The Caddo Western Boundary: Climate, Risk, andSettlement Patterns

Mary WhisenhuntOrigins of Agriculture

In the Northeast Texas region formerly occupied by the

Caddo Native American group, southwestern settlement

extended to, and largely ended at, the Neches River basin,

with only a few sites found further west on the eastern

floodplain of the Trinity River. That southwest border

coincided with the end of the Pineywoods, and the start of

the Post-Oak Savannah, ecotones. Both environments support

the farming of maize, a long-time contributor to the Caddo

diet. But if agriculture was feasible in multiple ecotones,

why didn’t the Caddo move further west and settle more

permanently in the Trinity River basin, or even further

west? In this paper, I examine some of the environmental

factors that may have influenced this western limit,

evaluating them against the archaeological record, Caddo

dental pathologies and recent stable isotope analysis of the

Caddo diet from those living in the Pineywoods, Post-Oak

Savannah, and the Blackland Prairie, where the northwest

boundary of the Caddo extends. I also explore paleoclimatic

considerations, as well as the possible impact of

precipitation variability on farming between the Pineywoods

and Post-Oak Savannah environments.

I then consider several possible constructs that could

explain or predict the spread of agriculture into the Post-

Oak Savannah region and its boundaries, including behavior

ecology, niche construction, Ideal Free Distribution (IDF),

and a terrestrial model of intensification in optimal

foraging theory. I outline the role that the desire for

predictable access to resources may have played in Caddo

decisions related to its borders and intensification

strategies. Based on modern data, rainfall measurements at

or west of the Trinity River Basin were somewhat lower than

those in the Caddo heartland. Those lower levels would have

been more sensitive to variability, potentially affecting

scheduled maize production. Agriculture west of the Trinity

likely would have been a riskier enterprise than in more

easterly environs, affecting Caddo settlement patterns.

texasbeyondhistory.n

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The Pineywoods, Post-Oak Savannah, and the Blackland

Prairie ecotones frame the western boundary of the Caddo.

Post-Oak Savanna regions have mostly claypan soils, with a

loamy or sandy surface layer and a more “clayey” subsoil.

Today, these areas are used mostly as improved pasture or as

rangeland, but also serve as habitats for a wide range of

wildlife including deer (USDA 2002:15). The flood plains

along the Trinity River have clayey, very slowly permeable,

and somewhat poorly drained soils. The U.S. Department of

Agriculture considers these soils to be poorly suited for

cropland because of frequent flooding (USDA 2002:19). The

Blackland Prairie has dark-colored loamy and clayey soils;

native vegetation is mid and tall grasses. The Pineywoods

has dominantly light-colored sandy and loamy soils and

native vegetation of oaks, pines and shade-tolerant grasses

(USDA 2002:11). Today, the land is used mostly for growing

commercial pine timber and for woodland grazing. Lime and

fertilizers are necessary for productive cropland and

pastures (Texas State Historical Association).

A major vegetative ecotone separates the pine forests

from the post oak regions in the southwestern boundary area

of the Caddo territory. Sorenson notes that this ecotone

coincides roughly with the winter position of the frontal

zone which separates Pacific and Maritime Tropical air

masses, suggesting a causal relationship between climate and

vegetation (Sorenson et al. 1976:141-142). The differences

between coniferous and deciduous/prairie environments is, in

part, due to seasonal climatic limitations of temperature

and moisture imposed by the dominant air passes of the Gulf

coastal region (Sorenson et al. 1976:143). That same

climatic variation would naturally impact not only the type

of wild resources available to Caddo inhabitants in each

zone, but could potentially impact cultigen success rates.

The Trinity River, whose northern half is roughly

analogous with the southwestern edge of the Caddo territory,

begins in Cross Timbers and Prairies vegetational areas,

extends through Blackland Prairies, Post-Oak Savannah, and

Pineywoods, ending in Gulf Prairies and Marsh ecotones. The

original vegetation of the Trinity River Basin was

bottomland hardwood forest, though farms and ranches are now

present there (Nixon et al. 1990:97). There is a

precipitation gradient from north to south, ranging between

89 cm in the upper basin to 135 cm in the lower basin (Nixon

et al. 1990:99).

