**1. Introduction**

The community of biologists has been eager to realize the promise of DNA barcodes since the concept of a rapid method for genetic identification of species was first proposed in 2003. As we approach twenty years of DNA barcoding, the application of these short, but highly variable sequences continue to increase and methods continue to be developed that utilize this ever-expanding resource for multiple fields of biology. The nearly ten million DNA barcodes for life on Earth available today provide a database that is especially useful for ecology and evolutionary biology. In particular, DNA barcodes provide a rapid resource to identify taxa; to quantify and understand species richness; and to determine community interactions in primary and secondary habitats. Many ecologists, who are concerned with the assembly and maintenance of species richness at local and regional scales, have driven empirical and conceptual advances in the field of community ecology. At the same time evolutionary biologists have focused on the description and classification of species diversity, factors controlling the origin and ancestry of biodiversity, and the network of interactions that connect evolutionary units through time and space.

Today, thanks to the ever-expanding and well-curated DNA barcoding resources now available, fundamental biological questions can be more rigorously addressed regarding community evolution, assembly, productivity, and species interactions across and among diverse habitats and organisms, including plants, animals, fungi, and microorganisms. DNA barcodes are now routinely used to discover new species, to determine phylogenetic patterns of community diversity, and to uncover the complexities of interactions in almost all domains of life to understand diets, symbioses, pollinator networks, and historically challenging biomes, such as below-ground soil and deep-water marine communities. This Special Issue of *Diversity* addresses the wide variety of applications of DNA barcodes, especially in plants. The eleven papers included in this Special Issue illustrate how the DNA barcode library continues to be expanded, the range of ecological and evolutionary questions that can be answered with DNA barcodes, and how plant-human interactions are better understood using DNA barcodes as a research tool.

### **2. Building the Plant DNA Barcode Library**

The diversity of gene regions that serve as DNA barcodes continues to expand from the original cytochrome oxidase 1 mitochondrial sequences applied to many groups of animals. To date, no single gene region fits all lineages of life as a universal DNA barcode. For that reason, researchers continue to experiment and search for the most effective DNA barcode for specific clades on the Tree of Life and particular type or condition of tissues within organisms. In this Special Issue Dal Forno et al. [1] explored the application of DNA barcodes in both fresh and historical collections of lichen-forming basidiomycetes. Their

**Citation:** Kress, W.J.; Gostel, M.R. Plant DNA Barcodes, Community Ecology, and Species Interactions. *Diversity* **2022**, *14*, 453. https:// doi.org/10.3390/d14060453

Received: 10 May 2022 Accepted: 1 June 2022 Published: 6 June 2022

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results demonstrate that barcode sequences can effectively be generated from both fresh and historical collections more than 100 years old and that fungal ITS barcode sequences provide powerful resources for species delimitation in an integrative taxonomic framework., Omelchenko and colleagues [2] continued the quest to identify the most efficient regions for metabarcoding members of the grass family and sugges<sup>t</sup> additional spacer regions from nuclear ribosomal DNA (e.g., ITS1 and ETS in addition to the more common ITS2 barcode) provide enhanced discriminatory power for species identification in mixed pollen samples of grass species. In a third paper Kenfack, Abiem and Chapman [3] tested the effectiveness and efficiency of applying the standard plastid plant barcode regions (rbcLa, matK and trnH-psbA) to over one hundred species of trees in a montane forest in Nigeria and concluded that the combination of rbcLa and matK is sufficient for species discrimination. As part of the Global Genome Initiative for Gardens, Gostel et al. [4] release in the Special Issue 2722 DNA barcode sequences from 174 families and 702 genera of land plants that represent taxa without previous barcode sequences in GenBank. Each of these papers represents a significant contribution to building the DNA barcode library for plants.
