Susceptibility of ponderosa pine, Pinus ponderosa (Dougl. ex Laws.), to mountain pine beetle, Dendroctonus ponderosae Hopkins,attack in uneven-aged stands in the Black Hills of South Dakota and Wyoming USA

Mountain pine beetle, Dendroctonus ponderosae Hopkins can cause extensive tree mortality in ponderosa pine, Pinus ponderosa Dougl. ex Laws., forests in the Black Hills of South Dakota and Wyoming. Most studies that have examined stand susceptibility to mountain pine beetle have been conducted in even-aged stands. Land managers increasingly practice uneven-aged management. We established 84 clusters of four plots, one where bark beetle-caused mortality was present and three uninfested plots. For all plot trees we recorded species, tree diameter, and crown position and for ponderosa pine whether they were killed or infested by mountain pine beetle. Elevation, slope, and aspect were also recorded. We used classification trees to model the likelihood of bark beetle attack based on plot and site variables. The probability of individual tree attack within the infested plots was estimated using logistic regression. Basal area of ponderosa pine in trees ≥25.4 cm in diameter at breast height (dbh) and ponderosa pine stand density index were correlated with mountain pine beetle attack. Regression trees and linear regression indicated that the amount of observed tree mortality was associated with initial ponderosa pine basal area and ponderosa pine stand density index. Infested stands had higher total and ponderosa pine basal area, total and ponderosa pine stand density index, and ponderosa pine basal area in trees ≥25.4 cm dbh. The probability of individual tree attack within infested plots was positively correlated with tree diameter with ponderosa pine stand density index modifying the relationship. A tree of a given size was more likely to be attacked in a denser stand. We conclude that stands with higher ponderosa pine basal area in trees >25.4 cm and ponderosa pine stand density index are correlated with an increased likelihood of mountain pine beetle bark beetle attack. Information form this study will help forest managers in the identification of uneven-aged stands with a higher likelihood of bark beetle attack and expected levels of tree mortality.     

Physiological responses of ponderosa pine in western Montana to thinning, prescribed fire and burning season

Low-elevation ponderosa pine (Pinus ponderosa Dougl. ex. Laws.) forests of the northern Rocky Mountains historically experienced frequent low-intensity fires that maintained open uneven-aged stands. A century of fire exclusion has contributed to denser ponderosa pine forests with greater competition for resources, higher tree stress and greater risk of insect attack and stand-destroying fire. Active management intended to restore a semblance of the more sustainable historic stand structure and composition includes selective thinning and prescribed fire. However, little is known about the relative effects of these management practices on the physiological performance of ponderosa pine. We measured soil water and nitrogen availability, physiological performance and wood radial increment of second growth ponderosa pine trees at the Lick Creek Experimental Site in the Bitterroot National Forest, Montana, 8 and 9 years after the application of four treatments: thinning only; thinning followed by prescribed fire in the spring; thinning followed by prescribed fire in the fall; and untreated controls. Volumetric soil water content and resin capsule ammonium did not differ among treatments. Resin capsule nitrate in the control treatment was similar to that in all other treatments, although burned treatments had lower nitrate relative to the thinned-only treatment. Trees of similar size and canopy condition in the three thinned treatments (with and without fire) displayed higher leaf-area-based photosynthetic rate, stomatal conductance and mid-morning leaf water potential in June and July, and higher wood radial increment relative to trees in control units. Specific leaf area, mass-based leaf nitrogen content and carbon isotope discrimination did not vary among treatments. Our results suggest that, despite minimal differences in soil resource availability, trees in managed units where basal area was reduced had improved gas exchange and growth compared with trees in unmanaged units. Prescribed fire (either in the spring or in the fall) in addition to thinning, had no measurable effect on the mid-term physiological performance and wood growth of second growth ponderosa pine.     

Growth and water relations of Eucalyptus marginata (jarrah) stands in response to thinning and fertilization

