Background
Amazon palm swamp peatlands are major carbon (C) sinks and reservoirs. In Peru, this ecosystem is widely threatened owing to the recurrent practice of cutting
Mauritia flexuosa
palms for ...fruit harvesting. Such degradation could significantly damage peat deposits by altering C fluxes through fine root productivity, mortality, and decomposition rates which contribute to and regulate peat accumulation. Along a same peat formation, we studied an undegraded site (Intact), a moderately degraded site (mDeg) and a heavily degraded site (hDeg) over 11 months. Fine root C stocks and fluxes were monthly sampled by sequential coring. Concomitantly, fine root decomposition was investigated using litter bags. In the experimental design, fine root stocks and dynamics were assessed separately according to vegetation type (
M. flexuosa
palm and other tree species) and
M. flexuosa
age class. Furthermore, results obtained from individual palms and trees were site-scaled by using forest composition and structure.
Results
At the scale of individuals, fine root C biomass in
M. flexuosa
adults was higher at the mDeg site than at the Intact and hDeg sites, while in trees it was lowest at the hDeg site. Site-scale fine root biomass (Mg C ha
−1
) was higher at the mDeg site (0.58 ± 0.05) than at the Intact (0.48 ± 0.05) and hDeg sites (0.32 ± 0.03). Site-scale annual fine root mortality rate was not significantly different between sites (3.4 ± 1.3, 2.0 ± 0.8, 1.5 ± 0.7 Mg C ha
−1
yr
−1
at the Intact, mDeg, and hDeg sites) while productivity (same unit) was lower at the hDeg site (1.5 ± 0.8) than at the Intact site (3.7 ± 1.2), the mDeg site being intermediate (2.3 ± 0.9). Decomposition was slow with 63.5−74.4% of mass remaining after 300 days and it was similar among sites and vegetation types.
Conclusions
The significant lower fine root C stock and annual productivity rate at the hDeg site than at the Intact site suggests a potential for strong degradation to disrupt peat accretion. These results stress the need for a sustainable management of these forests to maintain their C sink function.
Mauritia flexuosa palm swamp, the prevailing Peruvian Amazon peatland ecosystem, is extensively threatened by degradation. The unsustainable practice of cutting whole palms for fruit extraction ...modifies forest's structure and composition and eventually alters peat‐derived greenhouse gas (GHG) emissions. We evaluated the spatiotemporal variability of soil N2O and CH4 fluxes and environmental controls along a palm swamp degradation gradient formed by one undegraded site (Intact), one moderately degraded site (mDeg) and one heavily degraded site (hDeg). Microscale variability differentiated hummocks supporting live or cut palms from surrounding hollows. Macroscale analysis considered structural changes in vegetation and soil microtopography as impacted by degradation. Variables were monitored monthly over 3 years to evaluate intra‐ and inter‐annual variability. Degradation induced microscale changes in N2O and CH4 emission trends and controls. Site‐scale average annual CH4 emissions were similar along the degradation gradient (225.6 ± 50.7, 160.5 ± 65.9 and 169.4 ± 20.7 kg C ha−1 year−1 at the Intact, mDeg and hDeg sites, respectively). Site‐scale average annual N2O emissions (kg N ha−1 year−1) were lower at the mDeg site (0.5 ± 0.1) than at the Intact (1.3 ± 0.6) and hDeg sites (1.1 ± 0.4), but the difference seemed linked to heterogeneous fluctuations in soil water‐filled pore space (WFPS) along the forest complex rather than to degradation. Monthly and annual emissions were mainly controlled by variations in WFPS, water table level (WT) and net nitrification for N2O; WT, air temperature and net nitrification for CH4. Site‐scale N2O emissions remained steady over years, whereas CH4 emissions rose exponentially with increased precipitation. While the minor impact of degradation on palm swamp peatland N2O and CH4 fluxes should be tested elsewhere, the evidenced large and variable CH4 emissions and significant N2O emissions call for improved modeling of GHG dynamics in tropical peatlands to test their response to climate changes.
Over 3 years of monitoring, Peruvian Amazon Mauritia flexuosa palm swamp peatlands showed significant N2O emissions and large CH4 emissions, the later exhibiting an exponential rise with increased precipitation. Their degradation, which results from cutting whole palms for fruit extraction, induced microscale (hummock and hollow of live and cut palms) changes in N2O and CH4 emission trends and controls but no obvious changes at the macroscale. Monthly and annual emissions were mainly controlled by variations in: WFPS, water table level (WT), and net nitrification for N2O; WT, air temperature and net nitrification for CH4.
