Natural regeneration and recruitment of native Quercus robur and introduced Q. rubra in European oak-pine mixed forests.
The North-American Northern red oak (Quercus rubra L.) introduced to European forests over 200 years ago, spreads spontaneously in various types of forest ecosystems. At the same time the retreat of native pedunculate oak (Q. robur L.) has been observed. We aimed to recognize the extent of natural regeneration and recruitment of pedunculate (PO) and red oak (RO) in managed forests (central Poland) on sites typical of oak-pine mixed forest, as well as to identify habitat conditions which favour the occurrence of native oak and spread of the alien oak. The abundance and heights of juveniles of both oaks, as well as the impacts of the canopy, nurse and burial effects, on distribution and occurrence of oak specimens were studied on 1000 study plots (1 m2 each), in two transects crossing the Scots pine-pedunculate oak forest (POF) and the Scots pine-northern red oak forest (ROF). We revealed an advantage of introduced RO over native PO in natural regeneration, recruitment and spread. However, we did not find direct (interspecific) competition between PO and RO recruits. RO predominance over PO resulted from more effective colonization of empty niches by the alien oak in POF (suitable for both species), and the abundant establishment of RO seedlings and saplings within ROF. In contrast, PO was not able to re-colonize ROF. The regeneration of RO was microsite-limited in ROF gaps in the regeneration phase (probably due to site moisture deficiency) and under closed RO canopy in the recruitment phase (due to low light availability), and microsite-favoured under mature tree crowns due to the burial effect of the litter layer (and higher light availability under heterogeneous canopy). In POF, RO regeneration was microsite-favoured due to the burial effect of a dense moss layer and the nurse effect of shrubs, including conspecifics. PO regeneration was also microsite-favoured due to the burial effect of the moss layer, but its recruitment was herbivore-limited there (due to browsing). The co-occurrence of PO and RO specimens, both in POF and ROF, resulted from forest community structure, oak ecology and behaviour of acorn dispersers. Because the native PO is at a disadvantage in its native range caused by a lower share of mature trees in forested areas, strong impacts of fungal pathogens and high pressure by insects and large herbivores (also noted in our study), the introduced oak is able to become a serious competitor.