Click here to access the report. Here’s the abstract:
Tree Mortality Decreases Water Availability and Ecosystem Resilience to Drought in Piñon-Juniper Woodlands in the Southwestern U.S.
Climate-driven tree mortality has increased globally in response to warmer temperature and more severe drought. To examine how tree mortality in semiarid biomes impacts surface water balance, we experimentally manipulated a piñon-juniper (PJ) woodland by girdling all adult piñon trees in a 4 ha area, decreasing piñon basal area by ~65%. Over 3.5 years (2009–2013), we compared water flux measurements from this girdled site with those from a nearby intact PJ woodland. Before and after girdling, the ratio of evapotranspiration (ET) to incoming precipitation was similar between the two sites. Girdling altered the partitioning of ET such that the contribution of canopy transpiration to ET decreased 9–14% over the study period, relative to the intact control, while noncanopy ET increased. We attributed the elevated noncanopy ET in the girdled site each year to winter increases in sublimation and summer increases in both soil evaporation and below-canopy transpiration. Although we expected that mortality of a canopy dominant would increase the availability of water and other resources to surviving vegetation, we observed a decrease in both soil volumetric water content and sap flow rates in the remaining trees at the girdled site, relative to the control. This postgirdling decrease in the performance of the remaining trees occurred during the severe 2011–2012 drought, suggesting that piñon mortality may trigger feedback mechanisms that leave PJ woodlands drier relative to undisturbed sites and potentially more vulnerable to drought.
From Water Deeply (Matt Weiser):
Piñon trees have been dying in droves across the West. Laura Morillas, lead author of the new study, found that losing piñon trees doesn’t necessarily free up more water in these arid habitats. It could mean the opposite.
This forest, in which piñon and juniper trees grow together, is a unique natural community common throughout the arid West. It covers millions of acres in nine states, but is most abundant in Colorado, Arizona, New Mexico, Nevada and Utah.
Piñon and juniper trees, somewhat shrubby and short, are not particularly majestic compared to a ponderosa pine or a sequoia. But they’re vitally important to people, wildlife and water supplies. By providing shade in sunny, high-elevation landscapes, piñon-juniper forests help ensure snow and rain last long enough to reach rivers and groundwater before evaporating.
Unfortunately, piñon trees seem to be particularly vulnerable to climate change. They’ve been dying in great swathes in recent years due to heat and drought. What does this mean for water supplies?
A new study led by scientists at University of New Mexico begins to answer that question. At a 10-acre research site, the researchers intentionally girdled the piñon trees (removed a strip of bark from their circumference) to kill them, then compared results over several years to a nearby control site. They found a surprising result: Losing piñon trees did not make more water available in the soil or for surviving juniper trees, as common sense would dictate. Instead, the entire test plot lost moisture more rapidly.