Víctor J. Jaramillo

BIOMASS, CARBON, AND NUTRIENT DYNAMICS OF SECONDARY FORESTS IN A HUMID TROPICAL REGION OF MÉXICO

Tropical secondary forests have the capacity to function as large carbon and nutrient sinks and may offset losses resulting from deforestation and land use. In the heavily deforested Los Tuxtlas Region of Mexico, aboveground biomass as well as aboveground and mineral soil C, N, S, and P pools were quantified in 11 secondary forest sites. These sites ranged in age from 6 mo to 50 yr following abandonment and had experienced between 1 and 30 yr of land use prior to abandonment. Total aboveground biomass (TAGB) increased with increasing site age and ranged from 4.8 Mg/ha in a recently abandoned site to 287 Mg/ha in the 50-yr-old secondary forest site. Results indicate that secondary forests would reach TAGB levels equivalent to those of primary forests in the Los Tuxtlas Region after 73 yr. Furthermore, mean annual aboveground biomass accumulation (ABA) of secondary forests was strongly and inversely related to the duration of prior land use. Aboveground pools of C, N, S, and P were also positively correlate...

Biomass, Carbon, and Nitrogen Pools in Mexican Tropical Dry Forest Landscapes

Tropical dry forest is the most widely distributed land-cover type in the tropics. As the rate of land-use/land-cover change from forest to pasture or agriculture accelerates worldwide, it is becoming increasingly important to quantify the ecosystem biomass and carbon (C) and nitrogen (N) pools of both intact forests and converted sites. In the central coastal region of México, we sampled total aboveground biomass (TAGB), and the N and C pools of two floodplain forests, three upland dry forests, and four pastures converted from dry forest. We also sampled belowground biomass and soil C and N pools in two sites of each land-cover type. The TAGB of floodplain forests was as high as 416 Mg ha–1, whereas the TAGB of the dry forest ranged from 94 to 126 Mg ha–1. The TAGB of pastures derived from dry forest ranged from 20 to 34 Mg ha–1. Dead wood (standing and downed combined) comprised 27%–29% of the TABG of dry forest but only about 10% in floodplain forest. Root biomass averaged 32.0 Mg ha–1 in floodplain forest, 17.1 Mg ha–1 in dry forest, and 5.8 Mg ha–1 in pasture. Although total root biomass was similar between sites within land-cover types, root distribution varied by depth and by size class. The highest proportion of root biomass occurred in the top 20 cm of soil in all sites. Total aboveground and root C pools, respectively, were 12 and 2.2 Mg ha–1 in pasture and reached 180 and 12.9 Mg ha–1 in floodplain forest. Total aboveground and root pools, respectively, were 149 and 47 kg ha–1 in pasture and reached 2623 and 264 kg ha–1 in floodplain forest. Soil organic C pools were greater in pastures than in dry forest, but soil N pools were similar when calculated for the same soil depths. Total ecosystem C pools were 306. The Mg ha–1 in floodplain forest, 141 Mg ha–1 in dry forest, and 124 Mg ha–1 in pasture. Soil C comprised 37%–90% of the total ecosystem C, whereas soil N comprised 85%–98% of the total. The N pools lack of a consistent decrease in soil pools caused by land-use change suggests that C and N losses result from the burning of aboveground biomass. We estimate that in México, dry forest landscapes store approximately 2.3 Pg C, which is about equal to the C stored by the evergreen forests of that country (approximately 2.4 Pg C). Potential C emissions to the atmosphere from the burning of biomass in the dry tropical landscapes of México may amount to 708 Tg C, as compared with 569 Tg C from evergreen forests.

Grazing History, Defoliation, and Competition: Effects on Shortgrass Production and Nitrogen Accumulation

Plants of Bouteloua gracilis were collected from heavily grazed prairie dog (Cynomys ludovicianus) colonies (ON—colony) and lightly grazed uncolonized sites (OFF—colony) in Wind Cave National Park, South Dakota. Our objectives were to determine (1) how grazing history affected production, nitrogen uptake, and biomass and nitrogen allocation following defoliation and (2) how such responses were affected by competing neighbors of the same population. The growth chamber experiment was factorial with three main treatments in 12 possible combinations. Main treatments were: population (plants from lightly and heavily grazed sites), defoliation (three intensities), and competition (with or without neighbors of the same population). There were no significant interactions among treatments. OFF—colony plants produced 121% as much biomass and their N. yield was 203% as great as ON—colony plants. ON—colony plants allocated a higher percentage of biomass and N to the roots from OFF—colony plants, while the latter allo...

