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Effects of water addition on biotic and abiotic components of a dry boreal forest in the Yukon Carrier, Patrick

Abstract

To test the response of the dry Kluane boreal forest ecosystem (Yukon, Canada) to increased rainfall as predicted from climate change scenarios I irrigated three 1.5 ha forest stands from 1995-1999, to double baseline summer rainfall levels. I tested if various biotic and abiotic components of this ecosystem would react to the reduction of the summer water deficit relative to three control stands. I predicted that in response to irrigation: 1) growth (or biomass) in some species of plants would increase. 2) redbacked voles would in turn increase in numbers with greater plant-food availability because they are food-limited herbivores. 3) mushroom biomass would increase. 4) decomposition would increase, and hence 5) net nitrogen (ammonium, NH4+) mineralization by soil microbes (i.e. nitrogen availability for plant nutrition) would also increase, further improving plant growth (1). Over the five years of irrigation, none of the selected species of plants nor the voles reacted to irrigation. Mushroom biomass increased 2.5-fold on irrigated stands. Litter decomposition increased log-linearly with the enhanced actual evapotranspiration (AET) following irrigation. Soil microbes immobilized NH4+ (rendering it unavailable to plants) as AET increased, while net NH4+ mineralization remained unchanged. The lack of increase in net NH4+ mineralization was responsible for the lack of plant response to irrigation in this study because plants in the boreal forest, including those at Kluane, are nitrogen limited. In turn, this lack of change in net NH4+ mineralization can be attributed to the increase in microbial NH4+ immobilization. Several ecological conditions may lead to NH4+ immobilization by microbes, including the chemistry of the dead organic matter in the soil. Organic matter of high carbon-to-nitrogen (C/N) ratio (20 or 30/1) typically leads to net nitrogen immobilization. The C/N ratio was not monitored in this study, and more research is therefore needed to substantiate this hypothesis that Kluane soils have high C/N ratios; other studies have found high C/N ratios in boreal forest soils. In the short-term, the detrital chain of this ecosystem would be stimulated by an increase in rainfall. No additional bottom-up effects are predicted to occur. Soil decomposers may therefore substantially enhance decomposition and CO2 emissions of the ecosystem, without any concurrent increase in atmospheric CO2 assimilation by plants. With climate change, the boreal forest ecosystem may become a net source of carbon to the atmosphere in the short-term, and hence exacerbate climate change through this positive feedback. In the longer-term however, the C/N ratio of the dead organic matter in the soil may decrease, reducing microbial NH4+ immobilization and allowing net NH4+ mineralization to occur. This could eventually lead to greater primary production and more CO2 assimilation by plants, hence counter-balancing the soil's positive feedback on climate change. The amplitude of this negative feedback through enhanced plant CO2 assimilation and the time span before it occurs are riot known. More experiments are therefore required to test whether the forest floor's C/N ratio would change in the longer term under increased precipitation and if net NH4+ mineralization and plant CO2 uptake would later increase, leading to an enhancement of the carbon sink activity in this part of the boreal forest.

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