Back in February I wrote a short post about our cottonwood tree planting efforts on the Missouri River. Here’s a more detailed description of that project, including some early (and interesting) results.
The project started as some informal discussions with Scott Josiah, the state forester for the Nebraska Forest Service. I mentioned to Scott my concerns that most of our cottonwood woodlands on Nebraska’s big rivers consist primarily of mature trees, and that we aren’t seeing many new cottonwood stands becoming established. As existing cottonwood trees die of old age, they’ll be replaced by the trees that are now filling the understory of the woodlands, including species such as ash, mulberry, hackberry, and eastern redcedar. Those are all fine trees, but provide a very different kind of habitat than a cottonwood woodland. Scott was interested in the same issue and, a few months later, approached me with an opportunity to join them on a grant application to fund a collaborative project.
We successfully obtained a grant from the U.S. Forest Service for a three year demonstration project, and the National Arbor Day Foundation jumped in with additional funding and support. Tyler Janke, our restoration ecologist along the Missouri River is spearheading the project, and will be restoring approximately 300 acres of cottonwood woodland over the next three years. Along the way, we’ll be trying and comparing a number of techniques to figure out what works best, including:
1. Planting cuttings and rooted seedlings. Directly planting young trees into the ground is the surest way to establish a woodland, but is also expensive, time-consuming, and difficult to do at large scales. We expect that cuttings (the harvested tips of young branches with buds on them) may survive less well than actual rooted seedlings, but they’re cheaper and easier to deal with. We’ll compare the two methods over three years and multiple sites.
2. Disking ground near existing mature cottonwoods. We hope to take advantage of the abundant seed that falls from existing trees by preparing good seed beds for them to land on and germinate in. Cottonwoods establish in moist bare soil. We can create the bare soil with disking, and then hope that the moisture will come from a combination of high groundwater (in some sites) and rainfall.
3. Irrigation. Rainfall may or may not come at the right times or amounts to stimulate seed germination in disked sites or to ensure survival of our tree plantings. Irrigation will allow us to narrow down the causes for good or poor tree establishment by controlling the amount of moisture available for young trees – at least in small areas. In addition, if irrigation turns out to make a really big difference in survival and establishment, there are significant areas of land along the Missouri River that have center pivot irrigation systems in place, and if those areas are converted to natural habitats in the future, those irrigation systems could be put to use.
4. Herbaceous seedings. Starting next year, we’ll be planting seed of grasses, sedges, and forbs within some of our cottonwood plantings. We want to see whether or not we can establish a sedge meadow-type community in the understory of developing cottonwood woodlands. If so, we might be able to avoid the more typical understory community of reed canarygrass and a few other invasive or monoculture-forming plant species. This is kind of a wild card portion of the project, but I think it’s an important one. Most woodland restoration projects focus solely on getting trees started, and I think there’s a danger that we might be missing our best (only?) opportunity to create a quality herbaceous vegetation layer beneath those trees by waiting until the trees are relatively mature before starting to worry about what’s growing beneath them.
Our demonstration project is taking place on private lands that are enrolled in the Wetland Reserve Program (WRP) but where converting cropland to grassland hasn’t worked very well for various reasons. All of them were underwater last year during the big flood, and there are a number of sites that have been flooded at least a couple times since being planted to prairie/wetland vegetation. It might turn out that cottonwood establishment is the best fit for land where flooding is too frequent or severe to establish and maintain grassland habitat. If so, we hope we can help that effort by developing and testing techniques that can make that conversion as efficiently and effectively as possible.
This spring, we harvested and planted about 30,000 cuttings (see the February post) in addition to about 1,000 rooted seedlings. Most of those were planted in early May with a mechanical tree planter, but some were hand-planted by volunteers from the National Arbor Day Foundation. At about the same time, Tyler had a contractor disk up about 100 acres across four different properties. In mid May, Tyler started irrigating 2 acres of disked land at one site with a big “traveling gun” irrigation system (used commonly on athletic fields - it has a sprinkler head on a long hose and a reel that winds the hose in as it waters).
