Growth and survival relationships of 71 tree species with nitrogen and sulfur deposition across the conterminous U.S

Abstract

Atmospheric deposition of nitrogen (N) influences forest demographics and carbon (C) uptake through multiple mechanisms that vary among tree species. Prior studies have estimated the effects of atmospheric N deposition on temperate forests by leveraging forest inventory measurements across regional gradients in deposition. However, in the United States (U.S.), these previous studies were limited in the number of species and the spatial scale of analysis, and did not include sulfur (S) deposition as a potential covariate. Here, we present a comprehensive analysis of how tree growth and survival for 71 species vary with N and S deposition across the conterminous U.S. Our analysis of 1,423,455 trees from forest plots inventoried between 2000 and 2016 reveals that the growth and/or survival of the vast majority of species in the analysis (n = 66, or 93%) were significantly affected by atmospheric deposition. Species co-occurred across the conterminous U.S. that had decreasing and increasing relationships between growth (or survival) and N deposition, with just over half of species responding negatively in either growth or survival to increased N deposition somewhere in their range (42 out of 71). Averaged across species and conterminous U.S., however, we found that an increase in deposition above current rates of N deposition would coincide with a small net increase in tree growth (1.7% per Δ kg N ha-1 yr-1), and a small net decrease in tree survival (-0.22% per Δ kg N ha-1 yr-1), with substantial regional and among-species variation. Adding S as a predictor improved the overall model performance for 70% of the species in the analysis. Our findings have potential to help inform ecosystem management and air pollution policy across the conterminous U.S., and suggest that N and S deposition have likely altered forest demographics in the U.S.

Fig 1

Gradients of N deposition, S deposition, mean annual temperature, and mean annual precipitation across the conterminous U.S. Panels are the a) mean total N deposition from 2000–2012, b) mean total S deposition from 2000–2012, c) mean annual temperature from 2000–2014, and d) mean annual precipitation form 2000–2014. Deposition data are from the TDEP product [23] and climate data are from PRISM (http://prism.oregonstate.edu). The averages for each tree in the analysis were a subset of these datasets that coincided with its measurement interval. The values used in the species-level empirical models correspond to the spatial distribution of the species.

Fig 2. Tree growth and survival versus N deposition by species and colored by ecological attributes.

Each curve represents the average tree growth (a-c) or survival (d-f) across the N deposition gradient for a single species. The relationships are separated by general curve shape; The (a,d) increasing, (b,e) unimodal, and (c,f) decreasing relationships with N deposition are shown. (For individual species and growth and survival curves by species and the curves associated with S deposition see S1 Fig). Note that two of the five species in panel d are functionally flat (< 0.01% change over deposition range) and are classified as flat in Table 1.

Fig 3

Relative change in growth and survival with N deposition of individual trees across the conterminous U.S. Maps show the relative mean, minimum, and maximum growth (a-c) and survival (d-f) responsiveness (percent change in growth or survival per change in annual N deposition rate) of trees within each 20 x 20 km pixel. Values were derived from the instantaneous rate of change in growth or survival with N deposition of the most parsimonious, species-specific models that allowed both N and S deposition (but did not necessarily contain N or S deposition). Red areas indicate where small increases in N deposition are associated with relatively large decreases in growth or survival while green areas indicate where small increases in N deposition are associated with relatively large increases in growth or survival. White areas are where no tree growth or survival data was available. Histograms indicate the fraction of individual trees by responsiveness to N deposition. Sample size for growth was 1,183,931 trees and for survival was 1,423,455 trees.

Fig 4

Relative change in growth and survival with S deposition of individual trees across the conterminous U.S. Maps show the relative mean, minimum, and maximum growth (a-c) and survival (d-f) responsiveness (percent change in growth or survival per change in annual S deposition rate) of trees within each 20 x 20 km pixel. Values were derived from the instantaneous rate of change in growth or survival with S deposition of the most parsimonious, species-specific models that allowed both N and S deposition (but did not necessarily contain N or S deposition). Red areas indicate where small increases in S deposition are associated with relatively large decreases in growth or survival. White areas are where no tree growth or survival data were available. Histograms indicate the fraction of individual trees by responsiveness to N deposition. Sample size for growth was 1,183,931 trees and for survival was 1,423,455 trees.

Fig 5

Spatial map of variance inflation factors (VIF): (a) VIF of the relationship between nitrogen (N) deposition and mean annual, temperature, mean annual precipitation, and sulfur (S) deposition; (b) VIF of the relationship between S deposition and mean annual, temperature, mean annual precipitation, and N deposition. VIF for N and S deposition was calculated for each of the 94 tree species, assigned to each tree in the FIA inventory based on species identity, and averaged across inventoried trees in each 20 x 20 pixel. Higher VIF values correspond to regions where the species present in the grid-cell have stronger covariation among predicters in the growth and survival models.