Twentieth century demographic changes in cirio and cardón in Baja California, México
Our purpose was to discern long-term large-scale patterns of survivorship and recruitment of two dominant, charismatic and useful desert plants, cirio [Fouquieriaceae; Fouquieria columnaris (Kell.) Kell. ex Curran] and cardón [Cactaceae; Pachycereus pringlei (S.Watson) Britton & Rose], and to evaluate the effects of physical and human factors. Location
The study included 77 sites distributed over c. 13,200 km2 in the northern Vizcaíno Region of the Sonoran Desert, state of Baja California, México. Method
Censuses used identified individuals (n = 3780 cirio and 2246 cardón) and were based on repeat photography. Time intervals between censuses ranged from 29 to 96 years, ending in 1996–2002. Earlier repetition (1963) of old photographs at 14 sites allowed analysis of temporal variation. The effect of elapsed time was modelled with Weibull functions for survivorship and linear functions for recruitment. To explain the residual variations, we tested categorical and continuous variables related to substrate, geomorphology, climate and geography. Human impacts were weighed with historical and quantitative analysis. Results
Elapsed time accounted for 45% of the variation of survivorship in cirio but only 35% in cardón. The fitted Weibull curves were used to estimate longevity: all-size cohorts of 1000 individuals would die out in 388 (cirio) and 723 years (cardón). Recruitment in cirio was linearly related to time (r2 = 0.71) and averaged < 0.003 year−1 but was less than mortality at 90% of the sites. For cardón, recruitment averaged > 0.005 year−1, was linearly related to time (r2 = 0.66) and was less than mortality at only 33% of the sites. Vital rates were not strongly correlated between mid- and late-century but were mostly similar to the long-term regional rates. Removing the factor of elapsed time, the residual variations showed that survivorship was greater on sites protected from winds for cirio and was less on very coarse soils for cardón. Recruitment increased with latitude in cirio and was greater on southern exposures for both species. The residual variations were not clearly affected, at a regional scale, by other factors thought to be important, including elevation, distance to the Pacific Ocean, geology, slope gradient, soil stability, older vs. young alluvial soils, and soil Ca : Mg and Na : K ratios. Human impacts have been sporadic and heterogeneous but locally strong; our quantitative indices of accessibility did not show regionally significant effects. Blowdown by hurricanes is a sporadic cause of major mortality for cirio but not necessarily for cardón. Main conclusions
At our scales, effects of time outweigh those of substrate, and human impact was scant. Large patterns were pronounced: cirio experienced a prolonged, widespread decline in the 1900s, while cardón fluctuated in different directions and degree among local populations. Cirio was found to be inherently much shorter-lived than cardón. We also suggest that recruitment in cirio was low due to long periods with relatively dry winters that affected the entire region, while spatial heterogeneity of cardón recruitment seemed more related to the variation in summer rains.