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.