publications

2024

1. Landsat assessment of variable spectral recovery linked to post-fire forest structure in dry sub-boreal forests

   Smith-Tripp, Sarah M., Coops, Nicholas C., Mulverhill, Christopher, White, Joanne C., and Axelson, Jodi

   ISPRS Journal of Photogrammetry and Remote Sensing Feb 2024

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   Forest disturbances such as wildfires can dramatically alter forest structure and composition, increasing the likelihood of ecosystem changes. Up-to-date and accurate measures of post-disturbance forest recovery in managed forests are critical, particularly for silvicultural planning. Measuring the live and dead vegetation post-fire is challenging because areas impacted by wildfire may be remote, difficult to access, and/or dangerous to survey. The difficulties of post-fire monitoring are compounded by the global increase in the frequency and severity of disturbances, as expansion of disturbed areas also increases the number and size of areas requiring post-disturbance monitoring. Methods that safely, efficiently, and extensively differentiate silviculturally beneficial coniferous growth from barren ground or deciduous shrubs are necessary to inform post-fire forest management. Satellite imagery can detect burn patterns, but monitoring changes in forest structure post fire is challenging due to complex vegetation responses. To overcome this challenge, this study combines post-disturbance spectral trajectory measures from a time series of historical Landsat imagery with field and remotely piloted aircraft (RPA) lidar (light detection and ranging) data to examine vegetation recovery of lodgepole pine (Pinus contorta) dominated sub-boreal forests after high-severity fires in 2006 in central British Columbia, Canada. Distinct spectral recovery trajectories were identified using data-clustering from a combination of seven Landsat spectral indices, with trajectories varying by recovery magnitude and rate. The forest structure associated with each distinct trajectory of spectral recovery was analyzed using 430 ha of spatially explicit forest structure measures (e.g., basal area, stem counts) and composition (e.g., percent coniferous) derived from 26 coincident field plots and high density RPA lidar (>200 points/m2) data. By comparing spectral trajectories to forest structure measures, we found the most spatially abundant cluster of spectral recovery coincided with a basal area of 0.62 m2/ha, high stem densities (>5000 stems/ha) and a high abundance of coniferous trees (>95 % coniferous). Around 10 % of the landscape was associated with relatively high abundance of deciduous vegetation (>20 %) in addition to very high conifer stem densities (>8000 stems/ha). By identifying the structural characteristics associated with unique Landsat spectral trajectories, we highlight the combined value of RPA lidar data and satellite image time series in providing a detailed and spatially explicit characterization of post-fire recovery relevant to managed forests.

2. Evaluating global vegetation products for application in heterogeneous forest-savanna landscapes

   Pletcher, Elise,  Smith-Tripp, Sarah, Evans, Damian, and Schwartz, Naomi B.

   International Journal of Remote Sensing Jan 2024

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   While satellite-derived global vegetation structure products are powerful and easy to use, their utility for studying spatial patterns within heterogeneous landscapes such as forest-savanna mosaics has not been extensively evaluated. We explored the application of global vegetation structure products in heterogeneous landscapes by comparing them with Airborne Laser Scanning. Specifically, we assessed the accuracy and bias of two fractional cover products, MODIS Vegetation Continuous Fields (VCF) and Hansen Global Forest Change (GFC), and one global canopy height model, Global Forest Canopy Height Model (CHM), in comparison with the same variables derived from local ALS point clouds. We found that there were limitations to all three products. MODIS VCF was less accurate than its reported accuracy by at least 3%, and GFC was 10% less accurate than MODIS VCF. While Global CHM had a similar magnitude of error to its reported product accuracy, product agreement was much lower (R2 0.19 vs. R2 0.61). We also found that the context of the analysis is important when choosing whether to use one fractional product over the other. Global products should be applied with caution in heterogeneous landscapes. Increased training and validation from these landscapes could improve the performance of these products and their utility for landscape-scale ecological research.

2022

1. Combining forest structure measurements with satellite spectral observations for forest recovery monitoring in burned environments of British Columbia, Canada

   Smith-Tripp, Sarah, Coops, Nicholas, and White, Joanne

   Jan 2022

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   Satellite-based methods to identify and map forest disturbances over broad spatial scales are well documented in the scientific literature and have improved in both spatial and temporal resolution with openly available time series data and computational capacity. Forest managers and silvicultural planners also require methods for post-disturbance monitoring of successful tree regeneration and ecosystem recovery. Broad-scale forest recovery monitoring is particularly important in western North America, which increasingly experiences large natural disturbance events, including wildfires, which pose a challenge for reforestation efforts Methods to monitor post-disturbance forest recovery using satellite imagery (including possible shifts in forest composition or type) are emerging and require verification and validation with complimentary indicators of successful regeneration, specifically measures of forest structure. Here, we focus on interior British Columbia, a timber dependent economic region that has experienced significant large wildfires over the last decade alongside historic mountain pine beetle outbreaks. We measured forest structure (e.g., tree density, height, etc.) using lidar data acquired from both aircraft and remotely piloted aircraft (RPA) in areas impacted by wildfires in 2010 and 2017. We then applied a space-for-time approach to relate spectral observations from Landsat time series data acquired five years post-fire to lidar and field measured forest structure. Our results indicate that the density of regenerating trees is accurately measured with lidar and that these measures of density relate to differences in spectral metrics observed at five-years post-fire. The overall goal of this research is to develop tools that aid landscapelevel silvicultural planning