While climate and other environmental factors are

clearly not singular factors in subsistence strategies,

culture change, and settlement patterns, there is little

doubt that they play a role. Global climate reconstructions

over the last 2000 years show relatively warm conditions

around 1000 AD, and then a relatively cold

Environmental Settings and Hydrologic Conditions in the Trinity River Basin (USGS Publication, http://pubs.usgs.gov/circ/circ1171/html/envhyd.htm)

period, or Little Ice Age, centered around 1700 AD. The

existence of a Little Ice Age from roughly 1500 to 1850 is

based on a variety of evidence, including pollen cores,

dendrochronology, glacier length records, and historical

sources (Committee on Surface Temperature Reconstructions

for the Last 2000 Years 2006:2). Data compiled by Collins

and Bousman suggests that modern climatic conditions were in

place about 3,000 years ago (Fields 2004:347).

Perttula describes how a 14,000-year stable isotopic

record from the Trinity River basin demonstrated significant

climate fluctuations over the millennia, with wetter

climates from 11,000 to 7500 B.P., again between 4000 and

2000 B.P., and after 1000 years ago. Brown’s oxygen

isotopic studies and Toomey’s pollen and fossil vertebrate

research tell a slightly different story. Their research

suggests that the Late Holocene period in North-Central

Texas was marked by two different environmental periods, one

from 5000 to 2500 B.P., and the other from 2500 to 1000 B.P.

(Brown 1998:167). Per Toomey, the earlier period culminated

a trend of general decreases in moisture, constituting the

driest phase of any time in Texas prehistory (Brown

1998:167-168). From 2500 to 1000 B.P., however, conditions

returned to more moderate, well-balanced moisture levels and

an altogether cooler, wetter climate. After 1000 B.P.,

Toomey’s research showed a return to a warmer, more xeric

environment (Toomey et al. 1993:299-320).

Perttula’s research indicates that droughts were not

uncommon in this more recent period. Dendrochronological

data from the past 1000 years shows evidence of many wet and

dry spells, with the worst droughts occurring in the late

A.D. 1200s, in the mid-1400s and 1600s, and in the mid-1700s

(Perttula 2004:370-371). Frequent dry conditions, both

between years, and even by season, could have significantly

impacted Caddo farming opportunities.

Brown examined four years of modern isotopic samples in

the Waco area in order to examine whether oxygen isotopic

values would show seasonal variation in north-central Texas.

He found that seasonal precipitation levels did indeed

affect those values, and that significant variation occurred

by season even within even a short, four-year span of time.

Brown’s research on climate change in Texas suggests that

seasonality of rainfall may be as important a variable in

climate as the amount of total rainfall—key to our

investigation of risk factors in the western Caddo

territories. Seasonality of rainfall can, Brown asserts,

dramatically affect vegetation in various ecotones,

potentially affecting not only wild plants and fauna

availability, but also cultigen success rates (Brown

1998:169).

One of the challenges in determining causal factors

involved in agriculture and Caddo boundaries is the lack of

maize and wild resource return rates by ecotone. Such data

would enable a more accurate assessment of the level of crop

availability and risk east and west of the Trinity River

basin. At a very general level, Nixon et al.’s research on

how species diversity correlated to precipitation levels

indicates that Post-Oak Savannah vegetational areas in Texas

had lower species diversity values than those in the more

moist Pineywoods vegetational areas (Nixon et al. 1990:104).

Wilson suggests that the Blackland Prairie had a relatively

poor natural resource base compared with other ecotones, as

it was dominated by prairie grasses (Wilson 2012:112).

As noted earlier in the paper, climate variability by

year and season, as well as precipitation levels, are

critical elements of the risk equation. James Hansen’s

research in Africa on dynamic seasonal climate prediction

models, ensuing crop yields, and land-use strategies provide

some intriguing observations that may be applicable in our

assessment of risk factors faced by the Caddo.

Hansen notes that climate has a critical impact on the

lives of rural subsistence farmers, particularly those

residing in marginal areas that are particularly susceptible

to weather and rainfall hazards (Hansen 2005:2037).

Variability in year-to-year rainfall is a serious impediment

to sustainability of rain-fed agricultural efforts.

Further, climatic extremes such as flooding or drought—both

factors potentially of concern on both sides of the Trinity

River—can have serious impacts on farmer livelihood.

