CSU Scientist: Longer Growing Season as Climate Warms May Not be the Boon that was Expected

A longer growing season is one of the most-well documented biological consequences of Earth’s warming climate. Earlier dates for leaf-opening and flowering for many plants, including garden plants, native trees and wildflowers, have been observed around the world during the past 30 years. Changes have been seen in yards, parks and even in the Colorado mountains. Many believe a longer growing season will result in healthier, more productive ecosystems. A closer look made by a Colorado State University researcher suggests it may not be that simple.

A new evaluation by CSU researcher Heidi Steltzer from the Natural Resource Ecology Laboratory and colleague Eric Post from Penn State University of recent research indicates that many plant species are shortening their annual growth cycle in response to climate warming. Plants that are greening and flowering earlier are also ending growth earlier in the year. A shortened period of growth may be a result of current environmental conditions, such as a mid-season drought in a warm summer, or could be the result of years of conditioning to past environmental conditions. Many species cannot extend the period over which they grow beyond a set number of days.

Surprisingly, these observations of the annual growth cycle of individual species are not inconsistent with a longer growing season observed, for example, in satellite images of Earth’s green season. Earlier springs may occur because early-season species are greening and flowering earlier due to warmer spring air temperatures. Later falls may result from a delay in the growth of other, late-season species in order to avoid extreme mid-season temperatures and summer droughts. The longer growing season can be the result of opposing responses by individual species within a common locale or community.

"Consider a meadow with two plants, which in the past grew and flowered over the same period of time.  As these plants shift the timing of when they grow in different ways, the period of time in which they are both flowering will decrease, and eventually there may be a gap between when they flower. The rate of decrease will be much more rapid and lead to gaps sooner if the plants are also shortening their annual growth cycles as has been observed in recent climate change experiments," said Steltzer. "This is what has piqued the interest of climate change ecologists. Like other changes in our environment due to climate warming, such as the loss of sea ice on the Arctic Ocean, change is more rapid than expected and the consequences are often unpredictable."

As time gaps open up when different species are growing and flowering, there will be visible changes to natural landscapes. For example, a Rocky Mountain meadow that at peak season is filled with diverse colors and shapes of flowers may instead flower in waves of reduced variation. Fall colors may similarly be visually less dramatic. The period of peak colors may decrease as well, potentially affecting the economy of regions dependent on ecotourism.

There will also be changes that are less visible that will reduce the recreational and health benefits of natural landscapes including Colorado’s mountains. Gaps in plant growth could reduce the quality of food for game animals, such as elk and caribou, and favorite birds, such as sage grouse, when their offspring are young. Insects that pollinate wildflowers may also find themselves without essential resources. These gaps could also lead to nitrogen losses to water supply, increasing exposure or filtration costs. Increased nitrogen losses would also lead to less productive ecosystems over time.  

"Though some may argue that a longer growing season will lead to greater plant growth, we expect this is unlikely, because individual species are not lengthening their annual growth cycles," Steltzer said.

The study was published in the May 15, 2009, issue of Science Magazine.