Case Study

Our case study is based on work in the Jornada Basin of south-central New Mexico. The Jornada Basin is part of the Mexican Highlands Section of the Basin and Range Physiographic Province within the extreme northern portion of the Chihuahuan Desert. The Jornada Basin was once dominated by warm season perennial grasses (e.g., Bouteloua eriopoda), but much of the area is now dominated by shrubs (e.g.,

Fig. 6.3. Relationship between the coefficient of variation in precipitation and the coefficient of variation of net productivity. (A) Based on data presented in Le Houerou et al15 (B) Results from PALS-FT based on three decades of precipitation (normal, wet, dry; see Fig. 6.4) and shown by functional types. Modified from Reynolds and Kemp.17

Fig. 6.3. Relationship between the coefficient of variation in precipitation and the coefficient of variation of net productivity. (A) Based on data presented in Le Houerou et al15 (B) Results from PALS-FT based on three decades of precipitation (normal, wet, dry; see Fig. 6.4) and shown by functional types. Modified from Reynolds and Kemp.17

Larrea tridentata and Prosopis glandulosa). This transition—which was initiated in the late 1800s or early 1900s— is believed to have been driven by overgrazing and global change (Fig. 6.2). The Jornada Basin now contains remnant grassland communities, and most areas are dominated by shrubs or codominated by shrubs, subshrubs, forbs, succulents, and grasses. The perennial grasses provide the most valuable forage for livestock in this region.

To address the importance of rainfall variability and rangeland production, with particular emphasis on the relative effect on different plant functional types, we used the Patch Arid Land Simulator (PALS) developed for the Jornada. PALS is a physiologically-based ecosystem model that contains the principal components of ecosystem carbon, water, and nutrient cycles.16,17 The results presented below are from our phenology-based version (PALS-FT), which includes the key plant functional types (FT) of the northern Chihuahuan Desert—shrubs, cool season annuals, summer annuals, forbs, and perennial grasses. Further details of the vegetation of the Jornada Basin, PALS-FT, and this modeling study are given elsewhere.16-18

In the first series of simulations, we examined the potential impacts of historical variations in precipitation on productivity, on both a year to year basis and over periods associated with decade-length climate shifts. We selected three periods from the long term records at the Jornada Basin (1914-1997, average annual rainfall = 247 mm): a "normal" decade (1968-1977, average = 250 mm); a "dry" decade (1947-1956, 33% below normal or 166 mm); and a "wet" decade (1984-1993, 32% above average rainfall or 325 mm) (see Fig. 6.4). Grouping all of these years together, our model simulations show that rainfall has large impacts on simulated annual net primary production (ANPP), although the absolute magnitude varies with plant functional type (Fig. 6.5). The scatter for these simulated results is substantial, which illustrates that, while there is a general increase in productivity with increasing annual precipitation, there is also considerable variation associated with the timing of that rainfall within individual years and with the utilization of this moisture by different functional types.

During the "dry" decade of 1947-1956, simulated ANPP was reduced by an average of 38%, but ANPP of the perennial grasses declined by 60%, whereas ANPP of the shrubs declined by only 25%. These results are consistent with the finding of Gibbens and Beck,19 who reported that above ground cover of the principal range grasses of the Jornada

Basin was reduced by 75% or more during this dry decade (they also speculated that this may have been a period favorable to increase in shrubs, and indeed our model simulations suggest that shrubs would have been less impacted than grasses—see Table 6.3). The reductions in simulated productivity of the seasonal annuals (50% for summer annuals and 20% for winter annuals) parallels the overall reductions of seasonal precipitation during the dry decade: Summer rainfall (July-September) was reduced by 60%, whereas winter rainfall (November-March) was reduced by about 25%.