Perhaps even more pertinent, Hansen suggests that the

uncertainty associated with climate variability actually

poses the most serious risk to farming sustainability

(Hansen 2005:2038). And within a paradigm of uncertainty,

crop success is more directly tied to precipitation levels

within a season, rather than on seasonal averages. Barrett

notes that the lack of predictable rainfall, in particular,

impacts risk-averse smallholder farmers—again, potentially

reflecting a Caddo agriculturalist template (Barrett

1998:1109-1112).

Understanding precipitation levels and the associated

risks and opportunities vis-à-vis cultivation in the Post-

Oak Savannah and Pineywoods areas is key to analyzing why

maize agriculture and Caddo settlements were few and far

between west of the Trinity River. One avenue to consider

is the premise that agriculture was tied to the Caddo’s

desire for increased food source predictability as a

mechanism to avoid risk. Marshall and Hildebrand’s

ethnographic research on why Kenyan and Ethiopian hunter-

gatherers adapted their subsistence strategy to include

cultigens may be particularly useful in exploring relative

levels of risk by region. In both case studies, the authors

found that people wanted to be able to schedule use of

certain resources in order to manipulate plants (and

animals) to ensure predictable access (Marshall and

Hildebrand 2002:102). This predictability may be affected

by climate and moisture; unpredictable rainfall causes great

variation in the productivity of African savanna ecosystems.

Marshall and Hildebrand note that fluctuations caused

by drought have been especially noticeable in arid regions

with highly variable precipitation levels (less than 30 cm

annually), with coefficients of variability often exceeding

30% (Marshall and Hildebrand 2002:106). They, like Hansen,

conclude that in more marginal areas, crops are more at risk

when experiencing even minor variations in rainfall, than

they are even in areas of more acute—but predictably acute—

aridity. In North Africa, for example, there was sufficient

rainfall for arid-adapted plants to survive and produce, but

not enough for them to be considered reliable resources.

Humans have to be able to plan, or schedule, plant resources

(Marshall and Hildebrand 2002:111-112). If agriculture

cannot be reliably scheduled, it becomes a less viable

subsistence option. Marshall and Hildebrand further suggest

that plant productivity is especially vulnerable to

variation in rainfall because of the importance of timing of

precipitation relative to plant growth phases--particularly

in maize agriculture (Marshall and Hildebrand 2002:112).

Weather variability tends to have an outsized impact on

localized agro-ecological niches. Under adverse growing

conditions such as environmental stress, maize row numbers

and size of corn ears tend to decrease. Eight-row corn

identified from the George C. Davis site, close to the

western edge of the Caddo territory, though 10- and 12-row

corn remains were apparently the most common form found.

The smallest and least productive corn ever found is from

the Prairie Caddo (non-Caddo) McGuire’s Garden site in the

Post Oak Savanna (Perttula 2008:79-80, 87, 93). Systematic

research into variability of regional and temporal variation

of maize phenotypes could provide insights into the impact

of climate-induced stress as well as regional and temporal

variation.

Dendrochronology records suggest that an extended

period of drought between A.D. 1430 and 1476 occurred in the

southern Caddo region, coinciding with what may have been

significant changes in Caddo settlement patterns (Perttula

2012:89-91). Around A.D. 1430 in the Late Caddo period, an

influx of Caddo farmers into the Pineywoods ecotone occurred

along the Big Cypress Creek, Sabine River, and Sulphur river

basins. At the same time, there is evidence of residential

movement from much of the Post-Oak Savanna. Perttula and

Rogers note that this emigration of Caddo people from more

marginal settings may have been the result of a period of

cooler, dryer weather that resulted in repeated crop

failures (Perttula 2012:91, Perttula and Rogers 2007:91).

Further, the archaeological record does not suggest that

Caddo settlements ever moved back to marginal, outlying

areas that had once been settled by Caddo farmers. Rather,

it demonstrates significant levels of emigration of from

west to east across Caddo territory (Perttula 2012:93-94).

