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.     

White spruce regeneration following a major spruce beetle outbreak in forests on the Kenai Peninsula, Alaska

Between 1987 and 2000, a spruce beetle (Dendroctonus rufipennis) outbreak infested 1.19 million ha of spruce (Picea spp.) forests in Alaska, killing most of the large diameter trees. We evaluated whether these forests would recover to their pre-outbreak density, and determined the site conditions on which spruce germinated and survived following the spruce beetle outbreak in forests of the Anchor River watershed, Kenai Peninsula, Alaska. White spruce (Picea glauca) and Lutz's spruce (Picea × lutzii), a hybrid between white and Sitka spruce (Picea sitchensis), dominate the study area. We measured the pre- and post-outbreak density of spruce in 108 3 m × 80 m plots across the study area by recording all live trees and all dead trees >1.5 m tall in each plot. To determine the fine scale site conditions on which spruce germinated and survived, we measured ground surface and substrate characteristics within 20 cm circular plots around a subset of post-outbreak spruce seedlings. The density of post-outbreak spruce (855/ha) was adequate to restock the stands to their pre-outbreak densities (643/ha) for trees >1.5 m tall. We could not accurately estimate recovery for pre-outbreak spruce seedlings because dead seedlings may have decayed in the 5–18 years since the beetle outbreak occurred. At the fine scale, spruce that germinated post-outbreak grew on a wide variety of substrates including downed log, stump, mesic organic mat, peat, hummocks and mineral soil. They exhibited a strong preference for downed logs (53%) and stumps (4%), and most (91%) of the downed logs and stumps that spruce rooted on were heavily decayed. This preference for heavily decayed logs and stumps was especially evident given that their combined mean cover was only 2% in the 3 m × 80 m plots. Within the 3 m × 80 m plots, spruce seedling survival was negatively correlated with bluejoint (Calamagrostis canadensis) litter cover.     

Population structure of Carapa guianensis in two forest types in the southwestern Brazilian Amazon

Carapa guianensis Aublet. is a tropical tree with strong multiple-use characteristics, and is valued for both the high quality oil extracted from its seeds and as a timber resource. This study compares the population structure of this economically important rainforest tree in two contrasting forest types: occasionally inundated and terra firme forests. Main study objectives were (a) to assess the density, spatial distribution, and size class structure of C. guianensis in these two forest types and (b) to use patterns of abundance, distribution and demographic structure to help infer key demographic stages or ecological variables that merit special focus for management. Four 400 m × 400 m plots, two in each forest type, were established to determine distribution and density patterns of C. guianensis ≥10 cm diameter at breast height (dbh) at the landscape level, and 32 10 m × 10 m subplots were randomly nested within each of the larger plots to measure individuals <10 cm dbh. Larger individuals (≥10 cm dbh) were found at higher densities in occasionally inundated forest than in terra firme forest: 25.7 trees ha⁻¹ and 14.6 trees ha⁻¹, respectively. Mean density of C. guianensis individuals <10 cm dbh was also higher in occasionally inundated forests, but variation of regeneration density among the subplots was high. Spatial distribution methods revealed a tendency toward clumping in both forest types, and both had similar size class structures, suggesting that both environments are suitable for C. guianensis. This new finding illustrates the potential for C. guianensis management in terra firme forests. High densities and clumped distributions in both forest types are also indices favorable for sustainable species management. Finally, several ecological variables (tree density and reproductive potential) were sufficiently different between terra firme and occasionally inundated forests to recommend stratification by forest type when conducting further studies on key ecological and management variables of C. guianensis.     

Vegetation structure, floristic composition and growth characteristics of Aucoumea klaineana Pierre stands as influenced by stand age and thinning

The coastal forest of Gabon abounds in monospecific secondary Aucoumea klaineana stands derived from natural regeneration after shifting cultivation. This paper aims to describe the changes in the structure and dynamics of these stands with age. It then assesses the impact of selective thinning in the upper storey on both structural and dynamic parameters.

