Earth Sciences
Volume 4, Issue 3-1, May 2015, Pages: 80-88

Taxus globosa Schltdl. (Taxaceae). Distribution and Diagnosis of an Endangered Yew

Javier López-Upton1, Xavier Garcia-Martí2

1Postgrado en Ciencias Forestales, Colegio de Postgraduados, Campus Montecillo, Texcoco, México

2Bioma Forestal, C / Zumedia, Etxauri, Navarra, Spain

Email address:

(J. López-Upton)
(X. Garcia-Martí)

To cite this article:

Javier López-Upton, Xavier Garcia-Martí. Taxus globosa Schltdl. (Taxaceae). Distribution and Diagnosis of an Endangered Yew. Earth Sciences. Special Issue: Conservation of Taxus Baccata Linn (Yew). Vol. 4, No. 3-1, 2015, pp. 80-88. doi: 10.11648/j.earth.s.2015040301.13


Abstract: After prospecting the vast majority of the populations and consulting herbarium collections and literature, data of distribution and demography of the Mexican populations of Taxus globosa is presented. Besides, the global distribution as the status of the species and its ecology is reviewed. Some populations are well preserved, however most of the prospected populations are under risk of anthropogenic disturbances. No real direct use of the species is performed, but other activities put pressure in their very small populations. With the data compiled, a preliminary diagnosis of its natural state is exposed towards the management and conservation of the species.

Keywords: Anthropogenic Disturbances, Conservation Ecology, Global Distribution, Mesoamerica, Mexican Yew, T. globosa


1. Introduction

The genus Taxus L. (Taxaceae) is distributed almost entirely throughout the northern hemisphere. The existence of four Taxus species is widely recognized in the American continent [1]. In the northernmost area of North America, T. Canadensis inhabits the Atlantic Coast of USA and Canada, while T. brevifolia is on the Pacific coast. T. floridana or T. globosa var. floridana sensu Spjut [2] - is located in the Apalachicola River Basin, a species displaying very restricted distribution at specific locations within the State of Florida [3]. Finally, the species considered as the subject of this work, T. globosa Schltdl. is one of the least studied species that inhabits the southernmost area, specifically in cloud forests located in Mexico and to the north of Central America [4].

This species, more widely known as Mexican Yew, has a disjoint distribution in Mexico along the length of the Sierra Madre Oriental (in the States of Nuevo León, Tamaulipas and San Luis Potosí, Querétaro, Hidalgo, Puebla and Veracruz) and the Juarez Sierra in Oaxaca and the sierras of Chiapas. Knowledge about the actual distribution of Mexican Yew is more diffuse and the size of its populations is much smaller in the Neotropical biogeographic region. There are numerous records with as little as 30-40 individuals per population in Sierra Juárez in Oaxaca. T. globosa herbarium specimens have been found in cloud forests belonging to Los Altos de Chiapas [5]. This yew also lives in Guatemala´s humid montane forests [6,7] located at Las Minas Biosphere Reserve, Sierra de los Cuchumatanes and the Volcanic Chain. In El Salvador, very small nuclei are residually located in remote areas near to the top of Cerro del Pital [8] at 2,670 m (ASL). A few kilometres away towards the west, in Honduras, the species lives in Santa Bárbara National Park, in addition to several locations inside the Celaque National Park, a place considered as the southernmost limit of its global distribution [22].

2. Morphological and Autoecological Nature

T. globosa is a tree that does not exceed 15 m in height and 60 cm in trunk diameter; so, it is a smaller species than T. baccata, and more related to the rest of the American species along the Eastern seaboard, such as the shrub-like T. canadensis. For the American plant, the needles are more shiny, elongated and acuminate at the apex, with a more visible dark green colour along the middle nerve areas and underside margins. The aril and seed are the T. globosa hallmark, these are more rounded and compressed; this is why, in 1838, Diederichvon Schlechtendal described it as Taxus baccata var. globosa [9]. The species grows preferably on well drained, acidic soils, rich in inorganic matter. The altitudinal range of distribution fluctuates from1,100 to 2,960 m (ASL). The highest altitude is found in the cloud forests of Oaxaca exceeding 2,600 m (ASL) which is the maximum altitude reached by T. baccata in the High Atlas of Morocco [10,11].