So was there, in fact, too much multi-year and seasonal

variability in rainfall levels west of the Trinity to ensure

maize crop reliability? To better understand this dynamic,

I compare current temperatures and precipitation levels

between the area west of the Trinity (vicinity of Fairfield,

Freestone County, Texas) and the Caddo heartland (vicinity

of Oak Hill, Rusk County, Texas), using U.S. Department of

Agriculture data from the period 1961-1990. Fairfield has

an annual precipitation of 101.14 cm (39.82 inches) as

compared to Henderson’s 114.5 cm (45.08 inches). In terms

of year-to-year variation, in two out of every 10 years,

Fairfield will have annual rainfall of less than 30.21

inches, as opposed to Henderson’s 35.3 inches. One also

sees seasonal variation in rainfall levels, per 2013 figures

found on weatherdb.com.

Further research designed to compare variation in

precipitation levels on a year-by-year basis, and by season

as well, is required to explore this phenomenon. The use

of statistical modeling will provide proxy data that should

help us determine how much risk the Caddo faced in

agricultural endeavors west of the Trinity, and whether or

not it was a relevant factor in westward expansion.

Paleobotanical data suggests that the process of maize

adoption in the region took place over several centuries to

nearly one thousand years, with maize arriving in Caddo

territory no earlier than A.D. 400. As Wilson and Perttula

note, the paleobotanical record indicates that maize didn’t

replace domesticated weedy plants, nuts, seeds and animal

foods in the Caddo diet until around A.D. 1300, when it was

intensively grown and became their most important food

source (Wilson 2013:3). From around A.D. 1450 to 1800, the

paleobotanical record indicates an overall reduction in wild

plant utilization, a de-emphasis on nuts, and a higher

reliance on beans, squash and gourd (Perttula 2008:94).

According to Rose et al., prior to A.D. 1100, maize

consumption among Caddo groups was less than 15 percent of

their diet. Recent stable isotope analysis from human

remains conducted by Perttula et al. indicates that after

A.D. 1200, the beginning of the Middle Caddo period,

consumption became uniformly high, peaking after around A.D.

1650 in the Historic Caddo period (Wilson and Perttula

2013:5; Pertulla et al. 2013:1). The greatest changes in

consumption patterns occurred between the Middle and Late

Caddo periods. While isotopic values show more corn in the

diet during the Late Caddo period, there is also significant

variability, indicating that some individuals ate

considerable amounts, while others ate little. Perttula et

al. suggest that such variation may reflect differential

maize production on family agricultural plots (Perttula et

al. 2013:6-7).

But other factors may be at work as well.

Environmental differences on a rather broad regional scale

were likely responsible for some of the diversity, as Caddo

territory included coastal plains, mountains, and of course

the river valley alluvial planes that dominated the

southwestern border area. Wilson and Perttula assess that

intensification of maize resources appeared highest in the

productive, broad alluvial valleys and Gulf Coastal plains

of Arkansas, Louisiana, Oklahoma and Texas. And as maize

production intensified throughout the Caddo territory, the

Blackland Prairie was abandoned in favor of more

agriculturally productive lands elsewhere (Perttula

2008:79).

Maize ubiquity values indicate that the use of maize

was more intensive in the eastern part of the Caddo area

than among the Caddo living along the western edge of the

eastern Woodlands (Perttula 2008:92). Johnson and Hard note

that except in isolated instances, maize did not play a role

in Savanna and Prairie regions (Johnson and Hard 2008:139).

Perttula also compared the stable carbon isotope values of

maize found at Pineywoods and Post Oak Savannah sites.

Evidence suggested significant variation, with lower C4

enriched maize values found at Post Oak Savannah locations.

Perttula suggests that subtle environmental differences

across the region may have resulted in depleted or enriched

stable isotope values in C4 pathway plants at different

times and places, depending on environmental stressors such

as drought (Perttula 2008:98).

Specific, comparative return rates and frequency of

wild and domesticated resources for the specific ecotones

are lacking in our assessment. Further, the paleobotanical

record as it stands often doesn’t match the

bioarchaeological evidence (Wilson 2012:88). Differential

preservation rates and the use of maize as fuel in smudge

pits may have caused it to be over-represented (Perttula

2008:88). Therefore, it makes sense to compare the relative

role that maize played in the diets of the early inhabitants

of the Post-Oak Savannah, Blackland Prairie, and Pineywoods

by examining stable isotope data and dental evidence.

