• Energy Research

AbstractTropical 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.

Forest fragmentation affects the heterogeneity of accumulated fuels by increasing the diversity of forest types and by increasing forest edges. This heterogeneity has implications in how we manage fuels, fire, and forests. Understanding the relative importance of fragmentation on woody biomass within a single climatic regime, and along climatic gradients, will improve our ability to manage forest fuels and predict fire behavior. In this study we assessed forest fuel characteristics in stands of differing moisture, i.e., dry and moist forests, structure, i.e., open canopy (typically younger) vs. closed canopy (typically older) stands, and size, i.e., small (10-14 ha), medium (33 to 60 ha), and large (100-240 ha) along a climatic gradient of boreal, temperate, and tropical forests. We measured duff, litter, fine and coarse woody debris, standing dead, and live biomass in a series of plots along a transect from outside the forest edge to the fragment interior. The goal was to determine how forest structure and fuel characteristics varied along this transect and whether this variation differed with temperature, moisture, structure, and fragment size. We found nonlinear relationships of coarse woody debris, fine woody debris, standing dead and live tree biomass with mean annual median temperature. Biomass for these variables was greatest in temperate sites. Forest floor fuels (duff and litter) had a linear relationship with temperature and biomass was greatest in boreal sites. In a five-way multivariate analysis of variance we found that temperature, moisture, and age/structure had significant effects on forest floor fuels, downed woody debris, and live tree biomass. Fragment size had an effect on forest floor fuels and live tree biomass. Distance from forest edge had significant effects for only a few subgroups sampled. With some exceptions edges were not distinguishable from interiors in terms of fuels.

In this study, we set up a wood decomposition experiment to i) quantify the percent of mass remaining, decay constant and performance strength of aspen stakes (Populus tremuloides) in dry and moist boreal (Alaska and Minnesota, USA), temperate (Washington and Idaho, USA), and tropical (Puerto Rico) forest types, and ii) determine the effects of fragmentation on wood decomposition rates as related to fragment size, forest age (and/or structure) and climate at the macro- and meso-scales. Fragment sizes represented the landscape variability within a climatic region. Overall, the mean small fragments area ranged from 10-14 ha, medium-sized fragments 33 to 60 ha, and large fragments 100-240 ha. We found that: i) aspen stakes decayed fastest in the tropical sites, and the slowest in the temperate forest fragments, ii) the percent of mass remaining was significantly greater in dry than in moist forests in boreal and temperate fragments, while the opposite was true for the tropical forest fragments, iii) no effect of fragment size on the percent of mass remaining of aspen stakes in the boreal sites, temperate dry, and tropical moist forests, and iv) no significant differences of aspen wood decay between forest edges and interior forest in boreal, temperate and tropical fragments. We conclude that: i) moisture condition is an important control over wood decomposition over broad climate gradients; and that such relationship can be non linear, and ii) the presence of a particular group of organism (termites) can significantly alter the decay rates of wood more than what might be predicted based on climatic factors alone. Biotic controls on wood decay might be more important predictors of wood decay in tropical regions, while abiotic constraints seems to be important determinants of decay in cold forested fragments.

AbstractTropical forests play a critical role in carbon and water cycles at a global scale. Rapid climate change is anticipated in tropical regions over the coming decades and, under a warmer and drier climate, tropical forests are likely to be net sources of carbon rather than sinks. However, our understanding of tropical forest response and feedback to climate change is very limited. Efforts to model climate change impacts on carbon fluxes in tropical forests have not reached a consensus. Here, we use the Ecosystem Demography model (ED2) to predict carbon fluxes of a Puerto Rican tropical forest under realistic climate change scenarios. We parameterized ED2 with species‐specific tree physiological data using the Predictive Ecosystem Analyzer workflow and projected the fate of this ecosystem under five future climate scenarios. The model successfully captured interannual variability in the dynamics of this tropical forest. Model predictions closely followed observed values across a wide range of metrics including aboveground biomass, tree diameter growth, tree size class distributions, and leaf area index. Under a future warming and drying climate scenario, the model predicted reductions in carbon storage and tree growth, together with large shifts in forest community composition and structure. Such rapid changes in climate led the forest to transition from a sink to a source of carbon. Growth respiration and root allocation parameters were responsible for the highest fraction of predictive uncertainty in modeled biomass, highlighting the need to target these processes in future data collection. Our study is the first effort to rely on Bayesian model calibration and synthesis to elucidate the key physiological parameters that drive uncertainty in tropical forests responses to climatic change. We propose a new path forward for model‐data synthesis that can substantially reduce uncertainty in our ability to model tropical forest responses to future climate.

