Linking responses of native and invasive plants to hurricane disturbances: implications for coastal plant community structure

Hurricane disturbances produce significant changes in forest microclimates, creating opportunities for seedling regeneration of native and invasive plant species alike. However, there is limited information on how changes in microclimates and pre-existing forest conditions affect native and invasive plants responses to hurricane disturbances. In this manipulative study, we examined the responses of three common shrub/small stature tree species, two of which are native to the coastal region of the southeastern USA (Baccharis halimifolia and Morella cerifera) and one that is invasive (Triadica sebifera), to two key components of hurricane disturbance (canopy damage and saline storm surge). In a greenhouse, we grew seedlings of these species under a range of shade levels that mimicked pre-and post-hurricane canopy conditions for wet pine forest and mixed hardwood forest, two forest communities common in coastal areas of the southeastern USA. Seedlings were subjected to saline storm surges equivalent to full strength sea water for 3 days. Seedling responses (mortality and growth) to the treatments were monitored for 16 months. All species benefitted from higher canopy openness. Storm surge effects were short-lived and seedlings readily recovered under high light conditions. The storm surge had stronger negative effects on survival and growth of all species when coupled with high shade, suggesting storm surge has greater negative impacts on seedlings where hurricane winds cause minimal or no canopy damage. The invasive T. sebifera was by far more shade tolerant than the natives. Survival of T. sebifera seedlings under highly shaded conditions may provide it a competitive edge over native species during community reassembly following tropical storms. Differential responses of native and invasive species to hurricane disturbances will have profound consequences on community structure across coastal forest stands, and may be regulated by legacies of prior disturbances, community structure, extent of canopy damage, and species’ tolerance to specific microclimates.

     

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

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.     

Original Articles: Nitrogen Availability and Old-Field Succession in a Shortgrass Steppe

The relationship between soil nitrogen (N) availability and plant community structure was investigated in old-fields in the shortgrass steppe of Colorado. Nitrogen availability was manipulated by N or sucrose additions for 4 years at three old-fields (early-seral, mid-seral, and late-seral) and at an uncultivated control site. The addition of N generally resulted in increased abundance of annual forbs and grasses relative to perennials at all of the previously cultivated sites. Conversely, experimental reduction of N availability generally increased the relative abundance of perennials. Despite a lack of detectable differences in N mineralization between sites and treatments, ion-exchange resin bags confirmed that sucrose additions reduced plant-available N and that N additions increased plant-available N. This was evidenced further by similar observations for plant tissue N content. The degree to which N additions increased N availability at the various sites supported the idea that late-seral plant communities are less effective at N capture relative to earlier-seral communities. The mid-seral old-field had the lowest rates of litter decomposition and a relatively large accumulation of litter on the soil surface. This mid-seral old-field was dominated by an exotic annual grass (Bromus tectorum), which appears to be a major hindrance to redevelopment of the plant-soil system. By experimentally reducing N availability at this stage, we were able, in 4 years, to change the plant community into one that more closely resembled the late-seral community. We also observed that the natural recruitment of weedy annual species on the uncultivated site during an unusually wet year was suppressed by reducing N availability. Our results suggest that available N is an important factor controlling the rate and course of plant and soil community redevelopment on abandoned croplands in the shortgrass steppe, and that manipulation of N availability might be useful in restoration of rangeland vegetation. Received 19 May 1998; accepted 27 August 1999.     

Effects of Multiple Disturbances (Fire and Hurricane) on Epiphyte Community Dynamics in a Subtropical Forest,Florida, U.S.A.1

