Frank P. Day

Day, F.P.; Adams, E.C.; Gibala-Smith, L.A.; Graziani, D.J.; McMillan, B.; Sedghi, N.; Shafer, J., and Smith, M., 0000. Determining change in coastal barrier island dune vegetation following a decade of nitrogen fertilization. Nitrogen deposition from agricultural and industrial sources is a threat to terrestrial biodiversity, and impacts are likely to be greatest in nitrogen-deficient systems. The results of a 10 year fertilization treatment on nitrogen-deficient coastal barrier island dunes and the decade following cessation of fertilizer application are reported here. The study objective was to determine the resilience of the dune plant communities. Would these communities return to their former state in terms of density, species composition, and species diversity after nitrogen additions ended, and if so, how rapid was their recovery? Plant density by species and species diversity were determined over a 20 year period from permanent plots established on three different-aged dunes on a Virginia barrier island. Two dominant species responded differently to nitrogen additions. Ammophila breviligulata abundance was enhanced by the additions, and, in most cases, Spartina patens abundance was repressed. On the older Hog Island dunes, the addition of nitrogen over a 10 year period had a dramatic and rapid negative effect on diversity. However, recovery to diversity values observed in the control plots was rapid, beginning almost immediately after nitrogen additions were stopped. The dune vegetation on a Virginia barrier island appears to be quite resilient, as it rapidly recovered from diversity loss after 10 years of nitrogen additions.

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.

Summary: Uncertainty surrounds belowground plant responses to rising atmosphericCO₂because 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 inFlorida exposed to 11 yr of elevated atmosphericCO₂using 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 elevatedCO₂occurred 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 elevatedCO₂. Increased resource availability associated with disturbance (nutrients, water, space) may facilitate greater responses of roots to elevatedCO₂. The disappearance of responses in fine roots suggests limits on the capacity of root systems to respond toCO₂enrichment.

Frank Day (2007) UMEB Program. Wetland Science and Practice: Vol. 24, No. 3, pp. 12-13. https://doi.org/10.1672/0732-9393(2007)24[12b:UP]2.0.CO;2

Elevated atmospheric carbon dioxide (CO₂) often stimulates the growth of fine roots, yet there are few reports of responses of intact root systems to long‐term CO₂ exposure. We investigated the effects of elevated CO₂ on fine root growth using open top chambers in a scrub oak ecosystem at Kennedy Space Center, Florida for more than 7 years. CO₂ enrichment began immediately after a controlled burn, which simulated the natural disturbance that occurs in this system every 10–15 years. We hypothesized that (1) root abundance would increase in both treatments as the system recovered from fire; (2) elevated CO₂ would stimulate root growth; and (3) elevated CO₂ would alter root distribution. Minirhizotron tubes were used to measure fine root length density (mm cm⁻²) every three months. During the first 2 years after fire recovery, fine root abundance increased in all treatments and elevated CO₂ significantly enhanced root abundance, causing a maximum stimulation of 181% after 20 months. The CO₂ stimulation was initially more pronounced in the top 10 cm and 38–49 cm below the soil surface. However, these responses completely disappeared during the third year of experimental treatment: elevated CO₂ had no effect on root abundance or on the depth distribution of fine roots during years 3–7. The results suggest that, within a few years following fire, fine roots in this scrub oak ecosystem reach closure, defined here as a dynamic equilibrium between production and mortality. These results further suggest that elevated CO₂ hastens root closure but does not affect maximum root abundance. Limitation of fine root growth by belowground resources – particularly nutrients in this nutrient‐poor soil – may explain the transient response to elevated CO₂.