One of the hypothesized benefits of seed dispersal is to escape density- and distance-responsive, host-specific, natural enemies near maternal plants where conspecific seed and seedling densities are ...high. Such high conspecific neighbor densities typically result in lower offspring growth and survival (i.e., negative density-dependent effects), yet many dispersal modes result in clumped seed distributions. New World leaf-nosed bats transport fruits to their feeding roosts and deposit seeds, thereby creating high-density seed/seedling patches beneath feeding roosts in heterospecific trees away from maternal trees, which seemingly nullifies a key benefit of seed dispersal. Such dispersal may still be adaptive if negative density-dependent effects are reduced under feeding roosts or if the benefit of being dispersed away from maternal trees outweighs negative effects of conspecific seed/seedling density below roosts. We mapped the entire post-germination population of a bat-dispersed tree species Calophyllum longifolium (Calophyllaceae) in a 50-ha plot on Barro Colorado Island, Panama in each of three successive years. We tested two hypotheses: (1) distance-dependent effects are stronger than density-dependent effects on seedling performance because seedlings far from conspecific adults are more likely to escape natural enemies even when at high densities and (2) negative density-dependent effects will be reduced far from vs. near conspecific adults. Density and distance were naturally decoupled, as expected. However, in contrast to our expectation, we found positive density effects on seedling survival and density-dependent effects did not differ with distance from conspecific adults. Both density and distance had positive effects on seedling survival when considered together, while only year had a significant effect on seedling growth. Thus, both being dispersed under bat feeding roosts and escaping the vicinity of conspecific adults were beneficial for C. longifolium seedling survival, supporting the directed dispersal and escape hypotheses, respectively. Despite resulting in high densities of conspecific seedlings, favorable habitat under bat feeding roosts and lack of negative density-dependent effects appear to provide evolutionary advantages in C. longifolium.
Physical dormancy in seeds can challenge restoration efforts where scarification conditions for optimal germination and seedling vigor are unknown. For species that occur along wide environmental ...gradients, optimal scarification conditions may also differ by seed source. We examined intraspecific variation in optimal scarification conditions for germination and seedling performance in koa (
Acacia koa
), which occurs across a wide range of environmental conditions. To evaluate scarification responses, we recorded imbibition percentage, germination percentage, germination time, seedling abnormalities, early mortality, seedling growth, and seedling survivorship. From these, we developed a scarification index (
SI
) that integrates these measures simultaneously. We hypothesized that seeds from lower elevation sources exposed to higher temperatures would have harder seed coats and would require more intense scarification treatments. To test this hypothesis, we repeatedly exposed seeds to hot water differing in temperature and time until seeds imbibed. Supporting the hypothesis, seeds from lower elevation sources generally required more intense scarification, although we found substantial variation among sources. Koa seeds germinated in about a week following imbibition. Boiling seeds (i.e., maintaining at 100°C) was effective for imbibing seeds but it also substantially reduced germination percentages. Repeated exposure to 90 to 100°C water did not reduce germination percentage but decreased seedling performance and increased early mortality. No seeds remained unimbibed after six attempts of boiling germinated whereas seeds remaining unimbibed after 15 attempts of exposure to 90 to 100°C water showed high germination percentages. Abnormalities in seedling development were rare but increased with treatment intensity. Exposure to 100°C water for 1 min overall generated the best
SI
values but the best treatment differed by elevation, and the treatment with the best
SI
was rarely predicted from the highest germination percentages. Seeds that imbibed without any treatment germinated at the same level as manually filed seeds but produced poor seedling quality. Variation in mother tree environments along an elevational gradient can lead to differences in seed coat characteristics, which may explain differing responses to treatments. Scarification treatments affected processes beyond imbibition and germination and using an index like
SI
may improve efficiency by identifying optimal scarification treatments while reducing seed waste.
Background
Tropical forests are repositories of much of the world’s biodiversity and are critical for mitigation of climate change. Yet, the drivers of forest dynamics are poorly understood. This is ...in large part due to the lack of long-term data on forest change and changes in drivers.
