Factors influencing tree species diversity and Betula alleghaniensis establishment in silvicultural openings

We examined the short-term effects of group-selection harvestingwith seed tree retention on the diversity, abundance and establishmentof tree seedlings in a northern hardwood forest in the UpperPeninsula of Michigan (49 openings, 20 closed canopy referencesites). Three opening sizes were examined – opening radius0.5 x canopy height (267 ± 62 m², n = 16), 0.75 x canopyheight (642 ± 85 m², n = 17) and 1.0 x canopy height(1192 ± 155 m², n = 16) (canopy height = 22 m). A singleyellow birch (Betula alleghaniensis Britt.) seed tree was retainedin the centre of each opening. Tree seedling density was significantlyhigher in the largest group-selection opening than at the closedcanopy reference sites (P < 0.05), the main factor for thiswas the increased proportion of yellow birch, red maple (Acerrubrum L.) and other minor species. Nevertheless, yellow birchwas still a minor component of the developing gap cohort, comprising5.9 per cent of the seedlings and 1.1 per cent of the saplings.Within openings, microsite variables, such as per cent coversof bare soil and coarse woody debris, were the best predictorsof yellow birch occurrence and density. Our results suggestthat microsite limitations and competing vegetation may greatlyreduce the efficacy of openings for ensuring the maintenanceof mid-tolerants.     

Topographic and compositional influences on disturbance patterns in a northern Maine old-growth landscape

Disturbance patterns are strongly coupled with forest composition and structure, and patterns change through time in response to shifts in climate, anthropogenic impacts and other factors. Knowledge of the natural disturbance patterns for establishing baseline conditions for a forest type or ecosystem facilitates change detection for other elements of the biophysical system important to management and conservation. Dendrochronological reconstructions from old-growth forest remnants throughout northeastern North America document average decadal rates of disturbance of 5%–<10% over the last 150–300 years. Relatively frequent, low severity disturbance characterized by small gaps representing canopy openings made by 1–3 trees prevail in these forests dominated by varying mixtures of late-successional tree species. Few studies, though, have explicitly characterized differences related to composition or topographic setting in old-growth landscapes. We addressed this by comparing the temporal and spatial disturbance patterns reconstructed from tree rings at two spatial resolutions (0.5 ha and 200 m²). Sites were selected to assess the influence of topography (slope) and cover type in stands where red spruce (Picea rubens Sarg) and balsam fir (Abies balsamea L. Mill) were key components. Low rates of disturbance (average <10% per decade) and small gap sizes (≤30 m²) prevailed in all stands during the decades from 1850–1980. Episodic pulses of disturbance, of nearly moderate intensity in some stands, opened ca. 20–30% of the canopy area and were associated with wind events and/or insect outbreaks that differentially affected stands. We found no significant difference in the average temporal rates of disturbance related to cover type or topography in 0.5-ha plots. However, the influence of these factors was evident in comparisons of gap areas estimated for 200-m² plot sections. At this resolution, the largest canopy openings (≥100 m²) occurred most frequently in slope sites, enabling pulses of canopy accession for Betula alleghaniensis (Britton). Whereas the smallest canopy openings (≤30 m²) dominated softwood stands, favoring red spruce, balsam fir and other shade-tolerant species throughout the forest. The variable effects of common disturbance agents, regardless of topographic position and/or cover types, points to the important role of biological legacies determining stand structure and composition on subsequent disturbance events and long-term patterns.     

Effects of green-tree retention on abundance and guild composition of corticolous arthropods

