Thermal acclimation of leaf respiration of tropical trees and lianas: response to experimental canopy warming,and consequences for tropical forest carbon balance

Climate warming is expected to increase respiration rates of tropical forest trees and lianas, which may negatively affect the carbon balance of tropical forests. Thermal acclimation could mitigate the expected respiration increase, but the thermal acclimation potential of tropical forests remains largely unknown. In a tropical forest in Panama, we experimentally increased nighttime temperatures of upper canopy leaves of three tree and two liana species by on average 3  ° C for 1 week, and quantified temperature responses of leaf dark respiration. Respiration at 25  ° C (R₂₅) decreased with increasing leaf temperature, but acclimation did not result in perfect homeostasis of respiration across temperatures. In contrast, Q₁₀ of treatment and control leaves exhibited similarly high values (range 2.5–3.0) without evidence of acclimation. The decrease in R₂₅ was not caused by respiratory substrate depletion, as warming did not reduce leaf carbohydrate concentration. To evaluate the wider implications of our experimental results, we simulated the carbon cycle of tropical latitudes (24 ° S–24 ° N) from 2000 to 2100 using a dynamic global vegetation model (LM3VN) modified to account for acclimation. Acclimation reduced the degree to which respiration increases with climate warming in the model relative to a no‐acclimation scenario, leading to 21% greater increase in net primary productivity and 18% greater increase in biomass carbon storage over the 21st century. We conclude that leaf respiration of tropical forest plants can acclimate to nighttime warming, thereby reducing the magnitude of the positive feedback between climate change and the carbon cycle.     

Urgent need for warming experiments in tropical forests

Although tropical forests account for only a fraction of the planet's terrestrial surface, they exchange more carbon dioxide with the atmosphere than any other biome on Earth, and thus play a disproportionate role in the global climate. In the next 20 years, the tropics will experience unprecedented warming, yet there is exceedingly high uncertainty about their potential responses to this imminent climatic change. Here, we prioritize research approaches given both funding and logistical constraints in order to resolve major uncertainties about how tropical forests function and also to improve predictive capacity of earth system models. We investigate overall model uncertainty of tropical latitudes and explore the scientific benefits and inevitable trade‐offs inherent in large‐scale manipulative field experiments. With a Coupled Model Intercomparison Project Phase 5 analysis, we found that model variability in projected net ecosystem production was nearly 3 times greater in the tropics than for any other latitude. Through a review of the most current literature, we concluded that manipulative warming experiments are vital to accurately predict future tropical forest carbon balance, and we further recommend the establishment of a network of comparable studies spanning gradients of precipitation, edaphic qualities, plant types, and/or land use change. We provide arguments for long‐term, single‐factor warming experiments that incorporate warming of the most biogeochemically active ecosystem components (i.e. leaves, roots, soil microbes). Hypothesis testing of underlying mechanisms should be a priority, along with improving model parameterization and constraints. No single tropical forest is representative of all tropical forests; therefore logistical feasibility should be the most important consideration for locating large‐scale manipulative experiments. Above all, we advocate for multi‐faceted research programs, and we offer arguments for what we consider the most powerful and urgent way forward in order to improve our understanding of tropical forest responses to climate change.     

Plants in a warmer world

Climate is a major determinant for the phenology, physiology, distribution and interactions of plants. The world's recent climate has shown a substantial increase in average temperature which is changing these processes in a perceptible way. The following review compiles and discusses studies reporting recently observed changes in the behaviour, ranges and interactions of species which are thought to be associated with climate change.The multitude of recently published studies providing evidence for the ecological impacts of climate change on many different continents strongly suggests that the last 30 years of warmer temperatures have had a substantial influence on both seasonal patterns, and altitudinal and poleward shifts in vegetation. Common features of change, but also some discrepancies in the response of plants to climate change, are discussed, as well as implications for biodiversity, higher level impacts on community structure and trophic interactions, and some ecosystem consequences.     