Methodology
We quantify changes in tree abundance, diversity, and stand structure along transects first enumerated in 1978 and resampled 2019 in Kibale National Park, Uganda. We tested five predictions. First, based on the purported role of seed dispersal and herbivory and our quantification of changes in the abundance of frugivores and herbivores, we tested two predictions of how faunal change could have influenced forest composition. Second, based on an evaluation of life history strategies, we tested two predictions concerning how the forest could have changed following disturbance that happened prior to written history. Finally, based on a 50-year climate record, we evaluate the possible influence of climate change on forest dynamics.
Results
More trees were present on the assessed transects in 2019 (508) than in 1978 (436), species richness remained similar, but diversity declined as the number of dominant species increased. Rainfall increased by only 3 mm over the 50 years but this had not significant effect on forest changes measured here. Annual average monthly maximum temperature increased significantly by 1.05 °C over 50 years. The abundance of frugivorous and folivorous primates and elephants increased over the 50 years of monitoring. Neither the prediction that an increase in abundance of seed dispersing frugivores increases the abundance of their preferred fruiting tree species, nor that as an increase in folivore abundance causes a decline in their preferred species were supported. As predicted, light-demanding species decreased in abundance while shade-tolerant species increased as expected from Kibale being disturbed prior to historical records. Finally, while temperature increased over the 50 years, we found no means to predict
a priori
how individual species would respond.
Conclusions
Our study revealed subtle changes in the tree community over 40 years, sizable increases in primate numbers, a substantial increase in the elephant population and an increase in local temperature. Yet, a clear picture of what set of interactions impact the change in the tree community remains elusive. Our data on tree life-history strategies and frugivore/herbivore foraging preferences suggest that trees species are under opposing pressures.
Studies have shown that median distances of plants from the nearest reproductive conspecific (recruitment distance) shifts outward with increasing age or size class, and that plant spatial ...distribution changes over time in a predictable manner. However, observations and empirical evidence for such predictable changes are limited, and underlying mechanisms explaining such patterns for a wide range of individual sizes are not fully explored. In Costa Rican premontane wet forests, I empirically tested whether recruitment distance changes in a predictable manner with increasing size for five animal‐dispersed tree species by considering all post‐germination sizes. Specifically, I tested the Janzen–Connell hypothesis and the colonization hypothesis by considering distance, density, size, herbivory, biotic infection, and light availability simultaneously. Recruitment distance increased with increasing size (16–22 m) for the four non‐pioneer species, suggesting eventual regeneration success for seeds dispersed away from reproductive conspecifics. During the 2 years of this study, I found positive distance‐dependent survivorship and light availability were important for post‐seedling survivorship, in agreement with the Janzen–Connell hypothesis and the colonization hypothesis, respectively, but only for seedlings. However, seedlings did not escape aboveground herbivory or biotic infection better when the seeds were dispersed greater distances. Results highlight the importance of seed dispersal for successful regeneration, and suggest that changes in spatial distribution over time may be predictable in the vicinity of maternal trees for some non‐pioneer tree species.
Abstract
With rising atmospheric CO
2
concentrations, strategies for greater carbon sequestration and long‐term storage are urgently needed. Current reforestation schemes typically use fast‐growing ...tree species with low wood density, limiting the amount of carbon stored. In most tropical forests, there are native species with relatively fast growth rates, high wood density and low mortality rates that also achieve large adult size—‘exceptional species’.
We present a simulated assessment of the carbon sequestration potential using these exceptional species in assisted secondary succession in the Neotropics.
In our example, the selected exceptional species could add up to 16.2 Mg C/ha of sequestered carbon in secondary wet forests at maturity.
Synthesis and applications
. By increasing the representation of exceptional species in reforestation efforts, we could potentially boost long‐term carbon storage in reforestation projects focussed on carbon sequestration. However, our approach is not a substitute for protecting existing old primary forests and reforested lands plus reducing deforestation and emissions.
