Daniel J. Zarin

ECOSYSTEM DEVELOPMENT AND PLANT SUCCESSION ON LANDSLIDES IN THE CARIBBEAN

Landslides are common in mountainous regions of the Caribbean and are triggered by heavy rains and earthquakes, and often occur in association with human disturbances (e.g., roads). Spatially heterogeneous removal of both substrate and vegetation is responsible for a variety of patterns of ecosystem development and plant successional trajectories within Caribbean landslides. Soil nutrient pools in exposed mineral soils reach levels comparable to mature forest soils within 55 yr but soil organic matter recovers more slowly. Plant colonization of landslides depends on the availability of propagules and suitable sites for germination, soil stability, and the presence of residual or newly deposited soil organic matter and associated nutrients. Once initial colonization occurs, the rate and trajectory of plant succession on landslides is strongly affected by plant/plant interactions. We present two conceptual models of landslide succession that summarize the major processes and pathways of ecosystem development and plant succession on landslides. Additional work is needed to characterize interactions between spatially heterogeneous zones, controls over soil development, impacts of key plant species, and the role of animals on Caribbean landslides.

Legacy of fire slows carbon accumulation in Amazonian forest regrowth

Amazonian farmers and ranchers use fire to clear land for agriculture and pasture as part of extensive land-use strategies that have deforested 500 000 km2 over the past 25 years. Ash from burning biomass fertilizes crops and pastures, but declining productivity often occurs after a few years, generally leading to land abandonment and further clearing. Subsequent forest regrowth partially offsets carbon emissions from deforestation, but is often repeatedly cleared and burned. In the first quantitative, basin-wide assessment of the effect of repeated clearing and burning on forest regrowth, our analysis of data from 93 stands at nine locations across the region indicates that stands with a history of five or more fires suffer on average a greater than 50% reduction in carbon accumulation. In the absence of management interventions, Amazonian landscapes dominated by this pronounced legacy of fire are apt to accumulate very little carbon and will remain highly susceptible to recurrent burning.

Post-Boom Logging in Amazonia

Recent analyses of timber exploitation in Amazonia conclude that a variety of socioeconomic and ecological factors in the region make a stable and profitable logging industry virtually impossible. Most of these studies focus on large-scale timber industries and their dependence on over-exploitation of a small number of high-value timbers. In this article we discuss the economic, ecological, and social aspects of Amazonian logging in a region where the timber industry appeared to have collapsed after stocks of high-value timber were exhausted. We show that forestry in a post-boom phase, currently found in many areas of Amazonia, differs from the better-described “boom” period in its scale of operations, in the range of timbers cut, in management practices employed, and in the costs and benefits of production. Results of a seven-year study show that when sawtimber, poles and firewood are produced in a management system that combines forestry and agriculture they can provide significant additional income for Amazonian smallholders.

Temporal variability of forest fires in eastern Amazonia

Widespread occurrence of fires in Amazonian forests is known to be associated with extreme droughts, but historical data on the location and extent of forest fires are fundamental to determining the degree to which climate conditions and droughts have affected fire occurrence in the region. We used remote sensing to derive a 23-year time series of annual landscape-level burn scars in a fragmented forest of the eastern Amazon. Our burn scar data set is based on a new routine developed for the Carnegie Landsat Analysis System (CLAS), called CLAS-BURN, to calculate a physically based burn scar index (BSI) with an overall accuracy of 93% (Kappa coefficient 0.84). This index uses sub-pixel cover fractions of photosynthetic vegetation, non-photosynthetic vegetation, and shade/burn scar spectral end members. From 23 consecutive Landsat images processed with the CLAS-BURN algorithm, we quantified fire frequencies, the variation in fire return intervals, and rates of conversion of burned forest to other land uses in a 32 400 km2 area. From 1983 to 2007, 15% of the forest burned; 38% of these burned forests were subsequently deforested, representing 19% of the area cleared during the period of observation. While 72% of the fire-affected forest burned only once during the 23-year study period, 20% burned twice, 6% burned three times, and 2% burned four or more times, with the maximum of seven times. These frequencies suggest that the current fire return interval is 5-11 times more frequent than the estimated natural fire regime. Our results also quantify the substantial influence of climate and extreme droughts caused by a strong El Niño Southern Oscillation (ENSO) on the extent and likelihood of returning forest fires mainly in fragmented landscapes. These results are an important indication of the role of future warmer climate and deforestation in enhancing emissions from more frequently burned forests in the Amazon.