We studied the effects of five thinning treatments (T1 = 5.5, T2 = 11, T3 = 16.5, T4 = 22.5 and T5 = 28.5 m(2) ha(-1) basal area under bark) x two fertilizer treatments (F0 = unfertilized and F1 = fertilized with 400 kg ha(-1) N plus 229 kg ha(-1) P) on growth and water relations of pole-sized Eucalyptus marginata J. Donn ex Sm. trees growing in southwestern Australia. Thinning reduced leaf area index (LAI) from 2.1 in the T4 and T5 treatments to 0.8 in the T1F0 treatment. Fertilizer had no effect on LAI in the T2, T4 or T5 treatments, but increased LAI by 45 and 20% in the T1 and T3 treatments, respectively. Thinning plus fertilizing increased diameter growth most in the fastest growing trees, from 0.4 cm year(-1) for trees in the T5F0 and T5F1 treatments to 0.7 and 1.2 cm year(-1) for trees in the T1F0 and T1F1 treatments, respectively. In both fertilizer treatments, stand basal area and volume growth increased with increasing stand density up to 15 m(2) ha(-1), and thereafter declined with increasing stand density, such that the growth rate of trees in the T5 treatment was only half of that at a stand density of 15 m(2) ha(-1). In response to fertilizer, growth rates of the slowest and fastest-growing trees increased from 0.35 and 3.5 m(2) ha(-1) year(-1) (F0) to 0.56 and 5.4 m(3) ha(-1) year(-1) (F1), respectively. Stand growth efficiency (growth per unit LAI) increased in response to thinning, and fertilizer increased stand growth efficiency at all stand densities. Throughout the dry season, T5 trees had lower predawn shoot water potentials (Psi(pd)) (minimum of -1.5 MPa) than T1 or T2 trees (minimum of -0.7 MPa). Fertilizer decreased Psi(pd) in T5 trees (by -0.9 and -1.5 MPa, respectively, in F0 and F1), but not in T1 or T2 trees. Stand growth rate was closely related to cumulative midday water stress (CMWS) over the dry season, and volume growth rate declined sharply from 6 m(3) ha(-1) year(-1) at a CMWS of 130 MPa days, to zero at a CMWS of 220 MPa days. Application of fertilizer to thinned stands increased LAI, stand growth efficiency and stand growth. In unthinned stands, fertilizer increased stand growth efficiency and stand growth; however, it also increased tree water stress, which limited the fertilizer-induced increases in LAI and growth. We attribute the increase in tree and stand growth in response to application of fertilizer to increased photosynthetic rates, increased allocation to stem wood, and in thinned stands also to higher LAIs.     

Restoration thinning and influence of tree size and leaf area to sapwood area ratio on water relations of Pinus ponderosa

Ponderosa pine (Pinus ponderosa Dougl. ex P. Laws) forest stand density has increased significantly over the last century (Covington et al. 1997). To understand the effect of increased intraspecific competition, tree size (height and diameter at breast height (DBH)) and leaf area to sapwood area ratio (A(L):A(S)) on water relations, we compared hydraulic conductance from soil to leaf (kl) and transpiration per unit leaf area (Q(L)) of ponderosa pine trees in an unthinned plot to trees in a thinned plot in the first and second years after thinning in a dense Arizona forest. We calculated kl and Q(L) based on whole- tree sap flux measured with heat dissipation sensors. Thinning increased tree predawn water potential within two weeks of treatment. Effects of thinning on kl and Q(L) depended on DBH, A(L):A(S) and drought severity. During severe drought in the first growing season after thinning, kl and Q(L) of trees with low A(L):A(S) (160-250 mm DBH; 9-11 m height) were lower in the thinned plot than the unthinned plot, suggesting a reduction in stomatal conductance (g(s)) or reduced sapwood specific conductivity (K(S)), or both, in response to thinning. In contrast kl and Q(L) were similar in the thinned plot and unthinned plot for trees with high A(L):A(S) (260-360 mm DBH; 13-16 m height). During non-drought periods, kl and Q(L) were greater in the thinned plot than in the unthinned plot for all but the largest trees. Contrary to previous studies of ponderosa pine, A(L):A(S) was positively correlated with tree height and DBH. Furthermore, kl and Q(L) showed a weak negative correlation with tree height and a strong negative correlation with A(S) and thus A(L):A(S) in both the thinned and unthinned plots, suggesting that trees with high A(L):A(S) had lower g(s). Our results highlight the important influence of stand competitive environment on tree-size-related variation in A(L):A(S) and the roles of A(L):A(S) and drought on whole-tree water relations in response to thinning.     

Ten-year responses of ponderosa pine growth,vigor, and recruitment to restoration treatments in the Bitterroot Mountains,Montana, USA