The experiment consisted of 34 Permanent Plots in stands from establishment to more mature stages (ca. 50 years old). Thirteen plots (17–45 years old) were thinned. More than 80% of the removal came from supernumerary dominant A. klaineana.

A. klaineana represented 60% of the total density in stands ca. 15 years old but >90% of the dominant trees in older stands. The changes with age in the floristic composition of the unthinned stands showed three successional stages during which pioneer species associated with A. klaineana (from establishment to ca. 15 years) were progressively replaced by mature forest species.

Basal area increased and density decreased with age before reaching stable values at ca. 40–45 years. Mortality was very high in young stands but decreased in the older ones. Mortality generally affected small diameter individuals in the dominated storey. Diameter and basal area increments showed that the stand growth resulted from the growth of dominant A. klaineana. Diameter increments of A. klaineana were elevated during the first years of colonisation (1.9 cm/year) and were still ca. 0.7 cm/year for 50-year-old dominant trees.

Thinning did not increase the mortality of the dominant population. It favoured the individual growth of A. klaineana. The gain was substantial for dominated trees and small dominant trees (from 60 to 100%) but was lower for large dominant trees (ca. 25–30%). Therefore, stimulation of individual growth did not compensate for the loss of basal area at the stand level.     

Effects of irrigation and coppicing on above-ground growth, physiology, and fine-root dynamics of two field-grown hybrid poplar clones

To determine the effects of irrigation and coppicing on above- and below-ground growth dynamics, a plantation of Populus × euramericana cv. ‘Eugenei’ and Populus tristis × Populus balsamifera cv. ‘Tristis 1’ was established in May 1984 on a level site in East Lansing, MI, USA. Supplemental water in the form of drip irrigation was applied to half the trees beginning the first growing season. All trees were cut down in March 1988 and the stumps allowed to coppice. Pre- and post-coppice height and diameter growth of ‘Eugenei’ always exceeded that of ‘Tristis’, and the former clone showed a positive response to irrigation, whereas the latter did not. The greater growth of ‘Eugenei’ was primarily due to its full use of the growing season. Post-coppice rates of photosynthesis were not affected by irrigation in either clone, but stomatal conductances were reduced in non-irrigated trees. Analysis of microvideo images taken in minirhizotron tubes buried in the soil close to the trees showed that ‘Tristis’ produced a greater length and number of fine roots in the top 30 cm of soil than ‘Eugenei’, regardless of treatment. Irrigated trees consistently produced more fine roots at 0–30 cm soil depth than non-irrigated trees only in ‘Eugenei’, but both clones showed greater fine-root production in non-irrigated trees at 30–100 cm. Both clones also showed substantial fine-root production in the spring immediately following coppicing, with no evidence of a shock-induced dieback of roots. The root systems of these two poplar genotypes apparently contain sufficient carbon and nitrogen reserves to fuel a spring flush of fine-root growth, even though the tops have been severed during the dormant season.     

Effects of white-tailed deer on vegetation structure and woody seedling composition in three forest types on the Piedmont Plateau

White-tailed deer (Ododcoileus virginiana) can substantially affect the structure and species composition of a forest. The tolerance of a forest community to browsing may vary by type as a result of varying biotic and abiotic factors of the environment. To date, no studies have compared the effects of browsing among forest communities within a physiographic region. We investigated the effects of browsing on vegetation structure and woody seedling composition in three forest types (oak–hickory, Virginia pine–eastern red cedar, bottomland hardwood) in Manassas National Battlefield Park (MNBP), Virginia, USA. We compared forb cover, vertical plant cover (0–1.5 m tall), and survival of tagged seedlings in 10 exclosed (2 m × 6 m) and 10 unexclosed plots in each forest type during a 5-year period. No differential effects of browsing were found among forest types. In all forest types, deer (67 deer/km²) suppressed forb and vertical plant cover to levels less than would be expected in the absence of deer. Seedling survival rates of most species were significantly reduced by browsing. By the 4th year of the study, box elder (Acer negundo), hickory (Carya spp.), and red maple (Acer rubrum) had been eliminated from unexclosed plots, and red and white oaks (Quercus spp.) dramatically reduced. Ash (Fraxinus spp.), black cherry (Prunus serotina), and hackberry (Celtis occidentalis), although significantly impacted, remained the most abundant species throughout the study. These findings suggest that white-tailed deer may be modifying the structure of the forest interior to the extent that it adversely affects wildlife species dependent on a dense understory to thrive. We predict that the future composition of forests in MNBP will shift towards stands with fewer species and a greater dominance of ash, black cherry, and hackberry, particularly in the oak–hickory and bottomland hardwood forests, where the majority of current dominants are most affected.     

