Chestnuts #2–Midseason

After two days of measuring seedlings, two major trends in survival became apparent. First, almost all of the seedlings planted in the forested plots were dead. I also noticed that they tended to die from the top down, with new stems regenerating from near the base of the original stem. Second, and much to my surprise, the fenced-in plots (protected from deer browsing) had actually suffered far higher rates of mortality than the unprotected plots.

The case of the forest mortality was especially interesting because it contradicted the expectations I had gleaned from the literature; most of the papers I read prior to my arrival classified the chestnut as either “shade tolerant” (Wang et al 2006) or “intermediate shade tolerant” (Joesting et al 2008, McCament and McCarthy 2005, and others). These terms have pretty straightforward meanings: shade tolerant species, such as sugar maple and American beech, can typically survive and grow under full shade, and intermediate shade tolerant species can tolerate some shading but do not grow quickly unless exposed to sun. Species in the latter group are also sometimes called “sit-and-waits,” meaning that they can survive for long periods of time in the understory, growing very slowly, until a gap opens in the canopy and they are “released” into a period of rapid growth. Based on what I had read, I was expecting the seedlings in the forest to be smaller than those in the cleared plots, but I was not expecting such an overwhelming difference in survival rates. I immediately set to pondering explanations. Perhaps the winter freezes were more intense and longer-lasting in the forested plots, both of which were situated on the slopes of stream valleys which could possibly trap cold air. Perhaps frost damage could explain the pattern of top-down seedling mortality that I had observed. I came up with an idea for a new part of my project: since cold tolerance can be measured in the lab, perhaps I could actually prove that the previous winter had been cold enough to kill the seedlings by comparing the lab results with local weather data. But I had other tasks to accomplish at the moment, so I set the idea aside for the time being.

A quick visual inspection was enough to give me a clue as to why the fenced plots had fared so poorly. In the cleared sites, where the difference in survival between the fenced and unfenced plots was most pronounced, the fenced plots almost resembled fountains: dense thickets of grass and shrubs had grown far taller than the vegetation outside the fences. I realized that, in attempting to protect the seedlings from herbivores, we had also removed the browsing pressure on the competing vegetation, which was now overwhelming the seedlings. It was fortunate, then, that the next phase of my project was to perform a census of this competing vegetation so that I could measure this effect. This was also the task which I most underestimated; it dragged on for more than a month, through some of the hottest days of the summer and could be almost maddeningly repetitive. In the end, though, this data was probably the most valuable information that I managed to gather this summer since it allowed me (and the lab) to learn what “forested” and “cleared” really meant in the context of the study.

To measure the vegetation, I used a device called a Daubenmire frame, which is really just a square of painted PVC pipes that makes it easier to estimate percent ground cover. In addition to ground cover, I also estimated stem density at ground level and at 25 cm height intervals. When estimating ground cover, I took both a cumulative measurement and also a species-by-species measurement for each plot. This process was especially time consuming because I was unfamiliar with most of the plants that I encountered: I returned to campus with 50 unidentified specimens that I collected, most of which still have to be keyed out in the herbarium (which is basically a library of plant specimens and identification guides).

Due to unforeseen changes in my schedule, I also realized at about this time that I would not be able to complete all of the work that I had intended to: photosynthesis and cold tolerance had to be abandoned. Neither was part of the original proposal that I had submitted to the Charles Center, but I was still rather discouraged about having to let them go, especially since both were mostly lab procedures and I was beginning to chafe at the high field: lab-work ratio. The decision wasn’t easy, but I realized that I had allowed my ambitions to get ahead of my capabilities and that I would have to cut the project down to size. Once I made the changes, though, I felt much better about the quality of the work which I had still resolved to do.