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. 2023 Jun 6:11:e15468.
doi: 10.7717/peerj.15468. eCollection 2023.

Isoscapes of remnant and restored Hawaiian montane forests reveal differences in biological nitrogen fixation and carbon inputs

Christopher B Wall  1   2 , Sean O I Swift  2 , Carla M D'Antonio  3 , Gerhard Gebauer  4 , Nicole A Hynson  2
Affiliations

Isoscapes of remnant and restored Hawaiian montane forests reveal differences in biological nitrogen fixation and carbon inputs

Christopher B Wall et al. PeerJ. .

Abstract

Deforestation and subsequent land-use conversion has altered ecosystems and led to negative effects on biodiversity. To ameliorate these effects, nitrogen-fixing (N2-fixing) trees are frequently used in the reforestation of degraded landscapes, especially in the tropics; however, their influence on ecosystem properties such as nitrogen (N) availability and carbon (C) stocks are understudied. Here, we use a 30-y old reforestation site of outplanted native N2-fixing trees (Acacia koa) dominated by exotic grass understory, and a neighboring remnant forest dominated by A. koa canopy trees and native understory, to assess whether restoration is leading to similar N and C biogeochemical landscapes and soil and plant properties as a target remnant forest ecosystem. We measured nutrient contents and isotope values (δ15N, δ13C) in soils, A. koa, and non-N2-fixing understory plants (Rubus spp.) and generated δ15N and δ13C isoscapes of the two forests to test for (1) different levels of biological nitrogen fixation (BNF) and its contribution to non-N2-fixing understory plants, and (2) the influence of historic land conversion and more recent afforestation on plant and soil δ13C. In the plantation, A. koa densities were higher and foliar δ15N values for A. koa and Rubus spp. were lower than in the remnant forest. Foliar and soil isoscapes also showed a more homogeneous distribution of low δ15N values in the plantation and greater influence of A. koa on neighboring plants and soil, suggesting greater BNF. Foliar δ13C also indicated higher water use efficiency (WUEi) in the plantation, indicative of differences in plant-water relations or soil water status between the two forest types. Plantation soil δ13C was higher than the remnant forest, consistent with greater contributions of exotic C4-pasture grasses to soil C pools, possibly due to facilitation of non-native grasses by the dense A. koa canopy. These findings are consequential for forest restoration, as they contribute to the mounting evidence that outplanting N2-fixing trees produces different biogeochemical landscapes than those observed in reference ecosystems, thereby influencing plant-soil interactions which can influence restoration outcomes.

Keywords: Acacia koa; Forests; Hakalau; Hawaii; Restoration; Stable isotopes.

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Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Figure 1
Figure 1. The Hawaiian Archipelago and sampling sites.
Site map of (A) the Hawaiian Archipelago highlighting the location of Hakalau Forest National Wildlife Refuge (red diamond) on the island of Hawai‘i; (B) Koa plantation and remnant forest sampling areas (KP, RK); (C, D) layouts (20 × 35 m), with plot corners in red circles and Acacia koa trees in green triangles; (E) images of the koa plantation and (F) perimeter of the remnant forest. Image credit: (A–D) GoogleMaps from R package ggmap, (E, F) L. Kersting.
Figure 2
Figure 2. Soil and foliar isotope values and elemental concentrations.
Soil and foliar (Acacia koa, Rubus spp.) (A) δ15N, (B) δ13C, (C) total nitrogen, and (D) total carbon in koa plantation (KP) and remnant (RK) Hakalau forests. Asterisks (*) indicate significant differences between forests within a sample type (p < 0.05). Values are mean ± SE, n = 48 (soil), 18 and 10 (Acacia koa), 28 and 14 (Rubus spp.) in KP and RK forests, respectively.
Figure 3
Figure 3. Soil and foliar isoscape density plots.
(A, B) Density plot of soil δ15Npredicted (left), soil δ13Cpredicted (middle), and (C) foliar δ15Npredicted samples (Acacia koa and Rubus spp., right) from isoscapes in the koa plantation (KP) and remnant (RK) Hakalau forests. Dashed lines indicate mean values in each forest.
Figure 4
Figure 4. Soil δ15N and δ13C isoscapes.
(A, B) Soil δ15N and (C, D) δ13C isoscapes for Hakalau koa plantation (KP, top) and remnant forests (RK, bottom). Color bar represents δ15N and δ13C values. Symbols represent locations where soil (open circles), Acacia koa (filled triangles), and Rubus spp. (open triangles) samples were collected; only soil samples were used in spatial interpolation.
Figure 5
Figure 5. Foliar δ15N isoscapes.
(A) Foliar δ15N for Hakalau koa plantation (KP, top) and (B) remnant forests (RK, bottom). Color bar represents δ15N values. Symbols represent locations where soil (open circles), Acacia koa (filled triangles), and Rubus spp. (open triangles) samples were collected; only foliar samples were used in spatial interpolation.
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