Burning of Amazonian forest in Ariquemes, Rondônia, Brazil: biomass, charcoal formation and burning efficiency

Biomass burning in tropical forests – the normal practice to prepare land for agriculture and ranching – has been a major source of CO₂ emitted to the atmosphere. Mass transformations by burning are still little studied in the tropics. The present study estimated parameters, such as the stock of carbon contained in the biomass, burning efficiency and the formation of charcoal and ashes in a tropical moist forest. Two sets of plots arranged in the form of ‘stars' (720 m² total) were installed in a 3.5 ha area of forest that had been felled for planting pasture at Fazenda Nova Vida, Ariquemes, Rondônia. Each ‘star' had six rays measuring 2 m × 30 m; alternating rays were designated for pre-burn and post-burn measurements. All above-ground biomass present in the plots was weighed directly before the burn in the pre-burn rays and after the burn in the post-burn rays. Pieces of wood with diameter ≥10 cm also had their biomasses estimated from volume estimates, using line-intersect sampling (LIS) in order to increase the area of sampling and to allow volume loss to be estimated as an increment based on individual pieces measured before, and after, the burn at the same point (as opposed to inferring change as a difference between independent estimates of stocks). The initial above-ground biomass (dry weight) before the burn was estimated at 306.5 ± 48.6 (mean ± SE) Mg ha⁻¹, with an additional 4.5 Mg ha⁻¹ for trees left standing. Carbon stock in the initial biomass (including trees left standing) was 141.3 (Mg C) ha⁻¹. After burning, carbon stock was reduced by 36.8% (burning efficiency). The stocks of charcoal and ash formed in the burn were, respectively, 6.4 ± 2.7 and 5.7 ± 1.0 Mg ha⁻¹. The destructive and nondestructive (LIS) methods did not differ significantly (t-test, p > 0.05) in estimating post-burn stocks of wood and charcoal. The results of this study contribute to improving the estimates of parameters needed for global carbon calculations and point to ways in which estimates of these parameters could be further improved.     

The site specific variables that correlate with the distribution of the Pacific Golden Chanterelle, Cantharellus formosus

Cantharellus formosus is one of the most abundantly collected commercial mushrooms in western North America. Despite its importance to commercial harvesting, little information is known about the habitat requirements of C. formosus. The purpose of this study was to identify the environmental factors that correlate with the distribution of the basidiomata of C. formosus. Fifty-five plots (5 m × 5 m) with basidiomata and 60 comparison plots without basidiomata (5 m × 5 m) were established in Sitka spruce stands in Patrick’s Point State Park. Thirty plots with basidiomata and 30 without basidiomata were randomly selected for measurement of all variables. The latter included total percent cover of the following categories: shrubs, forbs, bryophytes and canopy cover. Diameter at breast height (DBH), height of trees, and other factors were also measured including duff depth, exchangeable cations, exchangeable acidity and aluminum, pH, and organic matter. Data were analyzed using logistic regression analysis to determine which environmental variables significantly correlated to the distribution of basidiomata. The Chi-squared test of homogeneity was used to determine if presence of chanterelle basidiomata was related to soil classification characterisitcs. The results indicate that chanterelles are associated with areas with low exchangeable acidity (2.09 ± 0.30 cmol+/kg soil), moderate duff depth (11.01 ± 0.45 cm), and areas with bare humus and needle cover less than 30% (29.05 ± 3.04%). Identification of these variables is important to assist land managers in identifying habitats where C. formosus basidiomata are likely to occur.     

