DOI: 10.5061/dryad.zkh1893d5
title: Functional-trait based restoration alters nutrient cycling and invasion rates in Hawaiian lowland wet forest

description: Many degraded ecosystems have altered nutrient dynamics, due to invaders
 possessing a suite of traits that allow them to both outcompete native
 species and alter the environment. In ecosystems where invasive species
 have increased nutrient turnover rates, it can be difficult to reduce
 nutrient availability. This study examined whether a
 functional-trait-based restoration approach, involving the planting of
 species with conservative nutrient-use traits, can slow rates of nutrient
 cycling and consequently reduce rates of invasion. We examined a
 functional trait restoration initiative in a heavily-invaded lowland wet
 forest site in Hilo, Hawai'i. Native and introduced species were
 chosen to create four experimental hybrid forest communities, in
 comparison to the invaded forest, with a factorial design in which
 communities varied in rates of carbon turnover (slow or moderate [SLOW,
 MOD]), and in the relationship of species in trait space (redundant or
 complementary [RED, COMP]). After the first five years, we evaluated
 community-level outcomes related to nutrient cycling: carbon (C), nitrogen
 (N), and phosphorus (P) via litterfall, litter decomposition, and also
 outplant productivity and rates of invasion. We found that: (1) regardless
 of treatment, the experimental communities had low rates of nutrient
 cycling through litterfall relative to the invaded reference forest, (2)
 the MOD communities had greater nutrient release via litterfall than the
 SLOW communities, (3) introduced species had greater nutrient release than
 native species in the two MOD experimental communities, and (4) within
 treatments, there was a positive relationship between nutrient levels and
 outplant basal area, but outplant basal area was negatively associated
 with rates of invasion. The negative relationships among basal area and
 weed invasion, particularly for the two COMP treatments, suggest species
 existing in different parts of trait space may help confer some degree of
 invasion resistance. The diversification of trait space was facilitated by
 the use of introduced species; a new concept in Hawaiian forest
 management. Although challenges remain in endeavors to restore this
 heavily degraded ecosystem, this study provides evidence that functional
 trait-based restoration approaches using carefully crafted hybrid
 communities can reduce rates of nutrient cycling and invasion in order to
 reach management goals.
description: Nutrient cycling
 metrics
 Litterfall was measured as an index of productivity. Litter
 screens (40 cm x 40 cm, 1 mm mesh size) were placed in the plots along
 diagonal and crossed transect lines (n=20 per plot). Litter was collected
 monthly, bulked per plot, dried at 70°C for at least 48 hours, and then
 weighed. Starting in 2016, we separated out litter from the outplant
 species and sorted and weighed each species in order to calculate
 species-specific productivity and overall outplant contribution to the
 plots. Nutrients in the litterfall of outplant species were determined by
 grinding material in a Wiley mill (40 mesh), and then processing samples
 with a Costech CE Elemental Analyzer (Costech ECS 4010; Valencia, CA) to
 obtain the concentration of carbon (C) and nitrogen (N), and a Varian
 inductively coupled plasma spectrometer (Varian Vista-MPX inductively
 coupled plasma-optical emission spectrometry; Palo Alto, CA) to obtain the
 concentration of P. All nutrient analyses were conducted at the University
 of Hawai′i at Hilo Analytical Laboratory. We also
 examined the decomposition rate of each outplanted species using
 litterfall collected in 2016. Litter bags with the dimensions of 20 cm x
 30 cm (area inside) were constructed of 1-mm mesh fiberglass window
 screen, filled with 5 grams of litter each. Bags were placed in another
 area of the forest used for past experiments (Ostertag et al. 2009;
 Kandert et al. 2021).  Bags (n = 10 per time period per species) were
 collected at intervals of 4 and 12 months. Non-litter items such as roots
 and small rocks were discarded prior to drying (70°C for 48 hours) and
 weighing. Samples were ashed at 500°C for five hours (Muffle furnace
 Thermolyne 1400; University of Hawai′i at Hilo Analytical Laboratory) to
 determine the percentage of organic material and mass was corrected
 accordingly. The proportion of mass loss was calculated at each time
 point. Nutrient release for each species was determined
 with a series of two calculations that combined litterfall rates, litter
 nutrient concentrations, and decomposition rates per species. The series
 of equations used were: 1.     Litterfall mass (g/plot) *
 (C, N, or P (%)/100) = C, N or P in litterfall mass (g C, N, or
 P/plot) 2.     C,
 N or P in litterfall mass (g C, N, or P/plot) * (Mass Loss (%)/100) = C, N
 or P released from litterfall (g C, N, or P/plot) These
 equations provided the nutrient release per species per plot and for
 treatment comparisons, we added up values from all 10 species per
 plot.  Productivity & invasive resistance metrics In May
 2014 all outplants were tagged and their locations within plots mapped
 using ArcPad on Allegro MX field computers. All stems ≥ 1.3 m height that
 had a diameter at breast height (DBH) of at least 1.0 cm were measured to
 calculate basal area at the individual- and then plot-level. All existing
 natives and outplants were censused annually to monitor changes in
 survival and growth. Although the number of outplant individuals with a
 DBH ≥ 1.0 cm was minimal in 2014, 25% of outplants were recorded with a
 DBH by 2017. In using basal area/plot as a proxy for productivity it
 should be noted that this metric is not inclusive for all outplants but
 rather the most productive as gauged by those reaching the 1.0 cm DBH
 threshold. From the moment plots were originally
 cleared, it was necessary to hand-weed at approximately 3-4-mo intervals
 to sustain outplant growth. We removed all undesired recruits (any
 seedling or resprout that was not native or of a species outplanted in the
 experiment). After two years of plant community development, we were able
 to reduce weeding frequency to 6-mo intervals. At the start of 2016 and
 the years following, we began recording the number of weed species found
 in the plots, as well as the number of person-hours taken to weed a plot.
 These data points quantify invasion resistance in the analyses.
description: Excel