Summary
The initial visit by our first ESRM 492: Service Learning in New Orleans class to Woodlands Trail and Park in March of 2007 proved to be the start of a long-term collaboration between Woodlands Conservancy, the nonprofit organization that manages Woodlands Trail and Park, California State University Channel Islands (CSUCI), and Oregon State University. Our initial charge to survey the woody invaders that had been spread and/or released from competition by Hurricanes Katrina and Rita quickly morphed into an annual monitoring and long-term ecological restoration project. We now advise on the design and implementation of control efforts and are in the process of quantifying the efficacy of that restoration treatment. In short, control efforts in our initial 8 ha (20 ac) herbicide zone have suppressed the targeted woody invaders significantly. While suppression has not been total, the treated areas of Woodlands Trail and Park have far fewer invaders than when we initiated control efforts in late 2009. And while we have seen only limited success of our outplanted native tree seedlings, this treated region of bottomland hardwood forest is much more likely to recover given the reduced abundance of the aggressive invasive species.
Greg Bloom, Adam Robus and Stacey Alexander surveying Woodlands Trail band transects in March of 2012.
Focal Invasive Species
Our work emphasizes the control of three problematic woody invasive species from Asia; Triadica sebifera (formerly Sapium sebiferum, Chinese tallow), Melia azederach (Chinaberry), and Ligustrum sinense (Chinese privet). All of these species were present along the Gulf Coast for decades prior to Hurricanes Katrina and Rita. However, the hurricanes of 2005 drove an unprecedented seed dispersal event for each invasive species while simultaneously hobbling remnant intact, closed-canopy swamps ability to resist an invasion. Having lost their light-suppressing canopies, our bottomland hardwood forests were on track to become dominated by these non-native invaders. As these three invasive species appear to afford a poorer habitat for most native vertebrate and invertebrate species, the suppression of these invaders is key to assuring a healthy, well-functioning bottomland hardwood forest at Woodlands Trail and Park.
Our long-term monitoring sites across Woodlands Trail. Orange = Permanent Monitoring Plots; Green/Purple/Blue = start of 100-m long band transects running perpendicular to trail.
General Invasion Timeline
While we have no quantitative data prior to the hurricanes of 2005, ample qualitative and anecdotal evidence suggests that all three invasive species were limited to the periphery or frequently disturbed areas of Woodlands Trail and Park. Our initial surveys were conducted in March of 2007, 18 months after the twin blows of Hurricanes Katrina and Rita. During this initial sampling, we found abundant young individuals of tallow and Chinese privet. While Chinaberry was also present, it was growing at much lower densities on average.
During that first sampling effort in 2007, Rubus spp. (blackberry) was abundant at unusually high levels. It covered most low-lying plants and effectively smothered out numerous seedlings. During this period, blackberry seems to have greatly suppressed the growth of new woody recruits of all species. The somewhat different (“weedy”) growth pattern of our focal invasive species allowed them to more quickly grow upwards and pierce the suppressive blackberry canopy. By 2008, we saw many tallow already escaping their blackberry blanket. By March of 2009, any widespread suppressive effect of blackberry upon invaders was gone. Tallow were growing as fast as 2-3 m per year. Concern over this invasion motivated our first active control efforts in October of 2009.
The first two years’ surveys showed woody invaders spreading across our site and motivated our first restoration experiment. We decided to attack a 20-acre section close to the main entrance. In October (and then in follow-up treatments beginning as early as December) of 2009, we conducted a targeted herbiciding of all woody invaders.
Our on-going monitoring effort has been tracking the efficacy of our suppression efforts ever since. Our March 2010 sampling was the first post-herbiciding assessment effort. We conducted a major re-treatment of the area in May of 2011.
Russell Johnston hacking through Rubus sp. (blackberry) along Woodlands Trail Trail B on March 31, 2013. Photo: The Advocate
Methods
Herbicide Treatment
Certified herbicide applicators walked through our 8 ha (20 ac) treatment site in October of 2009, December 2009, and May 2011 in an effort to target and kill all individuals of our three target species. They used basal cuts and/or bark hacks to expose the cambium followed by painting with a mixture of 20% Garlon 4 and 80% Basal Oil for Chinese Tallow and Chinaberry. Habitat 1% was applied to Privet using a Foliar Application. Below are treatment descriptions:
Cut-stump treatment – This method was used for woody stems over 6 inches diameter at breast height (DBH). Herbicide was applied to the surface of freshly cut stems. Stems were cut at or near the ground and herbicide (generally Garlon 4) applied to the surface of freshly cut stems.
Hack and squirt– In sensitive areas where there was a concentration of non-targets, a hatchet or machete was used to make a series of spaced, downward cuts or incisions around the base of the target stem. Herbicide (generally Garlon 4) was immediately sprayed into these cuts.
Foliar application – Diluted herbicide (Habitat) was directly applied to the foliage of the target plants using a backpack sprayer. Spray is aimed at the target plant and applied until all leaves are wet.
Species ID cards are key…or are they?
Vegetation Monitoring
Our vegetation monitoring protocols have evolved over the years as we have come to know the site and the management needs of Woodlands Conservancy. We conduct both band transects and surveys within defined, permanent monitoring plots. Both categories of plant surveys show the same patterns of woody plants.
In September of 2009, immediately prior our initial herbiciding efforts (October 2009), we traveled to Woodlands Trail to establish six arrays of permanent monitoring plots inside and outside of our herbicide treatment. Each array consisted of a grouping of three plots. As with our band transects, our permanent monitoring plots are oriented relatively to trails. The first plot in each array hugs the trail and is 10 x 3 m deep. This small plot was designed to capture the response just adjacent to the trail. The next plot began 10 m in from the trail and covered an area 20 x 20 m. The final plot in each array was also 20 x 20 m, beginning 50 m from the trail (20 m from the far edge of the second plot). Owing to numerous logistical constraints, most of our plots differ from these idealized sizes. As such, density data here is standardized to individual plot area, measured after establishment. The corners of each permanent plot and center point of each plot was marked with a pvc stake and then surveyed with our sub-meter GPS.
Within each plot we measure the diameter at breast height (DBH) and approximate height of each living woody species. In addition to this tree demography, we collect a host of related data about the overstory and understory. These additional measurements are taken once within each quadrant of the larger plots (for a total of four data points per plot) or three times within the smaller, trail-adjacent plots. We quantify the amount of open sky with a densiometer, and then visually estimate the percent of native and non-native vegetative overstory (>3 m) cover. We then estimate the aggregate percent cover of blackberry (Rubus spp.) and maiden ferns (Thelypteris sp.) in the understory (<3 m). Blackberry is indicative of relatively disturbed areas while ferns are indicative of more mature, closed-canopy bottomland hardwood forest. Next, we note the presence of any downed logs (defined as woody debris with a diameter >10 cm) and their dimensions (diameter and length). The depth of woody debris (of any diameter) and leaf litter is estimated to the nearest centimeter. Mature bottomland hardwood forests will have moderate amounts of woody debris and thick leaf litter. Finally we note any recruit/out planted seedlings.
Sean photographing Chinaberry along Trail A of Woodlands Trail, March 24, 2009.