Years In Stands Research Articles

Nitrogen deposition suppresses ephemeral post-fire plant diversity.

Fire is a dominant force shaping patterns of plant diversity in Mediterranean-type ecosystems. In these biodiversity hotspots, including California's endangered coastal scrub, many species remain hidden belowground as seeds and bulbs, only to emerge and flower when sufficient rainfall occurs after wildfire. The unique adaptations possessed by these species enable survival during prolonged periods of unfavorable conditions, but their continued persistence could be threatened by nonnative plant invasion and environmental change. Furthermore, their fleeting presence aboveground makes evaluating these threats insitu a challenge. For example, nitrogen (N) deposition resulting from air pollution is a well-recognized threat to plant diversity worldwide but impacts on fire-following species are not well understood. We experimentally evaluated the impact of N deposition on post-fire vegetation cover and richness for three years in stands of coastal sage scrub that had recently burned in a large wildfire in southern California. We installed plots receiving four levels of N addition that corresponded to the range of N deposition rates in the region. We assessed the impact of pre-fire invasion status on vegetation dynamics by including plots in areas that had previously been invaded by nonnative grasses, as well as adjacent uninvaded areas. We found that N addition reduced native forb cover in the second year post-fire while increasing the abundance of nonnative forbs. As is typical in fire-prone ecosystems, species richness declined over the three years of the study. However, N addition hastened this process, and native forb richness was severely reduced under high N availability, especially in previously invaded shrublands. An indicator species analysis also revealed that six functionally and taxonomically diverse forb species were especially sensitive to N addition. Our results highlight a new potential mechanism for the depletion of native species through the suppression of ephemeral post-fire bloom events.

Rocky Mountain Bristlecone Pine (Pinus aristata) is a Confirmed Host to Mountain Pine Beetle (Dendroctonus ponderosae)

Mountain pine beetle (MPB) (Dendroctonus ponderosae Hopkins) is a native bark beetle that reproduces in pine (Pinus) species across western North America. High population levels can result in widespread host tree mortality. Over the past 2 decades, MPB has been responsible for pine mortality across millions of forested hectares in the western United States. Although a majority of the pine species found in the western United States are considered hosts to MPB, the host status of Rocky Mountain (RM) bristlecone pine (Pinus aristata) is unclear. We surveyed stands across the range of RM bristlecone pine in Colorado, USA, and quantified MPB-caused mortality within the past 10 years in stands where RM bristlecone and at least one other pine species co-occurred. We also evaluated in the field whether successful MPB brood production occurred in RM bristlecone pine. Our results confirm that RM bristlecone pine is susceptible to MPB attack and suitable for MPB reproduction. In mixed-species stands, pine species availability influenced MPB attack occurrence. The proportion of trees experiencing fatal beetle attack within a particular Pinus species was best predicted by the basal area proportion of that species in the stand prior to the most recent 10 years of mortality. These results indicate that RM bristlecone pine is vulnerable to ongoing climate change–induced contact with MPB.

Fire severity and changing composition of forest understory plant communities

AbstractQuestionsGradients of fire severity in dry conifer forests can be associated with variation in understory floristic composition. Recent work in dry conifer forests in California, USA, has suggested that more severely burned stands contain more thermophilic taxa (those associated with warmer and drier conditions), and that forest disturbance may therefore accelerate floristic shifts already underway due to climate change. However, it remains unknown how rapidly thermophilic taxa shifts occur following disturbance, how long such shifts are likely to persist, and how different thermophilic post‐disturbance communities are from pre‐disturbance communities.LocationColorado Front Range, USA.MethodsWe investigated these questions using a unique 15‐year vegetation plot dataset that captures pre‐ and post‐fire understory community composition across a gradient of fire severity in dry conifer forests, classifying taxa using the biogeographic affinity concept.ResultsThermophilization (defined here as a decrease in the ratio of cool‐mesic taxa to warm‐xeric taxa, based on biogeographic affinity of paleobotanical lineages) was observed as early as one year post‐fire for all fire severity classes, but was stronger at sites that burned at higher severity. The ratio of cool‐mesic to warm‐xeric taxa recovered to pre‐fire levels within 10 years in stands that burned at low severity, but not in stands that burned at moderate or high severity. The process of thermophilization after high‐severity fire appears to be driven primarily by the gain of warm‐xeric taxa that were absent before the fire, but losses of cool‐mesic taxa, which did not return during the duration of the study, also played a role.ConclusionsDecreases in canopy cover appear to be a main contributor to understory thermophilization. Fine‐scale heterogeneity in post‐fire forest structure is likely an important driver of floristic diversity, creating the microclimatic variation necessary to maintain floristic refugia for species mal‐adapted to increasingly warm and dry conditions.

