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Impacts and control of giant reed, Arundo donax, in riparian habitats. (02XN022)

Program

Exotic Pests and Diseases Research Program

Principal
investigator

J.S. Holt, Botany and Plant Sciences, UC Riverside

Host/habitat

Riparian Area

Pest

Giant Reed Arundo donax

Discipline

Weed Science

Review
panel

Natural Systems

Start year (duration)

2002 (Three Years)

Objectives

Determine ecophysiological traits correlated with invasiveness by quantifying seasonal resource use and physiological functioning of giant reed compared to representative native species that it displaces;

Determine how giant reed excludes native species by quantifying its seasonal impact on the microclimate of invaded sites both with native species present and where native species have been excluded;

Combine data from Objectives 1 and 2 to test timing and levels of giant reed removal required to provide the necessary microclimate for native species restoration by natural or artificial means.

Project
Summary

Arundo donax, giant reed, is a perennial grass that has spread widely in riparian ecosystems throughout California and severely impacted native vegetation, wildlife habitat, and water management. The goal of this research is to determine the ecophysiological traits of giant reed that are correlated with invasiveness and use this information to develop control strategies that will permit revegetation with native species to succeed. Experiments will quantify seasonal giant reed resource use compared to native riparian species and impacts of giant reed on the microclimate of invaded sites. Results will be used to test timing and levels of giant reed removal required to provide the necessary microclimate for native species restoration. This research will provide information to aid in controlling this weed and increase our understanding of the ecological and physiological basis of invasiveness.

Final report

Research was conducted to determine how giant reed affects the riparian microenvironment, how native species respond to the altered microenvironment, and the best timing and level of giant reed control needed to favor restoration of natives. The native species studied were bulrush and Goodding willow, both of which are common and widespread in riparian habitats. Experiments that evaluated seasonal photosynthetic activity of giant reed, a C₃ plant, showed that while activity slows in winter, no dormant period occurs in southern California.

Maximum photosynthetic rates were comparable to that of highly productive C₄ species, which explains the rapid growth rate and large biomass production of giant reed. Photosynthetic rates and water use efficiency were highest in March, and growth and biomass production increased from winter through summer when the highest growth rates occurred. Another experiment comparing nitrogen use of giant reed and the two native species showed that giant reed grew larger overall under both high and low nitrogen conditions.

However, willow benefited the most from added nitrogen, suggesting that giant reed removal could promote willow growth, particularly in areas near water containing nitrogen effluent.

Since field measurements indicated that the largest impact of giant reed on riparian habitats was reduction in light, an experiment was conducted to evaluate relative responses of the native species to shaded growth environments. Both species appeared to be relatively shade tolerant, making them good candidates for riparian restoration after suppression of giant reed. Results were used to design control treatments for giant reed that will alter the microenvironment to favor native species growth. Experiments testing various timings and levels of giant reed control are ongoing.

Third-year
progress

Research was conducted to evaluate ecosystem impacts of giant reedand physiological characteristics compared to two native riparian species, bulrush and Goodding willow. Results will indicate how giant reed excludes native species and what microclimate conditions are required for giant reed removal and native species restoration.

Experiments to quantify photosynthetic activity of giant reed, a C₃ plant, showed that while activity slows in winter, no dormant period occurred in southern California. Maximum photosynthetic rates varied from 18 to 32 micromol CO₂/m²/s depending on the season of measurement, which is comparable to that of highly productive C₄ species. Photosynthetic rates and water use efficiency were highest in March, although growth and biomass production increased from winter through summer corresponding to the period of maximal growth.

Another experiment comparing nitrogen use of giant reed, and the two native species showed that giant reed grew larger overall under both high and low nitrogen conditions. However, willow benefited the most from added nitrogen, suggesting that giant reed removal could promote willow growth, particularly in areas near water containing nitrogen effluent. Since field measurements indicated that the largest impact of giant reed on riparian habitats was reduction in light, an experiment was conducted to evaluate relative responses of the native species to shaded growth environments. Both appeared to be relatively shade tolerant, making them good candidates for riparian restoration after suppression of giant reed. Using results from these experiments, giant reed control treatments will be tested that create a suitable microclimate for native species restoration.

Second-year
progress

Research is underway to compare physiological activity of giant reed and native riparian species commonly found in communities invaded by giant reed. Experiments to quantify maximum potential and seasonal photosynthetic activity of giant reed have shown that while activity slows in winter, this species does not experience a dormant period in southern California. Maximum photosynthetic rates measured to date varied from 18.8 to 32.1 _mol CO₂/m²/s depending on the season of measurement. In an ongoing experiment, relative nitrogen use of giant reed and two riparian species (a bulrush, Scirpus americanus, and a willow, Salix gooddingii) is being evaluated. Results to date show that giant reed grew larger overall than the other two species under both high and low nitrogen conditions. However, willow benefited the most from added nitrogen, suggesting that giant reed removal could promote willow growth, particularly in areas near water containing nitrogen effluent. A similar experiment has been initiated to examine relative responses of these three species to shaded growth environments. A field experiment is also underway in which the effects of giant reed on microclimatic variables (light, temperature, and soil moisture and nitrogen) are being evaluated along six permanent transects oriented perpendicular to the river in two watersheds in southern California. Results of this research will be used to characterize seasonal fluctuations in giant reed productivity compared to native species and indicate the season during which it would be most effective to remove giant reed populations to maximize growth of natives while minimizing regrowth of giant reed.

First-year
progress

Experiments are underway to quantify photosynthesis and growth of giant reed from northern, central, and southern California. This information will be used to characterize seasonal fluctuations in giant reed productivity and indicate the season during which it would be most effective to remove or control giant reed populations to minimize regrowth. A second experiment has been planted to evaluate photosynthetic and growth responses to low or enriched levels of nitrogen by giant reed and two native riparian species, arroyo willow (Salix lasiolepis) and bulrush (Scirpus acutus), from three southern California sites. A third experiment is being initiated to evaluate responses of these three species to various growth light environments. Results of these experiments will help clarify the physiological mechanisms that result in giant reed's competitive advantage over native species (Objective 1). A field experiment has been initiated in which the effects of giant reed on microclimatic variables (light, temperature, moisture, and nitrogen) will be measured along replicate transects located in riparian habitats in three southern California counties. This experiment will reveal the seasonal impact of giant reed on resource levels in riparian habitats it invades (Objective 2). For Objective 3, we will integrate results from Objectives 1 and 2 to understand how and when during the year giant reed alters the microclimate to the detriment of native species and the seasonal physiological response of natives to altered resource availability. Using this information, we will test the timing and level of giant reed removal required for successful establishment of native species.

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