Restoring Forest Resilience in the Sierra Nevada Mixed-conifer Zone, with a Focus on Measuring Spatial Patterns of Trees Using Airborne Lidar

Restoring Forest Resilience in the Sierra Nevada Mixed-conifer Zone, with a Focus on Measuring Spatial Patterns of Trees Using Airborne Lidar
Author: Sean Medeiros Alexander Jeronimo
Publisher:
Total Pages: 308
Release: 2018
Genre:
ISBN:

In this dissertation I present three studies incorporating lidar data into different aspects of forest restoration. All studies use lidar individual tree detection as source data, in part to enable making measurements of tree spatial patterns in terms of tree clumps and canopy openings. This common focus exists because spatial patterns of trees influence fire and insect behavior, snow retention, tree regeneration, and other key ecosystem functions and services for which humans manage forests. In Chapter 1 I sought to provide this dataset by asking these questions: (1) What is the geographic and environmental distribution of restored active-fire forest patches in the Sierra Nevada mixed-conifer zone? (2) What are the ranges of variation in structure and spatial patterns across restored patches? (3) How do density, tree clumping, and canopy opening patterns vary by topography and climate in restored patches? I analyzed fire history and environmental conditions over 10.8 million ha, including 3.9 million ha in the Sierra Nevada mixed-conifer zone, and found that the 30,379 ha of restored patches were distributed throughout the range but were more abundant on National Park lands (81% of restored areas) than National Forest lands and were positively correlated with lightning strike density. Furthermore, 33% of restored areas were located in western Yosemite National Park and met our criteria for inclusion in this study only after being burned at low and moderate severity in the 2013 Rim Fire. Lidar-measured ranges of variation in reference condition structure were broad, with density ranging from 6-320 trees ha−1 (median 107 trees ha−1), basal area from 2-113 m2 ha−1 (median 21 m2 ha−1), average size of closely associated tree clumps from 1 to 207 trees (median 3.1 trees), and average percent of stand area >6 m from the nearest canopy ranging from 0% to 100% (median 5.1%). These ranges matched past studies reporting density and spatial patterns of contemporary and historical active-fire reference stands in the Sierra Nevada, except this study observed longer tails on distributions due to the spatial completeness of lidar sampling. Reference areas in middle-elevation climate zones had lower density (86 vs. 121 trees ha-1), basal area, (13.7 vs. 31 m2 ha-1), and mean clump size (2.7 vs. 4.0 trees) compared to lower- and higher-elevation classes, while ridgetops had lower density (101 vs. 115 trees ha-1), basal area (19.6 vs. 24.1 m2 ha-1), and mean clump size (3.0 vs. 3.3 trees) but more open space (7.4% vs. 5.1%) than other landforms. In Chapter 2 I developed new methods for integrating lidar data into silvicultural planning at treatment unit and project area scales, with a focus on dry forest restoration treatments. At the stand scale my objective was to delineate the decision space for prescription parameters like density, basal area, and spatial patterns given the soft constraints of reference conditions and the hard constraints of possible transitions given current structure. At the landscape scale my objective was to provide a framework for selecting from available treatment options, stand by stand, to meet different landscape-level goals. I applied the new methods to a case study area in the Lake Tahoe Basin, California and asked in this context: How do structural departures from reference conditions and associated treatment prescriptions vary with topographic position and aspect? I found that ridges and southwest-facing slopes in the study area had a greater degree of departure from the reference envelope and required more density reduction compared to valleys and northeast-facing slopes. In Chapter 3 I used pre- and post-Rim Fire data from the 25.6 ha Yosemite Forest Dynamics Plot (YFPD) to build a model of tree mortality predicted from lidar individual tree detection structural metrics. I calculated metrics at the scale of lidar-detected trees (termed tree-approximate objects, TAOs), at the scale of 0.1 ha plots centered on each TAO, and at the 90×90 m neighborhood scale. I used these to predict TAO mortality at the neighborhood scale and TAO mortality class – immediate or delayed mortality – at the TAO scale. I also tested the inclusion of a set of topoedaphic and burn weather predictors as well as a cross-scale interaction term between the TAO mortality model and the neighborhood-level mortality model. I asked these questions: (1) How does mortality progress 1-4 years post-fire in terms of rates, demographics, and agents? (2) What elements of forest structure and pattern predict immediate and delayed post-fire mortality at scales from TAOs to neighborhoods? (3) How does the prevalence of different mortality agents vary with changes in the important fine-scale predictors of fire mortality? I found that smaller trees were killed in the first year with a 40% mortality rate and the average diameter of killed trees increased each subsequent year while the mortality rate decreased. The topoedaphic and burn weather predictors as well as the cross-scale interaction improved model fit and parsimony, but that the improvement was directional, i.e., including neighborhood-level information improved the TAO-level model but not vice-versa. Important predictors fell into categories of fuel amount, fuel configuration, and burning conditions. Amounts of crown damage for immediately killed trees were higher for TAOs shorter than 51 m and in 0.1 ha areas where mean clump sizes was less than 21 TAOs. The amount of delayed mortality that was directly fire-related was higher when TAO crown base heights were less than 28 m and TAO density in 0.1 ha areas was greater than 170 TAOs ha-1. Crown base heights over 18 m and local TAO density of less than 180 TAOs ha-1 had more beetle kill and less rot. The model performed similarly well on an independent validation dataset of 48 0.25 ha plots spanning the footprint of the Rim Fire within Yosemite as on the YFDP training data, indicating that the model is widely applicable.