Dendrochronological reconstruction of fire history in coniferous forests in the Monarch Butterfly Biosphere Reserve, Mexico

BackgroundSome have proposed that fire return intervals lengthen with elevation in montane tropical coniferous forests, such as those found in central Mexico. This would generate patterns of synchronous tree establishment in higher elevation forests dominated by Abies sp. Mill., and patterns of continuous tree establishment in lower elevation forests dominated by Pinus sp. L. However, it is common to find codominant stands of both genera at intermediate elevations. The Monarch Butterfly Biosphere Reserve (MBBR) is located in this region, and is covered by Abies religiosa (Kunth) Schltdl. & Cham-dominant forest, A. religiosa–Pinus pseudostrobus Lindley-codominant forest, and P. pseudostrobus-dominant forest. Despite the ecological importance of the MBBR’s coniferous forests, little is known about their fire return intervals, including their relationship with climate, and their effects on tree establishment patterns according to the elevational range proposed above. In this study, using dendrochronological techniques, we reconstructed the historical fire return intervals for each forest type, evaluated their relationship with droughts and with the El Niño Southern Oscillation (ENSO), and assessed the effect of fire return intervals on tree establishment patterns. A total of 110 cross-sections were obtained, from 23 sites across the MBBR.ResultsThe fire chronology covered the period between 1925 and 2015. Fire return intervals averaged two years in stands dominated by A. religiosa, by P. pseudostrobus, and codominated by both species, regardless of elevation. No fire-free periods were detected, even after the establishment of the reserve in 2000. Fire frequency was not associated with periods of drought and ENSO. There was no relationship between fire return intervals and patterns of tree establishment, which was continuous in all three forest types.ConclusionsOur results suggest that coniferous forests of the MBBR have historically experienced frequent fires that are unrelated to climate and tree establishment. This may be evidence that natural fire regimes in these coniferous forests have been drastically altered by human activities. Although these frequent low-severity fires could adversely affect tree regeneration, and eventually decrease the integrity of the overwintering habitat of the Monarch Butterfly, the continuous tree regeneration observed suggests that these effects have not yet taken place.

Effects of partial harvesting on species and structural diversity in aspen-dominated boreal mixedwood stands

Abstract Partial harvesting is thought to result in more structurally diverse forest stands, providing a promising alternative to clearcutting and helping balance timber production and biodiversity conservation. Evidence for this hypothesis is based mostly on understory vegetation communities, with little information available on how partial harvesting affects tree species and structural diversity. In this study, we examined overstory and understory tree species and structural diversity of boreal mixedwoods over 20 years in stands that were unharvested, partially harvested with 40% canopy removal, and clearcut. As expected, clearcutting reduced overstory tree species and structural diversity relative to that of unharvested stands; partial harvesting, however, did not affect neighbourhood (area of 4 trees including one subject tree and 3 neighbours) or stand diversity until after a forest tent caterpillar outbreak that occurred during years 3–5 post-harvesting. The insect-induced trembling aspen (Populus tremuloides Michx.) mortality was higher in the partially harvested than in the unharvested areas decreasing structural diversity at both neighbourhood and stand scales. In comparison, within-stand variations (horizontal complexity or spatial heterogeneity) in neighbourhood diversity values (Shannon species diversity, Shannon structural diversity by 2-m height class, and structural diversity by height variation) increased following the insect outbreak. The post-harvesting temporal dynamics of overstory diversity appeared time dependent and related to harvest removal, forest tent caterpillar defoliation, and growth of released balsam fir (Abies balsamea (L.) Mill.) and aspen regeneration. In all treatments, understory diversity was decreased by clearcutting, which stimulated profuse aspen suckering, and abundant balsam fir regeneration. We conclude that partial harvesting with 40% canopy removal did not significantly change tree species or structural diversity of boreal mixedwood stands in northeastern Ontario, Canada. Changes in overstory diversity following a forest tent caterpillar outbreak, however, suggest that partial harvesting that removes more of the canopy and reduces individual tree dominance may increase species diversity and within-stand structural variation but reduce structural diversity in the overstory.

