On the transient role of plant xylem impairment over optimal root area and root depth distribution

Abstract:

By growing roots plants increase potential nutrient and water uptake, which facilitates biochemical and hydraulic health while adding respiration costs and potentially oversubscribing water use. It has been hypothesized that plants arrive at optimal root-to-leaf area (RLA) proportionality through tradeoffs between rhizosphere water and nutrient fluxes that maximize carbon gain while minimizing water loss. We encapsulated these concepts within an ecohydrological framework of soil-rhizosphere-plant hydraulic traits coupled to canopy biochemistry to test this apparent optimality of root allocation. The framework was implemented in TREES, a dynamic ecohydrology model that combines transience in hydraulic impairment with photosynthesis, respiration, and carbon allocation. We conducted model experiments in which total root area was distributed over shallow to deep root profiles, and under conditions where predawn xylem water potential either maintained homeostasis with soil water potential, an assumption of most ecohydrology models, or was fully dynamic with hydraulic impairment building during successive dry periods. The analysis was conducted on 6 conifer species of varying vulnerability to cavitation from climatically different sites in New Mexico, Wyoming, and Northern Wisconsin subject to climate change induced disturbance. Data included sap flux, eddy covariance fluxes, micrometeorology, and leaf level gas exchange. Species with lower vulnerability to cavitation had improved hydraulic status and transpiration at higher RLAs, but these improvements did not produce gains in carbon uptake, which suggested the need for more modest root areas. Distributing the same total root area to greater depth obviated hydraulic impairment in sites with larger precipitation events or lateral subsurface water flow, but only aided carbon uptake when nutrients were non-limiting. Except where chronic hydraulic impairment hindered photosynthesis, there was little advantage to growing deeper roots or maintaining homeostasis between xylem and rhizosphere water status, as both adjustments generally increased water use per unit gain in carbon. Based on the cross-site modeling experiments we offer some general scaling principles that consider tradeoffs between hydraulic and carbon uptake efficiency.