We addressed the interacting effects of a natural large–scale fire and a subsequent major hurricane on relative positions of epiphytes in a subtropical forest. In Everglades National Park, subtropical hammocks (hardwood tree “islands”; burned and unburned) during the Ingraham Fire (1989) were surveyed for trees and epiphytic bromeliads (Tillandsia spp.) one year before, as well as one and five years after, Hurricane Andrew (1992). We measured trees (species, diameter, and status [alive/dead]) and epiphytes (species, height, host tree characteristics, substrate life status, and density). The fire decreased the height of epiphytes during the hurricane because branches and bark of trees killed by the fire were unstable epiphyte substrates in the high winds. Proportions of epiphytes on Quercus virginiana were equally increased after the hurricane in both unburned and burned hammocks; the large size and bark characteristics resulted in greater proportional survival of epiphytes on this species. During the five years following the hurricane, changes in the distributions of epiphytes generally were toward pre–hurricane distributions, but recovery was faster in unburned than burned hammocks. We conclude that disturbances that kill trees are likely to amplify the vertical reduction of epiphytes during a subsequent hurricane and that effects of a single disturbance on plant populations can be influenced by the disturbance history of the system, including different types of disturbances.     

Hurricane Disturbance Alters Secondary Forest Recovery in Puerto Rico

Land-use history and large-scale disturbances interact to shape secondary forest structure and composition. How introduced species respond to disturbances such as hurricanes in post-agriculture forest recovery is of particular interest. To examine the effects of hurricane disturbance and previous land use on forest dynamics and composition, we revisited 37 secondary forest stands in former cattle pastures across Puerto Rico representing a range of exposure to the winds of Hurricane Georges in 1998. Stands ranged from 21 to>80 yr since agricultural abandonment and were measured 9 yr posthurricane. Stem density decreased as stands aged, while basal area and species richness tended to increase. Hurricane disturbance exerted contrasting effects on stand structure, contingent on stand age. In older stands, the basal area of large trees fell, shifting to a stand structure characteristic of younger stands, while the basal area of large trees tended to rise in younger stands with increasing hurricane disturbance. These results demonstrate that large-scale natural disturbances can alter the successional trajectory of secondary forest stands recovering from human land use, but stand age, precipitation and soil series were better predictors of changes in stand structure across all study sites. Species composition changed substantially between census intervals, but neither age nor hurricane disturbance consistently predicted species composition change. However, exposure to hurricane winds tended to decrease the abundance of the introduced tree Spathodea campanulata, particularly in smaller size classes. In all sites the abundance of the introduced tree Syzygium jambos showed a declining trend, again most strongly in smaller size classes, suggesting natural thinning through succession.     

Impacts of Hurricane Frances on Florida scrub-oak ecosystem processes: defoliation, net CO2 exchange and interactions with elevated CO2

Hurricane disturbances have profound impacts on ecosystem structure and function, yet their effects on ecosystem CO₂ exchange have not been reported. In September 2004, our research site on a fire‐regenerated scrub‐oak ecosystem in central Florida was struck by Hurricane Frances with sustained winds of 113 km h⁻¹ and wind gusts as high as 152 km h⁻¹. We quantified the hurricane damage on this ecosystem resulting from defoliation: we measured net ecosystem CO₂ exchange, the damage and recovery of leaf area, and determined whether growth in elevated carbon dioxide concentration in the atmosphere (C_a) altered this disturbance. The hurricane decreased leaf area index (LAI) by 21%, which was equal to 60% of seasonal variation in canopy growth during the previous 3 years, but stem damage was negligible. The reduction in LAI led to a 22% decline in gross primary production (GPP) and a 25% decline in ecosystem respiration (R_e). The compensatory declines in GPP and R_e resulted in no significant change in net ecosystem production (NEP). Refoliation began within a month after the hurricane, although this period was out of phase with the regular foliation period, and recovered 20% of the defoliation loss within 2.5 months. Full recovery of LAI, ecosystem CO₂ assimilation, and ecosystem respiration did not occur until the next growing season. Plants exposed to elevated C_a did not sustain greater damage, nor did they recover faster than plants grown under ambient C_a. Thus, our results indicate that hurricanes capable of causing significant defoliation with negligible damage to stems have negligible effects on NEP under current or future CO₂‐enriched environment.     

