Pinyon pine and juniper mortality impact understory vegetation and pinyon pine juvenile growth

Background/Question/Methods

Pinyon pine and juniper woodlands have experienced elevated levels of tree mortality in recent years.  While we are gaining a better understanding of the patterns and mechanisms associated with these events, some of the resultant ecological impacts are poorly understood.  Specifically, this study addressed the understory plant community and pinyon pine juvenile response to recent tree mortality.  Understory plant community effects were studied at two sites in northern Arizona: one juniper-dominated and one pinyon pine-dominated woodland.  At each site, we measured percent cover and species richness of understory vegetation beneath live and dead trees, as well as in open areas using a 2 x 3 m quadrat.  Also, pinyon pine establishment and juvenile growth were measured along a soil age chronosequence (930 – 750,000 y).  At each site, pinyon pine juvenile density and growth were quantified under live nurse plants, dead pinyon nurse plants, and open areas. 

Results/Conclusions

Understory plant cover and diversity had complex responses seven years after tree mortality across both sites.   At the juniper-dominated woodland site, understory plant species richness increased slightly but not significantly under dead junipers compared to live junipers.  Understory plant cover; however, increased by 1.3 times at this site and a portion of the cover and diversity response was due to a 150% increase in cheatgrass cover.  At the pinyon-dominated woodland, dead pinyon pine had five more species than live pinyon pines, while understory diversity beneath live and dead junipers was not significantly different.  Understory plant cover beneath dead pinyon pines on high stress locations increased by 400% compared to live pinyon pines.  However, cover was not significantly different under dead pinyon pines at low stress locations or under dead junipers at low and high stress locations.  Pinyon pine juvenile establishment and growth were impacted by nurse type, condition, and soil age.  On all sites, pinyon pine establishment was significantly reliant on nurse plants, with juvenile density under nurse plants increasing with soil age.  Also, juvenile pinyon pine growth was 20-40% greater beneath recently dead pinyon pine nurse plants compared to live pinyon pine nurses.   In conclusion, tree mortality events can have cascading effects on both understory plant community composition and pinyon pine establishment and growth.  Considering that climatic models predict further warming and increased incidences of drought over the coming decades, future tree mortality events will continue to have strong and likely lasting impacts on ecological communities.