Forest Structural Heterogeneity Research Articles

Hoverfly (Syrphidae) Communities Respond to Varying Structural Retention After Harvesting in Canadian Peatland Black Spruce Forests

Variable retention harvesting (VRH), in which trees are removed at variable intensity and spatial configuration across the landscape, retains greater forest structural heterogeneity than traditional clear-cut harvesting and is being recommended as an alternative for sustainable management of the boreal forest. Little is known about its effects on forest fauna; thus, we studied the influence of one type of VRH (harvesting with advanced regeneration [HARP]) on the Syrphidae (Diptera) community in northern Ontario forests of peatland black spruce (Picea mariana). We examined the effects of varying structural retention (from unharvested through partial retention to clear-cut) on syrphid species richness and abundance, and abundance of functional assemblages. Greater species richness and population abundances were found generally in harvested than in unharvested forests. Overall species richness and the abundance of four species (Platycheirus rosarum, Toxomerus marginatus, Xylota annulifera, and X. tuberculata) and larval predators were all higher in both clear-cut sites and those with structural retention than in unharvested sites. Similarly, overall species richness and the abundance of nine species were higher in clear-cut than in unharvested sites. Species responses are discussed in an ecological context. Differences among the levels of forest retention harvesting were relatively minor compared with those of the clear-cut and unharvested area, suggesting that local habitat characteristics may play a more important role in determining the syrphid community than the landscape configuration. However, a landscape level effect was evident, suggesting that syrphids may be useful in reflecting changes in stand structure at the landscape scale.

Hoverfly (Syrphidae) Communities Respond to Varying Structural Retention After Harvesting in Canadian Peatland Black Spruce Forests

Variable retention harvesting (VRH), in which trees are removed at variable intensity and spatial configuration across the landscape, retains greater forest structural heterogeneity than traditional clear-cut harvesting and is being recommended as an alternative for sustainable management of the boreal forest. Little is known about its effects on forest fauna; thus, we studied the influence of one type of VRH (harvesting with advanced regeneration [HARP]) on the Syrphidae (Diptera) community in northern Ontario forests of peatland black spruce (Picea mariana). We examined the effects of varying structural retention (from unharvested through partial retention to clear-cut) on syrphid species richness and abundance, and abundance of functional assemblages. Greater species richness and population abundances were found generally in harvested than in unharvested forests. Overall species richness and the abundance of four species (Platycheirus rosarum, Toxomerus marginatus, Xylota annulifera, and X. tuberculata) and larval predators were all higher in both clear-cut sites and those with structural retention than in unharvested sites. Similarly, overall species richness and the abundance of nine species were higher in clear-cut than in unharvested sites. Species responses are discussed in an ecological context. Differences among the levels of forest retention harvesting were relatively minor compared with those of the clear-cut and unharvested area, suggesting that local habitat characteristics may play a more important role in determining the syrphid community than the landscape configuration. However, a landscape level effect was evident, suggesting that syrphids may be useful in reflecting changes in stand structure at the landscape scale.

Coarse woody debris and canopy cover in an old-growth Jeffrey pine-mixed conifer forest from the Sierra San Pedro Martir, Mexico

The cultural practices associated with Euro-American settlement in the United States have altered forest structure and ultimately changed fundamental ecosystem processes. Coarse woody debris (CWD) and canopy cover are recognized as having great importance for many wildlife species and ecological processes. Little information is available from forests on historical levels of canopy cover and CWD before European settlement. A great deal of uncertainty exists concerning the long-term role of fire and the dynamics of CWD, especially in forests that once experienced frequent, low-moderate intensity fire regimes. The objective of this study was to quantify CWD and forest canopy cover in an area where harvesting has never occurred and limited fire suppression began in the 1970s. This study was done in Jeffrey pine-mixed conifer forests in the Sierra San Pedro Martir (SSPM) in northwestern Mexico. Canopy cover, canopy closure, and CWD were sampled on a grid of plots. Average canopy cover was 26.8%, average canopy closure was 40.1%. A total of 102 CWD pieces were measured, and nearly half of the plots (45.7%) had no CWD present. Average CWD density, percent cover, volume, and weight were 108 pieces ha −1, 1.5%, 47.5 m 3 ha −1, and 15.7 tonnes ha −1, respectively. All of the CWD sampled were in the later stages of decay. Less than average values for CWD density, percent cover, volume, and weight were recorded in 57%, 64%, 67%, and 69% of the plots, respectively. CWD dynamics in forests that experience frequent, low-moderate intensity fires are fundamentally different than those having long-interval, high-severity fires. There was a large amount of variability in all CWD and forest canopy cover measurements taken from Jeffrey pine-mixed conifer forests in the SSPM. Spatial heterogeneity in forest structure should be included in the desired conditions of xeric, pine-dominated forests in the United States that once experienced frequent, low-moderate intensity fire regimes. It should be noted that heterogeneity by itself may not lead to sustainable forests unless that heterogeneity includes stand structures that are resistant/resilient to high-severity fire, drought, insects, and disease.

