Managing forests infested by spruce beetles in south-central Alaska: Effects on nitrogen availability,understory biomass,and spruce regeneration

In Alaska, an outbreak of spruce beetles (Dendroctonus rufipennis) recently infested over one million hectares of spruce (Picea spp.) forest. As a result, land management agencies have applied different treatments to infested forests to minimize fire hazard and economic loss and facilitate forest regeneration. In this study we investigated the effects of high-intensity burning, whole-tree harvest, whole-tree harvest with nitrogen (N) fertilization, and conventional harvest of beetle-killed stands 4 years after treatment, as well as clear-cut salvage harvest 6 years after treatment. We measured available soil ammonium and nitrate and estimated N loss from leaching using in situ cation and anion resin exchange capsules. We also assessed spruce regeneration and responses of understory plant species. Availability and losses of N did not differ among any of the management treatments. Even a substantial application of N fertilizer had no effect on N availability. Spruce regeneration significantly increased after high-intensity prescribed burning, with the number of seedlings averaging 8.9 m⁻² in burn plots, as compared to 0.1 m⁻² in plots that did not receive treatment. Biomass of the pervasive grass bluejoint (Calamagrostis canadensis) was significantly reduced by burning, with burn plots having 9.5% of the C. canadensis biomass of plots that did not receive treatment. N fertilization doubled C. canadensis biomass, suggesting that N fertilization without accompanying measures to control C. canadensis is the least viable method for promoting rapid spruce regeneration.     

Stand and landscape level effects of a major outbreak of spruce beetles on forest vegetation in the Copper River Basin,Alaska

From 1989 to 2003, a widespread outbreak of spruce beetles (Dendroctonus rufipennis) in the Copper River Basin, Alaska, infested over 275,000 ha of forests in the region. During 1997 and 1998, we measured forest vegetation structure and composition on one hundred and thirty-six 20-m × 20-m plots to assess both the immediate stand and landscape level effects of the spruce beetle infestation. A photo-interpreted vegetation and infestation map was produced using color-infrared aerial photography at a scale of 1:40,000. We used linear regression to quantify the effects of the outbreak on forest structure and composition. White spruce (Picea glauca) canopy cover and basal area of medium-to-large trees [≥15 cm diameter-at-breast height (1.3 m, dbh)] were reduced linearly as the number of trees attacked by spruce beetles increased. Black spruce (Picea mariana) and small diameter white spruce (<15 cm dbh) were infrequently attacked and killed by spruce beetles. This selective attack of mature white spruce reduced structural complexity of stands to earlier stages of succession and caused mixed tree species stands to lose their white spruce and become more homogeneous in overstory composition. Using the resulting regressions, we developed a transition matrix to describe changes in vegetation types under varying levels of spruce beetle infestations, and applied the model to the vegetation map. Prior to the outbreak, our study area was composed primarily of stands of mixed white and black spruce (29% of area) and pure white spruce (25%). However, the selective attack on white spruce caused many of these stands to transition to black spruce dominated stands (73% increase in area) or shrublands (26% increase in area). The post-infestation landscape was thereby composed of more even distributions of shrubland and white, black, and mixed spruce communities (17–22% of study area). Changes in the cover and composition of understory vegetation were less evident in this study. However, stands with the highest mortality due to spruce beetles had the lowest densities of white spruce seedlings suggesting a longer forest regeneration time without an increase in seedling germination, growth, or survival.     

The effects of wildfires on wood-eating beetles in deciduous forests on the southern slope of the Swiss Alps

The effect of fires on Cerambycidae, Buprestidae and Lucanidae were studied at 23 sites within a chestnut forest in southern Switzerland. We compared six unburnt sites, two freshly burnt sites, eight sites which burned once at different times in the last 30 years, and seven sites where fires occurred repeatedly in the last 30 years. The diversity and the species composition of the three xylobiont families were related to various ecological variables at two levels of spatial scale, a small scale of 0.25 ha and a large scale of 6.25 ha. These variables were: fire frequency, time since the last fire, clear cutting after the fire, forest structure, amount of dead wood, and habitat mosaic. The fire does not have a direct effect on the xylobiont beetles community at small scale; however, fire has an indirect effect by maintaining a relatively open forest structure. The mosaic of forest areas burnt with different frequencies and at different times was an important factor influencing species richness and species composition at the large spatial scale.Data presented here supports the strategy to conserve the diversity and includes species composition of xylobiont fauna in deciduous forests: (i) at small spatial scale, to maintain highly structured and relatively open stands with large amounts of dead wood and big oak trees; (ii) at large spatial scale, to favour a mosaic of different forest habitats and successional stages. A forest offering a good structural diversity is important for maintaining landscape complexity and thus a high species richness of xylophagous beetles.     

