Hydrologic Tipping Points in Semi-Arid Biomes
Funding: NSF- Ecosystems
Research Team: Marcy Litvak (PI), Will Pockman (UNM), Susan Schwinning (Texas State), Andy Fox (UA), Rob Pangle (UNM), Cheng-Wei Huang (UNM), Robert Mullin (UNM), Amanda Sacks (UNM)
Objectives: Our goal is address an important and poorly-understood consequence of tree mortality, the effect of tree die-off on the environmental conditions that control future vegetation and ecosystem recovery from such a significant disturbance. The die-off of a large number of trees might suggest that the fraction of water previously consumed by the now-dead trees might be available to support growth of surviving trees, herbaceous vegetation, and seedlings. Yet data from an ecosystem-scale experiment simulating mortality, in which all pinon pines > 7cm diameter were girdled within a large plot in a pinon-juniper woodland, showed that despite the removal of over 30% of canopy area, the site became drier and hotter.
Hypotheses: We are testing the overarching hypothesis that piñon mortality in piñon-juniper woodlands (PJ) triggers a decline in both soil moisture and function in the remaining woody species by: 1a) reducing the movement of soil moisture via hydraulic redistribution (HR), connectivity between shallow and deep soil layers, and size of deep soil water storage; 1b) increased sublimation and below canopy ET causing an overall decline in shallow soil moisture and 1c) altered surface distribution due to soil water repellency of conifer litter. We also hypothesize that: 2a) there is a specific threshold density of trees where we see aridification following mortality; 2b) we should see more aridification following piñon vs. juniper mortality, due to the different functional roles these species serve in PJ woodlands: 3a) the presence of a petrocalcic layer reducing infiltration and 3b) precipitation regime that might reduce the size of soil water storage capacity (reduced winter precip and/or longer pre-monsoon dry period) are likely to facilitate aridification following mortality. Finally we hypothesize that aridification might trigger vegetation regrowth that favors more drought tolerant species in the long run, and the combination of structural changes and short term reduction in resilience of drought, might push PJ woodlands to an irreversible transition to a new ecosystem state.
Research Approach: To test these hypotheses, we are manipulating the species present in large field plots by girdling to simulate mortality of all trees of only one species per plot (piñon or juniper) to examine species specific roles these woody species play in regulating the hydrology of PJ woodlands. We will measure the response of hydraulic redistribution and sapflow in the remaining species, soil moisture profiles, surface energy balance components, both with and without litter manipulations. We will couple these field measurements to identify and quantify the mechanisms that contribute to increased aridity following mortality with experiments using Community Land Model, CLM 4.5, with an additional HR component, to: 1) quantify the role of HR in model simulations, how it impacts profiles of soil moisture, and affects transpiration and soil evaporation; 2) assess how species specific dieback, and changes in rooting distributions affect HR within the model; 3) assess how species specific dieback changes surface energy balance dynamics, including winter sublimation, and how this affects soil water recharge; 4) vary mortality rates to identify any threshold values leading to feedbacks causing or enhancing aridification; 5) investigate the role of litter on surface infiltration and runoff.
Hypotheses: We are testing the overarching hypothesis that piñon mortality in piñon-juniper woodlands (PJ) triggers a decline in both soil moisture and function in the remaining woody species by: 1a) reducing the movement of soil moisture via hydraulic redistribution (HR), connectivity between shallow and deep soil layers, and size of deep soil water storage; 1b) increased sublimation and below canopy ET causing an overall decline in shallow soil moisture and 1c) altered surface distribution due to soil water repellency of conifer litter. We also hypothesize that: 2a) there is a specific threshold density of trees where we see aridification following mortality; 2b) we should see more aridification following piñon vs. juniper mortality, due to the different functional roles these species serve in PJ woodlands: 3a) the presence of a petrocalcic layer reducing infiltration and 3b) precipitation regime that might reduce the size of soil water storage capacity (reduced winter precip and/or longer pre-monsoon dry period) are likely to facilitate aridification following mortality. Finally we hypothesize that aridification might trigger vegetation regrowth that favors more drought tolerant species in the long run, and the combination of structural changes and short term reduction in resilience of drought, might push PJ woodlands to an irreversible transition to a new ecosystem state.
Research Approach: To test these hypotheses, we are manipulating the species present in large field plots by girdling to simulate mortality of all trees of only one species per plot (piñon or juniper) to examine species specific roles these woody species play in regulating the hydrology of PJ woodlands. We will measure the response of hydraulic redistribution and sapflow in the remaining species, soil moisture profiles, surface energy balance components, both with and without litter manipulations. We will couple these field measurements to identify and quantify the mechanisms that contribute to increased aridity following mortality with experiments using Community Land Model, CLM 4.5, with an additional HR component, to: 1) quantify the role of HR in model simulations, how it impacts profiles of soil moisture, and affects transpiration and soil evaporation; 2) assess how species specific dieback, and changes in rooting distributions affect HR within the model; 3) assess how species specific dieback changes surface energy balance dynamics, including winter sublimation, and how this affects soil water recharge; 4) vary mortality rates to identify any threshold values leading to feedbacks causing or enhancing aridification; 5) investigate the role of litter on surface infiltration and runoff.