Effects of Microsite Characteristics on Tree Recruitment in the Rocky Mountains

Research Significance:

Many subalpine species are under threat as climate change causes shifts in these species’ suitable habitat ranges. Current regeneration models rely on regional climate conditions, rarely including fine-scale factors such as topography, soils, light quality and quantity, or moisture availability.

Effects of Microsite Characteristics on Tree Recruitment in the Rocky Mountains

Research Significance, Cont.

Poor understanding of these variations in microsite conditions and their effects in regeneration limits our ability to predict the future of biological functioning through global change. This also may hinder conservation efforts to preserve vulnerable tree species, as these microsite characteristics can act as both mitigators and intensifiers to climate change-related tree loss.

High elevation species like this lodgepole pine may be under threat from climate change as suitable habitats are lost and lower elevation species replace them.

Methods:

The research took place near Rocky Mountain National Park, in a mixed species stand around 8,800'.

Species studied included Ponderosa pine, lodgepole pine, Douglas fir, Englemann Spruce, subalpine fir, and limber pine.

This figure shows the elevation range in distribution of all tree species found in the research site. Figure by Alison Foster.

Methods, cont.

Locally sourced seeds of each of these species were planted across 24 microsites in the fall of 2018. At each site, seeds were planted in groups of 4 and repeated 6 times in the randomly generated pattern shown in Figure 3. Plots were protected from herbivory with rodent and bird-excluding cages. After winterizing, these sites were monitored regularly for germination and death throughout the 2019 growing season

Microsite Schematic, where PIPO = Pondersosa Pine, PSME = Douglas fir, PICO= lodgepole pine, PIEN = Engelmann Spruce, ABLA= subalpine fur, PIFL = limber pine. Made and designed by Alison Foster.

Methods, cont.

Light availability, surface temperature, and soil moisture were used to characterize each plot.

Hemispherical photos were used to calculate available light, thermometers left in the center of each microsite recorded daily temperatures from July to August, and soil moisture was recorded bi-weekly.

Findings:

Subalpine fir, Ponderosa pine, limber pine, and Douglas fir were the only species with any survival, however limber pine and Douglas fir were alone in having large numbers of survival across multiple plots.

A douglas fir germinant with cotyledons. Photo taken and germinant identified by Alison Foster.

Findings, cont.

Survival occurred in 13 of the 24 microplots. However, statistical analysis of microsite conditions between plots with and without survival found that the differences between these groups were not significant.

With no differences significant differences found in moisture availability, light availability, and surface temperature where seedlings survived and where no seedlings survived, differences in survival rates across species must reflect greater seed success in limber pine and Douglas fir trees.

A limber pine germinant with seed coat. Photo from High Elevation White Pines.

Findings, cont.

Greater seed mass often correlate with greater seed survival; however as shown above, Douglas fir, the most successful species, had relatively low seed mass compared to limber pine and Ponderosa pine seed mass. Further, Douglas fir had smaller seed mass than subalpine fir, which had no survival. Clearly more than one factor influenced survival in this study.

Findings, cont.

Interestingly, large numbers of the most successful species were herbivorized by insects while species that did not have any survival tended to have less herbivory. Most herbivory was early in the season, before these species died off following less rain at the end of the season.

Next Steps:

Further analysis of microsite characterists on the basis of germination, rather than survival, may lead to more understanding of the effects microsite conditions.

Additionally, further post-hoc testing will elaborate more on microsite characteristics and the role they may have played in regeneration.

Extensive death due to herbivory from insects occured across all plots. Removing this unexpected variable by upgrading herbivory cages to insect-excluding cages would prevent this loss of data.

This experiment should be expanded over time and space. The first half of the 2019 season was abnormally wet, which may have caused more germination than could normally be expected. Still, the germination rate across all plots was 14%. Having a larger sample size would allow more certainty with results, especially in coming years as the Rocky Mountains become drier with the onset of climate change.

Acknowledgements:

Madeline Almèn

University of Denver

Department of Biological Sciences

Thank you to my adviser Dr. Patrick Martin, my supervisor Alex Goke, and Alison Foster who allowed me to take over her research.

Thank you also to the Undergraduate Research Center for funding this research.

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