Switch from abrupt to diffuse treeline typology coincided with a switch from precipitation to temperature control of tree growth

Link To Share This Poster: https://cdmcd.co/v9DKgb

Karla N. Iruegas, Organismal Biology and Ecology, Colorado College, Colorado Springs, CO and Miroslav Kummel, Environmental Science, Colorado College, Colorado Springs, CO

Background/Question/Methods
Climate change is causing many treelines to advance to higher elevations. Many studies also show a change in the treeline typology from abrupt to diffuse. Diffuse treelines are more prone to advancing in response to rising temperatures than abrupt treelines.
Our research, conducted at a fast-moving diffuse treeline that has changed form from abrupt to island to diffuse, addressed the following questions: How does temperature/precipitation affect tree growth? Can such effects change over time? If so, can changes in the drivers of tree growth coincide with changes in treeline typology?
Our field site was located on Pikes Peak in the Front Range of the Colorado Rocky Mountains. The only species present in the study area was Picea engelmannii.
The study used tree cores extracted in 2011. Sixty tree cores were measured with a microscope and a linear Velmex bench. Crossdating was done visually using signal years and statistically using COFECHA. The crossdated cores were double detrended with a negative exponential model and then 67/n spline in dplR. The data were standardized. A master tree ring width chronology was created. The RStudio package Treeclim was used to correlate the master tree ring width chronology to monthly precipitation and temperature series.
Results/Conclusions
The tree rings were considerably complacent. The autocorrelation showed that the tree ring growth was influenced by its past with significant lag of up to 5 years.
The master series spanned 91 years (1924-2014) and had a series intercorrelation of 0.451, average mean sensitivity of 0.189, and mean series length of 70.0 years.
Regarding temperature (obtained from previously published data), tree growth had a statistically significant correlation with previous year June and current year March. Current year May and June were close to having a statistically significant correlation. In a seasonal scale, warm Summers and Winters were related with high tree growth. Springs and Falls with high precipitation were correlated with high tree growth and vice versa.
Analysis of thirty-five years long segments lagged by five years shows that up until the 1960s, precipitation had a major effect on tree growth. More precipitation in April led to a decrease in growth. Since 1980s, temperature was the factor causing the major effect. An increase in temperature during the months of March, May and June was correlated to an increase in tree growth.
These changes coincided with the change in treeline typology from abrupt treeline to diffuse treeline.