ABSTRACT
Understory assemblages associated with canopy-forming species such as
trees, kelps, and rockweeds should respond strongly to climate stressors
due to strong interaction strengths. Climate change can directly and
indirectly modify these assemblages, particularly during more stressful
seasons and climate scenarios. However, fully understanding the seasonal
impacts of different climate conditions on canopy-reliant assemblages is
difficult due to a continued emphasis on studying single species
responses to a single future climate scenario during a single season. To
examine these more complex interactions, we used mesocosm experiments to
expose intertidal assemblages associated with the canopy-forming golden
rockweed, Silvetia compressa , to elevated temperature and
pCO2 conditions reflecting two projected greenhouse
emission scenarios [RCP 2.6 (low) & RCP 4.5 (moderate)].
Assemblages were grown in the presence and absence of Silvetia ,
and in two seasons. Relative to ambient conditions, predicted climate
scenarios generally suppressed Silvetia biomass and
photosynthetic efficiency. However, these effects varied seasonally -
both future scenarios reduced Silvetia biomass in summer, but
only the moderate scenario did so in winter. These reductions shifted
the assemblage, with more extreme shifts occurring in summer.
Contrarily, future scenarios did not shift assemblages withinSilvetia Absent treatments, suggesting that climate primarily
affected assemblages indirectly through changes in Silvetia .
Mesocosm experiments were coupled with a field Silvetia -removal
experiment to simulate the effects of climate-mediated Silvetialoss on natural assemblages. Consistent with the mesocosm experiment,Silvetia loss resulted in season-specific assemblage shifts, with
weaker effects observed in winter. Together, our study supports the
hypotheses that climate-mediated changes to canopy-forming species can
indirectly affect the associated assemblage, and that these effects vary
seasonally. Such seasonality is important to consider as it may provide
periods of recovery when conditions are less stressful, especially if we
can reduce the severity of future climate scenarios.