John R. Butnor

Aboveground sink strength in forests controls the allocation of carbon below ground and its [CO2]-induced enhancement

The partitioning among carbon (C) pools of the extra C captured under elevated atmospheric CO2 concentration ([CO2]) determines the enhancement in C sequestration, yet no clear partitioning rules exist. Here, we used first principles and published data from four free-air CO2 enrichment (FACE) experiments on forest tree species to conceptualize the total allocation of C to below ground (TBCA) under current [CO2] and to predict the likely effect of elevated [CO2]. We show that at a FACE site where leaf area index (L) of Pinus taeda L. was altered through nitrogen fertilization, ice-storm damage, and droughts, changes in L, reflecting the aboveground sink for net primary productivity, were accompanied by opposite changes in TBCA. A similar pattern emerged when data were combined from the four FACE experiments, using leaf area duration (LD) to account for differences in growing-season length. Moreover, elevated [CO2]-induced enhancement of TBCA in the combined data decreased from ≈50% (700 g C m−2 y−1) at the lowest LD to ≈30% (200 g C m−2 y−1) at the highest LD. The consistency of the trend in TBCA with L and its response to [CO2] across the sites provides a norm for predictions of ecosystem C cycling, and is particularly useful for models that use L to estimate components of the terrestrial C balance.

EFFECT OF ELEVATED CO2 ON COARSE‐ROOT BIOMASS IN FLORIDA SCRUB DETECTED BY GROUND‐PENETRATING RADAR

Growth and distribution of coarse roots in time and space represent a gap in our understanding of belowground ecology. Large roots may play a critical role in carbon sequestration belowground. Using ground-penetrating radar (GPR), we quantified coarse-root biomass from an open-top chamber experiment in a scrub-oak ecosystem at Kennedy Space Center, Florida, USA. GPR propagates electromagnetic waves directly into the soil and reflects a portion of the energy when a buried object is contacted. In our study, we utilized a 1500 MHz antenna to establish correlations between GPR signals and root biomass. A significant relationship was found between GPR signal reflectance and biomass (R2 = 0.68). This correlation was applied to multiple GPR scans taken from each open-top chamber (elevated and ambient CO2). Our results showed that plots receiving elevated CO2 had significantly (P = 0.049) greater coarse-root biomass compared to ambient plots, suggesting that coarse roots may play a large role in carbon sequestration in scrub-oak ecosystems. This nondestructive method holds much promise for rapid and repeatable quantification of coarse roots, which are currently the most elusive aspect of long-term belowground studies.

The effects of 11 yr of CO2 enrichment on roots in a Florida scrub‐oak ecosystem

Uncertainty surrounds belowground plant responses to rising atmospheric CO₂ because roots are difficult to measure, requiring frequent monitoring as a result of fine root dynamics and long-term monitoring as a result of sensitivity to resource availability. We report belowground plant responses of a scrub-oak ecosystem in Florida exposed to 11 yr of elevated atmospheric CO₂ using open-top chambers. We measured fine root production, turnover and biomass using minirhizotrons, coarse root biomass using ground-penetrating radar and total root biomass using soil cores. Total root biomass was greater in elevated than in ambient plots, and the absolute difference was larger than the difference aboveground. Fine root biomass fluctuated by more than a factor of two, with no unidirectional temporal trend, whereas leaf biomass accumulated monotonically. Strong increases in fine root biomass with elevated CO₂ occurred after fire and hurricane disturbance. Leaf biomass also exhibited stronger responses following hurricanes. Responses after fire and hurricanes suggest that disturbance promotes the growth responses of plants to elevated CO₂. Increased resource availability associated with disturbance (nutrients, water, space) may facilitate greater responses of roots to elevated CO₂. The disappearance of responses in fine roots suggests limits on the capacity of root systems to respond to CO₂ enrichment.

Using Ground-Penetrating Radar to Detect Tree Roots and Estimate Biomass

Ground-penetrating radar (GPR) is a nondestructive means of detecting buried objects with electromagnetic waves. It has been applied to detect coarse woody roots, estimate biomass, root diameter, and spatial distribution of roots. This chapter discusses the development of root assessment techniques, basic methodology, and examples of field applications where GPR was successful.

Maximum growth potential in loblolly pine: results from a 47-year-old spacing study in Hawaii

Growth, allocation to woody root biomass, wood properties, leaf physiology, and shoot morphology were examined in a 47-year-old loblolly pine (Pinus taeda L.) density trial located in Maui, Hawaii,...

Phenotypic analysis of first-year traits in a pseudo-backcross {(slash x loblolly) x slash} and the open-pollinated families of the pure-species progenitors

A single test, including one pseudo-backcross (Pinus elliottii x Pinus taeda) x P. elliottii and open-pollinated families of the pure species progenitors, was established in North Central Florida in December 2007 to study the transfer of the fast-growing characteristics from a P. taeda L. (loblolly pine) parent into the P. elliottii Engelm. (slash pine) background. Several traits were measured in the first growing season: height growth, phenology, tip moth incidence, stem traits, crown architectural and needle traits. Heterosis was evaluated for each trait using analyses of variance by fitting a linear mixed model. All traits were significantly (p value < 0.05) different among families while the significance for heterosis varied by trait. Positive heterosis was found for average rate of shoot elongation (ASRE), total growth (TG), total height and number of needles per fascicle while the opposite was true for base diameter, top diameter, fascicle length, fascicle diameter, crown projected area and phenological traits (cessation, duration and day to reach 50% of the height). Average performance (i.e., no heterosis) was found for initiation of growth, number of branches, number of nodes, tip moth incidence, sheath length and specific leaf area. The analyses indicated that introgression of loblolly pine alleles into slash pine was effective and novel trait combinations were achieved. The pseudo-backcross had larger variation in early height growth than the slash pine families and was taller than all open-pollinated families at the end of the first season. Tip moth incidence was much lower than the loblolly pine family.

