Why are nitrogen‐fixing trees rare at higher compared to lower latitudes?

Abstract

Symbiotic nitrogen (N) fixation provides a dominant source of new N to the terrestrial biosphere, yet in many cases the abundance of N-fixing trees appears paradoxical. N-fixing trees, which should be favored when N is limiting, are rare in higher latitude forests where N limitation is common, but are abundant in many lower latitude forests where N limitation is rare. Here, we develop a graphical and mathematical model to resolve the paradox. We use the model to demonstrate that N fixation is not necessarily cost effective under all degrees of N limitation, as intuition suggests. Rather, N fixation is only cost effective when N limitation is sufficiently severe. This general finding, specific versions of which have also emerged from other models, would explain sustained moderate N limitation because N-fixing trees would either turn N fixation off or be outcompeted under moderate N limitation. From this finding, four general hypothesis classes emerge to resolve the apparent paradox of N limitation and N-fixing tree abundance across latitude. The first hypothesis is that N limitation is less common at higher latitudes. This hypothesis contradicts prevailing evidence, so is unlikely, but the following three hypotheses all seem likely. The second hypothesis, which is new, is that even if N limitation is more common at higher latitudes, more severe N limitation might be more common at lower latitudes because of the capacity for higher N demand. Third, N fixation might be cost effective under milder N limitation at lower latitudes but only under more severe N limitation at higher latitudes. This third hypothesis class generalizes previous hypotheses and suggests new specific hypotheses. For example, greater trade-offs between N fixation and N use efficiency, soil N uptake, or plant turnover at higher compared to lower latitudes would make N fixation cost effective only under more severe N limitation at higher latitudes. Fourth, N-fixing trees might adjust N fixation more at lower than at higher latitudes. This framework provides new hypotheses to explain the latitudinal abundance distribution of N-fixing trees, and also provides a new way to visualize them. Therefore, it can help explain the seemingly paradoxical persistence of N limitation in many higher latitude forests.

Competition alters predicted forest carbon cycle responses to nitrogen availability and elevated CO<sub>2</sub>: simulations using an explicitly competitive, game-theoretic vegetation demographic model

“…We used a tuning parameter, maximum leaf and fine root allocation, f LFR,max , to constrain the maximum allocation to leaves and fine roots in order to maintain a minimum growth rate of wood in years of low productivity. This is consistent with wood growth patterns in temperate trees, where new wood tissues must be continuously produced (especially early in the growing season) to maintain the functions of tree trunks and branches (Cuny et al, 2012;Michelot et al, 2012;Plomion et al, 2001). This parameter does not change the fact that leaves and fine roots are the priority in allocation, since allocation ratios to stems are around 0.4-0.7 in temperate forests (Curtis et al, 2002;Litton et al, 2007).…”

Constraining Carbon and Nutrient Flows in Soil With Ecological Stoichiometry

“…Mycorrhizal fungi and symbiotic N-fixing bacteria are soil microorganisms that provide nutrients to plants in exchange for photosynthetic C (Smith and Read, 2008;Menge et al, 2017). The activities of these specialized symbioses can alter soil stoichiometry.…”

“…In contrast to mycorrhizal fungi, which help plants acquire nutrients already present in soil, symbiotic N fixers introduce new N into the system (Menge et al, 2017). The majority of N fixation is performed by plant-associated symbionts, so these symbionts likely have the largest impact on SOM stoichiometry (Cleveland et al, 1999).…”

Phosphatase activity and nitrogen fixation reflect species differences, not nutrient trading or nutrient balance, across tropical rainforest trees

“…; Menge et al . ) and that plants can alter phosphatase activity to match soil phosphorus supply (Treseder & Vitousek ; Olde Venterink ; Turner & Wright ; Zalamea et al . ).…”

“…No study has explicitly examined the nutrient balance hypothesis for both fixation and phosphatase, although there is indication that fixers can adjust fixation to both nitrogen and phosphorus supply (Barron et al 2011;Batterman et al 2013a;Menge et al 2017) and that plants can alter phosphatase activity to match soil phosphorus supply (Treseder & Vitousek 2001;Olde Venterink 2011;Turner & Wright 2014;Zalamea et al 2016).…”