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Tropical forest understory regeneration occurs rapidly after disturbance with compositional trajectories that depend on species availability and environmental conditions. To predict future tropical forest regeneration dynamics, we need a deeper understanding of how pulse disturbance events, like hurricanes, interact with environmental variability to affect understory demography and composition. We examined fern and sapling mortality, recruitment, and community composition in relation to solar radiation and soil moisture using 17 years of forest dynamics data (2003–2019) from the Canopy Trimming Experiment in the Luquillo Experimental Forest, Puerto Rico. Solar radiation increased 150% and soil moisture increased 40% following canopy trimming of experimental plots relative to control plots. All plots were disturbed in 2017 by Hurricanes Irma and Maria, so experimentally trimmed plots presented the opportunity to study the effects of multiple hurricanes, while control plots isolated the effects of a single natural hurricane. Recruitment rates maximized at 0.14 individuals/plot/month for ferns and 0.20 stems/plot/month for saplings. Recruitment and mortality were distributed more evenly over the 17 years of monitoring in experimentally trimmed plots than in control plots; however, following Hurricane Maria demographic rates substantially increased in control plots only. In experimentally trimmed plots, the largest community compositional shifts occurred as a result of the trimming events, and compositional changes were greatest for control plots after Hurricane Maria in 2017. Pioneer tree and fern species increased in abundance in response to both simulated and natural hurricanes. Following Hurricane Maria, two dominant pioneer species, Cyathea arborea and Cecropia schreberiana, recruited abundantly, but only in control plots. In trimmed plots, increased solar radiation and soil moisture shifted understory species composition steadily toward pioneer and secondary‐successional species, with soil moisture interacting strongly with canopy trimming. Thus, both solar radiation and soil moisture are environmental drivers affecting pioneer species recruitment following disturbance, which interact with canopy opening following hurricanes. Our results suggest that if hurricane disturbances increase in frequency and severity, as suggested by climate change predictions, the understory regeneration of late‐successional species, such as Manilkara bidentata and Sloanea berteroana, which prefer deeper shade and slightly drier soil microsites, may become imperiled.

Collaboration between ecologists and learning scientists can give rise to powerful models for scientific outreach within ecology. This paper presents a process by which learning scientists and ecologists codesigned a science curriculum that invites students to join an ecological community of practice. In the Journey to El Yunque middle school science curriculum, students engage with simulation models generated from data gathered by Luquillo Long Term Ecological Research (LUQ LTER) scientists. Journey to El Yunque students can explore post‐hurricane population changes in yagrumo (Cecropia schreberiana), tabonuco (Dacryodes excelsa), coquís (Eleutherodactylus coquí), snails (Caracolus caracola), anoles (Anolis stratulus and A. gundlachi), veiled stinkhorn mushrooms (Dictyophora indusiata), and caterpillars (Historis odius). Ecology‐based revisions to Journey to El Yunque have included adding models of the effects of repeated hurricanes on limiting factors, based in part on findings from a canopy trimming experiment. Revisions based on classroom testing include simplifying student‐facing model controls to allow students to focus on the essential model components. The ongoing collaboration that keeps the Journey to El Yunque curriculum on the cutting edge of ecological and educational advances has been sustained for over two decades. We attribute the longevity of this work to (1) the long‐term nature of LUQ LTER, (2) a sustained interdisciplinary collaboration, and (3) our long‐term relationships with schools.

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 cm d.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-term recovery and may alter long-term structure and composition of the forest.

The trajectory of hurricane-induced succession was evaluated in a network of forest plots measured immediately before and 3 mo, 5, 10, and 15 yr after the direct impact of a Category 4 hurricane. Comparisons of forest structure, composition, and aboveground nutrients pools were made through time, and between species, life-history groups and geomorphic settings. The hurricane reduced aboveground biomass by 50 percent, causing an immediate decrease in stem density and diversity indices among all geomorphic settings. After 15 yr, basal area and aboveground biomass returned to pre-hurricane levels, while species richness, diversity indices, and stem densities exceeded pre-hurricane levels. Differences in species composition among geomorphic settings had not returned after 15 yr but differences in stem densities and structure were beginning to emerge. Significant differences were observed in the nutrient concentration of the three species that comprised the most aboveground biomass, and between species categorized as secondary high-light species and primary, low-light species. Species whose abundance was negatively correlated with the mature forest dominant also had distinct nutrient concentrations. When total aboveground nutrient pools were compared over time, differences in leaf nutrients among species were hidden by similarities in wood nutrient concentrations and the biomass dominance of a few species. The observed successional trajectory indicates that changes in species composition contributed to fast recovery of aboveground biomass and nutrient pools, while the influence of geomorphic setting on species composition occurs at time scales >15 yr of succession. Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp

