Quantification of Hydrologic Response to Forest Disturbance in Western U.S. Watersheds

Forested watersheds produce more than half of the water supply in the United States. Forests affect how precipitation is partitioned into available water versus evapotranspiration. This dissertation investigated how water yield and snowpack responded to forest disturbance following recent disturbances in western U.S. forests during the period 2000-2019. Chapter 2 systematically reviewed 78 recent studies that examined how water yield or snowpack changed after forest disturbances. Water yield and snowpack often increased after disturbance, but decreased in some circumstances. Decreased water yield was most likely to occur following disturbances that did not remove the entire forest canopy. It was also more likely to occur in more arid watersheds at lower latitudes, such as in the southwestern U.S., and on south-facing aspects. Chapter 3 examined 159 watersheds across the western U.S. to determine how often and where water yield increased or decreased following forest disturbance. Overall, more severe forest disturbances, particularly in relatively wet watersheds such as in the Northern Rocky Mountains or Pacific Northwest, were more likely to produce larger water yield. However, forest disturbances in very arid watersheds, such as those in the southwestern U.S., were more likely to result in less water yield. Chapter 4 developed a new method for more precisely mapping forest canopies and understory forest vegetation. This method used data collected by the U.S. Forest Service’s Forest Inventory and Analysis Program. The maps of separate forest canopy and understory vegetation layers are expected to allow hydrologists to make more accurate predictions regarding the effects of future vegetation changes on water supply. Previous studies that monitored water yield before and after clearcut timber harvests concluded that forest disturbances would lead to increased water yield. In contrast, the work presented here found that disturbances that do not remove the entire canopy (e.g., due to insects, drought, disease, thinning, low-severity wildfire) may lead to different water yield and snowpack responses than disturbances that remove the entire canopy (e.g., clearcut harvesting, severe wildfire). This work has therefore helped us better understand how future water supply, for people and for ecosystems, will be affected by future forest changes

Quantification of Hydrologic Response to Forest Disturbance in Western U.S. Watersheds

Forested watersheds produce more than half of the water supply in the United States. Forests affect how precipitation is partitioned into available water versus evapotranspiration. This dissertation investigated how water yield and snowpack responded to forest disturbance following recent disturbances in western U.S. forests during the period 2000-2019. Chapter 2 systematically reviewed 78 recent studies that examined how water yield or snowpack changed after forest disturbances. Water yield and snowpack often increased after disturbance, but decreased in some circumstances. Decreased water yield was most likely to occur following disturbances that did not remove the entire forest canopy. It was also more likely to occur in more arid watersheds at lower latitudes, such as in the southwestern U.S., and on south-facing aspects. Chapter 3 examined 159 watersheds across the western U.S. to determine how often and where water yield increased or decreased following forest disturbance. Overall, more severe forest disturbances, particularly in relatively wet watersheds such as in the Northern Rocky Mountains or Pacific Northwest, were more likely to produce larger water yield. However, forest disturbances in very arid watersheds, such as those in the southwestern U.S., were more likely to result in less water yield. Chapter 4 developed a new method for more precisely mapping forest canopies and understory forest vegetation. This method used data collected by the U.S. Forest Service’s Forest Inventory and Analysis Program. The maps of separate forest canopy and understory vegetation layers are expected to allow hydrologists to make more accurate predictions regarding the effects of future vegetation changes on water supply. Previous studies that monitored water yield before and after clearcut timber harvests concluded that forest disturbances would lead to increased water yield. In contrast, the work presented here found that disturbances that do not remove the entire canopy (e.g., due to insects, drought, disease, thinning, low-severity wildfire) may lead to different water yield and snowpack responses than disturbances that remove the entire canopy (e.g., clearcut harvesting, severe wildfire). This work has therefore helped us better understand how future water supply, for people and for ecosystems, will be affected by future forest changes

A Landscape-Level Assessment of Whitebark Pine Regeneration in the Rocky Mountains, USA

