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.

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

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
Vol. 28, No. 4, Part A. Special Issue: Long Term Responses of Caribbean Ecosystems to Disturbances (Dec., 1996), pp. 566-576

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.

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.

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.

Plant responses to simulated hurricane impacts in a subtropical wet forest, Puerto Rico

Shiels, Aaron B.; Zimmerman, Jess K.; García-Montiel, Diana C.; Jonckheere, Inge; Holm, Jennifer; Horton, David; Brokaw, Nicholas. 2010. Plant responses to simulated hurricane impacts in a subtropical wet forest, Puerto Rico. Journal of Ecology. doi: 10.1111/j.1365-2745.2010.01646.x.

1. We simulated two key components of severe hurricane disturbance, canopy openness and detritus deposition, to determine the independent and interactive effects of these components on woody plant recruitment and forest structure. 2. We increased canopy openness by trimming branches and added or subtracted canopy detritus in a factorial design. Plant responses were measured during the 4-year study, which followed at least 1 year of pre-manipulation monitoring. 3. The physical conditions of canopy openness and detritus deposition in our experiment resembled the responses to Hurricane Hugo, a severe category 4 hurricane that struck this forest in 1989. 4. Canopy detritus deposition killed existing woody seedlings and provided a mechanical barrier that suppressed seedling recruitment. The increase in understorey light caused by canopy trimming stimulated germination from the seed bank and increased seedling recruitment and density of pioneer species several hundred-fold when hurricane debris was absent. Many significant interactions between trimming and detritus deposition were evident from the manner in which seedling density, recruitment and mortality changed over time, and subsequently influenced the composition of woody stems (individuals ‡ 1 cmd.b.h.). 5. When the canopy was trimmed, stem densities increased> 2-fold and rates of recruitment into the stem size class increased> 25-fold. Trimming had no significant effect on stem mortality. The two dominant species that flourished following canopy trimming were the pioneer species Cecropia schreberiana and Psychotria berteriana. Deposition of canopy detritus had little effect on stems, although basal area increased slightly when detritus was added. There were no evident effects of the interactions between canopy trimming and detritus deposition on stems. 6. Synthesis. The separate and interactive effects of canopy openness and detritus deposition result in variable short-term trajectories of forest recovery. However, the short interval of increased canopy openness due to hurricane impacts and its influence on the recruitment of pioneer trees is the dominant factor that drives short-termrecovery and may alter long-term structure and composition of the forest.

Invertebrate communities in a tropical rain forest canopy in Puerto Rico following Hurricane Hugo

Schowalter, T. W., and L. M. Ganio. 1999. Invertebrate communities
in a tropical rain forest canopy in Puerto Rico
following Hurricane Hugo. Ecological Entomology 24:

1. Canopy invertebrate responses to Hurricane Hugo, tree species, and recovery time were examined at the Luquillo Experimental Forest in Puerto Rico during 1991–92 and 1994–95. Six tree species representing early and late successional stages were examined in paired plots representing severe hurricane disturbance (most trees toppled) and light hurricane disturbance (all trees standing and most branches intact). 2. Hurricane disturbance affected invertebrate abundances significantly. Sap-suckers and molluscs were more abundant, and defoliators, detritivores, and emergent aquatic insects were less abundant in recovering tree-fall gaps than in intact forest during this 5-year period. These changes in functional organisation are consistent with comparable studies of arthropod responses to canopy removal during harvest in temperate forests. 3. Tree species also affected invertebrate abundances significantly, but invertebrate communities did not differ significantly between the three early successional and three later successional tree species. 4. Most taxa showed significant annual variation in abundances, but only two Homoptera species showed a significant linear decline in abundance through time, perhaps reflecting long-term trends during recovery. 5. Leaf area missing, an indicator of herbivore effect on canopy processes, showed significant seasonal and annual trends, as well as differences among tree species and hurricane treatments. Generally, leaf area missing peaked during the wet season each year, but reached its highest levels during an extended drought in 1994. Leaf area missing also tended to be higher on the more abundant tree species in each disturbance treatment. 6. Herbivore abundances and leaf area missing were not related to concentrations of nitrogen, phosphorous, potassium, or calcium in the foliage. 7. This study demonstrated that invertebrate community structure and herbivory are dynamic processes that reflect the influences of host species and variable environmental conditions.

The Role of Disturbance, Topography, and Forest Structure in the Development of a Montane Forest Landscape

The Role of Disturbance, Topography, and Forest Structure in the Development of a Montane Forest Landscape
Keith S. Hadley
Bulletin of the Torrey Botanical Club
Vol. 121, No. 1 (Jan. - Mar., 1994), pp. 47-61

