Deforestation and fragmentation of tropical forest are the most serious threats to biodiversity. In the lowlands of the Huasteca Potosina region, Mexico, more than 95,000 ha of tropical forest have disappeared. This paper analyses tree species composition in tropical forest patches of this highly deforested region, located in northeast of Mexico. At present, only 57 remnants of arboreal vegetation larger than ten hectares still remain standing, which cover 6117 ha. These decreasing fragments host 140 floristic species, representing 42 families and 85 genera. Compared with previous inventories, these data show a sharp drop in species composition. Most of the forest remnants contain less than 50% of the floristic diversity recorded by previous surveys, particularly in the medium subperennial tropical forest.
Tropical forests are the most important terrestrial biomes, which contain the greatest specific diversity known―in all, and host more than half animal and plant species of the planet [
Mexico is one of the 17 mega-diverse countries and it is home to 10% - 12% of the world’s biodiversity [
By 2000, less than 11,000 ha of tropical forests were left standing and this patchy area contained only 71 forest relicts larger than 10 ha each (
The lowlands of the Huasteca Potosina region are located on the Gulf of Mexico coastal plain (21˚37' to 22˚28' N and 98˚07' to 98˚50' W); they cover about 305,600 ha and include the municipalities of Ébano, Tamuín and San Vicente Tancuayalab, in San Luis Potosí State (
According to the modified Köppen’s climate classification, the prevailing climate in the northern, central and western sections, which covers more than 85% of the study area, corresponds to a warm sub-humid climate with summer rainfall (Aw0(e)gw”). Towards the south, the climate is more humid and is classified as sub-humid with summer rainfall (Aw1(e)w”). The region’s rainfall pattern is characterized by a marked seasonality, with a short drought spell (canícula) amid the rainy season [
The vegetation types reported in the study region are tropical low thorn forest, the driest tropical forest characterized by lower canopy (less than 10 meters) and abundant thorny species. Tropical low dry deciduous forest whose main characteristic is deciduousness of most tree species, canopy between 10 and 30 meters, less dry than the previous one. Finally, the tropical medium semi-deciduous forest, the most humid is composed of semi-ever- green and deciduous tree species, canopy reaches 50 meters [
Concerning fauna, at least 17 different types of mammals have been reported [
Based on the location of 71 woody vegetation fragments identified in the year 2000 [
The deforested relicts registered in 2000 were eliminated and those relicts that were identified as standing fragmented were divided into smaller polygons. The divided or reduced fragments were delimited again, and their re-calculated area was used to calculate the total area of forest remnants. The annual rate of change of forest cover was calculated using the compound-interest-rate formula due to its explicit biological meaning [
where: P is the annual percentage of forest loss A1 and A2 are corrected forest cover estimates at time t1 and t2, respectively [
For this purpose, 36 fragments representative of the three main vegetation types (tropical low thorny forest, tropical low dry deciduous forest, and tropical medium semi-deciduous forest) with area ≥ 10 ha were selected randomly. Based on physical accessibility to each fragment and with its owner’s consent, 17 of them were selected to undertake the inventory of trees and shrubs.
In the field, within each fragment, 0.5 km transects were established. One observation point was established at one tip of each transect and all woody species observed (trees, shrubs) around this point (in a circular area with 10 m radius) were identified and quantified. The number of observation points depended on the vegetation changes within each fragment and its extent. On average, six transects per fragment were established (108 for all the measured fragments).
In order to determine the structure and age of the vegetation, measurement of tree diameter at breast height (DBH) was done for all observed species. The inventory also included the gathering of six samples of each woody species. Tree species were identified by a local expert botanist, and their vernacular/common names were recorded. Scientific names were determined for all collected tree species by comparing them with previous botanical inventories and consulting of species lists reported by Miranda & Hernández X [
Complementary information such as disturbance degree (for example, tree felling frequency, livestock foraging and firewood extraction), predominant environmental conditions (relief, slope, and soil type) and land-use history of each fragment, was collected for its characterization.
Woody species that could not be identified in the field were collected, labeled, pressed, and then taken to the “Isidro Palacios” Herbarium at the Research Institute of Arid Lands, Autonomous University of San Luis Potosí (IIZD-UASLP) for later ex situ identification. The eventual floristic identification was carried out following this procedure: 1) matching common and scientific names as previously determined by botanists, 2) dichotomous keys, representing two incompatible set of characters, 3) collection of specimens previously identified in book reviews/inventories, and 4) direct identification by local experts.
Of the samples, 81 were determined to the species level, 22 to genus level, 18 identified just by common name and 19 species were not determined due to the lack of reproductive organs.
Finally, the endangered tree species were identified through direct review of the Mexican Official Norm (NOM 059-SEMARNAT-2010), related to the protection of native species in Mexico. The environmental law classifies the endemic species in four categories: a) endangered, b) threatened, c) special protection, and d) probably extinct.