Whitebark pine (Pinus albicaulis Engelm.) has recently experienced high mortality due to multiple stressors, and future population viability may rely on natural regeneration. We assessed whitebark pine seedling densities throughout the US Rocky Mountains and identified stand, site, and climatic variables related to seedling presence based on data from 1,217 USDA Forest Service Forest Inventory and Analysis plots. Although mean densities were highest in the whitebark pine forest type, 83% of sites with seedlings present occurred in non-whitebark pine forest types, and the highest densities occurred in the lodgepole pine forest type. To identify factors related to whitebark pine seedling presence, we compared the results generated from three statistical models: logistic regression, classification tree, and random forests. All three models identified cover of grouse whortleberry (Vaccinium scoparium Leiberg ex Coville) as an important predictor, two models distinguished live and dead whitebark pine basal area and elevation, and one model recognized seasonal temperature. None of the models identified forest type as an important predictor. Understanding these factors may help managers identify areas where natural regeneration of whitebark pine is likely to occur, including sites in non-whitebark pine forest types

Natural regeneration of whitebark pine: Factors affecting seedling density across Idaho, Montana, and Wyoming

Whitebark pine (Pinus albicaulis) is an ecologically important species in high-altitude areas of the West due to the food source it provides for Clark’s nutcrackers, red squirrels, grizzly bears, and other animals. Whitebark pine stands have recently experienced high mortality due to wildfire, white pine blister rust, and a mountain pine beetle outbreak, leading several researchers and managers to question the species’ long-term viability. This study examined regeneration at over 1,000 Forest Inventory and Analysis (FIA) plots containing a whitebark pine component (i.e., any dead whitebark pine trees larger than 5 inches d.b.h. or live whitebark pines of any size) in the northern Rocky Mountains. Objectives were to characterize the population’s age and size structures, as well as identify factors that influence whitebark pine regeneration. Mean seedling density at FIA plots ranged from zero to over 3,000 seedlings per acre, with a mean density of about 300 seedlings per acre and a median density of about 110 seedlings per acre. At the landscape scale, whitebark pine’s age classes and size classes both show a steep reverse-j distribution. A two-stage modeling approach was used to relate site-specific and climate variables first to presence/absence of whitebark pine seedlings, and then to seedling density. Preliminary results suggest that regeneration is most strongly related to the density of understory vegetation, particularly the shrub Vaccinium scoparium, as well as seedling density of other tree species. Species composition of the overstory was more important than indicators of overstory density, including tree canopy cover and basal area. With respect to temperature and precipitation, the relative importance of mean versus variability metrics differed by season

Spatially Distributed Overstory and Understory Leaf Area Index Estimated from Forest Inventory Data

Forest change affects the relative magnitudes of hydrologic fluxes such as evapotranspiration (ET) and streamflow. However, much is unknown about the sensitivity of streamflow response to forest disturbance and recovery. Several physically based models recognize the different influences that overstory versus understory canopies exert on hydrologic processes, yet most input datasets consist of total leaf area index (LAI) rather than individual canopy strata. Here, we developed stratum-specific LAI datasets with the intent of improving the representation of vegetation for ecohydrologic modeling. We applied three pre-existing methods for estimating overstory LAI, and one new method for estimating both overstory and understory LAI, to measurements collected from a probability-based plot network established by the US Forest Service’s Forest Inventory and Analysis (FIA) program, for a modeling domain in Montana, MT, USA. We then combined plot-level LAI estimates with spatial datasets (i.e., biophysical and remote sensing predictors) in a machine learning algorithm (random forests) to produce annual gridded LAI datasets. Methods that estimate only overstory LAI tended to underestimate LAI relative to Landsat-based LAI (mean bias error ≥ 0.83), while the method that estimated both overstory and understory layers was most strongly correlated with Landsat-based LAI (r2 = 0.80 for total LAI, with mean bias error of -0.99). During 1984-2019, interannual variability of understory LAI exceeded that for overstory LAI; this variability may affect partitioning of precipitation to ET vs. runoff at annual timescales. We anticipate that distinguishing overstory and understory components of LAI will improve the ability of LAI-based models to simulate how forest change influences hydrologic processes

Spatially Distributed Overstory and Understory Leaf Area Index Estimated from Forest Inventory Data