Little is known about ponderosa pine forest ecosystem responses to restoration practices in the Northern Rocky Mountains, USA. In this study, restoration treatments aimed at approximating historical forest structure and disturbances included modified single-tree selection cutting, with and without prescribed burning. We compared the effectiveness of restoration treatments on growth, vigor, and composition of recruitment responses with untreated controls. We used a randomized block design to detect treatment differences in mean individual tree basal area increment (BAInc10), growth efficiency (GE), and recruitment abundance between two restoration treatments (Cut-only and Cut-burn) and a Control. We further examined treatment effects by tree age-class (Young, Mature, Presettlement) using a spatial ANOVA model that incorporates the spatial autocorrelation among trees within experimental units. Ten years after implementing restoration treatments, mean individual tree BAInc10 and GE were significantly higher for treated units relative to Control units; all three age-classes benefited similarly from restoration treatments relative to the Control, with the greatest response in the Cut-only and moderate response in the Cut-burn. When treated units were compared, Cut-burn negatively affected BAInc10 and GE relative to Cut-only. Presettlement trees responded positively to treatment relative to the Control, particularly for BAInc10, demonstrating the potential of these old trees to respond to reduced competition. The Cut-burn treatment, in contrast, negatively affected the BAInc10 and GE response of postsettlement trees when compared to Cut-only. Restoration treatments did not reduce the amount of Douglas-fir recruits. In addition, the recruitment of both ponderosa pine and Douglas-fir species was associated with the proximate cover of woody debris in Cut-only and Control treatments. Finally, special consideration needs to be taken for spring Cut-burn treatments, which appeared to dampen growth and vigor, relative to Cut-only, particularly for Young and Mature trees, and increased recruitment of ponderosa pine and particularly Douglas-fir.     

A simulation of the development and restoration of old-growth structural features in northern hardwoods

Thinning treatments in second-growth forest may be a practical means of accelerating the development of certain old-growth structural features in regions where old stands are presently uncommon. We used CANOPY, an individual-tree model calibrated with data from thinned and unthinned stands, to simulate effects of thinning on growth rates and development of old-growth structural features in second-growth northern hardwoods. Three simulated, moderately heavy thinnings over a period of 45 years nearly doubled the predicted mean radial increment of canopy trees, percent of stand basal area in large trees, and area of canopy gaps. Compared to untreated stands, thinned stands had fewer dead trees per ha, but the dead trees were larger in size and the overall volume of snags and logs was little affected. In a 77-year old even-aged stand, moderately heavy thinning was predicted to reduce the time needed to attain the minimum structural features of an old-growth forest from 79 to 36 years. Simulated treatments in an older, uneven-aged stand gave mixed results; the moderately heavy treatment stimulated individual tree growth, but the removal of some medium-sized canopy trees in conjunction with natural mortality delayed the development of old-growth structure. Total volume of dead wood may still be deficient under the thinning regimes investigated in this study, but predicted live-tree structure 45 years after moderately heavy thinning was typical of stands in the advanced transition and steady-state stages of old-growth development. Results suggest that thinning can substantially accelerate the development of old-growth structure in pole and mature northern hardwoods, but response in older, uneven-aged stands is more modest, and treatments in these stands may need to be more conservative to achieve restoration goals.     

Differences in leaf gas exchange and water relations among species and tree sizes in an Arizona pine-oak forest

We compared leaf gas exchange and water potential among the dominant tree species and major size classes of trees in an upland, pine-oak forest in northern Arizona. The study included old-growth Gambel oak (Quercus gambelii Nutt.), and sapling, pole, and old-growth ponderosa pines (Pinus ponderosa var. scopulorum Dougl. ex Laws.). Old-growth oak had higher predawn leaf water potential (Psi(leaf)) than old-growth pine, indicating greater avoidance of soil water stress by oak. Old-growth oak had higher stomatal conductance (G(w)), net photosynthetic rate (P(n)), and leaf nitrogen concentration, and lower daytime Psi(leaf) than old-growth pine. Stomatal closure started at a daytime Psi(leaf) of about -1.9 MPa for pine, whereas old-growth oak showed no obvious reduction in G(w) at Psi(leaf) values greater than -2.5 MPa. In ponderosa pine, P(n) and G(w) were highly sensitive to seasonal and diurnal variations in vapor pressure deficit (VPD), with similar sensitivity for sapling, pole, and old-growth trees. In contrast, P(n) and G(w) were less sensitive to VPD in Gambel oak than in ponderosa pine, suggesting greater tolerance of oak to atmospheric water stress. Compared with sapling pine, old-growth pine had lower morning and afternoon P(n) and G(w), predawn Psi(leaf), daytime Psi(leaf), and soil-to-leaf hydraulic conductance (K(l)), and higher foliar nitrogen concentration. Pole pine values were intermediate between sapling and old-growth pine values for morning G(w) and daytime Psi(leaf), similar to sapling pine for predawn Psi(leaf), and similar to old-growth pine for morning and afternoon P(n), afternoon G(w), K(l), and foliar nitrogen concentration. For the pines, low predawn Psi(leaf), daytime Psi(leaf), and K(l) were associated with low P(n) and G(w). Our data suggest that hydraulic limitations are important in reducing P(n) in old-growth ponderosa pine in northern Arizona, and indicate greater avoidance of soil water stress and greater tolerance of atmospheric water stress by old-growth Gambel oak than by old-growth ponderosa pine.     

