Fine root heterogeneity by branch order: exploring the discrepancy in root turnover estimates between minirhizotron and carbon isotopic methods

Abstract

Summary• Fine roots constitute a large and dynamic component of the carbon cycles of terrestrial ecosystems. The reported fivefold discrepancy in turnover estimates between median longevity (ML) from minirhizotrons and mean residence time (MRT) using carbon isotopes may have global consequences.• Here, a root branch order-based model and a simulated factorial experiment were used to examine four sources of error.• Inherent differences between ML, a number-based measure, and MRT, a massbased measure, and the inability of the MRT method to account for multiple replacements of rapidly cycling roots were the two sources of error that contributed more to the disparity than did the improper choice of root age distribution models and sampling bias. Sensitivity analysis showed that the rate at which root longevity increases as order increases was the most important factor influencing the disparity between ML and MRT.• Assessing root populations for each branch order may substantially reduce the errors in longevity estimates of the fine root guild. Our results point to the need to acquire longevity estimates of different orders, particularly those of higher orders.

Estimating fine root longevity in a temperate Norway spruce forest using three independent methods

“…Minirhizotron observations and isotope-based estimates of C age are currently considered as being the most promising approaches for investigating fine root longevity (Tierney and Fahey 2002;Guo et al 2008). However, it remains largely uncertain, why both methods produce very different results.…”

Whole-tree dynamics of non-structural carbohydrate and nitrogen pools across different seasons and in response to girdling in two temperate trees

“…The entire root systems can be separated into nine branch orders in both species, with the distal root tip being the first order. In this study, the first to third branch orders were defined as absorptive roots (mean diameter <0.5 mm), and the forth to ninth branch orders were defined as non-absorptive roots (mean diameter >0.5 mm) based on the relationships between the diameter, anatomy and lifespan of different branch orders for the two species (Wang et al 2006;Guo et al 2008;Xia et al 2010). After collection, all samples were immediately placed on dry ice, transported to the laboratory within 2 h and then killed in a microwave oven (5 min at 800 W, with a glass of water inside the oven to avoid overheating; Würth et al 2005).…”

“…Tree roots have a complex branching system with one to three distal non-woody branch orders devoted to resource uptake and several other woody root branch orders devoted to transport and long-term storage (Guo et al 2008;Xia et al 2010). Past studies documenting NSC and N dynamics in roots have not considered this branching architecture and have mostly separated root systems into coarse and fine roots based on root diameter (Latt et al 2001;Würth et al 2005;Kobe et al 2010).…”

“…Past studies documenting NSC and N dynamics in roots have not considered this branching architecture and have mostly separated root systems into coarse and fine roots based on root diameter (Latt et al 2001;Würth et al 2005;Kobe et al 2010). However, this diameterbased approach may not accurately distinguish roots of different functions (absorptive roots vs. non-absorptive storage roots) (Pregitzer et al 2002;Guo et al 2004Guo et al , 2008. To date, no published studies have reported NSC and N concentrations, pools and their seasonal dynamics across branch orders of the entire root system in mature trees.…”

Which is the best indicator for distinguishing between fine roots with primary and secondary development in <i>Cryptomeria japonica</i> D. Don: Diameter, branching order, or protoxylem groups?

“…Roots with primary developments have a living cortex, develop symbiotic associations with soil fungi, and are responsible for water and nutrient absorption (Hishi 2007, Guo et al 2008c). Roots with secondary developments have a cork layer and secondary xylem that provides protection from environmental stresses and carries out transport, anchorage, and storage functions (Brundrett 2002, Guo et al 2008a,b, Valenzuela-Estrada et al 2008. Roots with primary and secondary developments have been distinguished based on their branching order, number of protoxylem groups, and diameter within the fine root system (e.g., Hishi andTakeda 2005a,b, Guo et al 2008c).…”