Herbivores can sometimes alter the trajectory of entire ecosystems. In the northern-boreal mountain birch forest of Northern Fennoscandia, outbreaks by geometrid moths have caused mass mortality of mountain birch across large areas since the early 2000s. The severity of the outbreaks has been linked to the climatically facilitated range-expansion of the winter moth (Operophtera brumata), that has recently established itself as a new outbreaking defoliator in many areas that originally experienced outbreaks only by the native autumnal moth (Epirrita autumnata).
In the current study – focused on the Varanger region in northeast Norway – we show that the cumulative defoliation inflicted by such a novel two-species outbreak has exceeded a critical tolerance threshold (tipping point) of the forest, resulting in an abrupt increase in the mortality rate of birch stems. We also show that this severe mortality event has resulted in the loss of internal positive feedback mechanisms in forest regrowth, as regeneration by both basal sprouting and sapling production are positively affected by the presence of surviving stems and trees. Thus, reforestation may be very slow or fail altogether in areas that have been forced across critical tolerance thresholds and suffered mass mortality of trees. Accordingly, climatically induced alteration of moth defoliation regimes in the mountain birch forest could cause this ecosystem to undergo rapid and potentially persistent changes of state.
You can read the full paper here.
Reference: Vindstad, OPL, Jepsen, JU, Ek, M, Pepi, A, Ims, RA (2019) Can novel pest outbreaks drive ecosystem transitions in northern‐boreal birch forest? Journal of Ecology 107: 1141–1153.
https://doi.org/10.1111/1365-2745.13093
Picture: A conceptual model for how forest mortality induced by moth outbreaks may be followed by regeneration or a persistent transition to a treeless ecosystem state (photo: Jakob Iglhaut).