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

Deforestation and subsequent land-use conversion has altered ecosystems and led to negative effects on biodiversity. To ameliorate these effects, nitrogen-fixing (N 2 -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 N 2 -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 (δ 15 N, δ 13 C) in soils, A. koa , and non-N 2 -fixing understory plants ( Rubus spp.) and generated δ 15 N and δ 13 C isoscapes of the two forests to test for (1) different levels of biological nitrogen fixation (BNF) and its contribution to non-N 2 -fixing understory plants, and (2) the influence of historic land conversion and more recent afforestation on plant and soil δ 13 C. In the plantation, A. koa densities were higher and foliar δ 15 N 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 δ 15 N values in the plantation and greater influence of A. koa on neighboring plants and soil, suggesting greater BNF. Foliar δ 13 C also indicated higher water use efficiency (WUE i ) in the plantation, indicative of differences in plant-water relations or soil water status between the two forest types. Plantation soil δ 13 C was higher than the remnant forest, consistent with greater contributions of exotic C 4 -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 N 2 -fixing trees produces different biogeochemical landscapes than those observed in reference ecosystems, thereby influencing plant-soil interactions which can influence restoration outcomes.