browsed biomass was positively and significantly correlated to the total biomass available for browsing. Also, there were differences due to species, with ash being most browsed (44.6%), followed by Scots pine (18.9%) and silver birch (11.6%). Browsed biomass per browsed seedling, however, was largest for Scots pine. In addition, browsed seedlings were initially taller compared to unbrowsed seedlings for all species. The main management implication in this study is that the species mixture did not influence large herbivore browsing on Scots pine seedlings. Hence, removing or discouraging more (or less) attractive browse species in early stages of pine regeneration activities seems unnecessary from the point of large herbivore browsing.     

Influence of Cattle Stocking Rate on Browsing of Norway Spruce in Subalpine Wood Pastures

In the Swiss Alps, 15% of Swiss mountain forests are grazed during summer, mainly by cattle. The forest laws of various Swiss cantons characterise forest grazing as a detrimental form of land use and stipulate that this grazing practice should be restricted. However, little is known about tree damage actually caused by cattle. Seven subalpine ranges in the Swiss Canton Grisons, grazed by cattle at different stocking rates, were investigated. The condition of naturally regenerated young trees (Picea abies (L.) Karst.) was assessed before and after the cattle grazing period. In order to characterise the influence of wild ungulates on the young trees during winter, the assessment of tree condition was repeated in the proximate spring. In total, 4% of the young trees were browsed on the apical shoot, 10% were browsed on lateral shoots, 13% of the trees showed other damage. The variation among ranges could almost completely be explained by the cattle stocking rate (livestock units per hectare). During winter, wild ungulates browsed 3 times as many young trees as the cattle during summer. The results suggest that cattle stocking rates on subalpine wood pastures should not exceed one livestock unit per hectare in order to avoid intensive browsing and other damage by cattle on young Norway spruces.     

Early Influence of Wood Ash Application on the Carbohydrate Reserves and Composition in Needles of Scots pine {Pinus sylvestris)

Abstract

Wood ash treatment can probably increase forest productivity on low fertility sites. However, the resulting effect on the carbohydrate concentration as the main carbon and energy reserve in trees is little studied. In 2000, a square of 0.1 ha sandy soil below a 19-year-old Scots pine (Pinus sylvestris) plantation was treated with raw fly ash (0.5 kg.m^-2); untreated square was used as control. One year after the treatment, carbohydrates (glucose, fructose, sucrose, maltodextrins, starch and excess bound fructose) were analyzed enzymatically from current-year and one-year-old needles. The ratio K/N in needles suggested an improved balance between these elements in treated trees, in which the K concentration was higher. The largest relative differences (50% of control) were observed in glucose and fructose in summer. The squares did not differ in the concentration of the accumulated carbohydrate reserves in needles during low temperature stress in winter and before the growth of new shoots in spring. During the vegetation period decreased levels of soluble carbohydrates and starch were observed (max 70% of the control value around 100 mg total carbohydrates g^-1 dry mass). Because the experiment was designed without true replicates, reasons for the observed differences require further study.     

Minimum cuticular conductance and cuticle features of Picea abies and Pinus cembra needles along an altitudinal gradient in the Dolomites (NE Italian Alps)

Winter desiccation is believed to contribute to stress in coniferous trees growing at the treeline because cuticular conductance increases with altitude. To test whether winter desiccation occurs in high-altitude conifers of the Dolomites (NE Italian Alps), we measured minimum cuticular conductance (g(min)), needle wettability (contact angle) and cuticle thickness in Picea abies (L.) Karst. and Pinus cembra L. needles from December to August. Samples were collected from adult trees along an altitudinal gradient from valley bottom (1050 m a.s.l.) to the treeline (2170 m a.s.l.). The treeline site is one of the highest in the area and is characterized by a generally low wind exposure. Altitude had no effect on g(min) in either species. In P. abies, large seasonal variations in g(min) were recorded but no changes were related to needle age class. Pinus cembra had a low g(min) and appeared to be efficient in reducing needle water losses. There was a significant increase in g(min) with needle aging in P. cembra growing at low altitude that could be related to a shorter needle longevity compared with P. abies. High contact angles (> 110-120 ) suggested the presence of tubular epicuticular waxes on needles of both species. Contact angles were higher (low wettability) in high-altitude needles than in low-altitude needles. By the end of winter, there was no difference in contact angles between needles in the windward and leeward positions. Wax structures transformed toward planar shapes as demonstrated by the decrease in contact angle from winter to summer. In both species, the cuticle was thicker in needles of high-altitude trees than in needles of low-altitude trees and there was no correlation between g(min) and cuticle thickness. Because desiccation resistance did not decrease with altitude in either species, we conclude that they are not susceptible to winter desiccation at the tree line.     