Early post-hurricane stand development in Fringe mangrove forests of contrasting productivity

We examined the immediate effects of a hurricane (Hurricane Andrew, August 1992) in a coastal landscape in sub-tropical Florida, and then monitored stand recovery in Fringe mangrove sites of different productive capacity for 9 years after the disturbance. Structural impacts of the hurricane were confined almost entirely to forests within 200–300 m of the coast. Mortality and damage were concentrated on canopy individuals. Following the hurricane, rapid canopy recovery and the early onset of competition among Fringe forest stems, as evidenced by relatively high mortality of smaller individuals, magnified the initial dominance of hurricane survivors and early-established seedlings over later cohorts, and limited recruitment to the brief period prior to canopy closure. Changes in the relative abundance of the two dominant mangrove species following disturbance varied strongly along the productivity gradient. The shade-tolerant Rhizophora mangle L. generally became the overwhelming canopy dominant in the competitive environment of the recovering Coastal Fringe forest following hurricane, but the shade-intolerant Laguncularia racemosa (L.) C.F. Gaertn was better represented in less productive Interior Fringe sites, where canopy closure was delayed. Site productivity is an important determinant of the success of mangrove species during post-hurricane stand development, and consequently of the zonation of communities in the coastal landscape.     

兴安落叶松林火干扰后土壤有机碳含量变化

在大兴安岭兴安落叶松林区,选择不同恢复年限各种火烧强度(重度、中度、轻度)的火烧迹地进行调查并采集土壤样品,对火后有机层和矿质层有机碳含量变化进行研究,以期为进一步开展森林火灾对区域碳平衡影响的定量评估提供科学依据。研究结果表明:火干扰对土壤有机碳含量变化的影响包括火烧即时影响和火后生境条件变化带来的间接影响。火干扰样地有机层的积累与转化主要是通过火后林冠郁闭度的变化影响的,其有机碳总储量低于对照样地。对于矿质层土壤,重度和中度火干扰后,如果样地发生植被序列演替,即阔叶林植被入侵,样地郁闭度增加迅速,凋落物积累量增大,土壤有机碳含量将随着过火年限的增加而增加;如果样地发生自我更新,样地郁闭度增加缓慢,凋落物分解量大于积累量,土壤有机碳含量将会在一定时间内随着过火年限的增加而减少;但是,无论样地发生植被序列演替还是自我更新,土壤有机碳含量短时间内均无法恢复到火前水平。轻度火干扰后,土壤有机碳含量短期内先增加,随着植被更新情况的发展最终趋于平衡。     

Aboveground Biomass Accumulation in a Tropical Wet Forest in Nicaragua Following a Catastrophic Hurricane Disturbance1

Among their effects on forest structure and carbon dynamics, hurricanes frequently create large‐scale canopy gaps that promote secondary growth. To measure the accumulation of aboveground biomass (AGBM) in a hurricane damaged forest, we established permanent plots 4 mo after the landfall of Hurricane Joan on the Atlantic coast of Nicaragua in October 1988. We quantified AGBM accumulation in these plots by correlating diameter measurements to AGBM values using a published allometric regression equation for tropical wet forests. In the first measurement year following the storm, AGBM in hurricane‐affected plots was quite variable, ranging from 26 to 153 Mg/ha, with a mean of 78 (±15) Mg/ha. AGBM was substantially lower than in two control plots several kilometers outside the hurricane's path (331 ±15 Mg/ha). Biomass accumulation was slow (5.36 ± 0.74 Mg/ha/yr), relative to previous studies of forest regeneration following another hurricane (Hugo) and agricultural activity. We suggest that large‐scale, homogenous canopy damage caused by Hurricane Joan impeded the dispersal and establishment of pioneer trees and led to a secondary forest dominated by late successional species that resprouted and survived the disturbance. With the relatively slow rate of biomass accumulation, any tightening in disturbance interval could reduce the maximum capacity of the living biomass to store carbon.     

