Feb 27, 2009


Is it just me, or have the trees been dying more often lately?


This week's article: Widespread Increase of Tree Mortality Rates in the Western United States by Phillip J. van Mantgem, Nathan L. Stephenson, John C. Byrne, Lori D. Daniels, Jerry F. Franklin, Peter Z. Fulé, Mark E. Harmon, Andrew J. Larson, Jeremy M. Smith, Alan H. Taylor, Thomas T. Veblen. Published in the journal, Science, Volume 323, January 2009. See the on-line news story.


The plot line: Trees seem to be dying more frequently in recent times. It’s occurring all over old forests in western US (and southern BC), and the authors believe that is likely related to climate change (especially increasing temperatures).


Most important quote: “…increasing mortality rates could presage substantial changes in forest structure, composition, and function, and in some cases could be symptomatic of forests that are stressed and vulnerable to abrupt dieback.”


Relevance to landowners:

  • You can’t know from an eye-ball estimate if trees are dying faster or slower than normal
  • Very rapid change (i.e. LOTS OF TREES DYING) is a possibility in the near future worth thinking about
  • You don’t know how your forest will respond to climate change, but you can plan ahead

I often hear people lamenting about how all of their trees are dying (they usually expect me to tell them what the cause is and then try to revoke my PhD when I tell them I don’t know). While people may be able to notice outbreaks or other sudden pulses of tree death, a human being cannot notice through casual observation that trees are dying with any more or less frequency than normal. The only way to really know is to do what these authors did: track your trees over a long time. You should be aware that there are signals (like the ones pointed out by these authors) pointing towards rapid changes occurring in forests that have not been recently disturbed (i.e. thinned, burned). The changes are not always inevitable. Consult with a professional forester, or better yet, learn for yourself the kinds of treatments you might apply on your land and understand the tradeoffs between them. With so much uncertainty in how our forests will respond to climate change, the best approach might be a hedge-betting scenario. The idea with hedge-betting is that we try different approaches (including, perhaps, doing nothing) on different sections of the forest. For example, thinning in one area, clearing/planting in another area, and leaving some areas untouched. Different species will inevitably respond differently to climate change. Encouraging diversity in species is therefore also a form of hedge-betting.


Relevance to managers:

This article is relevant for all managers of western US forests. We have already seen insect epidemics that have impacted huge swaths of forests (e.g. mountain pine beetle) at undesirable rates. The timing or scale of the next epidemic is anybody’s guess. What’s a forester to do? A couple of suggestions:

  1. Support or start long-term demography plots on your land.
  2. Hedge your bets where forests are actively managed- structurally diverse forests may be more resilient against epidemics.

For the forest nerds in the house:

Relevance to science: This is a monitoring study. The authors tracked the fate of trees over a long period of time in lots of places and observed the patterns. Although there are no traditional experimental techniques involved, the study is nonetheless very important to science. The authors show clear patterns using tons of data, and use existing studies and databases to suggest an explanation for the observed pattern. I suspect that this will spur further studies aimed at confirming the reasons that the authors suggest (temperature increases).


Critique (I always have one, no matter how good the article is): One possible reason for the increase in tree death frequency is that the lack of fires (because of fire suppression) is not controlling insects and pathogens like they used to. In other words, the fires aren’t killing the bugs, so the bugs are free to kill the trees more often. The authors do not consider this mechanism to be likely, however. Their argument is that forests with fires that used to occur very frequently did not have a different mortality rate than forests with less frequent fires. The implication here is that bugs and pathogens are only controlled by fire when fire occurs frequently. Is it possible, however, that bugs and pathogens in forests without frequent fires are nonetheless controlled by fires? The control may just be on a different time scale. Insects and pathogens should reasonably be expected to have adaptations to fires, even if the fires don’t occur very often (depending on population cycles and lifespans of the species). In an article that is on the whole, very well supported by references from other research, there are no references given to support the assertion that fire suppression is not likely in this case. While I agree that it is not likely that fire suppression is the cause of increased tree death, perhaps it could be more of a contributor than the authors suggest.