No description available.

Previously disturbed and regenerating forests make up a significant proportion of the North American land area, and therefore play an important role in the exchanges of heat and trace gases between the terrestrial biosphere and the atmosphere. Assessment of local to global variability in CO2 exchanges by forests requires a combination of CO2 measurements made by eddy covariance (EC), field measurements, remote sensing data, and ecosystem models. The integration of these is problematic because of a mis-match in scale between measurement techniques. Despite the importance of regenerating forests on the global carbon balance, the processes affecting the carbon cycle within these forests is not well understood. Airborne scanning light detection and ranging (lidar) instruments provide new opportunities to examine three-dimensional forest characteristics from the level of individual trees to ecosystems and beyond. Lidar is therefore an effective link between plot measurements, eddy covariance, and low resolution remote sensing pixels. This thesis dissertation presents new science on the use of airborne lidar for evaluating remote sensing products within heterogeneous and previously clearcut ecosystems. The goals of this thesis were to first understand the processes affecting CO2 exchanges within a previously disturbed boreal jack pine chronosequence located in Saskatchewan, Canada and then to apply this understanding to evaluate low resolution remote sensing data products from the Moderate Resolution Imaging Spectroradiometer (MODIS) using airborne lidar. The first objective of this dissertation examined the factors that control light use efficiency (LUE) within the jack pine chronosequence during dry and wet years. The second objective examined the importance of vegetation structure and ground surface elevation on CO2 fluxes within a mature jack pine forest. The third objective developed and tested a simple model of lidar fractional cover and related this to the fraction of photosynthetically active radiation absorbed by the canopy (fPAR). This was then used to evaluate the MODIS fPAR product across the lower part of a watershed. Finally, the fourth objective was to model gross primary production (GPP) from airborne lidar. Lidar estimates of GPP were then compared with those from the EC system at the jack pine chronosequence and with the MODIS GPP (Collection 5) product. / Thesis (Ph.D, Geography) -- Queen's University, 2008-08-22 08:50:51.44

No description available.

This thesis provides a new explanation for the behaviour of radar backscatter of forests using vegetation structure models from the field of macroecology. The forests modelled in this work are produced using allometry-based ecological models with backscatter derived from the parameterisation of a radiative transfer model. This work is produced as a series of papers, each portraying the importance of macroecology in defining the forest radar response. Each contribution does so by incorporating structural and dynamic effects of forest growth using one of two allometric models to expose variations in backscatter as a response to vertical and horizontal forest profiles. The major findings of these studies concern the origin of backscatter saturation effects from forest SAR surveys. In each work the importance of transition from Rayleigh to Optical scattering, combined with the scaling effects of forest structure, is emphasised. These findings are administered through evidence including the transition’s emergence as the region of dominant backscatter in a vertical profile (according to a dominant canopy scattering layer), also through the existence of a two trend backscatter relationship with volume in the shape of the typical “saturation curve” (in the absence of additional attenuating factors). The importance of scattering regime change is also demonstrated through the relationships with volume, basal area and thinning. This work’s findings are reinforced by the examination of the relationships between forest height and volume, as collective values, providing evidence to suggest the non-uniqueness of volume-toheight relationships. Each of the studies refer to growing forest communities not single trees, so that unlike typical studies of radar remote sensing of forests the impact of the macroecological structural aspects are more explicit. This study emphasises the importance of the overall forest structure in producing SAR backscatter and how backscatter is not solely influenced by electrical properties of scatteres or the singular aspects of a tree but also by the collective forest parameters defining a dynamically changing forest.

Natural fire return intervals are relatively long in eastern Canadian boreal forests and often allow for the development of stands with multiple, successive cohorts of trees. Multi-cohort forest management (MCM) provides a strategy to maintain such multi-cohort stands that focuses on three broad phases of increasingly complex, post-fire stand development, termed “cohorts”, and recommends different silvicultural approaches be applied to emulate different cohort types. Previous research on structural cohort typing has relied upon primarily subjective classification methods; in this thesis, I develop more comprehensive and objective methods for three common boreal mixedwood and black spruce forest types in northeastern Ontario. Additionally, I examine relationships between cohort types and stand age, productivity, and disturbance history and the utility of airborne LiDAR to retrieve ground based classifications and to extend structural cohort typing from plot to stand-levels. In both mixedwood and black spruce forest types, stand age and age related deadwood features varied systematically with cohort classes in support of an age-based interpretation of increasing cohort complexity. However, correlations of stand age with cohort classes were surprisingly weak. Differences in site productivity had a significant effect on the accrual of increasingly complex multi-cohort stand structure in both forest types, especially in black spruce stands. The effects of past harvesting in predictive models of class membership were only significant when considered in isolation of age. As an age emulation strategy, the three cohort model appeared to be poorly suited to black spruce forests where the accrual of structural complexity appeared to be more a function of site productivity than age. Airborne LiDAR data appear to be particularly useful in recovering plot-based cohort types and extending them to the stand-level. The main gradients of structural variability detected using LiDAR were similar between boreal mixedwood and black spruce forest types; the best LiDAR-based models of cohort type relied upon combinations of tree size, size heterogeneity, and tree density related variables. The methods described here to measure, classify, and predict cohort-related structural complexity assist in translating the conceptual three cohort model to a more precise, measurement based management system. In addition, the approaches presented here to measure and classify stand structural complexity promise to significantly enhance the detail of structural information in operational forest inventories in support of a wide array of forest management and conservation applications.

