Endemic plant species on restricted soil types: 'early victims' or 'hardy survivors' of climate change?

One of the greatest challenges that land managers face today is anticipating how climate change will affect the diversity and composition of ecological communities to develop effective strategies for adaptation and mitigation. The direct effects of climate change on species via changes in temperature and precipitation have been the focus of many studies. Many conclude that altitudinal and latitudinal shifts in distribution will be necessary to survive the impacts of predicted climate change. 

 

Little attention, however, has been given to how plant species on 'restricted' soil (i.e. very infertile) will respond to climate change. Here, suitable habitats for such species are patchily distributed, and the dispersal distances required to move to newly suitable habitat are large, making successful migration unlikely. Are species confined to low-nutrient soils, which may reflect their tolerance of such conditions and intolerance of other biotic factors such as competition, particularly vulnerable to climate change?  Some studies suggest that soil specialists may be at less risk than species on 'normal' soils due to their stress-tolerant functional traits, but there is also contrary evidence.

 



Conceptual model of how present and future climate changes will affect (a) soil generalist and (b) soil specialist species differentially. Blue represents the current range of a species, Green is the future range of a species and Red is the area of overlap between the current and future ranges. Patchy suitable habitat for the soil specialist creates fewer colonization opportunities among the current and future ranges than for the soil generalist with contiguous suitable habitat.

Image: Damschen et al. (2012) Journal of Ecology 100: 1122-1130.



Plant communities on low-nutrient soils have two distinctive attributes that may cause them to respond uniquely to climate change, and I don't think we've really thought about these factors as they pertain to climate change responses.

 

First, they are often found in discrete areas making them more spatially isolated from one another than species on ‘normal’ soils that tend to be more contiguous. Hence, this spatial isolation may make it much more difficult for soil-specialist species to successfully migrate under climate change because suitable soils are isolated (or embedded in a hostile matrix to borrow an analogy from landscape ecology).

 

Second, because these species are on unproductive substrates, they may differ from communities on ‘normal soils’ in terms of limiting resources, functional traits, and the relative importance of disturbance, competition and other ecological processes. Plants in these special soil habitats often have traits associated with tolerance of drought and nutrient-limitation [e.g. small stature, low-specific leaf area (SLA), high allocation to roots relative to shoots] because nutrient availability is limited, water can be scarce, and soils may have additional unusual chemistries (e.g. particularly acidic pH). Special soil communities are often more strongly water-limited than others; therefore, they may be especially responsive to changes in available precipitation. On the other hand, because plants on special soils already have adaptations for stress tolerance, they may be particularly well-suited to withstand climatic changes.



Prostanthera galbraithiae (Wellington Mint-bush) is an erect to

spreading small shrub and is restricted to sandy soils of the Holey Plains State Park, Victoria

Asking questions about the fate of soil specialists in response to climate change is important for plant species such as the Wellington Mint Bush (Prostanthera galbraithiae), a vulnerable species in Australia. The species is endemic to a very small area of the Gippsland region of Victoria, restricted to sandy podzol soils typically low in macronutrients (especially N, P and K) and subject to long periods of soil moisture stress. Importantly, the sandy, nutrient-poor soils where Wellington Mint-bush occurs are restricted and embedded in a matrix of clay-based, more fertile soil types.

What are the potential responses to climate change of endemic plant species like the Wellington Mint-bush when soil factors appear to limit their current distribution?



To understand the role of climate factors on the fitness of soil specialists, it is necessary to compare the plasticity to water and temperature stress of the endemic Mint-bush to that of more widepread species (such as Prostanthera lasianthos, P. rotundifolia) to test the hypothesis that soil specialists are already well-adapted to environmental stress and hence, they may be particularly well-adapted to withstand climatic changes. Such studies, and life history study more generally, allows us to understand how plants respond to stress, and provides a powerful tool for making informed decisions about which species may need active intervention to ensure their persistence.

For species found in highly variable environments (such as those areas with frequent but unpredictable drought) where soil factors accentuate the magnitude of drought, a history of climate variability might confer rather general plasticity or tolerance of  future climate variation. Coupled with having to persist on infertile soils, some species might show more resilience to change in climate than those species from much more stable/predictable climates (i.e. where factors such as rainfall are more evenly distributed through the year) because they already have to exhibit 'stress-tolerance' to survive their specialised soils. Hence, it would be useful to ask whether range-restricted, soil specialists like the Wellington Mint-bush are more plastic in their response to climate variability than widespread species (where local adaptation might have developed – i.e. the ‘provenance’ concept), or are they simply better adapted to environmental stress.