Relationships between avian richness and landscape structure at multiple scales using multiple landscapes

Little is known about factors that structure biodiversity on landscape scales, yet current land management protocols, such as forest certification programs, place an increasing emphasis on managing for sustainable biodiversity at landscape scales. We used a replicated landscape study to evaluate relationships between forest structure and avian diversity at both stand and landscape-levels. We used data on bird communities collected under comparable sampling protocols on four managed forests located across the Southeastern US to develop logistic regression models describing relationships between habitat factors and the distribution of overall richness and richness of selected guilds. Landscape models generated for eight of nine guilds showed a strong relationship between richness and both availability and configuration of landscape features. Diversity of topographic features and heterogeneity of forest structure were primary determinants of avian species richness. Forest heterogeneity, in both age and forest type, were strongly and positively associated with overall avian richness and richness for most guilds. Road density was associated positively but weakly with avian richness. Landscape variables dominated all models generated, but no consistent patterns in metrics or scale were evident. Model fit was strong for neotropical migrants and relatively weak for short-distance migrants and resident species. Our models provide a tool that will allow managers to evaluate and demonstrate quantitatively how management practices affect avian diversity on landscapes.

Simulation and quantification of the fine-scale spatial pattern and heterogeneity of forest canopy structure: A lacunarity-based method designed for analysis of continuous canopy heights

Forests canopies are dynamic, continuously varying, three-dimensional structures that display substantial heterogeneity in their spatial arrangement at many scales. At the stand-level, fine-scale spatial heterogeneity influences key canopy processes and contributes to the diversity of niche space and maintenance of forest biodiversity. We present a quantitative method that we developed based on a novel application of two well-established statistical techniques – lacunarity analysis and principal component analysis (PCA) – to determine the fine-scale (0.5–33 m) spatial heterogeneity found in the outer surface of a forest canopy. This method was specifically designed for the analysis of continuous canopy height data generated by airborne LiDAR systems or digital photogrammetry; however, in this study we demonstrate our method using a large, well-documented dataset composed of simulated canopy surfaces only. We found that the magnitude of the lacunarity statistic was strongly associated with canopy cover ( R 2 = 0.85) and gap volume ( R 2 = 0.84), while the pattern of decline in lacunarity across discrete measurement scales was related to many size- and density-related attributes of stand and canopy structure (0.27 ≤ R 2 ≤ 0.58) and their diverse vertical and horizontal spatial distributions. PCA uncovered two major gradients of spatial heterogeneity from the 10 dimensions of our original lacunarity dataset. The stronger of these two gradients reflected the continuous variation in canopy cover and gap volume, while a second, more subtle gradient was associated with the array of possible vertical and horizontal spatial configurations that might define any one measure of canopy cover. We expect that this quantitative method can be used to support a broad range of practical applications in sustainable forest management, long-term ecological monitoring, and forest science. Further research is required to understand how these statistical estimates and gradients of measured spatial heterogeneity relate to other ecologically relevant patterns of forest composition, structure, and function.

Tree species diversity in a seasonally-dry forest: the case of the Pinkaití site, in the Kayapó Indigenous Area, Southeastern limits of the Amazon

This study investigates patterns of forest structure and tree species diversity in an anthropogenic palm grove and undisturbed areas at the seasonally-dry Pinkaití research station, in the Kayapó Indigenous Area. This site, managed by the Conservation International do Brasil, is the most southeastern site floristically surveyed in the Amazon until now. The secondary and a nearby undisturbed forest were sampled in a group of 52 floristic plots of 0.0625-ha (25x25-m) where all trees with DBH > 10 cm were measured and identified. The analyses were complemented with other two floristic plots of 1-ha (10x1000-m). The present study has shown that the Pinkaití, like other seasonally-dry forests, have great heterogeneity in forest structure and composition, associated with biotic characteristics of the most important tree species, natural disturbance and history of land-use. The palm grove, moderately dominated by the arborescent palm Attalea maripa (Aubl.) Mart., presented high tree species diversity and was floristically similar to undisturbed forests at the study site. It is discussed the importance of large arborescent palms for the seasonally-dry Amazon forests regeneration.