The role of light,soil and human factors on the probability of occurrence of an invasive and three native plant species in coastal transitions of coastal Mississippi,USA

Aims Understanding relationships between the distributions of species and their surrounding environment provides a basis for forecasting how species will respond to future environmental changes. In this study, we examined the effects of environmental factors and human developmental features associated with disturbances on probability of occurrence of juveniles of invasive Triadica sebifera and three native plant species, Baccharis halimifolia, Ilex vomitoria and Morella cerifera within a typical coastal transition in coastal Mississippi, USA.Methods We recorded presence of juveniles of focal species and measured environmental factors (soil salinity, canopy openness, soil texture and soil carbon to nitrogen ratio) along an 11.3 km transect located at Grand Bay National Estuarine Research Reserve. Further, we documented anthropogenic features and associated activities as a proxy for human disturbance.Important findings With the exception of I. vomitoria, all other species occurrences were significantly influenced by an interaction between soil salinity and canopy openness. The occurrence of I. vomitoria sharply decreased with salinity. B. halimifolia occurrence peaked at moderate levels of salinity and low to moderate levels of canopy openness. Occurrences of M. cerifera and the invasive T. sebifera were highest at low levels of salinity and canopy openness. These results indicate that salinity is a strong driver that limits distribution of juvenile native and invasive species in coastal transitions. Logistic regression confirmed the positive effects of anthropogenic disturbances on T. sebifera and I. vomitoria occurrence. It appears that while soil salinity is likely to drive retreat of the seaward boundaries of woody plant species distributions, increased human developments along the coast likely enhance the inland spread of species, in particular the invasive T. sebifera by increasing disturbances and facilitating dispersal. Results from this study can offer insight for the development of T. sebifera management and preventive measures for further spread along coastal areas of the southeastern USA.     

车八岭山地常绿阔叶林冰灾后土壤微生物群落功能多样性

研究了我国南方冰灾后常绿阔叶林林冠开度及土壤养分的空间异质性对土壤微生物功能多样性的影响.在受冰灾影响的粤北车八岭山地常绿阔叶林2 hm2固定样地中按照冠层受损程度选取16个20 m×20 m的样方,用半球面影像技术获取林冠开度,并取0～20 cm的表层土壤混合样品分析土壤的理化性质,同时应用Biolog技术分析微生物功能多样性.按林冠开度梯度对各样方土壤微生物群落利用单一碳源的分析发现,林冠开度大的样方土壤微生物的活性、丰富度、多样性和均匀度都较低,反之则较高.聚类分析的结果与林冠开度的梯度有高度的一致性.主成分分析表明各样方土壤微生物功能多样性具有显著差异(第一轴p<0.005;第二轴p<0.001),其结果与聚类结果基本吻合.冗余分析揭示了土壤全磷、全钾、全氮、速效氮、有机碳、容重、总孔隙度和林冠开度的综合作用对土壤微生物功能多样性有显著影响(p<0.005),其中林冠开度与土壤微生物群落功能多样性的关系最密切.土壤微生物功能多样性受土壤养分的影响,具体表现为:与土壤有机碳呈明显的正相关;与全氮正相关;与速效氮、全磷负相关.研究说明冰灾所造成林冠开度和土壤养分的空间异质性会影响到土壤微生物功能多样性,而土壤微生物功能多样性可用于对生境恢复的指示和评价.     

Effects of a Hurricane Disturbance on Aboveground Forest Structure, Arbuscular Mycorrhizae and Belowground Carbon in a Restored Tropical Forest

To better understand how management and restoration practices influence the response of terrestrial ecosystems to large-scale disturbances, it is critical to study above- and belowground effects. In this study, we examined the immediate effect of a major hurricane on aboveground forest structure, arbuscular mycorrhizae (AM) and belowground carbon pools in experimentally thinned plots in a tropical forest. The hurricane occurred five years after a thinning treatment, when thinned plots had similar aboveground carbon stocks but different forest structure compared to control plots. Thinned plots had more large diameter (>10 cm) trees compared to the control plots, which were characterized by a higher density of small diameter (<10 cm) trees. Despite pre-hurricane differences in forest structure, there were no significant differences between treatments in changes of canopy openness or number of affected trees following the hurricane. Thinned plots had larger belowground carbon pools than the controls plots before the hurricane, and these differences remained after the hurricane despite rapid decomposition of organic matter rich in nitrogen. There were no pre-hurricane differences in AM fungal spores or total AM root colonization. The hurricane reduced AM sporulation by nearly 50% in both treatments, yet we observed a significant increase in AM root colonization after the hurricane with greater AM colonization in the thinned plots. Hurricanes have well-known visible aboveground effects, but here we showed that less visible belowground effects are influenced by forest management and may play an important role in forest recovery.     

