Introduction

 The regional vegetation of the San Pedro Valley has experienced significant change through time, much of this a direct result of centuries of human land use and environmental manipulation.  In particular, the area around the large precolonial and early colonial-period pueblo LA 162, or Paa-ko (see Lycett, this session), has experienced some six episodes of significant vegetation change within the past seven or eight hundred years.  As part of a larger program of archaeobotanical and paleobotanical analysis in and around this important settlement, the authors analyzed a stratigraphic pollen record from low-energy silt and clay deposits located to the east of the South Division roomblock, in an area posited to be an old field location.  This record, described here, shows a sequence of dramatic vegetation change associated with arroyo cutting, erosion, deforestation, agriculture, the introduction of European weeds, and afforestation.  Pollen results reported here include a taxonomic summary (Morrison) as well as Poaceae pollen size statistics (Arendt).

Research Questions

 Stratigraphic pollen analysis was designed to complement ongoing work on archaeological pollen, especially the spatial analysis of pollen imbedded in superimposed colonial-period plaza surfaces and found in associated features.  These contexts showed, at least in the two uppermost historic plaza surfaces, a consistent presence of European-introduced weedy taxa (most notably, Plantago lanceolata, P. major, Tribulus terrestris, and Convolvulus) as well as pollen of indigenous plants of economic importance (Zea mays, Cleome, Berberis, etc.).  However, the overall historical context of both vegetation assemblages and plant introductions was not known.  Specific questions asked of these data include:

Analysis

  Twenty-one pollen samples were taken at 5 cm intervals from a freshly-cut section of the excavated unit.  The diagram presented here is based on analysis of 13 samples from throughout the sequence.  Pollen was extracted using standard techniques of Quaternary paleoecology, including acetolysis and HF extraction.   Acetolysis, in particular, can affect grain size and thus Poaceae size data should be considered in that light.  All samples were mounted in silicone oil.  Again, mounting media can affect grain size; size comparisons should be made only within these processing parameters.

  Pollen identifications were made against the University of Chicago Paleoecology lab reference collection. A minimum of 200 and a maximum of 600 grains per sample were counted.  Although Lycopodium spikes were used, here we report only on overall percentages.  Pollen data presented here are in summary form, grouped into ecologically significant categories.  Degraded grains are not included as part of the pollen sum.   All complete Poaceae grains were measured to the nearest .5 µm although this poster reports only on a more limited sample of grass pollen measurements.   Crumpled and folded grains are not included in the size statistics.
 

 The Sample

  LA 162 is located in the eastern foothills of the Sandia mountains, at an elevation of 1975 m (6500 feet), near the upper limit of the Pinon-Juniper woodlands but slightly below the transition to Ponderosa pine forests.   This region, including the pueblo itself,  was intensively grazed until the 1930s.  Although the site was fenced once it became state property, effective protection from grazing pressure dates back less than ten years.  Recent housing development in the area does not seem to have had a significant effect on forest cover and it is too early to say what specific changes this will make in the pollen record.

  Pollen samples were taken from unit 137E/-248N, a one by one meter test unit excavated in 1998.  This unit lay to the east of the South Division roomblocks, in a level area now covered with a relatively lush meadow.   The vegetation in this area varies from year to year with annual rainfall, but in general includes a range of grasses (Poaceae), herbaceous plants including Physalis, Monarda, Cleome, Solanum elaeagnifolium, Portulaca, Curcurbita foetidissima, and a variety of Asteraceae (Compositae).  Shrubs are more rare and include Chrysothamnus nauseosus (Chamisa, rabbit brush), Gutierrezia sarothrae (snakeweed), and Lycium pallidium (wolfberry).   In addition, we have observed  Triticum (wheat) growing wild in this area.   The San Pedro arroyo, less than 100 m to the east, is now deeply downcut.  It supports limited stands of Salix (willows), Populus fremontii (cottonwood), and other indigenous and introduced (e.g. Tamarisk, or Salt Cedar) riparian taxa.
 
 

 Introduction

  Although the Spanish brought a range of Old World cultigens such as peaches to the Southwest, as well as novel Mesoamerican cultigens such as chiles, perhaps the most important Spanish-introduced cultigens were food grains, especially wheat and barley.  Macrobotanical analysis of seeds from LA 162 reveals a large number of cobs, kernels, and cupules of Zea mays but only one (half) grain of wheat.   Other introduced food grains include an anomalous cache of wild rice grains (Zizania) in a feature in the third historic plaza surface (A3), a plant not grown anywhere in the region.  Thus the macrobotanical evidence suggests that the inhabitants of LA 162 continued to rely on corn-based agriculture and that they did not have significant access to European grains.   In order to follow out this possibility and to investigate more specifically which grains were being grown throughout the human occupation of the region, we undertook detailed size analysis of all grass pollen.