It’s still pretty early to start analyzing the results of the project, and we’ve got two more seasons to go. However, we have seen some interesting things already, and the very warm and dry weather this year has provided us with an interesting opportunity to see what happens on the extreme end (we hope) of the moisture continuum. Tyler has been watching the sites closely all season, and collecting a little data. More extensive data will be collected in the coming weeks to help us analzye our first year results. However, I had a chance to visit the sites with Tyler a few weeks ago, and can talk about what we saw at that point.
First, in this dry year, irrigation made a huge difference in the establishment and survival of trees. Between May 15 and August 1, Tyler figures he put a little more than 18 inches of “rainfall” on the two acres he irrigated. The rest of the sites only got about three and a half inches of rain during that same time period – most of that coming in a single rainfall event. Within the disked areas that were irrigated, germination from seed falling from nearby trees was very successful. Tyler’s winding down the irrigation now, so we’ll see how those young seedlings survive through the winter and into next year.
Second, we are seeing surprisingly good establishment of cottonwood seedlings in some portions of the disked/non-irrigated areas. Weed growth is very strong in most of the sites, so it’s hard to see the ground very well in many places. However, even accounting for that, it appears that the best seedling establishment is happening in places with sparse weed growth. What’s interesting about that is that those areas of sparse weeds appear to be in soils with the least organic matter – and most prone to drought stress. The fact that the cottonwood seedlings seem to be thriving where most annual weeds can’t is really intriguing. Tyler’s going to do some more thorough searching to confirm (or deny!) what we were seeing a few weeks ago, and will also have a soil scientist help him evaluate the soil properties where cottonwoods did and didn’t establish well.
Third, because of the tall weed growth in all four sites, Tyler mowed some plots in both the irrigated and non-irrigated portion of one site in early July. So far, it’s too soon to see whether or not that made a difference, but it gives us something else to look at. Tyler thinks there maybe a difference in seedling height between the mowed and unmowed portion of the disked/irrigated site, but whether or not that will affect long-term survival is unknown. Dealing with weeds may turn out to be a major challenge going forward. Tyler also sprayed some of the disked areas with Roundup – as well as the tree planting sites - early this spring, and that did seem to keep the weed growth suppressed somewhat, but we don’t know yet whether it was significant or not.
Finally, we saw a big difference between the survival of rooted seedlings and cuttings. Outside the irrigated area, most of the rooted seedlings are alive and growing, but very few of the cuttings are. Within the irrigated area, Tyler planted two small groups of cuttings – a group of 10 larger diameter cuttings and a group of 10 smaller diameter cuttings. So far, four out of the 10 big cuttings are alive and two out of the 10 small cuttings are alive. The vast majority of the rooted seedlings under irrigation have survived.
Very likely, the low survival of cuttings is due to the drought, but Tyler has a few other ideas to pursue before next year’s plantings. He’s wondering whether the warm dry winter affected the bud development on the branches we harvested for cuttings, and if so, whether we should have harvested the cuttings earlier in the season than we did.
Tyler also has some ideas about how to harvest higher quality cuttings next year – or at least to ensure more uniform quality. This year, we just harvested cuttings from the branches of small trees, but next year Tyler wants to harvest cuttings from “stool beds” created by cutting those small trees down this year and letting them create clusters of new stems from the ground. Harvesting those new stems will help ensure that we’re getting only one-year-old stems. He also wants to be more uniform about the length and diameter of cuttings we harvest, because that may be making a difference as well. Hopefully we can improve upon this first year’s survival rates by improving and standardizing cutting quality. A little rain would help too.
Overall, it was a great first year. The nice thing about a project like this is that because it’s an experiment/demonstration project, failure is still success. As the saying goes, we’ll learn more from our failures from our successes - and this first year provided us with an excellent opportunity to see failures! Of course, we also had some important successes and have the trees to show for it. In addition, the drought provided us an with an important opportunity to see and analyze where cottonwood seeds established on their own in our disked areas – under very stressful conditions. We hope to learn from that and incorporate those lessons into what we try in years two and three.
As Tyler continues to evaluate this first year, I’ll let you know if we find anything else interesting. Otherwise, stay tuned for year two!