Environmental Controls Over Acacia koa Productivity
Granting Agency: USDA, McIntire-Stennis and
Tropical-Subtropical Agriculture Research Programs
Project Period: Oct.
2003-Sep. 2008
Principal Investigators: Travis Idol and JB Friday
Co-PI:
Paul Scowcroft
Summary
Acacia koa is the second-most common tree in Hawaii, ranging from wet to dry-mesic forests at elevations from 500 to 6500 feet. It is also a nitrogen-fixing tree, able to overcome the inherent lack of N in young volcanic soils. Its beautiful and durable wood makes it an extremely valuable hardwood for fine furniture, musical instruments, and traditional Hawaiian canoes. It can regenerate from buried seedbanks in old pastures if exposed to light and protected from grazing. Koa restoration and reforestation projects are commencing on several of the major Hawaiian Islands. However, we do not yet know enough to model the growth and yield of koa forests across its historic range. Koa is known to be sensitive to mean annual rainfall, but no attempts have been made to relate productivity to elevation, which affects mean annual temperature. Additionally, few studies have looked at the soil and foliar nutrients in koa stands in order to determine their effect on koa productivity or their relationship to environmental variables.
With this in mind, we established study plots at 3 locations on the Big Island of Hawaii that differed in mean annual rainfall from 1000-2000 mm. Elevation ranged from 1200-2050 m, but the extent varied by location. Soil nutrient bioavailability was measured seasonally using ion exchange resin membrane probes, known commercially as PRS-probes. Foliar nutrients were estimated for fully-expanded leaves in the upper canopy. Soil N mineralization and soil P fractions were also determined in the laboratory.
Koa basal area vs exchangeable soil P 
Available P vs soil C:N ratio
Results
Results showed that koa basal area increased with rainfall, as expected. Soil P also appears to play an important role. Across all sites, basal area was linearly related to the pool of exchangeable soil P. Interestingly, the soil C:N ratio declined with increasing soil P availability, suggesting that P limitations may lead to poorer quality litter production and organic matter formation. Overall, it appears that at higher rainfall rates, the greater productivity of koa stands leads to a more rapid immobilization of this limiting nutrient in woody biomass than can be replaced via P weathering from primary minerals. Scavenging for available P likely depletes the exchangeable P pool size, which is reflected in low P availability.
Combined with data from other locations, we are using this information to calibrate standard ecological models of forest growth and productivity. The interactive effects of precipitation and temperature on nutrient status and forest productivity will have to be incorporated into a modified model in order to accurately reflect the true dynamics of these systems.
Publications:
Idol TW, Baker PJ, and Meason D. 2007. Indicators of forest ecosystem productivity and nutrient status across precipitation and temperature gradients in Hawaii. Journal of Tropical Ecology 23:693-704. doi:10.1017/S0266467407004439
Future Directions:
We hope to expand the standard growth metrics taken so far with remote sensing tools such as ground-based laser detection and ranging (lidar) to estimate details of stand structure and satellite imagery to rapidly assess forest health and productivity at the landscape scale. The goal is to use this information to calibrate a forest production and stand dynamics model, such as 3PG.