Ecology

Acclimation of tropical tree species to hurricane disturbance: ontogenetic differences

Wen, S.Y., Fetcher, N. & Zimmerman, J.K. (2008) Acclimation of tropical tree
species to hurricane disturbance: ontogenetic differences. Tree Physiology,
28, 935–946.

Abstract: 
We investigated acclimation responses of seedlings and saplings of the pioneer species Cecropia schreberiana Miq. and three non-pioneer species, Dacryodes excelsa Vahl, Prestoea acuminata (Willdenow) H.E. Moore var. montana (Graham) Henderson and Galeano, and Sloanea berteriana Choisy ex DC, following a hurricane disturbance in a lower montane wet forest in Puerto Rico. Measurements were made, shortly after passage of the hurricane, on leaves expanded before the hurricane (pre-hurricane leaves) and, at a later time, on recently matured leaves that developed after the hurricane (post-hurricane leaves) from both seedlings and saplings at sites that were severely damaged by the hurricane (disturbed sites) and at sites with little disturbance (undisturbed sites). Pre-hurricane leaves of the non-pioneer species had relatively low light-saturated photosynthetic rates (Amax) and stomatal conductance (gs); neither Amax nor gs responded greatly to the increase in irradiance that resulted from the disturbance, and there were few significant differences between seedlings and saplings. Pre-hurricane leaves of plants at undisturbed sites had low dark respiration rates per unit area (Rd) and light compensation points (LCP), whereas pre-hurricane leaves of plants at disturbed sites had significantly higher Rd and LCP. Post-hurricane leaves of plants at disturbed sites had significantly higher Amax and Rd than plants at undisturbed sites. Compared with seedlings, saplings had higher Amax and Rd and showed greater acclimation to the increase in irradiance that followed the disturbance. Post-hurricane leaves of the non-pioneer species had significantly lower Amax and were less responsive to changes in irradiance than the pioneer species C. schreberiana. Variation in Amax across light environments and stages was strongly related to differences in leaf mass per unit area (LMA), especially in the non-pioneer species. As indicated by Vcmax or Jmax per unit nitrogen, light acclimation of Amax was determined by leaf morphology (LMA) for the nonpioneer species and by both leaf morphology and leaf biochemistry for C. schreberiana. Ontogenetic changes in Amax were attributable to changes in leaf morphology. The ontogenetic component of variation in Amax across light environments and stages differed among species, ranging from 36 to 59% for the non-pioneer species (D. excelsa, 59.3%; P. acuminata var. montana, 44.7%; and S. berteriana, 36.3%) compared with only 17% in the pioneer species C. schreberiana.

Early successional woody plants facilitate and ferns inhibit forest development on Puerto Rican landslides

Walker, L.R., Landau, F.H., Velázquez, E., Shiels,
A.B. and Sparrow, A.D. (2010). Early successional
woody plants facilitate and ferns inhibit forest
development on Puerto Rican landslides. Journal
of Ecology 98, 625-35.

Abstract: 
1. The experimental removal of early successional species can explain how plant communities change over time. 2. During a 7.3-year period, early successional woody species, scrambling ferns and tree ferns were removed from a total of 10 landslides in the Luquillo Experimental Forest in north-eastern Puerto Rico. 3. Early successional woody plants in combination with tree ferns decreased species richness and cover of forbs and increased richness of late-successional woody plants compared to removals, facilitating long-term forest development. 4. Dense stands of scrambling ferns decreased both forb and woody plant richness compared to removals, inhibiting forest development. 5. Stands of monospecific tree ferns initially increased woody plant richness compared to removals, but overall decreased woody plant richness and cover, inhibiting forest development. 6. Synthesis. Early successional species both facilitate and inhibit succession on tropical landslides, but detailed predictions of successional trajectories remain elusive and are influenced by stochastic processes including arrival order, the life-form of colonizing species and their competitive interactions.

