Abstract
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, Corema album, to map regions of influence in a Portuguese dune ecosystem invaded by the N2-fixing Acacia longifolia. 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.
Highlights
Understanding the processes that determine spatial patterns in field data is a major goal in plant ecology [1]
Δ15N of C. album became substantially enriched in the vicinity of A. longifolia, with background values of uninfluenced vegetation of ca. -11‰ and -8‰ increasing to values close to 0‰ (Fig 1D and 1E), even though C. album itself has no capacity of N2-fixation
For δ13C, a distinct spatial pattern with values enriched by ca. 2.5‰ for C. album growing close to the A. longifolia canopies was evident for plot 1, while for plot 2, pronounced small-scale variation only weakly related to the presence of the invader was observed (Fig 1G and 1H)
Summary
Understanding the processes that determine spatial patterns in field data is a major goal in plant ecology [1]. One approach to analyze spatial patterns is to group spatially explicit measurements into separable homogeneous clusters, aiming at identifying biotic and abiotic factors that explain the variation between the clusters. It is seldom applied to study the spatial context of functional processes within plant communities. The spatial extent in which individual plants interact with one another or with the abiotic environment is often unknown [7]. One case where the area influenced by individual plants is of special interest is the invasion of exotic plant species into wellestablished native plant communities
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