Topographical shifts in fine root lifespan in a mixed, mesic temperate forest

Abstract

Root lifespan, often is estimated in landscape- and ecosystem-level carbon models using linear approximations. In water manipulation experiments, fine root lifespan can vary with soil water content. Soil water content is generally structured by complex topography, which is largely unaccounted for in landscape- and ecosystem-scale carbon models. Topography governs the range of soil water content experienced by roots which may impact their lifespan. We hypothesized that root lifespan varied nonlinearly across a temperate, mesic, forested catchment due to differences in soil water content associated with topographic position. We expected regions of the landscape that were too wet or too dry would have soils that were not optimal for roots and thus result in shorter root lifespans. Specifically, we hypothesized that root lifespan would be longest in areas that consistently had soil water content in the middle of the soil water content spectrum, while in soils at either very low or very high soil water content, root lifespan would be relatively short. We tested this hypothesis by collecting and analyzing two years of minirhizotron and soil moisture data in plots widely distributed in the Shale Hills catchment of the Susquehanna-Shale Hills Critical Zone Observatory in Pennsylvania. We found that fine root lifespans were longer in traditionally wetter topographic regions, but detected no short term (biweekly) effect of soil moisture on root lifespan. Additionally, depth in soil, soil series, slope face orientation, and season of birth strongly affected root lifespans across the catchment. In contrast, lifespan was unaffected by root diameter or mycorrhizal association. Failure to account for these variables could result in erroneous estimates of fine root lifespan and, consequentially, carbon flux in temperate forested regions.

Fig 1. Susquehanna Shale Hills Critical Zone Observatory macroplot site map.

Elevation map of Susquehanna Shale Hills Critical Zone Observatory with included topographical contoured lines. White, red, green, and blue dots represent macroplot sites that were established at ridge top, mid-slope planar, swale, and valley floor locations, respectively. Cyan diamonds represent macroplot volumetric time domain reflectometry (TDR) sensors locations, 2–3 per plot. TDR observations were also taken at sites with previously installed soil moisture sensors termed “GroundHOG” sites, which were represented on the map as asterisks. GroundHOG stands for ground hydrologic observation gear. GroundHOG sites have additional gas, water and measuring sensors used by the SSHCZO that were not used in this study.

Fig 2. Survivorship of roots at different topographic locations.

Survivorship curve of first-order roots in the top 10 cm of soil born in the spring of 2017 at different topographic regions, specifically ridge top (RT), midslope planar (MSP), swale (SW), and valley floor (VF) locations.

Fig 3. Survivorship of roots at different depths and topographic locations.

(a) Survivorship curves of first-order roots at different soil depths born in the fall of 2017 at swale locations. (b) Survivorship curves of first-order roots at different soil depths at valley floor (VF) and midslope planar (MSP) locations.

Fig 4. Influence of soil series on root survivorship in midslope locations.

Survivorship curves of first-order roots in the top 10 cm of soil born in the spring of 2018 across soils on the north slope with different slope steepness, specifically: Steep Weikert (Steeper (MSP)), flatter Weikert (Flatter (MSP)), and Rushtown (Swales).

Fig 5. Seasonal trends in first-order root lifespan across the Susquehanna Shale Hills Critical Zone Observatory.

Bayesian regression models were used in kriging. Maps displayed are posterior means of days since first appearance of a root in the upper 10 cm of soil. Number of plots with at least ten roots produced (n≥10) that were used in kriging were as follows: Spring 2017 (13 plots), summer 2017 (35), fall 2017 (25), spring 2018 (9), summer 2018 (38), and fall 2018 (27).