Direct effects of warming increase woody plant abundance in a subarctic wetland

Abstract

Both the direct effects of warming on a species’ vital rates and indirect effects of warming caused by interactions with neighboring species can influence plant populations. Furthermore, herbivory mediates the effects of warming on plant community composition in many systems. Thus, determining the importance of direct and indirect effects of warming, while considering the role of herbivory, can help predict long‐term plant community dynamics. We conducted a field experiment in the coastal wetlands of western Alaska to investigate how warming and herbivory influence the interactions and abundances of two common plant species, a sedge, Carex ramenskii, and a dwarf shrub, Salix ovalifolia. We used results from the experiment to model the equilibrium abundances of the species under different warming and grazing scenarios and to determine the contribution of direct and indirect effects to predict population changes. Consistent with the current composition of the landscape, model predictions suggest that Carex is more abundant than Salix under ambient temperatures with grazing (53% and 27% cover, respectively). However, with warming and grazing, Salix becomes more abundant than Carex (57% and 41% cover, respectively), reflecting both a negative response of Carex and a positive response of Salix to warming. While grazing reduced the cover of both species, herbivory did not prevent a shift in dominance from sedges to the dwarf shrub. Direct effects of climate change explained about 97% of the total predicted change in species cover, whereas indirect effects explained only 3% of the predicted change. Thus, indirect effects, mediated by interactions between Carex and Salix, were negligible, likely due to use of different niches and weak interspecific interactions. Results suggest that a 2°C increase could cause a shift in dominance from sedges to woody plants on the coast of western Alaska over decadal timescales, and this shift was largely a result of the direct effects of warming. Models predict this shift with or without goose herbivory. Our results are consistent with other studies showing an increase in woody plant abundance in the Arctic and suggest that shifts in plant–plant interactions are not driving this change.

Pathways for Ecological Change in Canadian High Arctic Wetlands Under Rapid Twentieth Century Warming

“…The increased dominance of shrubs in this coastal wetland~AD 1950 follows an increase in GDD 0 at AD 1931 (Figure 2b). Northward expansion of shrub communities and increased productivity in response to recent warming has been widely documented across the tundra (Elmendorf et al, 2012;Myers-Smith et al, 2011;Tape et al, 2006) and directly linked to increased shrub and decreased sedge abundance in an Alaskan coastal wetland (Carlson et al, 2018). Isostatic uplift can lead to a drop in relative sea level and lowering of the water table, increasing dryness to facilitate sedge to shrub succession (Klinger & Short, 1996).…”

Snowier winters extend autumn availability of high‐quality forage for caribou in Arctic Alaska

“…Plant communities are already responding to these temperature changes. Preferred summer forages like deciduous shrubs and graminoids (White et al 1975, Thompson and McCourt 1981, Denryter et al 2017) are thriving at the expense of less palatable evergreen shrubs and nonvascular plants as warming continues across much of the Arctic (Sturm et al 2001, Wahren et al 2005, Tape et al 2006, 2012, Hobbie et al 2017, Carlson et al 2018). While the acceleration of green‐up has become a foundation for understanding warming's direct impact on forage during the spring and summer (Prevéy et al 2017, 2019, Bjorkman et al 2018), the consequences of changes to winter weather and its subsequent effect on forage phenology, quality, and quantity are exceedingly important in a holistic perspective of the annual cycle of caribou forage (Walsh et al 1997, Sturm et al 2005, Gustine et al 2017).…”

Delayed herbivory by migratory geese increases summer‐long CO2 uptake in coastal western Alaska

“…Most studies involving passive OTC simulate season-long warming rather than advanced growing season and demonstrate small changes in ER, GPP, and NEE (Boelman et al, 2003;Cahoon et al, 2012;La Puma, Philippi, & Oberbauer, 2007;Leffler et al, 2016;Sharp, Sullivan, Steltzer, Csank, & Welker, 2013;Welker, Brown, & Fahnestock, 1999;Welker, Fahnestock, Henry, & O'dea, K. W., & Chimner, R. A., 2004) and increased plant biomass in Arctic systems (Doiron, Gauthier, & Lévesque, 2014;Hollister, Webber, & Bay, 2005;Wookey et al, 1995). At the same site, a concurrent study suggested seasonlong warming can increase abundance of Salix ovalifolia in the system (Carlson, Beard, & Adler, 2018). The nonsignificant season advancement effect on GPP/NEE (Figure 4) suggests that any increase in stem elongation in the season advancement plots (Figure 1) Supporting Information Tables S1-S5 Our experimental manipulation produced CO 2 exchange values similar to those observed in unmanipulated grazing lawns.…”