LOBLOLLY-PINE ROOT-GROWTH AND DISTRIBUTION UNDER WATER-STRESS

A rhizotron study was conducted to assess the relationship between soi l water availability and shoot and root growth of loblolly pine (Pinus taeda L.) under conditions where O-2 availability and mechanical resi stance were considered not limiting. Relationships between soil water availability and (i) root growth rate, (ii) root distribution, (iii) g rowth periodicity, and (iv) above- and belowground allocation were eva luated under four watering regimes ranging from maintaining profile ne ar field rapacity (WK 1) to withholding moisture for 4 wk (WK 4). The final distribution of cumulative root growth was similar among waterin g treatments, even though root growth shifted downward with depth duri ng the growing period due to water depletion in surface soils. Relativ e root elongation rates were found to be linearly related to soil wate r potential. The strength of this relationship was improved by conside ring only periods of active root growth. Reductions in above- and belo wground growth were proportional among water-restricting treatments. S tem volume and root growth in the driest treatments were reduced 46 an d 41%, respectively, compared with the WK 1 cells, indicating that pat terns of biomass allocation above and below ground were similar among treatments. In the absence of other limiting soil factors, flushes of root growth in this study were a function of relatively short-term cha nges in soil water potential. Our results indicate that when maximum t ree biomass is desired, management efforts in establishing pine should be aimed at reducing any restrictions to root growth during establish ment.