Biomass, Carbon, And Nutrient Dynamics Of Secondary Forests In A Humid Tropical Region Of Mexico

Ecology, Sept, 1999 by R. Flint Hughes, J. Boone Kauffman, Victor J. Jaramillo

INTRODUCTION

Secondary forests are rapidly becoming a common land cover type in tropical regions of the world. It has been estimated that [approximately]9 x [10.sup.6] ha of secondary forests are formed annually within these regions (Brown and Lugo 1990a). While secondary succession has always been an integral process in tropical forest ecosystems following natural disturbances (e.g., hurricanes and tree falls due to senescence), current increases in secondary forest cover are primarily the result of anthropogenic disturbances such as logging and conversion of forests to pasture and agricultural lands (Brown and Lugo 1990a, Skole and Tucker 1993).

In general, secondary forests have rapid rates of aboveground production, particularly during the initial stages of succession. Previous studies have documented rates of biomass accumulation of up to 100 Mg/ha after the first 15 yr of succession (Ewel 1980, Brown and Lugo 1990a, Lugo 1992, Lugo and Brown 1992). Consequently, secondary forests have the potential to assimilate and store relatively large fractions of the carbon and nutrients that are lost during deforestation and land use phases (Kauffman et al. 1995, Kauffman et al. 1998, Hughes et al., in press). Lugo and Brown (1992) estimated that forest fallows (i.e., secondary forests) may sequester between 0.6 and 1.4 Pg of C each year in tropical regions, or 40-90% of the estimated annual C emissions resulting from biomass burning in those regions. In this way, secondary forests in tropical regions have the capacity to play an integral role in ameliorating increasing levels of atmospheric C[O.sub.2] by assimilating and storing C in the terrestrial biosphere (Brown et al. 1996). However, Vitousek (1991) cautioned that while tree regeneration on deforested land results in potentially rapid carbon and nutrient accumulation, this storage is likely to be an ephemeral effect due to the shifting nature of land use in the tropics, which ultimately determines the long-term fate of stored C and nutrients. Given the growing prevalence of secondary forests in tropical regions and their potential importance to global carbon and nutrient cycles, a greater understanding of their development and functioning has been needed. Indeed, a body of knowledge regarding tropical secondary forest succession and function is accumulating in response to this need, and a number of developmental and functional patterns, as well as the potential controls on the variation of such patterns, have emerged from this growing database (Ewel 1980, Brown and Lugo 1990a, Lugo 1992, Zarin and Johnson 1995, Johnson et al. 2000).

While it is true that numerous studies have documented biomass and elemental pool dynamics and functional changes of tropical secondary forests through successional time (e.g., Ewel 1971, Toky and Ramakrishnan 1983, Uhl and Jordan 1984, Saldarriaga et al. 1988), less attention has been focused on the influence of prior land use on secondary forest succession. Since the majority of secondary forests in tropical regions establish and develop on lands that have experienced periods of management (e.g., as croplands or pastures), an increased understanding of how prior land use might constrain tropical secondary forest development, and thus constrain the potential rates of terrestrial C storage in tropical regions, is called for. Consequently, one of the primary objectives of this study was to determine the degree to which duration of prior land use affects accumulation rates of biomass, C, and nutrients in tropical secondary forests of the Los Tuxtlas Region of Mexico.

Additionally, accurate calculation of carbon budgets at both regional and national scales in the tropics depends on our capacity to quantify the accumulation of C by secondary forests now present in the regions and/or nations that have experienced high rates of deforestation. In Mexico, rates of deforestation in tropical evergreen forest systems have been exceedingly high. During the 1980s, [approximately]5000 [km.sup.2] of tropical forests were deforested annually (Masera et al. 1997). Hughes et al. (in press) showed that conversion of primary forests to pastures and cornfields in the Los Tuxtlas Region of Mexico resulted in dramatic reductions in total aboveground biomass as well as C, N, S, and P pools; declines ranged from 85 to 95% of the biomass and elemental pools contained in primary forest vegetation. Such losses of C, N, and S via combustion (i.e., anthropogenic biomass burning) and decomposition are of particular importance as these elements represent the major constituents of numerous radiatively active gas species currently accumulating in the earth's atmosphere (Levine 1990, Taylor and Penner 1994). While tropical secondary forests have been identified as a potential terrestrial C sink in Mexico (Masera et al. 1997), relatively little is currently known regarding potential rates of biomass and elemental accumulation by that nation's tropical secondary forests. To date only one study has quantified aboveground biomass and nutrient dynamics in secondary forests of the Mexican tropics (Williams-Linera 1983).