Plant waxes, such as n-alkanes, are used for a wide range of geologic applications including paleoclimate and paleovegetation reconstruction. Several common traits of n-alkanes that are used as paleo proxies include chain length distribution and average chain length (ACL). In addition, the carbon and hydrogen isotopic compositions of n-alkanes and other plant waxes are also commonly used. The effect of climate on plant wax traits has been the subject of many modern calibration studies, but a common challenge with studies is disentangling the competing effects of species, genetic variation, temperature, and precipitation. Here, we explore the effect of temperature and drought, independently and combined, on plant wax composition of the species Juniper monosperma with a large ecosystem-scale field manipulation experiment. Our results have important implications for using plant wax traits for reconstructing paleovegetation and paleoclimate. Trees were located at the Los Alamos Survival-Mortality (SUMO) field study in Los Alamos County, New Mexico (35.49º N, 106.18º W, 2175 m.). At this site, trees were selected for a manipulative study using open-top chambers and a rain shelter to examine the effects of drought and heat on J. monosperma. Selected J. monosperma trees were between 1.5 and 4.5 m tall and the mean tree age was 79 ± 7 years. In this study, a minimum of six tree replicates were assigned to each of the five treatments: (1) ambient control with trees growing in the open (no manipulation; 'ambient control'), (2) chambers with no manipulation, to test for ambient chamber effects ('chamber control'), (3) chambers with heat applied ('heat'), (3) trees growing in the open, but under a rain shelter to simulate drought conditions ('drought'), and (5) chambers with heat and where trees are located under a rain shelter to simulate drought ('heat+drought'). Manipulations started in June 2012 and continued for five years. Chamber temperatures were maintained at ambient for the chamber control and at 4.8 ºC above ambient for the heat and heat+drought treatments. Trees in the drought manipulations, grown in the open or in chambers, were situated under a precipitation exclusion structure that reduced precipitation by covering 45% of the overhead surface area. For this study, branch samples (~15 cm long) were collected from the distal foliar clump on the south-facing side of the trees at chest height in July and August of 2016 (Year 4 of the study). The total number of individuals varied by treatment from 4 to 8. Powdered leaves were extracted using an accelerated solvent extractor and the n-alkyl lipids (n-alkanes, n-alkanoic acids, and n-alkanols) were identified and quantified by gas chromatography mass spectrometry and flame ionization detection at the University of Cincinnati Organic Geochemistry Laboratory. The stable carbon isotopic composition of bulk leaf and n-alkanes were measured by isotope ratio mass spectrometry at the University of Cincinnati Stable Isotope Laboratory.
