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Heat load/water stress and leaf size in three mesquite woods in Sonora, MexicoPaul R. EarlFacultad de Ciencias BiológicasUniversidad Autónoma de Nuevo LeónSan Nicolás de los Garza, NL, Mexico. email@example.com
Is there enough rain ? NoReplacement of tall old trees by stunted ones of more desertic foliage is ongoing at Guaymas & Hermosillo, but not at Navajoa, Sonora. The reason is that Guaymas & Hermosillo have had their watertables lowered during the past half century by civic demand for water. In contrast, trees over 10 m in height at Navajoa live on Río Mayo.
IntroductionThe enormous change in 60 years in all Mexico is severe drops in the watertables such as over 20 m. Country water, some from deep wells, is canalized to the cities so that some rivers are dry beds, except for hurricane flooding. Many mesquite trees (Prosopis, Mimosaceae) more than 10 m tall are being replaced by ones of 2-4 m, all of unknown ages. Mesquite replacement trees in arroyos (streams) may not find enough groundwater to show rapid growth. The tall trees seen in a community were there long before the watertable dropped.
Mexico with almost 2 million km2 surface has vast northern areas of desertic matorral (short open thorn woodlands) with perhaps 250-600 mm rainfall/year distributed irregularly with most in the 3 months of July-September. The purpose of this work is to phenotypically describe a presumed major mutant with tiny leaves that arose in Guaymas and to its north in response to heat load/water stress.
Gates (1968) noted that, “The smallness of the leaves reduces the danger of leaf temperatures rising unduely high. Furthermore, if water becomes limiting, the stomata will close, the internal diffusion resistance will rise, the transpiration rate will drop and the leaf temperature will rise.” See also Sinha et al. (1997a, b, c, 1998) who wrote on the midday depression of photosynthesis, including stomatal closure due to the aridity of the air. Add that these mesquites fold in their leaflets at night. At sunrise, leaves open by 10-20 minutes. Mesquites close their leaves whenever light intensity drops sufficiently as in a daytime thunderstorm.
Drought occurred in 1940-60 killing many mesquites across the north of Mexico. This same period is when hydraulic works began diverting rural water to civic use in national antieconomic agriculture made oncompetitive by lack of rain. Now we have trees older than 50 years still standing, and the younger ones that have grown till today in much drier conditions range 5-30 years old. This ongoing replacement has strong genetic overtones (Earl, 1998, 2000, 2003) and fairly presents evolution in action.
The flora of much of the state of Sonora was given in a classic superpaper “Rio Mayo plants” by Gentry (1942). A before-and-after study is now strongly suggested. With Gentry’s predrought record, the record over 60 years later should provide acute insights into the Sonoran water stress story as well as important facts about the warming trend such as learning about the plants that have been extincted.
Mesquites combat heat load by genetically shortening the rachis to produce a smaller leaf so that 6 mm length of leaflets (6 LL) is found in the south and 36 LL in the north of Mexico. See also Hilu et al. (1982) on California mesquites. Velutine mesquites (not seen in this study area) defined by dark green overlapping leaflets about 9 LL have been often misidentified, because some have become dilute by repeatedly backcrossing P. glandulosa var. torreyana Benson.
Water stress leads to dry air then stomatal closure and less biomass that is seen as shorter trees less than 5 m. Again, see Sinha et al. (1997). When photosynthesis is shut down for long midday periods, the end result is a short tree of a few meters. See also Bohnert et al. (1995). If a permanent water table is available, mesquites can reach over 20 m, and height could be gained at even 1 m per year. Read also Felker et al. (1981). However in poor conditions, a tree of 80 years or more might be 8 m tall. Also, lowering a tall tree’s watertable will eventually kill it categorically.
Over centuries or even decades by water deficit, the more heat resistant mesquite races introgress the more northern mesquites, and plantation planners should know this. All Mexican races are, originally, mixtures of microphyllans and glandulosans, although some have different taxonomic species designations. See Table 1. Although leaf designs (Earl, 2003) vary radically by locality, all North, Central and South American mesquites could be considered as the same biological species as they cross. A taxonomic species results from an arbitrary decision. All Mexican & US mesquite trees cross freely.
