Contrasting impacts on the nitrogen cycle of co-occurring exotic species

Background/Question/Methods

Studying the differences in the nitrogen use strategy between native and exotic plant species can help to explain invasion success and to predict potential impacts on the nitrogen cycle of invaded communities. Plants adapted to nitrogen-rich ecosystems usually show leaf traits related to non-conservative nitrogen use strategies that may lead to an increase in soil nitrogen availability, as short leaf life spans, poorly defended leaves, high litter nutrient concentration or high litter decomposition rates. Previous literature suggests that invasive plants are close to this non-conservative strategy. Thus, if an invasive species producing large quantities of easy-to-decompose-nitrogen-rich litter invades an ecosystem dominated by nitrogen conservative species, it can increase nitrogen availability in soils, favoring the growth, establishment and spread of other new invasive species (i.e. invasion meltdown), even to the detriment of the conservative natives. We compared the nitrogen use strategy of native (Fraxinus angustifolia, Ulmus minor and Populus alba) and invasive co-occurring tree species (Ailanthus altissima, Robinia pseudoacacia and Ulmus pumila) to predict the consequences of shifts in community composition in river banks of inner Spain. We expected invasive trees to show leaf traits related to non-conservative strategies. We assessed the leaf lifespan, nitrogen resorption efficiency in leaves, nitrogen mean residence time, amount and timing of litter production and amount of nitrogen returned to soils.

Results/Conclusions

The native tree species followed a similar intermediate-conservative nitrogen use strategy. However, the exotics followed very different strategies, leading to contrasting impacts on the nitrogen cycle of the invaded community. U. pumila was the most nitrogen conservative species, meaning little impacts in case of dominance. In contrast, the N-fixer R. pseudocacia was the less conservative. However, R. pseudocacia would not increase nitrogen availability in the study area soils due to its low litter production and litter decomposition rates. Finally, despite showing leaf traits related to a nitrogen conservative strategy, A. altissima invasion would lead to an increase in soil nitrogen availability, as this species produces a high amount of nitrogen rich-labile litter. This study offers an example of the potential contrasting impacts of invasive species on the nitrogen cycle of invaded communities.