Topic 3.2: Forest Growth and Nutrient Cycling

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Topic Introduction

As trees grow and their structures are renewed, plant residues such as branches, leaves, bark and fruits accumulate on the forest floor and roots die and release organic matter into soil. These organic materials serve as an energy source for the decomposer community.  In forest ecosystems only about 1.5% to 5% of primary production is consumed by herbivores, leaving the bulk of organic materials to be consumed by organisms living in litter and surface soil. This interacting web of biota ranges from the relatively large earthworms and arthropods that mix detritus between soil layers and break it in to smaller pieces, through to fungal and bacterial microbes that mineralize organic matter, releasing CO2 to the atmosphere and inorganic nutrients in to the soil. Nutrients that were once bound in plant and animal structures become available again for plant uptake. This is the process of nutrient cycling that, over decades, centuries and millennia, acts to concentrate carbon and nutrients in the forest floor and surface soil. The occurrence of complex rainforests on pure white sands is good evidence of the importance of nutrient cycling in supplying nutrients for growth, rather than direct uptake from soil. Tropical ecosystems have a greater proportion of nutrients in biomass than temperate soils and consequently deforestation (felling plus burning) can result in a relatively infertile soil that is vulnerable to leaching losses and degradation.

The net result of forest growth is the accumulation of carbon and nutrients in litter and surface soil. Much of the annual demand for nutrients is met by tree internal cycling of nutrients, such as withdrawal of nutrients prior to leaf fall, and from nutrients released again from litter and soil organic matter. Consequently a large proportion of the annual demand for nutrients is drawn from nutrient cycling rather than uptake from soil reserves of nutrients. Viewed in this way, forests live on their past accumulation of nutrients.

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