Seasonally dry tropical forests (SDTF) are located in regions with alternating wet and dry seasons, with dry seasons that last several months or more. By the end of the 21st century, climate models ...predict substantial changes in rainfall regimes across these regions, but little is known about how individuals, species, and communities in SDTF will cope with the hotter, drier conditions predicted by climate models. In this review, we explore different rainfall scenarios that may result in ecological drought in SDTF through the lens of two alternative hypotheses: 1) these forests will be sensitive to drought because they are already limited by water and close to climatic thresholds, or 2) they will be resistant/resilient to intra- and inter-annual changes in rainfall because they are adapted to predictable, seasonal drought. In our review of literature that spans microbial to ecosystem processes, a majority of the available studies suggests that increasing frequency and intensity of droughts in SDTF will likely alter species distributions and ecosystem processes. Though we conclude that SDTF will be sensitive to altered rainfall regimes, many gaps in the literature remain. Future research should focus on geographically comparative studies and well-replicated drought experiments that can provide empirical evidence to improve simulation models used to forecast SDTF responses to future climate change at coarser spatial and temporal scales.
Purpose
A better knowledge of how deadwood decomposes is critical for accurately characterizing carbon and nutrient cycling in forests. Fungi dominate this decomposition process, but we still have ...limited understanding of fungal community structuring that ultimately controls the fate of wood decomposition. This is particularly true in tropical ecosystems. To address this knowledge gap, our study capitalized on an extreme storm event that caused a large and synchronized input of deadwood to the forest floor.
Methods
Here we report data for the first year of wood decomposition of trees in a Puerto Rican dry forest for nine tree species that were snapped by Hurricane Maria in 2017. We measured wood properties and the associated fungal communities after 12 months of decomposition and compared them with initial wood properties and stem-inhabiting fungal communities to identify the best predictors of wood decomposition rates and chemical changes.
Results
Changes in wood chemistry were primarily explained by rapid xylan losses, the main hemicellulose component for the studied tree species. Fungal communities were dominated by saprotrophic and plant pathogenic fungi and showed moderate changes over time. The initial relative abundances and ratios of different fungal functional guilds were significant predictors of both xylan and glucan losses, with plant pathogenic fungi accelerating cellulose and hemicellulose decomposition rates compared to saprotrophs.
Conclusion
Our results confirm that fungi present at the time of treefall are strong drivers of wood decomposition and suggest that plant pathogenic fungi might act as efficient early decomposers of hemicellulose in dry tropical forests.
Over the course of 16 d in the fall of 1998, Hurricane Georges made landfall on five Caribbean Island nations, two U.S. states, and two territories. Along its path, it impacted nearly every type of ...built environment and terrestrial and marine ecosystem found in the Caribbean and the southeastern United States. We reviewed ecological and sociological research related to Georges in order to demonstrate the potential power of regional synoptic networks despite notable gaps that existed at the time. Most studies examined various effects and responses within four years of the storm, though a few reported longer‐term results. Reduction in forest stem density was the most reported ecological effect and ranged from 7% to 51% among sites in different forest types. Forests previously impacted by Hurricane Hugo in 1989 showed lower mortality from Georges than forests with longer hurricane‐free intervals. Rivers in the storm's path exported heavy loads of sediment to marine systems. For example, 5–10 million tons of sediment was transported to marine systems from Puerto Rico, and suspended sediments increased tenfold in coastal Louisiana. Economic costs directly related to Hurricane Georges ranged from 5% to 200% of annual GDP in the year after the storm. Sociological research indicated that children and college students exposed to Hurricane Georges experienced elevated effects on mental health such as anxiety and depression for up to 2.5 yr. Established research areas and longitudinal studies were valuable in understanding hurricane effects in the context of long‐term trends but fragmented research capacity reduced both local and regional synthetic efforts. Georges provides a template of how future integrated research programs could provide a deeper understanding of how nature, urbanization, human culture, and societal norms interact, respond, and recover from a major hurricane. However, future studies should avoid using the Saffir‐Simpson scale as a shorthand indicator or predictor of storm effects because topographic, historical, ecological, political, infrastructural, and societal factors interact to alter storm effects. The breadth of topics addressed in the research produced after Georges shows the potential for transformative, regionally synthetic research that spans whole watersheds and nearshore areas while integrating ecological and social sciences.
Summary
Leaf habit has been hypothesized to define a linkage between the slow‐fast plant economic spectrum and the drought resistance‐avoidance trade‐off in tropical forests (‘slow‐safe vs ...fast‐risky’). However, variation in hydraulic traits as a function of leaf habit has rarely been explored for a large number of species.
