EPOW - Ecology Picture of the Week

Beneath the Tropical Leaf Litter

Forest floor leaf litter, Primary mixed evergreen forest
Nokrek National Park, Meghalaya, India

Explanation:  Welcome to the lush tropics!  Although the term "jungle" is not strictly a scientific term, it still aptly conjures the density and humidity of most old tropical forests of the world ... with its tangles of vines and impenetrable, almost claustrophobic concentrations of shrubs and trees. 

So one would expect to be walking on thick mats of leaves, twigs, and moss.  But this is typically not the case in tropical forests.  

Scrape away the top inch or the leaf litter, and what do you see?  Bare soil, often bare mineral soil with little organic matter!  

Tropical forests have what has been described as a "tight" nutrient cycle, where most of the vegetative biomass occurs above ground in the green stems and leaves of plants, and little occurs below ground.  As tropical plants shed their leaves or die, they decompose rapidly and their nutrients are quickly reabsorbed back into the living vegetation.  

This is one reason why many tropical forests are so fragile; they simply lack the robust thick organic soils found in many temperate zone forests.  
  

Leaf litter in an old secondary tropical forest in Mabali Forest Reserve,
on the shore of Lake Tumba, Democratic Republic of the Congo.
Here too the leaf litter layer is only an inch thick, covering 
a dense mineral soil beneath.

 
Still, as one study has found (Wieder et al. 2009), tropical forests play a significant role in the global carbon cycle.  The more rainfall that a tropical forest receives -- as with the monsoon-laden forests depicted in this week's photos -- the higher is the rate of decomposition of leaf litter on the forest floor, and the fast is the uptake of those nutrients back into the living forest above ground.  Moreover, it was found that availability of phosphorus often controlled the rate of litter decomposition by microbes.  

Litter decomposition is actually a complex process involving invertebrates that chew the dead leaves, microbes that decompose organic matter and change the structure of nitrogen and other macronutrients, rainfall that guides the rates of decomposition, and existing vegetation that takes back the nutrients into living tissues.  This is why, when many tropical forests are cleared and burned for agriculture, the soil can support only a few years of crops before it becomes devoid of nutrients and its structure lateralizes, that is, turns brick-hard.  

 
Information:
     Wieder, W. R., C. C. Cleveland, and A. R. Townsend. 2009. Controls over leaf litter decomposition in wet tropical forests. Ecology 90(12):3333-3341.