A national forest resources assessment for Costa Rica based on low intensity sampling

A goal of a National Forest Inventory (NFI) is the provision of information which is relevant and required for national level decision making and monitoring in forestry, but also for related sectors.

This paper presents and discusses a pilot study from Costa Rica where in 2000/2001 a low intensity sampling approach was used to generate national level forestry information. On a 15 km × 15 km grid air photo plots were interpreted for forest and land cover type. Readily available 1997 aerial photographs were used that were, however, only available for about 70% of the country: of the 228 grid points for the whole country only 159 could be aerial photo interpreted. Out of the 15 km × 15 km base grid of sample points, a 2 × 3 subset was selected for field assessment, resulting in a sample of 40 cluster plots, each comprising of four elongated rectangular sub-plots of 150 m × 20 m located on the perimeter of a square of 500 m side length.

Two novel components were integrated into the inventory: (1) the field plots were established on all lands, so that the tree resource was not only tallied inside forests but also on all other tree-bearing lands outside forests. (2) In addition to the biophysical information gathered on the traditional field plots, interviews were carried out with forest owners on the site of the field plots, in order to obtain data on the use of the forest resource.

Field work was carried out by 6 field crews and took altogether about 3 months. Results were generated from the field samples for the entire country. Aerial photo based area estimates were compared to the corresponding estimations from field sampling for the same area. According to the field sampling the forest cover for Costa Rica in 2001 is estimated to be 48.4% (simple standard error percent 9.3%). An estimated 8.2% of the total volume (dbh > 30 cm, all species) is outside forest.

This inventory took place with support from Food and Agriculture Organization (FAO) in the framework of FAO–Forest Resources Assessment's (FRA) Program Support to National Forest Assessments; it was carried out jointly by Sistema Nacional de Áreas de Conservación (SINAC), the Costa Rican authority responsible for forestry issues, and Centro Agronómico de Investigación y Enseñanza (CATIE), an international agricultural research center. Experiences of the study were subsequently used to implement similar inventories in three more countries (Guatemala, Cameroon, The Philippines).     

Regeneration of timber trees in a logged tropical forest in North Bolivia

For sustainable forest management, it is important to know the response of timber species to the change in environment caused by logging. We performed a 2-year study on germination, survival and growth of four timber species, Cedrela odorata, Swietenia macrophylla, Hymenaea courbaril, and Cariniana micrantha, and one non-commercial species Tachigali vasquezii. We sowed seeds of these species in five microenvironments: log landing, gap-crown and gap-trunk, skidder trail and understory, in a tropical lowland moist rain forest in northern Bolivia. We related seed and seedling performance to light availability, soil compaction, and plant competition. Germination did not differ significantly between microenvironments but survival of germinated seeds for most species was significantly higher (P < 0.05) in the log landing (46–100%) than in the understory (0–7%). After 2 years, the tallest plants were always found in the log landing (119–190 cm) and the smallest in the understory (12–26 cm) caused by a higher relative height growth rate (RHGR) in the log landing (0.003–0.004 cm cm⁻¹ per day) compared to the understory (0.000–0.001 cm cm⁻¹ per day). During the first year RHGR was positively related to canopy openness for all species and negatively to the number of overtopping competitors for three species. During the second year also water infiltration explained observed variation to RHGR. These results show that abandoned log landings and logging gaps are suitable environments for the regeneration of timber species studied. This finding suggests that the removal of competitors in log landings and logging gaps combined with leaving seed trees near these microenvironments or sowing seeds, will improve regeneration of timber species in tropical forests.     

