Carbon uptake by the terrestrial biosphere depends on supplies of new nitrogen (N) from symbiotic N fixation, but we lack a framework for scaling fixation accurately and for predicting its response to global change. We scaled symbiotic N fixation from individual N fixers (i.e. plants that host N‐fixing bacteria), by quantifying three key parameters—the abundance of N fixers, whether they are fixing N and their N fixation rates. We apply this framework to black locust, a widespread N‐fixing tree in temperate forests of the eastern United States, and harness long‐term data from southern Appalachian forests to scale fixation from trees to the landscape and over succession. Symbiotic N fixation at the landscape scale peaked in the first decade following forest disturbance, and then declined. This pattern was due to the declining density and declining fixation rates of individual black locust trees over succession. Independent of forest succession, and coincident with chronic atmospheric N deposition, we have evidence suggesting that nodule biomass produced by black locust trees has declined by 83% over the last three decades. This difference in nodule biomass translates to a maximum fixation rate of 11 kg N ha−1 year−1and a landscape average of 1.5 kg N ha−1 year−1in contemporary forests.
Most forests are recovering from human land use, making it critical to understand the effect of disturbance on forest recovery. Forests of the eastern United States have a long history of land use, but it is unknown whether historical disturbances have contributed to their transition from ectomycorrhizal (ECM) to arbuscular mycorrhizal (AM) tree dominance. Disturbance may promote nitrogen (N)‐fixing trees in early succession, which can elevate soil N availability even after they die. Higher soil N availability may facilitate the competitive success of AM trees over ECM trees, but such ‘N fixer founder effects’ have not been empirically tested. Here, we analysed data from three land‐use disturbances in a temperate forest historically dominated by ECM trees: selective‐cutting (ranging from 0 to 52 m2 ha−1), clear‐cutting and agricultural abandonment. These disturbances occurred at different times, but long‐term data capture 3–7 decades of forest recovery. We found that the AM tree fraction in contemporary forests was 2, 4, and 6‐fold higher following selective‐cutting, clear‐cutting and agricultural abandonment, respectively, compared to forest composition in 1934. Across these disturbances we also observed an increasing abundance of the N fixer black locust immediately following disturbance. Using a simulation model parameterized by data from black locust, we estimated historical rates of symbiotic N fixation to understand the relationship between N fixation and AM dominance in individual plots. We found that N fixation was positively associated with the growth of ECM trees generally, and oak and hickory specifically, only following light selective‐cutting (<12 or <18 m2 ha−1basal area extraction, respectively). Following higher levels of selective‐cutting and clear‐cutting, N fixation was positively associated with the growth of AM trees, particularly red maple and tulip poplar. Agricultural abandonment led to AM dominance regardless of N fixation rates.
- PAR ID:
- 10437576
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Applied Ecology
- Volume:
- 60
- Issue:
- 8
- ISSN:
- 0021-8901
- Page Range / eLocation ID:
- p. 1551-1561
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Synthesis . We find key controls on symbiotic N fixation by black locust over space and time, suggesting lower fixation rates in eastern deciduous forests than previous estimates. Our scaling framework can be applied to other N fixers to aid predictions of symbiotic N fixation and ecosystem response to global change. -
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Abstract Plant–fungal associations strongly influence forest carbon and nitrogen cycling. The prevailing framework for understanding these relationships is through the relative abundance of arbuscular (AM) versus ectomycorrhizal (EcM) trees. Ericoid mycorrhizal (ErM) shrubs are also common in forests and interactions between co‐occurring ErM shrubs and AM and EcM trees could shift soil biogeochemical responses. Here we test hypotheses that the effects of ErM shrubs on soil carbon and nitrogen either extend or are redundant with those of EcM trees.
Using regional vegetation inventory data (>3,500 plot observations) we evaluated the frequency, richness and relative abundance of ErM plants in temperate forests in the eastern United States and examined their relationship with EcM plant cover. We then used surface soil (7 cm) data from 414 plots within a single forest to analyse relationships between ErM plant cover, relative EcM tree basal area and soil carbon and nitrogen concentrations while accounting for other biogeochemical controls, such as soil moisture.
