Opportunistic Short-term Compensatory Water Uptake by Subalpine Trees
Observed via in situ Water Isotope Measurements
Abstract
Tree water source variation in semi-arid ecosystems is important to
understand because climatic shifts towards lower snowpack and increased
drought affect water availability in subalpine forests of the western
US.
Here, we use daily in situ measurements of stable isotopes (2H)
in soil and tree water, soil matric potential and sap flow to study tree
water uptake dynamics. We instrumented three soil profiles down to 90
cm, as well as three Aspen and Engelmann Spruce trees near Gothic,
Colorado, in the East River watershed. We observed the fate of natural
2H variations in rainfall, soil, and plants from June to October
2022, and in August 2023 we conducted a 2H labeled water irrigation
experiment.
Our observations showed that transpiration was reduced by all trees,
but partially compensated by shifting the dominant water source from 60
to 90 cm within days of a dry period. Intense rainfall quickly shifted
the plant water uptake partially to top soil layers. Changes in water
uptake depths were similar between aspen and spruce, but rainfall
infiltration was low in the spruce stand due to high canopy
interception. Therefore spruce transpiration was lower and relied more
on snowmelt. However, both species relied on snowmelt to sustain
transpiration and groundwater recharge from monsoonal rains was not
observed. These findings highlight the important role of snowmelt stored
in the deep soil layers for subalpine forest drought response and the
dominant fate of monsoonal rainfall to become transpiration rather than
recharging groundwater and streams in the Upper Colorado River.