The "wet" decade of 1984-1993 (Fig. 6.4) was a period of slightly increased summer moisture (10%) and greatly increased winter rainfall (50%). However, the perennial grasses, in which most growth occurs in the summer, were the most impacted in our simulations, having a 500% increase in productivity over this decade (Table 6.3), which seems counterintuitive. However, during this wet decade spring rainfall increased by 85%. Spring is normally the dry period in the northern Chihuahuan Desert, and a time of severe stress and tissue loss for grasses, which break dormancy in spring. The unusually wet spring periods of the wet decade not only alleviate drought stress in the grasses, but contribute to increased grass cover, allowing them to be more competitive with the other functional types for early summer moisture. However, these increases in grass cover are contingent upon having sufficient grass biomass in the community to take advantage of this moisture, and the model does not account for grass establishment. In fact, Brown et al20 reported that shrubs were the apparent beneficiaries of increased winter/spring rainfall during this period in another Chihuahuan desert community. In our simulations, shrubs also exhibited a large increase (150%) in ANPP during this period, largely by taking advantage of several periods of deep soil moisture recharge during heavy winter rainfalls. This is also consistent with some of our experimental findings.21

To more specifically assess the impacts of year to year variability of rainfall on productivity of plant functional types, we conducted ten series of 10 year model runs using the rainfall of the normal decade (1968-1977), but

Fig. 6.4. Annual precipitation at the Jornada Experimental Range (New Mexico, USA) for the last 80 years (average = 247 mm, dashed line). Maximum was 507 mm in 1984 and minimum was 79 mm in 1953. The seasonal distribution is approximately 65% in summer (July-October), 25% in winter (November-March) and 10% in Spring (April-June). The coefficient of variation (CV) of precipitation = 0.353 is identical to that reported by Le Houerou et al15 for 77 rangeland sites (see Fig. 6.3).

Fig. 6.4. Annual precipitation at the Jornada Experimental Range (New Mexico, USA) for the last 80 years (average = 247 mm, dashed line). Maximum was 507 mm in 1984 and minimum was 79 mm in 1953. The seasonal distribution is approximately 65% in summer (July-October), 25% in winter (November-March) and 10% in Spring (April-June). The coefficient of variation (CV) of precipitation = 0.353 is identical to that reported by Le Houerou et al15 for 77 rangeland sites (see Fig. 6.3).

in which each year's rainfall was modified by a random amount varying from -30 to +30% (leaving daily distributions fixed). Thus each 10 year rainfall series had a coefficient of variation (CV) varying from a low of 0.25 to a high of about 0.45, and for the 100 year period, the CV was 0.351 (identical to that of the natural rainfall for the period 1915-1997). A plot of CV of rainfall of the decade compared with the CV of production of plant functional types (Fig. 6.3B) illustrates three somewhat distinct patterns. First, both the shrubs and forbs in PALS-FT had low CVs (~0.4) that were statistically invariant with increasing variation of rainfall (p>0.05) (Fig. 6.3B). This CV is close to the long term average CV of rainfall in the Jornada Basin (0.35). These functional types best exemplify plants capable of utilizing moisture that may occur during any season (note that the dominant shrub is a drought-tolerant evergreen and the forbs are short-lived, herbaceous plants that can grow in any season). Second, both winter and summer annuals had production CVs that were much higher than that of rainfall (~1.45 for winter annuals and 1.25 for summer annuals) and, again, not dependent on rainfall. This high variation reflects the fact that productivity in these species depends on germination responses that are highly seasonal and rainfall specific. Small rainfall events within a season or large rainfall outside of their strict seasons result in almost no productivity because of lack of germination. Furthermore, their roots are located in the upper soil profile, which is subject to rapid drying. Thus, there are a number of seasons where substantial rain may not translate into any production; at other times, a small amount of rainfall that happens to be precisely timed for use by a seasonal annual species may result

Fig. 6.5. Predictions from PALS-FT for the relationship between net production and annual precipitation. Based on three decades of precipitation (normal, wet, dry; see Fig. 6.4) and shown by functional types. Modified from Reynolds and Kemp.17

in substantial productivity. The third pattern of variation is shown by grasses, which are the only functional type with a significant increase (p<0.05, r2 = 0.63) in the CV of production with increasing CV of precipitation (Fig. 6.3B). Because these perennial grasses grow mainly during the summer, they appear to be utilizing the most reliable moisture resource (see legend in Fig. 6.4). However, there is competition for this moisture from other plant functional types, and high evaporative demand can quickly remove moisture from small rainfall events. Many small events are not equivalent to the same amount of rain in a single event that percolates deeper within the profile. Thus, timing and amount of individual events becomes more important in summer, when grasses are growing. The length of the spring drought period may also impact grass growth in summer, as the death of roots and shoot tissue reduces the number of growing points capable of taking advantage of summer rainfall.

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