Torrey Pine and Escondido tree rings were diagrammed based on models projecting the soil moisture in the top 10 cm (4 inches) of soil. Why use soil moisture as an index of tree growth? My decision of which variable to use is a long and complicated story as evidenced by the long list of thank-yous to scientists below. To keep it simple, in the past Torrey pine tree rings have been highly correlated to precipitation (Biondi et. al. 1997). Fog was also significant in a study by Fischer et. al. 2009. But the recent drought was not only caused by lower precipitation than usual, but higher temperatures. While it is generally agreed that rising temperatures will have an increasing impact on tree growth, determining the best index to express this relationship is challenging. Soil moisture is recognized as a variable that expresses the influence of both precipitation and temperature. While it is difficult to predict how climate change will effect the coastal marine layer, if higher temperatures cause the marine layer to diminish, this should be reflected in soil moisture values.
The soil moisture data were provided by David Pierce of the Scripps Institution of Oceanography. The future projections average results from 32 different climate models, thereby reducing random chaotic variations of the weather and uncovering the consistent effects of global warming. Pierce suggests using the same models for both the historical period and future projections. As he explains, "When you compare model historical data to model future projections, systematic model errors cancel out to some degree. Using direct observations of soil moisture instead of model estimates is difficult due to the lack of good historical soil moisture observations.” Note that all of the projections show patterns of change and cannot be used to predict the climate in a given year.
Jeffrey Pine tree rings were diagrammed to represent the climate water deficit, an index that quantifies the amount by which plant's evaporative demand exceeds soil moisture. Diagrams of the past are based on actual historical data and those for the future, on one model's projections of the climate water deficit. The data is for the Cottonwood-Tijuana watershed, as opposed to the Laguna Mountains specifically. As explained on the California Climate Commons website, which was the source for this data, climate water deficit "effectively integrates the combined effects of solar radiation, evapotranspiration, and air temperature on watershed conditions given available soil moisture derived from precipitation. Climatic water deficit can be thought of as the amount of additional water that would have evaporated or transpired had it been present in the soils given the temperature forcing." Note that because the diagram of future projections is based on only one model there is considerable random variation from year to year, as we experience in the weather, unlike the soil moisture data used for the Torrey pine, which averages 32 models, thereby highlighting the impact of global warming. The CCSM4 RCP 8.5 model from which the data is derived is described as middle-range, closest to the mean of the models available on the California Climate Commons.
For the Jeffrey pine an additional diagram of historical data, based on the Palmer Drought Severity Index, extends through 2016 and is specific to the Laguna Mountains. This data was provided by Park Williams, Lamont-Doherty Earth Observatory of Columbia University. This index is based on a supply and demand model of soil moisture. Since many of the factors used in assessing demand, such as evapotranspiration and recharge rates, are difficult to calculate, the index uses an algorithm that approximates them using temperature and precipitation data.
The project is an artistic interpretation of scientific research. For instance, in nature, trees rings narrow as a tree ages, as well as due to years with an unfavorable climate, but to simplify the diagrams, I have not factored in the age of the tree. As the artist, I, Ruth Wallen, take fully responsibility for what is presented here. I would like to offer HUGE THANKS to all of the scientists and park managers who patiently answered questions, sent articles and/or data including: Michael Dettinger, Alexander Gershenov, David Pierce, Suraj Polade, Richard Somerville, from Scripps Institute of Oceanography, as well as Nathan Stephenson, Zhi-Yong Yin, Park Williams, Zhahai Stewart, Michael Wehner, Leroy Westerling, Darren Smith, and Talbot Hayes. For advice about technology and software, thanks to Brian Goldfarb UCSD, Bryan Kennedy, Science Museum of Minnesota,, Martin Baumgaertner, Angle Park Inc., and Michael Field, San Diego Natural History Museum. Additional thanks to Jinpa and Martha Heinz for help transporting the tree stumps.
For the Escondido updates huge thanks to Marian Sedio, Mayela Manasjan, Simon Breen and members of the North County Climate Alliance and Escondido Climate Action Alliance..
VERY SELECTED REFERENCES
A. Park Williams et. al., Temperature as a potent driver of regional forest drought stress and tree mortality, Nat. Climate Change 3, 292-297 (2013).
A Park Williams et. al., Forest responses to increasing aridity and warmth in the southwestern United States, PNAS, 107, 21289-21294.
F. Biondi et. al., Dendroclimatology of Torrey Pine, Amer. Midland Nat., 138 237-251. (1997).
D. Fischer et. al, Significance of summer fog and overcast for drought stress and ecological functioning of coastal California endemic plant species, J. of Biogeography, 36, 783-799. (2009)
D. Griffin and K. Anchukaltis, How unusual is the 2014-2016 drought?, Geophys.Res. Lett, 41, 9017-9023. (2014)
C. Millar and N. Stephenson, Temperate forest health in an era of emerging megadisturbance, Science, 349 823-236. (2015)
A. Potito and G. MacDonald, The Effects of aridity on conifer radial growth, recruitment and patterns in the eastern Sierra Nevada, California, Arctic, Antarctic and Alpine Res., 40 129-139. (2008)
The Copenhagen Diagnosis. 2009: Updating the World on the Latest Climate Science.