Changes in physiological attributes of ponderosa pine from seedling to mature tree

Plant physiological models are generally parameterized from many different sources of data, including chamber experiments and plantations, from seedlings to mature trees. We obtained a comprehensive data set for a natural stand of ponderosa pine (Pinus ponderosa Laws.) and used these data to parameterize the physiologically based model, TREGRO. Representative trees of each of five tree age classes were selected based on population means of morphological, physiological, and nearest neighbor attributes. Differences in key physiological attributes (gas exchange, needle chemistry, elongation growth, needle retention) among the tree age classes were tested. Whole-tree biomass and allocation were determined for seedlings, saplings, and pole-sized trees. Seasonal maxima and minima of gas exchange were similar across all tree age classes. Seasonal minima and a shift to more efficient water use were reached one month earlier in seedlings than in older trees because of decreased soil water availability in the rooting zone of the seedlings. However, carbon isotopic discrimination of needle cellulose indicated increased water-use efficiency with increasing tree age. Seedlings had the lowest needle and branch elongation biomass growth. The amount of needle elongation growth was highest for mature trees and amount of branch elongation growth was highest for saplings. Seedlings had the highest biomass allocation to roots, saplings had the highest allocation to foliage, and pole-sized trees had the highest allocation to woody tissues. Seedlings differed significantly from pole-sized and older trees in most of the physiological traits tested. Predicted changes in biomass with tree age, simulated with the model TREGRO, closely matched those of trees in a natural stand to 30 years of age.     

Bark beetle-caused mortality in a drought-affected ponderosa pine landscape in Arizona,USA

Extensive ponderosa pine (Pinus ponderosa Dougl. ex Laws.) mortality associated with a widespread severe drought and increased bark beetle (Coleoptera: Curculionidae, Scolytinae) populations occurred in Arizona from 2001 to 2004. A complex of Ips beetles including: the Arizona fivespined ips, Ips lecontei Swaine, the pine engraver beetle, Ips pini (Say), Ips calligraphus (Germar), Ips latidens (LeConte), Ips knausi Swaine and Ips integer (Eichhoff) were the primary bark beetle species associated with ponderosa pine mortality. In this study we examine stand conditions and physiographic factors associated with bark beetle-caused tree mortality in ponderosa pine forests across five National Forests in Arizona. A total of 633 fixed-radius plots were established across five National Forests in Arizona: Apache-Sitgreaves, Coconino, Kaibab, Prescott, and Tonto. Prior to the bark beetle outbreak, plots with mortality had higher tree and stocking compared with plots without pine mortality. Logistic regression modeling found that probability of ponderosa pine mortality caused by bark beetles was positively correlated with tree density and inversely related with elevation and tree diameter. Given the large geographical extent of this study resulting logistic models to estimate the likelihood of bark beetle attack should have wide applicability across similar ponderosa pine forests across the Southwest. This is particularly true of a model driven by tree density and elevation constructed by combining all forests. Tree mortality resulted in significant reductions in basal area, tree density, stand density index, and mean tree diameter for ponderosa pine and for all species combined in these forests. Most of the observed pine mortality was in the 10–35 cm diameter class, which comprise much of the increase in tree density over the past century as a result of fire suppression and grazing practices. Ecological implications of tree mortality are discussed.     

Prescribed burning and hexazinone herbicide as release treatments in a sapling hardwood-loblolly pine stand

To compare release treatments, a randomized complete block study was established in a 7-year-old hardwood-loblolly pine (Pinus taeda L.) stand in central Louisiana established using chopping and burning. There were 5 blocks of 3 treatments each: (1) check, (2) hexazinone applied once, and (3) prescribed backfiring applied twice. The first burn in December 1985 (7 years after site preparation) had a fire intensity of 90 kJ/s/m. The hexazinone herbicide was applied in April 1986 (the 8th year after site preparation) with a metered spotgun applicator at a rate of 3.0 kg active ingredient/ha. The second burn in March 1989 (the 11th year after site preparation) had a fire intensity of 106 kJ/s/m.The two prescribed burns increased the number of stems less than 1.5 m tall from 1,380 to 2,960/ha red maple (Acer rubrum L.), blackgum (Nyssa sylvatica var. bifora), and sweetgum (Liquidambar styraciflua L.). Burning reduced the number of loblolly pines less than 2.0 m tall, which resulted in a significant increase in average loblolly pine height and diameter. Loblolly pine comprised 62 and 59% of the basal area on the check and burn treatments, respectively, 11 years after site preparation. Hexazinone reduced the number of blackgum, sweetgum, and oak (Quercus spp.) from 6,100 to 4,560 stems/ha and resulted in significantly less hardwood tree basal area than found on the check or burn treatments. Therefore, the herbicide treatment resulted in principally a loblolly pine stand (over 80% of the total tree basal area was pine) four years after hexazinone application.     