The role of topkill in the differential response of savanna woody species to fire

Understanding the impact of fire on the demography of savanna trees and shrubs is necessary for understanding human impacts in tropical savannas. In a replicated experiment, we studied the impact of fire and vegetation cover on survival and growth of two subshrubs (Periandra mediterranea and Protium ovatum), two shrubs (Miconia albicans and Rourea induta) and three trees (Myrsine guianensis, Piptocarpha rotundifolia and Roupala montana) of the Brazilian cerrado savannas. Burning increased complete mortality (i.e. death of the individual) of five of the seven species, but primarily among individuals with stem diameters <4 mm. Stem mortality (i.e. topkill) was much more prevalent, primarily affecting individuals with stem diameter <32 mm, though all species experienced some topkill in even the largest size classes. Fires of higher intensity (flame length >2 m) caused greater mortality and topkill than fires of lower intensity (flame length <2 m). Pre-burn vegetation density had little effect on survival or resprout size, but did affect subsequent growth rates. Four species had greater growth rates in open sites, whereas only one species had greater growth rates in dense sites. For the three tree species and one shrub, resprouting individuals did not reach the minimum reproductive size within 1 year of burning, while the other shrub and the two subshrubs were able to reach reproductive size during this time, indicating that growth form largely determines the population response to frequent burning.     

Comparing ecological restoration alternatives: Grand Canyon, Arizona

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

Forest structure and C stocks in natural Fagus sylvatica forest in southern Europe: The effects of past management

Managed forests often differ substantially from undisturbed forests in terms of tree structure and diversity. By altering the forest structure, management may affect the C stored in biomass and soil. A survey of 58 natural stands located in the south-westernmost limit of European beech forests was carried out to assess how the C pools are affected by the changes in tree structural diversity resulting from past management. The mean tree density, basal area and the number of large trees found in unmanaged forests were similar to those corresponding to virgin beech forests in Central Europe, whereas large live trees were totally absent from partially cut stands. Analysis of the Evenness index and the Gini coefficient indicated high structural diversity in the three stand types. The results of the Kolmogorov–Smirnov test used to compare the diameter distributions of each group revealed significant differences between stand types in terms of distributions of total tree species and of Fagus sylvatica.

The mean C stocks in the whole ecosystem – trees, litter layer and mineral soil – ranged from 220 to 770 Mg ha⁻¹ (average 380 Mg ha⁻¹). Tree biomass (above and belowground), which averaged 293 Mg C ha⁻¹, constituted the main C pool of the system (50–97%). The statistical test (Kolmogorov–Smirnov) revealed differences in the distribution of C pools in tree biomass between unmanaged and partially cut stands. As a consequence of the presence of large trees, in some unmanaged stands the C stock in tree biomass was as high as 500–600 Mg C ha⁻¹. In the partially cut stands, most of the C was mainly accumulated in trees smaller than 20 cm dbh, whereas in unmanaged stands the 30% of tree C pool was found in trees larger than 50 cm dbh. Furthermore, many unmanaged stands showed a larger C pool in the litter layer. The C content of mineral soils ranged from 40 to 260 Mg C ha⁻¹ and it was especially high in umbrisols. In conclusion, the implementation of protective measures in these fragile ecosystems may help to maintain the highly heterogeneous tree structure and enhance the role of both soils and trees as long-term C sinks.     

Effect of natural disturbances on the abundance of Norway spruce (Picea abies (L.) Karst.) regeneration in nemoral forests of the southern boreal zone

The impact of natural disturbances on the canopy (trees ≥14 m high) and sapling stratum (>0.3 and ≤14 m high) composition was studied in nemoral old-growth forests located within the southern boreal zone in Central Russia (Central Forest Reserve, 32°29′–33°01′E, 56°26′–56°31′N). I hypothesized that the current disturbance regime does not allow the maintenance of current spruce abundance in the canopy, and, as a result, there is a continuous shift in the canopy composition towards a greater abundance of deciduous species. Three 300×20 m² transects were established to estimate the proportions of stand under non-closed unexpanded canopy gaps. Data on sapling composition of 49 canopy gaps were used to analyze pattern of gap refuting in these forests. Additionally, data from three forest inventories showed changes in canopy composition over a period from 1972 to 1990.

