Chaparral species that resprout after fire in the Santa Monica Mountains of California experience competition for light due to rapid growth of post-fire annuals or the presence of invasive species. If light limitation persists, post-fire resprouts may deplete carbohydrate stores and attempt photosynthetic compensation. Furthermore, they may shift leaf optical properties in response to shade. We tested these hypotheses by comparing the photosynthetic performance of post-fire resprouts of Heteromeles arbutifolia (toyon) under three treatments: shade, irrigated, and control. Shaded plants were grown under low Photosynthetic Photon Flux Density [PPFD] of ~200 µmol m^-2 s^-1, which represents 10 percent of the maximum. The irrigated treatment eliminated water stress as a confounding factor. Six plants of each treatment were examined for their photosynthetic response to increasing light levels [light response curve] and to increasing CO₂ levels [CO₂ response curves]. Parameters measured were maximum net photosynthetic rate (A_max), light compensation point (A_comp), dark respiration (R_dark), CO₂ inside the leaf at 2000 ppm external (C_i,2000), quantum yield (QY), chlorophyll fluorescence (F_v’/F_m’), stem elongation rate, stomatal conductance to water vapor diffusion (g_s), predawn water potential (Ψ_pd), and leaf absorptance (a).

Results/Conclusions

Significant differences were found among all three treatments, in all parameters measured, with the exception of Fv’/Fm’ (P < 0.05). These changes in photosynthetic performance were consistent with acclimation to shade and acclimation to water stress and may be an adaptation to compete for water and light with fast growing, post-fire annuals. Shifts in photosynthetic performance may be inadequate for survival under severe conditions such as the presence of vigorous invasive species.