Thesis (M.S.)--Oregon State University, 1993. / Typescript (photocopy). Includes mounted photographs. Includes bibliographical references (leaves 68-71). Also available on the World Wide Web.

Thesis (doctoral)--Swedish University of Agricultural Sciences, 2005. / Thesis documentation sheet inserted. Appendix reproduces four papers and manuscripts, three co-authored with others. Includes bibliographical references. Also partially available electronically via World Wide Web in PDF format; online version lacks appendix.

Thesis (doctoral)--Universität München, 1991. / Includes bibliographical references (p. 201-219).

Forest stands were studied to determine if old-growth forest structure could be mimicked in younger stands via overstory manipulation. Cover and species composition of understory plants were systematically sampled in sixteen thinned second-growth stands and sixteen adjacent unthinned second-growth Douglas fir (Pseudotsuga menziesii (Mirabel) Franco.) stands. The stands were thinned twenty-four to thirteen years ago. These were compared to seven nearby old-growth stands. Thinned and unthinned stands had matching elevations, aspect, and soils, yet differed primarily in management treatment. Leaf area indices were determined for these stands. Thinned stands differed from the old-growth and unthinned stands in having significantly higher cover values and species numbers, apparently resulting from increased light to the forest floor and a greater variety of microhabitats created by thinning. Young unthinned and old-growth stands were comparable in terms of cover and richness, but differed in species composition. Diversity indices showed no difference in species diversity between the three types of stands. Ordination of the species/sample data using Detrended Correspondence Analysis showed that understory species composition of the young unthinned and thinned stands was nearly identical. Species composition of old-growth stands differed from thinned and unthinned stands. The ordination indicated that age of the stands, structure of the canopy layers and climate were major determining factors in the species composition of the understory plant communities. Management manipulation of the second growth stands did not yield stands with understory vegetation communities that mimicked those of old-growth stands. The conclusions of this study were: 1) Shrub cover increased with thinning as compared to unthinned and old-growth stands. 2) Thinning increased the species richness of the stands, without increasing the number of exotics. 3) Diversity was not altered by thinning. Old-growth, thinned and unthinned stands did not differ in diversity values. 4) Patterns of community composition in thinned stands were more similar to unthinned equivalent stands than to nearby old-growth. / Graduation date: 1996

No description available.

This thesis contributes to the understanding of two of the most intriguing questions that forest ecologists have faced over recent decades: 1) how high diversity is maintained in species-rich ecosystems; and 2) what is the role of spatio-temporal environmental variation in structuring forest communities. The aims of the research were to ascertain how species composition varies both spatially and temporally and how changes in the vegetation can be understood in the context of species coexistence theories (niche versus neutral). A group of 38 sympatric species, including 9 species of Quercus, on which little ecological research has been undertaken, were used in this study. The data used in this project include eleven years of periodic remeasurements of permanent plots established in high-altitude oak forests in Mexico. Adult, sapling and seedling trees were studied as well as their environmental surrounding. Spatial and temporal variations in forest composition were analysed using multivariate statistical approaches. The results show that there are discrete communities in these mixed oak forests that correspond to specific environments. At a broad scale the study area can be classified into two floristic zones, a mesic zone characterised by associations that include Quercus candicans, Q. laurina and Q. castanea and; a xeric zone dominated by Q. crassipes. However of a finer scale of analysis important variation in composition was associated with different life stages of the trees, with adult trees showing much stronger environmental associations than seedlings and saplings. Successional pathways and rates vary at relatively fine scales. This may be as a result of dominance alternation between dominant canopy species. Micro-niche zonation processes caused by a high degree of environmental heterogeneity combined with individual species traits explain the coexistence of phylogenetically similar sympatric Quercus species. A hierarchy of processes, each acting at a different spatial and temporal scale, determines species diversity and coexistence. The overall findings support the idea that niche differentiation rather than chance events such as dispersal limitation, are more important in permitting species coexistence.