The use of chronosequences in studies of ecological succession and soil development

Walker LR, Wardle DA, Bardgett RD, Clarkson BD (2010) The
use of chronosequences in studies of ecological succession
and soil development. J Ecol 98:725–736

Abstract: 
1. Chronosequences and associated space-for-time substitutions are an important and often necessary tool for studying temporal dynamics of plant communities and soil development across multiple time-scales. However, they are often used inappropriately, leading to false conclusions about ecological patterns and processes, which has prompted recent strong criticism of the approach. Here, we evaluate when chronosequences may or may not be appropriate for studying community and ecosystem development. 2. Chronosequences are appropriate to study plant succession at decadal to millennial time-scales when there is evidence that sites of different ages are following the same trajectory. They can also be reliably used to study aspects of soil development that occur between temporally linked sites over time-scales of centuries to millennia, sometimes independently of their application to shorter-term plant and soil biological communities. 3. Some characteristics of changing plant and soil biological communities (e.g. species richness, plant cover, vegetation structure, soil organic matter accumulation) are more likely to be related in a predictable and temporally linear manner than are other characteristics (e.g. species composition and abundance) and are therefore more reliably studied using a chronosequence approach. 4. Chronosequences are most appropriate for studying communities that are following convergent successional trajectories and have low biodiversity, rapid species turnover and low frequency and severity of disturbance. Chronosequences are least suitable for studying successional trajectories that are divergent, species-rich, highly disturbed or arrested in time because then there are often major difficulties in determining temporal linkages between stages. 5. Synthesis. We conclude that, when successional trajectories exceed the life span of investigators and the experimental and observational studies that they perform, temporal change can be successfully explored through the judicious use of chronosequences.

Ecosystem Development and Plant Succession on Landslides in the Caribbean

Ecosystem Development and Plant Succession on Landslides in the Caribbean
Lawrence R. Walker, Daniel J. Zarin, Ned Fetcher, Randall W. Myster and Arthur H. Johnson
Biotropica
Vol. 28, No. 4, Part A. Special Issue: Long Term Responses of Caribbean Ecosystems to Disturbances (Dec., 1996), pp. 566-576

Abstract: 
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.

Natural disturbance and human land use as determinants of tropical forest dynamics: results from a forest simulator

Uriarte, M., C. D. Canham, J. Thompson, J. K. Zimmerman,
L. Murphy, A. M. Sabat, N. Fetcher, and B. L.
Haines. 2009. Natural disturbance and human land
use as determinants of tropical forest dynamics:
results from a forest simulator. Ecological Monographs
79:423–443.

Abstract: 
Forests are often subject to multiple, compounded disturbances, representing both natural and human-induced processes. Predicting forest dynamics requires that we consider how these disturbances interact to affect species demography. Here we present results of an individual-based, spatially explicit forest simulator that we developed to analyze the compounded effects of hurricane disturbance and land use legacies on the dynamics of a subtropical forest. We used data from the 16-ha Luquillo Forest Dynamics Plot in Puerto Rico, together with a reconstruction of historical wind damage, to parameterize the simulator. We used the model to ask two questions. (1) What are the implications of variation in hurricane frequency and severity for the long-term dynamics of forest composition, diversity, and structure? Both storm severity and frequency had striking effects on forest dynamics, composition, and structure. The periodicity of disturbance also played an important role, with periods of high hurricane activity fostering the establishment of species that may become rare in the absence of severe storms and quiescent periods allowing these species to reach reproductive size. Species responses to hurricane disturbance could not be predicted from their life history attributes. However, species perceived to be primary forest species exhibited low temporal variation in abundance through the simulations. (2) How do hurricanes and legacies from human land use interact to determine community structure and composition? Our results suggest that, over time, regardless of the storm regime, land use legacies will become less apparent but will lead to a forest community that contains a mixture of secondary and primary forest species formerly dominant in areas of different land use. In the long term, hurricane disturbance generated two communities with slightly greater similarity than those not subject to storms. Thus, the inclusion of hurricane disturbance does not alter the prediction that land use legacies in this tropical forest will diminish over time. Our simulations also highlight the contingent effects of human legacies on subsequent community dynamics, including the response to hurricane disturbance, therefore supporting the notion that compounded disturbances can interact in ways that cannot be predicted by the study of single disturbances. The widespread importance of land use as a large-scale disturbance makes it imperative that it be addressed as a fundamental ecological process.