The current status of nemoral forests is characterized by the high proportion of stand area under treefall gaps (71%). The loss of spruce from the canopy caused by treefalls (53% of the total basal area of gap-makers) was slightly greater than its canopy abundance (45%). Canopy gaps of all sizes encouraged spruce regeneration which might be due to a decrease in sapling mortality and/or more active recruitment of spruce seedlings. After a gap was formed, the presence of spruce in sapling strata increased. However, within both small (<200 m² in size) and large (>200 m²) gaps, tall (>6 m) spruce saplings did not reach the level of its abundance in the tree canopy. In gaps, tall (>6 m) saplings of lime (Tilia cordata) and elm (Ulmus glabra) grew more quickly than those of spruce and maple. These data suggested a decrease in canopy spruce and an increase in deciduous species in the near future which supported the original hypothesis. Analysis of forest inventory records revealed similar changes in the canopy structure over the past two decades. However, the observed high proportion of stand area under gaps implies that for the next few decades large areas of nemoral communities will be occupied by relatively young stands. This may, in turn, decrease the frequency of large-scale treefalls revegetated mainly by deciduous saplings.     

Formation of harvest units with genetic algorithms

Operational planning is normally based on stands as the primary unit of treatment. In recent years, interest has been directed towards the use of smaller area units such that the formation of treatment units becomes part of the operational planning. Here, the method of genetic algorithms was used in order to delineate harvesting units in a forest described as a grid of 20 m × 20 m pixels. Remote sensing data and inventory plots were employed to derive pixel estimates of forest variables. Appropriate parameter settings for the algorithm were investigated for two cases that consisted of 26 × 25 pixels. Based on the experience from these tests, the algorithm was employed in forming treatment units in an area encompassing 100 × 100 grid cells, or 400 ha. Due to the size of the problem, a two-stage procedure was suggested. The solutions of the genetic algorithm were compared with a heuristic, a cell removal algorithm, that was proposed in earlier studies of treatment unit formation. The genetic algorithm performed better than the heuristic for all the cases that were tested.     

Edge effects and forest water use: A field study in a mixed deciduous woodland

A field study was carried out in a mixed deciduous forest in order to measure the spatial variability of evapotranspiration in relation to distance from the nearest forest edge. Throughfall was collected in storage gauges in a transect across the edge. Transpiration was measured at the tree scale by means of the sap flux technique. Thermal dissipation probes were inserted into the hydro-active sapwood of 12–16 sample trees at a time covering four species. The sample trees were located close to a north- and a south-facing forest edge and between 3 and 69 m away from the nearest edge. The probes were moved to new trees about once a month and in total 71 trees were sampled. Sap flux densities were compared with potential evaporation and scaled up to the stand through multiplication with sapwood area per unit ground area. No significant edge effect on interception evaporation could be detected but there was a large influence on stand transpiration which increased towards the edge. In ash (Fraxinus excelsior L.), this increase resulted mainly from enhanced sap flux density (by 33–82%, depending on the size class) in trees located at the edge, whereas in oak (Quercus robur L.) the sap flux density was similar in edge and inner trees and an effect was only found at the stand scale in the way that the total basal area, per unit ground area, was larger near the forest edge than in the forest interior. Hawthorn (Crataegus monogyna L.) and field maple (Acer campestre L.), which occurred mainly in the understorey, were only weakly affected by the proximity to an edge. At the stand scale the total seasonal transpiration varied between 354 mm in the forest interior (>45 m away from the edge) and 565 mm at the forest edge (<15 m away from the edge), whilst the potential evaporation over the same period was 571 mm. This corresponds to Priestley–Taylor coefficients of 0.78 in the interior and 1.25 at the edge, whilst intermediate numbers were found for the area between the edge and inner zones. Using these results to calculate the average water loss per unit ground area of hypothetical woodlands of various sizes, it is shown that the edge effect dominates the water use of small forests and becomes negligible only for woodlands larger than 100 ha.     

