ECR feature: Katie Nigro on disturbances impact on pine distributions

Katie Nigro is a PhD candidate at the Colorado State University in the USA. She is an ecologist interested in plant distributions and their responses to impacts. Here, Katie shares her recent work on the effects of wildfire and beetle outbreaks on the range expansion of trembling aspen.

Katie Nigro in the field, after a day of searching for aspen seedlings.

Personal links. Twitter

Institute. Colorado State University

Academic life stage. PhD (4th year)

Major research themes. Disturbance impacts on vegetation recovery and plant species range shifts.

Current study system. I work in the montane and subalpine forests of Colorado and currently am focused on two tree species – trembling aspen and ponderosa pine. This system is experiencing rapid change via the increased frequency of large, severe wildfires, bark beetle outbreaks, and drought. Studying aspen and ponderosa simultaneously is super interesting because they both exhibit different responses to disturbances and thus are likely to face very different futures. In our recently published paper, we focus on aspen, which is great at repopulating after disturbance but needs a lot of water, whereas, in an ongoing study for my PhD, we are focusing on ponderosa pine, which is not as prolific a reproducer but is extremely drought tolerant.

A spruce-fir forest in the study area (Colorado, USA) with extensive canopy mortality due to spruce beetle outbreaks.

Recent JBI paper. Nigro, K. M., Rocca, M. E., Battaglia, M. A., Coop, J. D., & Redmond, M. D. (2022). Wildfire catalyzes upward range expansion of trembling aspen in southern Rocky Mountain beetle-killed forests. Journal of Biogeography 49(1), 201– 214

Motivation behind this paper. Until a decade or so ago, it was commonly thought that trembling aspen trees almost always reproduce asexually (by resprouting from their roots) and that sexual reproduction (aspen trees growing from seed) was extremely rare, occurring only under the perfect combination of soil and climate conditions. However, researchers have recently been stumbling upon aspen seedlings (from seed) more and more, especially in burned areas. Many of these seedling sightings have been far away from adult aspen, which led us to think that disturbance could facilitate aspen’s migration to cooler areas in response to climate warming. Tree migrations are usually projected to lag behind climate change because trees are long-lived and can’t just get up and walk to cooler sites, which leaves them vulnerable. We wanted to see if disturbances like fire and bark beetle outbreaks could promote faster migrations for tree species like aspen.

A severely burned area in the West Fork Fire Complex of 2013, Colorado, USA – where the study was located.

Key methodologies. This project hinged on two key methods – the first was figuring out what the upper elevational limit of aspen was in the study area via aerial imagery, and the second was to go out and survey areas above that upper elevational limit to see if signs of upward migration were occurring in fire and bark beetle impacted sites. Figuring out the local elevational limits of aspen’s distribution was novel for our study area and highlighted the fact that species elevations are highly localized and do not follow a strict minimum and maximum elevation profile for a given latitude. In addition, focusing the surveys on just those elevations above the local maximum allowed us to uncover patterns of range expansion at the same time as documenting post-disturbance forest recovery.

Katie conducting field surveys looking for aspen and characterizing site conditions.

Unexpected challenges. One surprising thing was the rarity of aspen seedlings overall. I was amazed that we did not find any aspen seedlings in sites that were beetle-killed but not burned. I expected less in beetle-killed sites than burned sites, given the documented benefit of bare soil for aspen seedling establishment. However, due to increased light availability, I still expected to find some seedlings in the beetle-killed forests. In general, the rarity of seedlings on the landscape (even in burned areas) was definitely a mental challenge at the beginning of the field season – we surveyed 25 plots before encountering an aspen seedling. You can imagine the adrenaline rush I felt when my field technician finally yelled over, “I think I found one!” 

Major results. We found that wildfire has the capacity to accelerate migrations for trembling aspen and likely other wind-dispersed, shade-intolerant tree species, thereby allowing them to catch up with climate changes that have already occurred. Interestingly, another widespread disturbance agent, bark beetles, did not facilitate the same expansion of aspen’s upward elevational limit as fire did. This reveals that tree species migrations will be differentially impacted by the increasing frequency and severity of disturbances in the coming decades. Importantly, we found that aspen regeneration not only depended on wildfire, but was also significantly impacted by local site conditions. Therefore, range shifts are unlikely to progress uniformly upwards in elevation, but rather will occur quite heterogeneously across the landscape.  

A burned aspen tree with abundant resprouts at its base, this is an example of aspen’s asexual mode of reproduction

Next steps for this research. Now that we have documented upward elevational shifts in a shade-intolerant wind dispersed species due to wildfire, I am looking into other tree species prominent in the western United States to see how disturbances and life history traits may impact their range margins. I am also investigating the potential for certain populations of trees to be better adapted to future climates than others. In the case where a species is unable to migrate fast enough, we can use this information to reforest disturbed areas with individuals that will be more resistant to future climate changes.

If you could study any organism on Earth, what would it be? I think I would still stick with plants – they’re generally agreeable, so resilient, and will sit still for hours on end while you measure anything and everything about them.

An aspen seedling found at a burned site, growing out from under a downed log. This is an example of sexual reproduction.

Anything else to add? I think I must shoutout once more to all the wonderful people that helped me collect field data for this project. The final data reflected in the paper is the result of steep, grueling off-trail hikes through snow, streams, and around downed logs, running from thunderstorms (more than once) and just generally sacrificing the comforts of home for the sake of science – I couldn’t have done it without the help of some seriously tough and adventurous folks.

Published by jbiogeography

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

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