Abstract
Species interactions in mixed plantations can influence tree growth, resources capture and soil fertility of the stands. A combined approach of tree-ring analyses and carbon stable isotope was used to check tree growth and water use efficiency of two species, Populus alba L. and Juglans regia L., intercropped with each other and with N-fixing or competitive production species. Furthermore, soil analyses were performed to understand how the different intercropping systems can influence soil characteristics, in particular soil carbon stock. Dendrochronological data showed that during the first years, the growth of principal species was favored by intercropping. This positive effect decreased in the following years in most of intercropped stands, due to light competition with the crown of companion species. Carbon isotope data showed that P. alba and J. regia had the highest intrinsic water use efficiency when growing with Elaeagnus umbellata Thunb, a shrubby species with a shallow root system that favors a non-competitive exploitation of soil water resources. Finally, the intercropping of the principal species with Corylus avellana L. promoted the highest soil C stock. Our findings confirmed the importance to consider the plantation dynamics and wood formation in the long-run and to apply appropriate thinning and pruning interventions to counteract interspecific competition.
Highlights
The wide spreading of secondary forests worldwide is connected to the huge depopulation of the countryside and mountain marginal lands in favor of big cities and to the abandonment of the agricultural and pastoral activities [1]
We examined distinct stands in which two valuable species widely used for the production of timber, white poplar (Populus alba L., Salicaceae) and common walnut (Juglans regia L., Juglandaceae), are intercropped with each other and with a shade tolerant shrub species such as hazel (Corylus avellana L., Betulaceae) and with N-fixing species such as Italian alder (Alnus cordata (Loisel.) Duby, Betulaceae) and autumn-olive (Elaeagnus umbellate (Thunb.), Elaeagnaceae)
Multi-species forest crops determineproduction”, favorable interactions highest. This effect is based on nicheofseparation among species, among productivity species through a mechanism known as “principle competitive production”, interspecific competition can be lower than intraspecific competition for a given triggering highest productivity [22,62,63]. This effect is based on niche separation among environmental factor, determining a more efficient use of species, interspecific competition can be lower than intraspecific competition for a resources with consequent greater production of biomass of studied tree species [22,64]
Summary
The wide spreading of secondary forests worldwide is connected to the huge depopulation of the countryside and mountain marginal lands in favor of big cities and to the abandonment of the agricultural and pastoral activities [1]. This land use change has certainly caused a series of positive effects, such as contributing to mitigate global warming through a higher absorption and storage capacity of atmospheric carbon [2]. The main disadvantages of monocultures are the high sensitivity to pest and disease attacks, a progressive reduction of soil fertility, the excessively simple plantation structure and the poor biodiversity which lead to low profits in the timber market [12,13,14]
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