Constraints
on nutrient cycling during recovery of dry tropical forests in Yucatan, Mexico:
Precipitation, soil fertility, and intensity of prior disturbance
Deborah
Lawrence and David Foster
Dry tropical forests have been characterized as slow to recover from, but highly
resilient to disturbance (Lugo and Murphy 1986). The forests of the Yucatan peninsula have
been chronically, if infrequently, disturbed by fire and hurricanes-similar in many ways
to current widespread disturbance associated with shifting cultivation. In addition, 2000
yrs of Mayan domination of the landscape may have preselected a group of species which
respond well to relatively large-scale, intense disturbances such as shifting cultivation.
We are investigating the role of precipitation, soil fertility, and recent disturbance
history in determining the rate and trajectory of ecosystem recovery in these forests.
Like many areas of the tropics, much of the land in our southern Yucatan study
region is in secondary forest which has regenerated from shifting cultivation of maize, or
milpa agriculture, following waves of colonization over the past 20-50 years. This system
of maize cultivation continues to be practiced by an ever-increasing number of colonists
in the region, thus it is and will continue to be a major driver in forest dynamics. Our
research on nutrient cycling and vegetation change will enhance our ability to make
projections about the ecological response of the forests themselves and about the future
productivity of land for non-forest and non-milpa alternatives, an important benefit in
this time of rapid development and land-use change.
We are building on an impressive body of work on ecosystem response following
shifting cultivation in the tropics (Ewel 1976, Uhl and Jordan 1984, Tiessen et al 1992)
with one of few studies in seasonally dry tropical forest. Our study is unique, however,
in its broad regional scope. We are undertaking process-oriented studies simultaneously at
locations spanning 10s-100s of kilometers. This regional approach allows us to investigate
directly the effect of differences in precipitation and soils without the confounding
effects of year to year differences in climate. It also enables us to understand
constraints on scaling up from site-specific studies to regional projections.
Using natural gradients in precipitation, depth to watertable, and soil
fertility, and human-induced gradients in patch age and land-use intensity, we are
investigating variability in the process of succession throughout the southern Yucatan
region. Our specific questions include: 1) how rapidly are nutrient stocks restored
following forest conversion and maize cultivation, and how are they partitioned between
plant biomass and soils? 2) how rapidly do the structure and composition of the tree
community recover, and how are they related to changes in nutrient-cycling? 3) how are
nutrient-cycling processes in secondary forests affected by cultivation history, water and
nutrient availability?
 Studies
of ecosystem recovery are underway in three areas of the southern Yucatan. These three
succession sequences comprise 38 stands ranging in age from 2-25 years since abandonment,
plus old growth. We are monitoring litterfall and forest floor dynamics at 10-15 sites in
each of the three regions. (Click on image to see larger version.) |
 At
each site, litter is collected monthly from four 1-m2 traps systematically arrayed within
one 500-m2 plot. All fallen debris will be collected twice (once in the wet season and
once in the dry) from four 1-m2 plots on the forest floor. Initial results suggest that
litterfall peaks after 5-8 years, then declines with age. It increases, as expected, with
precipitation. Forest floor accumulation increases with both age and precipitation. Our
results suggest greater production and more rapid decomposition in young stands, which may
contribute to quick recovery of these Mayan-legacy forests. |
 Future
work to investigate a general hypothesis of more rapid nutrient cycling in young forests
will examine several additional aspects of the nutrient cycle. These include rooting
dynamics, microbial biomass and nutrient transformations, and chemical allocation in leaf
tissue. |
References
Uhl, C. and C. F. Jordan. 1984. Succession and nutrient dynamics following
forest cutting and burning in Amazonia. Ecology 65: 1476-1490.
Tiessen, H., I. H. Salcedo, and E. V. S. B. Sampaio. 1992. Nutrient and organic
matter dynamics under shifting cultivation in semi-arid northeastern Brazil. Agriculture,
Ecosystems, and Environment 38: 139-151.
Ewel, J. 1976. Litter fall and leaf decomposition in a tropical forest
succession in Eastern Guatemala. Journal of Ecology. 64: 293-307 |