Understanding how structure and function change across environmental gradients is a fundamental goal of ecology, with important applications in a changing world. In this dissertation, I explore how environmental variations in temperature and precipitation affect three-dimensional canopy structure, and how this, in turn, affects forest function. Characterising how climatic variations affect forest structure and function is particularly important in tropical forests, which are globally important carbon stores that have already shown vulnerability to climate change. The future of tropical forest carbon stocks is highly uncertain, with plant physiological responses representing the largest source of model uncertainties. As such, my dissertation research comprises empirical investigations into how tropical forests will respond to high temperatures and drought. Firstly, I examine tropical forest response to high temperature by conducting a comparison of natural forest sites and a tropical forest mesocosm using eddy-covariance data. I present evidence that high temperature declines in tropical forest photosynthesis are not due to direct temperature effects (i.e., that cause damage to the photosynthetic machinery), but instead are predominantly due to indirect temperature effects that result from concurrent increases in vapour pressure deficit (VPD). While both mechanisms reduce photosynthesis, the impact of increased VPD under future climate may be partly mitigated by enhanced water-use efficiency associated with rising atmospheric CO2 concentrations, suggesting that tropical forests may have opportunities for resilience in the face of global warming. The second part of my dissertation research examines how tropical forest canopy structure responds to seasonal dry periods and anomalous droughts on seasonal and interannual timescales, using data from ground-based LiDAR (Light Detection and Ranging). I show that total leaf area index (LAI) does not represent the seasonality of forest structure, since the upper and lower canopy levels exhibit divergent seasonal responses. The seasonal pattern of upper canopy LAI shows good agreement with the seasonal pattern of enhanced vegetation index (EVI) measured from satellites, suggesting that satellites are not capturing the response of the lower canopy. These results indicate that smaller trees are responding to seasonal water limitations and larger trees to light availability. I found that the response of canopy structure to anomalous (El Niño-induced) drought was similar to seasonal dry periods, but that the trends in LAI and vertical canopy structure were amplified. In particular, I document a delayed loss of LAI from the upper canopy following extreme drought, which supports the idea that while smaller trees may be more responsive to shorter, less severe dry periods, larger trees are more susceptible to prolonged or more severe droughts. Finally, I combine a long-term ground-based LiDAR dataset with tree inventory data in order to identify the mechanisms (i.e., changes in leaf area and/or woody biomass) of structural changes caused by droughts. I present evidence that loss of lower canopy LAI following an El Niño-induced drought was due to the mortality of small trees, not loss of leaf area, while an increase in LAI in the upper canopy predominantly resulted from plastic leaf area changes. If small trees are susceptible to drought-induced mortality and the incidence of droughts increases, this could prevent the recovery of tropical forests from drought-induced disturbances.