All introduced European domesticated grains, as well as Zea mays and a large number of indigenous and introduced wild grasses belong to the family Poaceae (formerly known as Gramineae).   Although some morphological differences between the pollen of different grass taxa have been noted (chiefly in exine sculpturing and pore size and morphology), in general the pollen of all grasses  appears very similar under the light microscope, differing primarily in size.  Unfortunately, size differences are generally insufficient to make species or even genus-level identifications.  The sole exception to this generalization is Zea mays, which tends to have extremely large pollen grains.  There are, however, consistent size differences between broader groupings of grasses such that the pollen of the wild grasses tend to be smaller than that of the European domesticates (usually called the Cerealia group), which are in turn smaller than Zea mays.  There are some exceptions to this trend, but the specific wild grasses with very large pollen grains that have confounded analysts in other parts of the world appear not to be present in western North America.  The presence or absence, then, of European grains should be apparent in Southwest pollen records.

  Thus, one way to investigate the presence of broad categories of grass pollen in the environment (wild, Cerealia, maize) is to conduct systematic size comparisons.  This procedure is, however, complicated by the fact that the size ranges of these three categories overlap significantly, so that no individual grain can ever be identified with full confidence-aggregate analyses are always necessary and thus large sample sizes and population-level distinctions are required.  The common use of cut-off values for grass pollen categories tends to ignore the tails of actual size distributions, which follow the normal curve.  Cut-off values provide a falsely concrete identification of individual grains which may, in most cases, potentially belong to one or more general categories.

 Procedures

  A sample of reference pollen grains from the University of Chicago Paleoecology Lab, all subjected to acetolysis and mounted in silicone oil, were measured for comparison with the archaeological samples.  At least 200 grains of each reference samples was measured to the nearest .5 µm (Table 1).  Only complete, uncrumpled grains were measured.  The wild grasses studied had diameters between 12.5 and 52.5 µm while the Cerealia pollen (here represented by Sorghum bicolor) had diameters between 37.5 and 67.5 µm.  Curiously, the Zea mays studied here had diameters ranging from 26.3 to 80 µm, significantly smaller than the usual reported range of about 85 to 120 µm.  We are continuing to compile population-level size data for different corn varieties and this sample may either represent immature grains and/or a variety with anomalously small grains.  In any case, it does seem clear that most reported size ranges for Zea mays are unnecessarily restricted and that this is potentially a much greater range of size variation in this species than has previously been appreciated.   There is considerable overlap between these three categories, so assigning individual pollen grains to a category is not possible, although if one or more of these three categories are present, it should be apparent in the distributions of diameters as indicated by statistical tests.

Grass pollen grains from the stratigraphic column were studied using slides prepared for the more general taxonomic analysis.  Although additional grains have since been measured, we report here only on the 826 grains measured by Arendt.  The samples studied here come from eight of the original 21 samples, as compared to the 13 samples comprising the overall diagram. The overall mean grass pollen diameter from all levels is 20.0 µm ±0.32 with a standard deviation of 4.6, suggesting that, as expected, wild grasses predominate in all levels.  Even though the meadow containing 137E/-248N has been tentatively identified as the location of agricultural fields during one or both of the main occupational episodes at LA 162, if these fields were focused toward maize production, we still might expect the pollen of wild grasses, more prolific producers, to dominate the assemblage.  Both the Cerealia and Zea mays, should they be present, can be expected to appear in low concentrations.

  Depth-wise comparisons show minimal differences between levels though there are differences in the distributions of larger-sized outliers (Table 2).   The sample from -8.581 m bd occurs relatively late in the second clearance phase, when trees were already making a slow comeback (see main panel).  This level has the largest number of outliers, followed by -8.151 m bd, in the Pinon-Juniper woodland.  In general, however, there are not great differences between levels.  Because of the high positive kurtosis of these samples, a  Kruskal-Wallis ANOVA test was performed, p=0.0631.  At a .05 significance level, the null hypothesis cannot be discarded, and there thus appears to be little significant difference between levels.
 

