Effects of High-Priority Non-Native and Dominant Native Plant Species on the Water Cycle
Forested ridge with non-native and native plants
ʻĪao Valley near Kinihāpai Stream, Maui
The spread of highly-invasive non-native plant species in Hawaiʻi’s forests may be reducing freshwater availability across the islands. However, little information has been collected to determine the effects of highly-invasive non-native plant species on freshwater resources. The lack of information, in turn, limits the development of effective management strategies for preserving Hawaiʻi’s forests and the freshwater they provide. To help address this problem, a team of scientists initiated a study to better understand how selected non-native plant species may affect critical processes, such as infiltration and evapotranspiration, that influence freshwater availability.
The Water Cycle
Freshwater from precipitation including cloud-water interception can infiltrate to the plant-root zone where it may be released back to the atmosphere through evapotranspiration, or infiltrate deeper to recharge groundwater. Groundwater supplies 99 percent of all drinking water and 50 percent of all freshwater used in Hawaiʻi. Conversion of Hawaiʻi’s forests from native to non-native plant species may reduce infiltration rates and increase transpiration rates, which can reduce the amount of freshwater available for groundwater recharge.
Infiltration Capacity
Infiltration capacity describes the maximum rate at which soil absorbs water at the soil surface. Roots and burrowing organisms generally increase infiltration capacity by enhancing soil permeability, which can decrease surface runoff rates and increase recharge rates. However, infiltration capacity is also affected by soil hydrophobicity, which is a measure of a soil’s affinity for water and can be influenced by vegetation. Some non-native plant species increase soil hydrophobicity, which can decrease infiltration capacity, increase runoff rates, and decrease groundwater recharge rates when compared to other native plant species.
Transpiration
Plant species regulate transpiration through their stomata — tiny, closeable, pore-like structures on the surfaces of leaves. Some non-native plant species allow their stomata to remain open for longer periods of time, which can enable them to photosynthesize longer and grow more quickly when compared to other native plant species. Rapid-growth rates may indicate higher transpiration rates because increasing the amount of time the stomata remain open can also increase the amount of water vapor released to the atmosphere. Leaf area also affects transpiration because greater leaf area implies a greater number of stomata on a leaf surface and increased potential for higher transpiration rates. Some non-native plant species have greater leaf area that allows them to transpire greater quantities of water and decrease water available for recharge when compared to other native plant species.
Selected Native and Non-Native Plant Species
For this study, stakeholders and scientists selected seven high-priority non-native plant species and four dominant native plant species for field measurements.
Non-native plants
- Strawberry guava (Psidium cattleianum)
- Himalayan ginger (Hedychium gardnerianum)
- Tropical ash (Fraxinus uhdei)
- Christmas berry (Schinus terebinthifolius)
- Formosan koa (Acacia koa)
- Koster’s curse (Clidemia hirta)
- Kikuyu grass (Cenchrus clandestinus)
Native plants
- koa (Acacia koa)
- ʻohiʻa (Metrosideros polymorpha)
- uluhe (Dicranopteris linearis)
- ōlapa (Cheirodendron trigynum)
What are Scientists Learning?
Scientists are assessing how some specific non-native plant species in the watersheds may impact freshwater availability. To determine how infiltration, soil hydrophobicity, and transpiration rates are dependent on species type, the scientists collect field and laboratory measurements which include:
- Infiltration-rate testing
- Soil-hydrophobicity testing
- Soil particle-size analysis
- Leaf-level measurements to characterize exchanges of water vapor and carbon dioxide
- Leaf-area measurements
The research team involved in this study are scientists from the USGS Pacific Islands Water Science Center and the University of Hawaiʻi at Mānoa.
The spread of highly-invasive non-native plant species in Hawaiʻi’s forests may be reducing freshwater availability across the islands. However, little information has been collected to determine the effects of highly-invasive non-native plant species on freshwater resources. The lack of information, in turn, limits the development of effective management strategies for preserving Hawaiʻi’s forests and the freshwater they provide. To help address this problem, a team of scientists initiated a study to better understand how selected non-native plant species may affect critical processes, such as infiltration and evapotranspiration, that influence freshwater availability.
The Water Cycle
Freshwater from precipitation including cloud-water interception can infiltrate to the plant-root zone where it may be released back to the atmosphere through evapotranspiration, or infiltrate deeper to recharge groundwater. Groundwater supplies 99 percent of all drinking water and 50 percent of all freshwater used in Hawaiʻi. Conversion of Hawaiʻi’s forests from native to non-native plant species may reduce infiltration rates and increase transpiration rates, which can reduce the amount of freshwater available for groundwater recharge.
Infiltration Capacity
Infiltration capacity describes the maximum rate at which soil absorbs water at the soil surface. Roots and burrowing organisms generally increase infiltration capacity by enhancing soil permeability, which can decrease surface runoff rates and increase recharge rates. However, infiltration capacity is also affected by soil hydrophobicity, which is a measure of a soil’s affinity for water and can be influenced by vegetation. Some non-native plant species increase soil hydrophobicity, which can decrease infiltration capacity, increase runoff rates, and decrease groundwater recharge rates when compared to other native plant species.
Transpiration
Plant species regulate transpiration through their stomata — tiny, closeable, pore-like structures on the surfaces of leaves. Some non-native plant species allow their stomata to remain open for longer periods of time, which can enable them to photosynthesize longer and grow more quickly when compared to other native plant species. Rapid-growth rates may indicate higher transpiration rates because increasing the amount of time the stomata remain open can also increase the amount of water vapor released to the atmosphere. Leaf area also affects transpiration because greater leaf area implies a greater number of stomata on a leaf surface and increased potential for higher transpiration rates. Some non-native plant species have greater leaf area that allows them to transpire greater quantities of water and decrease water available for recharge when compared to other native plant species.
Selected Native and Non-Native Plant Species
For this study, stakeholders and scientists selected seven high-priority non-native plant species and four dominant native plant species for field measurements.
Non-native plants
- Strawberry guava (Psidium cattleianum)
- Himalayan ginger (Hedychium gardnerianum)
- Tropical ash (Fraxinus uhdei)
- Christmas berry (Schinus terebinthifolius)
- Formosan koa (Acacia koa)
- Koster’s curse (Clidemia hirta)
- Kikuyu grass (Cenchrus clandestinus)
Native plants
- koa (Acacia koa)
- ʻohiʻa (Metrosideros polymorpha)
- uluhe (Dicranopteris linearis)
- ōlapa (Cheirodendron trigynum)
What are Scientists Learning?
Scientists are assessing how some specific non-native plant species in the watersheds may impact freshwater availability. To determine how infiltration, soil hydrophobicity, and transpiration rates are dependent on species type, the scientists collect field and laboratory measurements which include:
- Infiltration-rate testing
- Soil-hydrophobicity testing
- Soil particle-size analysis
- Leaf-level measurements to characterize exchanges of water vapor and carbon dioxide
- Leaf-area measurements
The research team involved in this study are scientists from the USGS Pacific Islands Water Science Center and the University of Hawaiʻi at Mānoa.