Will they come? Long-term response by forest birds to experimental thinning supports the “Field of Dreams” hypothesis

Intensifying forest management practices via use of tree plantations represents a common response to rising global demand for wood and paper products. However, a large proportion of tree plantations, globally, exist in closed-canopy mid-seral stages, which are frequently associated with reduced biodiversity. In some regions, silvicultural techniques such as thinning have been used to accelerate the development of complex vertical structure and composition (e.g., shrubs, understory, midstory, large trees) in these plantations; these attributes are expected to constitute suitable habitat for species associated with particular forest structures. We report on the results of a 15-year randomized-block manipulative study based in Oregon, USA, designed to test the hypothesis that ‘if you build it they will come’ (i.e., the “Field of Dreams” hypothesis; the ‘creation’ of complex vertical structure in forests via thinning will result in colonization by bird species associated with these structures). We conducted point counts of forest birds 7 times each year in stands thinned at two different intensities and in unharvested controls. Responses to thinning by some bird species [e.g., hermit warbler (Setophaga occidentalis), Swainson’s thrush (Catharus ustulatus) and Wilson’s warbler (Cardellina pusilla)] changed from being negative in the short term (<6years), to being positive over the longer term. However, some species were sensitive to thinning disturbance even after 15years of recovery [e.g., Hutton’s vireo (Vireo huttoni)]. Disturbance-associated species initially increased following thinning; but over the longer-term, detections of several of these species [American robin (Turdus migratorius) and Townsend’s solitaire (Myadestes townsendi)] notably declined or ceased. Our findings indicate that results from short-term studies (⩽6years) cannot be used to predict the longer-term response of birds to forest management treatments, particularly for species of birds whose response to thinning over time was non-linear. Further, none of the management prescriptions tested were alone sufficient to maintain long-term occupancy for all of the species in the study. Implementing thinning at intervals across landscape scales to develop different seral stages and stand-structures, while also maintaining unthinned areas for species negatively impacted by thinning, will likely have the greatest positive impact on beta diversity of birds in managed plantation landscapes.

Long-term effect of wetland drainage on the productivity of Scots pine stands in Poland

The aim of this study was to estimate the long-term effect of wetland drainage on the productivity of Scots pine stands in Poland. Data were obtained from the forested area of the Niepolomice Forest District, which was drained in 1964–1968. To describe the long-term changes in site productivity, a local site index model was developed. The effect of drainage depended primarily on the age of the stand at the time of drainage. A positive growth response (i.e., an increase in height) was observed in stands that were younger than 45–50years at the time of drainage, with younger stands showing stronger responses. Drainage caused an increase in the site productivity of young stands that can be expressed as a change of 5.4m in site index. For fully stocked stands, the average effect of drainage expressed as the predicted increase in total wood production at age 100years was estimated to be approximately 200m3ha−1 (25%) in those stands planted after drainage. The predicted increase in the total volume of production at total stand age 100years for stands that were 30years old at the time of drainage was estimated to be approximately 125m3ha−1 (15%). For stands that were 40years old at the time of drainage, the predicted increase in volume production was approximately 50m3ha−1 (6%). Stands older than 50years at the time of drainage were assumed to be unaffected.