Corticolous, or bark-dwelling, arthropods may be useful indicators of environmental changes associated with variable-retention harvests. We studied the effects of varying levels and patterns of green-tree retention on the community composition of bark-dwelling arthropods. Arthropods were sampled with crawl traps installed on 280 live trees and 260 snags (all Douglas-fir) at three locations (experimental blocks) in the western Cascade Range of Oregon and Washington. Sampling coincided with the breeding season of the brown creeper (Certhia americana), a primary avian predator, in 2003 and 2004. Within each block, arthropods were collected in five, 13-ha experimental units—a control (uncut forest) and four treatments representing one of two levels of retention (15% vs. 40% of original basal area) and one of two spatial patterns (trees dispersed vs. aggregated in 1-ha patches). In total, 166,234 arthropods – predominantly Collembola (70%) – were collected over the course of study. With the exception of Collembola, arachnids were the most abundant arthropods (23% of individuals); spiders (Araneae) accounted for >95% of arachnids. Other common insect orders included adult Coleoptera (16%), Hemiptera (15%), and Diptera (12%). For all insects combined (excluding Collembola), activity-density (daily catch per trap, an index of abundance) was significantly greater in harvest treatments than in controls. The greatest increases were found at 15% retention—densities were 2.6 times greater than in controls and 1.8 times greater than at 40% retention. Pattern of retention did not affect abundance of most arthropod groups, although two families of spiders (Linyphiidae and Thomisidae) were more abundant in dispersed than in aggregated treatments. Traps on live trees yielded 2.2 times more arthropods than did traps on snags likely reflecting differences in food resources. A high proportion of herbivorous taxa showed negative associations with local density and basal area of overstory trees and positive associations with cover of herbs, suggesting that many corticolous arthropods originate in the understory and respond positively to increases in vegetation cover following retention harvests. The numerical dominance of Collembola and high abundance of Diplopoda also suggest important ecological ties between communities of corticolous and detrital (litter-dwelling) arthropods.     

Enrichment planting as a silvicultural option in the eastern Amazon: Case study of Fazenda Cauaxi

Liana-dominated forest patches constitute 15–20% of old-growth forests in the Eastern Amazon but are generally excluded from management for timber production. Here we ask if liana-dominated patches may be brought into production by clearing lianas and conducting enrichment planting (EP) of native timber species. We present growth results from 8 years of such EP trials. Rapid growth and low mortality of all species in this study suggest that EP in cleared liana patches can contribute to timber stocks in second and third harvests of managed forests. The most vigorous individuals of Parkiagigantocarpa and Schizolobium amazonicum in each enrichment site grew more than 1 cm diameter per year (rates were initially >2 cm yr⁻¹), and attained dominant canopy positions and diameters equal to those of small canopy trees in the surrounding forest within 8 years of planting (mean dbh ∼18 cm and ∼20 cm, respectively, at year 8). Limited data on Ceiba pentandra plantings indicate a similar trajectory for this species (dbh ∼40 cm in 8 years). The most vigorous Swietenia macrophylla grew at least 1 cm per year in enrichment plots (mean dbh ∼10 cm in 8 years), but take longer to attain dominant positions. Tabebuia serratifolia may take much longer to reach the canopy than other species tested (rates <1 m yr⁻¹). We attribute the excellent performance to light availability; planting in intact soil with minimal compaction and abundant organic material; and low competition rates maintained by periodic thinning of competing vegetation.     

Canopy disturbance and tree recruitment over two centuries in a managed longleaf pine landscape

Disturbance history was reconstructed across an 11300 ha managed longleaf pine (Pinus palustris Mill.) landscape in southwestern Georgia, USA. Our specific objectives were to: (i) determine forest age structure; (ii) reconstruct disturbance history through the relationship between canopy disturbance, tree recruitment and growth; and (iii) explore the relationship between canopy disturbance and climate. Age structure, canopy disturbance events and initial growth patterns at coring height were examined by randomly sampling 1260 trees in 70 1.3 ha plots. Principal component analysis was used to group plots with similar age structures to gain insight into the dynamics between canopy disturbance and recruitment. Disturbance events were detected by large and rapid increases in radial growth. We tested the following hypothesis to investigate whether these growth increases could have been triggered by improved climatic conditions: precipitation and drought are positively correlated to radial growth releases. Only four stands (comprising <6% of the study area) had an even-aged structure. Further, tree recruitment prior to European settlement indicates that longleaf pine naturally recruited into areas 1.3 ha or less, supporting early-20th century observations that the primary longleaf pine forest was uneven-aged. Contrary to our hypothesis, growing season precipitation and drought was significantly and negatively correlated with canopy disturbance (radial growth releases), which indicates that a reconstruction of disturbance history could proceed with some confidence. Most trees sampled were recruited at coring height from 1910 to 1935. Of the 67 canopy disturbances detected from 1910 to 1935, the average growth release ranged from 139 to 277% per half decade suggesting the occurrence of large canopy disturbances. Rapid initial growth patterns of young trees during these years show evidence of reduced overstory competition and support the detected disturbance intensity. Our reconstruction of stand dynamics is markedly similar to independent records of local oral and written history, which gives an additional set of evidence that the disturbance detection methodology used can be useful in open-canopied forests. Stands with multiple cohorts reveal a mix of continuous minor and major canopy disturbances leading to continual tree recruitment, suggesting their applicability as models for long-term forest management. The significant relationship between climate and disturbance in our data suggests that with the expected warming over the next 100 years, climatic impacts on stand dynamics should be incorporated into long-term longleaf pine forest restoration and management.     

