Since the turn of the century, more than a million hectares of pinyon pine-juniper woodland in the Southwest, U.S.A. have been killed by beetle infestations or direct drought. We hypothesize that interannual climatic variation, in combination with the soil water-holding capacity, account for most of the spatial variation recently recorded in mortality. To test this hypothesis, we first map areas within the pinyon-juniper woodland type in Utah, Arizona, Colorado and New Mexico that have shown significant variation spatially and temporally in mortality. Next, we overlay maps classifying differences in soil water-holding capacity and tally the corresponding areas with observed mortality. Finally, to assess the importance of soil water storage capacity versus soil texture in explaining the observed patterns of mortality, we utilize a simplified process-based growth model (3-PG) to analyze variations in gross photosynthesis (and mean tree vigor) over the period between 1985 and 2005 at a representative site in New Mexico.
Results/Conclusions
GIS analysis shows that 84% of the pinyon-juniper mortality recorded in 2003 and 2004 occurred on soils with an available soil water capacity (As) of <150 mm , with 70% on soils with <100 mm. For the six classes of As, a negative power function describes the percentage decrease in the area on which mortality was recorded between 2003-2004). It was not possible to assess variation in soil texture because that classification was incomplete, but considerable variation was noted in areas with similar values of As. A sensitivity analysis in 3-PG identified sustained periods of drought and supported field observations that the majority of mortality should be largely restricted to soils with Ac values < 100 mm once canopy leaf area approaches a maximum value. Additional analyses indicated that differences in soil texture play a small part (<10%) in accounting for variation in gross photosynthesis and that consecutive years of drought, even if less severe than during a single year, result in substantially more mortality.