Post-1935 changes in forest vegetation of Grand Canyon National Park, Arizona, USA: Part 1 - ponderosa pine forest

Vegetation plots originally sampled in Grand Canyon National Park (GCNP), Arizona, USA in 1935 are the earliest-known, sample-intensive, quantitative documentation of forest vegetation over a Southwest USA landscape. These historical plots were located as accurately as possible and resampled in 2004 to document multi-decadal changes in never-harvested Southwestern forests. Findings for ponderosa pine forest (PPF) differed among three forest subtypes (dry, mesic, and moist PPF), indicating that understanding the ecology of PPF subtypes is essential for development of ecologically based management practices. Dry PPF, which is transitional with pinyon-juniper vegetation at low elevation, exhibited no changes from 1935 to 2004. Mesic PPF, the core subtype of PPF, had increased densities of total trees, ponderosa pine (Pinus ponderosa), and white fir (Abies concolor) in the 10-29.9 cm diameter class from 1935 to 2004 that may have induced decreased densities of larger ponderosa pines and total tree and ponderosa pine basal areas. Moist PPF, which is transitional with mixed conifer forest at high elevation, was the most dynamic PPF subtype with decreases from 1935 to 2004 in total density and total basal area that are largely attributable to decreases in quaking aspen (Populus tremuloides). Graphical synthesis of datasets with historical and modern values for density and basal area indicates that overall PPF (all subtypes combined) increased in sapling density of all species combined and conifers with canopy potential and decreased in density of quaking aspen trees since the late 19th century. PPF of GCNP has passed through an accretion phase of forest development with increases in density and, depending on PPF subtype and variable being examined, is at or past the point of inflection to recession of density and basal area. Increases in small diameter ponderosa pine and white fir from 1935 to 2004 portend potential additional accretion, but decreases in total basal area, density and basal area of quaking aspen, basal area of ponderosa pine, and density of larger diameter ponderosa pine indicate PPF has passed the inflection point from accretion to recession. Uncertainties about 19th-century PPF structure and composition and about future ecological and societal environments lead to the conclusion that resource managers of GCNP and other natural areas should consider a change in focus from the objective of achieving desired future conditions to an objective of avoiding undesired future conditions.     

Age-related changes in ecosystem structure and function and effects on water and carbon exchange in ponderosa pine

As forests age, their structure and productivity change, yet in some cases, annual rates of water loss remain unchanged. To identify mechanisms that might explain such observations, and to determine if widely different age classes of forests differ functionally, we examined young (Y, approximately 25 years), mature (M, approximately 90 years) and old (O, approximately 250 years) ponderosa pine (Pinus ponderosa Dougl. ex P. Laws.) stands growing in a drought-prone region of central Oregon. Although the stands differed in tree leaf area index (LAIT) (Y = 0.9, M = 2.8, O = 2.1), cumulative tree transpiration measured by sap flow did not differ substantially during the growing season (100-112 mm). Yet when water was readily available, transpiration per unit leaf area of the youngest trees was about three times that of M trees and five times that of O trees. These patterns resulted from a nearly sixfold difference in leaf specific conductance (KL) between the youngest and oldest trees. At the time of maximum transpiration in the Y stand in May-June, gross carbon uptake (gross ecosystem production, GEP) was similar for Y and O stands despite an almost twofold difference in stand leaf area index (LAIS). However, the higher rate of water use by Y trees was not sustainable in the drought-prone environment, and between spring and late summer, KL of Y trees declined fivefold compared with a nearly twofold decline for M trees and a < 30% reduction in O trees. Because the Y stand contained a significant shrub understory and more exposed soil, there was no appreciable difference in mean daily latent energy fluxes between the Y stand and the older stands as measured by the eddy-covariance technique. These patterns resulted in 60 to 85% higher seasonal GEP and 55 to 65% higher water-use efficiency at the M and O stands compared with the Y stand.     