Effects of an intense ice storm on the structure of a northern hardwood forest

A major ice storm in January 1998 provided an opportunity to study the effects of a rare, intense disturbance on the structure of the northern hardwood forest canopy. Canopy damage was assessed using visual damage classes within watersheds of different ages at the Hubbard Brook Experimental Forest (HBEF) and changes in leaf area index in two of these watersheds. Ice thickness was measured, and ice loads of trees were estimated using regression equations. In the 60- to 120-year-old forests (mean basal area 26 m2·ha–1), damage was greatest in trees >30 cm diameter at breast height and at elevations above 600 m. Of the dominant tree species, beech (Fagus grandifolia Ehrh.) was the most damaged, sugar maple (Acer saccharum Marsh.) was the most resistant, and yellow birch (Betula alleghaniensis Britt.) was intermediate. Trees with advanced beech bark disease experienced heavier ice damage. Little damage occurred in the 14-year-old forest, while the 24- to 28-year-old forest experienced intense damage. In the young stands of this forest, damage was greatest between 600 and 750 m, in trees on steep slopes and near streams, and among pin cherry (Prunus pensylvanica L.). Recovery of the canopy was tracked over three growing seasons, and root growth was monitored 1 year after the storm. Because of the high density of advance regeneration from beech bark disease and root sprouting potential in ice-damaged beech, HBEF will likely see an increase in beech abundance in older forests as a result of the storm. There will also be a more rapid change from pioneer species to mature northern hardwoods in the younger forests. These predictions illustrate the ability of rare disturbances to increase heterogeneity of forest structure and composition in this ecosystem, especially through interactions with other disturbances.

The effects of harvest-created gaps on plant species diversity, composition, and abundance in a Maine oak–pine forest

In forests where large, stand-replacing disturbances are infrequent, small-scale disturbances associated with the mortality and replacement of individual trees are a primary source of heterogeneity in forest composition and structure. The disturbances considered in this study were canopy gaps created by a partial harvest in the winter of 1987–1988 in an oak–pine ( Quercus–Pinus) forest that is part of a long-term ecosystem study at the Holt Research Forest in coastal Maine. This study examined the gap phase of the forest turnover cycle: the brief episodes of rapid change when processes determining the structure and composition of a forest community occur. The objective was to evaluate the effects of harvest-created gaps and soil moisture (as reflected by soil drainage classes) on woody and herbaceous species diversity, composition, and abundance relative to unharvested control areas. Because the vegetation was sampled in both 1993 and 1998, detection of short-term changes was possible. Harvest gaps had higher total plant species richness in the understory (vegetation <1 m tall) than controls, primarily due to more infrequently-occurring species. Average species richness varied by soil drainage class but not treatment, whereas the Shannon–Wiener diversity index was not affected by treatment, drainage class, or time. Species abundance was more evenly distributed (i.e., higher evenness) in controls than in gaps. Species unique to harvest gaps decreased in number and abundance over time. Detrended correspondence analysis (DCA) revealed that harvest gaps exhibited greater variability in species composition than controls, although many of the gaps and controls were similar in composition. Shifts in individual species abundance were evident with treatment, drainage, and time. Diversity measures calculated for sapling density showed a significant difference among drainage classes for species richness, but no significant difference between treatments for species richness, the Shannon–Wiener index, or the evenness index. Gap size was positively correlated with the total density of saplings, the density of several species, and species richness. There were significant differences between treatments for the density of all species tested in at least one of the four height classes. DCA of sapling data revealed some separation of harvest gaps and controls but there was considerable overlap. Axis 1 tended to separate early successional species in the harvest gaps from shade tolerant species in the controls. The understory plant community at the HRF appears to be resilient to the partial harvest that created a variety of gap sizes. Silvicultural objectives were met; the harvest gaps created sites for establishment of seedlings and sprouts of tree species, particularly early successional species.

Earthquake impacts in old‐growth Nothofagus forests in New Zealand

Abstract. Six stands located on different land forms in mixed old‐growth Nothofagus forests in the Matiri Valley (northwest of South Island, New Zealand) were sampled to examine the effects of two recent large earthquakes on tree establishment and tree‐ring growth, and how these varied across land forms. 50 trees were cored in each stand to determine age structure and the cores were cross‐dated to precisely date unusual periods of radial growth. The 1968 earthquake (M = 7.1, epicentre 35 km from the study area) had no discernible impact on the sampled stands. The impact of the 1929 earthquake (M = 7.7, epicentre 20 km from the study area) varied between stands, depending on whether or not they had been damaged by soil or rock movement. In all stands, the age structures showed a pulse of N. fusca establishment following the 1929 earthquake, with this species dominating establishment in large gaps created by landslides. Smaller gaps, created by branch or tree death, were closed by both N. fusca and N. menziesii. The long period of releases (1929–1945) indicates that direct earthquake damage was not the only cause of tree death, and that many trees died subsequently most likely of pathogen attack or a drought in the early 1930s. The impacts of the 1929 earthquake are compared to a storm in 1905 and a drought in 1974–1978 which also affected forests in the region. Our results confirm that earthquakes are an important factor driving forest dynamics in this tectonically active region, and that the diversity of earthquake impacts is a major source of heterogeneity in forest structure and regeneration.

