Penstemon Distribution on the Devils Slide Trail
On 16 August 2019, Tom GPS'd all the locations of the three penstemon species on the Devils Slide Trail, with a new location defined as being at least 0.02 miles (100 feet) from another GPS point for that species. The only exception was if there was a group of plants spanning a distance of less than 0.02 miles. In that case, Tom GPS'd the edges of that group of plants.
Fig. 1 shows the result of that survey.
Fig. 1. A view looking east using Google Earth showing the locations of the three penstemon species on the Devils Slide Trail excluding the lowermost two locations on the trail. Click on the picture to get a larger version without labels showing the complete Devils Slide Trail.
Note that the view is to the east, with north to the left.
Because Google Earth plots the symbols slightly above the ground surface, the pink ball and the cluster of red balls that appear to be beyond Saddle Junction are actually on the Devils Slide Trail.
The dominant vegetation of the Devils Slide Trail in most places is a dense forest of sugar pine and canyon live oak. The densest canopy of those species is on the southwest-facing slopes, as can be seen in Fig. 1, where there is unbroken green along the trail. The only penstemon found in those areas are a few P. grinnellii in the upper part of the trail, where rock cliffs open up the canopy a bit.
The northwest-facing slopes are more open, and P. rostriflorus grows in most of those areas, along with some P. grinnellii.
The drainages are favored by P. grinnellii, which typically grows on only the northwest-facing slope part of the drainage, although it will grow on the southwest-facing slope if it is open enough.
P. labrosus doesn't like slopes at all; it is a plant of quite-level surfaces at San Jacinto. Thus the only location on the Devils Slide Trail for it is at the very uppermost Devils Slide Trail, in the flattish part just west of Saddle Junction. You can see that the flattish Tahquitz Valley, at the top of Fig. 1, contains mostly P. labrosus, with P. rostriflorus restricted to ridgelines and the steeper drainages there.
Penstemons are strong fire-followers, especially P. grinnellii, whose abundance increases by many orders of magnitudes in the first years after a fire here. Since the Devils Slide Trail area hasn't burned for many decades, the current penstemon distribution there probably represents the long-term equilibrium of those species. It is quite possible that immediately after a burn, P. grinnellii might be found in many more locations here.