The Science

By warming mature spruce trees and surrounding plants in containment tents in a Minnesota peat bog, researchers examined what happens when the growing season is extended. They found it makes plants more vulnerable to cold snaps. Trees and shrubs green up earlier in spring and stay green longer in autumn, indicating potential extension of the growing season by up to 3 to 6 weeks by the end of the current century. However, as plants greened up earlier, some also lost their winter hardiness. These plants were exposed to damage when a spring frost hit in early April 2016.

The Impact

Recent warming trends lengthen the growing season in temperate and boreal ecosystems, such as the Ozark Highlands and Alaska’s interior forests. Scientists wondered if the longer growing season trend would be limited by the shorter days (less daylight) in the early spring and late autumn. This study resolves that debate. It shows that with warming of up to +9 degrees Celsius above ambient, vegetation responses to increased temperature were linear and not limited by day length.

Summary

The SPRUCE experiment is applying warming (0 to +9 degrees Celsius above ambient) and carbon dioxide (ambient and elevated) treatments to intact communities of mature vegetation in a boreal black-spruce sphagnum bog in Minnesota (upper Midwest). Digital cameras mounted in each of the 10 experimental plots show that warming treatments linearly extend the period of vegetation activity in both spring and autumn. There was little evidence that the length of the day (photoperiod) limited these phenological shifts. The camera observations are consistent with ground observations of the timing of flowering and growth by a variety of bog plant species. In spring 2016, unusually warm weather in March was followed by extreme cold in early April. Vegetation in the warmest chambers (+6.75, +9.0 degrees Celsius) suffered severe frost damage as the temperature dropped to -3 degrees Celsius, indicating a premature loss of frost hardiness. By comparison, vegetation in the cooler chambers (0, +2.25, +4.5 degrees Celsius) was undamaged, despite experiencing dramatically colder temperatures (up to -15 degrees Celsius). Thus, because phenological transitions—including loss of frost hardiness—appear to be driven by the temperature, rather than cued by photoperiod, vegetation may be exposed to greater risk of frost damage in a warmer world. These experimental results are of particular significance because boreal forests have a circumpolar distribution and play a key role in the global carbon cycle.

Funding

The Department of Energy, Office of Science, Office of Biological and Environmental Research supported the research. The National Science Foundation supported PhenoCam.

Publications

A.D. Richardson, K. Hufkens, T. Milliman, D.M. Aubrecht, M.E. Furze, B. Seyednasrollah, M.B. Krassovski, J.M. Latimer, W.R. Nettles, R.R. Heiderman, J.M. Warren, and P.J. Hanson, “Ecosystem warming extends vegetation activity but heightens vulnerability to cold temperatures.” Nature 560, 368 (2018). [DOI: 10.1038/s41586-018-0399-1]

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