We sampled leaf and branch functional traits of 97 tropical dry forest tree species from four sites to investigate whether patterns of trait variation varied consistently in relation to leaf habit along the ‘slow‐safe vs fast‐risky’ trade‐off.
Leaf habit explained from 0% to 43.69% of individual trait variation. We found that evergreen and semi‐deciduous species differed in their location along the multivariate trait ordination when compared to deciduous species. While deciduous species showed consistent trait values, evergreen species trait values varied as a function of the site. Last, trait values varied in relation to the proportion of deciduous species in the plant community.
We found that leaf habit describes the strategies that define drought avoidance and plant economics in tropical trees. However, leaf habit alone does not explain patterns of trait variation, which suggests quantifying site‐specific or species‐specific uncertainty in trait variation as the way forward.
Caribbean tropical forests are subject to hurricane disturbances of great variability. In addition to natural storm incongruity, climate change can alter storm formation, duration, frequency, and ...intensity. This model-based investigation assessed the impacts of multiple storms of different intensities and occurrence frequencies on the long-term dynamics of subtropical dry forests in Puerto Rico. Using the previously validated individual-based gap model ZELIG-TROP, we developed a new hurricane damage routine and parameterized it with site- and species-specific hurricane effects. A baseline case with the reconstructed historical hurricane regime represented the control condition. Ten treatment cases, reflecting plausible shifts in hurricane regimes, manipulated both hurricane return time (i.e. frequency) and hurricane intensity. The treatment-related change in carbon storage and fluxes were reported as changes in aboveground forest biomass (AGB), net primary productivity (NPP), and in the aboveground carbon partitioning components, or annual carbon accumulation (ACA). Increasing the frequency of hurricanes decreased aboveground biomass by between 5% and 39%, and increased NPP between 32% and 50%. Decadal-scale biomass fluctuations were damped relative to the control. In contrast, increasing hurricane intensity did not create a large shift in the long-term average forest structure, NPP, or ACA from that of historical hurricane regimes, but produced large fluctuations in biomass. Decreasing both the hurricane intensity and frequency by 50% produced the highest values of biomass and NPP. For the control scenario and with increased hurricane intensity, ACA was negative, which indicated that the aboveground forest components acted as a carbon source. However, with an increase in the frequency of storms or decreased storms, the total ACA was positive due to shifts in leaf production, annual litterfall, and coarse woody debris inputs, indicating a carbon sink into the forest over the long-term. The carbon loss from each hurricane event, in all scenarios, always recovered over sufficient time. Our results suggest that subtropical dry forests will remain resilient to hurricane disturbance. However carbon stocks will decrease if future climates increase hurricane frequency by 50% or more.
Ectomycorrhizal (ECM) symbioses support forest functioning globally, yet both the structure and function of ECM fungal communities in seasonally dry neotropical forests (SDTFs), known for extreme ...heterogeneity in vegetation and edaphic properties, remain under characterized.
Here, we evaluated the relative influences of seasonal versus spatial variation in ECM fungal community structure in soils from four environmentally divergent SDTFs. We also assessed the importance of biotic and abiotic drivers of SDTF ECM fungal community structure at regional scales, as well as ECM impacts on soil carbon (C) and nitrogen (N) cycling.
ECM fungal frequency, relative abundance and richness all increased in the wet season, but spatial rather than seasonal effects explained more variation in community composition. Across the four SDTFs investigated, differences in tree communities drove ECM fungal community turnover more than geographic distances, site abiotic conditions or soil chemistry. Although soil moisture and ECM tree basal area were stronger predictors of soil biogeochemistry, incorporating ECM fungal community composition and relative abundance added explanatory power to models of soil C and N cycling in the wet season.
Synthesis: Our results highlight the importance of seasonality and plant community composition in shaping different aspects of SDTF ECM fungal community structure and diversity as well as the potential for both the plant and fungal components of ECM symbioses to impact soil functioning across heterogenous SDTFs. Furthermore, our findings suggest that alterations in SDTF plant community composition due to climate or land‐use change will have important consequences for ECM fungal diversity and associated effects on soil biogeochemical cycling.
RESUMEN
Las ectomicorrizas mantienen el funcionamiento de los bosques a nivel global, sin embargo, pocos estudios han caracterizado la estructura y función de las comunidades de hongos ectomicorrícicos (ECM) en los Bosques Secos Neotropicales (BST), los cuales poseen una alta heterogeneidad en su composición florística y propiedades edáficas.
En este trabajo se evaluó la influencia relativa de la variación estacional vs. espacial sobre la estructura de la comunidad de hongos ECM en suelos de cuatro BST que divergen en sus características ambientales. También se estimó la importancia de los factores bióticos y abióticos sobre la estructura de las comunidades de hongos ECM de estos bosques a escalas regionales, y su impacto en el ciclo del carbono (C) y nitrógeno (N) en los suelos.
