Responses of wetland productivity to changes in water table depth (WTD)
are controlled by complex interactions among several soil and plant
processes, and hence are site-specific rather than general in nature.
Hydrological controls on wetland productivity were studied by
representing these interactions in connected hummock and hollow sites in
the ecosystem model ecosys, and by testing CO2 and energy
fluxes from the model with those measured by eddy covariance (EC) during
years with contrasting WTD in a shrub fen at Lost Creek, WI. Modelled
interactions among coupled processes for O2 transfer,
O2 uptake, C oxidation, N mineralization, N uptake and C
fixation by diverse microbial, root and mycorrhizal populations enabled
the model to simulate complex responses of CO2 exchange to
changes in WTD that depended on the WTD at which change was occurring.
At the site scale, greater WTD caused the model to simulate greater
CO2 influxes and effluxes over hummocks vs. hollows, as has
been found at field sites. At the landscape scale, greater WTD caused
the model to simulate greater diurnal CO2 influxes and
effluxes under cooler weather when water tables were shallow, but also
smaller diurnal CO2 influxes and effluxes under warmer
weather when water tables were deeper, as was also apparent in the EC
flux measurements. At an annual time scale, these diurnal responses to
WTD in the model caused lower net primary productivity (NPP) and
heterotrophic respiration (Rh), but higher net ecosystem
productivity (NEP = NPP – Rh), to be simulated in a cooler
year with a shallower water table than in a warmer year with a deeper
one. This difference in NEP was consistent with those estimated from
gap-filled EC fluxes in years with different water tables at Lost Creek
and at similar boreal fens elsewhere. In sensitivity tests of the model,
annual NEP declined with increasing WTD in a year with a shallow water
table, but rose in a year with a deeper one. The model thus provided an
integrated set of hypotheses for explaining site-specific and sometimes
contrasting responses of wetland productivity to changes in WTD as found
in different field experiments.