2. Canopy height impacts on the growing season and monthly microclimate in a burned forest of British Columbia, Canada

   Smith-Tripp, Sarah M., Eskelson, Bianca N. I., Coops, Nicholas C., and Schwartz, Naomi B.

   Agricultural and Forest Meteorology Aug 2022

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   Forest canopies can buffer seedlings from extreme climate conditions. Yet, how disturbed forest canopies influence microclimate is not well understood, despite the important implications of microclimate for seedling establishment and post-disturbance successional trajectories. Better understanding of the relationship between a forest canopy and sub-canopy temperature and moisture conditions requires easily acquired and continuous forest canopy data, which is increasingly available due to new technology. Here, we measured canopy height using a remotely piloted aircraft (RPA) and monitored microclimate with low-cost temperature and soil moisture sensors in a sub-boreal forest impacted by fires of variable severity. We used regression models to investigate how differences in canopy height influenced microclimate variables. Mean growing season temperatures at -8 cm (soil), 0 cm (surface), and 15 cm (near-surface) relative to the ground surface were higher under shorter more disturbed canopies. Soil temperature was most sensitive to canopy height differences: linear models for the observed data range predicted a 2.0 °C increase in mean growing season soil temperature with every 10 m decrease in canopy height. We observed a weak negative relationship between canopy height and mean growing season soil moisture. We found that canopy height summarized at moderate resolution (15 m) better explained differences in temperature in our disturbed landscape. This work informs future methods to produce gridded microclimate datasets and outlines the impact of disturbed forest structure on microclimate variables. Our results show that the characteristics of the forest canopy remaining after a burn impact microclimates, which has important implications for post-fire ecosystems.

2021

1. Impacts of a regional multiyear insect defoliation event on growing-season runoff ratios and instantaneous streamflow characteristics

   Smith-Tripp, Sarah, Griffith, Alden, Pasquarella, Valerie J., and Matthes, Jaclyn Hatala

   Ecohydrology Aug 2021

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   Repeated moderate severity forest disturbances can cause short- and long-term shifts in ecosystem processes. Prior work has found that stand-replacing disturbances (e.g., clear-cutting) increase streamflow in temperate forests, but streamflow responses to repeated moderate severity disturbances are more equivocal. This study examined a moderate disturbance caused by an unexpected population irruption of the invasive insect Lymantria dispar (common name: gypsy moth) in 2015–2017. This irruption resulted in defoliation that was locally severe in some areas but spatially heterogeneous at a regional scale. L. dispar larvae consume leaves during the summer growing season, which effectively reduces tree leaf area and associated evapotranspiration. Our regional approach in Southern New England, USA, used data from 83 US Geological Survey (USGS) stream gages to assess whether changes in growing-season watershed runoff ratios and instantaneous streamflow characteristics during the 2015–2017 L. dispar irruption were associated with satellite-derived metrics of changes in forest condition (i.e., defoliation), compared to a 20-year baseline streamflow period. We found a small, linear increase in growing-season runoff ratio anomalies that was associated with defoliation intensity. The association between defoliation intensity and runoff ratio anomalies was magnified in less anthropogenically impacted reference watersheds. We also found that defoliation intensity was associated with larger volumes of high and medium instantaneous streamflows compared to baseline mean flow conditions. This study provided important insights into the impacts of moderate disturbance on ecohydrology in mesic temperate forests with a unique methodological approach that assessed the impacts of spatially heterogeneous and repeated moderate disturbance at a regional scale.

2. Global maps of soil temperature

   Lembrechts, J. J, Hoogen, J., Lenoir, J., Smith-Tripp, S., and others.,

   Global Change Biology Dec 2021

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   Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km² resolution for 0-5 and 5-15 cm soil depth. These maps were created by calculating the difference (i.e., offset) between in-situ soil temperature measurements, based on time series from over 1200 1-km² pixels (summarized from 8500 unique temperature sensors) across all the world’s major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (-0.7 ± 2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in-situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications.

2020

1. 2020-22 Remote sensing as a tool for efficient forest health and landscape monitoring in Metro Vancouver’s water supply areas.

   Smith-Tripp, Sarah

   Dec 2020