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.     

Tree Regeneration and Sapling Damage of Pedunculate Oak Quercus robur in a Grazed Forest in Galicia, NW Spain: A Comparison of Continuous and Rotational Grazing Systems

Forest grazing has been recognised as being a useful tool in fire-risk reduction, in addition to having the potential to preserve or enhance forest biodiversity if managed correctly. Concern for natural regeneration of forest trees in Europe has also prompted interest in the effects of large herbivores on seedling and sapling growth and mortality. An investigation was carried out into sapling damage and density of natural regeneration of oak (Quercus robur) in a mature, pony-grazed, Pinus radiata forest in Galicia, NW Spain under two different grazing regimes (continuous and rotational). In all treatments significantly more oak seedlings and saplings were recorded in areas of grass sward than areas dominated by taller ground flora species. Damage to oak saplings was assessed from the form (height and canopy) relative to stem diameter. The height and average canopy diameter of similar-aged saplings were found to be significantly greater in ungrazed (control) than both continuous and rotationally grazed treatments. Height and canopy diameters of similar-aged oak were not significantly different between the two grazing treatments. Significant differences were observed in tree form, with unbrowsed saplings having the greatest height to canopy width ratio and those in the continuously browsed plots having the smallest. An obvious decrease in the goodness of fit (R²) of regression analyses were found in continuously grazed areas compared to rotational and control plots for both height and canopy data. The differences in damage observed were not significantly different enough to suggest one method of grazing over the other as being better for minimising sapling damage. Management requirements are more likely to dictate grazing regime. Overall, stock density is likely to have a more significant effect on damage than stocking system.     

Uptake,distribution and retranslocation of nitrogen by Pinus radiata from 15N-labelled fertilizer applied to podzolized sandy soil

The efficiency of uptake of nitrogen from fertilizer by Pinus radiata (D. Don) was measured in an intensively managed young plantation. Ammonium sulphate labelled with ¹⁵N was applied to trees at 2 or 13 months after planting, and the uptake of N by trees up to an age of 4.8 years (canopy closed) was measured by harvesting trees sequentially.Fertilizer application increased the growth of trees. The amount of labelled N recovered in the trees was low: a maximum of 6.4% when fertilized 2 months after planting and 17.7% when fertilized 13 months after planting. The uptake of ¹⁵N ceased 12 months after application, although tree sizes and total N uptake increased greatly until canopy closure. Nitrogen taken up from the fertilizer as a proportion of total N content in the trees decreased from 34.0 to 10.8% as the size of the tree increased over a period from 6 to 56 months after planting.Nitrogen in the needles accounted for 58 to 72% of the absorbed ¹⁵N, and was highly mobile. Of the ¹⁵N in the needles on trees 12 months after fertilizer was applied, 22.1% remained in the same needles, 71.6% was retranslocated to new tissues, and 6.3% was lost from trees in litter by age 56 months.Although the concentration of total N in the soil was very low, the uptake of soil-N by trees was high, suggesting that establishment practices increased mineralization of soil-N.The understanding of nitrogen cycling obtained in this study emphasizes the need to design fertilizer application strategies to maximise uptake, minimize leaching losses and to capitalize on internal storage and retranslocation.     

Co‐occurrence of the ascomycete Lophodermium piceae and the rust fungus Chrysomyxa abietis in Norway spruce needles

The frequency of needles and the proportion of needle segments infected by Lophodermium piceae were compared in symptomless and Chrysomyxa abietis‐infected, 1‐year‐old needles of Picea abies. In late spring, symptomless needles from both rust‐infected and healthy saplings were sampled. In addition, rust‐infected, totally chlorotic needles and needles with chlorosis along about half their length from the diseased trees were examined. In all three stands, the proportion of segments infected by L. piceae was larger in the rust‐infected half of the needle than in the symptomless half; but the difference was statistically significant in only one of the stands. The proportion of L. piceae‐infected segments among the nonrust‐infected needles was the same as that found for the uninfected half of rust‐infected needles (after correction for size differences). No differences in the proportion of L. piceae‐infected segments were found between the totally chlorotic, rust‐infected needles and the green needles of diseased or healthy trees.     

