richness and diversity of natural regeneration of trees in pastures
A significant number of tree species are able to regenerate naturally within managed pastures. In 1 ha of grazed pasture, we found an average of 10 630 seedlings and 354 saplings belonging to 60 and 30 tree species, respectively. The extensive cattle management traditionally used in Muy Muy (manual weeding, infrequent use of herbicides, large paddocks and infrequent rotation of cattle) probably favours the natural regeneration of trees in the pastures. The fact that such management practices characterize a large proportion of the cattle production regions of Central America (Carr & Langhammer 2005) suggests that it is possible to maintain populations of native tree species in managed pastures and illustrates the potential for managing natural regeneration as a means to conserve tree diversity within tropical agricultural landscapes (Gordon et al. 2003).
Our results suggest that there are two different groups of tree species present in pastures: (i) those with an adequate natural regeneration and (ii) those with a limited natural regeneration. These two groups reflect different capacities of tree species to establish seedlings, saplings and adults in active pastures, because of ecological conditions in pasture systems and farmers’ management practices.
Forty-six per cent of the 85 tree species found in pastures appeared to have high probabilities of maintaining their populations in active pastures through natural regeneration, as individuals of these species were found in all three size classes. The tree species with the highest importance indices (IVI and IVIs) within this group were pioneer species that are dispersed by cattle or wind and have high resprouting capacities (Stevens et al. 2001). These characteristics favour the regeneration of these species, allowing them successfully to overcome the low availability of seeds, high solar radiation, high evapotranspiration, degraded soils, grazing and trampling by cattle, and the extraction and pollarding of trees by farmers, which are considered important barriers to tree establishment on pastures elsewhere (Nepstad et al. 1996; Holl 1999).
Tree species with the highest IVI showed different strategies to overcome natural regeneration barriers in grazed pastures. Species dispersed by cattle (e.g. Guazuma ulmifolia, Cassia grandis, Enterolobium cyclocarpum and Leucaena shannonii) are probably favoured by the presence of cattle and may be widely dispersed in cattle dung in the paddocks (Janzen 1981; Somarriba 1986; Radford et al. 2001; Traba, Levassor & Begoña 2003). Wind-dispersed pioneer species, such as Tabebuia rosea and Cordia alliodora, are readily dispersed from nearby forest borders into the adjacent pastures (Augspurger & Franson 1988). The presence of remnant forest patches and isolated trees in pastures also enhances seed dispersal by wild animals into the pastures (Holl 1999; Harvey 2000). Tree species with high resprouting capacity, such as Guazuma ulmifolia and Gliricidia sepium, have a reduced mortality of seedlings and saplings caused by cattle browsing and trampling (Hobbs & Mooney 1985; Cordero & Boshier 2003). In addition, many of the common tree species that regenerate in pastures are retained by farmers because of their value as forage (leaves or fruits), timber or shade for cattle (Harvey & Haber 1999; Cordero & Boshier 2003).
Fifty-six per cent of the 85 tree species found in these pastures were represented in only one or two of the three size classes evaluated, suggesting that they have limited natural regeneration and low probabilities of maintaining their populations on active pasturelands. This group of species includes both species that seem to be unable to regenerate within the pastures, and others that may be regenerating but are removed by current management practices. For example, 16 species within this group appear to be unable to recruit new individuals to maintain their populations in pastures. These species are unable to regenerate in these pastures because of low densities of adults in the landscape, inbreeding or unfavourable microsite conditions in pastures (Janzen 1986). However, these trees could still disperse pollen to other trees in the landscape (fragments or continuous forest) and may contribute to gene exchange and species population dynamics at the landscape level (White, Boshier & Powell 2002).
Another 18 tree species appear able to colonize pastures but do not survive to an adult stage. The absence of adult trees of these species, despite the presence of seedlings and saplings, could be the result of natural growth patterns (e.g. small tree species that rarely reach a diameter of 10 cm, such as Capparis frondosa) or of limited regeneration conditions (Stevens et al. 2001). The absence of individuals in the largest size class could also indicate that these species have just recently colonized the pastures and have not yet reached the adult stage, as has been observed in other colonization studies with pioneer species (Lichstein, Grau & Aragón 2004). Alternately, the high temperature and humidity fluctuations could limit establishment and growth of seedlings and saplings of these tree species in open pastures (Conrado, Chavelas & Camacho 1993; Gerhardt 1999; Hooper, Legendre & Condit 2005). In addition, the absence of adult trees of these species on active pasturelands could be the result of cattle activity (trampling, browsing) and/or pasture management practices (e.g. manual weeding with machetes), which have been identified as strong selective factors driving population dynamics of some tree species in active pastures elsewhere (Camargo et al. 1999; Kindt, Simon & Van Damme 2004).
