Abstract
Rapid climate change imperils many small-ranged endemic species as the climate envelopes of their native ranges shift poleward. In addition to abiotic changes, biotic interactions are expected to play a critical role in plant species’ responses. Below-ground interactions are of particular interest given increasing evidence of microbial effects on plant performance and the prevalence of mycorrhizal mutualisms. We used greenhouse mesocosm experiments to investigate how natural northward migration/assisted colonization of Rhododendron catawbiense, a small-ranged endemic eastern U.S. shrub, might be influenced by novel below-ground biotic interactions from soils north of its native range, particularly with ericoid mycorrhizal fungi (ERM). We compared germination, leaf size, survival, and ERM colonization rates of endemic R. catawbiense and widespread R. maximum when sown on different soil inoculum treatments: a sterilized control; a non-ERM biotic control; ERM communities from northern R. maximum populations; and ERM communities collected from the native range of R. catawbiense. Germination rates for both species when inoculated with congeners' novel soils were significantly higher than when inoculated with conspecific soils, or non-mycorrhizal controls. Mortality rates were unaffected by treatment, suggesting that the unexpected reciprocal effect of each species’ increased establishment in association with heterospecific ERM could have lasting demographic effects. Our results suggest that seedling establishment of R. catawbiense in northern regions outside its native range could be facilitated by the presence of extant congeners like R. maximum and their associated soil microbiota. These findings have direct relevance to the potential for successful poleward migration or future assisted colonization efforts.
Highlights
In the face of rapid anthropogenic climate change, large numbers of species will need to shift their geographic distributions poleward to track the climatic conditions that have supported their populations in the past (Chen et al 2011; Corlett and Westcott 2013; Diffenbaugh 2013)
The R. catawbiense germination rate was significantly higher than R. maximum germination in the live treatment (P < 0.001); there was no difference in germination rate between species in the sterilized control (P = 0.950) (Fig. 2a)
In the face of climate change, at-risk endemic plants with specialized mutualisms are of significant concern, as geographic disassociation between mutualist partners might result in lower performance and survival rates, or even extinction
Summary
In the face of rapid anthropogenic climate change, large numbers of species will need to shift their geographic distributions poleward to track the climatic conditions that have supported their populations in the past (Chen et al 2011; Corlett and Westcott 2013; Diffenbaugh 2013). In the context of biotic limits on migration and range shifts, species dependent on obligate mutualisms, or those with facultative mutualisms important to seedling establishment, are of particular concern (Dunn et al 2009) For such species, it is possible that a lack of required mutualists in a new region, or the presence of poorly-matched novel partners, could hinder the range shifts necessary to track rapid changes in the geographic distribution of abiotically-suitable habitat (Parker 2001; van der Putten 2012). These dynamics could lead to geographical decoupling of co-dependent mutualistic partners, for example, if the species involved responded differently to changing abiotic conditions (van der Putten 2012; Lankau 2016) or if they differed in dispersal and migration ability, risking accelerated decline or even coextinction
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