Comparing ecological restoration alternatives: Grand Canyon, Arizona

Three treatments designed to initiate the process of restoring the surface fire regime and open forest structure of a southwestern ponderosa pine forest were compared on the Kaibab National Forest along the Grand Canyon’s South Rim. The treatments were: (1) full restoration (FULL)—thinning trees to emulate stand structure prior to fire regime disruption ca. 1887, forest floor fuel treatment, and prescribed burning, (2) minimal thinning (MIN)—removing young trees only around living old-growth (pre-1887) trees, fuel treatment, and prescribed burning, (3) burn-only (BURN)—representing the current management policy in Grand Canyon National Park (GCNP), and (4) CONTROL. Each treatment was applied to a 12 ha unit. Compared to reconstructed 1887 conditions, all study sites were much more dense prior to treatment (94–176 trees/ha in 1887, compared to 783–3693 trees/ha in 1997). However, basal area increases were less striking (12.6–20.3 in 1887, 17.5–27.0 m²/ha in 1997), reflecting past harvest and dwarf mistletoe reduction treatments that removed many large pines. In 2000, 1 year after treatment, tree densities were reduced to 11, 23, and 37 of pre-treatment levels in the FULL, MIN, and BURN treatments, respectively. Understory plant communities showed significant declines in richness and plant frequency across years, probably due to a severe drought in 2000 (60% of average precipitation). No differences in plant communities were observed across treatments, despite the mechanized disturbance associated with tree removal in the FULL treatment. Prescribed fire behavior (flame length, flaming zone depth) and effects (bole char, crown scorch) were similar across all three burned treatments. Simulated fire behavior under dry, windy conditions was reduced in all three treatments compared to the control. The FULL treatment was much less susceptible to crownfire due to reduced crown bulk density and crown fuel load and increased crown base height. Crownfire susceptibility of the BURN treatment was only slightly reduced, while the MIN treatment was intermediate. Compared to the reference conditions of forest structure, the FULL treatment represented the most rapid and comprehensive restoration treatment, although the residual stand was at the low end of historical density. The BURN treatment thinned many small trees but had minor effects on crownfire susceptibility. Effects of the MIN treatment fell between FULL and BURN. The experimental treatments may be useful for the creation of defensible firebreaks near developments, roads, and boundaries with the FULL treatment, supplemented by MIN and BURN treatments over larger areas.     

Conserving genetic diversity in ecological restoration: a case study with ponderosa pine in northern Arizona,USA

Our study used allozyme analyses to evaluate potential impacts of ecological restoration treatments on genetic diversity of ponderosa pine (Pinus ponderosa var. scopulorum) populations within the Fort Valley Experimental Forest near Flagstaff, AZ. Allele frequencies varied among pre-settlement clumps, with trees more closely related to each other within clumps. This clumpy spatial stand structure typical of reference conditions thus represents “genetic neighborhoods” and suggests restoration of clumpy versus more evenly dispersed trees in naturally regenerating stands will protect evolutionary genetic patterns. Compared to pre-settlement populations, post-settlement trees had slightly greater heterozygosity, and allelic richness and allele frequencies varied between these two age groups. These genetic differences could have resulted from different selective conditions under which the two age groups became established. Genetic diversity of populations created using different selection criteria for residual post-settlement trees did not vary but simulated removal of 75% of post-settlement trees decreased allelic richness. Maintaining more clumps created from post-settlement populations with higher tree densities across the landscape would be more effective at conserving allelic richness. Overall results of this study indicate use of genetic structure and diversity can help guide restoration treatments to help ensure adaptive potential is conserved.     

Fine root biomass in relation to site and stand characteristics in Norway spruce and Scots pine stands

Variations in fine root biomass of trees and understory in 16 stands throughout Finland were examined and relationships to site and stand characteristics determined. Norway spruce fine root biomass varied between 184 and 370 g m(-2), and that of Scots pine ranged between 149 and 386 g m(-2). In northern Finland, understory roots and rhizomes (< 2 mm diameter) accounted for up to 50% of the stand total fine root biomass. Therefore, the fine root biomass of trees plus understory was larger in northern Finland in stands of both tree species, resulting in a negative relationship between fine root biomass and the temperature sum and a positive relationship between fine root biomass and the carbon:nitrogen ratio of the soil organic layer. The foliage:fine root ratio varied between 2.1 and 6.4 for Norway spruce and between 0.8 and 2.2 for Scots pine. The ratio decreased for both Norway spruce and Scots pine from south to north, as well as from fertile to more infertile site types. The foliage:fine root ratio of Norway spruce was related to basal area and stem surface area. The strong positive correlations of these three parameters with fine root nitrogen concentration implies that more fine roots are needed to maintain a certain amount of foliage when nutrient availability is low. No significant relationships were found between stand parameters and fine root biomass at the stand level, but the relationships considerably improved when both fine root biomass and stand parameters were calculated for the mean tree in the stand. When the northern and southern sites were analyzed separately, fine root biomass per tree of both species was significantly correlated with basal area and stem surface area per tree. Basal area, stem surface area and stand density can be estimated accurately and easily. Thus, our results may have value in predicting fine root biomass at the tree and stand level in boreal Norway spruce and Scots pine forests.     