One of the most notable autecological features of Mexican Yew is its distinctly nemoral nature. T. globosa occurs exclusively in sheltered and shady bottoms of canyons that shows permanent or seasonal standing water; the species presence depends on the canopy above filtering out a large amount of the direct solar radiation. The species needs an annual rainfall over 800 mm but which can exceed 2,500 mm. For example, in forests of the State of Hidalgo, where we detected a recent expansion of the species, yew inhabits a temperate and sub-humid climate under a canopy of Abies religiosa (Kunth) Schltdl. & Cham. This protective cover significantly reduces evapotranspiration rates inside the forest. In this area, rainfall ranges from 1,000 to 1,200 mm excluding horizontal precipitation that should increase considerably the total amount. The average annual temperature in those well preserved forests is 14° C [4].

3. T. globosa Habitat

The Mexican yew woods are characterized by their distribution within a broad and discontinuous latitudinal gradient. They are isolated by lower and warmer areas including semi-desert areas. Usually, these formations record a high orographic precipitation, mild climate and high floristic complexity that varies even between nearby sites [12]. For example, Luna-Vegaet al. [13] describes these forests as typical vegetal communities that exist in temperate zones, where there is a subordinated floristic element of subtropical nature. Structural formations coexisting with yew in Mexico are mixed pine-oak forests: Pinus pseudostrobus, P. ayacahuite, P. patula, Arbutus xalapensis, Quercus crassifolia, Q. germana, Q. rysophylla, Q. laurina and Tilia mexicana among many others and fir forests (Abies religiosa and A. vejarii). The yew also occurs in cloud forests alongside species belonging to this type of formations such as Ostrya virginiana, Liquidambar styraciflua, Cornus disciflora to name some widely distributed species [9, 13, 14]. In Mesoamerican montane forests (located at southern Mexico, Guatemala, Honduras and El Salvador), the species is typically associated with Pinus ayacahuite and Abies guatemalensis [7].

Figure 1. Global distribution of T. globosa.

4. Methodology

After reviewing herbarium collections, analysing related literature, and several interviews in local communities, a complete census of T. globosa in Mexico is achieved. In 29 of these populations we made a survey and in situ characterization, gathering data about number of individuals, geographic location, altitude, regeneration recruitment and other parameters. With this information the conservation value of each location (classified as favourable, inadequate and unfavourable) was created following a model already tested for T. baccata [33]. After this, an analysis of the data collected was performed in order to offer a wide view from ecological aspects and the conservation of the species and its habitat. Further, geographic data was gathered to create a map of the natural distribution of T. globosa including the localization of the species in Guatemala, Honduras and El Salvador (Figure 1).

5. Results and Discussions

To date, in the Mexican territory, a total of 75 populations of T. globosa have been located in Mexico (Table 1). The vast majority of these nuclei contain less than 100 adult individuals. From the 29 characterized and inventoried populations, only 11 of them showed spontaneous regeneration and its overall proportion of male/female individuals is approximately 1/1.3 [15]. 12 of these characterized populations were considered under favourable condition, 12 unfavourable and 5 with inadequate information (Table 2). Consequently, we can say that demographic bottlenecks, that require a more detailed and specific study, are frequent. However, there are a few yew stands that currently exhibit a remarkable regeneration due to diminished human intervention and their difficult access. These are roughly focused in at least two large areas: one is located in the central and southern area of Nuevo León in north-eastern Mexico, including many nuclei within the municipalities of Zaragoza and Santiago. Here there are T. globosa populations with many individuals registered in a census: Las Tinajas with 3,035 individuals and Potrero Redondo with 3,460 [14]. These same authors conducted a demographic study in several northern populations that included a classification by forest mensuration with class intervals (height-diameter) that allows inference of the inter-annual evaluation of the recruitment dynamics, as well as disturbance changes that occurred in a spatio-temporal scale for the studied yew populations. The second zone is located hundreds of miles towards the south and represents what might constitute a set of meta-populations distributed in the headwaters of the deepest valleys that define El Chico National Park in Hidalgo. This zone is located in the central-eastern area of Mexico and includes populations showing abundant regeneration such as those located in two ravines: "Cañada Los Ayacahuites"and "Pueblo Nuevo Canyon", each of them with next to 150 mature trees in a very well preserved area.