The effects of slash burning on ecosystem nutrients during the land preparation phase of shifting cultivation

The most commonly observed change in soil following slash-and-burn clearing of tropical forest is a short-term increase in nutrient availability. Studies of shifting cultivation commonly cite the incorporation of nutrient-rich ash from consumed aboveground biomass into soil as the reason for this change. The effects of soil heating on nutrient availability have been examined only rarely in field studies of slash-and-burn, and soil heating as a mechanism of nutrient release is most often assumed to be of minor importance in the field. Few budgets for above and belowground nutrient flux have been developed in the tropics, and a survey of results from field and laboratory studies indicates that soils are sufficiently heated during most slash-and-burn events, particularly in dry and monsoonal climates, to cause significant, even substantial release of nutrients from non-plant-available into plant-available forms in soil. Conversely, large aboveground losses of nutrients during and after burning often result in low quantities of nutrients that are released to soil. Assessing the biophysical sustainability of an agricultural practice requires detailed information about nutrient flux and loss incurred during management. To this end, current conceptual models of shifting cultivation should be revised to more accurately describe these fluxes and losses.

ECOSYSTEM-SCALE IMPACTS OF DEFORESTATION AND LAND USE IN A HUMID TROPICAL REGION OF MEXICO

Deforestation of tropical evergreen forests is a major contributor to increasing levels of greenhouse gases in the atmosphere. However, large uncertainties currently exist concerning the quantities of C and other elements lost to the atmosphere due to the conversion of primary forests to pastures and agricultural lands. Elemental losses associated with land conversion in the heavily deforested Los Tuxtlas Region of Mexico were quantified. Total aboveground biomass (TAGB) as well as carbon and nutrient pools in aboveground vegetation and soils were measured along a land-use gradient that included primary forests as well as pastures and cornfields, which represent the dominant land-use types in the region. TAGB of primary forests in the Los Tuxtlas Region averaged 403 Mg/ha; pasture and cornfield sites averaged 24 and 23 Mg/ha, respectively. Approximately 80% of TAGB of forests was composed of trees >30 cm in diameter at breast height (dbh), while trees >70 cm dbh accounted for 44% of TAGB. Conversion of fo...

Seasonally Dry Tropical Forests: Nutrient cycling in tropical deciduous forests

Introduction Nutrient cycling processes have been well documented for tropical moist forest (Vitousek & Sanford, 1986; Bruijnzeel, 1991) but few comprehensive syntheses exist for tropical and subtropical dry and deciduous forests (Lugo & Murphy, 1986; Singh, 1989). Tropical dry forests are considered among the most threatened tropical ecosystems (Janzen, 1988) because they experience considerable exploitative pressure (Murphy & Lugo, 1986). In India, such pressures have been responsible for the transformation of vast areas of deciduous forest into savanna (Singh, 1989). The current rate of destruction of deciduous forest makes it imperative that we gain a thorough understanding of nutrient cycling in the remaining intact and successional forests. At present one of the principal agricultural practices relies on forest slash burning, resulting not only in recurrent nutrient losses thereby affecting the long-term productivity of the system but also substantially contributing to emissions of C and N to the atmosphere (Kauffman, Sanford & Sampaio, 1990; Maass, Chapter 17). Seasonally dry forest production is controlled by the amount and distribution of annual rainfall (Martinez-Yrizar, Chapter 13), and this may explain why nutrients have not been considered in detail (Murphy & Lugo, 1986; Singh, 1989). For example in a recent review of forest nutrient cycling there was only one reference to tropical dry forest (Vogt, Grier & Vogt, 1986). If we assume that water availability alone limits primary production in tropical deciduous forest, we could conclude that nutrient limitation is not important. However, multiple resource limitation of plant growth is common in natural communities (Chapin et al., 1987).

