Mistletoe negatively impacts vigor, growth and reproduction of silver fir forests at regional and local scales

Mistletoes impact their tree hosts in different ways and affect distinct processes such as growth and reproduction. However, comparative assessments of mistletoe effects on these two processes are scarce, particularly at different spatial scales. Here, we quantified how mistletoe (Viscum album L.) affected radial growth and cone production of silver fir (Abies alba Mill.), a major European conifer. We assessed these impacts at continental (2004 −2019 period, 301 plots across Europe), regional (western Spanish Pyrenees, 0.5º W−0.5º E and 42.5º−43.0º N, 30 sites and 360 trees) and local scales (two Pyrenean stands with severe mistletoe infestation − VI and SN sites). Mistletoe abundance enhanced crown defoliation at all scales. At continental scale, the percentage of infested trees has been increasing reaching values close to 15 % in 2019. At regional scale, mistletoe infestation was severe in 19 % of sampled trees. At local scale, mistletoe infestation contributed to forest decline through growth loss and decreased cone production. Silver fir stands with more trees heavily infested by mistletoe showed abundant winter fleshy-fruited species such as Sorbus aria. Mistletoe led to lasting (12 years prior to sampling) and irreversible growth reduction (−78 % in site VI, −44 % in site SN) in severely infested trees as compared to lightly infested coexisting trees. The acute defoliation of the upper crown in severely infested trees negatively impacted cone production, but this effect was significant only at local scale. Thus, continental-wide assessments of mistletoe infestation and how it impacts defoliation, growth and cone production should be carefully validated and refined by regional and local data.