Rainfall partitioning into throughfall and stemflow was studied during a year in a primary forest (tall forest) and in two adjacent secondary forests with contrasting structure (medium and low ...forests) in the Gran Sabana. Mean annual concentrations of nutrients in throughfall and stemflow were significantly higher (
P
<
0.05) than those in incident rainfall, particularly for K. Concentrations of N were very low not only in rainfall but also in throughfall and stemflow, indicating absence of N leaching in these forests. Concentration of P in rainfall was under the detection limit, and in throughfall and stemflow was very low. With very few exceptions, the mean annual concentrations of nutrients in throughfall and stemflow did not differ significantly (
P
>
0.05) between the forests, which seem to indicate that differences in structure and floristic composition do not play an important role in the nutrient dynamic at the canopy level. Throughfall and stemflow amounts represented 71–77% and 2–8% of the annual incident rainfall, respectively. Annual volumes of throughfall were quite similar in tall, medium and low forests, while the annual volume of stemflow decreased from tall to medium and low forests. With some exceptions, the annual inputs of nutrients in throughfall tended to be quite similar between the studied forests. Stemflow input tended to decrease with decreasing tree density from tall to medium and low forests, which is more related to the decrease in the volume of this flow from tall to medium and low forests than with changes in the element concentration of the stemflow among the forests. We concluded that incident rainfall is an important means of nutrient transfer from the forest canopy to the soils, and that a decrease in the tree density affected negatively the inputs of nutrients in stemflow but not in throughfall.
With the objective to contribute to a better understanding of ecological consequences of deforestation on the below-ground system in the Western Llanos of Venezuela, we evaluated the vertical ...distribution, nutrient concentration and seasonal changes of total fine root mass (FRM) (<2 mm diameter) in a semi-deciduous tropical dry forest and in 2 adjacent pastures of Cynodon nlemfuensis: a young pasture (YP, 5 years old) and an old pasture (OP, 18 years old) in the Obispo municipality, Barinas State. This evaluation included measurements at the end of the rainy season, during the dry season and during the subsequent early rainy season in 2005/2006. Highest FRM was recorded during the dry season, which probably indicates a plant water-stress response mechanism. The highest proportion (63‒88%) of FRM was concentrated in the 10–20 cm soil layer at all studied sites, probably due to a higher nutrient and moisture content at that depth. Non-significant differences (P>0.05) were observed in the total concentrations of organic carbon, nitrogen, phosphorus, calcium and magnesium in the FRM in soils supporting forest, OP and YP at the evaluated depths. Non-significant changes in the total FRM and nutrient concentrations were observed between the sampling periods and the 3 study sites. YP soils showed a slight increase in FRM that could be associated with the root growth of secondary vegetation, which is considered a weed and is periodically removed. Our results suggest that the land use change from tropical forest to pastures has not significantly affected the mass of fine roots and their carbon and nutrient concentrations. Further studies are needed to determine if these findings apply to other ecosystems.
We evaluated changes of different soil nitrogen forms (total N, available ammonium and nitrate, total N in microbial biomass, and soil N mineralization) after conversion of semideciduous dry tropical ...forest in 5- and 18-year-old pastures (YP and OP, resp.) in the western Llanos of Venezuela. This evaluation was made at early rainy season, at end rainy season, and during dry season. With few exceptions, no significant differences were detected in the total N in the three study sites. Compared to forest soils, YP showed ammonium losses from 4.2 to 62.9% and nitrate losses from 20.0 to 77.8%, depending on the season of the year. In OP, the ammonium content increased from 50.0 to 69.0% at the end of the rainy season and decreased during the dry season between 25.0 and 55.5%, whereas the nitrate content increased significantly at early rainy season. The net mineralization and the potentially mineralizable N were significantly higher P < 0.05 in OP than in forest and YP, which would indicate a better quality of the substrate in OP for mineralization. The mineralization rate constant was higher in YP than in forest and OP. This could be associated with a reduced capacity of these soils to preserve the available nitrogen.
Litter decomposition and nutrient dynamic were studied in tall primary forest (TF) and in adjacent slightly fire-affected (MF) and strongly fire-affected (LF) forests of the Gran Sabana, southern ...Venezuela. The aim of the study was to compare the mass and nutrient loss of litter in undisturbed forest and adjacent fire-disturbed forests growing under the same soil conditions. The results showed no significant differences in the dry-mass reduction among TF, MF, and LF after 1-year of decomposition. At the end of the decomposition period, the mass loss was 31% in TF, 24% in MF, and 25% in LF. With few exceptions, the initial nutrient content of the litter did not show significant differences among the studied forests. The initial litter was poor in nutrients, especially in P, with C/N ratio extremely high, particularly in LF. Both residual P content and C/N ratio were the most effective predictors of dry-mass loss. The general trend in the three studied forests was net N and to less extend P immobilization and release of K, Mg, and Ca. We concluded that during a 1-year period, the decomposition process was similar in undisturbed and adjacent fire-disturbed forests in the Gran Sabana and that the low litter decomposition seems to be mainly controlled by the low chemical quality of the decomposing litter.