5.1. Key Mechanisms of Ecological Interaction

Once some of the more "healthy" Mexican Yew stands in ideal situations were pinpointed, one wonders, what could be the factors involved in the success of its establishment? The average production of fruits per tree for T. globosa is very limited: 40-50 seeds/adult female per year [16]. This fact was subsequently confirmed from seed collection from multiple locations and across years. Mast year occurred every two and three year, and some trees do not have any seed in a year. This circumstance may be due to the purely shade-tolerant nature (sciophilous) of the species in the sense of [32] who detected that lack of light causes a decreasing production of flowers in some European yew populations. Seed viability has been investigated using the tetrazolium test to analyze the seed viability from different populations [15,17] and those obtained in 2011 for the present work. The maximum percentages of viable seed obtained globally were 98%, 75% and just 21% respectively. Further, germination capacity is very low especially in the stands from central Mexico [17]. Thus, in these vigorous yew stands, a successful recruitment of new individuals, having so little seed rain and fluctuating viability values, is a fact that catches the eye, especially when it is compared to European species that easily reach seed production averaging several thousand seeds per individual per year.

Table 1. Located populations of Taxus globosa in Mexico.

Population State Latitude Longitude Height (msnm)
1 La Ciénega Nuevo León 25º 22' 35" 100º 13' 15" 1460
2 El Manzano Nuevo León 25º 22' 29" 100º 12' 39" 1460
3 El Manzano-VitroParque Nuevo León 25º 22' 27" 100º 13' 05" 1470
4 El Cilantrillo Nuevo León 25º 21' 15" 100º 20' 00" 1890
5 San Isidro Nuevo León 25º 20' 50" 100º 18' 15" 2055
6 El Tejocote Nuevo León 25º 19' 20" 100º 15' 30" 1970
7 La Camotera Nuevo León 25º 16' 45" 100º 15' 10" 1760
8 Cañada La Trinidad Nuevo León 25º 14' 49" 100º 09' 20" 1361
9 La Trinidad Nuevo León 25º 14' 00" 100º 08' 30" 1400
10 El Butano Nuevo León 25º 10' 50" 100º 08' 00" 2000
11 Cañada Agua Fría Nuevo León 24º 02' 20" 99º 42' 50" 1890
12 Cañada La Era Nuevo León 24º 02' 15" 99º 44' 05" 2350
13 Cerro El Viejo Nuevo León 23º 59' 10" 99º 44' 30" 2 375
14 La Yerbabuena Nuevo León 23º 55' 31" 99º 47' 58" 2100
15 La Encantada Nuevo León 23º 55' 20" 99º 48' 20" 2385
16 La Tinaja (Cañada) Nuevo León 23º 53' 25" 99º 47' 29" 2650
17 Puerto Purificación Tamaulipas 23º 55' 20" 99º 28' 18" 1300
18 Galindo Tamaulipas 23º 54' 00" 99º 27' 00" 2300
19 Los Caballos-Puerto Purificación Tamaulipas 24° 04' 00" 99° 27' 00" 1 245
20 Reserva "El Cielo" N Tamaulipas 23º 12' 25" 99º 13' 35" 1550
21 Reserva "El Cielo" S Tamaulipas 23º 11' 46" 99º 13' 43" 1620
22 Joya de Salas Tamaulipas 23° 10' 48" 99° 16' 26" 1 650
23 Los Tres Cerritos Tamaulipas 23° 07' 10" 99° 12' 00" 1350
24 Rancho El Cielo Tamaulipas 23° 07' 00" 99° 12' 00" 1450
25 Ojo de Agua del Indio Tamaulipas 23° 05' 45" 99° 12' 40" 1430
26 Sierra de Guatemala Tamaulipas 23° 09' 00" 99° 18' 00" 1550
27 San José Tamaulipas 23° 04' 48" 99° 13' 40" 1340
28 Casa de Piedra Tamaulipas 23° 02' 48" 99° 14' 50" 1400
29 Arroyo del Tepaxtle San Luis Potosí 21º 55' 40" 100º 16' 40" 2000
30 Cañada de las Avispas Querétaro 21º 27' 26" 99º 07' 48" 1980
31 Cerro Grande Querétaro 21º 25' 00" 99º 07' 00" 2300
32 Llano Chiquito Querétaro 21º 24' 30 99º 07'30" 2120
33 El Salto Querétaro 21º 14' 50'' 99º 38' 30'' 2 200
34 La Joya del Hielo Querétaro 21º 13' 30" 99º 09' 35" 1 900
35 Cañada Los Granadillos Querétaro 21º 12' 40'' 99º 41' 00'' 2400
36 Barranca El Bosque Querétaro 21º 09' 40'' 99º 40' 44'' 2.600
37 Cañada de Agua Fría Querétaro 21º 08' 14'' 99º 41' 06'' 2620
38 Agua Blanca Cerro Hidalgo 20°22'01'' 98°20'22'' 2260
39 El Remudadero Hidalgo 20º 22' 00" 98° 19' 00" 2250
40 Cañada Los Zorrillos Hidalgo 20° 13' 16" 98° 43' 08" 2400
41 Cañada Los Ayacahuites Hidalgo 20° 12' 25" 98° 43' 10" 2.630
42 Los Corrales-El Chico Hidalgo 20° 12' 13" 98° 43' 33" 2550
43 El Conejo Hidalgo 20°12'11'' 98°43'28'' 2500
44 Pueblo Nuevo Hidalgo 20º 11' 04" 98º 40' 42" 2.530
45 Cuyamalolla Hidalgo 20º 05' 23" 98º 32' 04" 2650
46 Barranca Rio Seco Hidalgo     2 630
47 Estación de Honey Puebla 20º 15' 00" 98° 12' 00" 2000
48 El Salto, Helechales, Veracruz 20º 36' 00" 98° 26' 20" 1720
49 Viborillas Veracruz 20° 29' 00" 98° 29' 20" 2 200
50 Buenavista Veracruz 19º 44' 00" 96° 49'00" 2150
51 Cerro La Tolva Veracruz 19° 39'00" 97° 08' 00" 2000
52 Pueblo Viejo Veracruz 19º 31' 55" 97º 03' 15" 2140
53 El Encinal II Veracruz 19º 31' 16" 97º 03' 08" 2500
54 Rancho Las Golondrinas Veracruz 19º 30' 53" 97º 03' 33" 2649
55 Oxtlapa Veracruz 19º 25' 20" 97º 06' 15" 2140
56 El Rincón Atotonilco Veracruz 19° 09' 00" 97° 14' 00" 2 500
57 Cerro Mirador (C. Hueso) Oaxaca 17° 40' 29" 96° 33' 54" 2 400
58 Río Perfume-Oeste Oaxaca 17° 39' 00" 96° 32' 00" 2660
59 Río Perfume Oaxaca 17° 38' 49" 96° 32' 26" 2 800
60 Puerto La Soledad Oaxaca 17° 35' 15" 96° 34' 40" 2 500
61 Mirador Oaxaca 17° 34' 43" 96° 29' 51" 2 800
62 Cerro Humo Chico Oaxaca 17° 33' 00" 96° 30' 10" 2 800
63 Parte alta de la brecha Oaxaca 17° 30' 00" 96° 30' 20" 2 950
64 El Carrizal Oaxaca 17° 29' 20" 96° 41' 30" 2540
65 Llano Verde Oaxaca 17° 29' 15" 96° 24' 00" 2200
66 Cruz de la Raya Oaxaca 17° 18' 00" 96° 19' 00" 2460
67 Tepitongo Oaxaca 17° 18'00" 96° 02'00" 1700
68 La Natividad Oaxaca 17° 17' 40" 96° 26' 00" 2.500
69 El Quelite Oaxaca 17° 17' 15" 96° 26' 30" 2 372
70 Capulalpa-Llano Verde Oaxaca 17° 11' 30" 96° 22' 44" 2 400
71 Cañada de Torres Oaxaca 17º 11' 03" 96º 22' 44" 2700
72 Cumbres de los Frailes Oaxaca 17º 11' 00" 96° 24' 00" 2 500
73 Coapilla Chiapas 17º 10' 00" 93º 10' 00" 1750
74 Tenalchen Chiapas 16º 37' 00" 93º 20' 00" 2200
75 Jocosic Chiapas 16º 50' 00" 92º 30' 00" 2 300

