Drought and mistletoe reduce growth and water-use efficiency of Scots pine

To what extent do mistletoes contribute to growth decline in drought-prone forests? Can the rising atmospheric CO2 concentrations offset the negative impacts of drought and mistletoe infestation on tree growth? Long-term data on growth and intrinsic water use efficiency (iWUE) may allow answering both questions. We used dendrochronology to assess long-term changes in radial growth and iWUE in Scots pine (Pinus sylvestris) trees severely infested by mistletoe (Viscum album) as compared to non-infested trees. The relationships among tree variables and mistletoe infestation were quantified using structural equation models. Linear mixed-effects models of basal area increment as a function of climate were fitted to severely infested and non-infested trees. Infested trees showed higher stem and crown diameters because they grew faster than non-infested trees in the past. Mistletoe infestation enhanced defoliation and reduced radial growth for more than 10 years prior to sampling, while iWUE was significantly lower on severely infested trees only for the last 5 years. Severely infested trees had higher growth responsiveness to drought stress than non-infested trees. Although infested and non-infested trees displayed similar rising iWUE temporal trends, the combined effect of drought stress and mistletoe infestation caused a reduction in growth and reversed the CO2-induced increase of iWUE in infested trees. We conclude that rising atmospheric CO2 concentrations cannot compensate for the impacts of drought and mistletoe on tree growth and iWUE.

► Does mistletoe (Viscum album) induce growth decline in xeric Scots pine forests?
► We analyze growth and intrinsic water-use efficiency (iWUE).
► Infested trees grew faster than non-infested trees in the past.
► Mistletoe reduced growth, enhanced defoliation and increased growth sensitivity to drought.
► Drought and mistletoe infestation reversed the CO2-induced increase of iWUE.