There have been several ecological studies in forests of the Guayana Shield, but so far none had examined the changes in structure and composition of evergreen forests with altitude. This study ...describes and analyzes the structure, species composition and soil characteristics of forest stands at different altitudinal zones in Southeastern Venezuelan Guayana, in order to explain the patterns and the main factors that determine the structure and composition of evergreen forests along the altitudinal gradient. Inventories of 3 948 big (>10cm DBH) and 1 328 small (5-10cm DBH) woody stems were carried out in eleven plots, ranging from 0.1 to 1.0ha, along a 188km long transect with elevations between 290 and 1 395masl. It has been found that 1) hemiepihytes become more dominant and lianas reduce their dominance with increasing altitude and 2) the forest structure in the study area is size-dependent. Five families and 12 genera represented only 9% of the total number of families and genera, respectively, recorded troughout the gradient, but the two groups of taxa comprised more than 50% of the Importance Value (the sum of the relative density and the relative dominance) of all measured stems. Moreover, the results suggest that low species richness seems to be associated with the dominance of one or few species. Stand-level wood density (WD) of trees decreased significantly with increasing elevation. WD is an indicator of trees'life history strategy. Its decline suggests a change in the functional composition of the forest with increasing altitude. The Canonical Correspondence Analysis (CCA) indicated a distinction of the studied forests on the basis of their altitudinal levels and geographic location, and revealed different ecological responses by the forests, to environmental variables along the altitudinal gradient. The variation in species composition, in terms of basal area among stands, was controlled primarily by elevation and secondarily by rainfall and soil conditions. There are other interacting factors not considered in this study like disturbance regime, biological interactions, productivity, and dispersal history, which could affect the structure and composition of the forests in the altitudinal gradient. In conclusion, it appears that the structural and floristic variability observed in the studied transect is produced by a combination of different climates and randomly expressed local processes interacting across a complex physical landscape.
Tropical peat swamp forests are major global carbon (C) stores highly vulnerable to human intervention. In Peruvian Amazonia, palm swamps, the prevalent peat ecosystem, have been severely degraded ...through recurrent cutting of
Mauritia flexuosa
palms for fruit harvesting. While this can transform these C sinks into significant sources, the magnitude of C fluxes in natural and disturbed conditions remains unknown. Here, we estimated emissions from degradation along a gradient comprising undegraded (Intact), moderately degraded (mDeg) and heavily degraded (hDeg) palm swamps. C stock changes above- and below-ground were calculated from biomass inventories and peat C budgets resulting from the balance of C outputs (heterotrophic soil respiration (Rh), dissolved C exports), C inputs (litterfall, root mortality) and soil CH
4
emissions. Fluxes spatiotemporal dynamics were monitored (bi)monthly over 1–3 years. The peat budgets (Mg C ha
−1
year
−1
) revealed that medium degradation reduced by 88% the soil sink capacity (from − 1.6 ± 1.3 to − 0.2 ± 0.8 at the Intact and mDeg sites) while high degradation turned the soil into a high source (6.2 ± 0.7 at the hDeg site). Differences stemmed from degradation-induced increased Rh (5.9 ± 0.3, 6.2 ± 0.3, and 9.0 ± 0.3 Mg C ha
−1
year
−1
at the Intact, mDeg, and hDeg sites) and decreased C inputs (8.3 ± 1.3, 7.1 ± 0.8, and 3.6 ± 0.7 Mg C ha
−1
year
−1
at the same sites). The large total loss rates (6.4 ± 3.8, 15.7 ± 3.8 Mg C ha
−1
year
−1
under medium and high degradation), originating predominantly from biomass changes call for sustainable management of these peatlands.
P fractions and sorption processes were studied in samples taken from the organic surface layer and in the underlying mineral soil of a forest-savanna sequence consisting of: (1) tall primary forest ...(TPF), (2) tall secondary forest (MSF), (3) low secondary forest (LSF), and (4) open savanna (S) in la Gran Sabana, South Venezuela. The organic surface layer in the TPF and MSF showed the highest P concentrations in all analysed P fractions. P in this organic layer was mainly associated with inorganic forms, suggesting that this layer is an important source of bio-available P. The organic surface layer was not present in LSF and S probably because of the occurrence of recurrent surface fires. The conversion of forest to savanna influenced the distribution of the different forms of P in the soil. While non-occluded (resin-+NaOH-P extractable) and organic (NaHCO^sub 3^-+NaOH-+HCl-Po) P declined from the forest to savanna, occluded (concentrated HCl-extractable+residual P) forms increased. The correlation between sorption maxima and soil organic C was not significant; however, organically bound forms of Al were the main component that explained the adsorption capacity of these soils. The above findings suggest that the organic surface layer and the soil organic matter are important for maintaining P fertility in the undisturbed and little disturbed forests. However, when the system is heavily perturbed by fire the organic surface layer, the main P source, disappears and the patterns of P cycling change.PUBLICATION ABSTRACT
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