Burnett subdivided the Southern Caddo area by these regions

and conducted bioarchaeological analysis of remains found by

ecotone, examining the relationship between maize

consumption and the physical environment. Burnett and

Rose’s findings suggested that the diets of Post-Oak

Savannah and Blackland Prairie residents showed a higher

dietary contribution of maize than did those from the

Pineywoods area, results that were later contradicted by

Wilson’s research (Burnett and Rose et al. 1998, Wilson

2012:88).

Contradictions exist even within this limited dataset.

Wilson examined dental remains of Caddo in East Texas,

finding that vegetative region was an important variable

only for the Blackland Prairie population, who had the

lowest caries and highest dental attrition rates of the

three regional groups. The results suggested less

dependence on carbohydrate-rich maize and more on grit-

producing wild nut resources (Wilson and Perttula 2013:4).

Pineywoods populations had lower mean dental wear.

Temporally, dental attrition showed that food processing

and/or diet was coarser in the Formative Caddo period and

remained so in the Blackland Prairie into the Early Caddo

period, while those residing in the Pineywoods either had a

softer diet, used wooden grinding tools, or both (Wilson

2012:91-92). These results suggest that local environments

impacted maize consumption more than temporal factors in the

southern Caddo area.

Biotic Region

Dental Wear

Delta C❑13

CollagenDelta C❑

13

ApatiteDelta C❑

13

Apatite-CollagenSpacing

Delta C❑15

Collagen

Blackland Prairie

28.7 -16.07 -8.67 7.15 8.92

Post-OakSavannah

16.8 -16.36 -8.10 8.26 --

Pineywoods

17.2 -15.95 -8.98 6.78 8.71

From Wilson 2012:109

However, as noted in the chart above, isotopic mean

values between the three regional groups are comparatively

uniform in terms of maize and protein consumption (Wilson

2012:108). Within the populations, Wilson and Perttula’s

examination of mean maize isotope values of Caddo

populations show a high standard deviation, suggesting a

high degree of variability within those groups (Wilson and

Perttula 2012:17). Thus, while dental remains show

variation in maize consumption between ecotones, isotopic

data does not reflect the same results.

While we lack some of the data that would enable a

conclusive determination of why Caddo farmers bounded their

operations at or near the Trinity River, we can at least

consider theoretical frameworks that provide productive ways

to think about that process. The principles of human

behavior ecology predict that the adoption of domesticated

plants outside the parent region are contingent on the

availability of wild resources in the new area, the

adaptability of the cultigen to be planted, and population

effects on wild food resources (Kennett et al. 104). We

know that there was a long period of transition in the Caddo

western region as inhabitants moved from a dependence on

wild plants to maize--sufficient time for the cultigens to

adapt to local environments, and for inhabitants to modify

those environments to encourage that transitional process.

Following Smith’s description of niche construction within

small-scale societies, foragers who settle in resource-rich,

productive “hotspot” settings, often along river floodplain

corridors, are primed to shift to agriculture, just as the

Caddo did (Smith 2012:8). Predictable and wetter weather

enabled the same process in the Blackland Prairie and in the

more marginal Post-Oak Savannah ecotones. The Caddo then

entered into an extended phase of exploitation and

experimentation, optimizing growing processes and modifying

cultigens in a gradual move to a diet focused on maize.

But why did that transition occur? In Winterhalder and

Kennett’s behavioral ecology framework, they suggest that

domesticates were regarded as a risk minimizer in small-

scale societies, and were initially adopted to diversify

existing resource bases. Eventually, domesticates became

the dominant subsistence resource, with the diversification

of wild resources then serving as the primary mechanism for

mitigating risk (Winterhalder and Kennett 2006:198). This

emphasis on risk could explain why the Caddo settled only

infrequently outside the Pineywoods ecotone. It would also

foreground why, where the Caddo did live in the Blackland

Prairie and Pineywoods ecotones, they departed with the

onset of variable, drier and colder weather patterns.

