Title: Global climate change will increase the abundance of symbiotic nitrogen‐fixing trees in much of North America
Abstract
Symbiotic nitrogen (N)‐fixing trees can drive N and carbon cycling and thus are critical components of future climate projections. Despite detailed understanding of how climate influences N‐fixation enzyme activity and physiology, comparatively little is known about how climate influences N‐fixing tree abundance. Here, we used forest inventory data from theUSAand Mexico (>125,000 plots) along with climate data to address two questions: (1) How does the abundance distribution of N‐fixing trees (rhizobial, actinorhizal, and both types together) vary with mean annual temperature (MAT) and precipitation (MAP)? (2) How will changing climate shift the abundance distribution of N‐fixing trees? We found that rhizobial N‐fixing trees were nearly absent below 15°CMAT, but above 15°CMAT, they increased in abundance as temperature rose. We found no evidence for a hump‐shaped response to temperature throughout the range of our data. Rhizobial trees were more abundant in dry than in wet ecosystems. By contrast, actinorhizal trees peaked in abundance at 5–10°CMATand were least abundant in areas with intermediate precipitation. Next, we used a climate‐envelope approach to project how N‐fixing tree relative abundance might change in the future. The climate‐envelope projection showed that rhizobial N‐fixing trees will likely become more abundant in many areas by 2080, particularly in the southernUSAand western Mexico, due primarily to rising temperatures. Projections for actinorhizal N‐fixing trees were more nuanced due to their nonmonotonic dependence on temperature and precipitation. Overall, the dominant trend is that warming will increase N‐fixing tree abundance in much of theUSAand Mexico, with large increases up to 40° North latitude. The quantitative link we provide between climate and N‐fixing tree abundance can help improve the representation of symbiotic N fixation in Earth System Models.
What are the primary biotic and abiotic factors driving composition and abundance of naturally regenerated tree seedlings across forest landscapes of Maine? Do seedling species richness (SR) and density (SD) decrease with improved growing conditions (climate and soil), but increase with increased diversity of overstorey composition and structure? Does partial harvesting disproportionately favour relative dominance of shade‐intolerant hardwoods (PIHD) over shade‐tolerant softwoods (PTSD)?
Location
Forest landscapes across the diverse eco‐regions and forest types of Maine,USA.
Methods
This study usedUSDAForest Service Forest Inventory Analysis permanent plots (n = 10 842), measured every 5 yr since 1999. The best models for each response variable (SR,SD,PIHDandPTSD) were developed based onAICand biological interpretability, while considering 35 potential explanatory variables incorporating climate, soil, site productivity, overstorey structure and composition, and past harvesting.
Results
Mean annual temperature was the most important abiotic factor, whereas overstorey tree size diversity was the most important biotic factor forSRandSD. Both mean annual temperature and overstorey tree size diversity had a curvilinear relationship withSRandSD. Average overstorey shade tolerance and percentage tolerant softwood basal area in the overstorey were the top predictor variables ofPIHDandPTSD,respectively. Partial harvesting favouredPIHDbut notPTSD.
Conclusions
This is one of the first studies to comprehensively evaluate a number of factors influencing naturally established tree seedlings at a broad landscape scale in the Northern Forest region of the easternUSAand Canada. Despite limitations associated with relatively small plot size, large seedling size class and lack of direct measurements of light, water and nutrients, this study documents the influence of these factors amid high variability associated with patterns of natural regeneration. The curvilinear relationship between mean annual temperature withSRandSDsupports the argument that species richness and abundance usually have unimodal relationships with productivity indicators, whereas the curvilinear relationship between overstorey tree size diversity andSRandSDsuggest that moderate overstorey diversity incorporates multiple species as well as higher seedling individuals.
Mueller, Rebecca C.; Scudder, Crescent M.; Whitham, Thomas G.; Gehring, Catherine A.(
, New Phytologist)
Summary
Successive droughts have resulted in extensive tree mortality in the southwestern United States. Recovery of these areas is dependent on the survival and recruitment of young trees. For trees that rely on ectomycorrhizal fungi (EMF) for survival and growth, changes in soil fungal communities following tree mortality could negatively affect seedling establishment.
We used tree‐focused and stand‐scale measurements to examine the impact of pinyon pine mortality on the performance of surviving juvenile trees and the potential for mutualism limitation of seedling establishment via alteredEMFcommunities.
