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Roles of Understory Biodiversity
by Jason Sircely
Spring 2007

The ecosystems of our planet currently face dramatic and unprecedented changes. Two of these changes, the loss of biodiversity and biological invasions, will alter how ecosystems function. My master’s thesis research at Columbia examined the impact of understory plant diversity on ecosystem processes and on the abundance of exotic invasive plants. My work also provided information for Black Rock’s study on the threat of oak loss in northeastern forests [Ed. Note: See “What Will Happen If the Oaks Die?,” Spring 2006].

I looked at the influence of plant diversity on several fundamental ecosystem processes (productivity, or growth, of understory plants; decomposition; and the conversion of nitrogen and calcium to inorganic forms during decomposition), and on exotic plant invasions. I also examined the influence of environmental conditions such as light, soil moisture, soil depth, and soil nitrogen content to compare their importance with plant diversity.  Finally, I assessed the role of scale (scale of ecological association) by examining how the effects of plant diversity and environmental conditions differed for wetter (seeps) and drier (mesic) areas and analyzing them both separately and together.

Plant diversity appeared to increase productivity and decomposition in the forest understory; decomposition was unaffected by diversity in seeps. In contrast, plant diversity appeared to decrease nitrogen mineralization in seeps, possibly indicating greater uptake of nitrogen (calcium results are forthcoming).  Still, other mechanisms could be responsible, which future studies could test.

While environmental differences resulted in greater productivity in seeps than in mesic areas, they had no effect on productivity, decomposition, or nitrogen mineralization within either zone. This suggests that loss of understory plant diversity could reduce productivity, slow mesic decomposition, and alter seep nitrogen cycling. 

Both invasive plant abundance and the number of native species were greater in the more fertile seeps, so invasion and native plant diversity were positively correlated. However, there were no such correlations within the seep and mesic areas. This supports an emerging consensus that plant diversity does not facilitate invasions at finer ecological scales, despite positive relationships between diversity and invasibility at greater scales.

The study provides a step forward in understanding how biodiversity loss will affect our ecosystems over coming decades. Losses of understory plant diversity appear likely to decrease ecosystem process rates and, depending on the ecological scale, to exert even more control over ecosystem processes than fine-scale environmental differences.