ESA 2013 Preview: Predicting Plant Invasions in Sierra Nevada Forests

Jens Stevens, 4th year PhD candidate in the Davis Graduate Group in Ecology, continues our ESA 2013 preview posts. You can see Jens’ talk at ESA as part of an organized session on  3:40 PM, Thursday, August 8, in room 101B (OOS 28-7). 

If you are a non-native plant in California, chances are you like to hang out where most of the people do- in the valleys and along the coast. As a proportion of the total flora, there are fewer non-natives at higher elevations, a trend that has been observed worldwide and holds true in California’s own Sierra Nevada mountains.  There are several suggested reasons why this might be, but some of the popular ones include a harsher climate, and less disturbance, as you move up-slope. This is certainly true in the Sierra Nevada, where lower elevations are mild and rainy, and higher elevations are cold and can develop deep  snow-packs. And the most common disturbance, fire, also decreases in frequency as you move up in elevation through the Sierra Nevada, both because of climate and because people have gotten really good at putting them out in forests.

If you have spent any time driving around the foothills of the Sierra Nevada mountains in the spring, you’ve probably noticed this plant:

[mantra-multi][mantra-column width=”1/2″] Stevens_ScotchInvasion3[/mantra-column] [mantra-column width=”1/2″] Scotch broom is a tenacious little shrub in in the pea family, and it is a pretty aggressive invader. But we noticed something else- that as you drive up the west side of the Sierra Nevada after a snowstorm, there’s a big pulse of Scotch broom, and then right around the time you start seeing snow (somewhere between 4000 and 5000 feet, on average), you stop seeing Scotch broom. In fact, the distribution of Scotch broom stops incredibly abruptly below 4000 feet! (the gray shading in the picture below).

[/mantra-column] [/mantra-multi]

Stevens_Map2Given that snowpack is widely expected to decrease in extent and duration in response to future climate warming, I started wondering how that might affect the Scotch broom distribution into the future. On top of that, as managers and the public increasingly recognize the need to reduce fuels and put fire back into these forests to restore ecological processes, we might expect to start seeing more disturbance by fire as well, which could favor Scotch broom, which has a tough seed coat but likes to germinate after a fire. So, along with Andrew Latimer at UC Davis and Rob York at the Berkeley Center for Forestry, I designed an experiment to manipulate snow and fire, and see if certain combinations of those forces led to more likelihood of invasion. So, we’d burn some plots, not burn others, then we’d increase snow on some plots, decrease it on others, and hold it constant on the rest. That’s called a factorial experiment, kids. Piece of cake.

We were fortunate that the UC Berkeley’s Blodgett Forest (just uphill from the Scotch broom infestation, see map above) agreed to let us transplant some Scotch broom, and its hotter, drier cousin Spanish broom, from the lowlands up the hill, in the name of science. We planted seeds of each species into forest stands that had been thinned as part of a fuel-treatment program. Once all the seeds had been carefully planted and had their carefully marked with  locations marked with nails, half of the plots were lit on fire! As should be the case with prescribed fire, it wasn’t very dramatic.

Turns out the fire was the easy part, and the snowpack manipulations were the hard part. After each snow storm, I convinced friends, colleagues, and several undergraduates who knew no better, to grab a shovel and follow me up into the mountains. We’d spend a few days doing heavy manual labor, and then spend the rest of the time skiing, buildng snowmen and drinking beer. If that sounds like fun to you, let me know, because it’s happening again this winter!

[mantra-multi][mantra-column width=”1/2″]


Two weeks after snow manipulation, the increased-snow plot is still obvious

[/mantra-column] [mantra-column width=”1/2″]


One of these people didn’t help at all with the shoveling, and it’s not who you think.

[/mantra-column] [/mantra-multi]


I counted which individuals were alive (out of a few thousand seeds in the plots!) three times during that first year- right after the burn, in the spring after the snow had all melted, and in the fall. I used that data to describe the probability of seeds germinating and surviving to that fall census. Here’s a summary of what we found:


Just to orient you- those red bars are the snow-removal plots, the yellow bars are the ambient snow plots, and the blue bars are the snow-addition plots.

Contrary to our expectation, Scotch broom, that hardy bastard from the grey north, didn’t seem to care how much snow we dumped on it. But it did like that fire- the probability of establishment after one year jumped from around 10% in the unburned plots to around 20% in the burned plots (those “error bars” are in fact 95% confidence intervals around the predicted probability generated from a mixed-effects binomial model, in case you were wondering).


But Spanish broom, that invader from the southlands, had a clear favorite from all the combinations: It liked the reduced snow conditions, and it especially liked them after a fire.

What does it all mean? Well, we want to wait until these plants start flowering to say whether their full probabilities of successful invasion are affected by these different conditions. It could be that it all evens out after a few years. But based on this first year’s data, it looks like the lower-elevation invader, Spanish broom, might find more habitat to its liking farther up the mountain if forecasts of decreased snowpack and increased use of fire are correct.

About Jens Stevens

I'm a plant ecologist interested in the effects of disturbance and climate on plant diversity and demography. Much of my work currently is in forests of western North America, investigating patterns of understory diversity in response to fire and forest management.

I came to UC Davis by way of Carleton College, and a Master's in Plant Biology at the University of Vermont with Brian Beckage. I did my PhD with the Graduate Group in Ecology in Andrew Latimer's lab, where I studied plant invasions and the effects of fuel treatments and wildfire on ecological processes in montane forests of the Sierra Nevada. I am currently a postdoctoral researcher with Malcolm North at the UC Davis John Muir Institute for the Environment. Get in touch!
Tagged , . Bookmark the permalink.

Leave a Reply