Effects of nurse-tree crop species and density on nutrient and water availability to underplanted Toona ciliata in northeastern Argentina

Cultivation of high-value hardwoods is often more difficult than cultivation of many pioneer species commonly used in fast-growing plantations. On some sites, the facilitative effects of nurse trees can be necessary for initial crop species establishment, but their competitive effects can also reduce juvenile growth rates of the crop species. To improve establishment success in mixed-species plantations, we tested the effects of the nurse-tree species Grevillea robusta A.Cunn. ex R.Br., Pinus elliottii Engelm. × Pinus caribaea Morelet, and Pinus taeda L. and four densities (0%, 25%, 50%, and 75% of the initial density) on Toona ciliata M.Roem. light, soil water, and soil nutrient availability. Growth of T. ciliata tended to increase with decreasing nurse-tree density and increasing light availability. However, growth was greater under G. robusta than under the pines, even where light conditions were similar, corresponding to mostly higher nutrient availability and higher soil water contents underneath G. robusta. Wood δ13C of T. ciliata was positively correlated with growth, foliar nutrient contents (N, P, K, Mg, Ca), and soil water content at a depth of 20–40 cm. Our results suggest that G. robusta is less competitive for soil nutrients and water than the pine nurse-tree species.