Hydraulic redistribution of soil water during summer drought in two contrasting Pacific Northwest coniferous forests

The magnitude of hydraulic redistribution of soil water by roots and its impact on soil water balance were estimated by monitoring time courses of soil water status at multiple depths and root sap flow under drought conditions in a dry ponderosa pine (Pinus ponderosa Dougl. ex Laws) ecosystem and in a moist Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) ecosystem. The fate of deuterated water applied to small plots to create a strong horizontal soil water potential gradient was also monitored to assess the potential for horizontal redistribution of water and utilization of redistributed water by co-occurring shallow-rooted plants. In a 20-year-old Douglas-fir stand, approximately 28% of the water removed daily from the upper 2 m of soil was replaced by nocturnal hydraulic redistribution during late August. In an old-growth ponderosa pine stand, approximately 35% of the total daily water utilization from the upper 2 m of soil appeared to be replaced by hydraulic redistribution during July and August. By late September, hydraulic redistribution in the ponderosa pine stand was no longer apparent, even though total water use from the upper 2 m of soil was nearly identical to that observed earlier. Based on these results, hydraulic redistribution would allow 21 and 16 additional days of stored water to remain in the upper soil horizons in the ponderosa pine and Douglas-fir stands, respectively, after a 60-day drought. At both sites, localized applications of deuterated water induced strong reversal of root sap flow and caused soil water content to cease declining or even temporarily increase at locations too distant from the site of water application to have been influenced by movement of water through the soil without facilitation by roots. Xylem water deuterium values of ponderosa pine seedlings suggested utilization of redistributed water. Therefore, hydraulic redistribution may enhance seedling survival and maintain overstory transpiration during summer drought. These first approximations of the extent of hydraulic redistribution in these ecosystems suggest that it is likely to be an important process in both wet and dry forests of the Pacific Northwest.     

Water limitations to carbon exchange in old-growth and young ponderosa pine stands

We investigated the impact of seasonal soil water deficit on the processes driving net ecosystem exchange of carbon (NEE) in old-growth and recently regenerating ponderosa pine (Pinus ponderosa Doug. ex Laws.) stands in Oregon. We measured seasonal patterns of transpiration, canopy conductance and NEE, as well as soil water, soil temperature and soil respiration. The old-growth stand (O) included two primary age classes (50 and 250 years), had a leaf area index (LAI) of 2.1 and had never been logged. The recently regenerating stand (Y) consisted predominantly of 14-year-old ponderosa pine with an LAI of 1.0. Both stands experienced similar meteorological conditions with moderately cold wet winters and hot dry summers. By August, soil volumetric water content within the upper 30 cm had declined to a seasonal minimum of 0.07 at both sites. Between April and June, both stands showed similar rates of transpiration peaking at 0.96 mm day(-1); thereafter, trees at the Y site showed increasing drought stress with canopy stomatal resistance increasing 6-fold by mid-August relative to values for trees at the O site. Over the same period, predawn water potential (psi(pd)) of trees at the Y site declined from -0.54 to -1.24 MPa, whereas psi(pd) of trees at the O site remained greater than -0.8 MPa throughout the season. Soil respiration at the O site showed a strong seasonal correlation with soil temperature with no discernible constraints imposed by declining soil water. In contrast, soil respiration at the Y site peaked before seasonal maximal soil temperatures and declined thereafter with declining soil water. No pronounced seasonal pattern in daytime NEE was observed at either site between April and September. At the Y site this behavior was driven by concurrent soil water limitations on soil respiration and assimilation, whereas there was no evidence of seasonal soil water limitations on either process at the O site.     

Forest thinning and subsequent bark beetle-caused mortality in Northeastern California