Land Use History, Environment, and Tree Composition in a Tropical Forest

Thompson, Jill; Brokaw, Nicholas; Zimmerman, Jess K.; Waide, Robert B.; Everham, Edwin M. III; Lodge, D. Jean; Taylor, Charlotte M.; Garcia-Montiel, Diana; Fluet, Marcheterre 2002. Land use history, environment, and tree composition in a tropical forest. Ecological applications. Vol. 12, no. 5 (2002): pages 1344-1363.

Abstract: 
The effects of historical land use on tropical forest must be examined to understand present forest characteristics and to plan conservation strategies. We compared the effects of past land use, topography, soil type, and other environmental variables on tree species composition in a subtropical wet forest in the Luquillo Mountains, Puerto Rico. The study involved stems > 10 cm diameter measured at 130 cm above the ground, within the 16-ha Luquillo Forest Dynamics Plot (LFDP), and represents the forest at the time Hurricane Hugo struck in 1989. Topography in the plot is rugged, and soils are variable. Historical documents and local residents described past land uses such as clear-felling and selective logging followed by farming, fruit and coffee production, and timber stand improvement in the forest area that now includes the LFDP. These uses ceased 40-60 yr before the study, but their impacts could be differentiated by percent canopy cover seen in aerial photographs from 1936. Using these photographs, we defined four historic cover classes within the LFDP. These ranged from cover class 1, the least tree-covered area in 1936, to cover class 4, with the least intensive historic land use (selective logging and timber stand improvement). In 1989, cover class 1 had the lowest stem density and proportion of large stems, whereas cover class 4 had the highest basal area, species richness, and number of rare and endemic species. Ordination of tree species composition (89 species, 13 167 stems) produced arrays that primarily corresponded to the four cover classes (i.e., historic land uses). The ordination arrays corresponded secondarily to soil characteristics and topography. Natural disturbances (hurricanes, landslides, and local treefalls) affected tree composition, but these effects did not correlate with the major patterns of species distributions on the plot. Thus, it appears that forest development and natural disturbance have not masked the effects of historical land use in this tropical forest, and that past land use was the major influence on the patterns of tree composition in the plot in 1989. The least disturbed stand harbors more rare and endemic species, and such stands should be protected.

RIPARIAN VEGETATION AND STREAM CONDITION IN A TROPICAL AGRICULTURE–SECONDARY FOREST MOSAIC

Heartsill-Scalley T, Aide TM. 2003. Riparian vegetation and stream condition
in a tropical agriculture–secondary forest mosaic. Ecological
Applications 13: 225–234.

Abstract: 
Changes in land cover from forest to agriculture often alter riparian vegetation, which modifies the physical conditions of streams. To understand the impacts of different categories of land cover on riparian and stream habitats, we sampled riparian vegetation and stream conditions in three adjacent watersheds in southeastern Puerto Rico. Land cover categories (pasture, mixed, and forest) were determined using aerial photographs. Vegetation structure and composition and characteristics of streams were assessed for 35 riparian sites. Sites were located along first-order streams, at 400–600 m elevation in the wet-forest life zone. Understory vegetation in the forest sites was mainly shrubs, herbs, and ferns, whereas the mixed and pasture sites were dominated by grasses, vines, and bare soil. Syzygium jambos and Spathodea campanulata, nonnatives, and Guarea guidonia, a native, were the most common tree species in the riparian areas. Surrounding land cover explained .60% of the variation among stream sites. There was a positive relationship between tree cover and percentage of dissolved oxygen, and a negative relationship between tree cover and percentage of substrata covered by sediments from eroded soil. The amount of woody debris in the streams tended to increase with forest cover. Overall, land cover is a landscape feature that effectively characterized riparian understory cover, tree species composition, and stream condition

Possible Environmental Factors Underlying Amphibian Decline in Eastern Puerto Rico: Analysis of U.S.Government Data Archives

STALLARD, R. F. 2001. Possible environmental factors
underlying amphibian decline in eastern Puerto Rico:
analysis of U.S. government data archives. Conservation
Biology 15:943–953.

