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Effects of nurse tree species on growth environment and physiology of underplanted Toona ciliata Roemer in subtropical Argentinean plantations Dordel, Julia

Abstract

Toona ciliata Roem. plantations require a nurse-tree overstorey to minimize damage from drought and frost in north-eastern Argentina. Nurse-species vary in their facilitative and competitive abilities, thus influencing trade-offs among T. ciliata establishment success, stem quality and productivity, and ultimately stand revenues. The first objective was to quantify the influence of nurse species (G. robusta A. Cunn., Pinus elliottii Englm. x Pinus caribaea Morelet, P. taeda L.) and thinning density on mortality, stem morphology and growth of under-planted T. ciliata. Trade-offs among performance traits were considered in developing recommendations for nurse species selection and density management. The second objective was to examine interspecific interaction mechanisms by determining the effects of nurse species and thinning intensities on light, soil water and nutrient availability to T. ciliata. The final objective was to link T. ciliata establishment success and early growth to soil water availability, by calibrating the two-dimensional forest hydrology model, ForWaDy. To develop a stand management tool for guiding establishment of T. ciliata plantations it was found that T. ciliata responded more to nurse species identity than density. G. robusta was less competitive with T. ciliata for soil nutrients and soil water than either of the pine species: P. elliottii x P. caribaea was an strong competitor for water while P. taeda was a strong competitor for nutrients. G. robusta, by contrast, increased water and phosphorus availability to T. ciliata. Density effects varied with nurse species and performance traits. ForWaDy had good predictive capability for early T. ciliata performance, and was especially discriminatory among nurse species effects. This study shows that nurse species and thinning effects depend on management objectives, but that nurse species choice can be more important than thinning for increasing early T. ciliata performance. Future research employing a greater range of overstorey species, thinning densities, tree ages and environmental conditions will improve predictions of mixed stand productivity under a broader range of conditions. The study contributes to a better understanding of competitive and facilitative interactions in nurse tree plantations and emphasize the need for long-term empirical and modeling studies of mixed species stand dynamics.

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