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

1. 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. 2. Evergreen forest and fynbos species were grown under three light availability treatments (24%, 54% and 100%) in a glasshouse with their photosynthetic response to continuous and fluctuating light, dark respiration, root respiration, biomass and leaf traits quantified. 3. 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 vs. 3.4 mu mol CO2 m(-2) s(-1)). 4. 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 specialisations that promote the emergence and maintenance of alternative stable states.

Automated summary

Contrasting light availability between forest and fynbos results in selection of dissimilar traits, with fynbos species unable to efficiently intercept light and maintain a positive carbon balance under low, dynamic light conditions.