Douglas-fir trees in the Yosemite area may eventually be a thing of the past if they are unable to adapt to increasing drought stress under climate change. I have begun a project to evaluate the adaptive potential of Yosemite trees in collaboration with the National Park Service, and this summer we conducted initial fieldwork. The field season had its fair share of frustration–a huge wildfire, two medium wildfires, and a government furlough, to name a few–but all told, it was a huge success.
We collected tree cores, needle samples, and size measurements from over 300 Douglas-fir trees from 12 sites in and around Yosemite in just 20 days of fieldwork (not counting site scouting, break days, and travel days). And this was just a warm-up for the real work planned for next year.
The tree cores offer a window into each tree’s relationship with climate. It is often possible to find correlations between the width of each annual growth ring and each year’s climate (e.g., drought intensity). For instance, the amount of growth reduction observed in drought years can indicate a tree’s drought tolerance and help predict how it will respond to anticipated increases in drought stress given ongoing climate change. We extract cores from trees using a hand-powered borer–essentially a long, hollow drill bit.
To get data from the cores, we mount them on grooved boards, sand them, scan them with a high-resolution scanner, and load the images into a program that automatically identifies ring boundaries and measures ring widths.
To make sure we identified the rings correctly, we “cross-date” the cores by comparing ring widths across cores. In the graph below, the horizontal axis represents year (going back in time from 2012) and the vertical axis represents relative ring width. Each color represents a different core. In this example, the two cores show good agreement in growth response from one year to the next.
The next step is to correlate ring widths with climate variables to determine each tree’s sensitivity to climate. Among other factors, differences in climate-sensitivity from one tree to the next may be due to genetic differences among trees. By relating each tree’s genetic composition to its drought tolerance, we can identify mutations that increase drought tolerance and use that information to infer a population’s capacity to adapt to future increases in drought stress. The first step in obtaining genetic data is collecting tissue–in this case, needles. The task–my favorite–requires a very specialized skill that involves slinging a weight (tied to a thin rope) over a lower canopy branch (sometimes 20 meters up!) and shaking until a twig falls down. We then clip the needles off the twig and store them with desiccant in a fancy barcoded vial for inventorying and storage in the Forest Tree Genetic Stock Center until we are ready for DNA extraction and sequencing.
We also map the precise location of each tree by designating a “reference point” (e.g., a tall, dead tree), obtaining an accurate GPS reading for it, and measuring the distance and bearing from each sampled tree to the reference using a compass and laser rangefinder. I wrote some custom computer scripts to process the bearing-and-distance records and compute UTM coordinates for each tree. Here is a map sampled trees in one plot, with circle size proportional to tree diameter. The points don’t line up perfectly with trees from the aerial photograph, but the locations are precise enough to re-locate the trees in the field on future visits.
After we finished most of our sampling in and around the northern part of the park, the Rim Fire tore through and burned more than half of our sampled plots. We’ll have to return next spring to assess mortality of the sampled trees. If mortality was high, we’ll face an interesting decision: what do we do with the DNA from a bunch of dead trees? Jurassic Park schemes aside, there are a few options. 1) Proceed with the analysis because it will provide general insights into the dynamics of climate adaptation in the Sierra Nevada conifers despite the fact that the specific sampled stands no longer exist; 2) Select new sampling sites with lower mortality to replace those that burned at high severity–this will ensure managers have data on real-world stands they are interested in managing; or 3) Return to the high-mortality sites in a few years to sample regenerating seedlings for an inter-cohort comparison–does fire promote adaptation to changing climate by shortening generation time, or does it reduce adaptive potential by depleting the pool of potential parent trees? I’d be quite excited to address the latter question, but of course it all comes down to funding.