Human set fires beginning in the mid 1800s and repeated insect outbreaks of western spruce budworm (Choristoneura occidentalis Free.) and Douglas-fir bark beetle (Dendroctonus pseudotsugae Hopk.) during the past 50 years have resulted in a dramatic change in the montane (<ca. 2900 m) forest landscape of the Colorado Front Range. Here, I examine the historical and spatial relationship between these disturbance agents and topography using stand structure and dendroecological data from 38 contiguous stands. These data suggest that aspect and relief are important factors determining the spatial and temporal patterns of disturbance, succession, and rates of stand development. The rate of postfire stand development and hence, subsequent stand susceptibility to insect outbreaks appears to be related to aspect. North-facing stands experience rapid postfire development and greater susceptibility to insect attack due to higher host tree densities, larger mean tree size, and a more uniform distribution of host trees over larger contiguous areas. Postfire stand recovery on south facing slopes appears to be slower and stand susceptibility to insect attack is less due to lower host densities, smaller mean tree size, and a less uniform distribution of host trees over smaller areas. Relief, independent of aspect, enhances the structural diversity of the forest landscape by promoting irregular burn patterns and intensities, thus creating a fire-induced mosaic of different aged stands. As these different aged stands continue to grow older, they reach a stage of development susceptible to insect outbreaks at different times. As a result, insect-induced changes in the structural characteristics of the current landscape emulate fire-induced landscape patterns that developed largely due to human activities beginning in the 1860s.

Pervasive alteration of tree communities in undisturbed Amazonian forests

Laurance, W.F. et al. (2004) Pervasive alteration of tree communities
in undisturbed Amazonian forests. Nature 428, 171–175

Amazonian rainforests are some of the most species-rich tree communities on earth1. Here we show that, over the past two decades, forests in a central Amazonian landscape have experienced highly nonrandom changes in dynamics and composition. Our analyses are based on a network of 18 permanent plots unaffected by any detectable disturbance. Within these plots, rates of tree mortality, recruitment and growth have increased over time. Of 115 relatively abundant tree genera, 27 changed significantly in population density or basal area—a value nearly 14 times greater than that expected by chance. An independent, eight-year study in nearby forests corroborates these shifts in composition. Contrary to recent predictions2–5, we observed no increase in pioneer trees. However, genera of faster-growing trees, including many canopy and emergent species, are increasing in dominance or density, whereas genera of slower-growing trees, including many subcanopy species, are declining. Rising atmospheric CO2 concentrations6 may explain these changes, although the effects of this and other large-scale environmental alterations remain uncertain. These compositional changes could have important impacts on the carbon storage, dynamics and biota of Amazonian forests.

Hurricane Effects on Soil Organic Matter Dynamics and Forest Production in the Luquillo Experimental Forest, Puerto Rico: Results of Simulation Modeling

Hurricane Effects on Soil Organic Matter Dynamics and Forest Production in the Luquillo Experimental Forest, Puerto Rico: Results of Simulation Modeling
Robert L. Sanford, Jr., William J. Parton, Dennis S. Ojima and D. Jean Lodge
Vol. 23, No. 4, Part A. Special Issue: Ecosystem, Plant, and Animal Responses to Hurricanes in the Caribbean (Dec., 1991), pp. 364-372

The forests and soils at Luquillo Experimental Forest (LEF), Puerto Rico, are frequently disturbed by hurricanes occurring at various frequencies and intensities. We have derived a forest version of the Century soil organic matter model to examine the impact of hurricanes on soil nutrient availability and pool sizes, and forest productivity in the tabonuco forest at Luquillo. The model adequately predicted aboveground plant production, soil carbon, and soil nitrogen levels for forest conditions existing before Hurricane Hugo. Simulations of Hurricane Hugo and of an historical sequence of hurricanes indicated a complex pattern of recovery, especially for the first 10 yr after the hurricanes. After repeated hurricanes, forest biomass was reduced, while forest productivity was enhanced. Soil organic matter, and phosphorus and nitrogen mineralization stabilized at higher levels for the LEF than for hurricane-free tabonuco forest, and organic soil phosphorus was substantially increased by hurricanes. Results from these simulations should be regarded as hypotheses. At present there is insufficient data to validate the results of hurricane model simulations.

Variations in Belowground Carbon Storage and Soil CO2 Flux Rates along a Wet Tropical Climate Gradient

McGroddy, Megan; Silver, Whendee L. 2000. Variations in Belowground Carbon Storage and Soil CO2 Flux Rates along a Wet Tropical Climate Gradient. BIOTROPICA 32(4a): 614-624 .

We used a humid tropical elevation gradient to examine the relationships among climate, edaphic conditions, belowground carbon storage, and soil respiration rates. We also compared open and closed canopy sites to increase the range of microclimate conditions sampled along the gradient, and determine the effects of canopy openings on C and P storage, and C dynamics. Total soil C, the light C fraction, and all of the component fractions of the P pool were significantly related to soil moisture, and all but total soil C were also significantly related to temperature. Both labile and recalcitrant soil P fractions were negatively correlated with the light C fraction, while the dilute HCl-extractable P pool, generally thought of as intermediate in availability, was positively correlated with light C, suggesting that P may play an important role in C cycling within these systems. Total fine root biomass was greatest at 1000 m elevation and lowest at 150 m, and was strongly and positively correlated with soil moisture content. Soil respiration rates were significantly and negatively correlated with fine root biomass and the light C fraction. In forested sites, soil respiration rates were strongly and negatively correlated with total belowground C pools (soils 1 roots 1 forest floor). Belowground C pools did not follow the expected increasing trend with decreases in temperature along the gradient. Our results indicated that in humid tropical forests, the relationships among soil C and nutrient pools, soil respiration rates, and climate are complex. We suggest that frequent and prolonged anaerobic events could be important features of these environments that may explain the observed trends.
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