Recovery of photosynthesis in 1-year-old needles of unfertilized and fertilized Norway spruce (Picea abies (L.) Karst.) during spring

Photosynthetic O(2) evolution and chlorophyll a fluorescence were measured in 1-year-old needles of unfertilized and fertilized trees of Norway spruce (Picea abies (L.) Karst.) during recovery of photosynthesis from winter inhibition in northern Sweden. Measurements were made under laboratory conditions at 20 degrees C. In general, the CO(2)-saturated rate of O(2) evolution was higher in needles of fertilized trees than in needles of unfertilized trees over a wide range of incident photon flux densities. Furthermore, the maximum photochemical efficiency of photosystem (PS) II, as indicated by the ratio of variable to maximum fluorescence (F(V)/F(M)) was higher in needles of fertilized trees than in needles of unfertilized trees. The largest differences in F(V)/F(M) between the two treatments occurred before the main recovery of photosynthesis from winter inhibition in late May. The rate of O(2) evolution was higher in needles of north-facing branches than in needles of south-facing branches in the middle of May. Simultaneous measurements of O(2) exchange and chlorophyll fluorescence indicated that differences in the rate of O(2) evolution between the two treatments were paralleled by differences in the rate of PS II electron transport determined by chlorophyll fluorescence. We suggest that, during recovery of photosynthesis from winter inhibition, the balance between carbon assimilation and PS II electron transport was maintained largely by adjustments in the nonphotochemical dissipation of excitation energy within PS II.     

Nitrogen allocation and the fate of absorbed light in 21-year-old Pinus radiata

We investigated effects of nitrogen (N) fertilizer and canopy position on the allocation of N to Rubisco and chlorophyll as well as the distribution of absorbed light among thermal energy dissipation, photochemistry, net CO2 assimilation and alternative electron sinks such as the Mehler reaction and photorespiration. The relative reduction state of the primary quinone receptor of photosystem II (QA) was used as a surrogate for photosystem II (PSII) vulnerability to photoinactivation. Measurements were made on needles from the lower, mid and upper canopy of 21-year-old Pinus radiata D. Don trees grown with (N+) and without (N0) added N fertilizer. Rubisco was 45 to 60% higher in needles of N+ trees than in needles of N0 trees at all canopy positions. Chlorophyll was approximately 80% higher in lower- and mid-canopy needles of N+ trees than of N0 trees, but only approximately 20% higher in upper-canopy needles. Physiological differences between N+ and N0 trees were found only in the lower- and mid- canopy positions. Needles of N+ trees dissipated up to 30% less light energy as heat than needles of N0 trees and had correspondingly more reduced QA. Net CO2 assimilation and the proportions of electrons used by alternative electron sinks such as the Mehler reaction and photorespiration were unaffected by N treatment regardless of canopy position. We conclude that the application of N fertilizer mainly affected the biochemistry and light-use physiology in lower- and mid-canopy needles by increasing the amount of chlorophyll and hence the amount of light harvested. This, however, did not improve photochemistry or safe dissipation, but increased PSII vulnerability to photoinactivation, an effect with likely significant consequences during sunflecks or sudden gap formation.     

Influence of fertilizer timing on seasonal nutrient uptake and dry-matter production by young Pinus radiata in southern western Australia

Seasonal changes in above ground dry-matter, nitrogen (N), and phosphorus (P) accumulation were measured following application of N and P in autumn or spring to 1-year-old Pinus radiata (D. Don). Dry-matter production and nutrient accumulation were measured eight times over two years following fertilization.

All trees produced dry-matter throughout the year, but during the summer, fertilized trees produced more dry-matter than unfertilized trees. In contrast to dry-matter production, nutrient accumulation showed a distinctly seasonal pattern with maximum accumulation of N and P occurring in winter and spring, when rainfall and soil moisture were highest. Accumulation of N and P either slowed markedly or ceased during summer depending on fertilizer treatment. Continued dry-matter production during summer, when nutrient accumulation was low, resulted in the decline of N and P concentrations in needles, branches and stems of all trees. This indicated that nutrients required for new growth during summer were mobilized from existing foliage and wood. Fertilization increased the concentrations of N and P in foliage and wood, and these higher concentrations persisted through summer. Spring fertilization increased N accumulation to a greater extent than autumn fertilization, this effect lasting two years. The greater dry-matter production by fertilized trees during summer indicated that growth during summer was limited by nutrient supply.     

