Capacitive recharge, hydraulic redistribution, and nighttime transpiration as competing water sinks in Pinus ponderosa
Abstract
Forests in northern Idaho experience seasonal drought from late-spring to mid-fall, the effects of which are mitigated by deep ash-capped soils. Despite not being recharged through the grow season, these soils supply enough water for five months of transpiration. We used this five-month dry down to observe the water fluxes within three adjacent 35-year-old Pinus ponderosa trees in detail. Our goal was to determine the importance of three potentially competing water sinks: capacitive recharge, hydraulic redistribution, and nighttime transpiration and their impact on the trees' functionality at different points as their water source declined. Heat ratio method sap flow probes, used for their ability to determine directional flow, were installed in roots, lower trunks, upper trunks, lower branches, and upper branches. The timing and magnitude of the organ-level water fluxes were compared to predawn and midday leaf water potentials, which were recorded every two weeks, to indicate an increase in the ratio of capacitive recharge to overall transpiration as predawn water potentials declined. This was corroborated by volumetric water content of the sapwood, measured continuously, that showed daily cycles and a seasonal decline. Hydraulic redistribution, which has been observed in a nearby stand, was not perceptible at this site. Twenty-four hour diurnal leaf-level gas exchange showed significant nighttime transpiration in July, before predawn water potential values began to decline (-0.83 MPa), but not in September, when predawn water potential values were at their most negative (-1.6 MPa). The trees exhibited a classically isohydric strategy to maintain relatively non-negative water potentials, decreasing the magnitude of daily transpiration through the dry down, with midday water potentials not declining below -2.1 MPa, a common threshold for the species.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.B51I2047B
- Keywords:
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- 0428 Carbon cycling;
- BIOGEOSCIENCESDE: 0439 Ecosystems;
- structure and dynamics;
- BIOGEOSCIENCESDE: 0476 Plant ecology;
- BIOGEOSCIENCESDE: 1813 Eco-hydrology;
- HYDROLOGY