Root Distribution of Two Tree Species Under a Heterogeneous Nutrient Environment

Potted sweetgum Liquidambar styracflua and loblolly pine Pinus taeda seedlings were grown for 4 months in a greenhouse to test the effects of soil nutrient heterogeneity on plant root development. The same quantity of fertilizer was applied either uniformly to the entire pot soil surface or to just one quarter of it. Pots were placed in full sunlight or underneath shade cloth that attenuated light to 30% of full sunlight. The spatial development of root systems of both species was strongly influenced by the fertilization arrangements, but they reacted to soil nutrient heterogeneity differently; sweetgum roots concentrated in the fertilized patch, while loblolly pine roots extended much more to the unfertilized patches. Root system morphology was also affected by nutrient arrangements. For both species, root interbranch length increased and fine roots were thinner in nutrient-rich patches. Root branching angles were either unaffected by nutrient patchiness or too variable to detect a trend. Root proliferation in the rich patches was primarily due to fine root elongation and development of higher order lateral roots. Fertilization arrangements under full light did not significantly affect most aspects of total plant growth or allometry, including total plant mass, above-ground mass, leaf mass, total fine root mass, total root mass, fine root mass/shoot mass and fine root mass/leaf mass. However, interspecific differences were usually significant. In shade, large fertilization treatment effects appeared in total plant mass, above-ground mass, leaf mass and height. Root development in heterogeneous soils differed between light treatments. Sweetgum was more plastic than loblolly pine in its reaction to soil nutrient patchiness under full light, but both had similar plasticity in shade. This study suggests that sweetgum and loblolly pine might have different nutrient foraging strategies and that the strategies shift as light availability changes. These results suggest that competitive interactions between sweetgum and loblolly pine may be affected by the spatial heterogeneity of soil resources. Since sweetgum is a more efficient soil forager, silvicultural methods that encourage uniform distribution of soil resources may also increase the competitive ability of loblolly pine, the more commercially valuable of the two species.