Hot moments and hot spots of CO2 dynamics along the boreal forest and mire ecotone

Boris Tupek, Kari Minkkinen, Pasi Kolari, Mike Starr, Minna Pulkkinen, Jukka Alm, Jukka Laine, Eero Nikinmaa

Tutkimustuotos: Artikkeli kirjassa/raportissa/konferenssijulkaisussaKonferenssiartikkeliTieteellinen

Abstrakti

We studied the poorly known mesoscale spatial variation of forest - mire ecotone formed by the upland forests adjacent to transitional forest and mire margin and to lowland mires. Especially little is known of the contribution of the forest and mire margin, frequently fluctuating between wet and dry conditions, into the mesoscale CO2 budget. This transitional forest -mire zone has relatively little area, causing difficulties to study by eddy-covariance systems, but could represent landscape hot spot if contributing significantly different from the adjacent typical ecosystems.
Is the boreal forest - mire margin acting as a sink or source of CO2? Is the forest - mire margin tree assimilation larger than organic matter decomposition (when water table draws down) during dry year?
This study brings insights on CO2 exchange processes of forest - mire ecotone for hot moments such as typical (2005), exceptionally wet (2004), and exceptionally dry (2006) year, which is not evident from typical landscape CO2 studies. Additionally this study evaluates the role of forest floor vegetation along the forest and mire ecotone as a potential trigger-point in mesoscale CO2 balance sensitive to moisture variation.

While between wet and dry vegetative seasons the xeric, subxeric, mesic, and herbrich upland forests cut in half the forest floor respiration (Rff), the sparserly forested wet mires enhanced their total forest floor CO2 efflux. Although the forest floor photosynthesis (Pff) during the summer 2006 ceased by drought to early spring levels uniformly in all stations along the forest - mire ecotone, the recovery of Pff (associated with increased soil moisture) was observed only on sparsely forested mire and partially on mesic spruce forest. While the Pff recovered sporadically after the soil moisture increased, the Rff after soil rewetting was rapidly reaching and exceeding pre-drought levels along the ecotone. The measured instantaneous forest floor net exchange (NEff) as the sum of total respiration (including soil,tree roots, and forest floor vegetation) and forest floor photosynthesis was typically positive (CO2 source to atmosphere) and rarely negative (sink). However, during dry summer 2006 the daytime NEff measurements were generally lowered and often negative (sinks) by the downfall of Rff and in two stations (in mesic upland spruce forest, and in sparserly forested mire) also by the enhanced Pff values.
The aboveground net primary production (NPPa) derived from woody increments showed along the forest - mire ecotone typical levels associated with the soil fertility gradient and opposing trends among the most fertile and poor forest types between wet year 2004, dry year 2006, and intermediate year 2005.
While for the fertile mesic spruce forest the NPPa peaked during dry year 2006, likely due to the root access to the ground water level, in xeric or wet poorly fertile stations with pine dominated tree stands NPPa was steady during 3 years with the slight increase in intermediate year 2005. The opposing tree stand production trend was observed also within the narrow range of the forest and mire edge. The upper (closer to stands on mineral soils) forest- mire transitional herbrich paludified spruce - birch forest demonstrated NPPa maximum in wet year 2004 , while the lower (closer to stands on peat soils) forest- mire transitional herbrich paludified spruce -pine- birch forest demonstrated NPPa maximum in dry year 2006.
The annual net ecosystem exchange (NEe) based on the forest floor net exchange (NEff), the aboveground net primary production (NPPa), and the gross primary photosynthetic production allocated below ground (GPPb) revealed that in spite of upland forests and lowland mires being mostly sinks of CO2 to atmosphere, the forest - mire transitions were sources of CO2 to atmosphere during wet, dry, and intermediate years.

This work was supported by the Nordic Centre for Studies of Ecosystem Carbon Exchange and its Interactions with the Climate System (NECC), Nordic Centre of Excellence (NCoE), REBECCA by Helsinki University Environmental Research Centre (HERC), Finnish Centre of Excellence in Physics, Chemistry, Biology and Meteorology of Atmospheric Composition and Climate Change (FCoE).
Alkuperäiskielienglanti
OtsikkoProceedings of the Finnish Doctoral Programme in Atmospheric Composition and Climate Change ACCC
Sivumäärä1
Vuosikerta129
JulkaisupaikkaHelsinki
Julkaisupäivä2012
Sivut70
TilaJulkaistu - 2012
OKM-julkaisutyyppiB3 Vertaisarvioimaton artikkeli konferenssijulkaisussa
Tapahtuma Finnish Doctoral Programme in Atmospheric Composition and Climate Change ACCC - , Suomi
Kesto: 1 tammikuuta 1800 → …

Julkaisusarja

NimiReport series in aerosol science
Numero129

Lisätietoja


Volume:
Proceeding volume: 129

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