Shifting Window of Growing Seasons

Timing of spring & autumn in the Great Smoky Mountains National Park

spring greening
Spring greenup advances earliest on the moist slopes of the Park’s lower elevations. Photo by Kristina Plaas, courtesy of National Park Service.

When winter comes to an end, it’s no mystery that warming temperatures and spring rains bring new life. Wildlife emerges, flowers bloom, and brilliant green leaves begin to fill the ground and the forest canopy—all part of their seasonal cycle known as phenology.

Observers know those green leaves don’t appear at the same time every spring, nor do they begin to fade away at the same time every fall. U.S. Forest Service researchers now have a better understanding of the variation in the timing of spring and autumn across a diverse mountain landscape.

Steve Norman, a research ecologist with the Eastern Forest Environmental Threat Assessment Center led a study that was recently published in a special issue of Remote Sensing. Along with Eastern Threat Center research ecologist Bill Hargrove and biological scientist Bill Christie, he analyzed changes in vegetation greenness — or land surface phenology — from satellite-based data collected daily between 2000 and 2015.

Researchers used the ForWarn monitoring and assessment tool to view the data on maps generated every eight days for the study area: the Great Smoky Mountains National Park in western North Carolina and eastern Tennessee.

“The Great Smoky Mountains National Park supports rich vegetation communities and other biodiversity due to complex terrain and ample rainfall, though seasonal variation in temperature and precipitation is considerable from year to year,” says Norman. “We examined the influence of seasonal weather variation as well as forest cover types, topography, and disturbance history on the Park’s land surface phenology.”

The researchers determined that the timing of spring vegetation greenup and autumn browndown in the Park can vary by about 2 and a half weeks each year. In general, spring warmth accelerates vegetation greenup. Early autumn warmth delays browndown.

Land surface phenology in the Park may also depend on cross-seasonal weather. For example, a preceding cold winter may accelerate spring greenup. If spring was cold, vegetation may stay green longer into autumn. If spring was warm, vegetation may brown down earlier.

yearly greening
Vegetation greenness rises and falls each year in the Park’s deciduous forests. Researchers found that spring greenup and autumn browndown can vary by about 2 and a half weeks each year. Image courtesy of Remote Sensing.

Researchers say elevation appears to drive greenup and browndown more so than moisture, which differs by slope positions. “Forest greenup advances earliest on the moist slopes of the Park’s lower elevations. Greenup on sunny slopes advances next, and then continues progressively upslope,” says Norman.

This pattern seems consistent with on-the-ground observations of tree species. Oaks in moist coves regularly green up days to weeks earlier than oaks that that dominate dry sunny sites, for example.

In autumn, this pattern reverses. “Shaded sites, especially at high elevations, lose vegetation greenness before the sunny slopes do. Once autumn browndown begins, it progresses quickly at all elevations,” says Norman.

Satellite-based data and maps are important tools for monitoring the timing of seasonal vegetation changes across a landscape.  With this information, resource managers can understand ecosystem productivity, anticipate wildlife movement and behavior, and prepare for potential disturbances, such as growing season insect defoliation and wildfire activity.

This insight is also useful for planning around the timing of spring flowering and autumn foliage, which are huge draws for recreation-based tourism and critical for local economies. “Our ability to efficiently and accurately track growing season dynamics is particularly critical as climate and disturbance regimes are changing,” says Norman.

Read the full text of the article.

For more information, email Steve Norman at stevenorman@fs.fed.us.

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