Effects of post-fire salvage logging on boreal mixed-wood ground beetle assemblages (Coleoptera,Carabidae)

The frequency and intensity of salvage logging has recently increased in burned forests of the Canadian boreal so that post-fire areas make up a significant annual share of all harvested forest land in some years. However, little is known about how this practice affects re-establishment of animal and plant communities that already have been strongly altered by the fire. We pitfall-trapped carabid beetles (Coleoptera, Carabidae) for two consecutive years following a large-scale boreal wildfire (276,000 ha) in burned mixed-wood forests of 12 landscape units (2.5 km × 2.5 km each). These units included four salvage treatments, each replicated three times: control (no salvaging), low (ca. 20–30% of merchantable mixed-wood removed), moderate (40–50%) and high salvage intensity (60–70%). We established 16 sampling sites in each unit: on the control landscapes, all 16 sites were in un-logged stands; in low-salvage units, 4 out of 16 sites were in salvaged areas; in moderate- and high-salvage units, 8 and 12 out of 16 sites, respectively, were in salvaged areas. Salvage logging positively affected carabid species richness. However, there was an overall salvage-caused decrease in the abundance of many common forest-dwelling carabids, and an increase in the abundance of disturbance or open-habitat specialists. Interestingly, the effects of salvage logging on the total catch and several abundant forest species (e.g., Calosoma frigidum, Harpalus laevipes, Pterostichus punctatissimus, P. adstrictus and Platynus decentis) appeared to be more important at the landscape level (the four landscape treatments) than at the level of sampling sites (site logged versus not). We suggest that this observation resulted from ambiguous site-level responses to salvage that collectively contributed to the (mostly negative) significant responses at the landscape level. Effects of fire severity (estimates based on tree survival) on carabids were species-specific; however, the impact of this measure was most often significant at the site, and not at the landscape, level. A Multivariate Regression Tree revealed that fire severity and the overall (pre-fire) amount of mixed-wood forest on the landscape were significant determinants of assemblage structure, with the local effects of fire severity being strongest.     

Quantifying economic losses from wildfires in black pine afforestations of northern Spain

We implemented a fire risk assessment framework that combines spatially-explicit burn probabilities, post-fire mortality models and public auction timber prices, to estimate expected economic losses from wildfires in 155 black pine stands covering about 450 ha in the Juslapeña Valley of central Navarra, northern Spain. A logit fire occurrence model was generated from observed historic fires to provide required fire ignition input data. Wildfire likelihood and intensity were estimated by modeling 50,000 fires with the minimum travel time algorithm (MTT) at 30 m resolution under 97^th percentile fire weather conditions. Post-fire tree mortality due to burning fire intensity at different successional stages ranged from 0.67% in the latest stages to 9.22% in the earliest. Stands showed a wide range of potential economic losses, and intermediate successional stage stands presented the highest values, with about 124 € ha^− 1 on average. A fire risk map of the target areas was provided for forest management and risk mitigation purposes at the individual stand level. The approach proposed in this work has a wide potential for decision support, policy making and risk mitigation in southern European commercial conifer forests where large wildfires are the main natural hazard.     

Distribution of saproxylic beetles in a recently burnt landscape of the northern boreal forest of Québec