Effects of winter buds on winter nitrogen uptake and allocation in Pinus densiflora saplings

Studies of nitrogen (N) use by plants have confirmed some winter N uptake; however, the mode of regulation of plant N use in winter is unknown. The regulation of N use by plants during winter may differ from that in the growing season, as plant growth strongly affects N use. We investigated the effects of winter buds on winter N use by Japanese red pine (Pinus densiflora), as a previous study demonstrated that N absorbed during winter contributes significantly to leaf growth in the following spring. We conducted a bud pruning experiment during winter to examine the effects of winter buds on winter N uptake and allocation among plant organs using ¹⁵N labeling. Over a three-week labeling period, the ¹⁵N content in roots increased to 0.20 ± 0.12 mg N g DW^?1, which is equivalent to 1.8 ± 1.1 % of the total N content in the roots. However, this absorbed ¹⁵N rarely appeared in needles and buds. Bud pruning did not affect ¹⁵N uptake and allocation. On the other hand, significant total N retranslocation was found within the crowns of saplings without bud pruning, but N was not retranslocated in bud-pruned plants. The bud pruning experiment indicated that N was retranslocated from needles into winter buds. Since soil N availability changes dramatically and is unstable in many forest ecosystems, N contained in needles would be a more stable source of N than newly absorbed N.     

The influence of fertilizers on the occurrence of Armillaria mellea in Scotch pine plantations

The chemical analysis of needles and wood of roots of trees belonging to four stages of disease caused by A. mellea in fertilized (N, P, K, Ca, Mg) and non-fertilized three Scotch Pine plantations were made. The attack caused gradual interruption of absorption and resulted in the exhaustion of some elements (N, P) and concentration of other ones (K, Ca) in needles and in the decrease of P and transitional growth of Mg-level in wood of roots. Considerable growth of concentration of some chemical substances in wood of roots of dead trees was observed. This referred to the fertilized and nonfertilized trees as well. The rate of these changes, however, was different after treatment: fertilization had no effect on the concentration of K in needles of trees in intermediate stages of disease, but it influenced in the lower rate the exhaustion of Mg in wood of roots. The role of K and Mg in defensive reactions is discussed.     

Growth of Young Sitka Spruce (Picea sitchensis (Bong.) Carr.) and the Effect of Simulated Browsing, Staking and Treeshelters

Sitka spruce (Picea sitchensis (Bong.) Carr.) 1 + 1 transplantswere grown at 1 x 1 m for 3 years with 100 per cent chemicalweed control on a very fertile site. Treatments were control,firm staking, staked with 15 cm diameter netting with simulatedbrowsing, staked with simulated browsing to 15 cm diameter crown,1.2 m treeshelter, 1.2 treeshelter with holes and no browsing.No leaders were browsed. Trees were assessed for annual heightgrowth, stem diameter at 0.5 m and 1.0 m, total above and belowground biomass, root:shoot ratio, root plate diameter, rootdepth and root volume. No 3-year heights were significantlydifferent but third-year increments were; the greatest finalheight was in the control at 200.7 cm. All other tree parametershad significant treatment differences demonstrating the needto select carefully the criteria of success in experiments.Treatments which restricted the horizontal development of thecanopy either physically or by simulated browsing seriouslyreduced stem diameter and root development. The controls hada root:shoot ratio of 0.418; treeshelter treatments had thesmallest root:shoot ratio of 0.238; staked trees were 0.379while staked browsed trees had the highest value at 0.447. Acceptanceof browsing or use of shelters or guards to protect trees frombrowsing may seriously reduce growth.     

Genetic and environmental control of seasonal carbohydrate dynamics in trees of diverse Pinus sylvestris populations

We explored environmental and genetic factors affecting seasonal dynamics of starch and soluble nonstructural carbohydrates in needle and twig cohorts and roots of Scots pine (Pinus sylvestris L.) trees of six populations originating between 49 degrees and 60 degrees N, and grown under common garden conditions in western Poland. Trees of each population were sampled once or twice per month over a 3-year period from age 15 to 17 years. Based on similarity in starch concentration patterns in needles, two distinct groups of populations were identified; one comprised northern populations from Sweden and Russia (59-60 degrees N), and another comprised central European populations from Latvia, Poland, Germany and France (49-56 degrees N). Needle starch concentrations of northern populations started to decline in late spring and reached minimum values earlier than those of central populations. For all populations, starch accumulation in spring started when minimum air temperature permanently exceeded 0 degrees C. Starch accumulation peaked before bud break and was highest in 1-year-old needles, averaging 9-13% of dry mass. Soluble carbohydrate concentrations were lowest in spring and summer and highest in autumn and winter. There were no differences among populations in seasonal pattern of soluble carbohydrate concentrations. Averaged across all populations, needle soluble carbohydrate concentrations increased from about 4% of needle dry mass in developing current-year needles, to about 9% in 1- and 2-year-old needles. Root carbohydrate concentration exhibited a bimodal pattern with peaks in spring and autumn. Northern populations had higher concentrations of fine-root starch in spring and autumn than central populations. Late-summer carbohydrate accumulation in roots started only after depletion of starch in needles and woody shoots. We conclude that Scots pine carbohydrate dynamics depend partially on inherited properties that are probably related to phenology of root and shoot growth.     