Satellite-based estimation of biomass carbon stocks for northeast China''s forests between 1982 and 1999

Northeast China maintains large areas of primary forest resource and has been experiencing the largest increase in temperature over the past several decades in the country. Therefore, studying its forest biomass carbon (C) stock and the change is important to the sustainable use of forest resources and understanding of the forest C budget in China. In this study, we use forest inventory datasets for three inventory periods of 1984–1988, 1989–1993 and 1994–1998 and NOAA/AVHRR Normalized Difference Vegetation Index (NDVI) data from 1982 to 1999, to estimate forest biomass C stock and its changes in this region over the last two decades. The averaged forest biomass C stock and C density were estimated as 2.10 Pg C (1 Pg = 10¹⁵ g) and 44.65 Mg C ha⁻¹ over the study period. The forest biomass C stock has increased by 7% with an annual rate of 0.0082 Pg C. The largest increase in the C density occurred in two humid mountain areas, Changbai Mountains and northern Xiaoxing’anling Mountains. Climate warming is probably the key driving force for this increase, while anthropogenic activities such as afforestation and deforestation may contribute to variations in the C stocks.     

The impact of hemlock looper (Lambdina fiscellaria fiscellaria (Guen.)) on balsam fir and spruce in New Brunswick, Canada

In 1989, the first recorded outbreak of hemlock looper (Lambdina fiscellaria fiscellaria (Guen.)) occurred in New Brunswick, Canada. Data were collected from ten plots established in an area infested from 1992–1994, to assess impacts of hemlock looper. Ocular and branch sample assessments of current defoliation and ocular assessments of total defoliation (all age classes of foliage) were conducted for balsam fir (Abies balsamea [L.] Mill.), white spruce (Picea glauca [Moench] Voss), and black spruce (Picea mariana (Mill.) B.S.P.). Stand response was assessed and related to cumulative defoliation. Ocular assessments were found to accurately estimate defoliation, which was significantly related to tree mortality. Ninety-two percent of balsam fir trees that had cumulative defoliation >90% died. Mortality of balsam fir was significantly (p < 0.05) related to tree size, in both lightly and severely defoliated plots; trees with DBH <11 cm sustained 22–48% higher mortality than larger trees. Mortality of balsam fir, in terms of both percent stems/ha and m³/ha merchantable volume, increased exponentially in relation to three estimates of cumulative (summed) plot mean defoliation. The strongest relationships (r² = 0.75–0.79) were between mortality and the ocular defoliation assessment for 1990–1993 foliage. Tree mortality caused by the looper outbreak ranged from 4–14% stems/ha in lightly defoliated and from 32–100% in severely defoliated plots; merchantable volume killed was 3–14 m³/ha and 51–119 m³/ha, respectively. Relationships between mortality and defoliation were similar when defoliation was assessed for 1987–1993 and 1990–1993 foliage age classes.     

Tree Life History Patterns and Forest Dynamics

Abstract

Forest trees are usually classified into broad ecological groups (e.g., pioneers vs. mature-phase species) based on the importance of natural disturbance for their regeneration. These classifications only tie into account a small component of the plant's life-cycle, usually seeds and seedlings, and therefore may over-simplify the role of natural disturbance in the evolution of life-history traits. We present a conceptual model for the study of plant demography within heterogeneous environments, and illustrate its use by reviewing how natural disturbance influences the demography of several lowland tropical tree species at Los Tuxtlas, Mexico. Our model assumes that habitat patches such as treefall gaps in tropical forests can affect a plant at all stages of its life-cycle, and reveals many potential life history patterns in relation to forest dynamics. We propose that only detailed demographic studies of whole life histories and genetics of tree populations, as well as forest disturbance regimes, will allow us to elucidate convergent life histories and the existence of discrete ecological groups among tropical forest trees.