Biotic Communities in the Sub-Mogollon Region of the Inland Southwest Author(S): Charles H

Arizona-Nevada Academy of Science

Biotic Communities in the Sub-Mogollon Region of the Inland Southwest Author(s): Charles H. Lowe Jr. Source: Journal of the Arizona Academy of Science, Vol. 2, No. 1 (Aug., 1961), pp. 40-49 Published by: Arizona-Nevada Academy of Science Stable URL: http://www.jstor.org/stable/40025670 Accessed: 21/05/2010 20:41

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http://www.jstor.org BIOTIC COMMUNITIES IN THE SUB-MOGOLLON REGION OF THE INLAND SOUTHWEST

Charles H. Lowe, Jr.* The University of Arizona

The Sub-Mogollonregion is a highly varied land- Table 1. Five World Ecologic Formation-types in North scape that is common to those fractions of Arizona, America, Their Ecologic (Biotic) Formations in New and Sonorawhich lie essen- the Inland Southwest, and the Major Association- Mexico, Chihuahua, types (= Vegetation Types) that Comprise Them. tially between the ColoradoPlateau to the north and the SierraMadre to the south (Fig. 1). The natural- Desert Formation-type ist is immediately struck by the wide expanses of 1. Northern Desert Scrub Formation1 lowland habitats (deserts and grasslands) which Sagebrush associations the or Saltbush associations separate highly disjunctive upland highland Blackbrush associations areas (woodlands and forests). An unusuallyfertile 2. Southern Desert Scrub Formation2 laboratoryis presented here for the study of evolu- Creosotebush associations tionary rates in diverse forms of animals and plants Saltbush associations in the biotic communities isolated in the Tarbush associations recently Sandpaperbush associations disjunctiveranges. Joshuatree associations The Basin and Range Province is an ecological Paloverde associations and evolutionaryprize. On the other hand, the evolu- Mesquite associations tionary prize is not yet won in the present almost Grassland Formation-type3 total absence here of a recent paleoclimaticpicture 3. Desert Grassland Formation such as could be based on either plant megafossils Desert Grassland associations or microfossils In to 4. Plains Grassland Formation (e.g., leaves) (pollen). regard Shortgrass Plains associations this, Dr. Martin has joined the symposiumto discuss 5. Mountain Grassland Formation paleoecologicalaspects of southwesternbiogeograph- Mountain Grassland associations ical problems. Woodland Formation-type3 BIOTIC COMMUNITIES.-The biotic commun- 6. Chaparral Formation4 ities of the Sub-Mogollonregion are listed in Table Chaparral associations 1. The northerndesert scrubformation of the Inland 7. Evergreen Woodland Formation Oak Woodland associations Southwest is the only formation listed in the table Encinal associations5 that is not representedin the Sub-Mogollon region. Pine-oak Woodland associations0 The classificationof these plant-animalcommunities Pinyon-juniper Woodland associations is a classificationof the for 8. Riparian Woodland Formation representedvegetation, woodland associations7 the mappablereality of the biotic communityis only Riparian the mappablereality of its vegetation. Forest Formation-type What is 9. Coniferous Forest Formation actually classified in Table 1? Does a Pine Forest associations8 biotic communityreally exist? In this symposiumon Fir Forest associations9 bioecology, we are already pledged by at least our Spruce-fir Forest Associations10 title to the existence of biocommunities,i.e. recog- Aspen associes11 nizable and predictableplant-animal associations and Tundra Formation-type formationsof various orders of magnitude.There is, 10. Alpine Tundra Formation12 in fact, a large body of convincing evidence in both Alpine Tundra associations the New and Old World literaturein supportof this ecologic principle (see Allee, et al., 1949; Dice, 1952; 'Great Basin Desert (Shreve, 1942). Dansereau,1957). 2Mojave, Sonoran, and Chihuahuan Deserts (Shreve, 1942). The general vertical relationships of the several "Savannas" (whether or not to be recognized as forma- formations in western North from lower tions) are, in this region, grassland-woodland and grass- America, land-desert and the desert to elevations,are clear. One of the formations ecotones, grassland (part). higher 4Petran or Interior Chaparral (riparianwoodland), however, differs from the oth- (auct.). "Shreve (1915); see text. ers in that it occurs throughoutall of the rest with Mexican Pine-Oak Woodland (auct.); see text. '"Developmental associes" (allogenic), with stability as per- * Paper presented in a symposium on "Bioecology of the manent as the drainage characteristics; see text. Arid and Semiarid Lands of the Southwest," arranged by sMontane Forest (auct.). Linton Gardner J. and Charles H. Lowe, Jr. of the Com- 9Montane Forest (auct.), Abies and/or Pseudotsuga. mittee on Arid Zone Research of the Southwestern and Forest Rocky Mountain Division of the American Association For 10Subalpine (auct.). The Advancement of Science, held at New Mexico High- ^Developmental (autogenic). lands University, Las Vegas, New Mexico, April 29, 1958. Petran or Rocky Mountain Alpine Tundra (auct.).

