South-West Rift Zone

The Silene Population on the North-East Side of the Crater Rim Road

June 17, 1999

Kim Bridges and Mike Huddleston

 

The South-West Rift Zone Silene population has been under our surveillance for several years.  Observations made over the past few weeks indicate that this population is healthy, with most of the plants showing strong growth and what should soon be abundant flowering.

We haven't previously visited the opposite (NE) side of the Crater Rim Road.  In the past we focused our observations on the SW Rift zone area on the "outside" of the road.  However, on this trip we particularly noticed a large "green area" of short-stature but relatively lush growth in an area that we would have expected to find few, if any, plants.  This area is adjacent to Halema`uma`u Crater.

The notes and pictures here is the record of our observations in this area.  Our primary concern has been an examination of the Silene, but we include preliminary observations on the other plant species here.

The order is approximately the same as we traveled, from along side the Crater Rim Road, through the "green area," on to the rim of Halema`uma`u Crater, and back to the road.

We were able to spot our first Silene hawaiiensis while walking along the "inside" of the road from the parking area.

This is a relatively large plant.  It is growing vigorously, as shown by many long green shoots.  There are lots of flower buds.  Some flowers have recently opened.

The proximity to the road may supply this individual with more moisture.

 Mike examines a roadside Silene hawaiiensis
Silene hawaiiensis showing vigorous growth and flowering This is a large Silene hawaiiensis
Mike looks on the ground at evidence of past flowering events and finds a few seed capsules. Roadside Silene hawaiiensis showing flowers
The topography of this region is much more varied than on the other side of the road.  There are many large cracks, generally filled with a variety of good-sized plants.

There is often a strong wind blowing across this area.  These cracks provide a refuge for species that might otherwise have difficulty with such an environmental challenge.

 Cracks hold many species
The "green area" is several hectares in extent.  If is at a slightly lower elevation than Crater Rim Road, which is located just behind the rise on the background of the photo on the right.  Note the gentle slope of Mauna Loa in the background.

The vegetation forms a closed canopy with a variety of shrubs and grasses.  It is a denser community than is found on the nearby shrublands (such as around the Volcano Observatory).

There is an abrupt margin which can be seen in the photo below on the left.  Halemaumau Crater is just off the picture to the right.

There is evidence of many dead plants.  These are the dead remains with the cause of death not obvious.  There are even a few downed tree ferns.

 "Green area" with a closed canopy of shrubs and grasses
Abrupt edge of the "green area" "Green area" shrubs, including many large dead plants
Dicranopteris linearis (Common Name: false staghorn fern; Hawaiian: uluhe) covers part of the "green area."  You can also see grasses and, in the foreground, some Dodonea viscosa (Hawaiian: `a`ali`i). Dicranopteris covering part of the "green area"
One of the surprising finds in the "green area" was a Coprosma menziesii (Hawaiian: pilo).

This individual had both fruits and flowers.

Stone and Pratt (1994: 183) list this as occurring in mid-elevation woodlands, but below 3,000 feet.  This individual was at approximately 3,800 feet elevation.

 Coprosma menziesii
Coprosma menziesii Coprosma menziesii
The two photos below show the "green area" in the background and Silene plants in the foreground.  No Silene were found growing in the "green area."  They did occur nearby in the open area.  These individuals appeared to be vigorous and preparing to flower.
Silene hawaiiensis in the open areas near the "green area" Silene hawaiiensis in the open near the "green area"
Many of the Silene were seen growing in areas that have a surface substrate with a small gravel or course sand texture.

It appears that these are water washed areas, and the Silene are growing where these finer sediments have accumulated.  However, it may not be water that has moved the sediments (since this is a low-rainfall area), but wind (which is abundant).

 Silene hawaiiensis in an open area
Silene hawaiiensis growing in small gravel-like substrate Silene hawaiiensis growing in a coarse sand-like substrate
Mike takes a look at a vigorously growing individual.

Below are pictures of the two Silene plants that we found that had gall-like accretions on their stems.

Both plants are in the general region between the "green area" and the Halemaumau Crater.

 Vigorously growing Silene hawaiiensis
Silene hawaiiensis with gall-like accretions Silene hawaiiensis with gall-like accretions
There are small-stature Metrosideros polymorpha (Hawaiian: `ohi`a) scattered through the area.  Many of these have reddish-colored leaves.

This characteristic was seen in a Metrosideros plant on the other side of the highway; it is also apparent in some individuals in the Sulfur Bank area.

 Metrosideros polymorpha with red leaves
This panoramic shot shows the proximity of this site to the Halema`uma`u Crater.  The "green area" is about 3/4 of the distance from the left side of the picture; if you visually extend the slope of Mauna Loa down, this line runs into the "green area."

Scattered individuals of Silene occur over this entire area.

Panoramic view of the entire area studied
Mike looks over the landscape with Halema`uma`u Crater in the background.

This shows the proximity of this site to the crater.  Just a few steps to Mike's left is a high cliff.  Don't wander close to the edge!

Silene individuals extend quite close to this point.

 Mike near Halemaumau Crater
This is a vigorously growing plant shown from close to ground level. The prominent root is characteristic.  Numerous stems branch from this root.  Here you can see that although the stems are short, they are covered with new leaves.

It appears likely that the shoots are periodically removed from the plant -- either through death (perhaps when it is dry or through acid rain deposition) or grazing.

 Silene hawaiiensis showing stout root and vigorous growth
The picture on the right shows the most exposed Silene root that we've found.  This is a small plant (notice how few stems there are), so the size of this root may not be representative of other roots in either extent or general condition.

This tap root must be an important survival characteristic of this species.

 Small Silene hawaiiensis showing an exposed root
Many of the Silene were seen growing in areas that had a "rock-like" crust.  Apparently this is ash that has been subjected to acid rainfall.  It makes a hard layer with few cracks.

The plant shown below right also has a section of an exposed root.
Silene hawaiiensis growing in an area with a rock-like crust Silene hawaiiensis growing in an area with a rock-like crust
A brief examination of the cracks in this area shows a deep layer of ash (about 3 to 4 m deep) overlaying basalt.  The stony crust, mentioned above, covers the ash layer and appears to be about 1 cm thick.

This area received a major ashfall as a result of the 1790 Kilauea event.
A view of the rock-like crust The rock-like crust over deep ash layers
This Metrosideros polymorpha (Hawaiian: `ohi`a) is growing deep in a hole.  It is quite a large individual but it the top of the plant is not above the ground surface. Metrosideros polymorpha growing in a hole

References

Last Updated: 06/26/99