Identifying the mechanisms that prevent competitive exclusion in tropical forests is a key goal of tropical ecology. Because trees are long-lived organisms, it is complicated to test theory related to coexistence. However, the seedling stage, during which tree mortality is the highest, offers an ideal proxy to evaluate mechanisms that promote or hinder tree species coexistence. This dissertation utilizes both theory and empirical approaches to investigate two mechanisms thought to influence seedling dynamics and tree species coexistence: negative feedbacks from tree natural enemies and positive feedbacks from arbuscular mycorrhizal (AM) fungi. Tree natural enemies might promote tropical tree species coexistence by acting as agents of negative density-dependent mortality. Simultaneously, tropical seedlings associate with arbuscular mycorrhizal (AM) fungi, which can increase seedling growth and survival through enhanced nutrient and water uptake. However, little is known about the effects of AM fungi on tropical tree community dynamics. In chapter 1, I developed a theoretical model that combines competition between trees, predation by tree natural enemies, and mutualism with AM fungi, and showed that a subtle balance between negative and positive feedbacks is required to reach tree species coexistence. In chapters 2 and 3, I used empirical data collected from El Yunque (Puerto Rico) to gain a better understanding of the distribution of soil fungi and tree natural enemies in a secondary tropical forest and to test some of the assumptions of my theoretical model. In chapter 2, I found evidence that soil characteristics and the tree community were important to structure soil fungal communities, and I demonstrated long-lasting effects of past human land use. If AM fungi are important to promote tropical tree species coexistence as suggested by my theoretical model, past land use could influence tree species coexistence by altering AM fungal communities, emphasizing the need for additional studies about land use legacy effects on AM fungal communities. In chapter 3, I showed that seedlings at intermediate conspecific density and from intermediate abundance tree species, hosted the richest community of natural enemies, suggesting that negative density-dependent processes might be non-linear, and partially supporting my theoretical model. Finally in chapter 4, I investigated seedling mortality and showed that natural enemies increase seedling mortality, whereas AM fungal diversity decreases seedling mortality, counteracting the local effects of natural enemies. I also found evidence that AM fungal diversity rescues rare tree species, and natural enemies reduce survivorship of more abundant species, thereby preventing competitive exclusion. Therefore, at the community scale, AM fungal diversity and natural enemies act in the same direction, promoting tropical tree species coexistence, which is consistent with the findings of my theoretical model. In conclusion, this dissertation jointly investigated the effects of negative and positive feedbacks on tropical tree species coexistence, and demonstrated the importance of combining demographic processes that are known to occur simultaneously.

This thesis explores issues relating to the application of physiological-process models (???process models???) of forest growth to mixed species, mixed age forests, in particular the coastal blackbutt forests of New South Wales. Using a dataset provided by State Forests of New South Wales (Carter 1994 unpubl.) a numeric description of the forest was developed and stand-level parameters of interest were derived, in particular the plot by plot stemwood volume growth from 1975 to 1999. The amounts of harvested volume, volume that died and volume that grew into the measurement population were identified separately, and several different means of accounting for volume change over time were investigated. A method for quantifying the impact of harvesting and other silvicultural practices on the growth of the forest was developed and programs were written to convert the stand-level summary of the harvest impact into a semi-random selection of trees that would be ???harvested??? from the database under the set of silvicultural assumptions (Dore et al. 1999). A number of process models were investigated and reviewed before selecting one particular model, SUSTAIN (Dewar 1997) for adaption to these forests. This model is a relatively simple process model with a small number of input parameters. The model was adapted so that it could be used to compare the SUSTAIN estimate of growth with the growth of an individual stand of trees in the Kendall Forest Management Area, between Wauchope and Taree on the mid-north coast of NSW. To improve the accuracy of the prediction of growth by SUSTAIN, a method of re-setting the state of the stand to the actual condition at the time of remeasurement was developed. In addition, the SUSTAIN model was extended to enable two separate levels of canopy to be described and grown separately. Ultimately the model was only partially successful in mirroring the growth predicted by the empirical data. Its partial success is attributed primarily to the difficulties associated with correctly determining the allocation parameters used by the model to assign net photosynthate to the roots, foliage and stemwood. The nature of the change in allocation parameters when the forest stand is disturbed by harvest or fire needs further investigation.

