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Monsoon Season

August 15, 2014

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Rainy season has arrived in Portal, AZ.  For those who are unfamiliar with the area, the majority of the year’s precipitation in the Chihuahuan Desert comes in July-August, also referred to as monsoon season.  Instead of the steady, prolonged rains that the word monsoon usually calls to mind, the Arizona version consists of relatively short but intense storms interspersed with periods of blue sky.  These storms also tend to be highly localized: you may watch the rain coming at you down the valley all morning, only to have it skirt around you to the west and miss you completely.  See http://www.wrh.noaa.gov/twc/monsoon/monsoon_NA.php for more information on the North American Monsoon. 

So far this year the Portal experimental site seems to be sustaining more hits than misses, and it’s shaping up to be a fairly “good” rainy season.  To see how the current season is measuring up to recent years, I’ve plotted total monthly precipitation through time in the figure below, with monsoon seasons highlighted as grey bands.  July 2014 was the 5th rainiest month in the past 14 years, exceeded only by the rainy seasons of 2004, 2006, 2008, and 2011.  And we still have August to look forward to! 

Time series of precipitation at Portal

Time series of precipitation at Portal

So what will this mean for our study?  One may predict that a pulse of precipitation in a moisture-poor ecosystem should cause an increase in plant growth.  Indeed, we have found evidence that the rainy season is correlated to a peak in vegetation, and that more precipitation generally means higher peaks. The following plot shows this (approximately) linear relationship between summer precipitation and summer vegetation.  The astute observer may notice that 2002 and 2007 managed to reach high levels of plant activity with low levels of precipitation.  I would first like to note that the site’s weather station was not operational during 2002 and so precipitation during this time was estimated using data from a nearby weather station which, due to the patchiness of storms mentioned above, may not be a valid estimation.  For 2007 I have no such excuse; either there are anomalies in the vegetation dataset I’m using (which is satellite-derived Normalized Difference Vegetation Index), or the rain-plant relationship is somewhat more complicated than I’ve assumed.  For the present season, if it keeps raining at the same rate we’ve seen so far, we will get about 180mm in total, putting my prediction of vegetation index at 0.42-0.43.  We’ll see how this pans out in another month or two. 

Precipitation vs. Vegetation linear regression

Precipitation vs. Vegetation linear regression

The response of the rodent community to rainy season is much more difficult to observe than the plant response.  The plant activity inspired by rainy season provides a surplus of food to the rodents, which encourages reproduction and should result in an increase in rodent abundance.  However, current research suggests that short-term weather events have little to do with rodent population dynamics, that models must incorporate weather patterns from the past year or more to detect subsequent changes in rodent abundance.  For example, numbers are not likely to increase after a single “good” summer monsoon, but they may if the site experiences a wet winter followed by a wet summer or two wet summers in a row. 

Researcher and researchee enduring inclement weather

Researcher and researchee enduring inclement weather

I want to conclude by thanking local contractor Bob Walton for the beautiful new roof on our ramada.  Just in time to keep our heads dry during this past sampling weekend! 

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Spring 2014 Plant Census

April 23, 2014

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Last month brought members of the weecology group (Dr. Morgan Ernest, recent graduates Dr. Glenda Yenni and Dr. Sarah Supp, and current graduate student Erica Christensen) back to Portal for the (approximately) semi-annual plant census.  Winter rains had provided us with a decent selection of annual plants to count and identify, which was a welcome contrast to the sparse plant census in spring 2013.  The breakout stars this year were smallflowered milkvetch (Astragalus nuttallianus), which showed up hundreds-strong in some quadrats, Esteve’s pincushion (Chaenactis stevioides), and the invasive redstem stork’s bill (Erodium cicutarium).  One of the most striking desert flowers found in the area, the California poppy (Eschscholzia  californica Mexicana), was also present on many plots.

Eschscholzia  californica Mexicana

Eschscholzia californica Mexicana

In total, we identified 28 annual plant species, a marked improvement from the 16 species recorded in the spring census of 2013 and quite similar to the 30 species recorded in 2012.  However while species composition was similar in years 2012 and 2014, plant abundances were much lower in 2014.  This is likely due in part to differences in total winter precipitation; precipitation was high in 2012, very low in 2013, and somewhere in the middle in 2014.

Abundances of the top 10 species for the past 3 years: Astragalus nuttallianus, Erodium cicutarium, Chaenactis stevioides, Lesquerella gordoni, Descurainia pinnata, Phacelia arizonica, Linanthus bigelovii, Eschscholzia californica mexicana, Eriastrum diffusum, Lepidium lasiocarpum

Abundances of the top 10 species for the past 3 years: Astragalus nuttallianus, Erodium cicutarium, Chaenactis stevioides, Lesquerella gordoni, Descurainia pinnata, Phacelia arizonica, Linanthus bigelovii, Eschscholzia californica mexicana, Eriastrum diffusum, Lepidium lasiocarpum

We also used the trip to perform some much-needed maintenance on the site.  The low fences around each of our 24 experimental plots require frequent upkeep to prevent the rodents from tunneling under or climbing over them, but there is also a barbed wire cattle fence around the perimeter of the 20 ha site that has gotten very little care in the past decade or so.  It only came to my attention in March, when I was surprised to find a very large herbivore chewing his cud on my study site.  We were able to chase him off with minimal damage (either physical or emotional) to man and beast, but the experience inspired me to request the help of my PhD adviser in repairing some conspicuous holes in the perimeter fence.  So how many PhDs does it take to mend a barbed wire fence?  Turns out one and a half will get it done.

Dr. Morgan Ernest deftly handling some baling wire for fence repair

Dr. Morgan Ernest deftly handling some baling wire for fence repair

The cow wasn’t the only unexpected herbivore we encountered during the March rodent census.  Caught in one of the traps, seen for the first time on the site since 2009, was a tawny-bellied cotton rat (Sigmodon fulviventer).  Three species of cotton rat have been recorded at the site (S. fulviventer, S. hispidus, and S. ochrognathus), all of which prefer grass-dominated habitat to shrub-dominated.  They have been uncommon but persistent at Portal since its inception in 1977, and were even fairly abundant when the area was dominated by lush grasses in the mid to late 2000s.  However recent years have seen decreased rainfall and therefore the plots have been more characteristic of shrubland than grassland, and the Sigmodons moved elsewhere.  Does the return of the cotton rat mean a return to grassland is on the horizon?

S. fulviventer wasn't as excited to see me as I was to see him

S. fulviventer wasn’t as excited to see me as I was to see him