Resumen
Con el aumento de las concentraciones atmosféricas de CO
2
, se necesitan urgentemente estrategias para una mayor captura y almacenamiento a largo plazo del carbono. Los esquemas de reforestación actuales suelen utilizar especies de árboles de rápido crecimiento con baja densidad de madera, lo que limita la cantidad de carbono almacenado. En la mayoría de los bosques tropicales, hay especies nativas con tasas de crecimiento relativamente rápidas, alta densidad de madera y bajas tasas de mortalidad que también alcanzan un gran tamaño adulto—‘especies excepcionales’.
Presentamos una evaluación simulada del potencial de secuestro de carbono usando estas especies excepcionales en la sucesión secundaria asistida en el Neotrópico.
En nuestro ejemplo, las especies excepcionales seleccionadas podrían sumar hasta 16.2 Mg C/ha de carbono secuestrado en bosques húmedos secundarios en su madurez.
Síntesis y aplicaciones
. Al aumentar la representación de especies excepcionales en los esfuerzos de reforestación, podríamos impulsar potencialmente el almacenamiento de carbono a largo plazo en proyectos de reforestación centrados en el secuestro de carbono. Sin embargo, nuestro enfoque no reemplaza la protección de los bosques primarios antiguos existentes y las tierras reforestadas, ni reduce la necesidad de disminuir la deforestación y las emisiones.
Natural selection for plant species in heterogeneous environments creates genetic variation for traits such as cold tolerance. While physiological or molecular analyses have been used to evaluate ...stress tolerance adaptations, combining these approaches may provide deeper insight. Acacia koa (koa) occurs from sea level to 2300 m in Hawaiʻi, USA. At high elevations, natural koa populations have declined due to deforestation, and freeze tolerance is a limiting factor for tree regeneration. We used physiology and molecular analyses to evaluate cold tolerance of koa populations from low (300–750 m), middle (750–1500 m), and high elevations (1500–2100 m). Half of the seedlings were cold acclimated by exposure to progressively lowered air temperatures for eight weeks (from 25.6/22.2°C to 8/4°C, day/night). Using the whole plant physiology‐freezing test and koa C‐repeat Binding Factor CBF genes, our results indicated that koa can be cold‐acclimated when exposed to low, non‐freezing temperatures. Seedlings from high elevations had consistently higher expression of Koa CBF genes associated with cold tolerance, helping to explain variation in cold‐hardy phenotypes. Evaluation of the genetic background of 22 koa families across the elevations with low coverage RNA sequencing indicated that high elevation koa had relatively low values of heterozygosity, suggesting that adaptation is more likely to arise in the middle and low elevation sources. This physiology and molecular data for cold tolerance of koa across the elevation gradient of the Hawaiian Islands provides insights into natural selection processes and may help to support guidelines for conservation and seed transfer in forest restoration efforts.
Abstract
Forest fragmentation is pervasive in tropical landscapes, and one pathway by which fragmentation may negatively impact populations is via edge effects. Early life‐stages are particularly ...important for species regeneration as they act as bottlenecks, but how edge effects may act differentially on different life‐stages is unknown. This study evaluated edge effects on multiple early life‐stages of a currently common animal‐dispersed, shade‐tolerant tree
T
apirira mexicana
(
A
nacardiaceae). The study was conducted in tropical premontane wet forest fragments in a highly deforested region of
C
osta
R
ica. The stages assessed were pre‐dispersal predation, primary dispersal, post‐dispersal predation, secondary dispersal,
ex situ
germination,
in situ
seed longevity, first and second year seedling abundance, second year seedling survivorship, and basal diameter growth. Results showed that impacts of edge effects were not equal across stages, but were limited to specific stages and times. One stage which may act as a bottleneck for species regeneration was pre‐dispersal predation. Over 60 percent of the seeds were predated by larvae, and predation was higher near the edge than interior habitat. Seeds lost viability within 10 d in the forest. Germination to first year seedling stage was also lower near edges, but such effect was eliminated within a year after that. Primary dispersal, seedling survivorship, and growth were not affected by proximity to edges, and both secondary dispersal and post‐dispersal predation were rare. This study demonstrates that current population abundance may not guarantee future species persistence and the importance of considering multiple life‐stages for a comprehensive assessment of forest fragmentation effects on species regeneration.