Patterns of spread of Pinus contorta Dougl. ex Loud. invasion in a Natural Reserve in southern South America

Pinus species, which have formed the foundation of commercial forestry industry in many countries, are known to be invasive in natural ecosystems, especially in the Southern Hemisphere. Pinus contorta is considered one of the most aggressively invasive plantation species. In this paper we aim to: (a) determine patterns of P. contorta invasion in relation to its size and age structure and spatial distribution and (b) determine the effect of vegetation cover on its regeneration. For this purpose, we have chosen the Malalcahuello National Reserve in south-central Chile, which is dominated by Araucaria araucana forests. In the three P. contorta trial plots, attributes were measured in order to describe its current structure. We selected one of the three P. contorta trial plots and set eight 50 m wide and 125 m centrifugal transects starting at the North Azimuth (0°) orientation and then one each 45°. In each transect 25 circular plots of 2.5 m radius were established every 25 m. In each plot, we measured collar diameter (CD) for each P. contorta individual and adjusted a diameter–age function. We recorded the presence of cones for each individual P. contorta within the plots and the total number of P. contorta individuals. In each plot, we measured percent vegetation cover (grass, understory and canopy). The selected parent stand showed a decrease in density from the original plantation spacing, from 2500 to 150 plants ha⁻¹. Of all sampled individuals, only a 38% had cones. We found reproductive structures in trees as young as 5-year-old. Regeneration was found in all transects. Mean density for the area was 1600 plants ha⁻¹, and the greater plant number was found in the West transect, reaching an average of 6600 plants ha⁻¹. Through the interpolation performed with the kriging method, a map of the area with the spatial gradient of plant density was obtained. Naturally regenerated individuals of P. contorta occupied an area of 78 ha where the most distant individual is located in the southeast direction at an average distance of 1200 m from the three study plots. We determined that a positive association exists between P. contorta and the species A. araucana and Nothofagus antarctica. The capacity for early and consistent reproduction and the establishment of individuals dispersed at great distances from the original plots allow us to conclude that P. contorta has great potential as an invasive species in forests of this area of the Andes of southern South America. Invasion of P. contorta has many important implications for the conservation of native forests in our region including diminished regeneration of A. araucana.     

Crown transparency, tree mortality and stem growth of Pinus sylvestris, and colonization of Tomicus piniperda after an outbreak of Gremmeniella abietina

In the year 2000, large areas of forest in Sweden, mainly 30-50 year old Pinus sylvestris (L.) stands, were attacked by the fungus Gremmeniella abietina (Lagerb.) Morelet. The aims of this study were to investigate: (i) the relationship between G. abietina-induced tree crown transparency (CT) and P. sylvestris (L.) tree mortality; (ii) the influence of CT levels on stem growth; (iii) the recovery of the crown; and (iv) the association of CT and colonization by Tomicus piniperda (L.). Thirty-five permanent sample plots were established in five P. sylvestris stands (38-46 years old), infested by G. abietina, and 23 plots in four reference stands, not obviously infested.During the 5 years following the attack, the total mortality amounted to 454 trees ha⁻¹ and 7.8 m² ha⁻¹, on average, in the five infested stands, corresponding to 42% of the trees and 34% of the basal area at the time of the attack. Most of the mortality occurred within 2 years of the attack. The mortality of individual trees (2002-2005) was found to be related to the crown transparency (CT), the position of needle loss within the crown and the tree diameter at breast height. Based on our modeling, the probability of mortality was substantially increased if the initial CT-value was higher than 85%.Growth reductions were detected for individual trees with an initial CT of >c. 40%. In contrast, trees with a low initial CT (<c. 40%) were not affected and even exhibited increased growth. In the five infested stands, the reductions in basal area and volume increment were estimated to be 26-58%, and, 42-73%, respectively, during the five growing seasons after the attacks.The trees in the infested stands that were still alive in spring 2005 had started to recover in terms of CT. Breeding of T. piniperda on the P. sylvestris (L.) stems occurred almost exclusively on stems with a CT > 90%.The data from this study suggest that when a P. sylvestris (L.) stand has been attacked by G. abietina, trees with a CT above 80% should be felled; the remaining trees will have a high probability of survival and resistance to successful breeding by the T. piniperda.     