Physical Disturbance and Community Structure of Exposed and Protected Reefs: A Case Study from St. John, U.S. Virgin Islands

Major physical disturbances impacted fringing reefs at 4 to12 m depth off the south coast of St. John, at intervals of28, 14 and 12 months over a 6 year period (1985–1991).The most wave exposed habitats were dominated by the hydrocoralMillepora alcicornis and the encrusting gorgonian Erythropodiumcaribaeorum. Both of these species displayed high resistancestability in response to the 1987 bleaching event and to HurricanesGilbert (1988) and Hugo (1989). Although Hurricane Hugo wasranked as the most severe hurricane of the past century, itcaused comparatively little damage to the shallow exposed reef,illustrating that the effects of physical disturbance may notalways conform to a priori predictions based on wave exposure.This counter-intuitive result is explicable when the chronologyof disturbance is considered. For example, the shallow exposedcommunity was more heavily damaged than the protected reef byHurricane Gilbert occurring one year prior to Hurricane Hugo.Sponges and thick mats of E. caribaeorum had accumulated onthe protected reef during a 54 month period of little disturbanceand were vulnerable to disturbance by virtue of their elevationabove the substratum. These organisms were dislodged by extremewater motion generated by Hurricane Hugo. In contrast, the shallowexposed community was still recovering from Hurricane Gilbertwhen Hurricane Hugo struck. Monitored populations of two coralspecies, Agaricia agaricites and Tubastrea coccinea, declinedover the six year period, but recruitment in 1990 partiallycompensated for high mortality caused byHurricane Hugo. Therewere significant increases in the abundance of algal turf onthree out of the four study reefs (4 m exposed and protected,12 m protected) in response to the creation of patch space bythe multiple physical disturbances. This study demonstratesthe need to interpret recent disturbances in light of the historyof past disturbance on the reef.     

Changes in woody vegetation abundance and diversity after natural disturbances causing different levels of mortality

Questions: How does woody vegetation abundance and diversity differ after natural disturbances causing different levels of mortality? Location: Abies balsamea–Betula papyrifera boreal mixed‐wood stands of southeast Quebec, Canada. Methods: Woody vegetation abundance and diversity were quantified and compared among three disturbance‐caused mortality classes, canopy gap, moderate‐severity disturbances, and catastrophic fire, using redundancy analysis, a constrained linear ordination technique, and diversity indices. Results: Substantial changes in canopy tree species abundance and diversity only occurred after catastrophic fire. Shade‐tolerant, late‐successional conifer species remained dominant after canopy gap and moderate‐severity disturbances, whereas shade‐intolerant, early‐successional colonizers dominated canopy tree regeneration after catastrophic fire. Density and diversity of mid‐tolerant and shade‐intolerant understory tree and shrub species increased as the impact of disturbance increased. Highest species richness estimates were observed after catastrophic fire, with several species establishing exclusively under these conditions. Relative abundance of canopy tree regeneration was most similar after canopy gap and moderate‐severity disturbances. For the sub‐canopy tree and shrub community, relative species abundances were most similar after moderate‐severity disturbances and catastrophic fire. Vegetation responses to moderate‐severity disturbances thus had commonalities with both extremes of the disturbance‐caused mortality gradient, but for different regeneration layers. Conclusions: Current spatio‐temporal parameters of natural disturbances causing varying degrees of mortality promote the development of a complex, multi‐cohort forest condition throughout the landscape. The projected increase in time intervals between catastrophic fires may lead to reduced diversity within the system.     

