Are we at risk of losing much of our western forests? – Accelerating mortality of forests in and around the Rocky Mountain Biological Lab

Something odd and very worrisome caught my eye scrolling through my photos DSC_4115 2of my labs work up at the Rocky Mountain Biological Lab . . . Every year since 2002 I and my lab have been heading up to the Rocky Mountain Biological Lab in Gothic Colorado to conduct repeated measures across an elevational gradient. Here we measure diversity, ecosystem carbon and water flux as well as measures traits and soil and local climate measures.  The goal of the project is to monitor community and ecosystem form, function, and diversity as well as to assess prominent hypotheses for what drives spatial and temporal variation in diversity and ecosystem function.

I became interested in calculating the mortality rate of forests found near our sites via photography. Since we started working at RMBL, at each of our sites I’ve been taking photos of each site. I was curious to pair photos of the forest nearby our study sites. For the past decade western Colorado (and the west in general) has experienced several unprecedented drought years and climate forecasts for the west predict the drought to intensify (van Mantgem eval. 2009Cook et al. 2015). As a result, tree mortality has increased throughout the west and elsewhere (Williams et al. 2013). The area around RMBL is some of the wettest in Colorado so we may not expect that tree mortality to be as severe here. However, are we starting to witness mortality in some of the wetter areas of Colorado? When I paired the photos from one of our sites across from a large stand of subalpine fir forest at approximately 11,000ft. and counted the trees it was clear that the forest is showing signs of heightened mortality. Pfeiler_forest_1 Next, I zoomed into the photo and identified each canopy in 2003 and 2013. I then highlighted each dead tree (IDed from the lack of green needles) in the same area in both 2003 and 2013. Highlighting the trees makes the dead trees stand out Pfeiler_Forest2 Next, I calculated the annualized mortality rate for this forest as the mortality rate,  mu = [ln(N_0)ln(N_t)]/t, where N_0 is the initial number of stems at time 0 and N_t is the number of surviving stems at time t (Condit et al., 1995). I then compared this annualized mortality (mu) to the ‘natural’ mortality rate for subalpine forests.  The results are striking . . . Pfeiler_Forest3The mortality rate observed over the past decade is 5-6 times higher than the ‘normal’ mortality rate expected for a typical ‘normal’ subalpine forest. These results are consistent with an increase in mortality due to the heightened drought conditions experienced by this forest over the last 10 years. Since 2013 I have continued to watch this patch of forest and will continue to better estimate mortality rates of the forests surrounding RMBL. My worry is that these are the first signs of a continued and directional dieback of most forests throughout the west due to increased warming and extreme drought.

This mortality is consistent with forecasts for accelerating collapse of subalpine forests throughout the Western United States.  I then used our new forest forecasts tool (see forestforecasts.org ) developed by my lab group in collaboration with the Aspen Center for Environmental Studies to then generate an animated forecast for the suitability of current and future climate for Engleman Spruce. ezgif-1324617589Our site uses the most recent climate forecasts at 10 year increments until the year 2081. See Forestforecasts.org for more information about the methodology. In the animation shown here – you can see the mountains around RMBL and Crested Butte looking North-north west. This animated ‘weather channel-like’ forecasts shows the decrease in suitable habitat for Engleman Spruce, a dominant tree species of the subalpine forests of Colorado, through time till 2081.  The results are sobering – according  to this forecast, Engleman Spruce and subalpine forests in general are expected to exhibit a precipitous decline around Crested Butte, The Rocky Mountain Biological Lab, and throughout Colorado and the Western United States in general.
These results are consistent with other recent studies (Notaro et al. 2012Jiang et al. 2013; ). The heightened mortality around our study sites may be the first signs of this forecasted decrease.

Next we summarised the change in area climatically suitable for the main tree species of high elevation forests of Colorado and elsewhere in the west. Here is what we find. . .  if we continue the current pace of greenhouse gas emissions the total forested area could be reducedsubalpine_FFDM dramatically. However, if we stabilise greenhouse gas emissions so that they peak around mid-21st century, then decline to zero by the end of the 21st century then the loss of forest cover, while significant, is not as dramatic.  In short, any reduction of greenhouse gas emissions will directly translate to slowing the reduction of forested area in the West, Colorado, and in and around RMBL.  The choice is up to us.

This past summer I gave presented some of these initial findings at the Aspen Ideas Festival. If you are interested in learning more about how we are developing new visualisation and big data approaches to understanding the fate of our Western Forests see the Huffington Post article or watch a high definition video of my AIF talk here.

The take home message here is not entirely dour or pessimistic – it appears that our Western Forests will respond to our climate change policies. As we show in our ForestForecasts.org site the forecasted steady decline of the forests around the Rocky Mountain Biological Lab as well as throughout the Western United States can be slowed if not halted if we enact policies that increasingly slow and stabilize greenhouse gas emissions.

ForestForecasts.org allows you to explore how different greenhouse gas emissions scenarios will then translate to different forest distributions in the near future. What we are excited about is that forest forecasts.org site clearly shows that if we choose to stabilise greenhouse gas emissions the heightened mortality and loss of forests can be stopped.

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