Effect of species and spacing of fast-growing nurse trees on growth of an indigenous tree, Hopea odorata Roxb., in northeast Thailand

We tested the effects of species and spacing of nurse trees on the growth of Hopea odorata, a dipterocarp tree indigenous to Southeast Asia, in a two-storied forest management system in northeast Thailand. Eucalyptus camaldulensis, Acacia auriculiformis, and Senna siamea were planted as nurse trees in 1987 at spacings of 4 m × 8 m, 2 m × 8 m, 4 m × 4 m, and 2 m × 4 m in the Sakaerat Silvicultural Research Station of the Royal Forest Department, Thailand. Seedlings of H. odorata were planted in the nurse tree stands at a uniform spacing of 4 m × 4 m and in control plots (no nurse trees) in 1990. Stem numbers of some nurse trees were thinned by half in 1994. The stem diameter and height of all trees were measured annually until 1995 and again in 2007. The mean annual increment (MAI) in volume was estimated as 8.2–10.1 m³ ha⁻¹ year⁻¹ for E. camaldulensis and 0.9–1.2 m³ ha⁻¹ year⁻¹ for S. siamea, smaller than reported elsewhere. This suggests that the site properties were not suitable for them. The MAI of A. auriculiformis was 7.9–9.8 m³ ha⁻¹ year⁻¹, within the reported range. Survival rates of H. odorata in the S. siamea stands and the control plots decreased rapidly during the first 2 years but then stayed constant from 1992. In contrast, survival rates of H. odorata in the E. camaldulensis and A. auriculiformis stands were initially high (>70%), but then decreased after 1995. Stem diameter, tree height, and stand basal area of H. odorata were large in both the S. siamea stands and the control plots from then. The growth of H. odorata was largest in the 2 m × 8 m S. siamea stands. In contrast, it was restricted in the E. camaldulensis and A. auriculiformis stands owing to strong shading by their canopies. Thinning by 50% tended to facilitate the growth of H. odorata temporarily in the E. camaldulensis and A. auriculiformis stands. The stand basal areas of nurse trees and of H. odorata showed a trade-off. These results suggest that the growth of H. odorata was maximized in the S. siamea stands. We assume, however, that the growth of H. odorata could be improved even in the E. camaldulensis and A. auriculiformis stands by frequent or heavy thinning.     

Short-term effects of group-selection harvesting on breeding birds in a northern hardwood forest