Forest change affects the relative magnitudes of hydrologic fluxes such as evapotranspiration (ET) and streamflow. However, much is unknown about the sensitivity of streamflow response to forest disturbance and recovery. Several physically based models recognize the different influences that overstory versus understory canopies exert on hydrologic processes, yet most input datasets consist of total leaf area index (LAI) rather than individual canopy strata. Here, we developed stratum-specific LAI datasets with the intent of improving the representation of vegetation for ecohydrologic modeling. We applied three pre-existing methods for estimating overstory LAI, and one new method for estimating both overstory and understory LAI, to measurements collected from a probability-based plot network established by the US Forest Service’s Forest Inventory and Analysis (FIA) program, for a modeling domain in Montana, MT, USA. We then combined plot-level LAI estimates with spatial datasets (i.e., biophysical and remote sensing predictors) in a machine learning algorithm (random forests) to produce annual gridded LAI datasets. Methods that estimate only overstory LAI tended to underestimate LAI relative to Landsat-based LAI (mean bias error ≥ 0.83), while the method that estimated both overstory and understory layers was most strongly correlated with Landsat-based LAI (r2 = 0.80 for total LAI, with mean bias error of -0.99). During 1984-2019, interannual variability of understory LAI exceeded that for overstory LAI; this variability may affect partitioning of precipitation to ET vs. runoff at annual timescales. We anticipate that distinguishing overstory and understory components of LAI will improve the ability of LAI-based models to simulate how forest change influences hydrologic processes

Pinus albicaulis Engelm. (Whitebark Pine) in Mixed-Species Stands throughout Its US Range: Broad-Scale Indicators of Extent and Recent Decline

We used data collected from >1400 plots by a national forest inventory to quantify population-level indicators for a tree species of concern. Whitebark pine (Pinus albicaulis) has recently experienced high mortality throughout its US range, where we assessed the area of land with whitebark pine present, size-class distribution of individual whitebark pine, growth rates, and mortality rates, all with respect to dominant forest type. As of 2016, 51% of all standing whitebark pine trees in the US were dead. Dead whitebark pines outnumbered live ones—and whitebark pine mortality outpaced growth—in all size classes ≥22.8 cm diameter at breast height (DBH), across all forest types. Although whitebark pine occurred across 4.1 million ha in the US, the vast majority of this area (85%) and of the total number of whitebark pine seedlings (72%) fell within forest types other than the whitebark pine type. Standardized growth of whitebark pines was most strongly correlated with the relative basal area of whitebark pine trees (rho = 0.67; p < 0.01), while both standardized growth and mortality were moderately correlated with relative whitebark pine stem density (rho = 0.39 and 0.40; p = 0.031 and p < 0.01, respectively). Neither growth nor mortality were well correlated with total stand basal area, total stem density, or stand mean diameter. The abundance, extent, and relative growth vs. mortality rates of whitebark pine in multiple forest types presents opportunities for management to encourage whitebark pine recruitment in mixed-species stands. The lodgepole pine forest type contained more whitebark pine seedlings (35%) than any other forest type, suggesting that this forest type represents a potential management target for silvicultural treatments that seek to facilitate the recruitment of whitebark pine seedlings into larger size classes. National forest inventories in other countries may use a similar approach to assess species of concern

Western Larch Regeneration Responds More Strongly to Site and Indirect Climate Factors Than to Direct Climate Factors

Substantial shifts in the distribution of western larch (Larix occidentalis Nutt.) are predicted during the coming decades in response to changing climatic conditions. However, it is unclear how the interplay between direct climate effects, such as warmer, drier conditions, and indirect climate effects, such as predicted increases in fire disturbance, will impact fire-adapted species such as western larch. The objectives of this study were (1) to compare the relative importance of stand, site, and indirect versus direct climatic factors in determining western larch seedling recruitment; (2) to determine whether seedling recruitment rates have changed in recent years in response to disturbance, post-fire weather, and/or climate; and (3) to determine whether seedlings and mature trees are experiencing niche differentiation based on recent climatic shifts. We addressed these objectives using data collected from 1286 national forest inventory plots in the US states of Idaho and Montana. We used statistical models to determine the relative importance of 35 stand, site, and climatic factors for larch seedling recruitment. Our results suggest that the most important predictors of larch seedling recruitment were indicative of early-seral stand conditions, and were often associated with recent fire disturbance and cutting. Despite indications of climatic niche compression, seedling recruitment rates have increased in recent decades, likely due to increased fire disturbance, and were unrelated to post-fire weather. Compared to sites occupied by mature trees, seedling recruitment was positively associated with cooler, drier climatic conditions, and particularly with cooler summer temperatures, but these climatic factors were generally less important than biotic stand variables such as stand age, basal area, and canopy cover. These results suggest that, for fire-dependent species such as western larch, increased heat and drought stress resulting from climatic change may be offset, at least in the near term, by an increase in early-seral stand conditions resulting from increased fire disturbance, although localized range contraction may occur at warm, dry extremes