Date of Award:

8-2022

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Committee Chair(s)

Janis L. Boettinger

Committee

Janis L. Boettinger

Committee

Grant E. Cardon

Committee

Astrid R. Jacobson

Committee

Dennis L. Newell

Committee

Jonathan L. Deenik

Abstract

The soils of western Haleakalā are incredibly diverse due to two primary reasons: 1) they receive varying levels of rainfall based on location relative to Haleakalā, and 2) volcanic vents upwind of the area indicate that volcanic ash has blanketed much of western Haleakalā in the past. Ash can weather to form short-range-order materials, which contribute to the classification of andic soil properties. Due to their structure, short-range-order materials impart soil behavior that creates benefits and challenges in land management. Therefore, an understanding of how these soils formed and differ, and where andic soils occur is crucial.

The objectives of this study were to observe soil properties to determine how they vary across the precipitation gradients of the study area and how they relate to soil formation and diversity; and to utilize spatial statistics to predict where andic soils may occur to improve soil mapping.

Sixteen soils were sampled on the same mapped substrate of northwest-facing slopes < 5% across gradients in elevation (7-1362 m) and mean annual precipitation (283-2768 mm MAP). Soils were described by horizon to at least 1-m depth, or a rootrestricting layer, and rock fragments at the base of each soil pit were sampled. Soil chemical, physical, and mineralogical properties were performed, and soil data across depth and location were compared. Using climate raster data and soil property data used to classify andic soils for the 16 soils sampled, we created and validated models to predict the areal extent of andic soils in the study area.

Soil organic carbon increased with increasing precipitation before decreasing at 2300 mm MAP. Base saturation and pH remain high until a threshold at 1500 mm MAP before decreasing abruptly. The degree of mineral weathering and soil development increased with increasing MAP. The lowland soils of central Maui are likely much older than soils of upcountry Maui. Ash-influenced soils of upcountry Maui were less developed, more likely to have andic soil properties, and had disrupted pedogenic thresholds for pH and base saturation. Spatial predictions of soil properties used to classify andic soils accurately classified Natural Resources Conservation Service soils to 88.9%.

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