We used a before-after, control-impact design (one year pre-harvest, two years post-harvest) and unlimited-radius point counts to study the effects of typical group-selection harvesting (0.5 gaps ha⁻¹ placed near seed trees within a standard single-tree selection harvest) and intensive group-selection harvesting (4 gaps ha⁻¹ placed on a grid with no harvesting between gaps) on the composition and abundance of breeding birds in tolerant hardwood forests in Algonquin Provincial Park, Ontario. Percent similarity between pre- and post-harvest bird communities was 5–9% lower in selection harvested stands than in reference stands. Differences in percent similarity among the three treatments were not significant, however, suggesting that the changes in the bird community in stands harvested with group selection were not substantially different than those in reference stands. Abundance of aerial foragers and tree-and-shrub nesters increased in response to typical and intensive group selection in the second year post-harvest. By contrast, bark foragers and cavity-nesters decreased in the first year post-harvest and then increased in the second year post-harvest in response to typical group selection. Abundance of 16 (73%) of 22 species was not affected by harvesting. Blue Jay (Cyanocitta cristata), Chestnut-sided Warbler (Dendroica pensylvanica), Least Flycatcher (Empidonax minimus), and White-throated Sparrow (Zonotrichia albicollis) increased in response to intensive group selection in the first or second year post-harvest, whereas Chestnut-sided Warbler, Hairy Woodpecker (Picoides villosus), and White-throated Sparrow increased in response to typical group selection in the first or second year post-harvest. Ovenbird (Seiurus aurocapilla) decreased slightly in response to typical group selection in the second year post-harvest. Our short-term data suggest that intensive, rather than typical, group-selection harvesting is preferred for maintaining densities of cavity-nesting birds and Ovenbird; whether these advantages continue through the remainder of the cutting cycle and beyond requires further investigation.     

Defoliation and growth relationships for mid-rotation Sitka spruce attacked by the green spruce aphid, Elatobium abietinum (Walker) (Homoptera: Aphididae)

Green spruce aphid (Elatobium abietinum) feeds primarily on the 1-year-old and older needle leaves of spruce (Picea spp.) and is a major defoliator of commercial Sitka spruce plantations in the UK and other maritime regions of north-west Europe. The impact of E. abietinum on mid-rotation (23-28-year-old) Sitka spruce in Radnor Forest, in mid-Wales, was determined by comparing aphid population densities, defoliation and growth between plots of trees that were either treated with insecticide or were left untreated. The experimental treatments were maintained for 5 years and over this period (2002-2006) peak E. abietinum densities in the untreated plots varied from 5 to 36 aphids per 100 needles. These densities, which were representative of low to moderate rates of infestation, were associated with low rates of defoliation (0-8%), but they had a significant (P ? 0.01) impact on mean radial increment (RI) and mean volume increment (VI). In 2005, the year with the highest aphid populations, peak densities in the untreated plots averaged 14 aphids per 100 needles and this rate of infestation reduced RI by 17%, VI by 10% and the dry weight of current-year needles by 10%. On average, across all years, infestation by E. abietinum reduced mean annual VI by 6%. Comparisons with previous studies on the impact of the aphid on 4-year-old, 7-year-old and 9-year-old Sitka spruce (Straw et al., 2005) indicate that defoliation by E. abietinum, on a per capita basis, decreases as trees grow older, but that the impact on VI increases. The different growth response of young and mid-rotation Sitka spruce to infestation is related to differences in canopy structure, particularly in the ratio of current-year needles to older needles, and the greater demands on photosynthetic production in older trees that arise from the need to support an increasing quantity of non-photosynthetic structural tissues in branches, stem and roots.     

Polylepis australis’ regeneration niche in relation to seed dispersal,site characteristics and livestock density

Establishment of Polylepis forests endemic to the mountains of South America may be affected by seed dispersal, site characteristics and livestock density. Polylepis australis (“tabaquillo”) grows in the high mountains of central Argentina, where we set up 76 square study plots of 900 m². To determine dispersal distance, we distributed 6 seed traps per plot in and around 20 plots. To determine the best site characteristics and livestock stoking rates, at two river basins differing in historic stocking rates, we analyzed the presence of seedlings in 56 plots and recorded topography, vegetation types and indicators of livestock activity. We also measured microsite characteristics in a sample of 32 comparable pairs of 1 m² quadrats, with and without seedlings. Maximum recorded dispersal distance of P. australis seeds was 6 m, and seedlings were found no more than 10 m from seed trees. The numbers of seedlings and seed trees were 3.5 and 4 times higher, respectively, in the basin with less livestock. At the 900 m² plot scale, a Poisson regression indicated a positive relationship between seedling number and P. australis canopy cover. At the quadrat scale (1 m²), seedlings were found in quadrats with significantly lower evidence of soil erosion than comparison quadrats without seedlings. We conclude that the main limitations to recruitment are short seed dispersal distances, lack of seed trees and extreme soil erosion. Management should therefore aim at preserving seed trees and reducing livestock density to prevent erosion.     

