Light demands of shrub seedlings and their establishment within scrublands

Light demands, i e. shade tolerance and response to light, of seedlings of six shrub species (Berberis vulgaris L., Cornus sanguinea L., Crataegus monogyna JACQ., Ligustrum vulgare L., Rosa canina L., Sambucus nigra L.) were investigated during establishment beneath scrub. Four methods were used: (1) microclimatic measurements in two patches of a scrub sere, (2) recording of density and survival of seedlings of the six species naturally occurring in this sere, (3) transplantation of seedlings of Rosa to these sites, and (4) gas exchange measurements on seedlings of all six species grown in a glasshouse. Penetration of light decreased strongly with scrub development whereas temperature and relative air humidity were only slightly different in old scrub compared to pioneer stages. Density and survival of naturally occurring seedlings were highest in an intermediate stage of scrub development, while growth and survival of transplanted seedlings decreased significantly with increasing cover of scrub. The seedlings of the six shrub species differed in their light demands, indicated by dark respiration, light compensation point, photosynthetic capacity, and quantum efficiency near light-saturated photosynthesis. Cornus and Rosa were slightly more shade-tolerant, whereas Berberis, Crataegus, Ligustrum and Sambucus apparently had higher light demands, but none of them was particularly adapted to establish in a strongly shaded environment. The differences among species in survival of the naturally occurring seedlings did not agree generally with the results of the gas exchange experiment. However, we conclude that light availability is a crucial factor for growth and survivial of seedlings in old scrub, but additional factors (e.g. soil water content) have to be considered in order to explain the species-specific differences.