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
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.
Engineering resilience, a forest's ability to maintain its properties in the event of disturbance, comprises two components: resistance and recovery. In human‐dominated landscapes, forest resilience ...depends mostly on recovery. Forest recovery largely depends on autogenic regulation, which entails a negative feedback loop between rates of change in forest state variables and state variables themselves. Hence community dynamics change in response to deviations from forest equilibrium state. Based on the premise that autogenic regulation is a key aspect of the recovery process, here we tested the hypothesis that combined old‐growth forest (OGF) and secondary forest (SF) dynamics should show autogenic regulation in state variables, and thus convergence towards OGF‐based reference points, indicating forest resilience.
We integrated dynamic data for OGF (11‐year monitoring) and SF (16‐year monitoring) to analyse three key state variables (basal area, tree density and species richness), their annual rates of change and their underlying demographic processes (recruitment, growth and mortality). We examined autogenic regulation through generalized linear mixed‐effects models (GLMMs) to quantify functional relationships between rates of change of state variables (and underlying demographic processes) and their respective state variables.
State variables in OGF decreased moderately over time, against our prediction of OGF constancy. In turn, the three state variables analysed showed negative relationships with their respective rates of change, which allows the return of SF to OGF values after disturbance. In all cases, recruitment decreased with increasing values in state variables, while mortality increased.
The observed negative relationships between state variables, their rates of change and their underlying demographic processes support our hypothesis of integrated OGF and SF dynamics showing autogenic regulation for state variables. Competition seems to be a major driver of autogenic regulation given its dependence on a resource availability that declines as forest structure develops.
Synthesis. Based on a straightforward and comprehensive approach to quantify the extent to which tropical forest dynamics is self‐regulated, this study highlights the importance of autogenic regulation for tropical dry forest as a basic component of its resilience. This approach is potentially valuable for a generalized assessment of engineering resilience of forests world‐wide.
Resumen
La resiliencia ingenieril, es decir, la capacidad de un bosque para mantener sus propiedades ante un evento de disturbio, incluye dos componentes: resistencia y recuperación. En paisajes dominados por los humanos, la resiliencia de los bosques depende principalmente de su recuperación. La recuperación de un bosque depende en gran medida de la regulación autogénica, la cual implica un ciclo de retroalimentación negativa entre las tasas de cambio de las variables de estado del bosque y las propias variables de estado. Por lo tanto, la dinámica comunitaria cambia en respuesta a las desviaciones respecto al estado de equilibrio del bosque. Con base en la premisa de que la regulación autogénica es un aspecto clave del proceso de recuperación, pusimos a prueba la hipótesis de que la integración de la dinámica del bosque maduro (BM) y del bosque secundario (BS) debería mostrar regulación autogénica en sus variables de estado, y por lo tanto una convergencia hacia los valores de referencia del BM, lo cual indicaría que el bosque es resiliente.
Integramos datos de la dinámica del BM y del BS (11 y 16 años de monitoreo, respectivamente) para analizar tres variables de estado clave (área basal, densidad de árboles, riqueza de especies), sus tasas anuales de cambio y sus procesos demográficos subyacentes (reclutamiento, crecimiento, mortalidad). Examinamos la regulación autogénica a través de modelos lineales generalizados de efectos mixtos (GLMM) para cuantificar las relaciones funcionales entre las tasas de cambio de las variables de estado (y los procesos demográficos subyacentes) y sus respectivas variables de estado.
Las variables de estado en el BM disminuyeron de forma moderada con el tiempo, en contra de nuestra predicción de constancia para el BM. A su vez, las tres variables de estado analizadas mostraron relaciones negativas con sus respectivas tasas de cambio, lo que permite el retorno de los valores del BS a los del BM después del disturbio. En todos los casos, el reclutamiento disminuyó al aumentar los valores en las variables de estado, mientras que la mortalidad aumentó.
Las relaciones negativas observadas entre las variables de estado, sus tasas de cambio y sus procesos demográficos subyacentes apoyan nuestra hipótesis de que la integración de la dinámica del BM y del BS debería mostrar regulación autogénica en las variables de estado. La competencia parece ser uno de los principales determinantes de la regulación autogénica dada su dependencia de una disponibilidad de recursos que disminuye a medida que se desarrolla la estructura del bosque.
Síntesis. A partir de un enfoque sencillo e integral para cuantificar el grado en que la dinámica de los bosques tropicales está autorregulada, este estudio destaca el papel de la regulación autogénica en el bosque seco tropical como un componente básico de su resiliencia. Este enfoque es potencialmente valioso para hacer evaluaciones generalizadas de la resiliencia ingenieril de los bosques en todo el mundo.