This study evaluated the importance of burned habitat characteristics as well as the likely dispersal from specific habitats in the distribution of saproxylic beetles the same year as a fire occurred, in burned black spruce stands (Picea mariana [Mill] B.S.P.) in the northern boreal forest of Québec. The distribution of early post-fire saproxylic species was mainly driven by burned habitat attributes at the plot scale (0.04 ha), especially fire severity, suggesting that the effect of environment attributes can act at a relatively fine scale. Some xylophagous and most predaceous species were more abundant in severely burned stands whereas fire severity had the opposite effect on several common mycophagous species. The amount of newly fire-killed trees that could be used as breeding substrates in the burned stands had only a weak positive influence on these functional groups. The great majority of early saproxylic species were weakly associated with the distance from unburned forests or other recently burned patches that could act as potential “source habitats”. Indeed, these variables were of lesser importance than the attributes of the burned habitat. Woody debris that were already present in plots before the fire, potentially serving as local of source-populations for early colonizers, had virtually no influence on the local abundance of species. Many saproxylic species, including some true pyrophilous, clearly showed higher abundance as distance from unburned stands increased. This unexpected relation may reflect that dispersal of insects toward the burnt landscape very shortly after fire could be driven by the higher amount of volatiles released by severely burned forests, which are more likely as distance from unburned forest increased.     

The effects of partial cutting on stand structure and growth of western hemlock–Sitka spruce stands in southeast Alaska

The effects of partial cutting on species composition, new and residual-tree cohorts, tree size distribution, and tree growth was evaluated on 73 plots in 18 stands throughout southeast Alaska. These partially cut stands were harvested 12–96 years ago, when 16–96% of the former stand basal area was removed.Partial cutting maintained stand structures similar to uncut old-growth stands, and the cutting had no significant effects on tree species composition. The establishment of new-tree cohorts was positively related to the proportion of basal-area cut. The current stand basal area, tree species composition, and stand growth were significantly related to trees left after harvest (p<0.001). Trees that were 20–80 cm dbh at the time of cutting had the greatest tree-diameter and basal-area growth and contributed the most to stand growth. Diameter growth of Sitka spruce and western hemlock was similar, and the proportion of stand basal-area growth between species was consistent for different cutting intensities.Concerns about changing tree species composition, lack of spruce regeneration, and greatly reduced stand growth and vigor with partial cuts were largely unsubstantiated. Silvicultural systems based on partial cutting can provide rapidly growing trees for timber production while maintaining complex stand structures with mixtures of spruce and hemlock trees similar to old-growth stands.     

Plant species diversity on logged versus burned sites in central Alaska

Natural fires and logging are two of the main disturbances affecting upland boreal forest in Alaska. The objectives of this study were to determine whether logged sites differ from burned sites in (1) overall plant species richness, (2) successional trajectories, and (3) species diversity at particular stand structural development stages. We compared plant species diversity on sites burned in natural fires to sites that were logged and not subsequently burned in central Alaska. We sampled 12 logged and 12 burned former upland white spruce (Picea glauca (Moench) Voss) forests in four stand development stages representing stand initiation (stage A), early stem exclusion (stage B), understory reinitiation (stage C), and mature hardwood (stage D) stages. In this study the dates of disturbance varied from 1990 to 1994 in stage A, 1978 to 1983 in stage B, 1957 to 1965 in stage C, and 1900 to 1920 in stage D plots. All sites were similar in slope, aspect, and soil type. Vascular plants were identified to the species level (except for certain willows) and bryophytes and lichens were identified to the level of presumptive (usually unknown) species within family groups. Organic layer thickness was significantly greater on logged sites compared to burned sites overall and at each stage. Burned sites (all stages combined) supported more species (146) than logged sites (111), and more species at each stand development stage. Burned plots in stages A and B supported abundant cover of a few apparent fire specialist species (Ceratodon purpureus (Hedw.) Brid., Marchantia polymorpha L. and Leptobryum pyriforme (Hedw.) Wils.) that were present in only minor amounts on logged sites. Burned plots exhibited higher species turnover from stage to stage and among all stages than logged plots. Species dominant in burned stage A plots were nearly absent in burned stage C and D plots, while logged stage A dominants, which were common mature forest species, increased in each subsequent stage. We compared floristic similarity between our disturbance plots and mature upland white spruce stands in Bonanza Creek Long-Term Ecological Research (LTER) site. Only five species found in the LTER dataset were not also present in this study, which suggests that nearly all species compositional change in our study area occurs during the first century after disturbance. Logged sites appear to begin and continue succession with a greater share of the original mature forest understory plants, while burned sites initiate succession with more distinctive and specialized plant species.     