Carbon and nitrogen stocks in soil,trees and field vegetation in conifer plantations 10 years after deep soil cultivation and patch scarification

Abstract

Biomass, total nitrogen (N) and total carbon (C) stocks were determined in trees, roots, field vegetation and soil in plots given two different site preparation treatments, deep soil cultivation (DSC) approximately 50?cm deep and patch scarification (PS), at three locations in Sweden 10?years after treatment. One location was planted with Pinus contorta, one with Picea abies and one with a mixture of P. abies and Pinus sylvestris. No differences were found in total ecosystem (trees, roots, field vegetation and soil) C and N stocks between the DSC and PS plots. In the DSC plots the tree biomass, tree N and C contents and total biomass were higher than in the PS plots, but the opposite was found for stocks in field vegetation. Biomass and C stocks in the total vegetation (trees, roots and field vegetation) were higher in the DSC plots. However, vegetation N stocks did not differ between the soil treatments, probably because the combined amount of leaf tissue in the trees and field vegetation did not differ between them. The proportions of biomass allocated to roots, stems and needles did not differ between the two treatments. However, the rooting was deeper in DSC plots, possibly because nutrient availability was higher, and subsoil density lower, following DSC than following PS.     

Needle anatomy changes with increasing tree age in Douglas-fir

Morphological differences between old-growth trees and saplings of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) may extend to differences in needle anatomy. We used microscopy with image analysis to compare and quantify anatomical parameters in cross sections of previous-year needles of old-growth Douglas-fir trees and saplings at the Wind River Canopy Crane site in Washington and at three sites in the Cascade Mountains of Oregon. We also compared needle anatomy across a chronosequence of 10-, 20-, 40- and 450-year-old Douglas-fir trees from the Wind River site. Anatomy differed significantly between needles of old-growth trees and saplings at all sites, suggesting a developmental change in needle anatomy with increasing tree age. Compared with needles of old-growth trees, needles of saplings were longer and had proportionately smaller vascular cylinders, larger resin canals and few hypodermal cells. Astrosclereids, which sequester lignin in their secondary cell walls and occupy space otherwise filled by photosynthetic cells, were scarce in needles of saplings but abundant in needles of old-growth trees. Needles of old-growth trees had an average of 11% less photosynthetic mesophyll area than needles of saplings. The percentage of non-photosynthetic area in needles increased significantly with increasing tree age from the chronosequence of 10-, 20-, 40- and 450-year-old trees at the Wind River site. This reduction in photosynthetic area may contribute to decreased growth rates in old trees.     

The relationship between Swiss needle cast symptom severity and level of Phaeocryptopus gaeumannii colonization in coastal Douglas‐fir (Pseudotsuga menziesii var. menziesii)*

This study examined 108 15‐year‐old Douglas‐fir (Pseudotsuga menziesii var. menziesii) trees to investigate whether trees exhibiting less severe Swiss needle cast (SNC) symptoms were more resistant (had less fungal colonization) or more tolerant (maintained healthy foliage under similar infection levels). Trees were sampled from six open pollinated families that were categorized into three disease severity groups (two families for each group; mild, moderate and severe disease symptoms). The amount of retained foliage and level of discoloration were visually assessed on trees in the field. Fungal colonization (as determined by proportion of stomata occluded with pseudothecia and by amount of Phaeocryptopus gaeumannii DNA in sampled needles) was measured on 1‐ and 2‐year‐old needles in the laboratory. Trees in the different disease severity groups were similar with respect to amount of fungus in their needles, yet the trees in the mild symptom group retained higher proportions of needles and maintained greener foliage. The relationship between amount of P. gaeumannii in needles and SNC symptom severity was statistically significant (p < 0.05) for amount of fungal DNA in 1‐year‐old needles and average needle retention (NR) over the last four growing seasons. Average NR decreased with increased amount of pathogen DNA in the mild disease symptom families. This relationship was reversed in the severe disease symptom group and there was no relationship in the moderate disease symptom group. Because the amount of P. gaeumannii DNA in foliage did not differ significantly among the groups, differences in symptom severity were attributed to tolerance, not resistance. Visual scoring of individual trees for average NR over the past four growing seasons could be used to effectively assess for SNC tolerance in Douglas‐fir.