Comment on the Gabbro Soils of Pine Hill

2012·0.82 MB·English

by  Earl B Alexander

Madrono, Vol. 59, No. 1, p. 1, 2012
POINT-OF-VIEW
Comment on the Gabbro Soils of Pine Hill ascertained from aqua regia digestion ofthe soils
to evaluate the total elemental reserves in the
Burge and Manos (2011) investigated the soils. The soil with C. roderickii had subsoil P
genetic relationships of Ceanothus roderickii W. similar to that in the other soils, but the surface
Knight and Ceanothus cuneatus Nutt. var cunea- soil in the C. roderickii plant community had
tus and sampled surface soils where the plants much more P than in the surface soils at the sites
were found. They claimed to have shown that the lacking C. roderickii. The surface soil at the C.
two species are associated with chemically roderickiisite also had much more organic matter
different gabbro soils. than the soils at the other two sites. Evidently, the
The mineralogical and chemical differences amounts of P in the surface soils was largely
among gabbro rocks are great and plant distri- dependent on the amounts of plant detritus that
butions from those dominated by olivine to those had been incorporated into them, which is a
dominated by Ca-feldspars might be expected to
function of entire ecosystems, not only a single
be different. With respect to C. roderickii and species. Perhapsthesoil parent materials at the C.
oqCu.neswcthuiinoecnash:tuCs(.1)rvoaWdreh.raictcukniiiesawttiuhlsel,grartnohgewre?eo(2fa)rgeWahbtabhrrteoeissoktiehlyes mtroahdeteerrsiiucarklfisaicseoiftseosoitlwshheerhraedgaBbausbrrgmoeuacsonhidls,PMabanustotshtehseapmapprlleaenndtt
range of gabbro soils on which C. cuneatus var.
communities at C. roderickii sites were cycling
cuneatus will grow? (3) Considering gabbro rocks less P than the plant communities at the
and soils where the ranges ofC. roderickiiand C.
cuneatus var. cuneatus do not overlap, what are wedgeleafceanothus sites?
Unfortunately, the methods of Alexander
the mineralogical differences in the rocks and the
(2011) are too intensive to apply broadly and
chemical differences in the soils that might limit
plant distributions. A final test would be to plant the low- intensive methods of Burge and Manos
the two species in soils from different kinds of are inadequate to show gabbro petrologic and
gabbro rocks under climatically similar or soil differences related to the distributions of
endemic plants. Perhaps future investigations
controlled.
Burge and Manos (2011) did not identify the that are less intensive than that of Alexander,
specific gabbro rock mineralogies; they sampled but comprehensive enough to identify the kinds
only surface soils, and they ascertained only the of gabbro parent rocks and both surface and
readily extractable portions of the chemical subsoil reserves of key elements, will identify
elements. Locations where they sampled surface what gabbro rock and soil features lead to
soils may have been in areas where the distribu- different plant distributions.
tions of C. roderickii and C. cuneatus var. —
cuneatus do not overlap, but the methods were Earl B. Alexander, Soils and Geoecol-
not adequate to distinguish different kinds of ogy 1714 Kasba Street, Concord, CA 94518;
rocks and soils. Their data indicate that the [email protected].
greatest differences between the surface soils at
sites with different ceanothus species were differ- Literature Cited
eEnDtTaAm)ouenxttrsactoafblMeehP.lich III (dilute acids and Alexdaisntdriebru,tionEs. oBn.th2e0m1.1.MaGdarbobiiroo 5s8o:i1l1s3-a1n2d2. plant
Alexander (201 1) sampled the parent rocks and Burge, D. O. and P. S. Manos. 2011. Edaphic
both surface (0-15 cm) and subsoils (30^5 cm) at ecology and genetics of the gabbro-endemic shrub
one site with C. roderickii and two sites without Ccanotluis roderickii (Rhamnaceae). Madrono
it on the Pine Hill gabbro. Phosphorus was 58:1-21.