Conclusion: Grass Pollen

  It does not seem that the inhabitants of LA 162 ever grew wheat or other European food grains.  In addition, we recovered very few grains within the size range of Zea mays, a finding consistent with other studies carried out in field contexts.   In contrast, pollen from the historic plaza surfaces was relatively much richer in Zea mays-sized pollen and macrobotanical remains of corn were abundant in the site, suggesting that low numbers of very large grass grains does not necessarily indicate an absence of corn cultivation.  These results should be considered preliminary; ongoing research with larger sample sizes should improve our ability to make stratigraphic distinctions within this core.

 European Plants: 137E/-248N and the Historic Plaza

  Although the pollen record suggests that there was never any cultivation of  introduced plant domesticates in this region, European weeds did establish themselves early in the colonial period and, along with the grazing animals they seem to have followed, have significantly affected the herbaceous flora of northern New Mexico.

  In the 137E/-248N stratigraphic sample, introduced taxa were found only in the two uppermost levels: one grain each of Plantago lanceolata (ribwort plantain) and Tribulus terrestris (goathead) at -8.151 and one grain of Plantago major at -8.221.  Both these levels fall into zone VI.  Although zone IV is posited to represent the initial part of the second clearance phase, corresponding to the colonial-period reoccupation of Paa-ko pueblo, no introduced taxa were found in samples from levels in this zone or in zone V.  In part, this may be a reflection of sample size, as pollen from introduced weeds may be expected to be quite rare in the early years of their introduction to the region.  This supposition is borne out by ongoing analysis of pollen grains from the three superimposed historic  plaza surfaces of the north division (Lycett, Johansen, this session.  Here, deposits from a relatively short span of time (early to mid-seventeenth century) have been subjected to intensive pollen analysis such that contemporaneous sample sizes total at well over 6000 grains.  At present, only surface 1 (the uppermost "dung surface" consisting largely of animal dung and representing later use of the plaza as a sheep camp) and surface 2 have been fully analyzed.  Surface 3, the lowest complete historic plaza surface, is only partially analyzed.

In this work, we see systematic differences in grass sizes, reflecting the specialized nature of the dung surface.  Zea mays-sized pollen is also represented in much larger numbers on the plaza, especially in specific features which may have been used for storage.  Further, there is a good representation of introduced plants on surfaces 1 and 2, primarily weedy taxa that may have been accidently introduced along with grazing animals and whose spread may be attributed, in part, to increased grazing pressure on indigenous taxa.  As noted above, the only evidence for European domestic plants at LA 162 is a single charred grain of wheat although there are abundant remains of animal domesticates (Sunseri and Gifford-Gonzales, this session).  The plaza pollen should be approximately contemporaneous with zones IV and V.
 
 

  The stratigraphic pollen record from 137E/-248N shows a complex record of vegetation history, much of it attributable to centuries of human action.  The lowest part of the core, Zone I, represents a riparian vegetation association and suggests that the floodplain of the Arroyo San Pedro was once more extensive than it is now, extending as far as the sample unit.   This further suggests more recent arroyo cutting.  Following the decline of the riparian vegetation, Zone II represents a period of forest clearance and of open, disturbed vegetation associated with the precolonial occupation of Paa-ko pueblo.  This was the period of maximal population size at the site and it is probable that maize agriculture was the mainstay of agricultural production in the area.

Zone III is a brief period of forest regeneration, not, however, equivalent to the heavy cover of Pinon and Juniper now present at this elevation.  Zone IV, the second clearance zone, represents a return to a more open vegetation and appears to correlate with the colonial-period reoccupation of the pueblo.  In this period, erosion is severe, a trend associated perhaps with new grazing pressures as well as with removal of the forest cover.  Erosion is indicated by more rapid sediment infill, a high proportion of degraded pollen grains, and the presence of large numbers of pre-colonial ceramics washed down from the south division roomblocks.  Although there are in this sample no introduced European weeds, data from the historic plazas suggests that they were present at this time.

Zone V begins a slow, steady afforestation, perhaps coincident with declining regional populations and a transition to more land-extensive forms of production.  This trend culminates in Zone VI, when the present Pinon-Juniper woodland became established.  Historic records document that this latter period was a time of extensive grazing and use of the region for cattle ranching; the present development of the area for housing has yet to make a significant impact on the pollen record.

Grass pollen size studies show that the colonial-period inhabitants of LA 162 did not grow, at least to any significant extent, European domestic grains such as wheat or barley.  The macrobotanical record confirms the limited access of the site's inhabitants to such exotica.   Both the presence of European animal domesticates, however, and the growing importance of introduced weedy taxa, point to significant landscape changes associated with the introduction of grazing animals to the area.