Carbon cycling and storage in world forests: biome patterns related to forest age

Forest age, which is affected by stand‐replacing ecosystem disturbances (such as forest fires, harvesting, or insects), plays a distinguishing role in determining the distribution of carbon (C) pools and fluxes in different forested ecosystems. In this synthesis, net primary productivity (NPP), net ecosystem productivity (NEP), and five pools of C (living biomass, coarse woody debris, organic soil horizons, soil, and total ecosystem) are summarized by age class for tropical, temperate, and boreal forest biomes. Estimates of variability in NPP, NEP, and C pools are provided for each biome‐age class combination and the sources of variability are discussed. Aggregated biome‐level estimates of NPP and NEP were higher in intermediate‐aged forests (e.g., 30–120 years), while older forests (e.g., >120 years) were generally less productive. The mean NEP in the youngest forests (0–10 years) was negative (source to the atmosphere) in both boreal and temperate biomes (?0.1 and –1.9 Mg C ha^?1 yr^?1, respectively). Forest age is a highly significant source of variability in NEP at the biome scale; for example, mean temperate forest NEP was ?1.9, 4.5, 2.4, 1.9 and 1.7 Mg C ha^?1 yr^?1 across five age classes (0–10, 11–30, 31–70, 71–120, 121–200 years, respectively). In general, median NPP and NEP are strongly correlated (R²=0.83) across all biomes and age classes, with the exception of the youngest temperate forests. Using the information gained from calculating the summary statistics for NPP and NEP, we calculated heterotrophic soil respiration (R_h) for each age class in each biome. The mean R_h was high in the youngest temperate age class (9.7 Mg C ha^?1 yr^?1) and declined with age, implying that forest ecosystem respiration peaks when forests are young, not old. With notable exceptions, carbon pool sizes increased with age in all biomes, including soil C. Age trends in C cycling and storage are very apparent in all three biomes and it is clear that a better understanding of how forest age and disturbance history interact will greatly improve our fundamental knowledge of the terrestrial C cycle.     

Long‐term nitrogen addition modifies microbial composition and functions for slow carbon cycling and increased sequestration in tropical forest soil

Nitrogen (N) deposition is a component of global change that has considerable impact on belowground carbon (C) dynamics. Plant growth stimulation and alterations of fungal community composition and functions are the main mechanisms driving soil C gains following N deposition in N‐limited temperate forests. In N‐rich tropical forests, however, N deposition generally has minor effects on plant growth; consequently, C storage in soil may strongly depend on the microbial processes that drive litter and soil organic matter decomposition. Here, we investigated how microbial functions in old‐growth tropical forest soil responded to 13 years of N addition at four rates: 0 (Control), 50 (Low‐N), 100 (Medium‐N), and 150 (High‐N) kg N ha^?1 year^?1. Soil organic carbon (SOC) content increased under High‐N, corresponding to a 33% decrease in CO₂ efflux, and reductions in relative abundances of bacteria as well as genes responsible for cellulose and chitin degradation. A 113% increase in N₂O emission was positively correlated with soil acidification and an increase in the relative abundances of denitrification genes (narG and norB). Soil acidification induced by N addition decreased available P concentrations, and was associated with reductions in the relative abundance of phytase. The decreased relative abundance of bacteria and key functional gene groups for C degradation were related to slower SOC decomposition, indicating the key mechanisms driving SOC accumulation in the tropical forest soil subjected to High‐N addition. However, changes in microbial functional groups associated with N and P cycling led to coincidentally large increases in N₂O emissions, and exacerbated soil P deficiency. These two factors partially offset the perceived beneficial effects of N addition on SOC storage in tropical forest soils. These findings suggest a potential to incorporate microbial community and functions into Earth system models considering their effects on greenhouse gas emission, biogeochemical processes, and biodiversity of tropical ecosystems.     

Carbon balance in the tundra, boreal forest and humid tropical forest during climate change: scaling up from leaf physiology and soil carbon dynamics

Carbon exchange by the terrestrial biosphere is thought to have changed since pre-industrial times in response to increasing concentrations of atmospheric CO₂ and variations (anomalies) in inter-annual air temperatures. However, the magnitude of this response, particularly that of various ecosystem types (biomes), is uncertain. Terrestrial carbon models can be used to estimate the direction and size of the terrestrial responses expected, providing that these models have a reasonable theoretical base. We formulated a general model of ecosystem carbon fluxes by linking a process-based canopy photosynthesis model to the Rothamsted soil carbon model for biomes that are not significantly affected by water limitation. The difference between net primary production (NPP) and heterotrophic soil respiration (R_h) represents net ecosystem production (NEP). The model includes (i) multiple compartments for carbon storage in vegetation and soil organic matter, (ii) the effects of seasonal changes in environmental parameters on annual NEP, and (iii) the effects of inter-annual temperature variations on annual NEP. Past, present and projected changes in atmospheric CO₂ concentration and surface air temperature (at different latitudes) were analysed for their effects on annual NEP in tundra, boreal forest and humid tropical forest biomes. In all three biomes, annual NEP was predicted to increase with CO₂ concentration but to decrease with warming. As CO₂ concentrations and temperatures rise, the positive carbon gains through increased NPP are often outweighed by losses through increased R_h, particularly at high latitudes where global warming has been (and is expected to be) most severe. We calculated that, several times during the past 140 years, both the tundra and boreal forest biomes have switched between being carbon sources (annual NEP negative) and being carbon sinks (annual NEP positive). Most recently, significant warming at high latitudes during 1988 and 1990 caused the tundra and boreal forests to be net carbon sources. Humid tropical forests generally have been a carbon sink since 1960. These modelled responses of the various biomes are in agreement with other estimates from either field measurements or geochemical models. Under projected CO₂ and temperature increases, the tundra and boreal forests will emit increasingly more carbon to the atmosphere while the humid tropical forest will continue to store carbon. Our analyses also indicate that the relative increase in the seasonal amplitude of the accumulated NEP within a year is about 0–14% year^?1 for boreal forests and 0–23% year^?1 in the tundra between 1960 and 1990.     