Effects of wood-ash on the tree growth,vegetation and substrate quality of a drained mire: a case study

The effects of wood-ash fertilisation on tree stands, soil characteristics and ground vegetation were studied on a drained pine mire in Finland (64°51′N, 26°04′E, 62 m a.s.l.). The original site type was a treeless, mesotrophic Sphagnum papillosum fen. The site was drained in 1933 and the wood-ash fertilisation experiment was started in 1947. The treatments were: (i) unfertilised, (ii) wood-ash 8 t ha⁻¹, and (iii) wood-ash 16 t ha⁻¹.Drainage and ash application had radical and long-lasting consequences on the biological activity on the site and the vegetation compartments studied. The understorey vegetation had been profoundly affected by the ash with almost complete transformation of the species and other life forms. Even 50 years after the ash treatment the changes in vegetation/site type and the tree stand were clearly visible. On the unfertilised plot, the biomass of ground vegetation consisted mostly of mosses and dwarf shrubs, but on the ash-treated plots it consisted mostly of herbs and grasses typical of upland forests.Ash treatment had promoted stem volume growth of Scots pine (Pinus sylvestris L.) substantially and for a long time. The total wood production on the ash plots during 1947–1994 was 13 and 17 times over that of the control plot. Unfertilised pine trees suffered from P and K deficiency throughout the study period. The concentrations of some plant nutrients (P, K) decreased during the past years on Ash8. No nutrient shortage afflicting the tree stand was observed on Ash16 during the study period.Ash application has also led to increased concentration of nutrients in the peat. A sizeable proportion of the mineral nutrients applied were still in the 0–20 cm peat layer. On the ash-treated plots the amount of soil nitrogen (0–20 cm) was 18 and 29 times and the amount of soil phosphorus 9 and 13 times over the amount bound by the tree stand and the ground vegetation (Ash8 and Ash16, respectively). The stock of potassium was generally small in the surface peat—only 60–90% of the amount of potassium bound in the tree stand and the ground vegetation.It was concluded that wood-ash had powerfully influenced the biological processes in surface peat. The decomposition of cellulose was significantly accelerated by both ash treatments. Ash fertilisation also increased the emissions of CO₂. The intensified decomposition rate in the litter, vegetation and peat explained to a large extent the accelerated growth of the Scots pine stands studied.     

Fire history of white and Lutz spruce forests on the Kenai Peninsula,Alaska, over the last two millennia as determined from soil charcoal

The presence of over 429,000 ha of forest with spruce (Picea spp.) recently killed by spruce beetles (Dendroctonus rufipennis) on the Kenai Peninsula has raised the specter of catastrophic wildfire. Dendrochronological evidence indicated that spruce beetle outbreaks occurred on average every 50 years in these forests. We used 121 radiocarbon-dated soil charcoal samples collected from throw mounds of recently blown over trees to reconstruct the regional fire history for the last ca. 2500 years and found no relation between fire activity and past spruce beetle outbreaks. Soil charcoal data suggest that upland forests of white (Picea glauca) and Lutz (Picea x lutzii) spruce have not on average burned for 600 years (time-since-fire range 90 to ∼1500 years, at 22 sites) and that the mean fire interval was 400–600 years. It would thus appear that 10 or more spruce beetle outbreaks can occur for every cycle of fire in these forests. We caution, however, that a trend of warmer summers coupled with an increasing human population and associated sources of ignitions may create a greater fire risk in all fuel types than was present during the time period covered by our study. We suggest that forest management focus on creating fuel breaks between valued human infrastructure and all types of forest fuels, both green and dead.     

Factors affecting stand structure in forests – are there climatic and topographic determinants?

Multi-aged stands are not a common structural type of mountain-ash forest in the Central Highlands of Victoria, southeastern Australia, but they are nevertheless important, particularly as habitat for wildlife. Extensive field data and information generated from spatial models of climate, topography and radiation regimes were examined to identify factors which related to the occurrence of stands of multi-aged mountain-ash forest. The probability of occurrence of multi-aged stands increased significantly (p < 0.001) with the age of the forest. There also was evidence that multi-aged stands were more likely to occur on steeper slopes (p = 0.01). When actual on-ground field measurements were ignored and program-generated climate, topography and radiation data only were modeled, a decrease in the shortwave radiation ratio (a measure of the estimated solar radiation budget) was associated with a significantly increased probability of occurrence (p = 0.03) of multi-aged stands. Our analyses indicated there are particular parts of mountain-ash forest landscapes where complex multi-aged stand structures are more likely to develop. This has implications for the methods used to harvest mountain-ash forests for timber and pulpwood, particularly the need for increased retention of structural components of stands targeted for logging.     

