Sahara Mustard Reduction in Numbers in the Borrego Desert Floor in 2015
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| Pictures taken just northeast of the end of the pavement on Di Giorgio Road of approximately the same location.
Left: A field of nearly-solid Sahara mustard, Brassica tournefortii, on 23 February 2010 that had mostly wiped out the native annual display beginning in ~2007. Right: A field of mostly-native young annuals on 22 January 2015. The previous three years of severe drought in this area (2011-2012, 2012-2013 and 2013-2014) mostly depleted the seed bank of Sahara mustard, allowing us to enjoy a native wildflower display once again here for at least a few years! Click on the pictures for larger versions. | |
The incredible good news this year is that in the desert floor, the three years without significant rain here has apparently mostly depleted the seed bank of Sahara mustard, Brassica tournefortii, and its numbers are significantly reduced on the desert floor!!
From his observations of weeded plots in the Mojave Desert, Chris McDonald had found that the seed bank was mostly depleted after three or four years. We had heard that, but found it very hard to believe until we saw it with our own eyes this year. Our Borrego Desert observations this year show that it does seem that three years is enough to kill most of the mustard seed bank in our area!
It has been depressing to have no annual germination on the Borrego Desert floor for three years, and we are well into a fourth year for areas east of Borrego Springs. But little did we know this was Nature's way of waving a magic wand to eliminate the mustard from many areas!
This year we have sparse to good annual germination in the Borrego Springs area and westward, including in areas such as the Hellhole Canyon Parking Lot (sparse; mostly under shrubs); the area south of the Visitor Center (decent, including out in the open); the desert sunflower sandy area along Henderson Canyon Road (sparse); the sandy area at the end of Di Giorgio Road (good); and Collins Valley above Lower Willows (good). In all of those areas, mustard is almost completely absent!!! There are still mustard seedlings here and there, but mustard is an extremely-minor component of the annual germination this year, whereas it was dominant before. Even filaree is not as abundant as it once was. Only Schismus is still abundant, but it is not dominant.
The absence of mustard is not due to any particular rainfall pattern this year. The non-native annuals of mustard, Schismus and filaree, Erodium cicutarium, are always the first of the annuals here to germinate, since they are adapted to a somewhat-different rainfall regime in their native land, allowing them to germinate with rainfall not heavy enough to germinate our native species. Hence if there was still a large viable seed bank of mustard, it would have germinated with or before our native annuals, as it always does.
The reason some mustard is still present in the immediate area of Borrego Springs is probably because there were some seeding mustards along roadsides in the last three years which replenished its seed bank there a little bit. Although we have had very little annual germination for the years 2011-2012, 2012-2013 and 2013-2014, what germination we did have was mostly these non-native annuals due to their lower moisture requirement for germination. The rainfall wasn't enough to germinate them away from roadsides; they needed the extra runoff from the road surface present along roadsides. Presumably, other areas with similar runoff, from buildings, boulders, hillsides or water channels will also have sparse viable mustard seed still present.
This depletion of the mustard seed bank also means that the area east of Borrego Springs, in the Borrego Badlands area, that was almost completely a sea of mustard by 2005 up to the north-south crest at the Thimble Trail, should now revert to being almost completely free of mustard the next time it rains enough there to germinate annuals. However, note that this is speculation, not observation, since there is no germination in that area so far this year.
Note that the mustard is mostly gone only from the desert floor in Borrego Springs and east; there is still plenty of mustard in the higher elevations west of Borrego Springs, including Borrego Palm Canyon and Henderson Canyon, and presumably also in the Santa Rosa Mountains north of the Borrego Badlands. But knowing that the seed bank lifetime is only three years means that any area that is kept free of seeding mustard plants for three years will no longer contain a viable seed bank of it! Hence all the mustard weeding that has been done by so many in so many areas is paying off more than expected at the time.
It obviously will be important to try to prevent the mustard from re-spreading into the desert floor in the future to delay as long as possible it taking over the Borrego Desert floor again.
The further information below was stimulated by an understandably-skeptical person who offered the following quote:
The absence of above ground plants ... can create the false impression that plant populations are no longer present, although they are merely lying in wait as viable seed.Kara A. Moore and James M. André, Fremontia Vol 2, No 1, 2014
I fully understand the skepticism about whether the mustard is truly extirpated in these areas! It is hard to believe that a major scourge of the desert floor might really have been knocked down significantly. We all fear that we might find in future years that it was only hiding this year. Skepticism is certainly warranted about any such prediction, since the only facts we have right now are that the mustard is much less abundant in the Borrego Springs area in January 2015 than it has been in the last decade.