The Warner Mountains of northeastern California on the Modoc National Forest experienced a high incidence of tree mortality (2001–2007) that was associated with drought and bark beetle (Coleoptera: Curculionidae, Scolytinae) attack. Various silvicultural thinning treatments were implemented prior to this period of tree mortality to reduce stand density and increase residual tree growth and vigor. Our study: (1) compared bark beetle-caused conifer mortality in forested areas thinned from 1985 to 1998 to similar, non-thinned areas and (2) identified site, stand and individual tree characteristics associated with conifer mortality. We sampled ponderosa pine (Pinus ponderosa var ponderosa Dougl. ex Laws.) and Jeffrey pine (Pinus jeffreyi Grev. and Balf.) trees in pre-commercially thinned and non-thinned plantations and ponderosa pine and white fir (Abies concolor var lowiana Gordon) in mixed conifer forests that were commercially thinned, salvage-thinned, and non-thinned. Clusters of five plots (1/50th ha) and four transects (20.1 × 100.6 m) were sampled to estimate stand, site and tree mortality characteristics. A total of 20 pre-commercially thinned and 13 non-thinned plantation plot clusters as well as 20 commercially thinned, 20 salvage-thinned and 20 non-thinned mixed conifer plot clusters were established. Plantation and mixed conifer data were analyzed separately. In ponderosa pine plantations, mountain pine beetle (Dendroctonus ponderosae Hopkins) (MPB) caused greater density of mortality (trees ha⁻¹ killed) in non-thinned (median 16.1 trees ha⁻¹) compared to the pre-commercially thinned (1.2 trees ha⁻¹) stands. Percent mortality (trees ha⁻¹ killed/trees ha⁻¹ host available) was less in the pre-commercially thinned (median 0.5%) compared to the non-thinned (5.0%) plantation stands. In mixed conifer areas, fir engraver beetles (Scolytus ventralis LeConte) (FEN) caused greater density of white fir mortality in non-thinned (least square mean 44.5 trees ha⁻¹) compared to the commercially thinned (23.8 trees ha⁻¹) and salvage-thinned stands (16.4 trees ha⁻¹). Percent mortality did not differ between commercially thinned (least square mean 12.6%), salvage-thinned (11.0%), and non-thinned (13.1%) mixed conifer stands. Thus, FEN-caused mortality occurred in direct proportion to the density of available white fir. In plantations, density of MPB-caused mortality was associated with treatment and tree density of all species. In mixed conifer areas, density of FEN-caused mortality had a positive association with white fir density and a curvilinear association with elevation.     

Relationship of site and stand characteristics to Armillaria root disease incidence on ponderosa pine in the Black Hills,South Dakota

Ponderosa pine (Pinus ponderosa) in the Black Hills National Forest, SD, USA, was surveyed for Armillaria root disease (ARD). The root pathogen Armillaria ostoyae occurred on ponderosa pine seedlings, saplings, pole‐size trees and large‐diameter trees. The mean incidence of aboveground disease symptoms by stem count was low (0.2%), but in certain areas, the incidence was higher, affecting the regeneration success and tree longevity. Symptomatic ponderosa pine were in areas characterized by having higher elevation, greater annual precipitation, more seedlings, bigger large‐diameter trees and greater odds of past harvesting activity than in areas without root disease. Stump density was positively spatially correlated with root disease incidence. No particular soil type was related to disease occurrence; though, in areas with symptomatic trees, soil available water holding capacity (AWC) was greater and soil permeability was less where root disease was present. Spatial analysis confirmed the relationships found in linear correlations, with soil AWC and stump density positively and soil permeability negatively correlated with per cent infected stems ha^?1 and basal area infected.     

Response of bark beetles and their natural enemies to fire and fire surrogate treatments in mixed-conifer forests in western Montana

Four treatments (control, burn-only, thin-only, and thin-and-burn) were evaluated for their effects on bark beetle-caused mortality in both the short-term (one to four years) and the long-term (seven years) in mixed-conifer forests in western Montana, USA. In addition to assessing bark beetle responses to these treatments, we also measured natural enemy landing rates and resin flow of ponderosa pine (Pinus ponderosa) the season fire treatments were implemented. All bark beetles were present at low population levels (non-outbreak) for the duration of the study. Post-treatment mortality of trees due to bark beetles was lowest in the thin-only and control units and highest in the units receiving burns. Three tree-killing bark beetle species responded positively to fire treatments: Douglas-fir beetle (Dendroctonus pseudotsugae), pine engraver (Ips pini), and western pine beetle (Dendroctonus brevicomis). Red turpentine beetle (Dendroctonus valens) responded positively to fire treatments, but never caused mortality. Three fire damage variables tested (height of crown scorch, percent circumference of the tree bole scorched, or degree of ground char) were significant factors in predicting beetle attack on trees. Douglas-fir beetle and pine engraver responded rapidly to increased availability of resources (fire-damaged trees); however, successful attacks dropped rapidly once these resources were depleted. Movement to green trees by pine engraver was not observed in plots receiving fire treatments, or in thinned plots where slash supported substantial reproduction by this beetle. The fourth tree-killing beetle present at the site, the mountain pine beetle, did not exhibit responses to any treatment. Natural enemies generally arrived at trees the same time as host bark beetles. However, the landing rates of only one, Medetera spp., was affected by treatment. This predator responded positively to thinning treatments. This insect was present in very high numbers indicating a regulatory effect on beetles, at least in the short-term, in thinned stands. Resin flow decreased from June to August. However, resin flow was significantly higher in trees in August than in June in fire treatments. Increased flow in burned trees later in the season did not affect beetle attack success. Overall, responses by beetles to treatments were short-term and limited to fire-damaged trees. Expansions into green trees did not occur. This lack of spread was likely due to a combination of high tree vigor in residual stands and low background populations of bark beetles.