At both scales, we found a positive relationship between ErM and EcM plants, and the majority of ErM plants were in the shrub layer. Within the forest site, ErM plants strongly modulated tree mycorrhizal dominance effects. We found negative relationships between EcM relative basal area and soil carbon and nitrogen concentrations, but these relationships were weak to negligible in the absence of ErM plants. Both EcM relative basal area and ErM plant cover were positively associated with the soil carbon‐to‐nitrogen ratio. However, this relationship was driven by relatively lower nitrogen for EcM trees and higher carbon for ErM plants. As such, the functional effects of ErM plants on soil biogeochemistry neither extended nor were redundant with those of EcM trees.
Synthesis . We found that ErM shrubs strongly influenced the relationship between tree mycorrhizal associations and soil biogeochemistry, and the effects of ErM shrubs and EcM trees on carbon and nitrogen were functionally distinct. Our findings suggest that ErM shrubs could confound interpretation of AM versus EcM tree effects in ecosystems where they co‐occur but also bolster growing calls to consider mycorrhizal functional types as variables that strongly influence forest biogeochemistry. -
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Abstract Aim Trees associating with ectomycorrhizal (ECM) fungi typically occur in infertile soils and use nutrients more conservatively than arbuscular mycorrhizal (AM) trees. We hypothesized that ECM trees would have greater nutrient resorption (i.e., proportion of nutrients resorbed during leaf senescence) than AM trees.
Location Global.
Methods We synthesized nitrogen (N) and phosphorus (P) resorption data from 378 species from sub/tropical, temperate and boreal forests, including 43 studies where ECM and AM trees co‐occurred, and conducted a meta‐analysis. Additionally, we quantified N resorption in 45 plots varying in ECM‐AM tree abundances in the temperate deciduous forests of southern Indiana, USA.
Results Overall, resorption patterns were driven primarily by mycorrhizal type, climate zone, and to a lesser degree, leaf habit. In the boreal forest, P resorption was 76% greater for ECM than AM trees (
p < .05). In the sub/tropics, AM trees resorbed 30% more N than ECM trees. At the sites where AM and ECM trees co‐occurred, ECM trees resorbed more N in temperate forests (15% greater;p < .001) whereas AM trees tended to resorb more N in sub/tropical forests (by 29%;p = .08). Besides, deciduous ECM trees resorbed more N (10%) and P (15%) than deciduous AM trees, while evergreen ECM and AM trees did not differ. In the deciduous forests of Indiana, where ECM and AM trees co‐occurred, the relative abundance of ECM trees in a plot was positively correlated to plot‐scale N resorption (R 2 = .25,p = .001), indicating greater nutrient conservatism with increasing ECM‐dominance.Main conclusions Our results indicate that mycorrhizal association – in addition to other factors – is correlated with the degree to which trees recycle nutrients, with the strongest effects occurring for N resorption by temperate deciduous trees.
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Tree species richness had a positive effect on community productivity across experiments, modified by the diversity of tree mycorrhizal associations. In communities with both AM and ECM trees, species richness showed positive effects on community productivity, which could have resulted from complementarity between AM and ECM trees. Moreover, both AM and ECM trees were more productive in mixed communities with both AM and ECM trees than in communities assembled by their own mycorrhizal type of trees. In communities containing only ECM trees, species richness had a significant positive effect on productivity, whereas species richness did not show any significant effects on productivity in communities containing only AM trees.
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At our α‐scale, sites recovering after agricultural abandonment only had 70% of the plant species richness (and ~30% of the evenness), compared to never‐ploughed sites. Within‐site β‐diversity recovered following agricultural abandonment to around 90% after 80 years. This effect, however, was not enough to lead to recovery at our γ‐scale. Richness in recovering sites was ~65% of that in remnant never‐ploughed sites. The presence of species characteristic of the never‐disturbed sites increased in the recovering sites through time. Forb and legume cover declines in years since abandonment, relative to graminoid cover across sites.
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