There can be a lot of reasons why the mustard is less abundant in the Borrego Springs area this year, that might not imply the seed bank is gone in other areas. For example, those areas might have more seed predators who scrounged out the seeds during the last three years of drought, and the seed bank might still have viable seeds in areas east of Borrego Springs.
Here are the reasons I don't think the mustard is hiding. But as always, predictions are never sure things.
That quote above is certainly true for our native plants, which often only appear in years when conditions are right. The extreme example are fire-followers in the chaparral, which can exist as seeds for almost a century! But we aren't talking about native plants here; we're talking about non-native plants which in some cases are not adapted to our rainfall regime with multiple-year droughts.
In order to make good conclusions about the absence of a plant population in a given year, one needs further information about the seed lifetime and the plant germination requirements, which we have in this case.
Note that no claim was made about the absence of B. tournefortii in 2012 when it mostly didn't appear, nor in 2013 when it mostly didn't appear, nor in 2014 when it mostly didn't appear.
The claim was made only this year, in 2015, when germination requirements were met in large areas in the immediate Borrego Springs area, and the mustard mostly wasn't there.
Note that Brassica tournefortii (henceforth B.t.) is not completely absent in the immediate Borrego Springs area; there has been very sparse germination just along roadsides, where there had been sparse germination in those previous severe drought years that replenished the seed bank a bit.
Furthermore, there is extensive B.t. germination to the west of Borrego Springs, where the seed bank is replenished every year.
From 10 years of observation of annuals in the desert, in a wide variety of weather and rainfall conditions, I've observed that B.t. and a few other non-native annuals are always the first to germinate, and need much less rainfall to germinate than our native annuals. In the severe drought years, B.t., and other non-native annuals, were the only plants to germinate in the immediate Borrego Springs area.
The observation that there are only a small number of B.t. plants in the immediate Borrego Springs area, along roadsides, and it is absent in immediately-nearby areas that have good native annual germination, says very clearly that its germination requirements were met this year there. If its seed bank was still viable, it would have germinated much more widely.
We also have good experimental evidence of the seed bank lifetime from Chris Mcdonald's work in the Mojave Desert.
This additional solid information about B.t. seed lifetime and germination requirements are what is needed in addition to the "absence of above-ground plants" in order to conclude that B.t. is extirpated from large areas of the desert floor where it hasn't germinated for three years.
There is good reason to expect that at least some non-native annuals would be extirpated in severe droughts, since some of them evolved in areas where the likelihood of severe droughts in repeated years is small. There was no pressure on them to evolve seeds that could survive repeated drought years, as there was on our native annuals.
For example, in the severe drought year of 2001 to 2002 on the coast, two non-native grasses, Gastridium ventricosum, nitgrass; and Lolium multiflorum, Italian rye-grass, were mostly extirpated from the Santa Rosa Plateau. Those two grasses were widespread there prior to 2001, with some areas a monoculture of rye-grass. They mostly disappeared for a number of years after 2001, coming back only very slowly. In those cases I have extensive personal observations of their absence in most areas through 2005 since I did mostly-full-day botanical surveys there every 3-4 days from late 2003 to early 2005.
In one of the areas formerly occupied by a monoculture of rye-grass along the Vernal Pool Trail, in a former vernal pool area, in 2003 there was an explosion of the native Euphorbia spathulata, warty spurge, when that area was opened up due to the absence of rye-grass.
The re-emergence of these two non-native species at the Santa Rosa Plateau was possibly from a small percentage of seeds that managed to live for two years, but more likely spreading from wetter areas (springs and creeks) where they were able to germinate even in 2001-2002. This is exactly what will happen in the Borrego Desert; B.t. will spread again to the low desert, unless efforts are made to stop it.
We've been granted a temporary reprieve from B.t. in some areas with a big helping hand from Mother Nature. It is up to us to try to stop B.t. from spreading again in as many areas as we can.
However, even though I think we are on solid ground in concluding that B.t. really is gone from many areas of the low desert, I'm sure we will all feel much better the next time we have a good rainfall year in the desert, and B.t. is still observed to be absent. Although I'm a strong believer in theoretical conclusions, the universe has a habit of making mockeries of such conclusions at times. In fact, I'm doing an germination experiment in February 2015 to see if there are viable B.t. seeds lurking in the soil.
By the way, there is a new weed in town, Canary Island knapweed, Volutaria canariensis, that can germinate from monsoonal rains, and has been found in the thousands in each of the past two years in Borrego Springs. The highest weeding priority will be to extirpate this so that it can never become the next Sahara mustard. Frank Harris, from Seattle, has been waging a one-man war against this new species in his winter sojourns to Borrego Springs. But he is leaving at the end of January, and we desperately need someone in Borrego Springs to take over this fight.