Soricid response to coarse woody debris manipulations in Coastal Plain loblolly pine forests

We assessed shrew (soricids) response to coarse woody debris (CWD) manipulations in managed upland loblolly pine (Pinus taeda) stands in the upper Coastal Plain of South Carolina over multiple years and seasons. Using a completely randomized block design, we assigned one of the following treatments to 12, 9.3-ha plots: removal (n = 3; all CWD ≥ 10 cm in diameter and ≥60 cm long removed), downed (n = 3; 5-fold increase in volume of down CWD), snag (n = 3; 12-fold increase in standing dead CWD), and control (n = 3; unmanipulated). Therein, we sampled shrews during winter, spring, and summer seasons, 2003–2005, using drift-fence pitfall arrays. During 1680 drift-fence plot nights we captured 253 Blarina carolinensis, 154 Sorex longirostris, and 51 Cryptotis parva. Blarina carolinensis capture rate was greater in control than in snag treatments. Sorex longirostris capture rate was lower in removal than downed and control plots in 2005 whereas C. parva capture rate did not differ among treatments. Overall, the CWD input treatments failed to elicit the positive soricid response we had expected. Lack of a positive response by soricid populations to our downed treatments may be attributable to the early CWD decay stage within these plots or an indication that within fire-adapted pine-dominated systems of the Southeast, reliance on CWD is less than in other forest types.     

Stand characteristics and downed woody debris accumulations associated with a mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak in Colorado

Lodgepole pine (Pinus contorta Dougl. ex Loud.)-dominated ecosystems in north-central Colorado are undergoing rapid and drastic changes associated with overstory tree mortality from a current mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak. To characterize stand characteristics and downed woody debris loads during the first 7 years of the outbreak, 221 plots (0.02 ha) were randomly established in infested and uninfested stands distributed across the Arapaho National Forest, Colorado. Mountain pine beetle initially attacked stands with higher lodgepole pine basal area, and lower density and basal area of Engelmann spruce (Picea engelmannii [Parry]), and subalpine fir (Abies lasiocarpa (Hook.) Nutt. var. lasiocarpa) compared to uninfested plots. Mountain pine beetle-affected stands had reduced total and lodgepole pine stocking and quadratic mean diameter. The density and basal area of live overstory lodgepole declined by 62% and 71% in infested plots, respectively. The mean diameter of live lodgepole pine was 53% lower than pre-outbreak in infested plots. Downed woody debris loads did not differ between uninfested plots and plots currently infested at the time of sampling to 3 or 4–7 years after initial infestation, but the projected downed coarse wood accumulations when 80% of the mountain pine beetle-killed trees fall indicated a fourfold increase. Depth of the litter layer and maximum height of grass and herbaceous vegetation were greater 4–7 years after initial infestation compared to uninfested plots, though understory plant percent cover was not different. Seedling and sapling density of all species combined was higher in uninfested plots but there was no difference between infested and uninfested plots for lodgepole pine alone. For trees ≥2.5 cm in diameter at breast height, the density of live lodgepole pine trees in mountain pine beetle-affected stands was higher than Engelmann spruce, subalpine fir, and aspen, (Populus tremuloides Michx.), in diameter classes comprised of trees from 2.5 cm to 30 cm in diameter, suggesting that lodgepole pine will remain as a dominant overstory tree after the bark beetle outbreak.     

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.     

Profound impoverishment of the large-tree stand in a hyper-fragmented landscape of the Atlantic forest