Looking at this phenomenon through the lens of

McClure’s ideal free distribution (IFD) model provides

additional insights. The IFD model outlines habitat

selection choices of individuals based on the evolutionary

assumption that individuals will choose a habitat in order

to maximize fitness. It helps analysts explore how people

use and distribute themselves in a landscape based on a

variety of social and environmental conditions. IFD assumes

that all individuals operate in a system of perfect

information that allows them to rationally choose their

habitat. It also assumes that individuals are, in fact,

free to move to that habitat. IFD therefore predicts that

“habitats will be settled in order of suitability, and

occupied in densities that equalize their marginal

suitabilities” (McClure et al. 2005:206). This framework of

explanation does seem to describe, in general terms, how the

Caddo habitat selection could have occurred.

While niche construction theory as outlined by Smith

can help explain how and why certain processes took place in

transitions to agriculture, it gives rather short shrift to

environmental factors that would have impacted forager and

early agriculturist decision-making. It also doesn’t

consider the element of risk that climate variability would

have introduced. The exploration of terrestrial models

within optimal foraging theory, mapped against modern

climate patterns, could enable us to better understand not

only the occurrence, but also the absence, of maize

cultivation in select regions. Johnson and Hard describe

how terrestrial models focus on the relative productivity of

wild resources, predicting the number of humans that could

be supported in a particular habitat. The model is derived

from a set of equations that involve climate data and

estimate plant and animal biomass (Johnson and Hard

2008:141). The model assumes that inhabitants would prefer,

and therefore inhabit, locations with the most abundant

resources. One would expect, then, that ecotones with

higher values of population density would reflect early

occupational density levels. Ideally, one would expect to

see higher Caddo density levels in the Pineywoods, and lower

levels in Post-Oak Savannah and Blackland Prairie regions—a

prediction that appears to be consistent with archaeological

plant and faunal assemblages in the Caddo territory.

However, there are inherent challenges in using optimal

foraging theory and terrestrial models to explain the Caddo

phenomenon. If one had the data, it would provide a sound

basis for comparing resource yields of the ecotones, but

lacking those analogs, it’s difficult to develop a detailed

understanding of seasonal patterns of wild and domesticate

resource availability and utilization (Smith 2006:300-301).

Additional research, perhaps including experimental

archaeology methods designed to estimate those yields, is

needed. Further, models would need to include not only

modern rainfall levels, but consider seasonal and year-by-

year variability in precipitation—key in assessing how risky

maize agriculture was in specific ecotones. It may be

feasible to use seasonal climate forecast models that

provide fine-scale spatial variability of rainfall and crop

response as a proxy for prehistoric micro-climate variation

and return rates between the Post-Oak Savannah and

Pineywoods regions (Hansen 2005:2039).

Weather patterns and the archaeological record suggest

that this climate-induced risk was the dominant factor

affecting Caddo settlement patterns. As noted earlier in

this paper, rainfall measurements at or west of the Trinity

River Basin were somewhat lower than those in the Caddo

heartland. Those lower levels would have been more

sensitive to variability, potentially affecting scheduled

maize production. Agriculture west of the Trinity likely

would have been a riskier enterprise than in more easterly

environs. Following long periods of drier and colder

weather, lower agricultural returns could also have been

responsible for the migration of Caddo from Blackland

Prairie and Post-Oak Savannah regions to the more

ecologically stable Pineywoods zones. Additional data

detailing exactly how variable ecotone weather is between

years, and seasonally, would provide credence to this

hypothesis, as would research on maize and wild plant

returns by ecotone. This is very much a moving research

target–and one that could provide productive insights into

our understanding of how foraging societies transition to

agricultural subsistence patterns.

The question remains, of course, how significantly

Caddo boundary delineation and subsistence patterns were

affected by environmental factors. The homogeneity of maize

consumer isotopic mean values across our three ecotones

suggests that a holistic answer may be more complex. It

remains to be seen how, or whether, the variability in

weather we suspect actually affected wild and cultigen

return rates. If return rates were not significantly

affected, social factors may have played a stronger role.

Thus, we should also consider exploring how spatial

boundaries of Caddo social networks functioned in mitigating

risk. Could boundaries be based on distance from ceremonial

centers? Or from larger villages whose more stable

agricultural returns provided additional resilience to

variable climate, assuming social networks were in place

that would ensure “hinterland” access to such resources?

Could defense considerations or clan dynamics have played a

role in these spatial boundaries? While further exploring

the validity of the premise that environmental factors were

a key variable, I leave the door open to social explanations

that could also illuminate this phenomenon.