Water relations of evergreen and drought-deciduous trees along a seasonally dry tropical forest chronosequence

Seasonally dry tropical forests (SDTF) are characterized by pronounced seasonality in rainfall, and as a result trees in these forests must endure seasonal variation in soil water availability. Furthermore, SDTF on the northern Yucatan Peninsula, Mexico, have a legacy of disturbances, thereby creating a patchy mosaic of different seral stages undergoing secondary succession. We examined the water status of six canopy tree species, representing contrasting leaf phenology (evergreen vs. drought-deciduous) at three seral stages along a fire chronosequence in order to better understand strategies that trees use to overcome seasonal water limitations. The early-seral forest was characterized by high soil water evaporation and low soil moisture, and consequently early-seral trees exhibited lower midday bulk leaf water potentials (Ψ_L) relative to late-seral trees (−1.01 ± 0.14 and −0.54 ± 0.07 MPa, respectively). Although Ψ_L did not differ between evergreen and drought-deciduous trees, results from stable isotope analyses indicated different strategies to overcome seasonal water limitations. Differences were especially pronounced in the early-seral stage where evergreen trees had significantly lower xylem water δ¹⁸O values relative to drought-deciduous trees (−2.6 ± 0.5 and 0.3 ± 0.6‰, respectively), indicating evergreen species used deeper sources of water. In contrast, drought-deciduous trees showed greater enrichment of foliar ¹⁸O (∆¹⁸O_l) and ¹³C, suggesting lower stomatal conductance and greater water-use efficiency. Thus, the rapid development of deep roots appears to be an important strategy enabling evergreen species to overcome seasonal water limitation, whereas, in addition to losing a portion of their leaves, drought-deciduous trees minimize water loss from remaining leaves during the dry season.     

Impact of a hurricane on the herpetofaunal assemblages of a successional chronosequence in a tropical dry forest

Land‐use change is the main cause of deforestation and degradation of tropical forest in Mexico. Frequently, these lands are abandoned leading to a mosaic of natural vegetation in secondary succession. Further degradation of the natural vegetation in these lands could be exacerbated by stochastic catastrophic events such as hurricanes. Information on the impact of human disturbance parallel to natural disturbance has not yet been evaluated for faunal assemblages in tropical dry forests. To evaluate the response of herpetofaunal assemblages to the interaction of human and natural disturbances, we used information of pre‐ and post‐hurricane herpetofaunal assemblages inhabiting different successional stages (pasture, early forest, young forest, intermediate forest, and old growth forest) of dry forest. Herpetofaunal assemblages were surveyed in all successional stages two years before and two years after the hurricane Jova that hit the Pacific Coast of Mexico on October 2011. We registered 4093 individuals of 61 species. Overall, there were only slight effects of successional stage, hurricane Jova or the interaction between them on abundance, observed species richness and diversity of the herpetofauna. However, we found marked changes in estimated richness and composition of frogs, lizards, and snakes among successional stages in response to hurricane Jova. Modifications in vegetation structure as result of hurricane pass promoted particular changes in each successional stage and taxonomic group (anurans, lizards, and snakes). Secondary forests at different stages of succession may attenuate the negative effects of an intense, short‐duration, and low‐frequency natural disturbance such as hurricane Jova on successional herpetofaunal trajectories and species turnover.     

A decade of belowground reorganization following multiple disturbances in a subtropical wet forest

Humid tropical forests are dynamic ecosystems that experience multiple and overlapping disturbance events that vary in frequency, intensity, and spatial extent. Here we report the results of a 10-year study investigating the effects of forest clearing and multiple hurricanes on ecosystem carbon reservoirs, nutrient pools and vegetation. The aboveground plant community was most heavily affected by multiple disturbances, with the 9-year-old stands showing high rates of hurricane-induced mortality relative to surrounding forest. Belowground pools were less affected. Live fine root biomass fluctuated in response to multiple disturbances, but returned to pre-disturbance levels after 10 years. Soil C was resilient to clearing and hurricanes, probably due to the large pool size and high clay content. Soil P fluctuated over time, declining during periods of rapid plant recovery and growth. With the exception of K, base cations recovered within 2 years following clearing and showed little response to hurricane disturbance.     

