BIG BEND The Impact of Human Use Upon the Chisos Basin and Adjacent Lands NPS Scientific Monograph No. 4

The climate of the Big Bend region is placed by Kendrew (1927) in a subtropical belt of high pressure which produces xeric climates. The seasonal high and low pressure centers over the Great Basin create the prevailing winds that influence Big Bend weather. The northerly winter winds produced by high pressures bring very little moisture to the region and only infrequent snow. The southerly, moisture-laden summer and early fall winds blow from the Gulf of Mexico and bring frequent, heavy rainfall to the cool, upper slopes of the mountains in the region. Typical of mountain regions, precipitation is of the convective shower type.

Long-range trend

In considering the role that climate has on vegetation, one must take into consideration both long-range and short-range cyclic trends. Martin (1963), using pollen analysis from Arizona sites, presents three major features of post-pluvial climatic history: an initial arid period climatically equivalent to the present and dating from 8000 to 10,500 B.P. (before the present); a less arid interval with intensified monsoon rainfall from 4000 to 8000 B.P.; and finally, an arid period closely resembling present conditions and lasting from 4000 B.P. to the present.

Data for the Chihuahuan Desert, and primarily from Big Bend National Park (Wells 1966), indicate a constant climatic condition from 36,600 B.P. to 11,560 B.P., using radiocarbon dating of vegetative materials from woodrat middens. Conditions became more xeric between 11,560 B.P. and 4200 B.P. as woodland species, such as Pinus cembroides (Mexican pinyon), Juniperus pinchoti, (redberry juniper), Quercus pungens (sandpaper oak), and Celtis reticulata (netleaf hackberry), present in the early middens, were excluded from the later middens at equivalent altitudes (1968 ft). Only xeric desert vegetation was found in the later middens, but was also present in the earlier middens along with the woodland species. The woodland, based upon its present lower elevational limits (4593 ft), has been shifted upward approximately 2624 ft since 11,560-14,800 B.P.

Several pertinent climatological studies cited by Hastings and Turner (1965) support the hypothesis that rainfall has been decreasing since at least the last half of the 19th century in the desert Southwest. There has been a notable decrease in winter precipitation and a slight decrease in summer moisture for areas in Arizona and New Mexico. Data presented by Schulman (1952) found a significant correlation between the amount of winter rainfall and growth in Big Bend National Park Pseudotsuga menziesii (Douglas fir). Since much fir growth occurs in the winter in response to winter precipitation, one wonders if the decreasing winter rainfall trend is affecting the growth of other mountain species in the park.

Accompanying the decreasing rainfall has been a worldwide warming trend (Hastings and Turner 1965). Studies of rural area temperature records from Arizona from 1893 to 1959 indicate that the temperature records from May to October increased 2.4° F in comparing the period 1903-13 with that of 1930-40. For the 6 cold months, the increase amounted to 1.75° F. These results correlated significantly with other Southwest temperature data. Since a temporal increase of 3° F in mean annual temperature is equivalent to dropping approximately 900-1000 ft in elevation, the observed temperature increase could be of major significance to the vegetation. These increasing temperatures would tend to favor extension of xeric species to even higher elevations than would be expected. Superimposing such climatic information upon grazing and human activities, one can expect even greater elevational increases by the vegetation types.

Short cycles

One cannot discredit the effects that short cyclic climatological changes have on vegetation of Big Bend National Park. A reduction in grass cover and in other vegetation is reported for drought years 1885-86, 1918-19, mid-1930s, and 1951-56. During these periods, changes occurred which altered the vegetation for decades, perhaps even centuries. Such an example will be presented later for the park.

Since climatological data are readily available in summary form from the U.S. Department of Commerce (1968) for the period 1947-66 in the basin, it will be only briefly discussed. A few major points which should be made concerning the winter months are that the mean daily maxima for November, December, January, and February are 60-70°F while the minima are 37-47°F. On an average of 11 days, the daily maxima do not exceed 50°F. Rainfall averages less than the 0.75 inch per month and occurs on only 4 days in both December and January and on only 2 in February. The sparse snowfall-sleet averages 3.7 inches annually.

For the months from March to October the mean daily maxima range from 68 to 88°F, with the highest being in June. Mean daily minima range from 40 to 58°F with the highest also being in June. A total average of 37 days can be expected above 90°F, Except for March and April, the rainfall averages greater than an inch for each month from May through October. July has the highest mean with 3 inches. The total annual average rainfall is 15.24 inches, with an average of 12.10 inches falling from May through October.

Included in the summary data for the basin is the record of one of the severest droughts since European man's entrance into the region. The drought began about July 1951 and lasted until September 1958. Conditions improved in 1957 and 1958 to annual averages of 16.80 inches from the previous 6 years' averages of 10.70 inches. These known drought years skew the 18-year average considerably. The average of the other 10 years (1949, 1950, 1959-66) is 17.92 inches, probably a more accurate annual average. Adding to these 10 years the totals for 1967 and 1968, the annual average is 18.26. This would be a difference of 7.56 inches less for the 6 severe years.

Communications with several persons living in the park during the drought indicate extensive deaths in the vegetation populations. McDougall (1953), after visiting the park in October 1953, states that oak trees were dying by the hundreds in Green Gulch, while pinyon pines and junipers were dying by the dozens. Similar deaths were reported for other species at desert elevations. Warnock (1967) reports similar information in his introduction. Certainly one cannot disregard the remains of many hundreds of trees seen from the trails in the basin.

A study conducted by Whitson (1965a,b) in Boot Canyon in 1964 presents an interesting response of the vegetation to the extended drought of the 1950s. The field work, conducted approximately 6-8 years after the end of the drought, showed a significantly high number of Pinus cembroides and Juniperus deppeana (alligator juniper) seedlings on the canyon floor where Quercus gravesii (Graves oak) trees were prevalent. He indicated that the more mesic oak vegetation was being replaced by a more xeric pinyon-juniper vegetation because very few oak seedlings were found. The evergreen seedlings were 4-6 years old at the time of the study.

Table 5 presents data for four permanent plots established in 1964 in the canyon and data collected in August for the same plots. At all plots the increase in Q. gravesii seedlings exceeded 100% of the 1964 census. Quercus grisea (gray oak) and Pinus cembroides increased by over 100% in one plot; however, Q. grisea showed the greatest percentage of death. Most of the new additions appeared to be from 1 to 3 years old. If the oak seedlings continue to grow, the original hypothesis of xeric replacement will be refuted. The data indicate that P. cembroides responded with great reproduction within 2-3 years after the drought, whereas Q. gravesii has required at least 10 years to respond. The recent additions of Juniperus flaccida (drooping juniper) and especially Acer grandidentatum (bigtooth maple) indicate perhaps a return to a more mesic time.

Table 5. Comparison of tree and seedling counts in four Boot Canyon permanent plots after a 5-year duration.