La frecuencia, abundancia relativa, y riqueza de los hongos ECM incrementó durante la estación lluviosa. No obstante, la variación en la composición comunitaria de hongos EM fue mayormente explicada por factores espaciales. En comparación con las distancias geográficas, las condiciones abióticas y la química de los suelos, las diferencias en la composición de especies de árboles tuvieron un rol más determinante en el recambio comunitario de los hongos ECM entre los cuatro BST estudiados. Aunque la humedad del suelo y el área basal de los árboles con ECM fueron los predictores más fuertes de la biogeoquímica de los suelos, la incorporación de la composición de las comunidades de hongos ECM y su abundancia relativa, mejoró el poder explicativo de los modelos de ciclado de C y N en los suelos durante la época de lluvias.
Síntesis: Los resultados de este estudio resaltan la importancia de la estacionalidad de las lluvias y la composición de las comunidades vegetales como determinantes de diferentes aspectos de la estructura y diversidad de las comunidades de hongos ECM en los BST, además del potencial que tanto las plantas, como el componente simbiótico de las ectomicorrizas tienen para impactar el funcionamiento de los suelos en los BST heterogéneos. Mas aún, los resultados sugieren que las alteraciones en la composición de las comunidades de plantas en los BST generados por cambios climáticos o en el uso del suelo, tendrán importantes consecuencias sobre la diversidad de hongos ECM y efectos asociados en el ciclado biogeoquímico en los suelos.
These results highlight the importance of seasonality and plant community composition in shaping different aspects of SDTF ECM fungal community structure and diversity as well as the potential for both the plant and fungal components of ECM symbioses to impact soil functioning across heterogenous SDTFs. Furthermore, these findings suggest that alterations in SDTF plant community composition due to climate or land‐use change will have important consequences for ECM fungal diversity and associated effects on soil biogeochemical cycling.Editor's Choice
The availability of nitrogen (N) and phosphorus (P) controls the flow of carbon (C) among plants, soils, and the atmosphere, thereby shaping terrestrial ecosystem responses to global change. Soil C, ...N, and P cycles are linked by drivers operating at multiple spatial and temporal scales: landscape-level variation in macroclimate and soil geochemistry, stand-scale heterogeneity in forest composition, and microbial community dynamics at the soil pore scale. Yet in many biomes, we do not know at which scales most of the biogeochemical variation emerges, nor which processes drive cross-scale feedbacks. Here, we examined the drivers and spatial/temporal scales of variation in soil biogeochemistry across four tropical dry forests spanning steep environmental gradients. To do so, we quantified soil C, N, and P pools, extracellular enzyme activities, and microbial community structure across wet and dry seasons in 16 plots located in Colombia, Costa Rica, Mexico, and Puerto Rico. Soil biogeochemistry exhibited marked heterogeneity across the 16 plots, with total organic C, N, and P pools varying fourfold, and inorganic nutrient pools by an order of magnitude. Most soil characteristics changed more across space (i.e., among sites and plots) than over time (between dry and wet season samplings). We observed stoichiometric decoupling among C, N, and P cycles, which may reflect their divergent biogeochemical drivers. Organic C and N pool sizes were positively correlated with the relative abundance of ectomycorrhizal trees and legumes. By contrast, the distribution of soil P pools was driven by soil geochemistry, with larger inorganic P pools in soils with P-rich parent material. Most earth system models assume that soils within a texture class operate similarly, and ignore subgrid cell variation in soil properties. Here we reveal that soil nutrient pools and fluxes exhibit as much variation among four Neotropical dry forests as is observed across terrestrial ecosystems at the global scale. Soil biogeochemical patterns are driven not only by regional differences in soil parent material and climate, but also by local-scale variation in plant and microbial communities. Thus, the biogeochemical patterns we observed across the Neotropical dry forest biome challenge representation of soil processes in ecosystem models.
► Performance of native and Leucaena saplings in subtropical dry forest habitats. ► Drought and fire caused high mortality of native species in grass-invaded areas. ► Unburned natives grew slightly ...faster in grass-invaded areas than forest understory. ► Leucaena outperformed native saplings, especially in unburned grass-invaded areas.
