Model-based Evidence for Cyclic Phenomena in a High-Elevation, Two-Species Forest

Abstract

Cyclic phenomena have been the focus of many studies in stressed conifer forests. In these systems, suppressed seedlings are released following the synchronous death of canopy trees. These cycles occur over hundreds of years, and thus studying them in the field is difficult, if not impossible in some cases. This difficulty highlights the advantages of vegetation modeling studies. We used the individual-based gap model, University of Virginia Forest Model Enhanced (UVAFME), to simulate forest dynamics over time at a high-elevation, subalpine forest (dominated by Engelmann spruce and subalpine fir) in southern Wyoming. Following model calibration, UVAFME was validated by running it up an elevation gradient to determine if it could simulate changes in species composition with elevation. UVAFME was then run exclusively at the high-elevation location for periods of 3000 years to simulate long-term forest dynamics at the site. It was found that without the intrusion of exogenous disturbances, the subalpine zone of the Rocky Mountains demonstrates cyclic phenomena, both at the plot scale and the landscape scale. By itself, Engelmann spruce demonstrates a natural periodicity of 300 years, whereas subalpine fir has a natural periodicity of 200 years. In the two-species forest, both species have a periodicity of 300 years. This output corresponds well with field data from similar high-elevation conifer sites. These results, along with other examples of cyclic phenomena in ecological systems, indicate that periodicities in ecosystems may be more common than previously thought, though they may be difficult to distinguish due to disturbances and the time-and space-scales at which they occur.

Modeling the interactive effects of spruce beetle infestation and climate on subalpine vegetation

“…As a traditional gap model, UVAFME is only spatially distributed in the vertical dimension. This model has been updated and parameterized for the southern Rocky Mountains (Foster et al 2015, and it was found that model output on size structure and speciesspecific biomass agreed with inventory data within the subalpine zone. A sample system for an actual forest area conforming to this assumptive framework would consist of a set of several hundred independent survey plots (in the present case, each 500 m 2 ) with species and size tallies of each tree on each plot measured annually.…”

“…The University of Virginia Forest Model Enhanced is an updated version of the gap model FAREAST (Yan and Shugart 2005) and has been successfully updated and tested for use in the southern Rocky Mountains (Foster et al 2015. The University of Virginia Forest Model Enhanced is an updated version of the gap model FAREAST (Yan and Shugart 2005) and has been successfully updated and tested for use in the southern Rocky Mountains (Foster et al 2015.…”

“…A sample system for an actual forest area conforming to this assumptive framework would consist of a set of several hundred independent survey plots (in the present case, each 500 m 2 ) with species and size tallies of each tree on each plot measured annually. Following this demonstration of fidelity in the southern Rockies (Foster et al 2015, UVAFME was chosen for this study to simulate forest response to shifting climate and disturbance regimes within the study region. Except for its more frequent update interval, output from the model resembles data from a system of replicate independent forest inventory plots Woodward 2011, Shugart et al 2018).…”

“…The University of Virginia Forest Model Enhanced (UVAFME) is an individual tree-based gap model, which simulates individual tree growth, mortality, and regeneration in response to climate, competition, and disturbances. The University of Virginia Forest Model Enhanced is an updated version of the gap model FAREAST (Yan and Shugart 2005) and has been successfully updated and tested for use in the southern Rocky Mountains (Foster et al 2015. Previous studies with UVAFME in the Rocky Mountains predicted upward shifts in species zonation with increasing temperatures ); however, increasing disturbances were not considered.…”

“…A complete description of the equations and subroutines within UVAFME can be found in Shuman et al (2015) and in the UVAFME User's Manual in Foster et al (2017). This model has been updated and parameterized for the southern Rocky Mountains (Foster et al 2015, and it was found that model output on size structure and speciesspecific biomass agreed with inventory data within the subalpine zone. The model was also found to accurately predict successional dynamics ❖ www.esajournals.org following bare-ground initiation as well as the change in species composition with elevation present within the southern Rockies (Daubenmire 1943, Veblen 1986, Aplet et al 1988.…”