Potential Use of a Spatially Constrained Harvest Scheduling Model for Biodiversity Concerns: Exclusion Periods to Create Heterogeneity in Forest Structure

The objective of this paper is to investigate potential use of a spatially constrained harvest scheduling model for biodiversity concerns. Change in the degree of biodiversity is represented only by spatial characteristics of harvesting patterns of forest stands with different exclusion periods applied to adjacent forest stands. A spatially constrained harvest scheduling model called SSMART (Scheduling System of Management Alternatives foR Timber-harvest) is used for the analysis. It is one of the heuristics to solve a spatially constrained harvest scheduling problem by using the partitioning heuristic. The algorithm incorporated into SSMART is designed to seek a solution for a multicriteria problem with present net value maximum, meeting spatial feasibility and minimizing period-to-period harvest flow fluctuation, approximating even-flow constraints within the 0–1 integer programming framework. Our experimental analysis shows that the longer exclusion period, the less the harvest flow level and the total present net value are derived and the more heterogeneous the forest structure becomes in terms of the forest stand age distribution. It is also shown that the three exclusion period results in a stable forest stand age distribution over the time horizon for our experimental forest.

A new method for modeling the heterogeneity of forest structure

Despite the critical ecological roles of structural complexity, ecologically relevant quantitative measures of structural complexity that allow comparisons among forest stands are still lacking. The objective of this study was to develop a method that allows comparisons of structural heterogeneity among stands. To encompass a broad range of potential structural complexities, we simulated three spatial point patterns each for tree-size distributions from five inventoried natural stands. Forest structure was modeled and analyzed by simulating point patterns of trees and constructing triangular networks to connect neighboring tree tops to one another. This method is based on the concept of spatial tessellation of tree positions, where point patterns are converted into 2-dimensional nearest neighbor triangles. A structural complexity index (SCI) was defined as the sum of the areas of 3-dimensional triangles (with x, y, and z coordinates) divided by the sum of the areas of 2-dimensional triangles. Vertical gradients were defined as the maximum size difference among the trees forming a triangle, that is, the greater the difference, the greater the structure and the larger the SCI value. Patch-types were defined as classes of structural gradients at different positions within the canopy. More patch-types are also indicative of more structurally heterogeneous stands. Both the SCI and the number of patch-types and patch-size heterogeneity related closely to conventional descriptors of forest structure. While the number of patch-types and patch-type heterogeneity related more to the vertical component of structural complexity, SCI integrated well with both the vertical and horizontal structure. SCI and the concept of patch-types complement one another and can be used to quantitatively compare the structure of different stands. The applicability of this modeling approach in characterizing the structural heterogeneity of forests across spatial scales is discussed.

Interactions between forest heterogeneity and surface fire regimes in the southern Sierra Nevada

Fire is a major agent of spatial pattern formation in forests, as it creates a mosaic of burned and unburned patches. While most research has focused on landscape-level patterns created by crown fires, millions of hectares of forests in North America are subject to surface fire regimes. A spatially explicit forest gap model developed for the Sierra Nevada was used to evaluate the influence of surface fire regimes on the heterogeneity of forest structure and composition within forest stands. Forest pattern was evaluated for a wide range of topographic positions in Sequoia National Park, California, to determine if repeated surface fires amplify existing spatial patterns. The spatial heterogeneity of some forest characteristics increased under a simulated fire regime relative to scenarios without fire. Although a distinct and regular fire-generated spatial pattern was not detected with an analysis of spatial autocorrelation, simulated surface fires did alter the spatial heterogeneity within a forest stand, primarily by degrading a regular structure that is imposed by competition for light in the absence of fire. The interaction between surface fires and forest pattern may be qualitatively different from that which occurs in forests subject to crown fires. As such, what has been learned about forests dominated by crown fires may not apply to forests subject to surface fire regimes.

