The full methods associated with this data set can be found in: Kuebbing SE and MA Bradford (2019) The potential for mass ratio and trait divergence effects to explain idiosyncratic impacts of nonnative invasive plants on carbon mineralization of decomposing leaf litter. Functional Ecology. Methods: Species Selection: We selected 24 native and nonnative invasive species that are regionally common and are found co-occurring in a deciduous forest stand in southeastern Connecticut, USA (Regional Water Authority Lake Gaillard Property; 41.336439 N, -72.783104W, decimal degree). Species Trait Data: We collected green leaves in the summer of 2014 and leaf litter from senescing plants in the fall of 2014, and restricted our leaf and litter collection to the ~17 ha forested area with similar site conditions to reduce potential variation arising from differences in environmental conditions. For each plant species, we measured a suite of green leaf (total nitrogen [N], total carbon [C]) and leaf litter traits ([C], [N], total leaf phosphorus [P], acid digestible lignin) that are strongly associated with litter decomposition dynamics. We report total C, N and P on a percent mass basis and acid detergent lignin (ADL) on a per gram dry mass basis (Table 1). Microcosm Litter Decomposition Experiments: We used laboratory experiments to assess litter decomposition dynamics for all microcosms over 160 days to capture both the faster initial decomposition dynamics, and then the slower second phase decomposition dynamics. For the all microcosms, we filled and mixed 50-mL plastic centrifuge tubes with 1 g of 2-mm milled leaf litter and 0.25 g of forest soil. The soil was collected from the surface mineral horizon from the same forest site and then sieved to 2 mm and stored at 4ûC after collection. It served as a common microbial inoculum for both experiments. For the single-species microcosms, we created 6 replicates for each species as well as 6 replicates of 6 g soil-only microcosms for a total of 150 microcosms. For the mixed-species litter microcosms, we included two different canopy litter mixtures with distinctive decomposition dynamics: 1) an oak-hickory canopy mixture comprised of Carya cordiformis, Quercus alba, and Q. rubra that are relatively slow-decomposing litters; and 2) a maple-poplar canopy mixture comprised of Acer rubrum, A. saccharum, and Liriodendron tulipifera that are relatively fast-decomposing litters. To each canopy litter mixture, we added understory litter from one of 18 understory plant species (Table 1). We varied the proportion of understory species in each litter mixture (1%, 5%, and 25%) to represent the possible range of reported values of the relative contribution of understory shrubs, trees and herbaceous plants to annual litterfall in northeastern deciduous forests. The remaining proportion of litter in each microcosm comprised each of the three canopy litter species in equal proportion. We created 4 replicates for each litter mixture microcosm (2 canopy litter mixtures ? 18 understory plant species litter ? 3 proportions [1%, 5%, and 25%] ? 4 replicates = 432 microcosms), as well as 4 replicates of 6 g soil-only microcosms, and 8 tree canopy litter control replicates (4 for each canopy litter mixture), bringing the total number of microcosms to 444. For single- and mixed-species litter microcosms, we regularly measured carbon mineralization rates across the 160-day experiment (single-species decomposition experiment: days 1, 4, 7, 10, 14, 20, 27, 35, 50, 65, 79, 110, 160 and mixed-species decomposition experiment: days 1, 4, 7, 11, 18, 25, 33, 54, 74, 95, 131, 160). For each carbon mineralization measurement, we fitted microcosms with gas-tight lids modified with septa for gas analysis. On each sampling day, we flushed each microcosm with CO2 free air for 3 min, allowed sealed microcosms to incubate for 4 h at 20¡C, sampled microcosm headspace with a syringe, and measured headspace air sample CO2 concentrations with infra-red gas analysis (IRGA; LI-COR model LI-7000, Lincoln, Nebraska, USA). This provided us a measure of carbon mineralization for each experimental microcosm at each sampling time. We created a carbon mineralization curve for each microcosm by plotting the measured headspace CO2 concentration across the 160-day period and intergrated the area under the curve to calculate the cumulative carbon mineralized for each microcosm. To account for soil respiration within each litter microcosm, we adjusted the total value of each litter microcosm by subtracting the average soil-derived cumulative CO2 respiration determined from the soil-only microcosms. Meta Data: File Name: Kuebbing and Bradford_Functional Ecology_Species Trait Data.csv Column Descriptions: Species: Latin binomial name of the 24 plant species included in the study. SpeciesID: Four letter code for each of the 24 plant species included in the study. NumberOfIndividualsSampled: The number of individuals of that species sampled to collect green leaves for green leaf trait measurements (i.e., specific leaf area, percent nitrogen, percent carbon, cargon:nitrogen). NumberOfLeavesSampled: The number of green leaves randomly selected from each individual for green leaf trait measurements. SpeciesOrigin: Whether the plant species is considered native or nonnative to Connecticut, USA (please see Methods in publication for full definition of native and nonnative). FunctionalGroup: The broad functional group of the plant species (canopy [tree species that grows >20m into forest canopy], tree [tree species typically <20m in height and found in the forest midstory], shrub, grass, vine, herb) TotalP_litter: The percent mass basis of total phosphorus of senesced litter. PercentN_litter: The percent mass basis of total nitrogen of senesced litter. PercentC_litter: The percent mass basis of total carbon of senesced litter. CNratio_litter: The carbon to nitrogen ratio of senesced litter. NPratio_litter: The nitrogen to phosphorus ratio of senesced litter. PercentN_leaf: The percent mass basis of total nitrogen of green leaves. PercentC_leaf: The percent mass basis of total carbon of green leaves. CNratio_leaf: The carbon to nitrogen ratio of green leaves. ADLdm: The acid detergent lignin content of senesced leaves (on a percent gram dry mass basis). File Name: Kuebbing and Bradford_Functional Ecology_Litter Mixture C Mineralization Data.csv Column Descriptions: TubeID: Unique ID for each independent mesocosm tube CanopyMIxture: The canopy litter mixture, either maple-poplar or oak-hickory, in each tube. UnderstorySpecies: The four letter code (see Species Trait Data for full species name) of the understory species added to the canopy litter mixture. For canopy litter mixtures the value is ÒnoneÓ. PercentUnderstory: The percent (1, 5, 25) of the litter mixture comprised of the understory species; for canopy litter mixtures this value is 0. Replicate: The replicate ID (A, B, C or D) of each unique canopy litter mixture, by understory species litter by percent understory combination. Cumulative_CO2: The cumulative carbon mineralization (CO2-C µg dry weight litter-1) of the mixture across 160 days of decomposition, adjusted for soil microbial respiration. File Name: Kuebbing and Bradford_Functional Ecology_Single-Species Litter C Mineralization Data.csv Column Descriptions: TubeID: Unique ID for each independent mesocosm tube SpeciesCode: The four letter code (see Species Trait Data for full species name) of the understory species added to the canopy litter mixture. For canopy litter mixtures the value is ÒnoneÓ. Replicate: The replicate ID (A-F) of each single-species litter. Cumulative_CO2: The cumulative carbon mineralization (CO2-C µg dry weight litter-1) of the mixture across 160 days of decomposition, adjusted for soil microbial respiration.