The potential of airborne LiDAR technology to quantify forest structure within eucalypt forests has been evaluated with a focus on the understorey stratum. To achieve this, three studies have been undertaken using multiple (4) LiDAR datasets acquired over three test areas located in Wedding Bells State Forest, Coffs Harbour, Australia. Initially, the effects of sensor configuration were evaluated using field measurements collected from three structurally and topographically differing field plots (40 x 90 m areas). Results indicated that canopy height profiles derived from LiDAR data at the plot scale were largely unaffected by a change in platform altitude from 1000 to 3000 m (p &gt 0.05). In addition, the derivation of individual tree attributes was found to be highly sensitive to the density of LiDAR observations whilst higher platform altitudes showed an increased proportion of single returns over forested areas. In the second study, an innovative field based approach was developed to sample the structure of the understorey (horizontally and vertically) for LiDAR validation purposes. Using two separate LiDAR datasets, this research confirmed that mean understorey height and understorey cover can be effectively mapped in areas of low to medium canopy cover whilst no significant relationship (p &gt 0.05) was identified between field and LiDAR estimates of maximum understorey height. In the third study, an optimised LiDAR beam interception model was developed and validated, and then applied to assess the interaction of extrinsic and intrinsic factors of the LiDAR survey. This demonstrated that the probability of beam interception through the forest canopy can be affected by factors both intrinsic (e.g. crown cover) and extrinsic (e.g. scan angle) to the structure of the canopy. Overall, the results of this research indicate that optimising the sensor configuration is important to the derivation of particular forest structural attributes and significantly, there is potential for LiDAR technology to provide quantitative and spatially detailed estimates of key understorey attributes such as mean height and cover.

The diverse lichen flora of the Pacific Northwest is being impacted by population growth and by forest management practices. Accumulating information about our lichen flora will improve our conservation strategies. This dissertation first collects information to improve our understanding of how lichen communities vary among forests of differing structure, and across the western Oregon landscape. It then proposes a method to predict species occurrence in unsampled sites by utilizing the information on forest characters and environmental gradients at sampled sites. Macrolichen communities sampled in coniferous forests revealed that old-growth stands (>200 yrs old) harbored communities that differed from those in young forests (50-110 yrs old). Even more atypical communities occurred in macrolichen hotspots, which were primarily in riparian zones. Many macrolichen species were associated with these hotspots, including numerous nitrogen-fixing cyanolichens. Macrolichen species associated with old-growth forested plots included the nitrogen-fixing lichen Lobaria oregana and several forage-providing alectorioid lichens. The presence of remnant old trees apparently increased the occurrence of old-growth associates in young stands. The calicioids, a group of microlichens investigated only in the Cascades, had a strong association with old growth forest and remnant trees. Diversity of calicioids may also be increased by legacy structures such as old snags and wolf trees. These structures increase continuity between current and previous stands. Macrolichen communities varied between the Coast and Cascade Mountain Ranges, following climatic gradients, particularly annual precipitation. Successional patterns in macrolichen communities appeared to differ between the mountain ranges. The modeling method proposed for using habitat associations to predict occurrence has several advantages over common modeling methods, such as regression. The method is simple, avoids parametric assumptions, provides easy updating of models as additional sites are sampled, and automatically accounts for interactions among predictor variables. It can be linked with GIS data and software to map estimated probability of occurrence across landscapes. The data on calicioids from the Cascades, supplemented with additional stand inventories, were used to test and demonstrate the modeling method. / Graduation date: 2001