Abstract: 
The past three decades have seen major declines in populations of several species of amphibians at high elevations in eastern Puerto Rico, a region unique in the humid tropics because of the degree of environmental monitoring that has taken place through the efforts of U.S. government agencies. I examined changes in environmental conditions by examining time-series data sets that extend back at least into the 1980s, a period when frog populations were declining. The data include forest cover; annual mean, minimum, and maximum daily temperature; annual rainfall; rain and stream chemistry; and atmospheric-dust transport. I examined satellite imagery and air-chemistry samples from a single National Aeronautics and Space Administration aircraftflight across the Caribbean showing patches of pollutants, described as thin sheets or lenses, in the lower troposphere. The main source of these pollutants appeared to be fires from land clearing and deforestation, primarily in Africa. Some pollutant concentrations were high and, in the case of ozone, approached health limits set for urban air. Urban pollution impinging on Puerto Rico, dust generation from Africa (potential soil pathogens), and tropical forest burning (gaseous pollutants) have all increased during the last three decades, overlapping the timing of amphibian declines in eastern Puerto Rico. None of the data sets pointed directly to changes so extreme that they might be considered a direct lethal cause of amphibian declines in Puerto Rico. More experimental research is required to link any of these environmentalfactors to this problem.

Migration patterns, densities, and growth of Neritina snails in Rio Espiritu Santo and Rio Mameyes, northeastern Puerto Rico

PYRON, MARK; COVICH, ALAN P. 2003. Migration Patterns, Densities, and Growth of Neritina punctulata Snails in Rio Espiritu Santo and Rio Mameyes, Northeastern Puerto Rico.. Caribbean Journal of Science, Vol. 39, No. 3, 338-347, .

Abstract: 
Snail size-frequency distributions in Rios Espiritu Santo and Mameyes, which drain the Luquillo Experimental Forest, Puerto Rico, showed that Neritina punctulata with shell lengths greater than 30 mm were the most abundant size class at upstream sites. The highest densities for all size classes were at the downstream sites. Growth rates were 0.015 mm/day for a large cohort (¡«25 mm shell length) and 0.035 mm/day for a small (¡«15 mm shell length). Minimum longevity estimates range from 3 to 7 years. Size distribution data suggest that snails migrate upstream. An August mark-recapture study resulted in most snails remaining at the release site, and some moved downstream. The greatest upstream distance reached was 200 m in 12 weeks. In a May mark-recapture study, when snails were observed moving in aggregations, the greatest distance moved was 200 m in 27 days, suggesting seasonality in movements and reproduction. Movement rates of snails in aggregates in May were between 0.08 and 0.17 cm/s. All of the snails marked in May moved upstream, and none were observed downstream from, or at the release site. We suggest that upstream movements can be explained by snails avoiding increased predation in the downstream reaches of these rivers from fishes, crustaceans, and birds

The potential for carbon sequestration through reforestation of abandoned tropical agricultural and pasture lands

Silver, W.L. et al. (2000) The potential for carbon sequestration
through reforestation of abandoned tropical agricultural and pasture
lands. Rest. Ecol. 8, 394–407

Abstract: 
Approximately half of the tropical biome is in some stage of recovery from past human disturbance, most of which is in secondary forests growing on abandoned agricultural lands and pastures. Reforestation of these abandoned lands, both natural and managed, has been proposed as a means to help offset increasing carbon emissions to the atmosphere. In this paper we discuss the potential of these forests to serve as sinks for atmospheric carbon dioxide in aboveground biomass and soils. A review of literature data shows that aboveground biomass increases at a rate of 6.2 Mg ha−1 yr−1 during the first 20 years of succession, and at a rate of 2.9 Mg ha−1 yr−1 over the first 80 years of regrowth. During the first 20 years of regrowth, forests in wet life zones have the fastest rate of aboveground carbon accumulation with reforestation, followed by dry and moist forests. Soil carbon accumulated at a rate of 0.41 Mg ha−1yr−1 over a 100-year period, and at faster rates during the first 20 years (1.30 Mg carbon ha−1 yr−1). Past land use affects the rate of both above- and belowground carbon sequestration. Forests growing on abandoned agricultural land accumulate biomass faster than other past land uses, while soil carbon accumulates faster on sites that were cleared but not developed, and on pasture sites. Our results indicate that tropical reforestation has the potential to serve as a carbon offset mechanism both above- and belowground for at least 40 to 80 years, and possibly much longer. More research is needed to determine the potential for longer-term carbon sequestration for mitigation of atmospheric CO2 emissions.
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