Species-specific Seedling Responses to Hurricane Disturbance in a Puerto Rican Rain Forest1

Seedling dynamics were followed in a Puerto Rican forest for 20 months following a severe hurricane to study the interactive effects of hurricane debris, nutrients, and light on seedling diversity, density, growth, and mortality. Three treatments (debris removal, an unaltered control with hurricane debris, and chemical fertilization added to hurricane debris) altered levels of forest debris and soil nutrients. Canopy openness was measured twice using hemispherical photographs of the canopy. We examined the demographic responses of six common species to treatments over time. Seedling densities increased for all six species but the only significant treatment effects were increased densities of the pioneer tree Cecropia and the shrub Palicourea in the debris removal treatment. Seedling growth declined with declining light levels for four species but not for the pioneer tree Alchornea or the non‐pioneer tree Dacryodes. Only Cecropia and the non‐pioneer tree Chionanthus had treatment effects on growth. Mortality also differed among species and tended to be highest in the fertilized plots for all but Cecropia and Dacryodes. We found only some of the expected differences between pioneer and non‐pioneer plants, as each species had a unique response to the patchy distributions of organic debris, nutrients, and light following the hurricane. High local species diversity was maintained through the individualistic responses of seedlings after a disturbance.     

Tree Species Composition and Diversity at Different Levels of Disturbance in Popa Mountain Park,Myanmar

Tropical forests are highly threatened by human activities, even within the protected areas that have been established to conserve biodiversity. Human activities may have different degrees of impact on vegetation structure, composition and diversity. Some studies have measured direct evidence of human activities in forming disturbance gradients, while others have quantified canopy cover as a proxy for disturbance. When measurement is confined to human activities, disturbance may be underestimated in areas of poor canopy cover where forest degrading activities have ceased. Where measurements are restricted to canopy cover, disturbance may be underestimated in areas of extensive canopy cover that may have been subject to past disturbances, and overestimated for areas where canopy cover is naturally sparse. Combining indicators of past and present disturbances is therefore necessary to examine the full spectrum of human disturbances. Forest vegetation in Popa Mountain Park, Myanmar was surveyed and classified into three levels of disturbance—undisturbed, medium disturbed and highly disturbed—derived from evidence of cutting and canopy cover. The forests are second or third growth after clearing for agriculture in the early 20th century. All trees with ≥10 cm diameter at breast height in 168 sample plots (10 m × 10 m) were identified and measured. Density, basal area and diversity decreased and dominance rose when disturbance increased. Canonical correspondence analysis (CCA) clearly discriminated three forest stands and CCA axes were significantly correlated to soil moisture, soil nitrogen, elevation, slope, aspect, distance from park boundary and distance from villages. Abstract in Myanmar is available at http://www.blackwell‐synergy.com/loi/btp .     

Resiliency and response to hurricane disturbance in atropical shrub, Ardisia escallonioides (Myrsinaceae), in southFlorida

The resiliency to hurricane disturbance and the response toposthurricane environmental conditions were examined in the tropicalshrub Ardisia escallonioides (Myrsinaceae). Hurricane Andrewstruck three of four study sites in subtropical hardwood forests insouth Florida on 24 August 1992. Posthurricane understory light levelsin the disturbed sites ranged from 21 to 53% of full light in1993 and remained high in 1994; in contrast, light levels averaged only9% in the undisturbed forest. Significant differences inmortality, damage, and defoliation were observed among the threehurricane-damaged populations. Mortality was low, but adults sufferedhigh levels of damage and defoliation. Following the hurricane,populations in the most severely disturbed forests had more growth thanpopulations in lightly damaged or undamaged forests. Seed germinationand seedling growth were not associated with light availability. Nolong-term seed dormancy was observed. The observed response toposthurricane environmental conditions is consistent with understoryspecies that show release following canopy opening, but are able topersist under periods of canopy closure. The local dominance of thisspecies in many coastal forests in south Florida may be due to the highfrequency of hurricanedisturbance.