Woody canopy transpiration in a piñon-juniper woodland: Variation due to experimentally imposed drought and mortality

Background/Question/Methods

Extended drought and ensuing tree mortality has been observed in Southwest piñon pine (Pinus edulis) and juniper (Juniperus monosperma) woodlands (P-J woodland) in recent years.  In an effort to assess the response of these two species to extended drought, a large-scale rainfall manipulation experiment was implemented in a P-J woodland located in Central New Mexico in 2007.  We experimentally reduced ambient rainfall on three replicated 1600 m² plots using passive rainfall exclusion structures (45% precipitation reduction).  Here we address variation in woody canopy transpiration (E_Cw) resulting from; 1) drought (experimentally imposed or natural), and 2) reductions in plot level woody cover due to drought induced mortality.  We measured tree level canopy transpiration via sap-flow for piñon pine and juniper in each plot, and scaled these measurements to the entire canopy of each plot using estimates of total basal area and sapwood area. 

Results/Conclusions

At experiment implementation (2007), P-J woody cover basal area and LAI were quite similar between drought and ambient plots averaging 18.0 (±0.4) m²/ha and 0.68 (0.03) m²/m² respectively across both treatments.  Annual precipitation at the site ranged from a high of 330.8 mm/yr (2008, wettest year) to a low of 252.0 mm/yr (year 2011, driest).  Maximum daily E_Cw varied between wet and dry years, with maximum E_Cw in ambient plots averaging 0.29 (0.03) mm/day in wet years and 0.14 (0.03) mm/day in the driest year (2011).  In contrast, drought plots had maximum daily E_Cw rates of 0.17 (0.05) mm/day in wet years and 0.07 (0.02) mm/day in the driest year (2011).  Daily and annual E_Cw were reduced due to experimentally imposed and naturally occurring drought, and effects were most pronounced in plots that had experienced extensive drought related piñon mortality.  Specifically, in drought plots that had experienced the most extensive piñon mortality, a maximum E_Cw rate of 0.11mm/day and 0.05 mm/day was observed in years 2010 (wet) and 2011 (dry).  Clearly, structural changes in piñon-juniper woodlands due to drought induced tree mortality will have long-term impacts on ecological processes irrespective of future climate variability.