Over the last few years Californians have grappled with how to manage lands during times of both drought and plentiful rainfall. At SFREC and on Central Valley rangelands, one question is whether management that promotes high forage in wet years alters ecosystem resilience in dry years. For example, promoting highly productive grasses is a common goal. While drought years can negatively affect productive grasses, less productive species, particularly forbs like filaree, do relatively well in drought years due to decreased competition. Over the last several years the Suding lab and SFREC crew have been building ever-larger drought manipulations to test how different management practices, and associated species mixes, affect forage across good and bad rainfall years.

 

In the first iteration of this project, we looked at how grazing practices and rainfall interact to affect forage over dry and wet years. We hypothesized that grazing practices that maintained a diverse mix of grasses and forbs would promote more stable forage across wet and dry conditions. To test this, we first varied grazing intensity over four years within a pasture to describe how grazing alters grass and forb abundances (Figure 1a). Second, we implemented rainout shelters and irrigation over three years to create “dry” and “wet” plots within areas of different grazing histories (Figure 1b). We found that moderate grazing practices maintained a diverse mix of grass and forb species. This mixture better maintained vegetation cover and biomass across rainfall conditions compared to low-grazed areas dominated only by grasses (Figure 2) (Hallett, Stein, Suding conditionally accepted, Oecologia).

 

In the second iteration of this project, we are exploring how rainfall timing alters grassland diversity and forage production. We hypothesized that early-season drought will alter which species recruit that year, with higher forb abundance in dry falls and higher grass abundance in wet years, whereas late-season drought would reduce overall production. To test this, we have implemented large shelters with roofs that are pulled in place to create early-season, late-season and continuous drought as well as a control (Figure 3). We are finding that periodic early-season drought helps to maintain forb diversity in California rangelands. Working with Dr. Whendee Silver, we are also testing the effect of rainfall timing on nutrient cycling and greenhouse gas emissions. We are finding that previous-season rainfall as well as current season alters greenhouse gas emissions, which may be important for managing rangelands for multiple ecosystem services going into the future.

Summer “drought” is a fact of life in the Sierra Foothills. Our Mediterranean climate is characterized by cool and (hopefully) wet winters and hot, dry summers.  While the higher elevations in the Sierra Nevada often receive summer moisture from thunderstorms, summers are typically dry in our foothill rangelands – we usually don't receive any measurable rainfall from late May or early June until October or even November. However, this year's summer drought is anything but normal. At the Sierra Foothill Research and Extension Center (SFREC) – and throughout the Sierra Foothills, we are starting to notice signs of stress after four consecutive dry years.

One of the most obvious signs of systemic stress, at least to me, are the water levels in our foothill reservoirs. After the first three years of this drought, we went into last winter with depressingly low water levels in most of our man made lakes. These reservoirs are critical for storing water for irrigation, human consumption, and downstream wildlife habitat. In normal years, they allow us to capture snow runoff and save it for use during the dry summer months. The winter of 2014-2015, however, brought virtually no snow to the Sierra Nevada – and consequently no spring runoff. I thought our reservoirs were low last summer; this summer is even worse. Last weekend, I drove across the Parrotts Ferry bridge over New Melones Reservoir (on the Stanislaus River between Calaveras and Tuolumne Counties). I was shocked to see that the water level had dropped below the old Parrotts Ferry bridge – I'd ridden across it as a kid, but didn't think I'd ever see it again once the New Melones Dam was built.  Perhaps even more shocking – I could see the Stanislaus River flowing under the bridge.

Another obvious sign of drought stress can be found in wildfire behavior. As I write this, many regions of Northern California have experienced another round of dry lightning.  Wildland firefighters report that this year's fires are exhibiting behavior that they've never experienced – fueled by incredibly dry vegetation and erratic winds. And the worst part of our fire season is still to come; while temperatures begin to moderate, September and October are generally dry and windy in the Sierra Foothills.

Closer to home, the oak trees in Auburn and at the Sierra Foothill Research and Extension Center are starting to turn color as if autumn were already here. When deciduous trees experience extremely dry conditions, they'll often shed their leaves early as a survival mechanism. The blue oaks and black oaks at SFREC are dropping leaves earlier this year than most of us can remember.  Some of the trees are in full color; they make the hillsides seem like we're already in October. In addition to turning color and dropping leaves, we've seen more trees dropping entire branches (and some are even falling down entirely).

Finally, we're also seeing our seeps and springs dry up at SFREC. The flow in some springs has stopped entirely, while others have slowed to a trickle. One spring in particular, which has been developed to feed a series of water troughs at the higher elevations of the Center, has never stopped flowing in the past – even in the 1976-77 drought. In the fourth year of this current drought, we seem to be in uncharted territory.

According to the National Weather Service, one of the strongest El Niños ever measured seems to be shaping up in the Pacific Ocean. Many in the media are reporting this phenomenon as if it's a sure bet – our drought is all but over!  Unfortunately, the impacts of El Niño aren't so certain – Northern California is just about as likely to experience dry conditions as wet weather in an El Niño year.  I'll believe we're having a wet winter when I'm still wearing my mud boots next April!  In the meantime, I'll keep an eye out for additional signs of stress.