Results show that there were 57 woody vegetation fragments larger than 10 ha in 2008, namely 14 less than in 2000. The mean size of forest fragments decreased from 110 ha in 2000 to 86 ha in 2008, as well as their perimeter (5050 m to 4480) in the same period. It is important to note that five of them (≥100 ha), were fragmented into 13 relicts with an area ≤ 20 ha. At least eight were reduced more than 40% from its original size such as (15) San Francisco Cuayalab, (4) El Jaguar and (9) La Reforma, which further increased the fragmentation of the dry tropical forests still remaining in the lowlands of the Huasteca Potosina (
The real impact of deforestation can be found in the overall decrease in area of forest fragments (31 of them were reduced from 7786 to 6117 ha in 2008, 1669 ha less than in 2000). The deforestation rate calculated for the 2000-2008 period was 2.9% per year. At the same time, just four fragments increased in size. According to ave- rage diameter of trees and land-use history of each fragment, 13 belong to primary vegetation and four to
secondary woody vegetation. The trees found in the last remaining fragments grew up in previously cleared lands for the Pujal-Coy hydro-agricultural project, but later abandoned by farmers.
In total, 140 different trees species were found in the fragments analyzed. These species belong to 42 families and 85 genera. Out of the collected species, 103 were trees and 37 were shrubs. Based on the identified species in each fragment, it was determined that five correspond to the tropical medium semi-deciduous forest (MSDF), eleven to the tropical low dry deciduous forest (LDDF), and two to tropical low thorny forest (LTF) (
In the low tropical thorny forest relicts, only seven tree species were registered which results in a poor floristic richness and the complete absence of thorny species, the presumed salient feature of this type of vegetation. The arboreal stratum is dominated by species such as: Tabernaemontana alba Mill., Bauhinia divaricata L., Pithecellobium calostachys Standl., Sabal mexican Mart, Ziziphus obtusifolia (Hook. ex. Torr & Gray) A. Gray, Celtis sp. and Guazuma ulmifolia Lam. (
In the low tropical deciduous forest relicts, 75 different tree species were registered. The dominant species were: Senna atomaria (L.) H. S. Irwin & Barneby, Achatocarpus nigricans Triana, Colubrina elliptica (Sw) Briz. & W. L. Stern, Euphorbia schendallii Boiss, Phyllostylon brasiliensis Capan, Acacia sp., Randia aculeata L., Ruprechtia cumingii Meisn., Guazuma ulmifolia Lam., Pithecellobium sp. and Sebastiana sp. (
In tropical medium semi-deciduous forest relicts, 57 different tree and shrub species were registered. The dominant tree species in the upper stratum were: Spondias sp., Tabernaemontana alba Mill., Ceiba pentandra (L.) Gaertn, Bursera simaruba (L.) Sarg., Garcia nutans Rhor, Acacia sp., Harpalyce arborescens A. Gray, Pithecellobium sp., Trichilia havanensis Jacq., Brosimun alicastrum Sw., Ficus sp., Piper sp., Coccoloba montana Standl., Ruprechtia cumingii Meisn., Randia aculeata L., Salix humboldtiana Willd., Populus arizonica Sarg., Pouteria durlandii (Standl.) Baehni, Guazum aulmifolia Lam., Carpodiptera ameliae Lundell. The dominant species in the medium stratum of the same forest were: Yucca sp. Enterolobium cyclocarpum (Jacq.) Griseb., Parmentiera aculeata (Kunth) Seem. and Tabebuia rosea (Bertol) DC (
Four forest fragments (1) CEMEX, (15) San Francisco Cuayalab, (4) El Jaguar and (11) Los Fomentos have at least more than 30 species, in contrast with (13) Pozos de Petróleo, (17) Tancojol and (3) El Hermanito where we found less than eight species in each fragment. Only one species (Cedrela odorata L.) found in San Francisco Cuayalab is under special protection according to NOM-059 for the protection of native species.
ID/Owner/Name of reference village | Type of vegetation | Surface ha in 2000 | Surface ha in 2008 | Current status | Total of species | Density species/ha |
---|---|---|---|---|---|---|
(1) CEMEX | LDDF | 862 | 862 | No change | 48 | 5133 |
(2) Crucero-Crucerito | LDDF | 70 | 59 | 21 ha less | 24 | 2181 |
(3) El Hermanito | MSDF | 23 | 23 | No change | 7 | 386 |
(4) El Jaguar | LDDF | 101 | 47 | 54 ha less | 33 | 2610 |
(5) El Porvenir | LDDF | 29 | 29 | No change | 26 | 485 |
(6) El Sasub | MSDF | 37 | 37 | No change | 13 | 279 |
(7) El Tulillo | LDDF | 249 | 238 | 11 ha less | 11 | 271 |
(8) Jose I. Nava | LTF | 46 | 36 | 10 ha less | 20 | 224 |
(9) La Reforma | LDDF | 103 | 77 | 26 ha less | 15 | 350 |
(10) Las Norias | LDDF | 158 | 158 | No change | 29 | 2646 |
(11) Los Fomentos | LDDF | 100 | 99 | 1 ha less | 31 | 1051 |
(12) Nuevo Ahuacatitla | LDDF | 373 | 354 | 19 ha less | 14 | 492 |
(13) Pozos de Petróleo | LTF | 99 | 99 | No change | 7 | 171 |
(14) Rancho Jumaisa | MSDF | 144 | 131 | 13 ha less | 20 | 160 |
(15) San Francisco Cuayalab | MSDF | 160 | 63 | 97 ha less | 34 | 395 |
(16) San Juan 1 | LDDF | 39 | 39 | No change | 14 | 1000 |
(17) Tancojol | MSDF | 100 | 100 | No change | 8 | 532 |
Type of vegetation: Tropical low thorn forest (LTF); Tropical low dry deciduous forest (LDDF); Tropical medium semi-deciduous forest (MSDF).