40 August 1961 LOWE- BIOTICCOMMUNITIES 4l

the exception of alpine tundra. The riparian wood- floodplains and which is further characterizedby land associationsof the Southwest are, in general, different species and/or life-forms than that of the relativelylitle known to most ecologists. The follow- immediately surrounding non-riparian climax. The ing is a brief discussionof distributionaland floristic southwestern riparian woodland formation is char- aspectsof this formation. acterizedby a complex of trees, and their plant and A riparian associationof any kind, is one which animal associates, restricted to the major drainage- occurs in or adjacent to drainagewaysand/or their ways that transgressthe landscapeof desert upward through forest. It is incorrect to regard this biotic Table 2. The Ecologic Formations Comprising Merriam's formation as merelya temporaryunstable, serai com- (1899) Life-zones in the Inland Southwest. See It is an with an Table 1 for formationsand formation- munity. evolutionaryentity enduring types. stabilityequivalent to that of the landscapedrainage- which form its habitat. Moreover, it Life-zones Formations ways physical Ecologic is, as are all ecologic formationsand their subidivions, to Lower Sonoran NorthernDesert Scrub locally subject and often dissolved by the vicissi- SouthernDesert Scrub tudes of human occupation. ( + disjunctDesert Grassland In the Sonoranand ChihuahuanDeserts in South- relicts)1 ern southern New Mexico and RiparianWoodland Arizona, adjacent Mexico, riparian woodland reaches its greatest ex- Upper Sonoran Desert Grassland2 pression of size and complexity in the immediate Plains Grassland proximity of permanentand semi-permanentstreams Chaparral at the lower elevations of conifer-clad mountains. EvergreenWoodland In the centerof the the RiparianWoodland Sub-Mogollonregion, riparian "big-five"are cottonwood (Populus fremonti), wil- Transition ConiferousForest ( = Ponderosa low (Salix bonplandiana and others), sycamore Pine Forest) {Platanus wrighti), ash (Fraxinus velutina and MountainGrassland and walnut Often three RiparianWoodland others), (fuglans major). or four of these species may occur together, and, Canadian v ConiferousForest (= Fir Forest) occasionally,all five. All of them are large, winter j MountainGrassland deciduous broadleaf trees of different genera and f RiparianWoodland families than the of the > Boreal species immediatelybordering non-riparian climax desert, etc., communities (see Fig. 2). This broadleaf associationforms the con- Hudsonian ConiferousForest (= Spruce-Fir spicuous and dominant plant component of a biotic Forest) community which includes characteristicspecies of MountainGrassland and terrestrial animals; for RiparianWoodland aquatic, semi-aquatic, example, the Summertanager (Piranga rubra), Bul- Arctic-Alpine Alpine Tundra lock oriole (Icterus galbula), Yellow warbler (Den- droica petechia), Black-neckedgarter snake (Tham- nophis cyrtopsis), Leopard frog (Rana pipiens), Canyon tree-frog (Hyla arenicolor), Longfin dace Table 3. Examples of Small Isolated Populationsof Trees (Agosia crysogaster), other vertebrate species, and Restrictedto Sheltered Slopes Near Summits of a number of invertebrates. Rangesin the Basin and Range Province,listed in large order of decreasing maximum range elevation Within the broadleaf riparian woodland, isolated (feet). oak trees (e.g., Q. emoryi, Q. arizonicd) occasionally down to as low as 3000 feet, and scrub oak Alt. Lemmon(9,163) Abies lasiocarpa Corkbarkfir finger Santa CatalinaMts. to 2500 feet or lower (e.g., Q. ajoensis). Mesquite (Prosopis juliflora), catclaw (Acacia greggi) and Clark Peak (7,903) Abies concolor White fir others often form a distinctivemicrophyllous border ClarkMountain associationon adjacentfloodplains (Fig. 2). The of the woodland Mt. Glen (7,512) Pinus cembroides Mexicanpinyon species composition riparian Dragoon Mts. changes with elevation. Riparian woodland distributed along major drainagewaysthroughout the ever- Wasson Peak (4,677) Quercusturbinella Scruboak green woodlands and conifer forests are characterized Tucson Mts. by such deciduous broadleaf trees as Texas mulberry (Morus microphylla), Arizona alder (Alnus oblongi- ^daphic control. folia), narrowleaf cottonwood (Populus angust i- 2The Desert Grasslandis zonally intermediatebetween Up- folia), southwestern chokecherry (Prunus virens), LowerSonoran. While referredto as per and usually Upper boxelder (Acer negundo), Rock Mountain maple Sonoran,in the original sense of Merriam,it has been con- and Scouler willow sideredLower Sonoranby some (see Wallmo, 1955; Hoff- (Acer glabrum), (Salix scoule- meister,1954). riana). 42 JOURNAL OF ARIZONA ACADEMYOF SCIENCE Vol. 2