In human‐dominated forests, the recovery component of resilience depends on autogenic regulation. The integration of old‐growth and secondary forest dynamics in a tropical dry forest confirmed autogenic regulation, expressed as negative functional relationships between forest state variables, their annual rates of change and their underlying demographic processes. These findings highlight the importance of autogenic regulation for tropical forest resilience.
Abandonment of agricultural lands promotes the global expansion of secondary forests, which are critical for preserving biodiversity and ecosystem functions and services. Such roles largely depend, ...however, on two essential successional attributes, trajectory and recovery rate, which are expected to depend on landscape-scale forest cover in nonlinear ways. Using a multi-scale approach and a large vegetation dataset (843 plots, 3511 tree species) from 22 secondary forest chronosequences distributed across the Neotropics, we show that successional trajectories of woody plant species richness, stem density and basal area are less predictable in landscapes (4 km radius) with intermediate (40-60%) forest cover than in landscapes with high (greater than 60%) forest cover. This supports theory suggesting that high spatial and environmental heterogeneity in intermediately deforested landscapes can increase the variation of key ecological factors for forest recovery (e.g. seed dispersal and seedling recruitment), increasing the uncertainty of successional trajectories. Regarding the recovery rate, only species richness is positively related to forest cover in relatively small (1 km radius) landscapes. These findings highlight the importance of using a spatially explicit landscape approach in restoration initiatives and suggest that these initiatives can be more effective in more forested landscapes, especially if implemented across spatial extents of 1-4 km radius.
Secondary tropical forests play an increasingly important role in carbon budgets and biodiversity conservation. Understanding successional trajectories is therefore imperative for guiding forest ...restoration and climate change mitigation efforts. Forest succession is driven by the demographic strategies—combinations of growth, mortality and recruitment rates—of the tree species in the community. However, our understanding of demographic diversity in tropical tree species stems almost exclusively from old‐growth forests. Here, we assembled demographic information from repeated forest inventories along chronosequences in two wet (Costa Rica, Panama) and two dry (Mexico) Neotropical forests to assess whether the ranges of demographic strategies present in a community shift across succession. We calculated demographic rates for >500 tree species while controlling for canopy status to compare demographic diversity (i.e., the ranges of demographic strategies) in early successional (0–30 years), late successional (30–120 years) and old‐growth forests using two‐dimensional hypervolumes of pairs of demographic rates. Ranges of demographic strategies largely overlapped across successional stages, and early successional stages already covered the full spectrum of demographic strategies found in old‐growth forests. An exception was a group of species characterized by exceptionally high mortality rates that was confined to early successional stages in the two wet forests. The range of demographic strategies did not expand with succession. Our results suggest that studies of long‐term forest monitoring plots in old‐growth forests, from which most of our current understanding of demographic strategies of tropical tree species is derived, are surprisingly representative of demographic diversity in general, but do not replace the need for further studies in secondary forests.
Tropical forests are globally important for biodiversity conservation and climate change mitigation but are being converted to other land uses. Conversion of seasonally dry tropical forests (SDTF) is ...particularly high while their protection is low. Secondary succession allows forests to recover their structure, diversity and composition after conversion and subsequent abandonment and is influenced by demographic rates of the constituent species. However, how these rates vary between seasons for different plant sizes at different successional stages in SDTF is not known. The effect of seasonal drought may be more severe early in succession, when temperature and radiation are high, while competition and density-dependent processes may be more important at later stages, when vegetation is tall and dense. Besides, the effects of seasonality and successional stage may vary with plant size. Large plants can better compete with small plants for limiting resources and may also have a greater capacity to withstand stress. We asked how size-dependent density, species density, recruitment and mortality varied between seasons and successional stages in a SDTF. We monitored a chronosequence in Yucatan, Mexico, over six years in three 0.1 ha plots in each of three successional stages: early (3–5 years-old), intermediate (18–20 years-old) and advanced (>50 years-old). Recruitment, mortality and species gain and loss rates were calculated from wet and dry season censuses separately for large (diameter > 5 cm) and small (1–5 cm in diameter) plants. We used linear mixed-effects models to assess the effects of successional stage, seasonality and their changes through time on demographic rates and on plant and species density. Seasonality affected demographic rates and density of large plants, which exhibited high wet-season recruitment and species gain rates at the early stage and high wet-season mortality at the intermediate stage, resulting in an increase in plant and species density early in succession followed by a subsequent stabilization. Small plant density decreased steadily after only 5 years of land abandonment, whereas species density increased with successional stage. A decline in species dominance may be responsible for these contrasting patterns. Seasonality, successional stage and their changes through time had a stronger influence on large plants, likely because of large among-plot variation of small plants. Notwithstanding the short duration of our study, our results suggest that climate-change driven decreases in rainy season precipitation may have an influence on successional dynamics in our study forest as strong as, or even stronger than, prolonged or severe droughts during the dry season.
