Last fall, I installed a series of six “common gardens” along an elevation gradient in the Sierra Nevada near Foresthill. At each site, I planted Jeffrey pine seeds collected from four different elevations–from 4000 ft to 7500 ft–in the northern Sierra. The goal of the experiment is to see whether seeds perform best in the climate where they are produced (that is, they are “locally adapted”) or in a different climate (that is, they are “maladapted” to their site). Because the reproductive trees in any given site established long ago (trees take decades to reach maturity and can survive for hundreds of years), it is possible that the climate today is quite different than it was when those trees established. If locally adapted populations are unable to track changes in climate, we can expect a significant decline in forest tree performance under climate change.
I installed the plots in areas that burned at moderately high severity during the 2008 American River fire, as Jeffrey pine seedlings usually establish soon after a disturbance–usually fire–kills mature trees and frees up important resources such as light and water. Working in burned patches can be quite unpleasant; due to the lack of overstory, there is little shade, and the shrubs that often establish soon after fire can be thorny and dense. But burned patches can also be strikingly beautiful.
This August, the American Fire started burning near my plots. I watched the fire perimeter expand each day, enveloping one, then two, then three of my common gardens. Last week, the main road to my plots was finally re-opened, so I went to survey the damage. Given the Sierras are a naturally fire-prone system, I understood the risks of planting seedlings here, and I established numerous plots over a relatively wide area to hedge my bets. The lowest- and highest-elevation plots were spared, but the intermediate plots got toasted. Here is (was) one of my common gardens:
My plots were all within the perimeter of the 2008 American River fire, so this summer’s American Fire cleared away the remaining litter and woody debris, leaving behind the proverbial “moonscape.” Other areas nearby hadn’t burned in 2008 and contained abundant mature trees. This year’s fire scorched most of those trees right to the crown, leaving behind some eerie scenery.
Naturally, fires in this type of forest, known as “mixed-conifer,” are frequent but quite mild–rarely are they so severe as to scorch needles all the way to the crown over huge swaths of land. However, a century of fire suppression by the Forest Service has allowed substantial fuel (fallen needles and woody debris) to accumulate so that when fires do occur, they are very severe and can cause high mortality, even in these fire-adapted species.
I collected seedling mortality data from the plots that didn’t burn. As long as some seedlings survive, those that die are also data points!
I haven’t thoroughly analyzed the data yet, but some interesting trends are already evident.
These figures are from the lowest-elevation common garden at 4800 ft. The figure on the left shows the proportion of planted seeds that germinated in the spring, emerged, and survived through June 10th. The plot on the right shows the proportion of seedlings alive on June 10th that survived to October 2nd. Interestingly, the higher-elevation seedlots initially performed better than the local seeds at this low-elevation site. However, later survival showed the expected pattern of local adaptation in which the seeds sourced from nearest the planting site performed better than those from higher elevations. This suggests that seedlings require different conditions at different stages of development and that overall adaptation to climate is not as straightforward as often assumed. Stay tuned for an integrated analysis of first-year survival at all sites.