Fine Root Dynamics in a Pinus palustris Mill. Ecosystem: The Role of Sampling Interval and the Soil Environment

Chapter 1 Abstract: We examined the impact of sampling interval on fine root production and mortality estimates by comparing data from a weekly minirhizotron sampling regimen to subsets of the same data representing biweekly, monthly, bimonthly, and quarterly sampling regimens. We also investigated possible sources of error involved in the root tracing technique and estimated root herbivory using the full weekly sampling regimen. Data were collected for eleven months from a Pinus palustris Miller woodland in southwest Georgia. As sampling interval increased, estimates of production and mortality declined, while estimates of mean fine root lifespan increased. Annual production values ranged from a maximum of 1.26 mm/cm2 for weekly sampling to 0.83 mm/cm2 for quarterly sampling. Total mortality varied from 0.97 mm/cm2 to 0.53 mm/cm2. Bias increased at a decreasing rate when sample interval was increased from weekly to monthly. The root tracing protocol added some small, random error to growth measurements; re-measuring roots returned values 0.16% smaller than initial measures. We also observed a root mortality and regrowth phenomenon that may be measurement error or short-term fluctuation in root length. Herbivory accounted for greater than 20% of fine root biomass produced. Our study suggests that increases in sampling frequency from monthly to weekly can provide substantial gains in accuracy for estimates of root dynamics.

Chapter 2 Abstract: We examined the impact of soil environmental variables (soil temperature, moisture, and available nitrate (NO3-) and ammonium (NH4+)) on the production, mortality, standing crop, turnover, and lifespan of Pinus palustris Miller fine roots using the minirhizotron technique. Data were collected for a full year from a P. palustris woodland in southwest Georgia. Mean soil temperatures appeared to have little influence on root processes, while temperature variance had a strong effect. More thermally variable microsites had increased root turnover and reduced root lifespans. Soil resources had a significant impact on demography; in particular, soil moisture and nitrate stimulated production, mortality, and turnover. High levels of soil resource availability also significantly reduced lifespan. Root lifespan was variable among individual roots based on root width, depth in the soil volume, and season of root production. Soil moisture had the strongest overall influence on root demography. This may result from the nature of our ecosystem (deep sands and subtropical climate); in addition, severe drought during our study may have enhanced the role of soil moisture, allowing environmental controls to increase in strength relative to within-plant controls on root demography.