Large tree species have a disproportional influence on the structure and functioning of tropical forests, but the forces affecting their long-term persistence in human-dominated landscapes remain poorly understood. Here we test the hypothesis that aging forest edges and small fragments (3.4–295.7 ha) are greatly impoverished in terms of species richness and abundance of large trees in comparison to core areas of forest interior. The study was conducted in a hyper-fragmented landscape of the Atlantic forest, northeast Brazil. Large tree species were quantified by recording all trees (DBH ≥ 10 cm) within fifty-eight 0.1-ha plots distributed in three forest habitats: small forest fragments (n = 28), forest edges (n = 10), and primary forest interior areas within an exceptional large forest remnant (n = 20). Large tree species and their stems ≥10 cm DBH were reduced by half in forest edges and fragments. Moreover, these edge-affected habitats almost lacked large-stemmed trees altogether (0.24 ± 0.27% of all stems sampled), and very tall trees were completely absent from forest edges. In contrast, large trees contributed to over 1.5% of the whole stand in forest interior plots (2.9 ± 2.8%). Habitats also differed in terms of tree architecture: relative to their DBH trees were on average 30% shorter in small fragments and forest edges. Finally, an indicator species analysis yielded an ecological group of 12 large tree species that were significantly associated with forest interior plots, but were completely missing from edge-affected habitats. Our results suggest a persistent and substantial impoverishment of the large-tree stand, including the structural collapse of forest emergent layer, in aging, hyper-fragmented landscapes.     

The effects of sudden oak death on foliar moisture content and crown fire potential in tanoak

The introduction of non-native pathogens can have profound effects on forest ecosystems resulting in loss of species, changes in species composition, and altered fuel structure. The introduction of Phytophthora ramorum, the pathogen recognized as causing Sudden Oak Death (SOD), leads to rapid decline and mortality of tanoak (Lithocarpus densiflorus) in forests of coastal California, USA. We tracked foliar moisture content (FMC) of uninfected tanoaks, SOD-infected tanoaks, SOD-killed (dead) tanoaks, and surface litter for 12 months. We found that FMC values differed significantly among the three categories of infection. FMC of uninfected tanoaks averaged 82.3% for the year whereas FMC of infected tanoaks had a lower average of 77.8% (ANOVA, P = 0.04). Dead trees had a significantly lower FMC, averaging 12.3% (ANOVA, P < 0.01) for the year. During fire season (June–September), dead tanoak FMC reached a low of 5.8%, with no significant difference between dead canopy fuels and surface litter (ANOVA, P = 0.44). Application of low FMC values to a crown ignition model results in extremely high canopy base height values to escape crown ignition. Remote estimation of dead FMC using 10-h timelag fuel moisture shows a strong correlation between remote automated weather station (RAWS) 10-h timelag fuel moisture data and the FMC of dead leaves (R² = 0.78, P < 0.01). Results from this study will help refine the decision support tools for fire managers in SOD-affected areas as well as conditions in other forests where diseases and insect epidemics have altered forest canopy fuels.     

Responses of oaks and tanoaks to the sudden oak death pathogen after 8 y of monitoring in two coastal California forests

Sudden oak death, caused by Phytophthora ramorum, is widely established in mesic forests of coastal central and northern California. In 2000, we placed 18 plots in two Marin County sites to monitor disease progression in coast live oaks (Quercus agrifolia), California black oaks (Q. kelloggii), and tanoaks (Lithocarpus densiflorus), the species that are most consistently killed by the pathogen in these areas. Through early 2008, the numbers of newly infected trees increased for all species. The infection rate for trees that were asymptomatic in 2000 was 5.0% y⁻¹ for coast live oaks, 4.1% y⁻¹ for black oaks and 10.0% y⁻¹ for tanoaks. Mortality rates were 3.1% y⁻¹ for coast live oaks, 2.4% y⁻¹ for black oaks, and 5.4% y⁻¹ for tanoaks. Mortality not attributed to P. ramorum was 0.54% y⁻¹ for coast live oaks, and 0.75% y⁻¹ for tanoaks. Weibull survival models of trees that were asymptomatic in 2000 provided overall median survival times of 13.7 y for coast live oaks, 13.8 y for black oaks, and 8.8 y for tanoaks. Survival of infected (bleeding) trees declined to 9.7 y for coast live oaks, 6.2 y for black oaks, and 5.8 y for tanoaks. Ambrosia beetle attacks on bleeding trees further reduced modeled survival times by 65–80%, reaffirming the earlier finding that beetle attacks on bleeding cankers considerably reduce survival. Across all plots, the modeled time for 90% of trees that were asymptomatic in 2000 to become infected is 36.5 y for coast live oaks and 15.4 y for tanoaks. There was a trend toward higher infection rates as tree diameter increased. Greater than 90% of living coast live oaks that failed during the study had extensive beetle tunneling at the site of the break. Disease intensity in coast live oaks at the plot level was positively associated with bay laurel (Umbellularia californica) basal area and negatively associated with Pacific madrone (Arbutus menziesii) basal area. This study demonstrates the use of survival modeling to characterize the effects of epidemic disease on different species and to project the future of forests infected with tree pathogens.     

