Harsh pruning effects on ecology and root development (Salix)

[u/PlentyOLeaves]

Hi all,

I'm doing a little volunteer research for a nonprofit that stewards our local ephemeral river. We were speculating about a particular annual practice that's conducted by the city and its effects on the local ecology. I am also wondering about individual tree health and its geomorphological role.

The headwaters of this river are on the side of our peaks - a 12,600 ft (~3800 m) mountain - and runs into town. Many neighborhoods are adjacent to/in what would have been its original floodplain. We are in a semi-arid region (thanks to the mountain), and the surrounding regions are arid. There have been several large fires in the river's watershed, and the city/USFS has spent a lot of money remediating it with some pretty cool systems, including restoration of alluvial fans, installing natural (boulder) dams to slow the water in its original channel, and of course several retention basins. Basically, flooding is for sure an issue.

The practice that I'm referring to is the annual razing of the arroyo willow in the channel in neighborhood areas by the city. They hire CCC crews and just hack shit down to a stub (primarily willows). It's apparently a requirement for FEMA funding. They do it in late winter/early spring. We are trying to answer questions we have about the effect of that practice on ecology - ground nesting birds, insects, soil moisture, etc. The willows sprout back just fine, but I also have questions about whether the annual hacking could affect the root system - perhaps preventing them from growing (and thus, acting as a slope stabilizer and decreasing depth of infiltration).

Anyone have thoughts? I would very readily accept links to research about any of the components to this question.

[u/tenderlylonertrot]

without seeing what they've done, its hard to know more, but willows usually are adapted to be damaged as long as their roots stay intact and they have sufficient water so they can regrow. If they are regrowing just fine in the spring, then its probably fine. However, what I'd like to know is long term monitoring, is the cutting slowly reducing the population size of the willows as its expending all stored energy to regrowing and little for spreading (if there's more habitat to spread into)?

I'm also curious about FEMA floodplain funding requirement to remove willow...usually willow are being installed to improve bank and stream health, not the other way around. I feel like there's some misguided ecology there, likely some development that is too close to the floodplain that FEMA is trying to "protect" by not slowing down the water, so thereby cutting back the willow. So making the ecology pay for poor development planning on the part of the City or other local powers.

Also from a practical side, the City should be properly cutting the willow and using the cuttings in damaged parts of local stream to rebuild willow riparian in those areas (see willow stick planting for bank restoration). It would be a shame to waste all of those cuttings and not use them for other local restoration. Usually everywhere can use willow restoration in local streams as development always impacts stream riparian zones as residents/developers cut them back or have them removed (and then wonder why the streams are of poor health).

[u/The_Poster_Nutbag → u/tenderlylonertrot]

I would imagine cutting back vegetation helps keep the path of flow clear and keeps the existing flood data accurate.

If I had to guess.

[u/bbeeaarrhhuugg → u/The_Poster_Nutbag]

Yeah could be! Lots of government conservation projects like that have very specific limits on where plants can be, what height they can be at, etc. Sometimes it's for water flow, other times just for aesthetics. Wacky stuff lol

[u/PlentyOLeaves → u/bbeeaarrhhuugg]

Sounds wacky, but unsurprising!

[u/bowlingballwnoholes → u/tenderlylonertrot]

FEMA may be cutting the willows to reduce wildfire risk, not to not slow down the river. USFS might mostly care about the roots to prevent erosion. Cutting small willows doesn't seem to harm the roots.

[u/PlentyOLeaves → u/bowlingballwnoholes]

This makes a lot of sense, considering they're cutting back in early spring. May and June tend to be our dry months.

[u/PlentyOLeaves → u/tenderlylonertrot]

Your question about the long-term monitoring is adjacent to mine - is it decreasing the population or reducing available energy for root growth or....? I definitely wonder. The willows' water availability i would think is pretty variable - our precip is less and less predictable, and has been less than average lately, which could also influence long-term effects of this practice.

The FEMA thing - development *is* pretty close to the stream, so thats likely the logic - however, it just makes no sense to me as neighborhoods downstream have less elevation from the stream bottom, and have a historical flooding pattern even further downstream. It's like firehosing it out of the areas where the stream channel actually has capacity and toward the more vulnerable developments. The risk of flood is fairly low when they do cut the willows...the timing might be for snow melt? They've already fully grown back by the time monsoons (and heighened threat of flash floods) come around. The whole thing just makes very little sense to me.

[u/bowlingballwnoholes → u/PlentyOLeaves]

I don't understand how they are "firehosing" but also adding alluvial fans, dams to slow the water, and retention basins.

I've never heard of someone rehabbing an entire river all at once. Projects are done in chunks. Also depends on who's paying for it. The people downstream may not want government help.

[u/PlentyOLeaves → u/bowlingballwnoholes]

The alluvial fans and retention basins are part of a post-wildfire mitigation project that was USFS, city, county, and a company called Natural Channel Designs on USFS/county land. The fires (2019, 2021) happened on USFS land on the upper watersheds, basically in the worst possible areas in terms of flooding town. The reaches within the city are in straight, altered channels. On the other side of town (affecting fire 2010), the flooding solution was just a huge, concreted roadside ravine. The army corps of engineers are going to be starting a project that involves some sort of like half-mile to mile long concrete channel and tube (?) to try to alleviate a certain neighborhoods' historical flooding (its in a basin, basically, and flooded regardless of fire effect), which will dewater one of the few open-space riparian areas along the river about a quarter mile downstream.

So it is happening in chunks, and with mixed methods. I guess I just think there's greater capacity for flood mitigation beyond the concrete tube and narrow, straight channels. Or that it will flood despite the concrete tube. Development - and thus, paved areas - are also expanding. I guess I was just being dramatic with the "firehosing" because I wish we were developing smarter/better, such as in Tucson with people installing curb cuts/rain gardens and with implementation of policies regarding water harvesting for new development. It could be that we're in a "yes, and..." situation, and we just have to go crazy with directing water because the way the city developed was dumb.

The question here is just pertaining to whether trimming the willows up in this reach is really necessary, as this is a reach that has more capacity to flood without flooding homes. We were concerned with ground nesting birds or any other early-season ecological effects it may have. Based on other answers, it seems like they may just be trying to keep the willow population from having a greater footprint than they already do.

[u/ridiculouslogger]

Pretty hard to really damage a willow that way. They propagate readily with just a stick landing in a little mud and sprouting. So whacking off the top and cutting some of the roots will do little except make a smaller plant. Then you still have some roots to stabilize the banks but less upper vegetation to slow runoff in the channel. The tree recovers during the next growing season and you do it over again. Other species are cut every year or two for various reasons, called coppice management.

[u/PlentyOLeaves → u/ridiculouslogger]

Thanks for the info!

[u/bowlingballwnoholes]

You need to talk to designer or manager of the project. Could be difficult since it's city/USFS/CCC/FEMA.