In September, I wrote a blog post about our research at the SPRUCE site in Minnesota, where a Department of Energy project is trying to answer the question:
What effects will warmer temperatures and rising atmospheric carbon dioxide have on boreal forests?
The SPRUCE experiment uses large chambers, about 30 feet wide and 25 feet high, to expose intact patches of mature boreal forest to environmental conditions that might exist in about 50 years. We mounted PhenoCams inside each chamber, to track how the experimental treatments—warming and elevated carbon dioxide—would influence vegetation phenology, and to provide a permanent record of the progression of the experiment. You can see, for example, that in early January there is little or no snow in the warmest chambers, but plenty of snow in the unwarmed chambers:
The imagery also shows us, quite clearly, the dramatic seasonal changes in vegetation color that occur even for “evergreen” vegetation. Compare these two pictures, taken in July and October:
We used this change in greenness to identify the timing of autumn leaf death in each chamber. And what we’ve learned so far, from the first six months of camera data, is that the warming treatments resulted in substantially delayed autumn leaf death. More specifically, leaf death was delayed by about 3.5 days per degree of warming—or, to put it another way, the growing season was about a month longer in the warmest (+9°C) chambers, compared to the unwarmed chambers.
What is interesting about these results is that they show a direct control of temperature on the timing of autumn senescence. This is a surprising result because it is generally assumed that changes in day length (i.e. shorter days in autumn) are what triggers senescence, not temperature.
Looking forward, we anticipate that different results may be obtained in the second year of the SPRUCE experiment, if warming treatments result in earlier spring onset, and increased water use by plants during spring and early summer, leading to drought conditions by late summer. It will be exciting to see what happens!