In Argentina, Saidman et al. (1998a, b) using molecular markers reported the accumulation (fixation) of mutants in mesquite hybrids not present in either parent population, sometimes called private genes. This case seems to be parallel to the origin of the race Tiny in Guaymas, but no genetic markers have been tried there. Race Tiny can have a private gene, a gene absent from the genomes of both parent population. One crucial point is that a large group of races are being examined that are simple hybrids. These taxonomic species are all interfertile. Interspecific hybridization is nonexistent and common backcrossing is interracial.
The emergence of a race with especially tiny leaves in Guaymas depended upon endemic dessication without outside influences, conforming to the model of private genes by Saidman et al. (1998a, b). Centerfire dispersal, a term from Simpson (1944), will extend the superior hybrid’s domain. Furthermore, in the case of Tiny, the niche is expanding, drying out. That is, desertification is extending to the north and northeast. Having gained the effecive heterozygosity, phenotypically expressed as a jump to a much shorter rachis, Tiny is swamping its torreyana parents. Still, we know nothing about the implied genes. In the past and less than a century ago, the Navajoan type (P. glandulosa var. torreyana Benson) dominated Guaymas. Navajoa has length of rachis of 83 mm (83 LR), whereas Tiny has 21-24 LR.
Outstanding papers in evolution that apply here are Stebbins (1950), Fisher (1958), Clausen & Heisey (1958), Clarke & Sheppard (1959), Maynard Smith (1975), Gottlieb (1984), Gould (1980), Gould & Lewontin (1979 ), Hill (1982), Kimura (1983), Lande (1983), Lande & Arnold (1983), Arnold & Wade (1984), Endler (1986), Mackay (1989), Abbott (1992), Orr & Coyne (1992), Rieseberg (1997), Orr (1998) and Kim & Rieseberg (1999).
There has been a sudden phenotypic jump in race Tiny, yet no genetic information in this study. Although the logic seems convincing, the genetic evidence awaits future studies. Not only is Lande’s statement confirmed, but also smaller leaves are gained gradually over great distances like Guerrero to Texas as well as by jumps. Two more papers that require consideration are Shrimpton & Robertson (1988) and Zeng et al. (1990).
Foliar morphometrics were studied. Fifty-six trees were studied with 1501 leaves, averaging about 27 leaves per sample. Hermosillo had 18 trees with 4 more trees at Mazatán and 3 at El Novillo. Navajoa has 16 and Guaymas 15 trees. The foliar variables, all in mm, were 1) leaflet length, LL, 2) length of the longer rachis, LR, 3) number of pairs of leaflets per rachis, PL, 4) number of pairs of raches, PR, and 5) length of the petiole, LP. PL & PR are counts, and only 1 PR occurred in Navajoan samples. For contrast, tree T113 of Mina, NL is added to Table 1. A guide to velutine mesquites as a single tree from Altar, Sonora that is # 10 in Table 1. Discriminant analysis was run in SPSS, Chicago, IL.
Fig. 1. The area of Sonora that was investigated. The bar measures 100 km. The distance from Hermosillo to Guaymas is 132 km, thus the bar is 76 % of this.
Fig. 2. Trees of Hermosillo, Navajoa and Guaymas, Sonora ordered by PL. Navajoa is undisturbed. Race Tiny is group 6. Group 7 is parents backcrossed by Tiny, and Group 8 is old torreyana parental trees.
Table 1. The foliar morphometrics of all 56 trees, grouped. Group 1 is CIAD torreyana trees at Hermosillo, Group 2 is Hermosillo intermediates, 3 is Mazatán & 4 Novillo. Navajoa is Group 5. Tiny is 6, 7 intermediates & 8 torreyanas of Guaymas. Single tree T113 of Mina, NL (9) is added for constrast as it converges by having a short rachis. # 10 is a standard for the velutine race from Altar, Sonora. SD is standard deviation. N is number of leaves.
