Exposure to Light

Light Requirements of King Ranch Bluestem
(Bothriochloa ischaemum var. songarica)

by Bob Harms email-here

My experience has been that King Ranch Bluestem (KR) thrives as a result of:

a burn
mowing (and grazing)

KR does not fare well:

under tree canopies
in the thatch of once dense KR
under mulch of any kind, even in strong sunlight

Although a need for exposure to light might seem to account for these circumstances influencing KR's development, there may be more factors at play than just exposure.

KR with constant shade under canopies between trees in late morning (right) and mid-afternoon (left).[Looking west; south edge on the left.]

The negative impact of location under tree canopies raises a number of issues. In the above 'shaded' KR patch, a gradual reduction in density occurs with distance from the east edge, with greatest exposure, with a more abrupt transition along the south edge, which also has numerous smaller woody plants and noticeable leaf litter.

KR under SW canopy of live oak NE side of the oak
- The adverse effect of shade on KR is a matter of degree. Since there is greater shade on the north & east side of a tree, KR will not be affected symmetrically on all sides, as illustrated in the above photos from two sides of a plateau live oak (Quercus fusiformis). Again, the northeast edge has numerous woody plants, which also benefit from the increased shade.
  
  KR under SW canopy of Spanish oak NE side of the oak
- Leaf litter is also a contributing factor, but the details may be complex. Leaf fall from live oaks occurs in a short period of time in the spring and deposits a layer of leaves immediately under the branches. Leaf fall from Spanish oaks occurs over a longer period in the fall, and the leaves are generally carried by winds away from the canopy area. In the images below, details from the above southwest exposures, there is noticeable leaf litter only under the live oak.
  
  Leaf litter detail under live oak Leaf litter detail under Spanish oak
  
  Under under one large Spanish oak (Quercus texana) in an area that is regularly mown, KR extends inward to the trunk. Perhaps mowing has contributed to KR's light requirements, thus minimizing the shading effects of the oak.
  
  KR in mown area continuous to the trunk of a large Spanish oak, at 2:30 p.m., August 18
KR thatch is never continuous, with scattered open spots, and in bright sunlight doesn't reduce the light reaching the ground in the same manner as mulch.
In burned areas KR seems to recover fairly uniformly over the entire burn area, not spreading from the periphery to the center. Perhaps seed dispersal from surrounding KR is facilitated by the bare ground, allowing wind to carry the spikelets along the surface for some distance. Wind dispersal of seeds might also account for the presence of KR along the very edge of paved roads. My photos from our summer burn seem to show that some KR bunches recover from a surviving root system.
How long does it take for mulch or other covering to create a fully 'dead' patch? My experience has been that such dead patches do not recover from the roots, and may take years to recover.

Light Requirements of King Ranch Bluestem(Bothriochloa ischaemum var. songarica)

Light Requirements of King Ranch Bluestem
(Bothriochloa ischaemum var. songarica)