The Effects of Late Spring Frost on Forest and Landscape Health of the Black Rock Forest, New York

Abstract:

Projected changes in climate are expected to increase the frequency of late spring frost events in the Northeast US. Such events can be harmful to trees because freezing temperatures that occur after leaf-out can damage or kill young leaves. The resultant defoliation typically forces a second flush of leaves but delays canopy development, which in turn delays the onset of canopy carbon uptake and alters canopy thermal properties. While forest response to defoliation events has been studied, much of this work has focused on insect-driven events (e.g., gypsy moth), which often occur later in the season and likely have different ecological implications than spring frost-induced defoliation events. Here, we use satellite-based remote sensing analyses to study the impacts of spring frost-induced defoliation on canopy green-up dynamics in a temperate deciduous forest.

In this study, we analyzed a recent freeze event that occurred on May 8-9, 2020 (DOY 129-130) at Black Rock Forest (BRF), which is located in the Hudson Highlands of southeastern New York State. We compared satellite images collected during the 2019 (no frost year) and 2020 growing season. The purview of this analysis includes: 1. A comparison of the current year to the previous years to determine the productivity of forest ecosystems and their ability to bounce back after the frost. 2. The impact of the frost event on landscape thermal properties by comparing growing season land surface temperature between the year with the frost event (i.e. 2020) to previous years.

We use the Landsat-8 Normalized Difference Vegetation Index (NDVI) to assess spatial and temporal patterns in canopy development. The land surface temperature (LST) is used to measure the temperature of the forest before and after the frost. The mean results of the NDVI value from Landsat-8 data highlights the differences in the timing of greenness between the two years. After DOY 129-2019 and DOY 130-2020, there is a decrease in NDVI for 2020 which suggests a delay in the canopy development. Analyses of ECOsystem Spaceborne Thermal Radiometer Experiment (ECOSTRESS) imagery indicates a rise in LST after DOY130-2020. LSTs after DOY129-2019 are still higher than that of 2020. The mean LST of low elevation forests which did not defoliate shows that higher elevation is hotter than other locations.