Clauss & Venable (2000, Am Nat) considered what a good year for a winter desert annual (Plantago insularis) would be in southern Arizona and California. Such a year would have a sequence, between September and March, of one month with at least 17 mm of rain and 24 mm in the following month. I wonder whether a similar estimation exists for summer desert annuals.
In our paper Crimmins TM, Crimmins MA, Bertelsen CD (2011) Onset of summer flowering in a “Sky Island” is driven by monsoon moisture. New Phytologist 191: 468-479, we found that alpha diversity is closely related to July precipitation--the greater the precipitation, the earlier the flowering across an elevation gradient of about 1265 meters. Of course there are always exceptions. We did not look at specific thresholds--without weather stations at various elevations within the canyon, it is not possible to get really specific. Because summer storms are so localized and the topography so varied in the American Southwest, it is really difficult to come up with any valid prediction for a particular site.
30 years of observation tell me that the timing of precipitation (onset/end), the amount of precipitation per event (large amounts = large runoff), the rate at which it falls (runoff increases with the rate), the number of precipitation events during the season (do soils stay moise?), the intervals between events (does soil dry out between events?), and the area covered by monsoon storms (storms can be highly localized)--all of these are at least as important as how much rain falls during the season. My guess at this point is that temperature is not a significant variable.
The other thing that is important is species are highly idiosyncratic in their responses to climate variables and topography, and I would doubt that thresholds for one species would be applicable to many others. This spring is an early one, likely due to very good rains last November, but many abundant species such as Phacelia distans are not doing particularly well this year while less common species such as Mentzelia jonesii are having their best year in 30 years. Another species, Plagiobothrys arizonica, is very common this year in my study area on the south slopes of the Santa Catalina Mountains, but the species is nearly absent in an area on the north slopes of the same range. Plantago patagonica (P. insularis has not been found in my study area) is having a very poor year, at least so far.
Many thanks Aria and Charles for your insights. I'll keep on looking for an estimation for a summer desert annual comparable to the proposed by Clauss and Venable.
My suggestions are:
Madrigal J, DA Kelt, P Meserve, FA Squeo & JR Gutiérrez. 2011. Bottom-up control of consumers leads to top-down indirect facilitation of invasive annual herbs in semi-arid coastal Chile. Ecology 92: 282-288.
Squeo, F.A., M. Holmgren, M. Jiménez, L. Albán, J. Reyes & J.R. Gutiérrez. 2007. Tree establishment along an ENSO experimental gradient in the Atacama desert. Journal Vegetation Science 18: 195-202.
Martínez-Tillería K, AP Loayza, DR Sandquist & FA Squeo. 2012. No evidence of a tradeoff between drought and shade tolerance in seedlings of six Coastal desert shrub species in north-central Chile. Journal of Vegetation Science 23: 1051-1061.
León MF, FA Squeo, JR Gutiérrez & M Holmgren. 2011. Rapid root extension during water pulses enhances establishment of shrub seedlings in the Atacama Desert. Journal of Vegetation Science 22(1): 120-129
Many thanks Francisco, though I couldn't find what I am looking for in these papers. I'll keep on looking.
I have a little over 100 annuals that flower in my study area in summer, but the elevation gradient is 945-2212 m. Species flowering from 945-1036 m include the following. (Some of these are found in more densely vegetated riparian scrub.) I've added an asterisk before species that may be useful. Note, though, that my area is higher than typical desert, so there are a number of different species in the genera indicated that may be useful. I'd recommend looking at a local flora to get more specific ideas. I would caution you not to select only very common species because they may be very well adapted to local climate variability that they won't tell you much.
*Amaranthus fimbriatus var. fimbriatus
*Amaranthus palmeri
Aristida adscensionis
Bidens bigelovii
*Boerhavia spicata
*Bouteloua aristidoides
Bromus rubens
Datura discolor
Dinebra panicea subsp. brachiata
Enneapogon cenchroides
*Eragrostis cilianensis
Eriochloa aristata var. aristata
Erythranthe nasuta
*Euphorbia florida
Helenium thurberi
*Heliomeris longifolia var. longifolia
*Heterotheca subaxillaris subsp. latifolia
*Hymenothrix wislizeni
*Ipomoea cristulata
*Ipomoea hederacea
*Kallstroemia californica
*Kallstroemia grandiflora
Lactuca serriola
*Muhlenbergia microsperma
Panicum hirticaule var. hirticaule
Pectis cylindrica
*Pectis papposa
*Phaseolus acutifolius var. tenuifolius
Poa annua
Polypogon monspeliensis
Salsola tragus
Sonchus asper
Sonchus oleraceus
*Tidestromia lanuginosa
*Urochloa arizonica
*Verbesina encelioides
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