Potential biomass accumulation in Amazonian regrowth forests.

Biomass accumulation in the secondary forests of abandoned pastures and slash-and-burn agricultural fallows is an important but poorly constrained component of the regional carbon budget for the Brazilian Amazon. Using empirical relationships derived from a global analysis, we predicted potential aboveground biomass accumulation (ABA) for the regions regrowth forests based on soil texture and climate data. For regrowth forests on nonsandy soils, the globally derived relationship provided a nearly unbiased linear predictor of Amazonian validation data consisting of 66 stands at seven sites; there was no significant difference between stands that regrew following use as pasture land and those that regrew following slash-and-burn agriculture. For regrowth forests on nonsandy soil, the 1 sigma error range of our ABA model was 58%–171% for the Amazonian validation data. For regrowth forests on sandy soils, the validation data were limited to 19 stands at one site, and the globally derived relationship was substantially biased multiplicatively and nonlinearly. Hence we developed a regional refinement by adding to our validation data ABA values from the two Amazonian sites with sandy soil that had previously been included in the global analysis. Based on a conservative jackknife goodness-of-fit assessment (leaving out one site at a time), we calculated a 1 sigma error range of 42%–158% for our sandy soil Amazonian regrowth forest ABA model. We present our predictions of potential regrowth forest ABA as a set of 0.5° resolution maps for the region at 5, 10, and 20 years following abandonment.

Forest community analysis and the point-centered quarter method.

The point-centered-quarter (PCQ) method has been applied in community analysis since the publication of the method nearly 50 years ago. This and other distance methods offer increased sampling efficiency over fixed-area plots (FAP), but have long been known to produce biased density estimates when plant distribution deviates from random spatial patterns. Spatial indices have been developed to quantify the direction of this bias when plant distributions are aggregated or evenly distributed. Its continued use, especially in community analysis, requires additional scrutiny in measurements of community structure. We measured 14 forest stands of varying age, elevation and disturbance regime using FAP and PCQ methods. Density estimates were biased, with the point-centered quarter method lower than fixed-area plot estimates when stems were aggregated and higher when stems were evenly spaced. In general the PCQ method underestimated species richness. The efficiency of the PCQ method makes it popular for ordination studies, although comparison of community structure varied from 18% to 90% similarity between the measurements of species basal area in the same stands using the two different methods. The bias observed in calculations of stem density, species abundance and community similarity indicate that use of the PCQ method should be approached with caution when used in community level analysis.

Recovery Of Forest Structure And Spectral Properties After Selective Logging In Lowland Bolivia

Effective monitoring of selective logging from remotely sensed data requires an understanding of the spatial and temporal thresholds that constrain the utility of those data, as well as the structural and ecological characteristics of forest disturbances that are responsible for those constraints. Here we assess those thresholds and characteristics within the context of selective logging in the Bolivian Amazon. Our study combined field measurements of the spatial and temporal dynamics of felling gaps and skid trails ranging from <1 to 19 months following reduced-impact logging in a forest in lowland Bolivia with remote-sensing measurements from simultaneous monthly ASTER satellite overpasses. A probabilistic spectral mixture model (AutoMCU) was used to derive per-pixel fractional cover estimates of photosynthetic vegetation (PV), non-photosynthetic vegetation (NPV), and soil. Results were compared with the normalized difference in vegetation index (NDVI). The forest studied had considerably lower basal area and harvest volumes than logged sites in the Brazilian Amazon where similar remote-sensing analyses have been performed. Nonetheless, individual felling-gap area was positively correlated with canopy openness, percentage liana coverage, rates of vegetation regrowth, and height of remnant NPV. Both liana growth and NPV occurred primarily in the crown zone of the felling gap, whereas exposed soil was limited to the trunk zone of the gap. In felling gaps >400 m2, NDVI, and the PV and NPV fractions, were distinguishable from unlogged forest values for up to six months after logging; felling gaps <400 m2 were distinguishable for up to three months after harvest, but we were entirely unable to distinguish skid trails from our analysis of the spectral data.

