The human footprint in the carbon cycle of temperate and boreal forests.
Temperate and boreal forests in the Northern Hemisphere cover an area of about 2 x 10(7) square kilometres and act as a substantial carbon sink (0.6-0.7 petagrams of carbon per year). Although forest expansion following agricultural abandonment is certainly responsible for an important fraction of this carbon sink activity, the additional effects on the carbon balance of established forests of increased atmospheric carbon dioxide, increasing temperatures, changes in management practices and nitrogen deposition are difficult to disentangle, despite an extensive network of measurement stations. The relevance of this measurement effort has also been questioned, because spot measurements fail to take into account the role of disturbances, either natural (fire, pests, windstorms) or anthropogenic (forest harvesting). Here we show that the temporal dynamics following stand-replacing disturbances do indeed account for a very large fraction of the overall variability in forest carbon sequestration. After the confounding effects of disturbance have been factored out, however, forest net carbon sequestration is found to be overwhelmingly driven by nitrogen deposition, largely the result of anthropogenic activities. The effect is always positive over the range of nitrogen deposition covered by currently available data sets, casting doubts on the risk of widespread ecosystem nitrogen saturation under natural conditions. The results demonstrate that mankind is ultimately controlling the carbon balance of temperate and boreal forests, either directly (through forest management) or indirectly (through nitrogen deposition).
Magnani
Federico
F
Department of Fruit Tree and Woody Plant Science, University of Bologna, Bologna I-40127, Italy. federico.magnani@unibo.it
Mencuccini
Maurizio
M
Borghetti
Marco
M
Berbigier
Paul
P
Berninger
Frank
F
Delzon
Sylvain
S
Grelle
Achim
A
Hari
Pertti
P
Jarvis
Paul G
PG
Kolari
Pasi
P
Kowalski
Andrew S
AS
Lankreijer
Harry
H
Law
Beverly E
BE
Lindroth
Anders
A
Loustau
Denis
D
Manca
Giovanni
G
Moncrieff
John B
JB
Rayment
Mark
M
Tedeschi
Vanessa
V
Valentini
Riccardo
R
Grace
John
J
D002244:Carbon / Q000378:metabolism*; D002980:Climate; D017753:Ecosystem; D006802:Human Activities; D009584:Nitrogen / Q000378:metabolism; D014197:Trees / Q000378:metabolism*
D002244:Carbon; D009584:Nitrogen