HAWAI`I VOLCANOES Invasion and Recovery of Vegetation after a Volcanic Eruption in Hawaii NPS Scientific Monograph No. 5

Vegetation Sampling

Plant species were listed periodically in permanent plots. These plots were arranged in a transect-system across the study area (Fig. 2).

On Kilauea Iki crater floor (habitat 1) four 1-m wide belt transects were laid out. Each of these begins at the crater floor margin; transect a at the NE side, b at the NW side, c at the SW side, and d at the SE side. The four transects join in the center of the crater floor in the form of a cross. Each 1-m wide belt transect was subdivided into 1-m segments, forming contiguous square meter plots. The number of square meter plots was determined by the transect length, transect a = 325 m, b = 385 m, c = 340 m, and d = 400 m.

The other five habitats were sampled with five transects. One transect, AA', extends in a NE direction parallel to the prevailing trade winds; a second one, BB', is at a right angle to the trade winds. This forms a cross with the first one (AA') on the cinder cone (habitat 2). The remaining three transects CC', DD', were laid out in the habitats next to the cinder cone.

A number of 10 x 10-m plots were established along these five transects. These are distributed by habitats as shown in Table 1.

Plots are arranged contiguously along the transects except along the long transect, AA'. Here, the plots are distributed at intervals, but uniformly, through habitats 4, 5, and 6.

A species list was prepared for the entire "Devastation Area," which was updated at each resurvey. The species found in the plots were checked off by assigning a Braun-Blanquet symbol (1965). In addition to the presence-record, this method provided for an estimate of abundance and cover of each species. These records began in January 1960 right after the eruption had ceased and were repeated five times in 1961, 1962, 1963, 1966, and 1968. The final year was 9 years after the eruption. The species list record was restricted by necessity to macroscopic plants. Algae were thus recognized only if they formed visible colonies, which appeared usually as dark blotches on the volcanic surfaces. It is quite possible that small-sized colonies may have been overlooked occasionally in a few quadrats. But the repeated observation of a large number of quadrats makes the appearance-record of the macroscopic algae for each habitat quite certain.

In addition to these records, photographs were taken periodically at 13 permanently fixed photo stations along the transects. The photographs were to serve as a visual documentation of the vegetation recovery in different habitats. The photographs were taken of exactly the same landscape segments in four quadrants around the central point of each photo station.

Environmental Measurements

A few environmental measurements were made in an attempt to find at least a partial explanation for the plant invasion and recovery patterns that were expected to emerge.

Climate

A general analysis of local climate (Doty and Mueller-Dombois 1966) gave an indication of a sharp rainfall gradient along transect AA', with a decrease from the Kilauea Iki area from about 2400 mm per year to about 1300 mm per year near the end of transect AA' in the upper Kau Desert. It seemed appropriate to verify this trend and to establish the amounts of rainfall for the observational years. Therefore, rain gauges were established in habitats 1, 4, 5, and 6 in 1964. Records were taken weekly or bi-weekly through 1968. The rain gauge locations are shown by the crosses on Fig. 2.

On the crater floor of Kilauea Iki, steam came from the cracks in the pahoehoe lava almost continuously throughout the period of observation. It was thought that this moisture source may have had an influence on plant establishment on the crater floor. In order to assess the contributions of this moisture source to objects standing out from the crater floor, Grunow fog interceptors were installed on rain gauges. Four such gauges equipped with an interceptor each were set out near the beginning of each transect. A rain gauge without interceptor was put next to an interceptor-gauge for comparison. Recordings were made from January 1967 through December 1968. A paired set of gauges was also established at the end of transect AA' in habitat 6.

Two hygrothermographs were installed in Stevenson screens (at 1.5 m heights), one on the floor of the crater in habitat 1, the other in habitat 6 at the end of transect AA'. In addition, the relative desiccation power of the environment was analyzed by Livingston white and black bulb atmometers mounted 30 cm above the ground near the rain gauges in habitats 1, 4, 5, and 6.

Substrate

Substrate sampling was done in various ways to obtain descriptive information on the edaphic properties of the new habitats and to verify hypotheses about the causes of certain observed plant invasion patterns.

Edaphic properties sampled in different locations were ash density, water-holding capacity and rate of water loss, available nutrients, pH, and degree of mineralization.

Ash density was used to separate the spatter area (habitat 3) from the pumice area (habitats 4, 5, and 6). The former showed densities greater than 1. Pumice was defined as ash with densities less than 1.

Water-holding capacity was tested on ash samples from habitats 2 through 6 by immersing pyroclastic fragments in water for 48 hours. After excess water had been allowed to drip off, the material was weighed for its holding capacity (here termed field capacity). Thereafter, it was reweighed every 24 hours in a room with a mean air temperature of 21°C and a mean relative humidity of 65% to determine the rate of water loss. Permanent wilting percentage was determined with a Richard's pressure plate at 15 atm.

Available nutrients tested were the cations (Ca, Mg, K, Na), nitrogen, and phosphorus. The method followed for cation extraction was the one described by Jackson (1958). Quantities were determined with a Perkin-Elmer spectrophotometer.

Available ammonia and nitrates were analyzed by Kjeldahl distillation (Harper 1924; Olsen 1929) and available phosphorus by a photometric method (Bray and Kurtz 1959). Substrate pH was determined for the upper 20 cm depths on a number of samples with a Beckman Expandomatic meter.

Degree of mineralization was analyzed with a Norelco X-ray diffractometer and thin sections of rocks were analyzed under the microscope for primary mineral occurrence and alterations.

Observed plant invasion patterns on the Kilauea Iki crater floor were believed to be related to substrate temperatures. Therefore, temperature measurements were made along the transects in numerous pahoehoe cracks with a Yellow Springs Telethermometer Model 43TC. These measurements were repeated several times during the warmest period of the day at intervals of 3, 30, 300, and 500 m from the crater floor edge to the center.

Another plant invasion pattern on the ash habitats showed a high correlation with the bases of tree snags. Soil moisture samples were taken at various points at the base of tree snags and away from them to verify the hypothesis that substrate moisture distribution was unequal over the ash surface and that the tree snags added to soil moisture by interception of rain water.