Throughout the tropics, non-native grasses invade, dominate, and persist in areas where subtropical and tropical dry forests have been highly degraded. In Central America and the Caribbean Islands, forests that regenerate in grass-invaded areas are generally composed of one to a few tree species, usually of the Fabaceae family and often non-native. We investigated the ecological factors that drive these successional patterns in southwestern Puerto Rico, where African grasses dominate extensive areas that are maintained by fires. The non-native legume tree Leucaena leucocephala commonly establishes in these areas and forms persistent, mono-dominant stands. We planted 455 saplings of 13 native tree species and Leucaena in native forest understory and in grass-invaded areas that were either subjected to or protected from prescribed fires 4months later. We measured growth and survival to test the following hypotheses: (1) saplings of native species are suppressed in grass-invaded areas compared to the forest understory, (2) Leucaena saplings outperform native species in grass-invaded areas, but not in the forest understory, and (3) Leucaena saplings are less susceptible to wildfires than native species. After 20months, 50%, 40%, and 3% of native saplings survived in the forest, unburned-grass, and burned-grass treatments, respectively. Over the same period, 90%, 80%, and 25% of Leucaena survived in the same treatments. The fires immediately killed 65% of native saplings and 50% of Leucaena saplings. The majority of unburned native saplings died during a seasonal drought, when high irradiance and reduced soil organic matter content likely led to higher mortality in grass-invaded areas than in forest understory. Native species’ growth was low in all treatments, yet diameter growth was slightly higher in the unburned grass than the forest understory. Leucaena saplings grew faster than the native saplings in all three treatments, especially in the unburned grass. Slow growth and high mortality of native saplings in grass-invaded areas contribute to the dominance of Leucaena in forests that regenerate on these sites and suggest that Leucaena may play a beneficial role in dry forest restoration in the Caribbean.
Questions
Tropical dry forests that experience severe disturbances (e.g., fires) often remain degraded for long time periods, during which non‐native grasses and trees dominate. One barrier to native ...tree regeneration in degraded areas may be seed dispersal limitation. To better understand how dispersal limitation influences recovery from degradation, we tested whether the mode and rates of seed dispersal differed in degraded sites dominated either by the exotic tree Leucaena leucocephala or open areas dominated by introduced pasture grasses. We also tested whether L. leucocephala stands facilitate the recruitment of native trees by increasing their seed input compared to open grass areas.
Location
Guánica Commonwealth Forest, Puerto Rico.
Methods
Seed rain was measured for one year in traps located within five vegetation types that ranged in degree of forest degradation from open grass to intact native forest.
Results
In open grass areas, seed rain density was similarly low for L. leucocephala and abiotically dispersed native trees (mean 95% CI = 50.9 15.1–171.0 vs. 34.2 10.3–113.5 seeds m−2 year−1), whereas it was even lower for animal‐dispersed native trees (0.14 0.03–0.67 seeds m−2 year−1). Compared to open grass areas, L. leucocephala‐dominated stands, even those with grass understories, had higher seed rain density of animal‐dispersed trees (43.0 12.9–143.6 seeds m−2 year−1), but not abiotically dispersed trees (20.8 6.3–68.5 seeds m−2 year−1).
Conclusions
The dominance of L. leucocephala in disturbed Caribbean dry forests does not appear to be mediated by disproportionate seed arrival in open areas compared to native tree seeds. Rather, subsequent factors such as seed and seedling survival likely favor L. leucocephala in highly degraded areas. Since L. leucocephala stands increase the seed rain of animal‐dispersed native trees, retaining them in highly disturbed Caribbean dry forests may facilitate the regeneration of native forests.
Forest regeneration partly depends on the ability of tree seeds to arrive in degraded sites. In Caribbean dry forests, the introduced tree species Leucaena leucocephala often dominates degraded sites decades after clearing or wildfire. More native tree seeds arrived in sites dominated by L. leucocephala than in open grass fields. Thus, L. leucocephala helps native forest regeneration in degraded sites.
This paper presents the analysis of a time series of hyperspectral images collected with the Hyperion sensor on board EO-1 to demonstrate how hyperspectral imaging can be used for studying seasonal ...variations of vegetation cover over the Guánica Dry Forest in Puerto Rico. The approach is based on a local unmixing procedure that splits the hyperspectral scene into tiles and performs endmember extraction on each tile. The main assumption is that within a tile, a single spectral signature is an adequate representation of an endmember. Local endmember signatures from each tile are then clustered to extract endmember classes that better account for endmember spectral variability across the scene and provide a better global description of the full forest scene. Within a scene, abundances are computed using all extracted spectral endmembers and the abundance of an endmember class is computed as the sum of the abundances for the spectral endmembers belonging to that class. Variations in abundance maps are used to understand seasonal changes in forest cover. The procedure was performed using eleven near-cloud-free Hyperion images collected in different months in 2008. Results from the analysis agreed with published knowledge of the phenological changes for this forest. Correlation analyses with NDVI and rainfall time series are used to understand variations in coverage of certain endmember classes with weather. Mangrove was shown to be uncorrelated with rainfall, whereas the upland forest endmember was highly correlated with rain. This study shows the potential for unmixing methods to exploit hyperspectral data for temporal analysis.
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