A Spatially Explicit Dual-Isotope Approach to Map Regions of Plant-Plant Interaction after Exotic Plant Invasion

<div><p>Understanding interactions between native and invasive plant species in field settings and quantifying the impact of invaders in heterogeneous native ecosystems requires resolving the spatial scale on which these processes take place. Therefore, functional tracers are needed that enable resolving the alterations induced by exotic plant invasion in contrast to natural variation in a spatially explicit way. ¹⁵N isoscapes, i.e., spatially referenced representations of stable nitrogen isotopic signatures, have recently provided such a tracer. However, different processes, e.g. water, nitrogen or carbon cycles, may be affected at different spatial scales. Thus multi-isotope studies, by using different functional tracers, can potentially return a more integrated picture of invader impact. This is particularly true when isoscapes are submitted to statistical methods suitable to find homogeneous subgroups in multivariate data such as cluster analysis. Here, we used model-based clustering of spatially explicit foliar δ¹⁵N and δ¹³C isoscapes together with N concentration of a native indicator species, <i>Corema album</i>, to map regions of influence in a Portuguese dune ecosystem invaded by the N₂-fixing <i>Acacia longifolia</i>. Cluster analysis identified regions with pronounced alterations in N budget and water use efficiency in the native species, with a more than twofold increase in foliar N, and δ¹³C and δ¹⁵N enrichment of up to 2‰ and 8‰ closer to the invader, respectively. Furthermore, clusters of multiple functional tracers indicated a spatial shift from facilitation through N addition in the proximity of the invader to competition for resources other than N in close contact. Finding homogeneous subgroups in multi-isotope data by means of model-based cluster analysis provided an effective tool for detecting spatial structure in processes affecting plant physiology and performance. The proposed method can give an objective measure of the spatial extent of influence of plant-plant interactions, thus improving our understanding of spatial pattern and interactions in plant communities.</p></div

Visualization of geographic barriers by plotting lines between the pairs of individuals that belong to both, the quartile with the highest genetic distances and the quartile with the lowest geographic distances.

<p>A. Using Jaccard distances based on AFLP data. B. Using GTR+G distances between <i>cox1</i> sequences.</p

Dendrogram illustrating results of the hierarchical cluster analysis of group medians derived from initial model-based clustering.

<p>The optimal solution with k = 3 clusters as identified by highest Silhouette value is indicated by different coloring. Labels specify plot and initial cluster membership in the form <i>plot</i>.<i>cluster</i> using Arabic and Roman numerals, respectively.</p

Scatterplots of the variables N concentration (g N*kg^-1), δ¹⁵N (‰) and δ¹³C (‰) derived from kriging maps in all combinations for the three study plots.

<p>Cluster membership to cluster I to VI (for plot 1) or cluster I to IV (for plots 2 and 3) is indicated by different symbols and coloring, sorted by median N concentration, with blue to red representing low to high N. Ellipses denote the normal-probability contours at probability = 0.5. Please note different scaling of the axes.</p

Boxplots illustrating N concentration (g N*kg^-1, A-C), δ¹⁵N (‰, D-F) and δ¹³C (‰, G-I) by cluster membership for plots 1, 2 and 3 as well as the distance to the closest <i>Acacia longifolia</i> canopy (m, J-K) for invaded plots 1 and 2.

<p>Different letters indicate significant differences at <i>P</i> < 0.05 (Kruskal-Wallis test, non-parametric multiple comparisons corrected for α-inflation).</p

First two dimension of a non-metric multidimensional scaling based on Jaccard distances between the AFLP data of the <i>Xerocrassa mesostena</i> individuals.

<p>Clusters delimited with Gaussian clustering are shown.</p

Geographic distribution of the genetic clusters within <i>Xerocrassa mesostena</i> on Crete.

<p>A. Distribution of the AFLP based clusters delimited using STRUCTURE. B. Distribution of the mitochondrial haplotype groups.</p

Results of the admixture analysis with the AFLP data of <i>Xerocrassa mesostena</i> with STRUCTURE for <i>K</i> = 5.

<p>Individuals are sorted into clusters and the clusters are ordered from west to east. The division of clusters 3 and 5 as found in the neighbor-net network (Fig. 3) is also indicated.</p

Maximum-likelihood tree based on partial <i>cox1</i> sequences of 95 <i>Xerocrassa mesostena</i> individuals and <i>X. subvariegata</i> and <i>X. grabusana</i> as outgroups.

<p>Haplotype clades are labelled to the right of the tree. Bootstrap support values larger than 75% are indicated by asterisks above the branches. For each individual the AFLP cluster as delimited using STRUCTURE is indicated by the same colour as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0062569#pone-0062569-g002" target="_blank">Figure 2</a>.</p

Results of the dbRDA within the <i>cox1</i> haplotype groups.

<p>Significances were obtained by 9,999 permutations.</p