Amazon Basin climate under global warming: the role of the sea surface temperature

The Hadley Centre coupled climate-carbon cycle model (HadCM3LC) predicts loss of the Amazon rainforest in response to future anthropogenic greenhouse gas emissions. In this study, the atmospheric component of HadCM3LC is used to assess the role of simulated changes in mid-twenty-first century sea surface temperature (SST) in Amazon Basin climate change. When the full HadCM3LC SST anomalies (SSTAs) are used, the atmosphere model reproduces the Amazon Basin climate change exhibited by HadCM3LC, including much of the reduction in Amazon Basin rainfall. This rainfall change is shown to be the combined effect of SSTAs in both the tropical Atlantic and the Pacific, with roughly equal contributions from each basin. The greatest rainfall reduction occurs from May to October, outside of the mature South American monsoon (SAM) season. This dry season response is the combined effect of a more rapid warming of the tropical North Atlantic relative to the south, and warm SSTAs in the tropical east Pacific. Conversely, a weak enhancement of mature SAM season rainfall in response to Atlantic SST change is suppressed by the atmospheric response to Pacific SST. This net wet season response is sufficient to prevent dry season soil moisture deficits from being recharged through the SAM season, leading to a perennial soil moisture reduction and an associated 30% reduction in annual Amazon Basin net primary productivity (NPP). A further 23% NPP reduction occurs in response to a 3.5 degrees C warmer air temperature associated with a global mean SST warming.     

森林生态系统碳循环的基本概念和野外测定方法评述

全球气候变化与森林生态系统碳循环息息相关,定量评估森林碳收支是生态系统与全球变化研究的重要任务。30年来森林生态系统碳循环研究已经取得了长足的进展,但全球和区域森林碳收支仍然存在很大的不确定性。这一方面与森林生态系统本身的复杂性有关,另一方面也与具体研究方法有关。评述了森林生态系统碳循环的基本概念和主要野外测定方法,为我国森林生态系统碳循环研究提供可参考的方法论。从生态系统碳浓度、密度、通量、分配和周转5个方面回顾了碳循环相关概念,指出碳浓度和碳储量是对碳库的静态描述,而碳通量和碳周转是对碳库的动态描述。净初级生产力是测量最普遍的碳通量组分,但大多数情况下因忽略了一些细节而被系统低估。普遍使用的净生态系统生产力,由于没有包含非CO2形式的水文、气象和干扰过程产生的碳通量,通常情况下高于生态系统净碳累积速率。在详细介绍碳通量组分的基础上,改进了森林生态系统碳循环的概念模型。重点讨论了碳通量的3种地面实测方法:测树学方法、箱法和涡度协方差法,并指出了其注意事项和不确定性来源。针对当前碳循环研究的突出问题,建议从4个方面减小碳循环测定的不确定性:(1)恰当运用生物量方程估算乔木生物量;(2)尽可能全面测定生态系统碳组分;(3)给出碳通量估算值的不确定性;(4)多种途径交互验证。     

