Effects of root severing treatments on loblolly pine

Abstract

Six-year-old loblolly pine seedlings were subjected to root severing treatments varying from 0 to 100% of first-order lateral roots. Separate treatments severed surface-oriented or deep-oriented roots. Plant water status was monitored periodically for several months. After all measurements were taken, gross root system structure was determined by excavation. Treatment responses were evident on all dates of measurement. Relationships between percentage of root system cut and leaf conductance or water potential were stronger when surface-oriented roots were cut than when deep-oriented roots were cut. Severing surface-oriented first-order lateral (SOFOL) roots probably resulted in greater impact on plant water status than severing deep-oriented first-order lateral (DOFOL) roots because (i) SOFOL roots had both surface-oriented and deep-oriented second-order lateral roots that could tap both surface and subsurface soil horizons for soil moisture, and (ii) the deep-oriented second-order roots (originating from the SOFOL roots) were spatially distributed over a much larger area than the DOFOL roots and thus would have access to soil water in a larger volume of soil. For SOFOL roots the relationship between percentage cut and leaf conductance or transpiration was strongly negative; for DOFOL roots, no relationship between these variables was observed. Initially water potential decreased with the percentage of roots cut in both groups; in later measurements, water potential was affected more by severing SOFOL than DOFOL roots. Calculation of soil moisture depletion by depth indicated that both surface- and deep-oriented second-order lateral roots were important for water uptake. Severing SOFOL roots significantly decreased nitrogen, phosphorus, and potassium levels in needles of the first growth flush of the year. Levels of these elements in terminal buds were not affected by severing SOFOL roots, but were significantly reduced by severing DOFOL roots. Secondary xylem production was reduced proportionately to the amount of root system cross-sectional area severed.