Soil and site factors influencing purple-flowered rhododendron ( Rhododendron ponticum L.) and eastern beech forests ( Fagus orientalis Lipsky) in Turkey

Abstract

Eastern beech ( Fagus orientalis Lipsky) is the major timber species growing in the Black Sea Region (BSR) forests of Turkey. Purple-flowered rhododendron ( Rhododendron ponticum L.) is native to the region and currently dominates the understory of almost the entire eastern beech forest, reducing tree regeneration and growth. An aging beech overstory with little or no regeneration is the current state of most of the beech–rhododendron forests. We examined whether and how major environmental and disturbance factors have influenced the distribution and growth of rhododendron and beech in the region by determining the effects of canopy light, topography (aspect, elevation, slope), soil (moisture content, depth, texture, pH), and disturbance (anthropogenic and fire) on the establishment, abundance, and growth rate of rhododendron and beech in two rhododendron-invaded beech forests with different anthropogenic disturbance histories in the western BSR. We also studied the age and size structures of current beech stands and rhododendron to understand their origins and their future under varying levels of anthropogenic pressure. The two sites had different stem-size class distributions. A weak relationship was found between diameter at breast height and age for beech, whereas there was a strong correlation between groundline diameter and age for rhododendron. The rhododendron understory was established after the beech overstory. Charcoal-density analysis and beech stem ages suggested that fire has not been a major regenerative disturbance for these sites, at least in the past 150–200 years. Rhododendron reduced both beech regeneration and long-term growth. The rhododendron-population structure suggested continuing domination in the near future, with increasing density and proportion of small stems. Soil moisture was an important environmental factor affecting rhododendron and beech abundance and growth. Adequate soil aeration was critical for rhododendron. Use of foliar herbicides with burning during tree-regeneration efforts will probably provide greater success on rhododendron control. Establishing mixed beech-conifer stands might lower expansion of current and future rhododendron populations in the long term.