April 17, 2009


Where have all the big trees gone?


This week's article: Twentieth-century decline of large-diameter trees in Yosemite National Park, California, USA by J.A. Lutz, J.W. van Wagtendonk, and J.F. Franklin. Published in the journal, Forest Ecology and Management, 2009, Volume 257, pp. 2296-2307 (preview here).



The plot line: A vegetation survey in Yosemite was done in the 1930’s. Surveys of the same areas (but not the same plots) were again done in the 1980’s-90’s. The authors used this quasi re-measurement to make inferences about how large diameter trees have changed over the last 50 to 60 years. Despite “issues” with using historical data that are biased, the authors are convinced from the data that the number of large trees has declined. They think that increased water stress from either increased competition from smaller trees and/or from climate change is the most probable cause of the decline.


Relevant quote: Plots in Pinus ponderosa–Calocedrus decurrens [ponderosa pine – incense cedar] forests that had experienced fire in the 20th century retained large-diameter P. ponderosa. Plots not experiencing 20th century fire had almost no large-diameter P. ponderosa.”


Relevance to landowners:

Big trees are often important to landowners. They look cool, they are resistant to fire, they provide a particular type of habitat, and when they die they provide an even more particular type of habitat (e.g. for cavity-nesting species). The most relevant results from this paper (I think anyway) are the way in which species composition of large trees changed, and the reminder that disturbances alter the rate of recruitment of large trees (i.e. how fast small trees grow into big trees). When smaller trees die from a disturbance, more resources are made available to the big trees that are left behind- and they often respond by growing faster. 


Where there was a fire, more large-diameter ponderosa pine were recruited. This was a specific disturbance (fire) that caused a shift in species composition. Every disturbance that kills a tree (or even a branch of a tree) in some way prefers one species over another. This occurs either because different species have different levels of resistance or because disturbances show certain “preferences” for one species or another (e.g. insects only attacking one kind of species).


Relevance to managers:

  • A new term was used that I hadn’t seen before – “fire debt.” A forest with a high fire debt is one that hasn’t had a fire in a long time, compared to how often it had fires in the past.
  • The use of historical data to see how things have changed probably has more relevance for parks than other areas. But even outside of park boundaries, it makes sense to try and understand how forest structure and composition has changed even if we are not trying to re-create the exact conditions of the past.
  • The focus of the analysis was on big trees, but from the diameter distributions you can also see big increases in densities of small diameter white fir.

Critique (I always have one, no matter how good the article is) for the pedants:

The authors clearly state the potential bias in the original data and the limitations that this created. Given the potential for bias, I basically tried to limit my personal inferences to looking for large enough changes to compensate for a potential bias. The changes in overall large tree densities weren’t huge. But the changes in species compositions were big enough to convince me that, at the least, there was a shift in the composition of large trees if not the number of large trees. For an alternative assessment (but with a similar outcome) for how old forests have changed, see the discussion of this article that used re-measurement of permanent plots.


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