Competitive effects of herbaceous species on water potential and growth of quercus ithaburensis ssp. Macrolepis seedlings

A noticeable decline of Quercus ithaburensis ssp. Macrolepis, a typical mediterranean species, has
been recorded in the past few decades throughout Greece. The triggers were mostly human
influences and specifically illegal cuttings, wildfires, and overgrazing. Regeneration and future
management of this species is therefore questionable. To investigate the effect of companion plants
on this decline as expressed by water availability and plant growth, a pot experiment was conducted
and oak seedlings were planted with Bromus sterilis and a mixture of Trifolium repens and T.
fragiferum as companion plants. Midday water potential from May to September and oak growth at
the end of the experiment were measured as factors affecting oak physiology. The results suggest a
different effect on the water potential of the oak seedlings depending on the companion plant
species and the time of the year. The presence of a companion plant that has completed its life cycle
by the end of spring may positively influence the water status of oaks during the summer months
due to shading and lower soil water evaporation. On the contrary, when the companion plants
continue their life cycle in the summer, the negative effect may be continued, specifically during the
very dry months. The negative effect of grass on the growth of oaks may be attributed to their
shading from the dense grass biomass. It can be concluded that control of competitive vegetation,
especially of grasses, is crucial for Quercus ithaburensis ssp. macrolepis regeneration by natural
(from acorns) or artificial (with up to one-year-old seedlings) methods. This control is necessary at
least in the first year after establishment, when seedling height is less than that of the competitive
vegetation. If the latter can not be removed by some ways (e.g. controlled grazing), then it is safer to
use taller oak seedlings of an older age in the reforestation projects.