Managing drought-induced mortality in Pinus radiata plantations under climate change conditions: A local approach using digital camera data

Pinus radiata D. Don is an internationally important plantation species. The predicted increases in the frequency, duration and/or severity of drought and heat stress associated with climate change could challenge the commercial viability of P. radiata plantations in several countries where existing plantations are grown in districts already vulnerable to periods of drought. We undertook an empirical approach to examine silvicultural and site factors associated with a significant drought-induced mortality event in southern New South Wales, Australia. The aim was to identify local, practical, risk prevention options for P. radiata plantations exposed to the climatic conditions predicted for this region. Our approach did not rely on ground-based assessment of plots but rather a total census of dead trees across two study areas totalling 10,000 ha of P. radiata, in compartments ranging in age from 0 to 35+ years. Dead tree density counts were derived both manually and automatically from high spatial resolution digital multispectral imagery acquired using a Leica ADS40 Airborne Digital Sensor. The results showed a strong correlation between dead tree densities obtained by the manual detection and the automated process ( R = 0.95, P < 0.0001). Two modelling approaches were applied: random forests (RF) and generalised additive models (GAM). For our study sites, both methodologies identified a similar set of parameters, with time since planting in unthinned stands being the most influential variable, and terrain variables playing a smaller role. Specifically, the models identified a threshold age at around 17–18 years for stands on good quality sites and before age 16 on poorer quality sites before the on-set of catastrophic mortality under severe drought conditions. Both modelling techniques also identified similar trends with respect to elevation and slope, and indirectly, site quality. These site attributes being likely to contribute to water availability. If stands in this area are to be planted at 1000 stems ha −1 and thinning schedules cannot be met, then we recommend avoiding sites having a mean elevation of below 600 m or to establish these sites with drought-tolerant genotypes. These recommendations cannot be extrapolated beyond the range of the data. However, the application of robust image classification techniques (e.g. automated tree counts) to high spatial resolution digital imagery, the increasing availability of high resolution climatic, terrain and edaphic GIS datasets, as well as readily available spatial modelling packages, helps off-set the limitations in transferability of our empirical approach.

FIRE, LOGGING, AND OVERSTORY AFFECT UNDERSTORY ABUNDANCE, DIVERSITY, AND COMPOSITION IN BOREAL FOREST

Understory vegetation plays a critical role in boreal ecosystems. Despite this, quantitative evaluation of the factors controlling understory vegetation abundance, diversity, and composition in the most diverse boreal forest region in North America is lacking. This study examined the dynamics of understory vegetation of stands of fire origin and tested effects of overstory composition and logging vs. fire on the understory vegetation dynamics in Ontario, Canada. Understory vegetation communities were sampled in 68 stands of conifer, mixed-wood, and deciduous overstory type ranging from 7 to 201 years postfire for stands of fire origin, and from 7 to 31 years for stands of logging origin. For stands of fire origin, total cover and species richness followed similar trends for the three overstory types and were highest in the intermediate-aged stands (72–90 years). Trends in cover and richness, however, differed significantly for vascular and nonvascular plant groups. Vascular cover and species richness were generally higher under deciduous stands, and lower on older stands, while nonvascular species richness was highest under conifer stands and increased with time since fire. Neither species richness nor compositional turnover was higher under mixed-wood stands; mixed-wood stands were compositionally intermediate to conifer and deciduous stands. Multivariate analysis using multiple-response permutation procedures indicated that understory communities were compositionally distinct for all overstory types and showed no convergence with increasing time since fire. Compared with postfire stands of similar ages, post-logged stands had similar total understory cover and richness. Vascular cover and richness, however, were higher on post-logged stands, and nonvascular cover and richness were lower. Stands of logging and fire origin were compositionally distinct for all overstory types and ages. Compositional differences appeared to be driven by higher preestablished rhizomatous species and few pyrophilic species on post-logged sites. Understory vegetation communities in the central boreal shield appear to support the intermediate disturbance hypothesis. Understory richness, however, was not negatively associated with high cover values as predicted by the intermediate disturbance hypothesis. Moreover, richness appears to be highest on sites with high light availability, suggesting that boreal understory communities are influenced more by plant tolerances for low resources, rather than by competition.

