Mexico: Querus Rugosa Restoration in Ecological Park of Mexico City

Project Stage:
Completed

Start Date:
1991-10-01

End Date:
1993-06-01

Primary Causes of Degradation

Degradation Description

Mexico has a great diversity of Quercus species, (between 130 and 200 many of which occupy areas particularly well suited to human settlement and agriculture. On the lower parts of the hills surrounding the Mexico City basin Quercus forests are disappearing at a high rate and those that remain suffer the impacts of increasing human disturbance. Seasonally dry oak forests are important in Mexico, both regarding extension and biodiversity and their ecological significance for society. The project used an experimental approach to assess the establishment, survival and growth of seedlings of Quercus rugosa at three different sites along an environmental gradient created by two disturbance events: (1) a natural disturbance (i.e. a lava flow which covered an area of 80 square km about 2000 years ago) and (2) a recent temporary human settlement that was in the area in 1987-1988. The disturbance caused by the human settlement included the removal of substrate in order to use the rocks for building huts and walls. An area of ca. 100 ha was left almost devoid of vegetation and an ecological restoration program was started in 1990.

Reference Ecosystem Description

The pre-disturbance condition was one of a closed oak forest toward a xeric shrubland with sparse oak trees, including a transition zone or border between them, formed by open oak forest with rich herbaceous and shrub vegetation. The forest understory includes mainly shrubs: Senecio barba-johanis, S. angulifolius, Eupatorium pazcuarense, E. petiolare Herbs: (Salvia mexicana S. microphylla all at low densities) S. elegans, Fuchsia thymifolia. Along the forest border, it is dominated by Q. rugosa, among others: Shrubs: Bouvardia ternifoli,a Eupatorium glabratum, E. pazcuarense, E. petiolare, Sedum oxypetalum, Verbesina virgata. Herbs: Dahlia coccinea, D. merckii, Fuchsia thymifolia, Oxalis lunata, Penstemon roseus, Salvia mexicana, Stevia salicifolia. The xerophytic shrubland, which was formerly characterized by sparse trees of Q. rugosa and the shrubs Senecio praecox, Agave inaequidens and Opuntia tomentosa, was severely disturbed from 1987-1988 by a temporary human settlement. As a result, the dominant trees are now Buddleia chordata, B. pawijlora and Dodonea viscosa. In addition, we find the following species:
Shrubs: Bouvardia ternifolia, Eupatorium glabratum, E. pycnocephalum, E. arsenei, Loeselia mexicana, Sedum oxypetalum, Verbesina virgata. Herbs: Asteraceae: Dahlia coccinea, D. rudis, Tagetes erecta, Piquena trinervia, Poaceae: Muhlenbergia macroura, Bromus exaltatus, Poa annua Forbs: Begonia gracilis, Castilleja tenuiflora, Commelina coelestis, Penstemon roseus.

Project Goals

The initial 1987-88 squatters were the motivation for the initial project and with its initiation it was realized that little was actually understood about the dynamics of Quercus rugosa in these environments.

Monitoring

The project does not have a monitoring plan.

Stakeholders

Very little is known about the ecology of these forests. In particular, we need to know the requirements for establishment and growth of the different Quercus species in order to achieve successful reintroduction as part of ecological restoration programs in the disturbed areas that are now under protection. Most studies concerned with Quercus regeneration have been carried out on high-quality mesic sites in the United States and Europe while less attention has been paid to the ecology of seasonally dry oak forests, except for some mediterranean-type areas. The problem is particulary acute in the Valley of Mexico because of the dire poverty that forces many of the squatters into the natural reserves that surround the city. The lack of political will to enforce environmental policy is further exacerbated by the economic realities driving the squatters in the first place.

Description of Project Activities:
During October and November 1991 acorns were collected from the study site and tested by floating them in water, which is a reliable means of identifying damaged acorns. Undamaged acorns were used to evaluate seed removal by predators on the ground. 10 replicates of each of three seed densities (groups of 1, 5 and 25 acorns) were placed on the ground at random points along a transect in each site in early March 1992 and the numbers of acorns remaining were recorded daily for the first week and weekly thereafter until late April. In order to study seed germination and seedling establishment 12 cages (25 cm x 30 cm x 30 cm) of 12 mm wire mesh, painted to avoid oxidation, were fixed to the soil at points chosen at random along a transect in each site. In March 1992, 10 acorns were placed inside each cage and covered with the litter present on the surface to mimic natural conditions. The cages were observed monthly and seedling emergence was recorded; they were opened at the end of August and the condition of the remaining seeds and seedlings was checked. The experiment was not truly replicated (i.e. there were replicates studied at one site in each of three habitat types) due to the labour-intensive nature of the study and the characteristics of the area which made it impossible to establish more than one 'forest border' site in the park. Seedlings grown in a local nursery from acorns collected in the area were used to study seedling survival and growth at each site. The seeds were previously tested for viability, numbered and assigned to small (< 1.5 g), medium (1.5 -2.5 g), or large (>2.5 g) size categories, as there is evidence that seed size affects seedling performance in this species. The acorns were germinated in trays containing agrolita, a porous inert material that retains moisture. When the seedlings were ca. 10 cm tall the agrolita was carefully rinsed away and the seedlings placed in black plastic bags filled with soil from the upper layer (ca. 15 cm) of the nearby oak forest. Both germination and seedling growth occurred in the nursery, which was covered by a black plastic mesh to provide partial shade. The seedlings in the nursery were watered frequently during the four-month period. In the second week of July 1991 ca. 100 seedlings were transplanted at each site at randomly chosen points along transects with a fixed orientation. Equal proportions of seedlings from each seed size class were planted at each site. Each seedling was tagged, and its initial size and location were recorded. From September 1991 seedling survival and growth (shoot height, basal diameter and crown area) were recorded monthly until September 1992 and every two months thereafter until the end of March 1993. Basal diameter was measured at a fixed, labelled height (ca. 1 cm from the soil), by placing a calliper on the wider axis of the shoot. Crown area was estimated from the mean of two measurements at right angles. Seedling damage was recorded as partial height loss (due to herbivory or top dieback) or total above-ground loss (due to desiccation, herbivory or some other cause, e.g. rotting). Subsequent censuses revealed whether the loss was temporary (i.e. the seedling resprouted) or permanent. Since the forest border is a heterogeneous habitat an attempt was made to classify the immediate habitat of each seedling in order to compare survivorship among them. Each seedling's microsite was classified as: (1) shaded, usually under the crown of established trees or large shrubs, (2) partially shaded by shrubs or close to the edge of the crown of established trees or (3) open.

Ecological Outcomes Achieved

Eliminate existing threats to the ecosystem:
The differences in canopy cover between sites were associated with differences in other physical characteristics. As expected, deep soils predominated at the closed forest site, while the disturbed site had a mainly rocky substrate, with a low proportion of microsites containing soil and litter. The open oak canopy of the border area had a more heterogeneous (patchy) substrate that can be considered as a transition between the former two sites. Throughout the year, maximum air temperatures were consistently 10 - 13 C higher at disturbed site than at the forest interior. After two months all acorns in the high density clusters had disappeared regardless of the site, while 30 % of the acorns remained in the 5- and l-seed clusters at the forest interior and in the l-seed clusters at the disturbed zone. Regardless of density, all seeds at the forest border had disappeared after two months. In all cases, however, some seeds remained on the ground for 35 days, which is enough time for a seed to germinate. After 35 days, the highest rate of removal was observed for the 25-seed clusters at the forest border, where 2 % of the seeds remained on the ground. Varying proportions, ranging from 10 % to 70%, of the seeds remained in the other cases. There were significant differences among sites for the 5- and 25-seed clusters, with more seeds than expected removed both at the forest border and the forest interior and an equal or lower number of seeds removed at the disturbed site. Seed germination and establishment (i.e, seedling rooting and elongation of the first flush of leaves) differed among sites, with significantly lower values in the disturbed zone, but no differences between the forest interior and the forest border. All seeds in completely open microsites had dried out in less than a week and only those under some kind of protective shade were able to germinate. The main causes of germination failure in the forest interior and border were rotting due to fungal attacks and predation of the emerging radicle (possibly by Diptera larvae). More than 95 % of the seedlings in the forest border and the disturbed site survived the first rainy season. At the forest interior mortality was slightly higher (12 %), mostly due to rabbits and small rodents that severed the shoots. The first dry season (November 1991- April 1992) had a great impact on survival at the disturbed site, where ca. 60 % of the seedlings died. The most conspicuous decline occurred in February and March, when a period of about 50 days without any appreciable precipitation occurred. Seedlings at the forest border and the forest interior had a higher survival, with only 13 % and 19 % of them dying during this dry season, respectively. During the second growing (rainy) season (May-November 1992) no seedlings died at the disturbed site and mortality was also very low at the forest border. As in the first growing season, more seedlings died at the forest interior during this period, mainly due to herbivory and fungal attacks. At the end of the two growing seasons the disturbed site had the lowest overall seedling survival (35 %), followed by the forest interior (60 %) and the forest border (76.5 %). The second dry season had a great impact on seedling survival, causing a drop from 76 % survival to 52 % at the forest border, similar to the final survival at the forest interior. Only 5 % of the seedlings at the disturbed site were alive after this drought period. In the two proportional risk analyses (one at the end of the second growing season and the other at the beginning of the third growing season), the forest interior was taken as the baseline against which comparisons were made. In both cases the model was highly significant, as judged by the likelihood ratio test. At the end of the second growing season (November 92) there were significant differences among the three sites, with the seedlings at the forest border having a 50 % lower relative risk compared to the forest interior, while at the disturbed site the risk was 80 % higher than at the forest interior. By April 1993, risk at the forest border had increased and was no longer significantly different from the forest interior, while at the disturbed site the difference increased, with a final risk 150 % higher than at the forest interior. The effect of the harsh second dry season (December 1992 -May 1993) is also evident in a comparison of seedling survival in each of the three types of microsite registered within the forest border. At the end of the second growing season (November 1992) there was relatively high survival (> 70 %) irrespective of the microsite, but at the end of March 1993 differences between partially shaded and open microsites had increased greatly (64 % and 25 % survival respectively). The relative importance of the different agents of mortality varied among sites. Overall, desiccation during the dry season was the main cause of seedling death at the disturbed site, while herbivory and rotting were particularly important at the forest interior, the main herbivore being the rabbit Sylvilagus floridanus. At the forest border there was not a single outstanding cause of death until the end of the second growing season, but after the 1993 dry season desiccation increased in importance, accounting for 62 % of the mortality observed. Out of a total of 204 dead shoots, 40 % resprouted. There were no significant differences among the proportions of seedlings resprouting at the disturbed site (32 %), the forest border (41 %) and the forest interior (49 %). The cause of shoot death significantly affected resprouting (P = 0.001), with a higher probability of resprouting after herbivory (59 %) than when a shoot desiccated (23 %) or rotted (29 %). At all three sites, there was a higher probability of resprouting if damage occurred during the rainy season (63 %) than during the dry season (28 %). At the disturbed site it is difficult to separate the effects of timing (i.e. shoot death occurring during the dry or rainy season) from desiccation, which leads to a low resprouting capacity. At the forest interior, where most damage to the seedlings was due to herbivory, this seasonal pattern persisted (64 % vs. 3 1 % resprouting after damage in the rainy and dry seasons, respectively). The period between damage and emergence of a new shoot varied from one to six months, with the majority of resprouting events occurring during the first three months. Late resprouting was more common at the forest interior than at the other two sites. In addition to the cases where all or most of the shoot died, there were frequent cases of partial dieback and partial removal by herbivores.

