Nitrogen-dependent recovery of subarctic tundra vegetation after simulation of extreme winter warming damage to Empetrum hermaphroditum
Aerts, R. 2010. Global Change Biology.16: 1071-1081
Abstract
Vast areas of (sub)arctic tundra are dominated by the ericoid dwarf shrub Empetrum
hermaphroditum. Recent experimental and observational data have shown that Empetrum
can be damaged heavily by recurrent extreme winter warming. In addition, summer
warming leads to increased soil N availability in tundra ecosystems. In a 7-year
experiment, I investigated the recovery of subarctic Empetrum-dominated tundra vegetation
using a factorial combination of various degrees of aboveground Empetrum removal
(simulating the damaging effects of extreme winter warming) and N addition (simulating
one of the effects of summer warming). After 7 years no new species had established in
the plots. The growth of planted Betula nana seedlings was stimulated by Empetrum
removal and reduced by N addition. This Empetrum-dominated tundra ecosystem was
resilient against severe disturbances. Only when Empetrum was 100% removed did it fail
to recover, and only in combination with high N supply the subordinate species (notably
Eriophorum vaginatum and Rubus chamaemorus, a graminoid and a forb) could benefit.
In the 50% removal treatment Empetrum recovered in 7 years when no N was supplied
and the cover of the subordinate species did not change. However, when N was added
Empetrum recovered faster (in 4 years) and the subordinates decreased. When Empetrum
was not removed and N was added, Empetrum even increased in abundance at the
expense of the subordinate species. Thus, profound changes in tundra ecosystems can
only be expected when Empetrum is very heavily damaged as a result of recurrent
extreme winter warming and when soil N availability is increased as a result of summer
warming. These changes in species composition upon extreme disturbance events may
lead to a wide variety of ecosystem feedbacks and cascade processes as this tundra
system is relatively species-poor, and can be hypothesized to have low functional
redundancy.
Key Words
arctic, climate change, disturbance, Empetrum hermaphroditum, extreme climatic events,phenolics, resilience, seedling establishment, soil nitrogen availability, summer warming