Herpetofaunal species richness responses to forest landscape structure in Arkansas

Species accumulation curves were used to study relationships between herpetofaunal richness and habitat characteristics on four watersheds in Arkansas that differed markedly with respect to management intensity. Selected habitat characteristics were estimated for stands containing the sample points and within buffers with radii of 250, 500 m, and 1 km surrounding the sample points. Richness of all three herpetofaunal groups (amphibians, reptiles, and all herpetofauna) was greater in hardwood forests than in pine or mixed pine–hardwood. For all three groups, the youngest forest age class had the most species. For amphibians and total herpetofauna, richness declined as stand ages increased. Reptiles had the highest richness at sample points with the lowest class of stand basal area (BA), whereas amphibians were richest at points having the highest BA. In contrast to expectations, there was no effect of distance from water on richness of any taxa; however, we may have had incomplete data on the spatial distribution of small ponds outside the sample plots because they were not recorded in the GIS data. Results for distance to roads were ambiguous, but with more separation of compared curves as more plots were sampled, a positive influence of road proximity was suggested. There was a positive effect of buffer age diversity at the 250 m scale for all three herpetofaunal groups, but less so at scales >250 m except for amphibians, which also showed a positive effect at the 1 km scale. The two most intensively managed watersheds had higher species richness than the two less intensively managed watersheds for reptiles, amphibians, and both groups combined. In this study landscape, where urban and agricultural influences were minimal, we did not observe negative impacts of forest management and associated activities, and local habitat heterogeneity created by silviculture often had a positive effect on herpetofaunal species richness.     

Productivity,nutrient cycling,and succession in single- and mixed-species plantations of Casuarina equisetifolia,Eucalyptus robusta,and Leucaena leucocephala in Puerto Rico

Tree growth, biomass productivity, litterfall mass and nutrient content, changes in soil chemical properties and understory forest succession were evaluated over a 8.5-year period in single- and mixed-species (50 : 50) plantations of two N₂-fixing species, Casuarina equisetifolia and Leucaena leucocephala, and a non-fixing species, Eucalyptus robusta. At the optimal harvest age for maximum biomass production (4 years), total aboveground biomass ranged from 63 Mg ha⁻¹ in the Eucalyptus monoculture to 124 Mg ha⁻¹ in the Casuarina/Leucaena mixture, and was generally greater in the mixed-species than in single-species treatments due to increased productivity of the N-fixing species in the mixed stands. Total litterfall varied from 5.3 to 10.0 Mg ha⁻¹ year⁻¹ among treatments, or between 5.9% and 13.2% of net primary production. Litterfall production and rates of nutrient return for N, P, K, Ca and Mg were generally highest for Leucaena, intermediate for Casuarina and lowest for Eucalyptus. These rates were usually higher in the mixed-species than in monospecific stands due to differences in biomass productivity, but varied considerably depending on their species composition. Total system carbon and nutrient pools (in biomass plus soils to 40-cm depth) for N, P, K, Ca, Mg, Mn at four years were consistently greater in the plantation treatments than in the unplanted control plots. Relative to the single-species plantations, these system pools were generally larger in the mixed-species plantations for C (−10% to +10%), N (+17% to +50%), P (−1% to +63%), K (−19% to +46%), Ca (−10% to +48%), Mg (+5% to +57%) and Mn (+19% to +86%). Whole-tree harvests at four years would result in substantial system carbon and nutrient losses, although these estimated losses would not exceed the estimated gains realized during the four-year period of tree growth at this site. At 7.5 years, soil organic matter and effective cation exchange capacity were reduced in all plantation treatments relative to the control. Changes in soil nutrient content from 0 to 7.5 years were highly variable and not significantly different among treatments, although stands containing Leucaena generally showed higher rates of nitrogen and phosphorus accretion in soils than those with Eucalyptus and/or Casuarina. Natural regeneration of secondary forest tree and shrub species increased over time in all plantation treatments. A total of 24 native or naturalized forest species were recorded in the plantations at 8.5 years. Woody species abundance at this age was significantly greater beneath Casuarina than either Eucalyptus or the Eucalyptus/Leucaena mixed stands. Species richness and diversity, however, were greatest beneath stands containing Eucalyptus and/or Leucaena than in stands with Casuarina.     