Mature pinyon mortality did not affect the survival of juvenile pinyons, but increased their growth. At both tree and stand scales, high pinyon mortality had no effect on the abundance ofEMFinocula, but led to alteredEMFcommunity composition including increased abundance ofGeoporaand reduced abundance ofTuber. Seedling biomass was strongly positively associated withTuberabundance, suggesting that reductions in this genus with pinyon mortality could have negative consequences for establishing seedlings.
These findings suggest that whereas mature pinyon mortality led to competitive release for established juvenile pinyons, changes inEMFcommunity composition with mortality could limit successful seedling establishment and growth in high‐mortality sites.
Trujillo, Diana I.; Silverstein, Kevin A. T.; Young, Nevin D.(
, New Phytologist)
Summary
Symbiotic nitrogen fixation in legumes is mediated by an interplay of signaling processes between plant hosts and rhizobial symbionts. In legumes, several secreted protein families have undergone expansions and play key roles in nodulation. Thus, identifying lineage‐specific expansions (LSEs) of nodulation‐associated genes can be a strategy to discover candidate gene families.
Using bioinformatic tools, we identified 13LSEs of nodulation‐related secreted protein families, each unique to eitherGlycine,ArachisorMedicagolineages. In theMedicagolineage, nodule‐specific Polycystin‐1, Lipoxygenase, Alpha Toxin (PLAT) domain proteins (NPDs) expanded to five members. We examinedNPDfunction usingCRISPR/Cas9 multiplex genome editing to createMedicago truncatulaNPDknockout lines, targeting one to fiveNPDgenes.
Mutant lines with differing combinations ofNPDgene inactivations had progressively smaller nodules, earlier onset of nodule senescence, or ineffective nodules compared to the wild‐type control. Double‐ and triple‐knockout lines showed dissimilar nodulation phenotypes but coincided in upregulation of aDHHC‐type zinc finger and an aspartyl protease gene, possible candidates for the observed disturbance of proper nodule function.
By postulating that gene family expansions can be used to detect candidate genes, we identified a family of nodule‐specificPLATdomain proteins and confirmed that they play a role in successful nodule formation.
Projected changes in temperature and drought regime are likely to reduce carbon (C) storage in forests, thereby amplifying rates of climate change. While such reductions are often presumed to be greatest in semi‐arid forests that experience widespread tree mortality, the consequences of drought may also be important in temperate mesic forests of Eastern North America (ENA) if tree growth is significantly curtailed by drought. Investigations of the environmental conditions that determine drought sensitivity are critically needed to accurately predict ecosystem feedbacks to climate change. We matched site factors with the growth responses to drought of 10,753 trees across mesic forests ofENA, representing 24 species and 346 stands, to determine the broad‐scale drivers of drought sensitivity for the dominant trees inENA. Here we show that two factors—the timing of drought, and the atmospheric demand for water (i.e., local potential evapotranspiration;PET)—are stronger drivers of drought sensitivity than soil and stand characteristics. Drought‐induced reductions in tree growth were greatest when the droughts occurred during early‐season peaks in radial growth, especially for trees growing in the warmest, driest regions (i.e., highestPET). Further, mean species trait values (rooting depth and ψ50) were poor predictors of drought sensitivity, as intraspecific variation in sensitivity was equal to or greater than interspecific variation in 17 of 24 species. From a general circulation model ensemble, we find that future increases in early‐seasonPETmay exacerbate these effects, and potentially offset gains in C uptake and storage inENAowing to other global change factors.
