Soil properties affect pinyon pine – juniper response to drought

Abstract

Since the late 1990s, drought‐driven dieback has affected more than a million hectares of pinyon pine‐juniper woodlands in the southwestern USA. Analysis of annual aerial surveys by the US Forest Service and soil survey data shows that most of the mortality occurred between 2003 and 2004 and that 70% was restricted to soils mapped as having available water storage capacities (Ac) <100 mm. We conducted a more refined analysis and found that as Ac increased in increments of 50 mm up to 300 mm, the distribution of areas with observed mortality decreased exponentially from 42% to 3% (n = 6 classes, r2 = 0.93). We used this information in a process‐based stand growth model, physiological principles predicting growth, to assess year to year variation in gross photosynthesis between 1985 and 2005 with climatic data at monthly intervals from four weather stations where pinyon‐juniper woodlands were confirmed by satellite imagery. A sensitivity analysis identified sustained periods of drought and supported field observations that once canopy leaf area approaches a maximum value, the majority of mortality should be restricted to soils with Ac values <100 mm. Additional analyses indicated that differences in soil texture played a small part (<10%) in the variation of gross photosynthesis and that consecutive years of drought may have a cumulative effect on pinyon pine vulnerability to bark beetle attack. Disturbances reducing canopy leaf area index should result in less pine mortality in the future, although conversion to shrub and grassland may occur if climate conditions continue to become less favorable. Copyright © 2012 John Wiley & Sons, Ltd.

Process-Based Modeling to Assess the Effects of Recent Climatic Variation on Site Productivity and Forest Function across Western North America

“…In the Southwest, much of the increase in tree mortality may be associated with consecutive years of drought [19,[53][54][55]. Our modeling analysis indicates that sustained increases in evaporative demand in the Southwest may be equally important as periodic drought (Figure 2b,c), which supports the conclusions by Weiss et al [51] drawn from longer-term climatic records.…”

“…Most process-based models take into account constraints imposed by seasonal variation in incident radiation, temperature and the evaporative demand of the atmosphere. Some also include a soil water balance to account for observed differences in growth and mortality during periods of extended drought [18,19]. Few models, however, account for spatial variation in soil fertility [20], although the above-ground growth responses to rising atmospheric CO 2 are recognized to be severely constrained on infertile soils [21], as well as on soils with extreme water deficits [22].…”

Water availability limits tree productivity, carbon stocks, and carbon residence time in mature forests across the western US

“…We acknowledge that this index has several shortcomings. For instance, the index does not account for spatial variation in soil water storage capacity, which can be crucial for determining plant performance during drought (Peterman et al, 2013). This might explain some of the variation in NPP and BIO among areas with similar CMI wy ; however, quantifying soil water storage capacity even at individual sites is challenging given uncertainty in soil structure and plant rooting capacity (Running, 1994).…”

Key landscape ecology metrics for assessing climate change adaptation options: rate of change and patchiness of impacts

“…This is a large impact considering the slow growth rates of these trees. The exact locations of areas of high piñon mortality were unpredictable when they occurred, and apparently relate to a combination of climate-driver and water-balance issues (Weiss et al 2009, Peterman et al 2013) but remain challenging to predict.…”

“…The uncertainty in this situation lay in when and where the vegetation die-off and concomitant crashes in ecosystem services would occur. Despite recent progress on determining the spatial patterns of climate and soil moisture that may have driven the patterns of tree mortality (Weiss et al 2009, Peterman et al 2013, Williams et al 2013, considerable uncertainty remains in precisely forecasting future such patterns of die-off, and this in turn constrains adaptation options.…”