Resumen
La fragmentación de los bosques es un fenómeno generalizado en los paisajes tropicales; una forma en que la fragmentación puede influir negativamente las poblaciones de plantas es a través de los efectos de borde. Las etapas de la vida tempranas son particularmente importantes para la regeneración de las especies ya que actúan como cuellos de botella, pero se desconoce cómo los efectos de borde pueden actuar diferencialmente sobre las distintas etapas de la vida. El presente estudio evaluó los efectos de borde en varias etapas de la vida temprana de
Tapirira mexicana
(Anacardiaceae), una especie de árbol común que es tolerante a la sombra, dispersada por animales. El estudio se realizó en fragmentos de bosque tropical premontano húmedo en una región altamente fragmentada de Costa Rica. Las etapas de evaluación fueron: la depredación previa a la dispersión, dispersión primaria, la depredación post‐dispersión, la dispersión secundaria, la germinación
ex situ
, la longevidad de las semillas
in situ
, la abundancia de plántulas de primer y segundo años, supervivencia de plántulas de segundo año, y crecimiento en el diámetro basal. Los resultados muestran que el impacto del efecto de borde no fue igual entre todas las etapas, pero se limita a etapas y épocas específicas. Una etapa que puede actuar como un cuello de botella para la regeneración de la especie fue la depredación antes de la dispersión. Más del 60 por ciento de las semillas fueron depredadas por larvas, y la depredación fue mayor cerca de los bordes que en el hábitat interior. Las semillas perdieron su viabilidad dentro del bosque en los primeros 10 días. La etapa desde la germinación a plántula de 1 año también se vio reducida cerca de los bordes, pero este efecto desapareció en el plazo de un año después de eso. La dispersión primaria, la supervivencia de las plántulas, y el crecimiento no se vieron afectados por la proximidad a los bordes, y tanto la dispersión secundaria como la depredación post‐dispersión fueron raros. Este estudio demuestra que la abundancia de la población actual no puede garantizar la persistencia de las especies futuras y la importancia de considerar varias etapas de la vida para una evaluación integral de los efectos de la fragmentación del bosque sobre la regeneración de las especies.
Forest fragmentation is pervasive in tropical landscapes, and one pathway by which fragmentation may negatively impact populations is via edge effects. Early life-stages are particularly important ...for species regeneration as they act as bottlenecks, but how edge effects may act differentially on different life-stages is unknown. This study evaluated edge effects on multiple early life-stages of a currently common animal-dispersed, shade-tolerant tree Tapirira mexicana (Anacardiaceae). The study was conducted in tropical premontane wet forest fragments in a highly deforested region of Costa Rica. The stages assessed were pre-dispersal predation, primary dispersal, post-dispersal predation, secondary dispersal, ex situ germination, in situ seed longevity, first and second year seedling abundance, second year seedling survivorship, and basal diameter growth. Results showed that impacts of edge effects were not equal across stages, but were limited to specific stages and times. One stage which may act as a bottleneck for species regeneration was pre-dispersal predation. Over 60 percent of the seeds were predated by larvae, and predation was higher near the edge than interior habitat. Seeds lost viability within 10 d in the forest. Germination to first year seedling stage was also lower near edges, but such effect was eliminated within a year after that. Primary dispersal, seedling survivorship, and growth were not affected by proximity to edges, and both secondary dispersal and post-dispersal predation were rare. This study demonstrates that current population abundance may not guarantee future species persistence and the importance of considering multiple life-stages for a comprehensive assessment of forest fragmentation effects on species regeneration.
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