This year, we have four distinct groups of cattle at SFREC.  Each group is part of one or more research projects:

• We have cow-calf pairs that belong to the UC Davis Animal Science Department.  These are fall-calving cows.  In normal years, the calves are weaned in late May, and the cows are grazed on dry forage during the summer and fall.  Currently, this herd is split into 3 groups – older cows with calves, second-calf heifers with calves, and open cows with calves.
• We have bred heifers also belonging to the UC Davis Animal Science Department.  These heifers are typically grazed on irrigated pastures during the summer and through fall calving.
• We have outside heifers that are part of a foothill abortion vaccine trial.  These heifers will be preg-checked in mid-April.  Normally, the open heifers would be kept at SFREC until late May.
• We have steers that are part of a long term adaptive management and targeted grazing research project.  Like the heifers, these steers would be kept at SFREC until late May in a normal year.

But this year is anything but normal!  After a very promising fall (with normal germination of our annual grasses and near-normal forage growth through the end of December), we are now coping with a fourth year of drought.  Since January 1, we've measured just 2.43 inches of precipitation (average for January-March is 11.69 inches).  With the lack of rainfall, forage growth has slowed (total production through April 1 was 1400 pounds per acre, which is about 100 pounds less than normal for this time of year).  Total production doesn't tell the whole story, however; the pastures that we've grazed since the first of the year haven't recovered as expected (which means we haven't returned to these pastures as expected).  My colleagues who have worked at SFREC for a number of years tell me that the vegetation is at least 30 days ahead of schedule – in other words, our annual rangeland looks more like mid-May than early April.  All of this means that this week's rain will help in some pastures, but it's “too little, too late” in others.  Our peak standing crop (the total amount of forage grown during this growing season) will likely be far below our long term average of 3000 pounds per acre.

Based on this year's reality, we started fine-tuning our drought plan in early March.  One of the key steps in our drought planning is to establish a critical date – a date by which we'll make some stocking decisions if we don't receive rainfall.  In early March, we decided that if we hadn't received at least an inch of precipitation by April 1, we'd need to start taking action.  Here are the steps we're looking at taking in the next 30 days (by May 1):

• Ship the open foothill abortion heifers 7-10 days following preg-check.  This will allow us to take as many as 150 heifers off the pastures at SFREC.
• Ship some of the steers (at least those in pastures where feed is not re-growing) by early May.  If we do happen to get some late-spring moisture, this would allow some regrowth for next fall.  This week's rain may be enough to get us through to the end of May.
• Ship open cows and calves.  There are only 13 pairs that fall into this category, but every little bit helps!
• Wean early.  Some of the Animal Science calves already weigh more than 500 pounds.  Another large group is between 450 and 500 pounds.  Weaning the calves now and shipping them off SFREC, will reduce the nutritional demand on the cows, and will reduce the forage demand (both from cows and from big calves) on our rangeland.

Because most of our forage base at SFREC (like much of California) is annual rangeland, we have to manage our spring grazing to ensure that we have enough dry forage through the summer and fall months.  We also want to make sure that we leave enough residual dry matter to protect our soils and provide a microclimate for germination when it does rain again next fall.  On commercial ranches, decisions like those I've outlined take on profound economic consequences – the impacts of shipping lighter calves, selling replacement heifers, and culling the cow herd more deeply than normal can be felt for years to come.  At SFREC, these decisions can impact long term research into important topics.  Ultimately, however, the long-term health of our rangelands is our greatest responsibility.

For more information on long-term weather and forage production trends in the Sierra Foothills click here.  If you are interested in getting more information managing through low rainfall/forage years SFREC has a number of videos and publications that explore this topic.

To get more information about how producers can benefit from forage production data, click here.

SFREC visitors often notice and comment on the large "hoop house" structures that sit in a pasture just North of our headquarters. When precipitation is in the forecast, our staff works closely with the researchers associated with this particular project, to methodically construct the covers on these rain-out shelters in order to meet the specifications of the experimental design. 

Project Summary:

As precipitation more frequently departs from the historical range of variability, maintaining stability in forage production will be a critical management priority in Californian rangelands. A key mechanism that can lead to stability in forage production is compensatory dynamics, where plant functional groups respond differently to climate fluctuations (e.g.,tap­rooted forbs like Erodium tolerate drought, annual grasses are highly productive in non-drought conditions).

Researchers hypothesize that trade-off among functional groups should buffer the response of overall forage production to climate variability and better retain soil nutrients in dry-wet up cycles. The specific objectives are to: 1) quantify relationships among the fate and retention of nitrogen, soil moisture dynamics, and species compensatory dynamics in drought and non-drought conditions; and 2) incorporate these relationships into process-based models of nitrogen retention and forage production in rangelands.

This project will ultimately lead to more informed decision-making for ranchers interested in optimizing forage production and nutrient retention in the face of a highly variable future climate. It addresses an often overlooked but critical component to the stability of ecosystem processes: functional diversity of forage groups.

Researcher, Lauren Hallet explains the preliminary progress of this project in detail in the below video. Stay tuned for more information as samples are taken this spring.

Principle Investigator: Dr. Katherine Suding, ESPM, UC Berkeley