The role of shade in maintaining alternative stable states between open- and closed-canopy vegetation

Fire is commonly identified as strong driver of alternative stable states
such as adjacent open- versus closed-canopy vegetation types. The absence
of open-canopy species from closed-canopy understoreys, where light
availability is low and dynamic, however, suggests shade tolerance is an
integral determinant of such vegetation boundaries. While the importance
of light dynamics between alternative stable states has been acknowledged,
the physiological mechanisms behind sun versus shade-tolerance are
unclear. Here we investigated the differences in light interception and
carbon assimilation of open- and closed-canopy species, and whether this
contributes to the maintenance of alternative stable states. Evergreen
forest and fynbos species were grown under three light availability
treatments (24%, 54%, 100%) in a glasshouse with their photosynthetic
response to continuous and fluctuating light, dark respiration, root
respiration, biomass and leaf traits quantified.   Fynbos species
displayed smaller, thicker, and tightly aggregated leaves compared to
forest species. Under low light, fynbos species experienced some
mortality, and showed lower light-use efficiency with fluctuating light
and lower in situ photosynthetic rates under the 54%
light treatment compared to forest species (0.8 versus 4 mmol
CO2 m-2 s-1).   Synthesis. Contrasting
light availability in forest and fynbos selects for dissimilar traits,
where fynbos species are unable to efficiently intercept light and
maintain a positive carbon balance under low, dynamic light compared to
forest species. This shade intolerance is exacerbated by divergent
nutrient and fire regimes in forest and fynbos environments, requiring
incompatible trait specializations that promote the emergence and
maintenance of alternative stable states.