PRESCRIBED FIRE IN OAK SAVANNA: FIRE FREQUENCY EFFECTS ON STAND STRUCTURE AND DYNAMICS

Although it is well known that fire can exert strong control on stand structure, composition, and dynamics in savannas and woodlands, the relationship between fire frequency and stand structure has been characterized in few of the world's savanna and woodland ecosystems. To address this issue in temperate oak-dominated ecosystems, we studied the effects of fire frequency on stand structure and dynamics in oak savanna and woodland stands that had been burned 0–26 times in 32 yr, in the Anoka Sand Plain region of Minnesota (USA). Seedling densities declined with increasing fire frequency, but differentially, for northern pin oak (Quercus ellipsoidalis), black cherry (Prunus serotina), serviceberry (Amelanchier sp.), and red maple (Acer rubrum). Bur oak (Q. macrocarpa) seedling density was not sensitive to fire frequency. Frequent burning (at least three fires per decade) prevented development of a sapling layer and canopy ingrowth. Low-frequency burning (fewer than two fires per decade) produced stands with dense sapling thickets. Reductions in overstory density and basal area from 1984 to 1995 were observed for all stands burned two or more times during that period. Basal area declined by 4–7% per year, and density declined by 6–8% per year in stands burned four or more times. Mortality rates in burned stands were higher for northern pin oak (50%) than for bur oak (8%). Northern pin oak mortality was highest for small trees (< 20 cm dbh) and lowest for mature trees (30–40 cm dbh); mortality increased with fire frequency. Bur oak mortality declined with increasing fire frequency. Attempts to preserve and maintain savannas as a viable ecosystem type in this region will require a long-term commitment to restoration-based management, with prescribed fire as a central tool. Burn frequency treatments with four or more fires per decade produce similar reductions in stem density and stand basal area but may lead to unsustainable oak tree populations. Within this general range, fire frequencies at a decadal scale should be chosen to address other management objectives, including suppressing shrubs and promoting increased cover of grasses and other herbaceous species. Fire management with a long-term view may also require periodic respites to allow for new cohorts of mature oak trees.

Prescribed Fire in Oak Savanna: Fire Frequency Effects on Stand Structure and Dynamics

Although it is well known that fire can exert strong control on stand structure, composition, and dynamics in savannas and woodlands, the relationship between fire frequency and stand structure has been characterized in few of the world's savanna and woodland ecosystems. To address this issue in temperate oak-dominated ecosystems, we studied the effects of fire frequency on stand structure and dynamics in oak savanna and woodland stands that had been burned 0–26 times in 32 yr, in the Anoka Sand Plain region of Minnesota (USA). Seedling densities declined with increasing fire frequency, but differentially, for northern pin oak (Quercus ellipsoidalis), black cherry (Prunus serotina), serviceberry (Amelanchier sp.), and red maple (Acer rubrum). Bur oak (Q. macrocarpa) seedling density was not sensitive to fire frequency. Frequent burning (at least three fires per decade) prevented development of a sapling layer and canopy ingrowth. Low-frequency burning (fewer than two fires per decade) produced stands with dense sapling thickets. Reductions in overstory density and basal area from 1984 to 1995 were observed for all stands burned two or more times during that period. Basal area declined by 4–7% per year, and density declined by 6–8% per year in stands burned four or more times. Mortality rates in burned stands were higher for northern pin oak (50%) than for bur oak (8%). Northern pin oak mortality was highest for small trees (< 20 cm dbh) and lowest for mature trees (30–40 cm dbh); mortality increased with fire frequency. Bur oak mortality declined with increasing fire frequency. Attempts to preserve and maintain savannas as a viable ecosystem type in this region will require a long-term commitment to restoration-based management, with prescribed fire as a central tool. Burn frequency treatments with four or more fires per decade produce similar reductions in stem density and stand basal area but may lead to unsustainable oak tree populations. Within this general range, fire frequencies at a decadal scale should be chosen to address other management objectives, including suppressing shrubs and promoting increased cover of grasses and other herbaceous species. Fire management with a long-term view may also require periodic respites to allow for new cohorts of mature oak trees.