氮沉降对森林生态系统碳吸存的影响

工业化带来的大气氮沉降增加是影响森林生态系统碳吸存的重要因素。将森林碳库分为地上和地下两部分,从3个方面综述了国内外氮沉降对森林生态系统碳吸存影响的研究现状。(1)地上部分:氮限制的温带森林,氮沉降增加了地上部分碳吸存。氮丰富的热带森林,氮沉降对地上部分碳吸存没有影响。过量的氮输入会造成森林死亡率的上升,从而降低地上部分碳吸存。(2)地下部分:相比地上部分研究得少,表现为增加、降低和没有影响3种效果。(3)目前的结论趋向于认为氮沉降促进森林生态系统碳吸存,然而氮沉降所带来的森林生态系统碳吸存能力到底有多大依然无法确定,这也将成为未来氮碳循环研究的重点问题。分析了氮沉降影响森林生态系统碳吸存的机理,介绍了氮沉降对森林生态系统碳吸存影响的4种研究方法。探讨了该领域研究的不足及未来的研究方向。     

Colder soils in a warmer world: A snow manipulation study in a northern hardwood forest ecosystem

In this special section of Biogeochemistry, we present results from asnow manipulation experiment in the northernhardwood forest ecosystem at the Hubbard BrookExperimental Forest in the White Mountains ofNew Hampshire, U.S.A. Snow is important as aninsulator of forest soils. Later developmentof snowpacks, as may occur in a warmer climate,may result in increases in soil freezing (i.e.colder soils in a warmer world) and could causechanges in fine root and microbial mortality,hydrologic and gaseous losses of nitrogen (N),and the acid-base status of drainage water. Inour study, we kept soils snow free by shovelinguntil early February during the mild winters of1997/1998 and 1998/1999. The treatment producedmild, but persistent soil freezing and inducedsurprisingly significant effects on rootmortality, soil nitrate (NO₃ ^–) levelsand hydrologic fluxes of C, N and P. In thisspecial section we present four papersaddressing, (1) soil temperature and moistureresponse to our snow manipulation treatment(Hardy et al.), (2) theresponse of fine root dynamics to treatment(Tierney et al.), (3) theresponse of soil inorganic N levels, insitu N mineralization and nitrification,denitrification and microbial biomass to thetreatment (Groffman et al.)and (4) soil solution concentrations and fluxesof C, N and P (Fitzhugh et al.). In this introductory paper we: (1)review the literature on snow effects on forestbiogeochemistry, (2) introduce our manipulationexperiment and (3) summarize the resultspresented in the other papers in this issue.     

Response of soil surface CO2 flux in a boreal forest to ecosystem warming

Soil surface carbon dioxide (CO₂) flux (R_S) was measured for 2 years at the Boreal Soil and Air Warming Experiment site near Thompson, MB, Canada. The experimental design was a complete random block design that consisted of four replicate blocks, with each block containing a 15 m × 15 m control and heated plot. Black spruce [Picea mariana (Mill.) BSP] was the overstory species and Epilobium angustifolium was the dominant understory. Soil temperature was maintained (～5 °C) above the control soil temperature using electric cables inside water filled polyethylene tubing for each heated plot. Air inside a 7.3‐m‐diameter chamber, centered in the soil warming plot, contained approximately nine black spruce trees was heated ～5 °C above control ambient air temperature allowing for the testing of soil‐only warming and soil+air warming. Soil surface CO₂ flux (R_S) was positively correlated (P < 0.0001) to soil temperature at 10 cm depth. Soil surface CO₂ flux (R_S) was 24% greater in the soil‐only warming than the control in 2004, but was only 11% greater in 2005, while R_S in the soil+air warming treatments was 31% less than the control in 2004 and 23% less in 2005. Live fine root mass (< 2 mm diameter) was less in the heated than control treatments in 2004 and statistically less (P < 0.01) in 2005. Similar root mass between the two heated treatments suggests that different heating methods (soil‐only vs. soil+air warming) can affect the rate of decomposition.     

Meta-analysis shows forest soil CO2 effluxes are dependent on the disturbance regime and biome type

Forest soil CO₂ efflux (FCO₂) is a crucial process in global carbon cycling; however, how FCO₂ responds to disturbance regimes in different forest biomes is poorly understood. We quantified the effects of disturbance regimes on FCO₂ across boreal, temperate, tropical and Mediterranean forests based on 1240 observations from 380 studies. Globally, climatic perturbations such as elevated CO₂ concentration, warming and increased precipitation increase FCO₂ by 13% to 25%. FCO₂ is increased by forest conversion to grassland and elevated carbon input by forest management practices but reduced by decreased carbon input, fire and acid rain. Disturbance also changes soil temperature and water content, which in turn affect the direction and magnitude of disturbance influences on FCO₂. FCO₂ is disturbance- and biome-type dependent and such effects should be incorporated into earth system models to improve the projection of the feedback between the terrestrial C cycle and climate change.     