Table 2. Cases classified to their own group by discriminant analysis. Cases has the number of leaves.
Table 3. The 5 types of trees at Guaymas, including Tiny as # 1. In mm. N refers to number of leaves. SD is standard deviation. N is number of leaves.
DiscussionThe creation of a tiny leaf is taken as a major mutation that has been fixed. A uniquely small leaf has been selected in harmony with the newly changing drought environment at Guaymas. Algarroban mesquites with very small leaves, e. g., 6 LL, pop up in farflung parts of Mexico, and race Tiny has the smallest ones. Moderate-sized (17 LL) leaves occur on the Oaxacan Pacific coast and 6 LL leaves occur 100 km north after crossing the chain of Sierra Madre del Sur, whereas 36 LL occur in Coahuila, Mexico through New Mexico, USA (unpublished).
The Sierra Madre del Sur stops the coastal rain from entering the closeby Balsas river valley which is extremely desertic. Despite a 6-fold difference in leaflet length, all of these mesquites cross, meaning that no fertility reduction occurs. Tall old trees living in an environment now dry can do so since their roots have reached the watertable. Their offspring cannot establish in this site. What is a hybrid? What is a species? What is a macromutation? People can be talking at crosspurposes, because they are depending on different definitions of the same thing.
Mesquites seem to be a single species, although at least 83 taxonomic species are well-recognized, and some of these artificial species are claimed to be supported as even higher taxa by random amplified polymorphism DNA (RAPD) without sequencing (Ramírez et al., 1999), which is impossible.The change in leaf size from moderate to very small that is isolated at Guaymas is considered as caused by a major mutation that has been fixed by the especially dry niche. The same gene or one much like it is expected to be fixed in any near-identical niche, and sometimes called a private gene since it is absent phenotypically in the parent trees.
The scenario for the hot Balsas valley evolution of microphyllan mesquites from glandulosans follows the model of Tiny and its torreyana parents. A new type arises from an old one by the selection of mutants. Leaves are partially adapted to the humidity of the niche in all cases. As the Holocene warmed up starting 10,000 years ago, P. glandulosa all through Mexico including the entire Pacific coast seems to havemutated to P. microphylla when heat load became the survival factor. P. glandulosa occupied all Mexico and much of southwest US following the retreat of Wisconsin glatiation.
What appears to occur in the mesquites is heterozygosity by mutations, some of which are favored by the niche and fixed. This heterozygosity is induced in the original occupants by new selection pressures. Of course, crossing continues, but now new private genes (mutations) are being backcrossed. Tiny of Guaymas is swamping its parents, because it is better adapted to the new much drier niche. The rate of racial evolution seems great, assumedly because heat load/water stress selection has been strong over some 50 generations and is intensifying as the human population increases.
Selection for <6 LL occurs at El Infernillo, Michoacán, Victoria, Tamaulipas and Mina, NL (unpublished) as well as at Guaymas. These reoccurrences are the necessary and sufficient proof of convergence. The jump to about 24 LR from 83 LR might involve major genes. Read Orr & Coyne (1992), Hoffmann et al. (1995), Orr (1998) and Kim & Rieseberg (1999).Since convergent mesquites have been found most distant from each other by different people, many taxonomic species have been described.
The anterior scenario is a plausible story. It then can become the matter of sifting fact from fiction. The story qualifies as best information to date, even though mainly an impression. Also see Gould & Lewontin (1979). Still without doubt, swamping is the main mesquite activity, and this differs from hybridization with some sterility. Similar leaf types like small leaflet-packed, dark green 4 PR populations in Sonora, Chihuahua, New Mexico, Tamaulipas, northern Michoacán demonstrate convergence. The single migratory source of any character may be illusionary, and a convergent character may have emerged by mutation more than once.
Sonora has internal environmental deterioration due to increased civic use of water that is far more evident than the effects of competitive interracial mesquite migrations and racial introgression, even though introgression is the main feature in most other areas. The effects on all the flora and fauna remain unstudied, although Gentry’s (1942) superwork encourages such an effort.