Menge, Duncan N. L.; Wolf, Amelia A.; Funk, Jennifer L.; Perakis, Steven S.; Akana, Palani R.; Arkebauer, Rachel; Bytnerowicz, Thomas A.; Carreras Pereira, K. A.; Huddell, Alexandra M.; Kou‐Giesbrecht, Sian; et al(
, Ecological Monographs)
Abstract
Symbiotic nitrogen fixation (SNF) is a key ecological process whose impact depends on the strategy of SNF regulation—the degree to which rates of SNF change in response to limitation by N versus other resources. SNF that is obligate or exhibits incomplete downregulation can result in excess N fixation, whereas a facultative SNF strategy does not. We hypothesized that tree‐based SNF strategies differed by latitude (tropical vs. temperate) and symbiotic type (actinorhizal vs. rhizobial). Specifically, we expected tropical rhizobial symbioses to display strongly facultative SNF as an explanation of their success in low‐latitude forests. In this study we used15N isotope dilution field experiments in New York, Oregon, and Hawaii to determine SNF strategies in six N‐fixing tree symbioses. Nitrogen fertilization with +10 and +15 g N m−2 year−1for 4–5 years alleviated N limitation in all taxa, paving the way to determine SNF strategies. Contrary to our hypothesis, all six of the symbioses we studied sustained SNF even at high N.Robinia pseudoacacia(temperate rhizobial) fixed 91% of its N (%Ndfa) in controls, compared to 64% and 59% in the +10 and +15 g N m−2 year−1treatments. ForAlnus rubra(temperate actinorhizal), %Ndfawas 95%, 70%, and 60%. For the tropical species, %Ndfawas 86%, 80%, and 82% forGliricidia sepium(rhizobial); 79%, 69%, and 67% forCasuarina equisetifolia(actinorhizal); 91%, 42%, and 67% forAcacia koa(rhizobial); and 60%, 51%, and 19% forMorella faya(actinorhizal). Fertilization with phosphorus did not stimulate tree growth or SNF. These results suggest that the latitudinal abundance distribution of N‐fixing trees is not caused by a shift in SNF strategy. They also help explain the excess N in many forests where N fixers are common.
Liao, Wenying, Menge, Duncan N. L., Lichstein, Jeremy W., and Ángeles‐Pérez, Gregorio. Global climate change will increase the abundance of symbiotic nitrogen‐fixing trees in much of North America. Global Change Biology 23.11 Web. doi:10.1111/gcb.13716.
Liao, Wenying, Menge, Duncan N. L., Lichstein, Jeremy W., & Ángeles‐Pérez, Gregorio. Global climate change will increase the abundance of symbiotic nitrogen‐fixing trees in much of North America. Global Change Biology, 23 (11). https://doi.org/10.1111/gcb.13716
Liao, Wenying, Menge, Duncan N. L., Lichstein, Jeremy W., and Ángeles‐Pérez, Gregorio.
"Global climate change will increase the abundance of symbiotic nitrogen‐fixing trees in much of North America". Global Change Biology 23 (11). Country unknown/Code not available: Wiley-Blackwell. https://doi.org/10.1111/gcb.13716.https://par.nsf.gov/biblio/10037288.
@article{osti_10037288,
place = {Country unknown/Code not available},
title = {Global climate change will increase the abundance of symbiotic nitrogen‐fixing trees in much of North America},
url = {https://par.nsf.gov/biblio/10037288},
DOI = {10.1111/gcb.13716},
abstractNote = {Abstract Symbiotic nitrogen (N)‐fixing trees can drive N and carbon cycling and thus are critical components of future climate projections. Despite detailed understanding of how climate influences N‐fixation enzyme activity and physiology, comparatively little is known about how climate influences N‐fixing tree abundance. Here, we used forest inventory data from theUSAand Mexico (>125,000 plots) along with climate data to address two questions: (1) How does the abundance distribution of N‐fixing trees (rhizobial, actinorhizal, and both types together) vary with mean annual temperature (MAT) and precipitation (MAP)? (2) How will changing climate shift the abundance distribution of N‐fixing trees? We found that rhizobial N‐fixing trees were nearly absent below 15°CMAT, but above 15°CMAT, they increased in abundance as temperature rose. We found no evidence for a hump‐shaped response to temperature throughout the range of our data. Rhizobial trees were more abundant in dry than in wet ecosystems. By contrast, actinorhizal trees peaked in abundance at 5–10°CMATand were least abundant in areas with intermediate precipitation. Next, we used a climate‐envelope approach to project how N‐fixing tree relative abundance might change in the future. The climate‐envelope projection showed that rhizobial N‐fixing trees will likely become more abundant in many areas by 2080, particularly in the southernUSAand western Mexico, due primarily to rising temperatures. Projections for actinorhizal N‐fixing trees were more nuanced due to their nonmonotonic dependence on temperature and precipitation. Overall, the dominant trend is that warming will increase N‐fixing tree abundance in much of theUSAand Mexico, with large increases up to 40° North latitude. The quantitative link we provide between climate and N‐fixing tree abundance can help improve the representation of symbiotic N fixation in Earth System Models.},
journal = {Global Change Biology},
volume = {23},
number = {11},
publisher = {Wiley-Blackwell},
author = {Liao, Wenying and Menge, Duncan N. L. and Lichstein, Jeremy W. and Ángeles‐Pérez, Gregorio},
}
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