Figure 1.- Place names in the Sub-Mogollon region of the Inland Southwest. The Sonora-Chihuahua area is adapted after Marshall (1957, Fig. 1). August 1961 LOWE BIOTICCOMMUNITIES 43

Figure 2.- Diagrammatic profile of a foothill canyon in the Sonoran desert, as in the Galiuro and Santa Catalina Mountains in southeastern Arizona; at approximately 3000 feet elevation. Streamflow is either permanent or semi-permanent and in either case periodically torrential. The riparian woodland of cottonwood, willow, ash, sycamore, and walnut (Populus-Salix gallery association) is situated in and at the immediate edge of the channel. This broadleaf deciduous woodland is immediately bordered by a mesquite dominated flood plain characterized by deciduous microphylls {Proso pis- Acacia flood-plain association). The non-riparian climax on the adjoining slopes is Paloverde-Sahuaro (Cercidium-Cereus upland association) dominated primarily by lphyllous and microphyllous deciduous life- forms of trees, shrubs, and cacti.

In the direction of lower elevation and greater and Prosopis juliflora. These are short, deciduous, distance from the mountain base, with concomitant microphyllous, desert trees {Olneya is evergreen). reduction in immediate undersurfaceflow, the ripa- This is not a riparian woodland association.It is a rian communitychanges gradually in genetic com- desert riparianassociation. position and life-form. Larger arroyos, without LIFE-ZONES AND BIOMES.- The recogniz- streamsbut with sufficientundersurface flow, support able spatial deployment of characteristicspecies of mixturesof broadleaf and small-leaf deciduoustrees. plants and animals reoccuringunder similar environ- In both the desertand desert grassland,Celtis reticu- mental conditions as definable associations,provided lata, Sambucus mexicana, and Bumelia lanuginosa the basis for C. Hart Merriam's well-known life- take part in the make-up of the various riparian zones (Table 2). Such reoccurringvertical associa- species combinations. tions comprisedof the same species of animals and The extreme situation of deficient precipitation plants can be the experienceof anyone who cares to and its effect upon the local distributionof trees is take the trouble to verify vertical zonation in the seen in the Lower Colorado Valley region of the West and/or elsewhere where relief is fairly sharp. SonoranDesert. Here one finds not only the absence Should this be followed, however, by an attempt to of broadleaftrees along even the larges arroyos,but conscientiouslytrace in detail the upper or the lower that the microphylloustrees that line them include limit of such a vertically disposed biotic unit, then the foothill paloverde (Cercidium microphyllum), a the problem becomes knotty. And the greater the dominant on the upland slopes throughoutthe Ari- number of species to