The likely impact of elevated [CO2], nitrogen deposition, increased temperature and management on carbon sequestration in temperate and boreal forest ecosystems: a literature review

Temperate and boreal forest ecosystems contain a large part of the carbon stored on land, in the form of both biomass and soil organic matter. Increasing atmospheric [CO2], increasing temperature, elevated nitrogen deposition and intensified management will change this C store. Well documented single-factor responses of net primary production are: higher photosynthetic rate (the main [CO2] response); increasing length of growing season (the main temperature response); and higher leaf-area index (the main N deposition and partly [CO2] response). Soil organic matter will increase with increasing litter input, although priming may decrease the soil C stock initially, but litter quality effects should be minimal (response to [CO2], N deposition, and temperature); will decrease because of increasing temperature; and will increase because of retardation of decomposition with N deposition, although the rate of decomposition of high-quality litter can be increased and that of low-quality litter decreased. Single-factor responses can be misleading because of interactions between factors, in particular those between N and other factors, and indirect effects such as increased N availability from temperature-induced decomposition. In the long term the strength of feedbacks, for example the increasing demand for N from increased growth, will dominate over short-term responses to single factors. However, management has considerable potential for controlling the C store.     

菌根真菌影响森林生态系统碳循环研究进展

森林生态系统在全球碳(C)储量中占据极为重要的地位。菌根真菌广泛存在于森林生态系统中,在森林生态系统C循环过程中发挥重要的作用。阐述了不同菌根类型真菌在森林生态系统C循环过程中的功能,对比了温带/北方森林与热带/亚热带森林中菌根真菌介导的C循环研究方面新近取得的研究结果。发现温带和北方森林的外生菌根(EcM)植物对地上生物量C的贡献相对较小,然而是地下C储量的主要贡献者;以丛枝菌根(AM)共生为主的热带/亚热带森林地表生物量占比较高,表明AM植被对热带/亚热带森林地上生物量C的贡献相对较大。我们还就全球变化背景下,菌根真菌及其介导的森林生态系统C汇功能,以及不同菌根类型树种影响C循环的机制等进行了总结。菌根真菌通过影响凋落物分解、土壤有机质形成及地下根系生物量,进而影响整个森林生态系统的C循环功能。菌根介导的森林C循环过程很大程度上取决于(优势)树木的菌根类型和森林土壤中菌根真菌的群落结构。最后指出了当前研究存在的主要问题以及未来研究展望。本文旨在明确菌根真菌在森林生态系统C循环转化过程中的重要生态功能,有助于准确地评估森林生态系统C汇现状,为应对全球变化等提供重要的依据。     

Changes in seasonal precipitation distribution but not annual amount affect litter decomposition in a secondary tropical forest

In the tropics of South China, climate change induced more rainfall events in the wet season in the last decades. Moreover, there will be more frequently spring drought in the future. However, knowledge on how litter decomposition rate would respond to these seasonal precipitation changes is still limited. In the present study, we conducted a precipitation manipulation experiment in a tropical forest. First, we applied a 60% rainfall exclusion in April and May to defer the onset of wet season and added the same amount of water in October and November to mimic a deferred wet season (DW); second, we increased as much as 25% mean annual precipitation into plots in July and August to simulate a wetter wet season (WW). Five single‐species litters, with their carbon to nitrogen ratio ranged from 27 to 49, and a mixed litter were used to explore how the precipitation change treatments would affect litter decomposition rate. The interaction between precipitation changes and litter species was not significant. The DW treatment marginally accelerated litter decomposition across six litter types. Detailed analysis showed that DW increased litter decomposition rate in the periods of January to March and October to December, when soil moisture was increased by the water addition in the dry season. In contrast, WW did not significantly affect litter decomposition rate, which was consistent with the unchanged soil moisture pattern. In conclusion, the study indicated that regardless of litter types or litter quality, the projected deferred wet season would increase litter decomposition rate, whereas the wetter wet season would not affect litter decomposition rate in the tropical forests. This study improves our knowledge of how tropical forest carbon cycling in response to precipitation change.     