Factors limiting recovery of the ecosystem:
The Ajusco nature preserve has long been considered the "lung" of the Valley of Mexico with its abundant oak forests. With the pressure of the city's continued urbanization there are more and more impacts on the resources that surround the city, and the presence of squatter communities within the boundaries of the natural preserve itself. Additionally, the research found that high post-dispersal acorn predation is common and can limit oak regeneration. The rapid disappearance of high-density seed clusters, probably due to increased conspicuousness of clumped seeds, has been previously documented. The higher removal rates at the forest border and forest interior may reflect preferential foraging by small mammals, which are the main predators at the site, and the delayed consumption found at the disturbed site may be attributable to lower mammal densities and/or lower searching activities. These, in turn, could be related to a higher risk from small-mammal predators in the open and to the lack of acorn production at this site since the disturbance in 1988. Most of the removed acorns were eaten, which was determined by the frequent leftover shells. However, some removed acorns may be buried in caches by small mammals or birds and might escape predation if they germinate and become established seedlings before being recovered by a predator. In our study, this may occur at the forest interior and parts of the forest border, but not at the disturbed site, due to the impossibility of digging in the rocky substrate. A certain proportion of the seeds, most from the low-density clusters, remained in the ground long enough to germinate under adequate conditions. These conditions may be met in years with frequent rains during the period from November to January, simultaneous with or soon after seed shedding. Laboratory tests have shown that under favourable conditions, 85 % of viable Quercus rugosa acorns germinate after five weeks, with a mean of 15 days. It is very unlikely that a significant degree of germination and establishment occurs later than January, given that acorns do not remain viable in the soil for long periods and would also have to escape predation for 4 - 5 months, until the onset of the rainy season in May. If a viable acorn has escaped predation, the germination and establishment probabilities are relatively high at the forest border and the forest interior. The lower values found at the disturbed site are probably related to the high temperatures close to the ground and the relative scarcity of shaded microsites that protect the seeds and seedlings from desiccation. Additionally, the nature of the substrate at this site (basaltic rock) makes radicle penetration possible only in crevices where soil has been deposited. It has been reported at other localities that hard soils and high temperatures prevent oak regeneration and acorns exposed to full, day-long sunlight have little chance of germination.

Socio-Economic & Community Outcomes Achieved

Economic vitality and local livelihoods:
The difficulty assessing this particular case thirteen years on is the continual pressure on resources in the Valley of Mexico. Mexico City continues to grow and the squatter communities have continued to grow as the pressures of continued urbanization have been exacerbated. The need for resources and land will continue for the foreseeable future.