Figure 3.- Amplitudes (spans) of ecologic tolerance for trees dominating in communities on south-facing slopes on the south side of the Santa Catalina Mountains, Pima Co., Arizona. Ponderosa pine and western yellow pine are inter- changeable common names for P. ponderosa. mation in North America. In the widely distributed many others, have been more recently proposed. biome" in Arizona and New Mexico "spruce-moose THE GENETIC CLIMAX.- If the ecologic for- there is no moose at all, and in the Sub-Mogollon mations and associations of and animals as of Arizona there is on the Chiri- plants region spruce only given in Table 1 really exist, then, in final analysis, cahua and Pinaleno (Graham) Mountains (Fig. 1). to what does the owe its existence? I This "biome"enthusiasts, which over- community indulgence by have constructedFig. 3 to illustratean answerto this looks among other things the indirect affect of lati- The dominantsshown in the account tude and the essential of closed question. figure independence genetic for the of the upper story coveragein (= is but preponderance systems species), peculiarly interesting their respectiveareas on the south-facing slopes of irrelevant to serious investigation of biogeography the mountain. the at and Sub-Mogollon Using histogram communityecology. the right of the figure compare, for example, the The recognition of the obvious ecologic forma- number of species as well as the particularspecies tions and their associations in North America is which can occur together at, say, 5000 feet with long-standing.Their further classificationinto "life- those, say, at 6500, and those at 8000 feet. The zones" is not necessary.The terms "biome," and graph illustratesthe recognition that the community "biome-type"(sensu Allee, et al., 1949) are also owes its existence in final analysis to the overlap of not necessaryto representthe equivalentwell-known the spans of the genetically controlled ecological ecologic ranks for which these terms, as well as (physiological and morphological) tolerancesof its August 1961 LOWE - BIOTICCOMMUNITIES 45

Figure 4.- Diagrammatic profiles of San Francisco and O'Leary Peaks from southwest to northeast showing the effect of geomass and slope exposure (Merriam Effect) on the vertical distribution of biotic communities. After Merriam (1890). Note distribution of ponderosa pine and of spruce (Picea). comprised species. This is a genetic climax inter- individual closed genetic systems of the comprised pretation (see Mason, 1947; Lowe, 1959). species primarilyinteracting with a numberof limit- This concept is embodied in the works of Shreve ing or controlling environmentalfactors which are (1915, and elsewhere), Turreson (1922), Gleason essentiallyphysical in nature. (1926), Mason (1947), Whittaker (1953), in the ENVIRONMENTAL FACTORS. - Thus far, recent desertinvestigations of Yang and Lowe (1956, two environmental factors have been mentioned 1957), and in the contributionsof others. Complex which may account for part of the association dis- biosociological relationships between species cannot crepancy phenomena as noted by Shreve (quoted even begin to operate unless this first condition of above), and by naturalistsin general, viz., latitude tolerance overlap is met in space and time. Here and slope exposure. Edaphic (soil) control and fire "autecology"and "synecology"merge into one. control are among additional obvious factors. The relative positions of the vertical limits of the tolerancesfor the shown in 3 are con- Exceedingly important but possibly less obvious species Fig. environmental conditions relative to the vertical sistent regardlessof what the particularslope exposure be on For placementand displacementof species and commun- may any particularrange. example, ities of and animals are as follows: the the lower limit for P. ayacahuitewill be plants (1) consistently total elevation that the mountain attains, the higher than for P. ponderosa when both occur on (2) the same mountain That for P. cembroides size or mass of the mountain (geomass), and (3) slope. the elevation of the basin or from which the will be lower than P. leiophylla, Q. etnoryi lower plain than ar/zon/calower than mountainrises. Merriam (1890) and Shreve (1922) Q. arizonica,Q. Q. hypo- were the first to call attention to these fac- leucoides, etc. among tors. The effect of slope exposureon plant and animal The orderliness involved in this predictabilityis distribution is related to mountain side It characterizesall of those biotic com- complexly striking.1 base-level, mountain height and mass. munitiespresented in Table 1. Neverthless, as already alluded the den- Merriam (1890) provided an excellent color pro- to, quantitativecharacteristics, e.g., file of the of the San Francisco and sity, frequency, coverage) and qualitative or syn- vegetation thetic characteristics fidel- O'LearyPeaks area north of Flagstaff, Arizona. The (e.g., dispersion,presence, zone to 8,500 feet elevation on more ity) of associationsmay be affected so seriouslythat spruce dips the association at be either massive San Francisco Peak (to 12,800 ft.) but is any given place may absent on smaller Peak which reaches to unrecognizable or actually dissolved. In arid and O'Leary semi-aridlands, particularlyin deserts, the nature of a at moment is given biocommunity any essentially, ^onal inversion, and interdigitation (as in canyons), pro- if not only, the result of the inter-actionsof the duce local variations (see Hall and Grinnell, 1919). 46 MC DONALD CIVILDEFENSE HAZARDS Vol. 2