Aim
Tropical forest succession and associated changes in community composition are driven by species demographic rates, but how demographic strategies shift during succession remains unclear. Our ...goal was to identify generalities in demographic trade‐offs and successional shifts in demographic strategies across Neotropical forests that cover a large rainfall gradient and to test whether the current conceptual model of tropical forest succession applies to wet and dry forests.
Location
Mexico and Central America.
Time period
1985–2018.
Major taxa studied
Trees.
Methods
We used repeated forest inventory data from two wet and two dry forests to quantify demographic rates of 781 tree species. For each forest, we explored the main demographic trade‐offs and assigned tree species to five demographic groups by performing a weighted principal components analysis to account for differences in sample size. We aggregated the basal area and abundance across demographic groups to identify successional shifts in demographic strategies over the entire successional gradient from very young (<5 years) to old‐growth forests.
Results
Across all forests, we found two demographic trade‐offs, namely the growth–survival trade‐off and the stature–recruitment trade‐off, enabling the data‐driven assignment of species to five demographic strategies. Fast species dominated early in succession and were then replaced by long‐lived pioneers in three forests. Intermediate and slow species increased in basal area over succession in all forests, but, in contrast to the current conceptual model, long‐lived pioneers continued to dominate until the old‐growth stage in all forests. The basal area of short‐lived breeders was low across all successional stages.
Main conclusions
The current conceptual model of Neotropical forest succession should be revised to incorporate the dominance of long‐lived pioneers in late‐successional and old‐growth forests. Moreover, the definition of consistent demographic strategies that show clear dominance shifts across succession substantially improves the mechanistic understanding and predictability of Neotropical forest succession.
Land-use change occurs nowhere more rapidly than in the tropics, where the imbalance between deforestation and forest regrowth has large consequences for the global carbon cycle. However, ...considerable uncertainty remains about the rate of biomass recovery in secondary forests, and how these rates are influenced by climate, landscape, and prior land use. Here we analyse aboveground biomass recovery during secondary succession in 45 forest sites and about 1,500 forest plots covering the major environmental gradients in the Neotropics. The studied secondary forests are highly productive and resilient. Aboveground biomass recovery after 20 years was on average 122 megagrams per hectare (Mg ha(-1)), corresponding to a net carbon uptake of 3.05 Mg C ha(-1) yr(-1), 11 times the uptake rate of old-growth forests. Aboveground biomass stocks took a median time of 66 years to recover to 90% of old-growth values. Aboveground biomass recovery after 20 years varied 11.3-fold (from 20 to 225 Mg ha(-1)) across sites, and this recovery increased with water availability (higher local rainfall and lower climatic water deficit). We present a biomass recovery map of Latin America, which illustrates geographical and climatic variation in carbon sequestration potential during forest regrowth. The map will support policies to minimize forest loss in areas where biomass resilience is naturally low (such as seasonally dry forest regions) and promote forest regeneration and restoration in humid tropical lowland areas with high biomass resilience.
Regrowth of tropical secondary forests following complete or nearly complete removal of forest vegetation actively stores carbon in aboveground biomass, partially counterbalancing carbon emissions ...from deforestation, forest degradation, burning of fossil fuels, and other anthropogenic sources. We estimate the age and spatial extent of lowland second-growth forests in the Latin American tropics and model their potential aboveground carbon accumulation over four decades. Our model shows that, in 2008, second-growth forests (1 to 60 years old) covered 2.4 million km(2) of land (28.1% of the total study area). Over 40 years, these lands can potentially accumulate a total aboveground carbon stock of 8.48 Pg C (petagrams of carbon) in aboveground biomass via low-cost natural regeneration or assisted regeneration, corresponding to a total CO2 sequestration of 31.09 Pg CO2. This total is equivalent to carbon emissions from fossil fuel use and industrial processes in all of Latin America and the Caribbean from 1993 to 2014. Ten countries account for 95% of this carbon storage potential, led by Brazil, Colombia, Mexico, and Venezuela. We model future land-use scenarios to guide national carbon mitigation policies. Permitting natural regeneration on 40% of lowland pastures potentially stores an additional 2.0 Pg C over 40 years. Our study provides information and maps to guide national-level forest-based carbon mitigation plans on the basis of estimated rates of natural regeneration and pasture abandonment. Coupled with avoided deforestation and sustainable forest management, natural regeneration of second-growth forests provides a low-cost mechanism that yields a high carbon sequestration potential with multiple benefits for biodiversity and ecosystem services.
Summary
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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.
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