Long-term dynamics of a mixed conifer stand in Slovenia managed with a farmer selection system

The single-tree selection system is an important option for management of Norway spruce (Picea abies (L.) Karst.) and silver fir (Abies alba Mill.) forests because it provides continuous cover, requires low investments for tending, and promotes natural regeneration as well as high stand resistance and elasticity. It is often regarded as a very conservative system that usually results in only minor spatiotemporal changes in forest structure and composition. We studied management history, structural changes, regeneration dynamics, and light climate of a traditional single-tree farmer selection silver fir-Norway spruce forest (site typology Bazzanio-Abietetum). Stand structure was analyzed on five 0.25 ha permanent plots in 1994, 2001, and 2008. Regeneration density and height growth, forest floor vegetation, and light climate were also assessed on 1.5 × 1.5 m regeneration subplots in 2001 and 2008. Tree cores extracted from dominant trees from both species in two plots were used for reconstructing stand history and age structure of the canopy layer. We documented the forest response to three types of selection management regimes: excessive, normal, and conservative. Excessive management with harvest intensity significantly above the increment was documented until the late 1950s, including two peaks of heavy fellings (diameter limit cut) in the 1880s and 1930s, which favoured establishment of Norway spruce and released regeneration. The period that followed was characterized by normal selection management, but was nevertheless marked by a decline of silver fir as a result of air pollution and several droughts. This led to sanitary fellings that were carried out from the late 1970s to the early 1990s. In the last two decades conservative management followed, which led to suppression and decline of regeneration, especially of Norway spruce, and loss of selection structure. Although we recorded lower regeneration potential of silver fir compared with Norway spruce within the seedling category, silver fir outcompeted Norway spruce within the small-sized tree category (1 cm < dbh ? 10 cm) because of its superior height growth in low light levels (diffuse light <6%) and occupied a greater share of the canopy. Nevertheless, we anticipate that over the long-term the low light regime will also cause regeneration decline of silver fir and broadleaves. Our research revealed significant structural changes in a single-tree farmer selection forest during the last 150 years. These were a result of variable management regime and environment. A farmer single-tree selection system could better mimic the natural disturbance regime if spatiotemporal combinations of diverse felling regimes would be used.     

Long-term spatial dynamics of Acer saccharum during a population explosion in an old-growth remnant forest in Illinois

Species richness and evenness have greatly declined in oak–hickory forests in the central hardwood region in the U.S.A. in the past 100 years due to the rapid population growth of Acer saccharum. This study used a 50-year record of spatial dynamics to examine how demographic processes, particularly recruitment, may have contributed to this increase in an old-growth forest remnant, Brownfield Woods, Urbana, Illinois, U.S.A. The impact of canopy disturbance, including the outbreak of Dutch elm disease, on this increase was also evaluated. Historical maps of trees (≥7.6 cm DBH) from 1951, 1988, and 2001 in a 180 m × 280 m area were used to develop a series of univariate Ripley's L(d) functions to study changes in spatial patterns of three size classes of A. saccharum over time. Bivariate Ripley's L(d) functions were also utilized to evaluate spatial associations between recruitment and canopy disturbance. Our results indicated that A. saccharum was aggregated at most spatial scales up to 80 m during 1951–2001. Such aggregation arose mainly from small individuals. Furthermore, newly recruited individuals were aggregated at multiple spatial scales, and were significantly associated with canopy disturbance in general, as well as gaps created by Ulmus trees killed by Dutch elm disease. The aggregation of the 1951 initial group of small individuals changed via mortality to a random distribution over time. The results indicate that tree deaths caused by disturbances of different scales and types were the main cause of increased recruitment of A. saccharum in Brownfield Woods. The occurrence of Dutch elm disease further accelerated its population increase. This study demonstrated a direct spatial link between recruitment of A. saccharum and disturbance, and provided a long-term case study of a population explosion.