Thursday, July 12, 2012

On how I chose my samples - Part 1

One of the early challenges I had in doing field work was deciding how to chose plants to measure and tag for long-term monitoring.  At this point, I have read quite a bit about this topic and always take notice of descriptions of this process when they appear in papers. Sadly, such descriptions generally consist of a sentence or two buried somewhere between 'Study Species' and 'Study Site'. I did not have any formal field ecology experience prior to starting graduate school and my research adviser doesn't have any field based projects, so I felt a bit on my own in coming up with a sampling scheme. That's not to say that I didn't get advice from some very smart and experienced folks - just that at the end of the day, I had to cobble together a sampling scheme based on lots of reading, a fair bit of discussion, and a very small amount of experience.

My small bit of experience came by way of working as a Graduate Assistant to the chair of the department here at Stony Brook, Dr. Jessica Gurevtich. Dr. Gurevitch and her colleagues have been monitoring the post-fire recruitment and demography of pitch pine (Pinus rigida) here on Long Island since 1996.  The fire that affected these field sites was in 1995, and growing up near by, I remember it fairly well. Some of the results of this research have been published (Landis et al. 2005; Fang et al. 2006). As Dr. Gurevitch's GA, part of my job was to help out with the Pine Census during the summer of 2009, which that year was organized by my friends Adam and James.  In total, over the years well of 6000 pitch pine trees have been tagged with either plastic bird-band tags or aluminum tags, and once every two years a group of people would go out into the Long Island Pine Barrens, search for these marked plants, and take measurements - such as diameter at ankle height, diameter at breast height (DBH), height, presence/number of cones, etc.  When the census was initially established, 45 square 5x5m plots were distributed across various tree stands within three landscapes.  All pitch pine plants, and those that emerged during the study period, within these 45 plots have been tagged and measured.  It's a lot of work and a lot of data! So when I set to designe my sampling scheme, I had some notion that it would involve square plots and marking all of the Glossy Buckthorn plants inside of those plots with aluminum tags.  

Ok - plots and tags - that made sense.  However, the few papers actually related to Glossy Buckthorn that I had found (nearly all from groups at UNH associated with either Drs. Tom Lee or Robert Eckert) had used transects.  And transects seemed so much easier! Lay down a measuring line, then walk along it.  So, near the end of the summer of 2009, I head up north to the White Mountain National Forest (WMNF), walked into a field of Glossy Buckthorn and laid down a transect.
One end of a transect in WMNF
And this is when I learned that transects are not always the best option.  For one, I wanted to place the transect down in a random spot, and what that meant was that my tape was laid down right next, but not touching, a large patch of buckthorn.  Moving my tape would be biasing my data collection, but not having my tape touching any buckthorn made that transect useless.  Then, I couldn't help but notice that some of my transects went through incredibly dense stands of buckthorn, with trees that had pretty low fruit count, while other transects went through very sparse stands of buckthorn, but each tree had hundreds, if not thousands of fruit.  Hmm ... and this lead to a realization I should've had much earlier - before you decide on a sampling scheme, you have to know what you're planning on doing with the data you collect.  And I don't mean have some vague notion like, 'I'm going to make a demographic model!', I mean, you need to know what values you need to estimate, how much error is acceptable, how you're planning on dealing with outliers and missing data, and there's a whole slew of other things to think about carefully.  So I headed back to Stony Brook and gave this a lot more thought.  And that I'll talk about in the next post.

Landis et al. 2005. Variation in recruitment and early demography in Pinus rigida following crown fire in the pine barrens of Long Island, New York. Journal of Ecology 93(3):607-617.

Fang et al. 2006. Sources of variation in growth, form, and survival in dwarf and normal-stature pitch pines (Pinus rigida, Pinaceae) in long-term transplant experiments. American Journal of Botany 93(8):1125-1133.

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