After our braggart post last week, we thought that we should remind you all that despite all the lovely amenities afforded us at the posh Sells Recreation Center, it is dry...as in no beer before dinner while watching the sun set after a long hard day of work. Last week, we would have traded the showers for beer.
Valley-fill alluvium composed of decomposed granite and sand thus far has the most dense, continuous cover of low annuals (mostly Plantago ovata). These are the soils encountered in much of BMGR. Valley-fill alluvium of decomposed rhyolite, andesite, and other granitic volcanics seems to support a less continuous cover of annuals, but a more diverse assemblage. Desert pavement, generally composed of basalts, tuffs, and other volcanics are harsh environments and have the lowest cover, richness, fuel continuity, etc. Within the desert pavement, isolated patches of disturbed soils (directly linked to rodent, rabbit, and canid borrowing) offer refugia for annual plants, and are often colonized by native annuals and Schizmus barbatus. Also, within the desert pavement, linear erosional features offer similar conditions. Hillside soils, composed of volcanic rocks, bedrock, and gravel offer little space for plant colonization and thus far seem to be composed mostly of native annuals, especially our unknown ‘AST001’ which this week I confirmed as Calycoseris wrightii, as well as Eriogonum trichopes, which is abundant and widespread in desert pavement, seldom encountered in alluvium of volcanic origin, and almost nonexistent in decomposed granite alluvium.
Ocular estimation of fine fuel importance, based on general patterns observed (data will probably match this pretty well); from top to bottom the most important species in contribution to fine fuels:
1) Plantago ovata
2) Amsinckia intermedia
3) Eriogonum trichopes
4) Cryptantha & Pectocarya spp.
5) Lepidium lasiocarpum & Lepidium spp.
6) Schizmus barbatus
7) Native Brassicaceae spp.
8) Buffelgrass (mostly observed in roadside ditches and scattered in washes)
9) Brassica tournefortii
10) all others
*the contribution of native shrubs to fire activity cannot be overlooked, especially Ambrosia spp in the creosote valleys (drought-deciduous plants are very dry right now), and a wide range of shrubs in the upper Sonoran where the vegetation appears to be generally denser. Also, throughout the desert pavement areas, shrubs offer soil enrichment & microclimatic conditions that allow fine fuels to establish beneath them. I think the data will show this to a degree.
We have observed past fire evidence almost everywhere we have been. I am a keen observer of fires effects on forest and woodland structure and have applied my understanding of these systems to the Sonoran communities we’ve been in. The more obvious evidence we have seen includes recently killed shrubs (sometimes just charred micro-stumps persist, and sometimes, especially with creosote, the leafless skeleton persists), charred and fallen trees (mostly palo-verde & ironwood, which appear to have virtually no tolerance of fire), fire-scarred saguaro, and in the McDowell Mtn area, absolute devastation of most cacti, especially chainfruit cholla, buckhorn cholla, and Christmas cholla. The more subtle evidence consists of stand structural characteristics like saguaro age-class distribution, ocotillo that appear to be stump sprouts (does this species sprout following fire??), shrub re-sprouting (brittlebush, jojoba, and Celtis pallida respond favorably to fire, showing rapid re-growth and generally larger height attainment), and annual forb canopy continuity (only at McDowell Mtn Park did we see an obvious dominance of Schizmus in burnt areas). Fires appear to have been most prevalent in the valleys, and less common on the mountainsides. The alluvial valley-fill soils generally support more fine fuels than the mountainsides, so it may be that fires rage through the flats and only just barely creep up the slopes. However, we have had very few plots in the higher mountainside areas so it’s possible that we have just not yet seen any burnt slopes. Fire definitely simplifies these ecosystems through promotion of fire-tolerant shrubs, a reduction in cacti, increasing fine fuel continuity, and unseen effects on the wider suite of native forbs and grasses that I have not yet observed because of the dry conditions this year. The burnt Sonoran ecosystem loses its beauty to a noticeable degree, and at times looks haggard. It would be an interesting effort to overlay fire occurrence layers with our data, although accurate records probably do not exist, and our plots are too far spaced to really say anything too specific in regards to community response to fire.
Thanks folks-
Joe