Spatial heterogeneity in tropical forest structure: canopy palms as landscape mosaics

Spatial heterogeneity in tropical forest structure: canopy palms as landscape mosaics

Gap Dynamics of a Tropical Cloud Forest in Northeastern Mexico

Gap dynamics in a tropical cloud forest in Tamaulipas, Mexico were studied by describing the treefall regime and by measuring the temporal and spatial occurrence of gaps (1-yr-old lightgaps produced by treefalls) and patches (older lightgaps). The study plots comprised protected (4 ha) and exposed (2 ha) zones, where the frequency of gap and patch size per unit area was determined. The number of trunks on the forest floor was counted and their length and diameter were recorded. The causes of treefalls were determined. Fallen tree species were identified and a comparison of the specific composition between fallen and standing trees was made for the community. The results suggest that gaps play an important role in determining an environmental mosaic within the tropical cloud forest. Species-producing gaps are those dominating the canopy layer in approximately 80 percent of the treefalls. An average of 19 patches ha-' was registered, comprising 2 5 percent of the study site. The mean patch size as fitted to a lognormal distribution was estimated as 4.71 ? 0.6. Turnover rates were estimated as 158 yr for areas disturbed by natural causes and 90 yr for areas disturbed by joint causes (i.e., natural plus man-induced). The natural and the actual (natural plus man-induced) gap production rates were 0.5 and 0.63 gap ha-' yr-1. This means that an average of 1.27 percent of the forest area is disturbed every year by natural causes and that 2.22 percent of the area is disturbed by joint causes. FOREST REGENERATION PROCESSES frequently depend upon natural disturbance regimes. Wind, fire, and hurricanes kill trees and open gaps in the forest canopy (Hartshorn 1978, Bormann & Likens 1979, Lorimer 1980). The structural changes produced by treefalls create an environmental mosaic in the community determined by a gapage-dependent process (Watt 1947; Whitmore 1975, 1978; Halle et al. 1978; Oldeman 1978; Hartshorn 1980; Runkle 1981; Nakashizuka 1984a; Brokaw 1985; and Martinez-Ramos 1985). The spatial and temporal heterogeneity of forest structure depends on the rate of gap production as well as on gap sizes, shapes, and locations. According to the way a treefall occurs, gap formation provides a range of resources for the differential establishment of species within the community (Denslow 1980, Brokaw 1982, Putz et al. 1983, Nakashizuka 1984b, Moore & Vankat 1986, Veblen 1986). This paper describes the gap dynamics of a tropical cloud forest in northeastern Mexico. The work includes the study of the treefall regime which considers the factors influencing the way trees die and its effect on the species composition of the forest canopy. This paper also presents the temporal and spatial occurrence of gaps and patches based on the estimation of gap production and turnover rates, as well as on a description of patch sizes. STUDY AREA The study was conducted in the Biosphere Reserve El Cielo, a tropical cloud forest community which covers 10,000 ha (between parallels 23?12'N and 23?03'N; 99?18'W) at an altitude of 1250 m. The reserve is in the Sierra de Cucharas at the eastern slope of the Sierra Madre Oriental in the state of Tamaulipas, Mexico. The mean monthly rainfall in the region is 252.2 mm. A dry season occurs from late November to March, although there is considerable atmospheric humidity because of the frequent occurrence of mists. The mean monthly temperature is 13.03?C, with a minimum of 5.84?C and a maximum of 22.6?C. The climate is tropical-humid with a short, dry season (Puig 1976). The forest is greatly influenced by the easterly winds and polar air masses producing the cold winter winds locally called nortes. It is also affected by frequent, damaging hurricanes 1:34% of the total hurricanes that reach Mexican eastern coasts affect the state of Tamaulipas (Jauregui 1967)1. The cloud forest is in a mountainous region of volcanic origin. Soils are lithosols 10 cm deep with a sandy loam texture; rendzinas can sometimes be found 10 to 40 cm deep with a clay texture, both with a high organici-matter content (Puig et al. 1983). The vegetation has been described as a mixture of tropical and temperate species (Sharp et al. 1950, Hernandez X. et al. 1951, Sharp 1953, Rzedowski 1978, Lonard & Ross 1979) and more recently it was quantitatively described as a forest with abundant tropical tree species but with temperate canopy trees structurally dominating the community (Puig et al. 1983). This forest was I Received 10 October 1986, revision accepted 30 March 1987. 2 Present address: Centro de Investigaciones Biol6gicas de Baja California Sur, Apdo. Postal 128, La Paz 23060, Baja California Sur, Mexico. 178 BIOTROPICA 20(3): 178-184 1988 This content downloaded from 207.46.13.48 on Wed, 12 Oct 2016 05:31:54 UTC All use subject to http://about.jstor.org/terms