森林生态系统碳循环对全球氮沉降的响应

森林土壤和植被储存着全球陆地生态系统大约46%的碳,在全球碳平衡中起着非常重要的作用。过去几十年来,森林生态系统的碳循环和碳吸存受到了全球氮沉降的深刻影响,因为氮沉降改变了陆地生态系统的生产力和生物量积累。以欧洲和北美温带森林区域开展的研究为基础,综述了氮沉降对植物光合作用、土壤呼吸、土壤DOM及林木生长的影响特征和机理,探讨了森林生态系统碳动态对氮沉降响应的不确定性因素。热带森林C、N循环与大部分温带森林不同,人为输入的氮对热带生态系统过程的影响也可能不同,因此指出了在热带地区开展碳氮循环耦合研究的必要性和紧迫性。     

Roles of food web and heterotrophic microbial processes in upper ocean biogeochemistry: Global patterns and processes

The growth and dynamics of plankton in the ocean vary with natural cycles, global climate change and the long-term evolution of ecosystems. The ocean is a large reservoir for CO₂ and the food webs in the upper ocean play critical roles in regulating the global carbon cycle, changes in atmospheric CO₂ and associated global warming. Microheterotrophs are a key component of the upper ocean food webs. Here, we report on the results of an analysis of the distribution of bacteria and related properties in the World Ocean. We found that, for the data set as a whole, there is a significant latitudinal gradient in all field-measured and computed bacterial properties, except growth rate. Gradients were, for the most part, driven by an equator-ward increase in the Southern Hemisphere. The biomass, rates of production and respiration and dissolved organic carbon concentrations were significantly higher in the Northern than the Southern hemispheres. In contrast, growth rates were the same in the two hemispheres. We conclude that the lower biomass and production in the Southern Hemisphere reflects greater top-down control by microbial grazers, which would be due to a lower abundance or activity of omnivorous zooplankton in the Southern than Northern Hemispheres. These large spatial differences in dynamics, structure and activity of the bacterial community and the microbial food web will be reflected in different patterns of carbon cycling, export and air–sea exchange of CO₂ and the potential ability of the ocean to sequester carbon.     

Approaches for inclusion of forest carbon cycle in life cycle assessment – a review

Forests are a significant pool of terrestrial carbon. A key feature related to forest biomass harvesting and use is the typical time difference between carbon release into and sequestration from the atmosphere. Traditionally, the use of sustainably grown biomass has been considered as carbon neutral in life cycle assessment (LCA) studies. However, various approaches to account for greenhouse gas (GHG) emissions and sinks of forest biomass acquisition and use have also been developed and applied, resulting in different conclusions on climate impacts of forest products. The aim of this study is to summarize, clarify, and assess the suitability of these approaches for LCA. A literature review is carried out, and the results are analyzed through an assessment framework. The different approaches are reviewed through their approach to the definition of reference land‐use situation, consideration of time frame and timing of carbon emissions and sequestration, substitution credits, and indicators applied to measure climate impacts. On the basis of the review, it is concluded that, to account for GHG emissions and the related climate impacts objectively, biomass carbon stored in the products and the timing of sinks and emissions should be taken into account in LCA. The reference situation for forest land use has to be defined appropriately, describing the development in the absence of the studied system. We suggest the use of some climate impact indicator that takes the timing of the emissions and sinks into consideration and enables the use of different time frames. If substitution credits are considered, they need to be transparently presented in the results. Instead of carbon stock values taken from the literature, the use of dynamic forest models is recommended.     

Improving predictions of tropical forest response to climate change through integration of field studies and ecosystem modeling

Tropical 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.     

Detecting Tropical Forests' Responses to Global Climatic and Atmospheric Change: Current Challenges and a Way Forward

Because of tropical forests' disproportionate importance for world biodiversity and for the global carbon cycle, we urgently need to understand any effects on these ecosystems from the ongoing changes in climate and atmosphere. This review, intended to complement existing data reviews on this topic, focuses on three major classes of challenges that we currently face when trying to detect and interpret directional changes in tropical forests. One is the very limited existing information on the historical context of study sites. Lasting effects from past climate, natural disturbances, and/or human activities could be significantly affecting current-day processes in tropical forests and need to be investigated for all active field sites. Second, while progress has been made in recent years on standardizing and refining research approaches, a number of methods- and data-limitations continue to affect efforts both to detect within-forest changes and to relate them to ongoing environmental change. Important outstanding needs are improved sampling designs, longer time-series of observations, filling key data gaps, and data access. Finally, forest responses to ongoing environmental change are complex. The effects of many simultaneously changing environmental factors are integrated by the plants, and their responses can involve significant lags, carryovers, and non-linearities. Specifying effects of individual environmental changes, however, is required for accurate ecosystem-process models and thus for projecting future impacts on these forests. After discussing these several types of challenges and ways to address them, I conclude with a priority agenda for this critical area of research.
Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp .