Shrub encroachment negatively affects forage production and rangeland health in many semi-arid regions. Flourensia cernua (tarbush) has increased in the northern Chihuahuan Desert at the expense of grasslands. Our previous studies have shown a negative relationship between tarbush terpene concentrations and livestock herbivory. Concentrations of secondary compounds are affected by many biotic and abiotic factors, including amount and wavelength of solar radiation. A 3-year study was conducted to examine the impact of shade and UV light restriction on phenolic and terpene concentrations in tarbush. Our hypothesis was that UV restriction and partial shade would reduce carbon based secondary compounds. Sixty plants were randomly selected and assigned to 3 treatments (control, UV light restriction, or 50% incident light restriction) in a randomized complete block. Frames covered with clear plastic film (UV blocking treatment), black shade cloth (50% shade treatment) or frame only were placed over 20 plants each. Leaves were collected from each plant in late September each year and freeze-dried for total phenolic analysis (Folin-Denis method) or frozen for leaf surface terpene analysis (extracted with ethanol and identified and quantified with GCMS). Data were analyzed using repeated measures linear mixed effects models with treatment as the fixed effect and year as a repeated effect. Means were separated by LSD. Mean concentrations of total phenolics and total volatiles in tarbush were 82.4 and 12.5 mg/g DM, respectively. Tarbush leaves contained 102 individual terpenes (including 19 unknowns). No year x treatment interactions were detected for any variable (P > 0.05). Total phenolics did not differ between controls and UV restricted plants, but were lower for shaded plants than the other 2 treatments (P < 0.05). Total volatiles were higher for the UV restriction treatment than controls (P < 0.05), with shaded plants not differing from either treatment. Treatment effects were detected for 18 compounds (P < 0.05). Shade reduced total phenolics as predicted, but did not affect total volatiles. In contrast to our expectations, UV restriction had no effect on total phenolics, and increased total volatile concentrations.