ECR feature: Epiphytic lichens & atmospheric regimes with Rob Smith

Rob Smith is a postdoc in the Department of Botany and Plant Pathology at Oregon State University, who studies the effects of changing atmospheric regimes on forest vegetation. Rob’s recently published work in the Journal of Biogeography discusses how epiphytic macrolichen vulnerability to climate change can signal atmospheric stresses among a group of organisms that many assume to be indifferent to climate.

Early career researcher (Rob) in typical habitat

Links: Personal webpage | Google Scholar | GitHub

Institution: Oregon State University, Dept of Botany and Plant Pathology

Current academic life stage: Postdoc

Research interests: Forecasting how global changes to atmospheric regimes alter the functioning, persistence and distribution of forest vegetation.

Current study system: Epiphytic lichens – perched upon the branches and trunks of other plants – are tightly linked to the atmosphere.  Despite their mistaken “extremophile” reputation, these lichens live at the mercy of atmospheric nutrients, temperature and moisture.  Climate, therefore, imposes strict limits on individual performance and geographic distributions.  Since most lichens fit in the palm of the hand, they make excellent travel companions, in addition to being well-suited for reciprocal transplants and common garden experiments to better anticipate climate responses.

Recent paper in Journal of Biogeography: Smith RJ, Jovan S, McCune B. 2020. Climatic niche limits and community-level vulnerability of obligate symbioses. Journal of Biogeography, 47(2). DOI: 10.1111/jbi.13719.

Motivation for the paper: We previously looked at climate-driven historical changes in lichen community compositions using a large-scale national forest inventory (FIA, the US Forest Service’s Forest Inventory and Analysis).  Yet, we still lacked the basic ability to forecast future changes at large scales.  Our new vulnerability approach arose from the observation that populations at the physiological “edges” of a species’ realized niche can also reveal its geographic edges – places on the landscape where incremental warming or drying could push a population to local extinction.  For many species together, co-occurrence of such “vulnerable” populations would signal a community on the verge of compositional turnover.  Forest workers would like to anticipate the location and magnitude of compositional changes before they occur.

(A) Climate stress from prolonged drying or warming can change lichen community compositions, including these pale green Alectoria hair-lichens draping western hemlocks in the mist near Cascade Pass, Washington, USA. (B) Bryoria horsehair lichens festoon a subalpine fir, with the glaciers of Mount Rainier in the distance, Washington, USA.  Direct exposure to the atmosphere makes epiphytic lichens sensitive to climate variation. (C) Charismatic epiphytic lichens (like this “oakmoss” Evernia prunastri) are easily transplanted, making them ideal for examining climate tolerance. (D) Installing cameras for daily census of lichen dynamics as part of new work with the Epiphytic Lichen Observation Network (ELON).

Key methodologies: Ecologists often depict environmental responses as community-weighted means (e.g., temperature optima).  Yet, central mean values ignore the “tails” of realized niches – boundaries which define the outer limits of persistence.  By contrast, our vulnerability scores are explicit about how far populations are from their climatic edges, and therefore the conditions under which incremental climate changes would lead to local declines.  Across 400+ epiphytic macrolichen species, and combined with climatic exposure at thousands of FIA plots nationwide, vulnerability scores let us identify geographic “hotspots” of expected compositional changes.

Unexpected challenges: One challenge was to adequately depict the realized climatic niche of several hundred species, including species whose ranges extended well beyond the study area.  We resolved this by introducing hundreds of thousands of herbarium records from all of North America, from tropical to polar regions.  This introduced its own challenge: how best to standardize the unequal sampling efforts typical of natural history collections?  For this, we binned observations into spatial grid-cells of gradually increasing sizes, finally arriving at an acceptable cell-size that “smoothed away” unequal sampling efforts while preserving climate information.

Major result and contribution to the field: We found remarkably high climate-change vulnerability among a group of organisms that many folks assume are indifferent to climate!  This supports the emerging perspective that epiphytic macrolichens can sufficiently signal atmospheric stresses.  Unexpectedly, we also found that communities most vulnerable to warming were concentrated in low‐elevation and southerly locations.  This suggests that warm-edge communities – commonly assumed to be “thermophilic” – may in truth be perilously close to exceeding their climatic limits.  Could the vulnerability approach work for the organisms you study?  You can try it for yourself, using the freely available R package:

What are the next steps? We are working to generalize the vulnerability concept to admit multiple drivers that interact.  This is because interactions among atmospheric stressors (climate and nitrogen excesses) and disturbances (wildfires) not only violate species’ tolerances directly, but can also aggravate pest/pathogen risks.  We also need to compare vulnerability among different lineages and morphogroups (forest trees, shrubs, forbs, graminoids, lichens) to identify the most responsive organisms.  Finally, we are developing user-friendly, open-source mapping tools that let forest workers quickly identify hotspots of compositional changes for adaptation purposes.

If you could study any organism on Earth, what would it be and why? To me, the most fascinating organisms are forest trees, which breathe our air and cycle our carbon.  Even for these everyday organisms, there is some hint of mystery in simple acts like passive water uptake (solely by transpiration! no active pumping!).  Despite being the dominant characters in forests, we are still far from accurately forecasting tree growth, survival and reproduction over large scales in the face of global changes.

Any other little gems you would like to share? It’s always fun to hear origin stories, how each person first began to engage the natural world.  My first real entry to forest thinking was the simple result of being an introverted kid – stealing off down the leafy railroad tracks paralleling the Patapsco River to decipher Lao Tzu or Albert Schweitzer under red oaks.  And then, looking up at red oaks, wondering.  There are hundreds of ways to enter your own forest.

Vulnerability scores combine local climate exposure with sensitivities of individual species, like these Bryoria horsehair lichens above White River at Mount Rainier, Washington, USA.

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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