Key Lessons Learned

These results have clear implications for the restoration of oaks to the Ajusco region. As safe sites are scarce, planting acorns is not a sound restoration technique. Substrate characteristics will prevent adequate burial of acorns and high-density sowing while acorn desiccation and predation are to be expected. Other restoration techniques, based on seedling and juvenile introduction, may prove more successful and are discussed in detail below. Seedling survival differed strongly by site. The high mortality found at the disturbed site during both dry seasons strongly suggest that low water availability, especially during the period from December to April, is the main factor limiting seedling survival. In the absence of frequent rains, high temperatures near the ground and the porous rocky substrate that does not retain moisture cause extensive seedling death. The low death rates registered during the rainy season at this site in both years reinforce this view, as high temperatures continue until July without causing seedling death with adequate precipitation. Dehydration has also been reported as an important cause of death of Quercus engelmanii and Q. douglasii seedlings in California’s mediterranean climate. The higher survival of seedlings planted in the forest border until the end of the second growing season is probably related to the presence of a semi-open canopy that protects the seedlings from solar radiation and desiccation. Comparison between the two years suggests that, as a whole, this environment can maintain relatively high seedling survival when the drought is not too severe. However, the pervasive drought of 1993 (the growing season was delayed until June, because of unusually low rainfall during May), caused differences in survival associated with the specific microsites occupied by the seedlings. In those conditions persistence is closely associated with the relative position of the seedling under the canopy of established trees or shrubs. Muick (1991) and Callaway (1992; Callaway & Davis 1998) also found that shade and shrubs exert a facilitative effect on Quercus seedlings. This facilitation process is frequent in habitats such as deserts and sand dunes, where seedlings are likely to experience water deficiency. At the forest interior, canopy shade and litter favour the maintenance of soil humidity throughout most of the year and herbivory and rotting, which are common in seedlings growing under closed canopies, increase in importance. Resprouting, which is common in oaks, made an important contribution to seedling survival. The proportion of seedlings resprouting was high, similar to the 35 % reported for Q. engelmanii in California. The lack of a significant association between ability to resprout and site could be influenced by the fact that all seedlings had been grown in the nursery for approximately four months before being planted, and had probably accumulated similar levels of photosynthates. Quercus leucotrichophora seedlings growing beneath a closed canopy are less likely to possess enough carbohydrates to resprout. While biomass removal by herbivores does not imply any harm to the root, shoot desiccation probably entails root desiccation and thus seedling death. This is likely to account for the relationship observed between resprouting and cause of shoot loss. Resprouting capacity and water availability were also correlated, as shown by the differences found between the dry and rainy seasons. A significant correlation between timing of aerial biomass reduction and seedling survival and growth has been described in other tree species. A relationship between seed size and survival was
evident only under the relatively favourable conditions of the forest border. Other studies have shown a significant correlation between seed mass and oak seedling survival in the field and under experimental reduction of biomass, although it is not clear whether it is a general phenomenon in the genus. Our results point to the conclusion that under conditions favourable for seedling survival, differences established early during seedling development increase with time. As with survival, seedling growth was limited by moisture deficiency where open conditions prevailed, as shown by the abrupt declines in seedling size during each dry season at the disturbed site. Although surviving seedlings were able to recover at varying rates, the numbers surviving after two periods of drought were too low to make a contribution to a substantial recovery of the population. At the forest border the decline in size was less pronounced and a lower proportion of seedlings experienced size reductions. The low seedling growth found at the forest interior confirms the view that the shaded conditions of a closed canopy are inappropriate for oak seedling growth.
Significant reductions in the growth of other oak seedlings have been observed when grown in 8- 10% of full sunlight. The behaviour of planted seedlings of Q. rugosa under the forest canopy suggests that it is a relatively shade-intolerant species and the lack of persistence of naturally established seedlings under the forest canopy supports this assertion. Undamaged seedlings showed no differences in height between the forest border and the disturbed site, but the larger crown area of intact seedlings growing at the disturbed site and data on basal diameter (which is related to root biomass) suggest that they probably had larger total dry mass at the end of the second growing season, as has been described for other Quercus seedlings growing in full sunlight. However, overall growth was not severely reduced under the semi-open canopy of the forest border and the significantly higher proportion of seedlings growing and surviving at this site indicate that these conditions promote oak regeneration. It is very likely that the ability to survive in open conditions increases in later stages of the life-cycle, such as saplings and young trees. Neither seed sowing nor high-density seedling input seem to be good techniques for improving regeneration in this case. An alternative and ecologically more sound method should include selection of suitable microsites along the border of the forest or the selection or duplication of environmental conditions that mimic the forest border in disturbed sites. The partial shade provided by trees or shrubs seems to be especially relevant and established trees of Buddleia cordata and other large shrubs could be used; they are also a source of litter that may help to ameliorate fluctuations in soil temperature. Substrate conditions could also be improved by soil addition while planting. Selection of large seeds is advisable if one-year old or younger seedlings are to be used. In view of the importance of resprouting for survival, it is desirable to grow seedlings in relatively large containers and particular care of the seedlings’ roots is required during
transplanting, as starch reserves used for resprouting are stored in the roots. Much of the common practice of reforestation programs in Mexico relies on large-scale production of seedlings which are then transplanted extensively without a proper selection of adequate species, planting sites, or seedling quality. This project points to a different strategy, which might be generalized to seasonally dry and other xeric disturbed sites. This requires a previous knowledge of what a ‘safe site’ represents for the relevant species, which is crucial for the restoration of damaged ecosystems. Other relevant components of the program include better-trained personnel, more effort per plant and a lower overall density of seedlings planted. Under present conditions, such a method would likely yield more success in restoration programs.

Long-Term Management

Like many ecological problems in Mexico, the necessity that hinges on day-to-day survival often trumps the need for understanding long-term ecological sustainability. This case illustrates the often misunderstood nature of ecological problems in Mexico because it was caused and continues to be caused simply by these pressures of economic necessity. Perhaps the lesson to be learned here is that Mexico is ripe for a restoration economy and that through education in restoration techniques and the application of well targeted restoration monies, the country would be put to work restoring their forests and their future.

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