9,000 feet.2 The profile also shows Merriam'sobser- Table 1. Ecotones occur between all categories in vation of the reverse situation in the case of the the ecologic hierarchy. The initial decision as to warmer and drier yellow pine belt which encircles whether or not a given situationin the field is eco- the peaks.P. ponderosaonly reachesto 8,200 feet on tonal, and subsequent decisions as to the spatial more massive San Francisco,but up to 8,800 feet on limits of the ecotones (and, therefore, the limits to smaller O'Leary (Fig. 4). These distributions are the biotic entities concerned), are decisions which results of what may be termed the Merriam Effect. have rarely been made with sufficient objectivity. Following the "spruce zone" southward into the Some recent recognition that the problem of the Sub-Mogollonregion we find it occurringas low as ecotone can and should be handled so as to obtain 9,000 feet on two massive ranges, the Grahams (to quantitative data for estimation of parametersis a 10,720 ft.) and the Chiricahuas(to 9,795 ft), while large step in the right direction. completely absent from the nearby Santa Catalinas, Mexican pine-oak woodland.- A recent major Rincons, Santa Ritas, and Huachucas, all of which contributionto bioecologyin the Inland Southwestis reach to over 9,000 feet, but all of which are also Marshall's (1957) excellent of birds of the of study much smallergeomass (Fig. 1). Similarsituations Mexican pine-oak community in the are for Sub-Mogollon found other specieson mountainsin the West. region (pine-oak woodland, in Table 1). This is an Returning again to Plate 1 in Merriam'sclassic outstanding contribution to avian bioecology. Mar- study (see Fig. 4), it will be noted that the fir zone, shall evaluated all of the birds and con- as is restrictedon pine-oak such, O'LearyPeak to the moisture cluded that the pine-oakwoodland lacks a distinctive protectednorth-facing slope (comparewith Fig. 5). avifauna. The species of birds most abundantin the This typical slope position for peak-pocketrelicts in pine-oak also occur its borders.Most of the the beyond West is found repeated for many genera and species are derived from the zones which on bordering species many rangesof varied size in New Mexico, are pine forest above and encinal below. Arizona, California,other states, and in Mexico; not The same is true for the and mam- are herpetofauna all, however, completelyrestricted to north slopes. mal fauna in Mexican Of the Some that are in mind are in Table 3. represented pine-oak. examples given several Madrean species of mammals and reptiles Recently we discovered a particularlyinstructive of of the Merriam Effect while on a (e.g., species Peromyscus, Nasua, Sceloporus, example working Eumeces, Elaphe, Crotalus) restricted in southern vegetation transectin the Tucson Mountains (Yang, Arizona and New Mexico to the "Sierra 1957). In this case, scrub oak outlying (Quercus turbinella) Madreanmountains" (Baboquivari,Santa Rita, Hua- barelysurvives as a population of a few individuals on to the north under Wasson chuca, Chiricahua,Peloncillo, Animas, Big Hatchet, hanging slope just and others) as distinct from the outlying "Rocky Peak (4,677 ft). The Tucsons (to 4,677 ft.) are a Mountains" Rincons and Santa desert to the much conifer-clad (e.g., Catalinas), range adjacent larger, none appear to*be adaptive constituentsof a clear- Santa Catalinas (to 9,163 ft); the two ranges are cut Mexican biotic Mexican about a 10-mile minimum between their pine-oak community. separatedby pine-oakwoodland is essentiallya broadecotone with 1000 meter base levels. Quercus turbinella also oc- a of and animal which curs on the SantaCatalinas, to paucity plant species might dipping approximately otherwise strongly characterizeit in its relatively 3,900 feet elevationon the north side. The difference mid-elevation in elevation for the lowermostoccurrence of the oak high position. on the two mountainsis about 700 feet The following statementon this problem, written (Fig. 5). as a first some time The Merriam Effect on the ecologic distribution impression ago (Lowe, 1952), of is here. Three contains the essence of what we have more recently populations nicely exemplified found: typical aspects of the ecological restriction imposed on the same where it occurs "Mexican Pine-Oak Woodland (see Leopold, genetic system (species) is restricted in in the on the smaller of two adjacentmountains are clearly 1950) highly development seen in this On the smaller Santa CatalinaMountains, which are near the north- example (see Fig. 5). ern limit of ChihuahuaPine chi- mountain,the populationoccurs at a higher minimum (Pinus leiophylla it occurs on the north huahuand) and just beyond the northern limit of elevation, protected -facing Pine Pine-Oak be con- slope of the highest peak(s) and the maximum size Apache (P. latifolid). may of the is small with sidered either (a) a distinctive vegetation type, or population exceedingly (compare an ecotone. The facts are observable: pine and spruce in Fig. 4). (b) following There can be little that such differences (1) Pine-Oak is a climax over wide area, including question area in northern Mexico than in southern in vertical distributionof populations are related to greater the MerriamEffect. Whether or not also be Arizona and New Mexico; its ecological metropolis they may is in the SierraMadre Occidental and related to Whittaker's (1956) post-glacial warming outlying ranges remains to be determined. of northwesternMexico. (2) The physiognomy of hypothesis this is that of an Oak-Woodland - Con- ECOTONES.- The ecotones are not listed in vegetation iferous Forest ecotone. (3) The floristiccomposition of its life-form dominantsis distinctivein part (e.g., 2Elevations of Merriam (1890, Plate 1). ChihuahuaPine and Apache Pine). (4) There are - August 1961 LOWE BIOTICCOMMUNITIES 47

