INTRODUCTION
Environmental stressors associated with climate change can influence the
performance and survival of populations (Wernberg et al. 2011, Kim et
al. 2016, Cavalcanti et al. 2018, Spooner 2018, Dudgeon 2019, Doney et
al. 2020, Heijmanns et al. 2022). When these populations are
foundational, as with trees, rockweeds, kelps, corals, and seagrasses
(Hoegh-Guldberg 1999, Sunny 2017, Metzger et al. 2019, Hultine et al.
2020), these changes can alter community structure, ecosystem
productivity, nutrient cycling, and energy flow (Ehrenfeld 2003, Lister
& Garcia 2018, Boukal et al. 2019, Spector & Edwards 2020, Sullaway &
Edwards 2020). The extent of these impacts, however, will depend on the
severity of environmental change (i.e., which of the projected climate
change scenarios actually occurs; Reum et al. 2020; Ángeles-González et
al. 2021) and the characteristics of the species being considered (i.e.,
how species interactions shift under these climate change scenarios;
Brown et al. 2014, Kim et al. 2016, Edwards 2022). Unfortunately, the
consequences of climate change on communities with foundation species
remain largely uncertain because most previous studies focused on the
impacts of a single future climate scenario in a single season and on a
single population (Bass et al. 2021).
Simulating different future climate scenarios will better model climate
change impacts by incorporating different levels of severity. The
Intergovernmental Panel on Climate Change (IPCC) provided several
Representative Concentration Pathways (RCP) that predict changes in
temperature and ocean pH by the year 2100 relative to present-day
levels. For example, RCP 2.6 (+1 °C/-0.1 pH units from
ambient conditions) represents a low-impact scenario where emissions are
stabilized by the 2020s while RCP 4.5 (+2 °C/-0.2 pH
units) represents a moderate scenario where emissions are stabilized by
the 2040s (IPCC 2022). Observing the effects of climate change under
multiple scenarios can reveal potential thresholds and offer greater
predictability for management and conservation efforts (Thurman et al.
2020). Given 1) the uncertainty in the severity of future climate
change, and 2) that small differences in temperature and/or pH can be
biologically and ecologically meaningful (Wang et al. 2015, Araújo et
al. 2018, Harrington et al. 2020), multiple scenarios need to be
considered.
The severity of future climate change impacts on natural ecosystems may
vary among seasons, but this variation also remains understudied
(Russell et al. 2012). Well-known climate change alterations to seasonal
events such as droughts, coastal upwelling, and growing periods have
already disrupted phenological cycles and restructured communities
across a wide range of ecosystems, but even more nuanced effects could
be similarly impactful (Ernakovich et al. 2014, Ooi et al. 2014, Donham
et al. 2021). For example, we identify at least three season-specific
mechanisms that could impact intertidal communities. First, warming may
have a stronger effect in summer because higher temperatures will become
problematic for species living near their thermal maxima (Madeira et al.
2012). Second, periods of peak low tide may result in seasonally harsher
environments during the summer because they tend to occur between
morning and noon when irradiances are greatest, but in the winter, they
tend to occur during the late afternoon when irradiances have decreased
(Flick 2016). Third, the reproduction, dispersal, and recruitment of
marine species are often seasonal, and this seasonality may interact
with climate (Ådahl et al. 2006, Edwards 2022), resulting in
differential species assemblages.
Although population-level studies have provided important insights, such
as taxa-specific effects of elevated pCO2 (Ragazzola et
al. 2012, Fernández et al. 2015, Shukla & Edwards 2017, Kim et al.
2020), they may not accurately predict impacts on whole communities
because they do not allow for species interactions that may mitigate or
magnify the impacts of climate change. For example, giant kelp,Macrocystis pyrifer a, may reduce the effects of climate change on
benthic coralline algae by absorbing excess CO2 (Hirsh
et al. 2020). Likewise, feeding on higher quality kelp grown under
future climate scenarios may remove the direct negative effect of
climate on grazer growth and gonad development (Brown et al. 2014).
Despite the staggering increase in climate change related research
during the past two decades, the ratio of single species studies to
community level studies remained nearly the same (i.e., single species
studies continue to comprise ~60% of studies in this
field, Bass et al. 2021). When papers published between 2010 and 2019
were subdivided into those focusing on single species versus species
assemblages, single species studies were three times more common
(Wernberg et al. 2012, Bass et al. 2021). Successfully predicting the
impacts of climate change on natural populations will require increased
efforts towards studying these impacts on natural assemblages.
Canopy-forming species and their understory assemblages form a critical
set of interactions, which could influence the impacts of climate change
on individual species within the community (Edwards & Connell 2012).
Canopy-forming species modify their physical and chemical environments
(Edwards 1998, Gonzales et al. 2017, Hondolero & Edwards 2017, Joly et
al. 2017, Ørberg et al. 2018) and can provide a more favorable habitat
for shade-adapted understory species (Clark et al. 2004, Flukes et al.
2014, Kitao et al. 2018, Roberts & Bracken 2021). In turn, understory
species can affect canopy-forming species through various mechanisms,
such as augmenting recruitment and survival of juvenile life stages
(Barner et al. 2016, Beckley & Edwards 2021). In intertidal
environments, canopy-forming species may allow lower elevational species
to expand into higher elevations by providing a refuge from thermal and
desiccation stress during emersion at low tide (Watt & Scrosati 2013).
This may be particularly true for fleshy and calcareous seaweeds that
are sensitive to desiccation and photoinhibition (Short et al. 2014,
Kram et al. 2016). As a consequence, the performance of understory
species can be directly and/or indirectly affected by climate-mediated
changes (Edwards & Connell 2012, Ragazzola et al. 2012, Koch et al.
2013, Kim et al. 2020). Community-level approaches should therefore be
especially pertinent for these canopy-dependent assemblages.
A community that might be sensitive to future conditions is the
canopy-forming, intertidal rockweed, Silvetia compressa(henceforth Silvetia ), and its understory assemblages. Rockweed
canopies transform inhospitable areas into refuges by trapping moisture
and stabilizing substrate temperature (Bertness et al. 1999). The algal
assemblage associated with these refuges includes fleshy, turfing, and
calcifying seaweeds (Sapper & Murray 2003). These understory species
enhance primary productivity (Tait & Schiel 2018), provide settlement
cues and substrate for commercially important invertebrate larvae (Morse
& Morse 1984), and feed higher trophic levels (Ellis et al. 2007). Such
interactions and corresponding services will be heavily altered shouldSilvetia populations decline. Recently, Silvetia declines
have co-occurred with ocean warming associated with the 2015-16 El Niño
(Graham et al. 2018, MARINe 2023). Future climate conditions resulting
in similar levels of warming but across a prolonged period would likely
exacerbate the decline of Silvetia communities.
To understand the impacts of multiple climate change scenarios onSilvetia communities, we used mesocosms to expose Silvetiaand its understory assemblages to three levels of ocean change
conditions (Ambient, RCP 2.6, and RCP 4.5). These experiments also
manipulated Silvetia presence to distinguish between direct and
indirect effects of climate change on the dominant understory species.
We repeated this experiment in the summer and winter to assess seasonal
variation in these effects. Because future climate scenarios were
expected to suppress Silvetia growth, we also conducted field
manipulations of Silvetia to understand the consequences of
canopy loss on natural understory assemblages at two levels of
understory biomass.