Nitrogen‐fixing trees have no net effect on forest growth in the coterminous United States

Abstract

Nitrogen (N)‐fixing trees fulfil a unique and important biogeochemical role in forests through their ability to convert atmospheric N2 gas to plant‐available N. Due to their high N fixation rates, it is often assumed that N‐fixing trees facilitate neighbouring trees and enhance forest growth. This assumption is supported by some local studies but contradicted by others, leaving the overall effect of N‐fixing trees on forest growth unresolved. Here we use the US Forest Service's Forest Inventory and Analysis database to evaluate the effects of N‐fixing trees on plot‐scale basal area change and individual‐scale neighbouring tree demography across the coterminous US. First we discuss the average trends. At the plot and individual scales, N‐fixing trees do not affect the relative growth rates of neighbouring trees, but they facilitate recruitment and inhibit survival rates, suggesting that they are drivers of tree turnover in the coterminous US. At the plot scale, N‐fixing trees facilitate the basal area change of non‐fixing neighbours. In addition to the average trends, there is wide variation in the effect of N‐fixing trees on forest growth, ranging from strong facilitation to strong inhibition. This variation does not show a clear geographical pattern, though it does vary with certain local factors. N‐fixing trees facilitate forest growth when they are likely to be less competitive: under high N deposition and high soil moisture or when neighbouring trees occupy different niches (e.g. high foliar C:N trees and non‐fixing trees). Synthesis. N‐fixing trees have highly variable effects on forest growth and neighbour demographics across the coterminous US. This suggests that the effect of N‐fixing trees on forest development and carbon storage depends on local factors, which may help reconcile the conflicting results found in previous localized studies on the effect of N‐fixing trees on forest growth.

N supply mediates the radiative balance of N2O emissions and CO2 sequestration driven by N‐fixing vs. non‐fixing trees

“…Our study has important implications for reforestation. While our results apply directly to the common practice of monoculture reforestation (Liu et al 2018), recent empirical studies suggest that N-fixing trees, which are often recommended for reforestation to relieve N limitation of neighboring plant growth (Hulvey et al 2013, Cunningham et al 2015, do not always promote neighboring plant growth (Staccone et al 2021). Overall, our results suggest caution in planting N-fixing trees during reforestation and emphasize the need for further study.…”

Nitrogen‐fixing trees increase soil nitrous oxide emissions: a meta‐analysis

“…N-fixing trees are generally suggested for reforestation based on the theory that they relieve N limitation of plant growth and facilitate neighbouring trees (Hulvey et al 2013, Cunningham et al 2015, which would counteract their stimulation of N 2 O emissions. However, recent studies have found a negative influence or no influence of N-fixing trees on neighbouring trees, presumably due to strong competitive interactions (Xu et al 2020, Staccone et al 2021 (Liao et al 2017), the expansion of arid ecosystems (Pellegrini et al 2016, Gei et al 2018, Steidinger et al 2019, and the encroachment of N-fixing trees into grasslands, savannahs, and shrublands (Liu et al 2013). mitigation via reforestation due to uncertainties in its calculation (Bastin et al 2019a, Veldman et al 2019.…”

A novel representation of biological nitrogen fixation and competitive dynamics between nitrogen-fixing and non-fixing plants in a land model (GFDL LM4.1-BNF)

“…6; Knoepp, 2009aKnoepp, , b, 2018Swank and Waide, 1988), and a meta-analysis of temperate forests (Fig. 6; Stehfest and Bouwman, 2006); (3) compared ecosystem C fluxes from the numerical experiments of LM4.1-BNF and LM3-SNAP to eddy covariance observations from CHL at the hourly timescale (Fig. 7; Oishi, 2020); and (4) compared ecosystem C fluxes from the numerical experiments of LM4.1-BNF and LM3-SNAP to a meta-analysis of temperate forests (Fig.…”

“…Soil N 2 O and NO emissions are from Stehfest and Bouwman (2006). Data from deciduous temperate forests were selected.…”