Conflicting impacts of nitrogen fixing trees on neighbors in temperate US forests

Background/Question/Methods

Nitrogen-fixing (N-fixing) trees have been proposed to relieve ecosystem-scale nitrogen limitation thereby fueling forest growth and carbon storage. However, the impact of N-fixing trees on the growth rates of immediate neighboring trees is unclear. Recent work in tropical rainforests suggests that N-fixing trees inhibit growth of neighboring trees, contradicting a long-held assumption that N-fixing trees facilitate forest generation. Here we use the immense (>70,000 N-fixing trees) Forest Inventory and Analysis (FIA) dataset to address the nature of neighbor interactions of N-fixing trees in temperate forests of the US. Specifically, we address the question: To what degree do N-fixing trees facilitate or inhibit the growth rate of neighboring trees in temperate forests? We study forests across the US to examine the influence of N-fixing tree types (rhizobial and actinorhizal), genera, age classes, and a range of environmental conditions on the interaction between N-fixing and non-fixing trees.

Results/Conclusions

We found that, overall, N-fixing trees have a facilitative impact relative to non-fixing trees on the average growth rate of neighboring non-fixing trees when examined at the individual tree scale (p<0.01). This facilitative effect occurred across forest age classes and dominant N-fixing tree types. The average increase in growth at the individual scale was 2.36x10^-3 cm² per % increase in neighborhood crowding index comprised of N-fixer. When aggregated at the plot scale, however, N-fixing trees decreased the plot-level basal area growth rate by 0.7% per % increase in plot basal area comprised of N-fixers. The effect of N-fixing trees was highly variable across geography. When the effects were aggregated into 0.5° by 0.5° grid cells, 35.2% of grid cells with a significant effect from N-fixing trees exhibited a negative effect (inhibitory effect) while the other 64.8% were positive. This high variability suggests that the effect of N-fixing trees on their neighbors is driven by site-specific factors such as limitations in nitrogen, water, other soil resources, light, or herbivory.