Standard practice calls for maintaining pasture grasses chopped down in a 3 ft circle around tree seedlings to prevent competition. But might the grasses actually facilitate?

What is the effect of pasture grasses on tree seedlings?  Do they help or hinder?

Two of 6 experimental treatments on the target tree Inga.

Two of 6 experimental treatments on the target tree Inga.

In the early years we kept pasture grasses cut back from our tree seedlings for at least two years because we assumed once the grasses were released from control by cattle grazing, they would grow so fast as to overwhelm the small trees. 

However, we decided to test that assumption, since grasses also might help shade vulnerable tree seedlings during the three-month dry season.  We lose many during these droughts. We did two experiments, one in 1998-1999 on Inga edulis (non-native legume tree) and the other in 2008-2010 on two natives, Inga spectabilis and "maria" (Calophyllum brasiliense).

The photo at right shows two treatments with the native Inga as the target tree.  The cut bamboo sticks mimic the shade offered by grass without competition below ground with roots.  The bags of soil around the Inga protect the soil from drying out, as does standing pasture grass, but do not compete with the target either above-ground for light or below-ground with roots.  Four other treatments allowed us to determine whether competition was more important than the benefits of shading, and whether above- or below-ground competition was more important.

In fact, a slight facilitation effect by grasses seemed to happen over the dry season, but it was overwhelmed by the negative impact of competition during the rainy season. The lessons we took from the results were to:

  • keep grass cleared around seedlings until they are taller than the grasses, as do the local foresters, and
  • do not clear the grasses before the impending dry season but allow them to grow tall enough to shade the seedlings.

The 1998-1999 work is published in a MS thesis at UCI by Satya Rhodes-Conway; the 2008-2010 work is published in a MS thesis at UCI by Mary Anderson.

Several plots of the 2008-2010 experiment, fenced from cattle and planted with maria.