Across the southern slope of the Nepal Himalaya lie large areas of upper-elevation broadleaf evergreen forest. Resources extracted by subsistence villagers include hand-cut tree-leaf fodder for livestock, fuelwood, and poles for construction of herders’ shelters. Indigenous use of forest products may be altering forest structure and resource availability in Nepal. This research examined forest structure and its relationships with human use of forest products near the upper-elevation village of Chimkhola in west-central Nepal. In the 150-ha forest study area, mean density and standard error (SE) of trees >10 cm dbh was 817 (30) stems ha⁻¹ and mean (SE) total basal area was 44 (3.2) m² ha⁻¹. Cluster analysis of density data for 32 tree species suggested three forest communities: a Symplocos-Quercus community, a Symplocos-mixed evergreen community, and a Rhododendron-Symplocos community. Ordination by principal components analysis of tree species densities indicated a relationship between community structure and a cutting index that increases with harvest intensity. Size-class distributions of important fodder-resource oak species suggested Quercus lamellosa is in decline, but Q. oxyodon and Q. semecarpifolia may be reproducing successfully. Age-diameter regression equations of three evergreen tree species showed growth rate of Symplocos ramosissima to be approximately twice that of Machilus duthei or that of Lindera pulcherrima. The fast growth rate and shade tolerance of S. ramosissima appear to confer a successional advantage. I assessed sustainability of harvest of pole-size (5-10 cm dbh) S. ramosissima used in herders’ shelters, by comparison of harvest and replenishment rates. Mean (SE) density of pole-size S. ramosissima was 375 (32) stems ha⁻¹ in the study area. If the total harvest was distributed evenly across the forest area, it would be 34 stems ha⁻¹ yr⁻¹ and would not exceed the replenishment rate. However, harvest was concentrated in the Symplocos-Quercus community, closest to the village and agricultural fields. Using stump counts as a proxy of relative harvest intensity, harvest rates were sustainable in the Symplocos-mixed evergreen and Rhododendron-Symplocos communities, but not sustainable in the Symplocos-Quercus community.

Natural fire return intervals are relatively long in eastern Canadian boreal forests and often allow for the development of stands with multiple, successive cohorts of trees. Multi-cohort forest management (MCM) provides a strategy to maintain such multi-cohort stands that focuses on three broad phases of increasingly complex, post-fire stand development, termed “cohorts”, and recommends different silvicultural approaches be applied to emulate different cohort types. Previous research on structural cohort typing has relied upon primarily subjective classification methods; in this thesis, I develop more comprehensive and objective methods for three common boreal mixedwood and black spruce forest types in northeastern Ontario. Additionally, I examine relationships between cohort types and stand age, productivity, and disturbance history and the utility of airborne LiDAR to retrieve ground based classifications and to extend structural cohort typing from plot to stand-levels. In both mixedwood and black spruce forest types, stand age and age related deadwood features varied systematically with cohort classes in support of an age-based interpretation of increasing cohort complexity. However, correlations of stand age with cohort classes were surprisingly weak. Differences in site productivity had a significant effect on the accrual of increasingly complex multi-cohort stand structure in both forest types, especially in black spruce stands. The effects of past harvesting in predictive models of class membership were only significant when considered in isolation of age. As an age emulation strategy, the three cohort model appeared to be poorly suited to black spruce forests where the accrual of structural complexity appeared to be more a function of site productivity than age. Airborne LiDAR data appear to be particularly useful in recovering plot-based cohort types and extending them to the stand-level. The main gradients of structural variability detected using LiDAR were similar between boreal mixedwood and black spruce forest types; the best LiDAR-based models of cohort type relied upon combinations of tree size, size heterogeneity, and tree density related variables. The methods described here to measure, classify, and predict cohort-related structural complexity assist in translating the conceptual three cohort model to a more precise, measurement based management system. In addition, the approaches presented here to measure and classify stand structural complexity promise to significantly enhance the detail of structural information in operational forest inventories in support of a wide array of forest management and conservation applications.

No description available.

The effects of two forms of mid-rotation release on thinned, fertilized loblolly pine (Pinus taeda L.) plantations in Virginia were examined: aerial imazapyr and basal triclopyr application. Imazapyr measurement plots were installed in nine Piedmont and twelve Coastal Plain plantations operationally released with imazapyr, and triclopyr measurement plots were installed within a controlled fertilization/release study spanning both regions. No differences in volume were detected following triclopyr release. All release dates combined, Piedmont released areas averaged 0.06 m3/tree (18%) greater than the control and Coastal Plain released areas averaged 0.05 m3/tree (14%) greater than the control. Reductions in hardwood basal area, stem density, and shrub stratum cover were observed for both forms of release. Reductions in shrub stratum richness and diversity were also documented for imazapyr release; however, trends indicate that richness and diversity, as well as stem density and shrub stratum cover, may recover to pre-treatment levels. Herbaceous vegetation was increased on triclopyr sites, which was reflected in an elevated turkey (Meleagris gallopavo sylvestris L.) food/brood index. Following imazapyr release, habitat suitability index (HSI) values for pine warblers (Dendroica pinus L.) and black-capped chickadees (Parus atricapillus L.) increased due to reductions in canopy hardwoods and increases in snags. Reduced shrub stratum density resulted in a lower bobwhite quail (Colinus virginianus L.) cover index on imazapyr-released areas. / Master of Science