 

Table 2. Number of trees and conservation condition of T. globosa in Mexico.

Population Mature trees Condition
1 La Ciénega 27 inadequate
2 El Manzano 9 unfavorable
3 El Manzano-VitroParque 162 favorable
4 El Tejocote 436 favorable
5 La Camotera 100 inadequate
6 Cañada La Trinidad 208 favorable
7 La Trinidad 106 favorable
8 Potrero redondo 3460 favorable
9 Cañada La Era 657 favorable
10 La Yerbabuena 357 favorable
11 La Encantada 543 favorable
12 La Tinaja (Cañada) 3,035 favorable
13 Reserva "El Cielo" 100 inadequate
14 Mesas de San Isidro 123 unfavorable
15 Cañada de las Avispas 330 favorable
16 La Joya del Hielo 2 unfavorable
17 Cañada Los Granadillos 120 inadequate
18 Barranca El Bosque 4 unfavorable
19 Cañada de Agua Fría 55 unfavorable
20 Los Corrales-El Chico 57 unfavorable
21 Cañada Los Zorrillos 41 inadequate
22 Cañada Los Ayacahuites 153 favorable
23 Pueblo Nuevo 166 favorable
24 El Remudadero 10 unfavorable
25 Cuyamalolla 110 unfavorable
26 El Encinal II 90 unfavorable
27 Barranca de la Funda 5 unfavorable
28 Capulalpa 11 unfavorable
29 Cañada de Torres 45 unfavorable

As noted above, the environments inhabited by T. globosa are characterized as moist microhabitats covered by an upper canopy formed by other species of larger size. This protecting cover could also serve as a bird perch accounting for the large number of yew juveniles thriving vigorously at short distances from adults of spruce, oak, pine, true fir (Figure 2) and to a lesser extent, from the existing yew adult individuals as seen in different species by other authors [18, 19]. Before the creation of new seedlings, the action of different dispersants is needed to make a decisive contribution to effective aril dispersal [20]. This means that the use as food of the few available arils could lead to the observed high recruitment rates. Thus, well-preserved, Mexican temperate forests are ecosystems with considerable capacity to shelter both forest- resident birds and bird species migrating through the Americas. From observations at Las Tinajas Mountain, Bluebirds (Sialia currucoides) eating yew arils during the fall have been detected (pers.obs.), which is likely complemented by the presence of other Turdidae birds - a very important family for yew dispersal – such as the Aztec Thrush (Ridgwayia pinicola), the Brown-backed Solitaire (Myadestes occidentalis), the Black Thrush (Turdus infuscatus) and the Clay-colored Trush (Turdus grayi) which are present in northern forestland communities of the Sierra Madre Oriental [21]. It is unknown whether there is some involvement in T. globosa dispersal by terrestrial mammals (foxes, badgers, weasels, etc.). On the other hand, hydrochory dispersal events should not be ruled out, due to the sub-riparian character of the species. We have not detected mechanical protection systems provided by other species (plant-plant mutualistic interactions) acting over young yew seedlings existing in the prospected area sensu García & Obeso [18].