Quantifying Forest Structure Parameters and Their Changes from LiDAR Data and Satellite Imagery in the Sierra Nevada

Quantifying Forest Structure Parameters and Their Changes from LiDAR Data and Satellite Imagery in the Sierra Nevada
Author: Qin Ma
Publisher:
Total Pages: 284
Release: 2018
Genre:
ISBN:

Sierra Nevada forests have provided many economic benefits and ecological services to people in California, and the rest of the world. Dramatic changes are occurring in the forests due to climate warming and long-term fire suppression. Accurate mapping and monitoring are increasingly important to understand and manage the forests. Light Detection and Range (LiDAR), an active remote sensing technique, can penetrate the canopy and provide three-dimensional estimates of forest structures. LiDAR-based forest structural estimation has been demonstrated to be more efficient than field measurements and more accurate than those from passive remote sensing, like satellite imagery. Research in this dissertation aims at mapping and monitoring structural changes in Sierra Nevada forests by taking the advantages of LiDAR. We first evaluated LiDAR and fine resolution imagery-derived canopy cover estimates using different algorithms and data acquisition parameters. We suggested that LiDAR data obtained at 1 point/m2 with a scan angle smaller than 12°were sufficient for accurate canopy cover estimation in the Sierra Nevada mix-conifer forests. Fine resolution imagery is suitable for canopy cover estimation in forests with median density but may over or underestimate canopy cover in extremely coarse or dense forests. Then, a new LiDAR-based strategy was proposed to quantify tree growth and competition at individual tree and forest stand levels. Using this strategy, we illustrated how tree growth in two Sierra Nevada forests responded to tree competition, original tree sizes, forest density, and topography conditions; and identified that the tree volume growth was determined by the original tree sizes and competitions, but tree height and crown area growth were mostly influenced by water and space availability. Then, we calculated the forest biomass disturbance in a Sierra Nevada forest induced by fuel treatments using bi-temporal LiDAR data and field measurements. Using these results as references, we found that Landsat imagery-derived vegetation indices were suitable for quantifying canopy cover changes and biomass disturbances in forests with median density. Large uncertainties existed in applying the vegetation indices to quantify disturbance in extremely dense forests or forests only disturbed in the understory. Last, we assessed vegetation losses caused by the American Fire in 2013 using a new LiDAR point based method. This method was able to quantify fire-induced forest structure changes in basal area and leaf area index with lower uncertainties, compared with traditional LiDAR metrics and satellite imagery-derived vegetation indices. The studies presented in this dissertation can provide guidance for forest management in the Sierra Nevada, and potentially serve as useful tools for forest structural change monitoring in the rest of the world.