vetrebratespecies which appearto be characteristically (in part) is consistent with the view that Pine-Oak Pine-Oak [e.g., Painted Redstart(Marshall, 1957)] is a distinctiveclimax vegetation type, ratherthan an although the number is not large and not yet com- ecotone between two vegetation types, it tends to pletely known. The majority, however, are animals favor the concept of ecotone for this vegetation. To predominantlyof the Oak-Woodland positioned be- consider it an ecotone, in its restricteddevelopment low it, and the Yellow Pine Forest positioned above. on the SantaCatalina Mountains of southernArizona "Distinctive characterizationby floristicallydiffer- does not contradictthe conceptof a distinctiveclimax ent climax dominants over wide geographic area vegetation type for Mexican Pine-Oak Woodland." favors the conceptthat MexicanPine- Oak Woodland On the Santa Catalinas (Lat. 32° 15 N.), pine- is a distinctiveWoodland vegetation type as is, for oak woodland is an example of a weak transition example, Pinyon-JuniperWoodland, Oak Woodland, association between the most mesic portion of the and Chaparral.(It may be noted that such floristic EvergreenWoodland Formationand most xeric por- differentiationis not necessarilyat complete variance tion of the ConiferousForest Formationin northern with the ecologic concept of ecotone as employed by Mexico and adjacent Arizona and New Mexico. some general ecologists.) The concept of Pine-Oak Southward,for example on the SierraOposura (Lat. as an ecotoneis favored by the evidence of vegetative 30° N.) in Sonora, if pine-oak is still an example life form (2, above), and also by animal distribu- of an inter-formationalecotone, it is one which tion in part (4, above). While animal distribution there exceeds in total geographicarea either one or

Figure 5.- Diagrammatic profiles of the Santa Catalina Mountains (after Shreve, 1915) and the adjacent Tucson Mountains, on a line northeast-southwest through Mt. Lemmon and Wasson Peak, respectively. The Merriam Effect on the vertical distribution of scrub oak (Quercus turbinella) is shown by arrows indicating the lowermost occurrence of the species on the two mountains. The small black area near Mt. Lemmon, indicated by the arrow, is the location of several individual corkbark firs (Abies lasiocarpa) all of which occur above 8,350 feet in the well-developed Abies (concohr; lasiocarpa) -Pseudotsuga fir forest which is restricted to north-facing slopes above 7,600 feet. 48 JOURNAL OF ARIZONA ACADEMYOF SCIENCE Vol. 2

