In California’s White Mountains, gnarled and weathered bristlecone pine trees dot the hillsides, stretching from the treeline downslope toward the valleys. The trees—some of which are more than 4,000 years old—offer an invaluable record of the past climate. Using the trees’ annual rings, scientists can reconstruct temperature patterns over thousands of years.
But precipitation can muddle interpretation of the tree rings. Particularly for high-elevation trees near the treeline, temperature and precipitation signals mix and make the climate record difficult to read.
To better understand the dueling temperature and precipitation signals in the rings, Bunn et al. evaluated the role of topoclimates—climate induced by topography—on the growth of bristlecone pines. The authors studied trees in the Ancient Bristlecone Pine Forest in California’s Inyo National Forest.
To derive the topoclimate data, the authors created digital surfaces modeled from data collected by over 50 temperature sensors placed throughout the treeline environment. The surfaces allowed the scientists to explore how topography affects temperature at scales of tens of meters. From the topoclimate surfaces, the authors extracted relevant climate variables, including the seasonal mean temperature (SMT), which describes the average daily temperature on days when the minimum temperature is above 0.9°C.
Past research has identified an SMT of 7.5°C as a critical threshold for bristlecone pines. In areas where SMT is greater than 7.5°C, the tree growth tends to be limited by moisture; below 7.5°C, growth tends to be limited by temperature. The authors used the SMT threshold to construct three separate tree ring chronologies from trees near the alpine treeline. One chronology sampled trees in “cold” zones (where the SMT is less than 7.5°C), one used trees from “warm” zones (where the SMT is great than 7.5°C), and the third sampled from both warm and cold zones. The chronologies were used to tease out the physiological limits to ring formation.
The authors confirmed that high-elevation bristlecone pines separate into two distinct modes of growth. Trees growing in the warmer topoclimates are moisture limited; their rings more closely resemble lower-elevation trees growing about 700 vertical meters downslope from the sample sites. In contrast, trees in the cold zones are growth limited by temperature. The results indicate that the difference in median SMT between the warm and cold zone trees is only 0.5°C, which suggests limits on growth respond to small changes in SMT. Slight variations in topography appear to have drastic effects on tree growth patterns.
The findings also revealed a trend that tree ring scientists might find disturbing: The sampled trees showed a recent loss of temperature sensitivity in their annual rings. The authors concluded the loss of sensitivity may be a result of temperature-induced drought stress as the number of high degree days increased without a change in precipitation.
Tree ring data remain a vital proxy for climate, and this new research could provide useful insight into the role of topography and microclimates on ring formation and dendochronology interpretation. (Geophysical Research Letters, https://doi.org/10.1029/2018GL080981, 2018)
—Aaron Sidder, Freelance Writer