Tropical forests play an important role in regulating the global climate and the carbon cycle. With the changing temperature and moisture along the elevation gradient, the Luquillo Experimental Forest in Northeastern Puerto Rico provides a natural approach to understand tropical forest ecosystems under climate change. In this study, we conducted a soil translocation experiment along an elevation gradient with decreasing temperature but increasing moisture to study the impacts of climate change on soil organic carbon (SOC) and soil respiration. As the results showed, both soil carbon and the respiration rate were impacted by microclimate changes. The soils translocated from low elevation to high elevation showed an increased respiration rate with decreased SOC content at the end of the experiment, which indicated that the increased soil moisture and altered soil microbes might affect respiration rates. The soils translocated from high elevation to low elevation also showed an increased respiration rate with reduced SOC at the end of the experiment, indicating that increased temperature at low elevation enhanced decomposition rates. Temperature and initial soil source quality impacted soil respiration significantly. With the predicted warming climate in the Caribbean, these tropical soils at high elevations are at risk of releasing sequestered carbon into the atmosphere.

Deadwood is a large global carbon store with its store size partially determined by biotic decay. Microbial wood decay rates are known to respond to changing temperature and precipitation. Termites are also important decomposers in the tropics but are less well studied. An understanding of their climate sensitivities is needed to estimate climate change effects on wood carbon pools. Using data from 133 sites spanning six continents, we found that termite wood discovery and consumption were highly sensitive to temperature (with decay increasing >6.8 times per 10°C increase in temperature)—even more so than microbes. Termite decay effects were greatest in tropical seasonal forests, tropical savannas, and subtropical deserts. With tropicalization (i.e., warming shifts to tropical climates), termite wood decay will likely increase as termites access more of Earth’s surface.