Initial impact of supervised logging and pre-logging climber cutting compared with conventional logging in a dipterocarp rainforest in Sabah,Malaysia

Pre-marked skid trails, directional felling and climber cutting when logging in tropical rainforests may be important ways of reducing damage to the forest, thus creating a healthier stand and improving future yields.This study, carried out in a virgin dipterocarp rainforest in the south of Sabah, Malaysia, compared two types of logging (both with and without pre-cutting climbers): conventional selective logging (CL) and supervised logging (SL). The latter is a selective logging system in which both pre-marked skid trails and directional felling were implemented. The pre-marked skid trails were aligned parallel to each other, spaced 62 m apart. A randomised complete block 2 × 2 factorial design was used in the experiment, consisting of 16 gross treatment plots, each of 5.76 ha with a 1 ha net plot in the centre.Fewer trees tended (0.050 < P ≤ 0.100) to be logged in SL plots than in CL plots (on average 9.4 and 13.0 trees ≥60 cm diameter breast height ha⁻¹). Pre-felling of climbers resulted in four more dipterocarp trees being logged ha⁻¹, compared with no climber cutting: a statistically significant difference (P ≤ 0.050). The basal areas lost of both large trees (≥ 60 cm dbh) and small dipterocarp trees (10–29 cm dbh) tended to differ between the logging systems, with CL leading to greater losses.There were significant differences in the residual stands left by the logging systems, with respect to the number of dipterocarps and their basal area in the diameter class 10–29 cm; ca 30% more stems being found after SL. No significant differences (or tendencies) in these variables were found in the residual stands in other diameter classes, or when trees of all species were considered.     

Nitrogen budgets for Scots pine and Norway spruce ecosystems 12 and 7 years after the end of long-term fertilisation

The magnitude of nitrogen storage and its temporal change in forest ecosystems are important when analysing global change. For example, the accelerated growth of European forests has been linked to increased nitrogen deposition, but the changes in the N inputs that cause long-term changes in ecosystems have not yet been identified. We used two Swedish forest optimum nutrition experiments with Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) to study the long-term fate of N applied to these forest ecosystems. In the pine experiment, in addition to fertiliser (NPK) application, soil acidity was manipulated by application of lime and dilute sulphuric acid. From the spruce experiment, we selected treatments with similar fertiliser doses as in the pine experiment and with and without lime addition.We quantified various terms in the N budget 12 years (pine) and 7 years (spruce) after the last N addition. In the pine stand the NPK-treatment was the only treatment to produce a significant increase in N in the tree biomass (97% above control), whereas in the spruce stand the N additions increased tree N in all treatment combinations (207% above control). In the pine stand the relative distribution of nitrogen between trees and soil did not vary across treatments, with trees containing around 12% of ecosystem N and humus containing around 44% of soil N. The increases in N stocks in the pine stands were mainly in the soil. In contrast, in the spruce ecosystem trees accumulated most of the added N and the increase in the soil was restricted to the humus layer.In the pine ecosystem, large losses of added N (between 254 and 738 kg ha⁻¹ out of 1040 kg ha⁻¹ added as fertiliser) occurred, whereas in the spruce ecosystem we recovered more N than could be accounted for by inputs (between 250 and 591 kg ha⁻¹). There was no clear pattern in the interaction between acidification/liming and N additions.     

Tropical forest burning in Brazilian Amazonia: measurement of biomass loading,burning efficiency and charcoal formation at Altamira,Pará