DROUGHT CONSTRAINTS ON LEAF GAS EXCHANGE BY MICONIA CILIATA (MELASTOMATACEAE) IN THE UNDERSTORY OF AN EASTERN AMAZONIAN REGROWTH FOREST STAND

Analyses of the effects of drought stress on Amazonian regrowth stands are lacking. We measured leaf gas exchange and leaf water potential of Miconia ciliata (Melastomataceae) in a dry-season irrigation experiment in 14-yr-old regrowth. In the dry season, irrigated plants maintained significantly higher leaf water potentials, photosynthetic capacity at light saturation (A(max)), stomatal conductance (g(s)), internal CO(2) concentration (C(i)), and lower A(max)/g(s) than control plants. The degree of dry-season down-regulation of control plant A(max), along with its fast recovery following rain, reveals the importance of occasional dry-season rains to the carbon budget of M. ciliata. During the wet season, we observed higher A(max) for control plants than for plants that had been irrigated during the dry season. We hypothesize that reduced drought constraints on photosynthesis of irrigated plants advanced the flowering and fruiting phenology of irrigated plants into the dry season. Flowers and fruits of control plants developed later, during the wet season, potentially stimulating a compensatory reproductive photosynthesis response in nearby leaves. The relative drought intolerance of M. ciliata may be a deciding factor in its ability to survive through the dynamic successional development of the regrowth stand studied.

Análise florística e estrutural de uma floresta em diferentes estágios sucessionais no município de Castanhal, Pará

The objective of this study is to understand secondary forest succession through florisitc and structural analysis of a forest in three successional stages (4, 8 and 12 years) located in Castanhal in the state of Para. This study considers wood species divided into two DBH classes: class I (DBH>1cm) and class II (DBH<1cm). Class I individuals were measured in twelve 10m x 10m plots in the 12-year-old successional forest, and in four 10 x 10 m plots in the 4 and 8-year-old secondary forest stands. Class II individuals were measured in 48 subplots of 1m x 1m in the 12-year-old forest stand, and in 16 subplots in the 4 and 8-year-old forest stands. In Class I, 18, 30 and 73 species were identified; and were found 12, 18 and 21 individuals/ha in the 4, 8 and 12-year-old forests, respectively. In class II, 17, 21 and 62 species were identified; and were found 50, 26 and 47 individuals/m2 in the 4, 8 and 12-year-old forests, respectively. For class I, Lacistema pubescens, Vismia guianensis and Myrcia sylvatica demonstrated the greatest abundance and relative dominance. In class II, Lacistema pubescens, Vismia guianensis, Miconia ciliata, Myrcia bracteata and Banara guianensis also displayed an elevated number of individuals. Myrcia sylvatica presented the greatest abundance in the three successional stages. Similarity among plots was approximately 60% for class I and 42% for class II. The results showed the forest with three stages of successional development: initiation phase (4 years), exclusion phase (8 years), and the beginning of the understory reinitiation phase (12 years).

Effects of seasonality, litter removal and dry-season irrigation on litterfall quantity and quality in eastern Amazonian forest regrowth, Brazil

Litterfall quantity and quality may respond to alterations in resource availability expected with ongoing land-use and climate changes. Here, we quantify the effects of altered resource availability on non-woody litterfall quantityandquality(nitrogenandphosphorusconcentrations)ineasternAmazonianforestregrowth(Brazil)through two multi-year experimental manipulations: (1) daily irrigation (5mm d −1 ) during the dry season; and (2) fortnightly litterremoval.Consistentwithothertropicalforestdatalitterfallexhibitedseasonalpatterns,increasingwiththeonset of the dry season and declining with the onset of the rainy season. Irrigation did not affect litterfall mass and had little impactonnitrogen(N)orphosphorus(P)concentrationsandreturn,exceptfordecreasinglitterPconcentrationatthe end of two irrigation periods. Litter removal did not alter litterfall mass or P concentration, but progressively reduced litterfall N during the course of the experiment. Overall, these results suggest significant resistance to altered resource availabilitywithintheboundsofourexperimentaltreatments;ourfindingsmayhelptoconstraincarbonandnutrient cycling predictions for tropical forests in response to land-use and climate changes.