Pinyon Pine Decline

Compared to the drought of the 1950s that caused significant ponderosa and pinyon pines mortality, the recent drought (1999-2009) in the southwest US was even warmer (Breshears et al. 2005). Mortality during the 1950s drought was documented on dry, often lower elevation sites, affecting older trees while the recent drought affected high elevation sites and all age classes. It caused rapid, regional-scale mortality of dominant pinyon pines (Pinus edulis) from infestations of bark beetles (Ips confusus) associated with the extreme drought stress. Regional aerial surveys conducted by the U.S. Forest Service confirm that there was widespread mortality in the four corners area (CO, NM, AZ, UT). 

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Even drought-tolerant junipers showed high mortality levels. Higher than normal precipitation levels between 1978 and 1995 could have exacerbated drought and temperature stress by having enhanced tree growth and resulting in increased competition for water and susceptibility to drought, beetle infestation, and associated pathogens. Furthermore, warmer temperatures and longer growing season could also have stimulated beetle population dynamics overwhelming the trees’ normal defenses.

To improve landscape resilience to projected future droughts, forest management practices have been proposed. Reducing stand density can increase water availability, nutrients and light for the remaining trees by decreasing competition. Thinning also decreases canopy interception of precipitation and therefore increases the amount of water reaching the ground. However, increasing light intensity through the canopy may lead to increased temperature at the ground level, higher soil water evaporation and development of understory shrubs and grasses competing with trees for water. Therefore, it is not clear if thinning could really mitigate drought in sensitive areas. Frequent prescribed fires could reduce understory growth after thinning but people are weary of escape fires and public support is lacking particularly during drought periods.

The massive pinyon die-off supports results from mechanistic vegetation models that have projected potential large-scale climate-induced vegetation shifts associated with disturbance such as fires or insect outbreaks caused by drought stress. But physiological explanations for the die-offs are still debated. Hydraulic failure (cavitation due to overly high tensions in the water columns of the xylem) and carbon starvation due to stomatal closure (adaptive process designed to reduce water loss and prevent leaf wilting) have been identified as likely culprits. In the summer of 2003, these mechanisms were identified as the leading cause of tree mortality in Western Europe. It is interesting to note that populations in southern New Mexico such as the Gila wilderness were not affected to the same extent by the drought and did not exhibit high mortality rates. What would explain such discrepancies?

When the map of pinyon mortality is overlaid with a soils map of the area, soils in areas of low mortality in southwest New Mexico appear to be young, fertile Mollisols that are deeper, finer textured and richer in nutrients than soils in regions of high mortality in northern New Mexico and southern Colorado. They have a cryic temperature regime (they freeze) that allows them to store water as ice and release it when temperatures rise. Their finer texture (pore size) as well as the freeze-thaw cycles give them increased long-term water storage potential. Soils in regions of very high pinyon mortality are generally coarse, sandy, skeletal, with low fertility while rich in Calcium (desert caliche layers). They have a torric moisture regime (hot and dry soils) and very little horizon development. Their large pores with little capillary forces promote rapid water evaporation or drainage rather than conservation. Consequently, water and nutrients seem far more available in soils of low mortality areas. Such considerations (both climate and soil characteristics) are important for land managers interested in protecting and maintaining sustainable populations and thus valuable seed sources under warmer drier future projected climate conditions.

Droughts are a long-standing feature of the Southwest’s climate that has included decades-long “megadroughts” during the last 2000 years. The likelihood of such events reoccurring has been enhanced by the release of anthropogenic emissions causing a general warming of the planet. As we discover around the world new areas of forest dieback (Allen et al. 2010) we can debate the various mechanisms that cause widespread mortality but the challenge rests in the hands of the land stewards who face the challenge of preserving healthy ecosystems that provide much needed ecosystem services to animal as well as human populations. And there is more than climate and agreement between models that need to be taken into account.

Co-author: Wendy Peterman (Conservation Biology Institute)

Photo credit: Craig Allen (USGS)

References:

Allen, C. D., A. I. Macalady, H. Chenchouni, D. Bachelet, N. McDowell, M. Vennetier, T. Kitzberger, A. Rigling, D. D. Breshears, E. H. Hogg, P. Gonzalez, R. Fensham, Z. Zhang, J. Castro, N. Demidova, J-H Lim, G. Allard, S. W. Running, A. Semerci, and N. Cobb.  2010.  A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests.  Forest Ecology and Management 259(4): 660-684.

Breshears, D. D., N. S. Cobb, P. M. Price, C. D. Allen, R. G. Balice, W. H. Romme, J. H. Kastens, M. L. Floyd, J. Belnap, J. J. Anderson, O. B. Myers, and C. W. Meyer. 2005. Regional vegetation die-off in response to global change-type-drought. Proceedings of the National Academy of Sciences, USA. 102: 15144-15148.