Understanding the interplay of rooting strategy and plant hydraulic traits on the response of forest stands to climate variation of California

It is critical to understand the Mediterranean ecosystem of California in a changing climate, where desynchronized supply of energy and water, combined the periodic drought events, poses specific challenge for trees. To deal with such situations, trees may co-adjust their leaf and root behavior as regulated by plant hydraulic traits. Rooting strategies are particularly important but among the least studied hydraulic traits. And, we do not know how below and above ground traits interact.

The plant-hydraulic version of the Functionally Assembled Terrestrial Ecopsystem Simulator (FATES-Hydro), integrates the dynamics of plant hydraulic processes and their feedbacks to the environment. FATES-hydro is used at the Sierra Nevada Critical Zone 2 (US-CZ2) site to address: 1. what combinations of plant hydraulic traits maximize carbon gain of pine stands during a normal year? 2. do such combinations lead to the greatest resistance and/or resilience of pines to drought? We conduct a global sensitivity analysis on five model parameters: root distribution, fine roots to shoot ratio, slope coefficient (g1) of Ball-Berry model, and plant vulnerability curve.

Based on sensitivity analysis, the best strategy to maximize NPP is deep roots with less stomatal sensitivity to vapor pressure deficit (i.e. anisohydric) and weak drought tolerance. At CZ2, soil storage capacity is close to annual rain fall. Deep roots allow trees to effectively deplete soil water storage and reduce drainage loss. Thus more water is available when energy peaks in summer. Weak drought tolerance forces stomata to close when soil gets dry thus maintains some soil moisture during dry season. Model predictions based on this strategy give the best match to the flux tower observations and soil moisture variation. However, this strategy results in a significant drop of NPP, loss of xylem conductance (indicating cavitation), and low leaf water potential during drought. Shallow roots also result in low NPP but allow the tree to maintain relatively high xylem conductance during repeated years of drought, and resume to pre-drought state quickly afterward.

Our study suggests that the best strategy to max. net carbon gain won't lead to good resistance/ resilience to drought at CZ2; and CZ property needs to be considered in understanding responding of trees to climate change.