Mass transformations were estimated in burns in the clearings of three colonist lots near Altamira, Pará, Brazil. In each lot, two groupings of six 60-m² plots were established in sites where the forest had been recently felled; plots were arranged as rays in a star-shaped pattern, with pre- and post-burn measurements made in alternate rays. Pre- and post-burn above-ground biomass was estimated by cutting and weighing the felled vegetation in 15 pre-burn and 18 post-burn plots (three pre-burn plots could not be weighed before one of the colonists burned the clearing) and by line intersect sampling (LIS) done along the axis of each of the 36 plots. Because of the high variability of the initial biomass present in the plots, volume data from LIS were more reliable for assessing change in the biomass of material over 10 cm in diameter (because this technique permits measuring the same trees before, and after, burning); other quantities relied on data from direct weighing. The best estimate of the mean pre-burn above-ground biomass at the site is 263 metric tons per hectare (t ha⁻¹); considering available measurements of the proportion of below-ground biomass elsewhere in Amazonia, the total dry weight biomass at the Altamira site corresponds to ≈322 t ha⁻¹. Assuming 50% carbon (C) content for biomass, the above-ground biomass at Altamira represents a carbon stock of 130 t ha⁻¹. Assuming a carbon content of 75% for charcoal, 1.3% of the pre-burn above-ground carbon stock was converted to charcoal, substantially less than is generally assumed in global carbon models.Measurements at Altamira imply a 42% reduction of above-ground carbon pools if calculated along with the scattered trees that farmers leave standing in their clearings, or 43% if these trees are excluded from the analysis. These values are substantially higher than the 27.6% measured in an earlier study near Manaus. However, most of the difference between results at the two sites is explained by differences in the distribution of initial biomass among the fractions, especially greater quantities of vines and of litter (including dead wood <5 cm in diameter) than at Manaus. Smaller diameter pieces burn more thoroughly than larger ones. At Altamira, the large percentage of above-ground carbon in vines (12.0%) is less typical of Amazonian forests than the lower percentage at Manaus (3.1%). The lower overall burning efficiency found at Manaus is, therefore, believed to be more typical of Amazonian burning. High variability indicates a need for further studies in many localities, and for perfecting less laborious indirect methods. Both a high biomass and low percentage of charcoal formation suggest significant potential contribution of forest burning to global climate changes from CO₂ and trace gases.     

Biomass and nutrient accumulation in pure and mixed plantations of indigenous tree species grown on poor soils in the humid tropics of Costa Rica

Aboveground biomass and nutrients and soil chemical characteristics were examined in young plantations of four indigenous tree species: Hieronyma alchorneoides, Vochysia ferruginea, Pithecellobium elegans, and Genipa americana, growing in mixed and pure stands at La Selva Biological Station, Costa Rica. Total tree biomass production rates ranged from about 5.2 Mg ha⁻¹ year⁻¹ for G. americana to 10.3 Mg ha⁻¹ year⁻¹ for H. alchorneoides pure stands, and for the species mixture it was about 8.9 Mg ha⁻¹ year⁻¹. Branches and foliage formed 25–35% of total tree biomass but they represented about 50% of total tree nutrients. H. alchorneoides, the four species mixture, and P. elegans had the greatest accumulations of total aboveground nutrients per hectare. The importance of the plantation floor as a nutrient compartment varied temporally. When forest floor litter biomass was at its peak, plantation floor litter N, Ca, and Mg were roughly equal to, or greater than stem nutrients for all species except for P. elegans. For P. elegans, the plantation floor consistently represented a very low proportion of total aboveground nutrients. G. americana and V. ferruginea trees showed 55–60% less biomass accumulation in mixed than in pure stands while H. alchorneoides and P. elegans trees grew 40–50% more rapidly in mixture. P. elegans foliage had 60% lower Ca but higher P concentrations in mixed than in pure stands, and G. americana had higher foliar Mg in mixed than in pure stands. V. ferruginea stands had the highest concentrations of soil Ca, Mg, and organic matter, particularly in the top layers. Relative to pure plantations, soil nutrient concentrations in mixed plantations were intermediate for N, P, and K, but lower for Ca and Mg. The results of this study can be used in the selection of tree species and harvest designs to favor productivity and nutrient conservation.     

Fuels and fire behavior in chipped and unchipped plots: Implications for land management near the wildland/urban interface