Résumé Les animaux, tels que les termites, ont été largement négligés en tant que moteurs à l'échelle mondiale des cycles biogéochimiques 1,2 , malgré les résultats spécifiques au site 3,4 . Le renouvellement du bois mort, une composante importante du cycle du carbone, est entraîné par de multiples agents de désintégration. Des études se sont concentrées sur les systèmes tempérés 5,6 , où les microbes dominent la désintégration 7 . La désintégration microbienne est sensible à la température, doublant généralement pour une augmentation de 10 °C (désintégration efficace Q 10 = ~2) 8–10 . Les termites sont des désintégrateurs importants dans les systèmes tropicaux 3,11–13 et diffèrent des microbes par leur dynamique de population, leur dispersion et leur découverte de substrat 14–16 , ce qui signifie que leurs sensibilités climatiques diffèrent également. En utilisant un réseau de 133 sites couvrant 6 continents, nous rapportons la première quantification mondiale sur le terrain des sensibilités à la température et aux précipitations pour les termites et les microbes, fournissant de nouvelles compréhensions de leur réponse aux changements climatiques. La sensibilité à la température de la désintégration microbienne se situait dans les estimations précédentes. La découverte et la consommation de termites étaient toutes deux beaucoup plus sensibles à la température (désintégration effective Q 10 = 6,53), ce qui entraînait des différences frappantes dans le taux de renouvellement du bois mort dans les zones avec et sans termites. Les impacts de termites ont été les plus importants dans les forêts tropicales saisonnières, les savanes et les déserts subtropicaux. Avec la tropicalisation 17 (c.-à-d., le réchauffement se déplace vers un climat tropical), la contribution des termites à la décomposition mondiale du bois augmentera à mesure qu'une plus grande partie de la surface de la terre deviendra accessible aux termites. Resumen Los animales, como las termitas, se han pasado por alto en gran medida como impulsores a escala mundial de los ciclos biogeoquímicos 1,2 , a pesar de los hallazgos específicos del sitio 3,4 . La rotación de la madera muerta, un componente importante del ciclo del carbono, es impulsada por múltiples agentes de descomposición. Los estudios se han centrado en los sistemas templados 5,6 , donde los microbios dominan la descomposición 7 . La descomposición microbiana es sensible a la temperatura, por lo general se duplica por cada aumento de 10 ° C (Q efectiva de descomposición 10 = ~2) 8–10 . Las termitas son desintegradores importantes en los sistemas tropicales 3,11–13 y difieren de los microbios en su dinámica de población, dispersión y descubrimiento de sustratos 14–16 , lo que significa que sus sensibilidades climáticas también difieren. Utilizando una red de 133 sitios que abarcan 6 continentes, informamos la primera cuantificación global basada en el campo de las sensibilidades a la temperatura y la precipitación para termitas y microbios, proporcionando una comprensión novedosa de su respuesta a los climas cambiantes. La sensibilidad a la temperatura de la descomposición microbiana estaba dentro de las estimaciones anteriores. El descubrimiento y el consumo de termitas fueron mucho más sensibles a la temperatura (descomposición efectiva Q 10 = 6.53), lo que llevó a diferencias sorprendentes en la rotación de madera muerta en áreas con y sin termitas. Los impactos de termitas fueron mayores en los bosques tropicales estacionales, las sabanas y los desiertos subtropicales. Con la tropicalización 17 (es decir, el calentamiento cambia a un clima tropical), la contribución de las termitas a la descomposición global de la madera aumentará a medida que más de la superficie de la tierra se vuelva accesible para las termitas. Abstract Animals, such as termites, have largely been overlooked as global-scale drivers of biogeochemical cycles 1,2 , despite site-specific findings 3,4 . Deadwood turnover, an important component of the carbon cycle, is driven by multiple decay agents. Studies have focused on temperate systems 5,6 , where microbes dominate decay 7 . Microbial decay is sensitive to temperature, typically doubling per 10°C increase (decay effective Q 10 = ~2) 8–10 . Termites are important decayers in tropical systems 3,11–13 and differ from microbes in their population dynamics, dispersal, and substrate discovery 14–16 , meaning their climate sensitivities also differ. Using a network of 133 sites spanning 6 continents, we report the first global field-based quantification of temperature and precipitation sensitivities for termites and microbes, providing novel understandings of their response to changing climates. Temperature sensitivity of microbial decay was within previous estimates. Termite discovery and consumption were both much more sensitive to temperature (decay effective Q 10 = 6.53), leading to striking differences in deadwood turnover in areas with and without termites. Termite impacts were greatest in tropical seasonal forests and savannas and subtropical deserts. With tropicalization 17 (i.e., warming shifts to a tropical climate), the termite contribution to global wood decay will increase as more of the earth's surface becomes accessible to termites. تم التغاضي إلى حد كبير عن الحيوانات، مثل النمل الأبيض، كمحركات عالمية النطاق للدورات الكيميائية الجيولوجية الحيوية 1،2 ، على الرغم من النتائج الخاصة بالموقع 3،4 . دوران الخشب الميت، وهو عنصر مهم في دورة الكربون، مدفوع بعوامل اضمحلال متعددة. وقد ركزت الدراسات على النظم المعتدلة 5،6 ، حيث تهيمن الميكروبات على الاضمحلال 7 . يكون الاضمحلال الميكروبي حساسًا لدرجة الحرارة، وعادة ما يتضاعف لكل زيادة 10 درجات مئوية (الاضمحلال الفعال Q 10 =~2) 8–10 . النمل الأبيض من المتحللين المهمين في الأنظمة الاستوائية 3،11-13 ويختلف عن الميكروبات في ديناميكياتها السكانية وانتشارها واكتشاف الركائز 14–16 ، مما يعني أن حساسياتها المناخية تختلف أيضًا. باستخدام شبكة من 133 موقعًا تمتد عبر 6 قارات، نبلغ عن أول قياس كمي ميداني عالمي لدرجات الحرارة وحساسيات هطول الأمطار للنمل الأبيض والميكروبات، مما يوفر فهمًا جديدًا لاستجابتها للمناخ المتغير. كانت حساسية درجة حرارة الاضمحلال الميكروبي ضمن التقديرات السابقة. كان اكتشاف النمل الأبيض واستهلاكه أكثر حساسية لدرجة الحرارة (التحلل الفعال Q 10 = 6.53)، مما أدى إلى اختلافات صارخة في دوران الأخشاب الميتة في المناطق التي تحتوي على النمل الأبيض أو لا تحتوي عليه. كانت آثار النمل الأبيض أكبر في الغابات الموسمية الاستوائية والسافانا والصحاري شبه الاستوائية. مع الاستوائية 17 (أي، يتحول الاحترار إلى مناخ استوائي)، ستزداد مساهمة النمل الأبيض في تحلل الخشب العالمي مع وصول المزيد من سطح الأرض إلى النمل الأبيض.