Phosphorus cycling in a Mexican tropical dry forest ecosystem

The study was conducted in five contiguous small watersheds (12–28 ha) gauged for long-term ecosystem research. Five 80 × 30 m plots were used for the study. We quantified inputs from the atmosphere, dissolved and particulate-bound losses, throughfall and litterfall fluxes, standing crop litter and soil available P pools. Mean P input and output for a six-year period was 0.16 and 0.06 kg⋅ha−1⋅yr−1, respectively. Phosphorus concentration increased as rainfall moved through the canopy. Annual P returns in litterfall (3.88 kg/ha) represented more than 90% of the total aboveground nutrient return to the forest floor. Phosphorus concentration in standing litter (0.08%) was lower than that in litterfall (0.11%). Phosphorus content in the litterfall was higher at Chamela than at other tropical dry forests. Mean residence time on the forest floor was 1.2 yr for P and 1.3 yr for organic matter. Together these results suggest that the forest at Chamela may not be limited by P availability and suggest a balance between P immobilization and uptake. Comparison of P losses in stream water with input rates from the atmosphere for the six-year period showed that inputs were higher than outputs. Balances calculated for a wet and a dry year indicated a small P accumulation in both years.

Small-scale heterogeneity in a semi-arid North American grassland. I : Tillering, N uptake and retranslocation in simulated urine patches

1. We examined the response of two grass species differing in growth form and photosynthetic pathway to simulated cattle urine deposition in a native semi-arid grassland. The species were Agropyron smithii, a spreading, rhizomatous C3 grass, and Bouteloua gracilis, a cespitose C4 grass. They are codominants in this grassland and A. smithii is the most important forage species in the study site. 2. Simulated urine increased tiller density of both species, with B. gracilis increasing in the first growing season while A. smithii was not increased until the second year. Both species had increased tiller densities and above-ground N concentrations in a ring outside the area wetted by urine. The percentage of retranslocated leaf N decreased, and standing dead litter N concentration increased in both species after urine deposition. 3. Above-ground biomass, N yield, and tiller height of A. smithii increased in urine patches. Leaf production was not affected by treatment. Both the increase in the live:dead biomass ratio and the higher number of live leaves at the end of the first growing season suggested urine deposition delayed senescence of A. smithii. 4. The implications of plant growth form in the spread of urine, the potential duration of urine effects in semi-arid grasslands, and the role of herbivore urine deposition in promoting grassland structural and functional heterogeneity are discussed.

Dissolved organic carbon affects soil microbial activity and nitrogen dynamics in a Mexican tropical deciduous forest

Seasonal variation of dissolved organic C (DOC) and its effects on microbial activity and N dynamics were studied during two consecutive years in soils with different organic C concentrations (hilltop and hillslope) in a tropical deciduous forest of Mexico. We found that DOC concentrations were higher at the hilltop than at the hillslope soils, and in both soils generally decreased from the dry to the rainy season during the two study years. Microbial biomass and potential C mineralization rates, as well as dissolved organic N (DON) and NH4+ concentrations and net N immobilization were higher in soils with higher DOC than in soils with lower DOC. In contrast, net N immobilization and NH4+ concentration were depleted in the soil with lowest DOC, whereas NO3− concentrations and net nitrification increased. Negative correlations between net nitrification and DOC concentration suggested that NH4+ was transformed to NO3− by nitrifiers when the C availability was depleted. Taken together, our results suggest that available C appears to control soil microbial activity and N dynamics, and that microbial N immobilization is facilitated by active heterotrophic microorganisms stimulated by high C availability. Soil autotrophic nitrification is magnified by decreases in C availability for heterotrophic microbial activity. This study provides an experimental data set that supports the conceptual model to show and highlight that microbial dynamics and N transformations could be functionally coupled with DOC availability in the tropical deciduous forest soils.

Small-Scale Heterogeneity in a Semi-Arid North American Grassland. II. Cattle Grazing of Simulated Urine Patches

1. This study examined the effects of simulated urine deposition on the selectivity and intensity of cattle grazing on tillers of Agropyron smithii, a spreading, rhizomatous C3 grass in a native semi-arid grassland. 2. Tiller density was not affected by treatment during 1987 but it increased in urine patches 12 months after treatment. 3. Up to July, 2 months after treatment, more tillers were grazed in urine patches than in controls, although this difference disappeared later in the growing season. Tillers were grazed to a lower height, and blade length was shorter, when growing in simulated urine patches under both stocking rates. 4. The mean height of grazed tillers and its variance were greater under moderate than under heavy stocking. 5. Above-ground N concentration of tillers was higher in urine patches. 6. The results suggest the interactions among selective grazing, urine deposition, and grazing pressure increased the structural heterogeneity of the grassland canopy.