干旱区生态环境知识资源中心

Aim 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 cardon [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 km(2) in the northern Vizcaino Region of the Sonoran Desert, state of Baja California, Mexico.

Method Censuses used identified individuals (n = 3780 cirio and 2246 cardon) 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 cardon. The fitted Weibull curves were used to estimate longevity: all-size cohorts of 1000 individuals would die out in 388 (cirio) and 723 years (cardon). Recruitment in cirio was linearly related to time (r(2) = 0.71) and averaged < 0.003 year(-1) but was less than mortality at 90% of the sites. For cardon, recruitment averaged > 0.005 year(-1), was linearly related to time (r(2) = 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 cardon. 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 cardon.

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 cardon fluctuated in different directions and degree among local populations. Cirio was found to be inherently much shorter-lived than cardon. 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 cardon recruitment seemed more related to the variation in summer rains.