Fourteen tree species were absent in the seedling size class and may be unable to colonize active pastures. There are several potential explanations for the absence of seedlings of these species. First, it is possible that some of these species were actively regenerating within the pastures but were not found as seedlings because (i) the trees were already taller than 30 cm at the time the study was conducted, (ii) these species only regenerate in specific microhabitats or under certain climatic conditions that were not present during the study period, or (iii) these species may regenerate in pulses (that occur supra-annually) and their seedlings were absent during the survey because it was potentially an unfavourable year for seedling establishment (Cornett et al. 2000). Secondly, these species may be unable to establish within the active pastures because of the pressures from cattle grazing and manual weeding, as has been reported for some tree species elsewhere (Camargo et al. 1999).
pasture management effects on tree seedlings
The species richness, density and composition of tree seedlings in the pastures were influenced by the composition of the grass layer, as well as the particular management practices associated with each pasture type. The fact that the greatest species richness of seedlings was found in pastures dominated by Brachiaria spp. compared with pastures with naturalized grass or pastures with Cynodon spp. is probably not only because of the growth characteristics of this grass species under active management (which may permit a large number of seeds to reach the soil and germinate) but because of the high density and species richness of adult trees in these pastures, as well as the cattle management and the closeness of these pastures to forest patches. The erect bunch-forming growth of Brachiaria spp. in this region (up to 1 m tall; Johnson et al. 2005) probably increases the availability of microsites appropriate for the germination and establishment of tree seedlings, particularly during the dry season (Gerhardt 1999), in comparison with the creeping, mat-forming architecture of Cynodon nlemfuensis (Fairfax & Fensham 2000), which impedes tree seeds from reaching the soil (M.J. Esquivel, personal observation). At the same time, the location of the Brachiaria pastures next to secondary forests and the fact that these pastures tend to have a high density and species richness of adult trees (Sánchez et al. 2005) increase the probability that these pastures receive a large seed rain (Holl 1999).
Finally, differences in the way in which the different types of pastures are managed may have an impact on seedling mortality. In Brachiaria pastures, where cattle are usually managed on a rotational basis (with individual paddocks being grazed for 4 days, followed by 30 days of recovery), seedling mortality by trampling is probably low (Simon et al. 1997). In contrast, the naturalized pastures are usually grazed continuously (i.e. without any recovery periods in dry seasons) and seedlings in these pastures are therefore likely to suffer greater mortality.
The history of fire use, on the other hand, showed no clear effect on seedling species richness or density. Studies elsewhere have shown that the use of fire in pastures can positively select for species with seeds that are fire tolerant or experience enhanced germination under fire (Conrado, Chavelas & Camacho 1993), while reducing the emergence of seedlings of other species that are negatively affected by the changes in understorey cover and microclimatic conditions in burnt areas (Setterfield 2002). However, our study found no clear effects of fire on regeneration patterns. There were no effects of burning on natural regeneration within either Cynodon or Paspalum pastures. The observed differences in seedling composition between Brachiaria pastures with different fire-use histories most probably reflects differences in the proximity of the pastures to forest, rather than a fire-history effect per se. The Brachiaria pastures that were not recently burned were located closer to secondary forests than Brachiaria pastures that had been burnt within the last 5 years, making it difficult to distinguish a fire-history effect from the effects of higher seed rain from the adjacent forest. It is possible, however, that the effects of burning are only obvious in the first months or years following burning, rather than the 5-year time frame used here, so additional research is merited.
The results of this study provide evidence against the commonly held notion that tropical trees in pastures are ‘living dead’ and incapable of maintaining their populations in active pastures, and instead point to the potential of natural regeneration in pastures to contribute significantly to the conservation of tree diversity in agricultural landscapes. Our results suggest that a significant number of the tree species censused can overcome the regeneration barriers that exist in pastures. In addition, our results show that the current pasture management conditions, such as grass composition, type of cattle grazing, farmer management of adult tree density and composition in the pastures and the presence of nearby forests, collectively determine the composition, richness and density of tree seedlings in active pastures.
However, unless specific measures are taken, the species richness and diversity of tree cover in these agricultural landscapes will gradually decrease over the long term because of the limited regeneration of certain tree species. Selective retention of saplings or seedlings during pasture management practices and protection against cattle activities could enhance the probability of some of these species reaching adult stages (Camargo et al. 1999). Manual dispersal of seeds into pastures, seedling transplantation from other habitats to favourable microsites on pastures, the use of more selective weeding practices (that retain more seedlings), the prevention of overgrazing (controlled rotation) and the physical protection of saplings from cattle could also enhance the natural regeneration of species with early colonization limitations (Simon et al. 1997; Martínez-Garza & Howe 2006). In addition, the retention of a high diversity of tree cover (particularly forest cover) within farms could help to ensure the continued dispersal of propagules into pastures (Holl 1999; Esquivel, Calle & Silverstone 2001; Harvey, Tucker & Estrada 2004). Collectively, these changes in pasture management could greatly enhance the natural regeneration of trees within active pastures and thereby contribute to the long-term conservation of tree diversity within the agricultural landscapes that dominate much of the tropics.