Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/21/2003
Publication Date: 4/1/2004
Citation: Kern, C.C., Friend, A.L., Johnson, J.M., Coleman, M.D. 2004. Fine-root dynamics in a developing populus deltoides plantation. Tree Physiology. 24:651-660. Interpretive Summary: Cottonwood trees are fast growing and can be used for paper production. Small non-woody or fine roots represent a relatively small percent of the total root biomass in a forested ecosystem. These roots can represent from 1 to 15% of the root biomass, depending on the age of the forest. In a young forest fine roots represent a larger percent of the total root biomass compared to a mature forest. Although fine roots are only a small part of the total root biomass, they play an integral role in carbon and nitrogen cycling. Growing roots utilize carbon and nitrogen. When the roots die, they release carbon and nitrogen. Nitrogen is a limiting nutrient in many forest systems. Nitrogen fertilizer use has environmental risks including migration of nitrogen into ground water systems. Roots are primarily responsible for nutrient and water uptake. Understanding root growth and life cycle dynamics in response to nitrogen management together with above-ground growth can improve nitrogen fertilizer use efficiency in tree plantation. Prudent management improves the potential for tree root systems to mitigate nitrate-contaminated ground and surface water as riparian or wastewater filters. This study sought to resolve some of the uncertainty in root production in response to nitrogen fertilizer using fast growing cottonwood trees propagated from the same genetic material. The growth and development was studied for two seasons to account for developmental affects and provide insight into the differences between seeding and ecosystem studies. Our results suggested that plantation studies provide a good model system to understand other forested settings and are more representative than potted seedling studies. The results will help define recommended fertilizer rates for tree plantations with maximum benefit and minimal environmental impact.
Technical Abstract: A cottonwood (Populus deltoides) plantation was established to evaluate nutrient availability on fine-root dynamics. Time-release fertilizer was applied to 225 m**2 plots at rates of 0, 50, 100, and 200 kg N ha**-1. Fine-root production, mortality, live-root standing crop, and lifespan were analyzed with monthly nondestructive photographic (minirhizotron) observations. Soil coring was used to measure fine-root biomass. Fine roots were controlled more by temporal, depth and diameter size factors rather than by fertilization. Cumulative fine-root production and mortality had strong seasonal patterns. Production was greatest during the mid-growing season and mortality was greatest after the growing season. Because cumulative production increased with fertilization more than cumulative mortality, the standing crop of live roots increased linearly with fertilizer treatment. However, cumulative production and mortality were high in control treatments, then fell to a minimum with intermediate levels of nutrient availability, and rose again to a maximum with 200 kg N ha**-1. Above-ground growth responded positively to fertilization, yet growth responses were equivalent despite a three-fold increase in application rate. The greatest fertilizer use and lowest fine-root activity occurred at the lowest treatment (50 kg N ha**-1). Fine-root lifespans ranged from 307 to over 700 days and increased with depth, diameter and nutrient availability. The strongest treatment effects were found with small diameter roots at shallow soil depths. The results of this plantation study are similar to results found in other forested settings suggesting it is a good model system to study factors controlling root system responses. Fine root turnover was not at steady state as fine-root production was four-fold higher than mortality during the last year of observation.