the other, or both, of the two formation-typesin- history, climatic control, structure and function as volved (see Marshall, 1957, Fig. 18). subtropical thorn forest4 in Sinaloa and Sonora, Encinal.- The encinal associations at first pinyon-juniper woodland in Colorado and Utah, may in New Mexico and Texas and desert in appear to be ecotones in as much as species of grassland and Pinus are the most California and Nevada. An obvious difficultywith Quercus, Juniperus, impor- Dansereau's abstraction"biochore" is summed tant dominants in the vegetation. On the contrary, partly the term encinal biotic communities dis- up in his own words (p. 81): "The biochores are represents too to be useful: to climatein too tinctive from pinyon-juniper, oak woodland, and large they respond in their climatic generalized a way to contain vegetation of a truly chaparraltypes, evolution, control, character." genetic composition,structure and physiognamy.The homogeneous associated animals and plants are characteristicand Axelrod (1950, 1958, and elsewhere) has recently the communities are marked to a greater or lesser described the composition, distribution, and evolu- extent by dry-tropicspecies of Acacia, Calliandra, tion of the extensive woodlands of western North Mimosa, Agave, Nolina, Yucca, and others. The America,present and past, which Dansereau(1958) modern encinal and Mexican pine-oak associations would bury under "Savana."Weaver and Clements correspond to the Arizonan Woodland Component (1929) buried them less ceremoniously under (part) of the Madro-TertiaryGeoflora (Axelrod, "Forest." 1950, 1958). Today they constitute communitiesin SUMMARY AND - Axelrod's broad Sierra CONCLUSIONS. Five (1958) Madrean Woodland world are in under a bi-seasonalrainfall dominated the south- ecologic formation-types represented by the Sub-Mogollon region of the Inland Southwest, western summer monsoon from late June into early October with in namely, tundra, forest, woodland, grassland, and peak precipitation either July or desert. The terms biome and are not August. biome-type preferable as substitutes for long standing ecologic The term encinal was first proposed by Shreve formation and formation-types.Within the world (1915) who distinguished a lower (open) encinal formation-typesthere are a number of formations and an upper (dense or closed) encinal on the (= biomes) which are arrangedwith reference to Santa Catalina Mountains. Until recently, the term Merriam'slife-zones in the Inland Southwest. encinal has been little used since Shreves time. In In the Sub-Mogollon region, as elsewhere, the recent years, biologists studying bioecological problems in recognizable biotic community, or any natural as- the Sub-Mogollon region (Lowe, Marshall, of have semblage plants and animals of whatever scope Martin,Phillips) recognizedits utility for plug- or more can a Encinal is a distinctive (two species), occurat any one point in ging terminological gap. and time as the of continuum on a vertical at mid-elevations. space only result the coincidence gradient of the of the controlled eco- It is dominated by the evergreen woodland "big amplitudes genetically oak logical tolerancesof the comprisedspecies. The nature three," namely, (Quercus*),juniper (Juniperus), of the climax is most revealed the and (Pinus). from its lowest genetic clearly by pinyon Progressing study of and animal distribution elevationallimit on the mountain to its highest (see plant population over relatively relief. Fig. 5), the encinal usually changes slowly from an sharp oak-grasslandscape upward through a relativelyopen The term Merriam Effect refers to an indirect oak-juniper associationto a more dense oak-juniper- environmental factor complex affecting association pinyon woodland into a relativelyclosed pinyon-oak- discrepancyphenomena within the genetic climax. juniperwoodland of varyingcomposition and density. This is briefly discussed as are some other better The encinal is a unique climax woodland continuum known environmentalfactors affecting both vertical in southern Arizona-New Mexico and Chihuahua- and horizontal biotic gradients. it is not an ecotone. Sonora; Some problems regarding southwestern inter- Woodland and savana.- Some of the broadest formational ecotones are discussed. Ecotones are ecotones in the Southwest occur between woodland prominent features of the southwestern landscape, and grassland and between desert and grassland. where, as inevitable results of the genetic basis of These and parts of the desert grassland (mesquite- the climax, they occur between all adjacent biotic grass, for example) are the closest approaches to communitiesregardless of rank. savanawhich occur in the Inland Southwest. Notwithstanding, Dansereau (1958) would include under a so-called "Savanabiochore,"3 the fol- 3Dansereau (1958 and elsewhere) would divide world vege- lowing diverse concepts: "... savanawoodland . . . tation on a structural basis into "four large units of equal thorn forest and scrub . . . savana . . . magnitude" and call them "biochores," as follows: forest, half-desert and desert. . . . heath . . . cold woodland." For western North savana, grassland, Americahis "Savanabiochore" includes biotic com- 4This is one of the most immediately obvious errors of mis- classified American placement by Dansereau. The semi-arid diciduous thorn munities, by ecologists, which are forst and short-tree forest (sensu Gentry, 1942) fall in as completely unlike and unrelated in evolutionary "Forest Biochore" rather than "Savana Biochore." August 1961 LOWE BIOTICCOMMUNITIES 49

LITERATURE CITED

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