Deforestation process in the Huasteca Potosina lowlands, that started 40 years ago, is still ongoing. Evidences show that the last forest remnants lost 1669 ha in eight year (2000/2008). This region had one of the highest deforestation rates (11% per year) in Mexico between 1973 and 1985, due to the implementation of the Pujal-Coy irrigation project (
This explains why only seven predominant tree species were found in the Tancojol fragment corresponding to tropical medium semi-deciduous forest (
Intensification of agriculture, livestock and forestry is heavily transforming this landscape and it is considered one of the main drivers of changes in species composition of ecosystems [
Ecological changes are two-fold. On the one hand, although most forest fragments still have the same area compared to 2000, most of them show a poor tree density. On the other hand, some smaller fragments (≤40 ha) have more trees species than the larger ones. This is the case of the (5) El Porvenir and (6) El Sasub fragments where a greater diversity of species was found despite their small size (
One of the explanations of the poor floristic diversity found in the low tropical thorny forest relicts is that the
tree and shrub strata were colonized by more aggressive species that established faster than the native vegetation. Species such as Phoebe tampicensis Mez. and Pithecellobium flexicaule (Benth.) J. M. Coult, are found in the low tropical thorny forest fragments only sporadically and as a part of the shrubby stratum, but previous studies [
Rzedowski [
Sabal mexicana is abundant where human set fires are common practice. The predominance of Cordia dentata Poiret., C. pringlei Robins., Croton sp., Harpalyce arborescens A. Gray and Rhacoma affscoparia, provide evidence of how these opportunistic species have substituted original species, facilitated by human-caused disturbance. The extraction of timber species or those with commercial value for local people has also modified the original composition and structure of vegetation.
Currently, most remnants of the low tropical deciduous forest contain less than 50% of the floristic diversity recorded by previous surveys conducted by Miranda & Hernández X. [
In addition, it is postulated that physical and structural characteristics, dominating shapes, forest productivity and seasonality have a great influence on distribution and abundance of wildlife [
The last standing forest remnants are an important reservoir of regional biodiversity in spite of their precarious conditions. If they were not protected in a short time, they will disappear forever, along with their associated biodiversity. Even though there are two state-protected natural reserves near to a biosphere reserve, it is still necessary to look for other legal ways of protection for these forest fragments. Some of the woody vegetation fragments work as small-scale biological corridors, which still make possible both the in-situ germplasm conservation and allow the survival of vanishing wild animal populations.
It is also possible that private fragments, such as (10) Las Norias or (4) El Jaguar, could be converted into Units for Environmental Management (UMA), specifically for wildlife conservation, as long as their owners are willing to become part of this environmental venture. Although one species under special protection was found in the (15) San Francisco Cuayalabremnant (Cedrela odorata L.) under special protection according to NOM- 059, it is practically impossible to propose a strategy of protection because this forest relict was divided among the several owners (ejidatarios) and fragmented after the vegetation sampling finished (
Although the deforestation process in the study region shows a significant decrease compared to previous years, the fragmentation still prevails. In eight years 14 forest fragments larger than 10 ha disappeared and their mean size and perimeter decreased, it means 1669 ha less than 2000. Overall, deforestation, extensive cattle raising, selective tree cutting, and opportunistic species invasion hamper the natural regrowth and expansion of forest remnants. These factors have caused a significant reduction of the tropical low thorny forest that once occupied a larger extent of this region. Furthermore, current floristic surveys, compared to previous studies, show an important decrease of tree species in the tropical deciduous forest. The 57 last standing forest remnants are an important reservoir of regional biodiversity, some of which are small-scale biological corridors, which still make the in-situ germplasm conservation possible and allow the survival of wild animal populations in spite of the prevailing conditions. The significant impact of deforestation in the composition and diversity of species of tropical low, thorny, and medium deciduous forests, force to look for legal ways of protection for the fragments that are still in good condition, otherwise they will disappear forever from the lowlands of the Huasteca Potosina.
We thank local botanist for help during the fieldwork, Tim Frazier and Augustine Avwunudiogba for their language correction as well as Laura Cano, Carlos Muñoz and Patricia Solis for their comments to improve the quality of this manuscript. The authors thank the Secretaría de Marina (SEMAR) for providing satellite images, FOMIX-SLP (FMSLP-2002-5715) and MyCOE-AAG (2007) (Biodiversity Initiatives in Central America) for providing the funds required to carry out the project.