Fire behavior was measured and modeled from eight 1 ha experimental plots located in the Francis Marion National Forest, South Carolina, during prescribed burns on February 12 and February 20, 2003. Four of the plots had been subjected to mechanical chipping during 2002 to remove woody understory growth and to reduce large downed woody debris from the aftermath of Hurricane Hugo in 1989. The remaining four (control) plots were left untreated. The burns were low intensity (mean flame length = 36.2 cm) and slow moving (mean spread rate = 1.18 m min⁻¹). Neither flame length nor rate of spread differed significantly between treatments (ANOVA F's < 0.5, P > 0.7, d.f. = 1,4). Post-burn observations provided somewhat more convincing evidence of treatment effects on fire behavior. According to transect data, only slightly more than half the area in the chip plots burned as compared to upwards of 80% in the burn-only plots. BehavePlus and Hough–Albini (HA) fire models correctly predicted the low intensity, slow moving fires given the observed wind and fuel moisture conditions. Accuracy of BehavePlus predictions depended on the value for fuel height entered in the model. Use of mean fuel height for the fuel depth parameter, as is typically recommended, somewhat overestimated fire hazard in the burn-only plots. However, limiting fuel height to the observed litter depth resulted in roughly accurate predictions. HA predictions for untreated fuels were close to correct even without adjusting fuel depth. When provided with two “high-risk” fuel and fire weather scenarios both models predicted more extreme fire behavior in the untreated fuels. In contrast, chipping appeared to protect against dangerous wildfires as long as fuel heights remained low. Smoke monitoring data from a companion study carried out in the same plots indicated a 60% reduction in smoke particulate production from chipped areas, roughly consistent with predictions of the fire effects model FOFEM. Mechanical chipping is apparently a useful method for limiting fire-hazard and smoke production in long-unburned fuels. However, questions remain concerning the long-term fate of heavy chip fuels and resultant effects on fire and smoke during severe drought.     

Silvicultural measures to increase the mechanical stability of pure secondary Norway spruce stands before conversion

This study evaluates the effect of two different thinning treatments (2 and 3), and a control without thinning (1), on stand stability of secondary even-aged Norway spruce stands, in relation to the main risk factors of snow and wind, which should be considered in the period of stand conversion. Treatment 2 is a heavy thinning at top heights of 10, 12.5 and 15 m; treatment 3 starts with the first heavy thinning at the top height of 10 m, but the second and third treatments are delayed till a top height of 20 and 22.5 m are reached. The experimental stands are in secondary Norway spruce forest growing on a site considered unsuitable for that species and especially at risk from snow and storm damage. The investigated thinning variants significantly influenced the stability of the experimental stands. Both thinning treatments encouraged diameter increment and therefore their h/d ratio reached lower levels than the control. In treatment 2, the h/d ratio stabilized in the period of intensive treatment at around 80; i.e., it is the most suitable treatment from the viewpoint of stem-break resistance. Treatment 3 did not stop h/d ratio increase, but slowed it compared to the control variant without thinning. Subsequently the later interventions at the top heights of 20 m and, especially, 22.5 m stopped the increase of the h/d ratio and kept it under the critical value of 90.     

Effect of burnt wood removal on the natural regeneration of Pinus halepensis after fire in a pine forest in Tus valley (SE Spain)

To determine the effect of burnt tree removal on post-fire natural regeneration of Pinus halepensis, two 2 500 m² areas were selected six months after the fire in a totally destroyed mature (>70 years) pine forest. In one area, all the trees were cut down and removed 10 months after the fire and, in the other, all the trees were left standing (control). In each area, 20 permanent plots of 20 m² each were randomly placed, and all seedlings emerging within them labeled by individual numbered plastic tags. Emergence, mortality, density and growth (height) of 6649 P. halepensis seedlings were monitored during the first four post-fire years. Seedling emergence was concentrated in the first post-fire autumn–winter period. No positive effect on seedling emergence was detected as a consequence of burnt trunk dragging and subsequent turning over of soil. Wood removal produced an immediate average seedling mortality of 33%, and notably increased seedling mortality during the subsequent summer, probably due to increased exposure of seedlings to sunlight and the possible debilitation of many individuals by mechanical contact during burnt wood removal. A negative correlation of pine seedling mortality with height was detected, which increased significantly on wood removal in the third post-fire year. That is, short seedlings (<10 cm) in treated plots were the most likely to die during this period. In spite of the detrimental effect of wood removal on sapling survival, seedling density four years after fire in the cleared area was large (3.3 seedlings/m²). Wood removal treatment reduced seedling growth: seedling height was significantly higher in the control stand, and differences in seedling height growth rate became particularly noticeable in the fourth post-fire year. The results denote that traditional wood removal practices do not threaten natural post-fire P. halepensis re-establishment if initial seedling density is large enough. However, further studies focused on wood removal effects on the final tree development level and other ecological aspects are necessary to choose adequate post-fire forest management.