Effects of Variable Density Thinning on Spatial Patterns of Overstory Trees in Mt. Hood National Forest

Effects of Variable Density Thinning on Spatial Patterns of Overstory Trees in Mt. Hood National Forest
Author:
Publisher:
Total Pages: 46
Release: 2018
Genre: Douglas fir
ISBN:

Variable density thinning (VDT) is a method of restoration thinning that attempts to increase ecosystem resilience and spatial heterogeneity in forest stands to more closely resemble mosaic-like patterns characteristic of late-successional forests, which consist of clusters of multiple trees, individual trees, and gaps. This study examines the spatial patterning of overstory trees resulting from VDT of conifer forests in Mt. Hood National Forest in the western Cascade Mountains and compares these patterns with reference conditions. Stem maps were created from field surveys of study plots within one mature stand and six thinned stands designated as Late-Successional Reserve (LSR) with varying minimum inter-tree spacing distances and implementation methods (designation by description and designation by prescription). A cluster analysis and global point pattern analysis were conducted for each of the seven stands. Spacing-based prescriptions below 15 feet resulted in approximately twice as many trees belonging to large clusters compared to reference conditions. Additionally, the results suggest that the designation by prescription method produces forest spatial patterns that are more similar to reference conditions than the designation by description method. This suggests that more flexible prescriptions that incorporate site-specific information should be utilized for restoration thinning in LSR stands.

Restoring Spatial Pattern to Southwestern Ponderosa Pine Forests

Restoring Spatial Pattern to Southwestern Ponderosa Pine Forests
Author: Dave Egan
Publisher:
Total Pages: 12
Release: 2008
Genre: Forest restoration
ISBN:

Until recently, forest managers have largely ignored the value of maintaining dynamic spatial patterns in forested ecosystems. In the America Southwest, where the norm in overstocked forests that are extremely susceptible to catastrophic fires and/or insect infestations and disease, restoring a spatial pattern of openings and tree groups would help alleviate these threats and move the forests within their historic range of variability. This ERI working paper focuses on restoring a dynamic spatial pattern to ponderosa pine forests in the American Southwest. It also addresses basic questions that land managers and others have about how to restore active spatial patterns across the forested Southwest.

Quantifying and Restoring Stand-level in Dry Forests of the Eastern Washington Cascades

Quantifying and Restoring Stand-level in Dry Forests of the Eastern Washington Cascades
Author: Derek John Churchill
Publisher:
Total Pages: 152
Release: 2013
Genre: Ecological heterogeneity
ISBN:

There is increasing evidence that spatial heterogeneity at multiple scales is a critical component of ecosystem resilience and adaptive capacity. In frequent-fire pine and mixed conifer forests in the western US, pre-settlement era forests were complex mosaics of individual trees, tree clumps, and openings. There is a broad scientific consensus that restoration treatments should seek to restore these mosaic patterns as these reference forests were adapted to frequent-fire and shifting climatic conditions. Yet, methods to quantify and incorporate spatial reference information into restoration treatments are not widely used. In addition, targets from reference conditions must be critically evaluated in light of climate change. In this dissertation, I develop a new set of spatial metrics to quantify within-stand pattern in terms of widely spaced individual trees, tree clumps, and openings (ICO). Within 0.5 ha tree neighborhoods, I found evidence that a definable range and distribution, or envelope, of pattern and structure was present. This envelope ranged from low density patterns with few clumps and high opening levels, to patterns with a mid-range of density and varying levels of clumping, to high density, highly clumped patterns. The envelope was constrained by an upper limit of clump size, maximum density levels well below site potential, and the presence of at least some clumping in all plots. Across 3 x 6ha plots, tree neighborhood patterns of clumps and openings were spatially dependent. Aggregations of large clumps formed sub-patches that occupied 7-16% of plot area. A gradient of low to moderate density with low levels of clumping was found on the remainder. A silvicultural approach to translating reference patterns into restoration prescriptions and monitoring protocols was also developed and applied in a case study. Treatments using this ICO approach resulted in a distribution of tree clumps and openings within the range of reference envelopes. I also developed a method based on climatic water balance parameters, downscaled climate projections, and plant associations to assess historical reference sites in the context of projected future climate and identify climate analogue reference conditions.