Figure 2. Juvenile yew growing beside an Abies religiosa. Promotion of hanger effect by the canopy individuals is very common in order to facilitate the recruitment of new plants.

The other location was chosen because it contains burgeoning yew stands situated at Los Ayacahuites Gully within El Chico National Park. This fact represents an a priori major figure of protection. This territory has vast formations of Abies religiosa and several pine species in the lower levels and is one of the oldest forest reserves in Mexico. Also, particularly characteristic, the park's managers have a special awareness of Mexican Yew. In addition, for years, it has been the preferred area for the study of the species [15,16,17]. However, the Protected Natural Areas do not cover or guarantee the protection of T. globosa and its associated ecosystems across its distribution [22]. In addition, even in areas where the Mexican yew has been until recently regarded as abundant, the species may now be suffering a reversal or at least a collapse in regeneration dynamics within its populations. This is the case of some nuclei present in the Sierra Gorda Biosphere Reserve in Queretaro (Figure 3), where we observed an aging population in Los Granadillos Gully and Agua Fría Gully, for example; this is due to increased livestock pressure and the cutting down of dozens of adult individuals. Paradoxically, some kilometres away, in Las Avispas Gully, there is an important yew nucleus-area managed by another communal group, different than that above, where the owner is aware of the environmental value of their surroundings.

Figure 3. Female tree of T. globosa characterized in Sierra Gorda. Queretaro. Mexico.

5.2. Importance of the Local Communities Implication

An obvious and predictable fact is that to promote optimal stability dynamics of this valuable yew stands needs the indispensable involvement of the existing human communities for conservation of the natural environment. In La Encantada Communal Land in Nuevo León - where, among others, the burgeoning yew population of Las Tinajas occurs - it is revealing to observe the management of the existing forest resources, which includes major logging of pines and oak species. However, this management is far less widespread throughout the whole territory; despite forest management by the community being a deeply-rooted custom, the reality of Mexican forests managed by local people is uneven and complex [23]. It has been highlighted that many human customs are the main limiting factor for the effective regeneration of the species. Consequently, most populations consist of adult specimens roughly dispersed in areas generally affected by disturbances such as timber extraction and excessive pressure by domestic cattle.

There are many voices warning about increased disappearance of these ecosystems in the American biogeographic and trans-frontier region known as Mega-Mexico, which is one of the global hotspots of biodiversity [24]. In recent decades, the deforestation problem has done nothing but worsen alarmingly: the area occupied by these cloud-forest formations has fallen by at least, 50% according to the National Commission for Knowledge and Use of Biodiversity-Conabio. Anthropic causes are clandestine and uncontrolled clear-cutting for wood extraction, recurrent forest fires and land-use changes generally for agriculture and livestock purposes [23]. To date, this increasing level of threat has been the major cause of decline of this unknown yew. Local extinction of the species has likely happened in the State of Mexico, where there are herbarium yew samples from the mid-20th century [9]. Recent disappearance of already isolated and fragmented populations has been noticed near to the northern El Cielo Reserve in Tamaulipas, in addition to the loss of a large number of individuals - only stumps remain - within some forests of Veracruz and Hidalgo. Both these previously mentioned Mexican States contain a high level of biodiversity; and in this sense, they are significantly affected by the relentless deforestation. A study of Contreras-Medinaet al. [25] has analyzed, through predictive modelling, habitats housing yews in a macro-territorial way: since a few decades ago, the potential niche of T. globosa has probably disappeared by up to 80% in the last forty years.