Nitrogen and phosphorus cycling in relation to stand age of Eucalypus regnans F. Muell.

Laboratory and in situ rates of N mineralization in soils from Mountain Ash (Eucalyptus regnans) forests were measured fortnightly for two years in stands aged 9, 40, 80 and about 250 years (overmature), and for one year in stands aged 5 and 46 years. Rates of anaerobic mineralization (the laboratory test) showed little seasonal or annual variation. In contrast, rates of in situ mineralization varied markedly with season, being highest in spring and summer. Anaerobic mineralization was highly correlated with stand age to 80 years, but decreased between ages 80 and 250 years (...)

Predicting Growth and "Success" of Coppice-Regenerated Oak Stems

Abstract Logistic regression models were derived from data collected in the Missouri Ozarks to estimate the probabilities that 5-year-old stems of red oaks and white oaks in thinned and unthinned sprout clumps would attain a specified dbh at ages 12 and 30. In deriving each model, several success criteria based on selected future dbh were specified. A stem was considered "successful" for a particular criterion if it attained that dbh or larger, but considered a "failure" if it did not achieve that criterion because of either slow growth or mortality. Consequently, each model accounted for both growth and mortality probabilistically. Growth of survivors was also estimated using linear regression models. These models were developed for two species groups using data for stems that were alive at both the beginning (age 5) and end of a measurement period (ages 12 and 30). The logistic models indicated that height at age 5 was a significant predictor of competitive success of stems in subsequent years in stands with either thinned or unthinned clumps. Diameter of the parent stump was also significantly correlated with success, but only in stands where clumps were thinned. Results indicate that under the conditions of this study, which was conducted on a single site, stems with high success probabilities could be identified early in the life of this coppice-regenerated stand, and that early clump thinning in this stand increased success probabilities and diameter growth. For. Sci. 33(3):740-749.

Control of the Pine Root Collar Weevil, Hylobius radicis1

The pine root collar weevil (Hylobius radicis Buchanan) has become increasingly important in recent years in stands of planted pines in southern Ontario, southeastern Manitoba, and the northeastern United States. Scots pine is most seriously affected, but native hard pines growing in association with Scots pine are sometimes damaged. The weevil breeds in the root collar of healthy pines, killing more than 90% of the trees in some plantations. Ten insecticides were tested on the weevil from 1954 to 1956. Ethylene dichloride was effective in killing the larvae, but failed to prevent re infestation, Lindane and dieldrin applied as water emulsions directly to the ground around the base of the trees effectively controlled the insect for at least 4 years.

THINNING SLASH AND LOBLOLLY PINES IN QUEENSLAND

SUMMARY A summary of the main conclusions resulting from twenty years of thinning research in slash and loblolly pines is given, and important principles are illustrated. Competition among the components of a stand commences at a very early age. Thinning on a commercial basis to 350 stems per acre at 12–15 years is recommended for 8 by 8 ft. plantings, and to 450 per acre at 13–15 years in stands planted at 7 by 7 ft. The reason for the difference is explained. A second thinning to 240 stems per acre is required five or six years later. If merchantable thinning is likely to be delayed, early non-commercial thinning is desirable. Thinning to 400–450 stems per acre at 3–4 years is advocated. No plantations are old enough to have been thinned more than twice, except experimentally. Subsequent thinnings are discussed, and it is probable that they will be designed to maintain the basal area of the stand within the range of 100–140 sq. ft. per acre; crop stems may be allowed to develop beyond this figure before...