Experimental Evaluation of Several Key Factors Affecting Root Biomass Estimation by 1500 MHz Ground-Penetrating Radar

Accurate quantification of coarse roots without disturbance represents a gap in our understanding of belowground ecology. Ground penetrating radar (GPR) has shown significant promise for coarse root detection and measurement, however root orientation relative to scanning transect direction, the difficulty identifying dead root mass, and the effects of root shadowing are all key factors affecting biomass estimation that require additional research. Specifically, many aspects of GPR applicability for coarse root measurement have not been tested with a full range of antenna frequencies. We tested the effects of multiple scanning directions, root crossover, and root versus soil moisture content in a sand-hill mixed oak community using a 1500 MHz antenna, which provides higher resolution than the oft used 900 MHz antenna. Combining four scanning directions produced a significant relationship between GPR signal reflectance and coarse root biomass (R2 = 0.75) (p < 0.01) and reduced variability encountered when fewer scanning directions were used. Additionally, significantly fewer roots were correctly identified when their moisture content was allowed to equalize with the surrounding soil (p < 0.01), providing evidence to support assertions that GPR cannot reliably identify dead root mass. The 1500 MHz antenna was able to identify roots in close proximity of each other as well as roots shadowed beneath shallower roots, providing higher precision than a 900 MHz antenna. As expected, using a 1500 MHz antenna eliminates some of the deficiency in precision observed in studies that utilized lower frequency antennas.

Current applications of GPR in forest research

Forests, both naturally regenerated stands and plantations are complex, long-lived systems, which can be difficult to assess and monitor over time. This is especially true of belowground biomass and internal features of trees which are inaccessible except by destructive sampling. Traditional methods are expensive, destructive, time-consuming, usually yield a small sample size and are not conducive to long-term monitoring. Since GPR was first used to map tree roots ten years ago, a variety of new applications have been introduced. On soils suitable for radar studies, root biomass surveys have been valuable means to quantify belowground biomass, spatial distribution of roots, measure root diameters and even map individual roots. Methods include collecting linear transects in reflection mode, interlacing grids of transects in order to create 3D reconstructions of roots and applying high frequency borehole antennas used in transmission mode to model vertically oriented roots. One of the more difficult problems we are currently considering is how to analyze permanently marked transects over time to monitor root development while soil moisture, temperature and surface conditions change seasonally. In a departure from subsurface analysis, we have recently employed a method using GPR to detect defects and moisture gradients in stems. Introduction Forests are complex, long-lived systems which present many challenges to analysis and inventory. Over the past hundred years, numerous methods of inventorying aboveground biomass have been developed ranging from allometric equations used to extrapolate wood volume from tree diameter to satellite-based sensors which can measure canopy height and density over a wide area. Methodology to quantify belowground biomass has lagged behind, until recently destructive excavations were the only option. Unlike annual crops; trees live for decades or longer, so destructive sampling is usually undesirable and not repeatable. The investment trees make in roots is considerable, up to half of the biomass in trees may be hidden belowground. Ground-penetrating radar (GPR) has been demonstrated to be a rapid means of detecting tree roots and measuring lateral root mass in well-drained, electrically resistive soils (Butnor et al. 2001; Butnor et al. 2003; Barton and Montagu 2004; Cox et al. 2005; Stover et al. 2007; Samuelson et al. 2008). Today, tree root biomass studies provide valuable insight into belowground productivity in forest systems and are used to test the effect of tree species, genetic selection, and subsequent management on carbon (C) allocation. In this paper we discuss several successful applications where GPR was used to quantify root mass, map root distribution, assess vertically oriented tap roots with borehole radar and detect defects in stems. Our purpose is to present how the technology is currently being used and highlight areas where the research is headed.

Integrating estimates of tree root mass predicted with ground penetrating radar and allometry

Ground penetrating radar (GPR) operated in reflection mode may be used to estimate lateral root biomass in forests. The technique has been very useful for quantifying belowground biomass and accounting for carbon in silivicultural studies. In general, surface-based GPR cannot detect fine roots (<2 mm diameter), vertical taproots, decayed roots or separate roots by species. This presents challenges when integrating GPR-based assessments of lateral roots (between trees) and below-stump biomass estimates (directly below trees) modeled from stand inventory data (e.g. diameter, height) as there may be overlap between the approaches. To support ongoing research in longleaf pine (Pinus palustris mill.) ecosystems, we scanned 11 longleaf pine trees aged 15 to 79 years with GPR and compared the results to excavations. A 16 m2 area around each tree was surveyed with 1500 MHz antenna via 9 parallel lines, 0.5 m apart with the tree located in the center. A root biomass map was made for each tree. The size of the excavated pit around each tree was calculated from the linear relationship between tree basal area and pit size e.g. younger trees < 25 yo 1.0 to 1.5 m2, older, larger trees 1.5 to 4.0 m2. Excavated roots were classified as lateral (roughly perpendicular to stem) and taproot (vertical roots) then weighed after drying. The area of the excavated pit was noted on the root biomass map and the mass detected in that area was calculated and compared to the mass of lateral roots. The proportion of roots “missed” by GPR was negligible for small trees ~10 cm diameter at breast height (DBH), increased to 87% for the largest tree (54 cm DBH). Fortunately, the proportion of longleaf pine lateral roots detected by GPR can be predicted using an exponential decay function fitted with tree DBH. The assumption that GPR detects all lateral roots may be valid for small trees (<10 cm DBH), though underestimation of mass is expected with larger trees.