5.3. Therapeutic Properties. Taxanes and T. globosa

As is known, yew is a potent anticancer agent. In the last two decades, studies of its biomedical properties have been extended to the Mesoamerican species [26]. Paradoxically, beginning forty years ago, this therapeutic attribute is reducing the populations of different species from the Taxus genus throughout North America; also, more recently, the Asian yew is decreasing even more dramatically in a pressing manner [27].

In Mexico, T. globosa populations do not seem to be currently significantly affected by this type of exploitation, except in El Tejocotal town, in Nuevo Leon (perhaps due to its easy access and its proximity to Monterrey City). However, this is possibly changing due to the growing demand for taxol intended for the global pharmaceutical industry. In other regions, there have been reports of selective extraction of yew in some Guatemalan forests [7]. Detailed and updated studies of the demographic situation of the species in the three Central American countries (Guatemala, Honduras and El Salvador) show high rates of deforestation and loss of biodiversity are happening. In our opinion, given the small size of the populations and their restricted global distribution, the commercial exploitation of the species will lead to rapid extinction in the short term. In this regard, T. globosa is included in the Official Mexican Norm (NOM-59-ECOL-2010) under its "in special protection" category [27]; and, in Guatemala, it appears in Conap [29] within the list of species requiring an environmental impact study before its possible use (included in Category 2 of the Red List of forest species). However, the economic incentive that will encourage plant extraction by certain members of local communities with high levels of marginality could be the trigger of illegal and uncontrolled exploitation, as is widespread in Asia [31].

6. Conclusions

In the 29 populations characterized in Mexico, 15 had a favourable conservation status, two an inadequate state and 12 populations are in unfavourable conditions. The region where T. globosa is best represented and best preserved is the Mexican state of Nuevo León followed by Hidalgo. In this sense, biggest problems of threat such as fragmentation and habitat degradation, low number of breeding individuals, and land use change is observed in a north-south gradient. Different authors confirm this spatial trend in the remaining populations of Guatemala, El Salvador and Honduras [7,9]. The poor fruit production suggests that the species may be subject to both inter-annual pulses of regeneration and there is a population requirement for a minimum number of adult individuals with an adequate sex ratio in order to achieve effective pollination and minimum fructification rates. This phenomenon has been confirmed in studies conducted in T. baccata [34]. A deeper and specific research on autoecology of T. globosa is needed.

In summary, our preliminary diagnosis suggests that well-preserved Mexican Yew could match the following pattern: i) a need for a strong contingent of reproductive individuals to counteract the low fruit production per adult individual due to innate abiotic factors of its autecology, ii) the existence of a protective canopy against direct sunlight that also provides the perch effect of other tree species, iii) the need of positive interactions with frugivore dispersing birds and low pressure by herbivore species. Therefore, for satisfactory renewal of T. globosa formations, the existence of a heterogeneous, fully structured and minimally altered ecosystem is essential.

In order to mitigate the human pressure, a possible solution to boost preservation of some of the most vulnerable and accessible yew stands could be the setting up of green pathways which could serve both as touristic attractions and to bring sustainable income to the local communities currently managing these areas, as can be observed in the population of Las Viborillas (Veracruz).

Different strategies to improve the knowledge of ecological behaviour are needed. The main objective is to mitigate - or avoid in the best of cases - the threats described above; so that, multifunctional action planning includes the in situ characterization and conservation of existing populations, the active promotion of genetic diversity by creating living collections and ex situ seed sources for reproduction, and the progress in biomedical research in order to reduce provisioning problem by exploiting the wild yew. Multidisciplinary collab oration between institutions and working groups are becoming increasingly necessary. In the case of T. globosa, this cooperation has allowed the use of some of the techniques successfully applied in the active conservation of T. baccata in the Mediterranean area [30], because although large distances exist between the two species, the reality and necessity of conservation is the same.

Acknowledgements

We thank to Dra. Lidia Osuna (CIBISS-IMSS) for their help during the field work. Dr. Pablo Ferrer (CIEF- Valencia) improved an early version of this manuscript. Dr. Peter Thomas (Keele University) looked over the English version of this manuscript.


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