Differential responses of pinon and juniper in a rainfall manipulation experiment in central New Mexico, USA

Background/Question/Methods

In the southwestern USA, severe drought in 2000 - 2003 led to widespread tree mortality.  In pinon-juniper woodlands, extensive pinon mortality occurred while juniper mortality was typically low.  To understand the mechanisms behind the differential mortality of these  species, we manipulated ambient rainfall on twelve 40 x 40 m plots in a pinon-juniper woodland at the Sevilleta National Wildlife Refuge and LTER to quantify species responses to treatments that are roughly equivalent to long term extreme drought or above-average precipitation.  Each block of four plots included an ambient control plot, a drought plot where 45-50% of rainfall was diverted by clear plastic troughs, and a cover-control plot with inverted plastic troughs to control for precipitation-independent effects in drought plots.  An irrigation plot in each block received 4-6 supplements per year, each equivalent to ~20 mm of rain, to increase growing season water availability.  To address species treatment responses , we combined continuous measurements of soil moisture, air and soil temperature and sap flow of target trees, with periodic measurements that included plant water potential, branch growth, leaf area and leaf level gas exchange. 

Results/Conclusions

In August 2008, one year after treatment structures were completed, we observed extensive pinon mortality in drought plots of the two blocks with most southerly exposure and shallowest soils.  Juniper trees in these plots remained active until progressive and ongoing mortality was observed in summer of 2010, three years after treatment began.  A hydraulic model parameterized with site data, indicated that drought pinon that died in 2008 exceeded the critical limits associated with hydraulic failure.  Interpretation of these data was complicated by high bark beetle activity in the Los Pinos mountains surrounding the site; all of the pinon that died exhibited high levels of bark beetle activity and infection by Ophiostoma fungi, which exacerbate the hydraulic limitation of drought by occluding the xylem.  Although clear treatment differences were evident in the third, relatively flat, block with deeper soils, neither pinon nor juniper there have exhibited drought related mortality through early 2011.  Over three years of treatment, trees on this block have exhibited clear shifts in growth and canopy leaf area consistent with availability of precipitation.