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

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.

Invasive acacias differ from native dune species in the hyperspectral/biochemical trait space

“…Within the native community, marked differences occurred within morphological species groups (Figures ), suggesting that ecosystem models based on few (putatively) representative species might not be accurate in diverse ecosystems (Díaz & Cabido, ). For example, variation in δ 15 N between species (Figures ) could indicate the use of different N sources, different rooting depths or association with different types of mycorrhizae (Craine et al., ), while high intraspecific variation in δ 15 N could suggest spatial heterogeneity controlled by, e.g., plant distribution patterns or abiotic factors that affect N availability and N cycling (Hellmann, Große‐Stoltenberg, Oldeland, Thiele, & Werner, ; Hellmann, Rascher, Oldeland, & Werner, ; Hellmann, Werner, & Oldeland, ; Ruiz‐Navarro, Barberá, Albaladejo, & Querejeta, ). Further, strong variation among native species in δ 13 C (Figure ) is related to different strategies of water use (Werner & Máguas, ).…”

Integrating anthropogenic factors into regional‐scale species distribution models—A novel application in the imperiled sagebrush biome

“…These covariates vary predictably along continental-scale environmental gradients and can be measured reliably at a variety of scales, facilitating spatial comparisons. However, in the Anthropocene, species' distributions are increasingly overridden by human influence, including altered wildfire regimes, land-use change, and invasive species (Crimmins, Dobrowski, Mynsberge, & Safford, 2014;Hellmann, Werner, & Oldeland, 2016;Jetz, Wilcove, & Dobson, 2007;Lewis & Maslin, 2015). Incorporating these human-induced factors into SDMs at a regional scale remains a major challenge.…”

Using nitrogen concentration and isotopic composition in lichens to spatially assess the relative contribution of atmospheric nitrogen sources in complex landscapes

“…To overcome this problem, Nr deposition in ecosystems can be used as a nitrogen deposition mapping tool. Plants have been shown to respond to different Nr sources by displaying different isotopic composition (Hellmann et al, 2016); yet, they exhibit a limited range of isotopic composition values (although larger variations exist when nitrogen uptake is mainly foliar; see Fogel et al, 2008). Unlike vascular plants, lichens and bryophytes exibit a large variation in nitrogen isotopic composition, and for a given area d 15 N values are more negative than those for vascular plants (Fogel et al, 2008).…”