One of the most significant challenges in developing a predictive understanding of the long-term effects of hurricanes on tropical forests is the development of quantitative models of the relationships between variation in storm intensity and the resulting severity of tree damage and mortality. There have been many comparative studies of interspecific variation in resistance of trees to wind damage based on aggregate responses to individual storms. We use a new approach, based on ordinal logistic regression, to fit quantitative models of the susceptibility of a tree species to different levels of damage across an explicit range of hurricane intensity. Our approach simultaneously estimates both the local intensity of the storm within a plot and the susceptibility to storm damage of different tree species within plots. Using the spatial variation of storm intensity embedded in two hurricanes (Hugo in 1989 and Georges in 1998) that struck the 16 ha Luquillo Forest Dynamics Plot in eastern Puerto Rico, we show that variation in susceptibility to storm damage is an important aspect of life history differentiation. Pioneers such as Cecropia schreberiana are highly susceptible to stem damage, while the late successional species Dacryodes excelsa suffered very little stem damage but significant crown damage. There was a surprisingly weak relationship between tree diameter and the susceptibility to damage for most of the 12 species examined. This may be due to the effects of repeated storms and trade winds on the architecture of trees and forest stands in this Puerto Rican subtropical wet forest. Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp.

Phoradendron hexastichum is a bird-dispersed mistletoe that infects the dioecious tree Cecropia schreberiana. Because both species share frugivore seed dispersers, we hypothesized that female Cecropia would have a greater probability and intensity of mistletoe infection than males due to more frequent visitation by shared frugivores. Over 50% of female Cecropia were infected, in contrast with 25% of males. On average, female trees had twice as many mistletoes as male trees. Infection probability and intensity increased with basal area in females but not in males, suggesting that lifetime reinfection was also female biased. We found mistletoe frugivores visiting uninfected fruiting females twice as often as males. Although mistletoes were mostly consumed by the mistletoe specialist Euphonia musica, we did not record Euphonia visiting uninfected Cecropia trees. Uninfected Cecropia trees were frequently visited by generalist frugivores (such as Spindalis portoricensis) that used both mistletoes and Cecropia fruits. The Cecropia-frugivores-Phoradendron network of interactions seems to have led to the spatial linkage of the two plant species through directional dispersal, to plant-plant facilitation through shared frugivores, and to bird-bird facilitation in which generalist frugivores start new foci of infection that specialist frugivores can use.

Los árboles tropicales pioneros constituyen un grupo funcional importante en los procesos de sucesión y restauración de los bosques. Sin embargo, muy poco se conoce sobre sus mecanismos de germinación bajo condiciones de estrés, y menos de los posibles tratamientos pregerminativos para mejorar el funcionamiento de las semillas. En el presente trabajo se examinaron los efectos de los tratamientos de hidratación-deshidratación y condiciones de estrés hídrico y calórico sobre la germinación de Guazuma ulmifolia, árbol pionero del Neotrópico. También se comparó la germinación de G. ulmifolia (semillas tratadas o no) con lo informado para otras especies pioneras tropicales (Cecropia schreberiana, Trichospermum lessertianum e Hibiscus elatus) con similares tratamientos de semillas y condiciones de siembra. La respuesta germinativa de G. ulmifolia se afectó significativamente por las condiciones de estrés, aunque lograron germinar en condiciones muy críticas de estrés hídrico (-1,46 MPa) y calórico (25/50 ºC). Sin embargo, la germinación se incrementó significativamente cuando las semillas fueron sometidas a ciclos de hidratación-deshidratación, siendo el tratamiento más efectivo un ciclo de hidratación parcial en agua. Este tratamiento pregerminativo también mejoró la germinación de C. schreberiana, T. lessertianum e H. elatus bajo condiciones de estrés, pero su efectividad dependió de la especie. El comportamiento germinativo de G. ulmifolia se correlacionó con su amplia distribución y características del hábitat, pero tal patrón no está claro en las otras especies. No existió una relación significativa entre la masa seminal y la respuesta al estrés hídrico. Se discute el papel ecológico de los ciclos de hidratación-deshidratación. Pioneer tropical trees constitute an important functional group in the succession and restoration processes of forests. However, very little is known about its germination mechanisms under stress conditions, and less about the possible pregermination treatments to improve the performance of the seeds. In the present work the effects of hydration-dehydration treatments and conditions of water and heat stress on the germination of Guazuma ulmifolia, pioneer tree of the Neotropic were examined. The germination of G. ulmifolia (seeds treated or not) was also compared with that reported for other tropical pioneer species (Cecropia schreberiana, Trichospermum lessertianum and Hibiscus elatus) with similar seed treatments and planting conditions. The final germination and start germination time of G. ulmifolia was significantly affected by stress conditions, although they were able to germinate under very critical conditions of water stress (-1.46 MPa) and heat stress (25/50 ºC). However, germination was significantly increased when the seeds underwent hydration-dehydration cycles, the most effective treatment being a partial hydration cycle in water. This pregermination treatment also improved the germination of C. schreberiana, T. lessertianum and H. elatus under stressful conditions, but its effectiveness depended on the species. The germination behavior of G. ulmifolia was correlated with its wide distribution and habitat characteristics, but this pattern is not clear in the other species. There was no significant relationship between the seed mass and the response to water stress. The ecological role of hydration-dehydration cycles is discussed.

The role of macroinvertebrates in the process of leaf breakdown is well studied in temperate streams, but less is known about their role in the tropics. We investigated the effect of reducing macroinvertebrate access to leaf material on leaf breakdown rates in a forested headwater stream in the Luquillo Experimental Forest, Puerto Rico. We measured leaf mass loss using fine and coarse mesh bags over 12 weeks for two common riparian species: Cecropia schreberiana (Moraceae) and Dacryodes excelsa (Burseraceae). Coarse mesh allowed freshwater shrimp and other macroinvertebrates to access leaf material, while fine mesh did not. Leaf breakdown rates did not differ between schreberiana and D. excelsa in coarse mesh bags (-0.0375/day vs. -0.0395/day, respectively), but C. schreberiana breakdown was significantly slower than D. exceha in fine mesh bags (-0.0159/day vs. -0.0266/day). Additionally, breakdown in fine mesh bags was significantly slower compared to coarse mesh bags for C. schreberiana, but less so for D. excelsa. Breakdown rates for all treatments were fast relative to those in temperate-zone streams indicating that both macroinvertebrates and macroinvertebrate-independent processing can strongly influence leaf decomposition in tropical streams. The difference between C. schreberiana and D. exceha indicates that the effect of macroinvertebrate exclusion can change with leaf type. /// El rol de los macroinvertebrados en el proceso de la descomposición de la hojarasca se ha estudiado extensamente en arroyos de climas templados, pero poco se conoce en los trópicos. Se investigó el efecto en las tasas de descomposición de las hojas a través de la reducción del acceso de macroinvertebrados a éstas en la cabecera de un arroyo cubierto de vegetación en el Bosque Experimental de Luquillo, Puerto Rico. Se midió la pérdida de masa de hojarasca utilizando empaques de malla gruesa y fina por 12 semanas en dos especies ripícolas comunes: Cecropia schreberiana (Moraceae) y Dacryodes exceha (Burseraceae). Los empaques de malla gruesa permitieron que camarones y otros macroinvertebrados tuvieran acceso libre a las hojas, mientras que los empaques de malla fina no. No hubo diferencia en las tasas de descomposición de las hojas para C. schreberiana y D. excelsa en los empaques de malla gruesa (-0.0375/día vs. -0.0395/día, respectivamente), pero en los empaques de malla fina la descomposición de C. schreberiana fue significativamente más lento que en D. excelsa (-0.0159/día vs. -0.0266/día). Además, la descomposición de schreberiana en los empaques de malla fina fue significativamente más lenta en relación a los empaques de malla gruesa, pero la diferencia fue menor para D. exceha. Las tasas de descomposición para todos los tratamientos fueron rápidas en relación a las encontradas en arroyos de climas templados, indicando que tanto los macroinvertebrados como algunos procesos independientes a los macroinvertebrados influyen grandemente en la descomposición de hojas en arroyos tropicales. La diferencia entre C. schreberiana y D. excelsa indica que el efecto de exclusión de macroinvertebrados puede variar entre especies.

In many forest ecosystems, green leaf deposition (greenfall) constitutes an enrichment over background levels of litterfall nutrients and may therefore influence key ecosystem processes. This study examined the litter quality and decomposition rates of green leaves compared to senescent litterfall for four dominant tree species (Dacryodes excelsa, Manilkara bidentata, Guarea guidonia, and Cecropia schreberiana) in a lower montane rain forest at El Verde Field Station, Luquillo Experimental Forest, Puerto Rico. Green leaves from the canopy and freshly senesced leaves from the forest floor were analyzed for carbon, nitrogen, and fiber and placed in litterbags in the field for up to 16 weeks. Green leaves displayed significantly higher rates of decomposition than did senescent litter among all four species. Green leaves also had significantly higher nitrogen concentrations and lower lignin to nitrogen ratios compared to senescent leaves. These results suggest that greenfall may have a major influence on decay processes and nutrient cycling in forests that experience large-scale green foliage removal.