Refining Estimates of Tropical Forest Greenhouse Gas Exchange using Plant Traits

热带森林在调节全球温室气体浓度方面发挥着至关重要的作用，与大气交换大量的二氧化碳（CO2）、甲烷（CH4）和一氧化二氮（N2O）。热带森林作为二氧化碳来源和大量汇的重要性已得到充分确认，但我们对热带森林对全球CH4和N2O收支的贡献知之甚少。量化热带森林中CH4和N2O的交换是很重要的，因为尽管湿地热带森林是CH4的主要来源，但自由排水的热带土壤是N2O的来源，但却是CH4的汇。树木也可以作为温室气体的管道，我们最近的工作表明，自由排水土壤上的热带树木可以排放或吸收CH4和N2O。然而，树木对热带森林温室气体收支的贡献是完全未知的，我们不知道自由排水土壤上的热带森林树木是否会作为CH4和N2O的总体汇或源。我们迫切需要解决这一知识差距，因为自由排水土壤上的热带森林覆盖了大片土地，正以惊人的速度受到人类活动的破坏或干扰。为了量化热带树木在全球温室气体预算中的作用，我们需要一种方法，使我们能够将当地或区域的测量结果转化为能够在更大范围内代表热带森林的模型。我们的项目将通过量化热带森林土壤和树木对CH4和N2O的排放和吸收，并确定这些温室气体通量受热带树木特定特征影响的程度，来开发这样一种方法。树木可以影响土壤中温室气体的产生或消耗，因为根系活动和植物凋落物的输入改变了土壤化学和微生物活动。同样，通过茎、枝和叶吸收或排放的温室气体在具有不同特征（如木材密度、生长速度或叶面化学）的树种之间差异很大。我们的项目将通过对树木和周围土壤的CH4和N2O通量进行详细测量，确定这些树木特性如何影响温室气体通量。在项目的第一年，我们将在中美洲巴拿马热带森林的一个集中研究地点确定温室气体通量与土壤化学或树木性状差异之间的关系。在计划的第二年，我们会将研究范围扩展至多个地点，以评估更多物种的土壤和树木的温室气体通量，并确定降雨变化的影响。因此，我们的野外测量将首次提供自由排水土壤上热带树木CH4和N2O通量的综合评估，这代表了热带温室气体收支的一个全新组成部分。在项目的最后一年，我们将利用我们关于地点、物种和单株树木内部和之间变化的数据来构建模型，利用相关植物性状和降雨数据的组合来估计森林温室气体总通量。我们将把我们的模型应用于沿降雨梯度的15个样地，以获得整个景观中森林温室气体交换的估计。通过使用树木特性而不是物种特性，我们的方法将提供一种估算热带广大地区温室气体吸收和排放的方法，这将改善热带森林在全球温室气体预算中的代表性，并为热带土地利用变化的影响评估提供信息。

Tropical forests play a crucial role in regulating global concentrations of greenhouse gases, exchanging vast amounts of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) with the atmosphere. The importance of tropical forests as both a source and substantial sink of CO2 is well established, but we know much less about the contribution of tropical forests to global CH4 and N2O budgets. Quantifying CH4 and N2O exchange in tropical forests is important because, although wetland tropical forests are a large source of CH4, free-draining tropical soils are a source of N2O but a sink for CH4. Trees can also act as conduits for greenhouse gases, and our recent work demonstrated that tropical trees on free-draining soils can emit or take up both CH4 and N2O. However, the contribution of trees to tropical forest greenhouse gas budgets is entirely unknown and we do not know whether tropical forest trees on free-draining soils will act as an overall sink or source for CH4 and N2O. We urgently need to address this knowledge gap because tropical forests on free-draining soils cover vast areas of land and are being destroyed or disturbed by human activities at an alarming rate. To quantify the role of tropical trees in global greenhouse gas budgets, we need an approach that allows us to translate local or regional measurements into models that can represent tropical forests across much larger scales. Our project will develop such an approach by quantifying the emissions and uptake of CH4 and N2O by tropical forest soils and trees, and determining the extent to which these greenhouse gas fluxes are influenced by specific characteristics of tropical trees.Trees can influence the production or consumption of greenhouse gases in the soil because root activity and plant litter inputs alter soil chemistry and microbial activity. Similarly, uptake or emission of greenhouse gases through stems, branches and leaves varies widely among tree species with distinct traits such as wood density, growth rate, or foliar chemistry. Our project will establish how such tree traits influence greenhouse gas fluxes by taking detailed measurements of CH4 and N2O fluxes from trees and the surrounding soil. In the first year of the project, we will determine the relationships between greenhouse gas fluxes and differences in soil chemistry or tree traits for six tree species at an intensive study site in tropical forest in Panama, Central America. In the second year of the project, we will expand our studies to multiple sites along a rainfall gradient to assess greenhouse gas fluxes from soils and trees for a larger range of species, and to identify the influence of changes in rainfall. Hence, our field measurements will provide the first ever comprehensive assessment of CH4 and N2O fluxes from tropical trees on free-draining soils, representing an entirely new component of the tropical greenhouse gas budget. In the final year of the project, we will use our data on variation within and among sites, species and individual trees to construct models that estimate total forest greenhouse gas fluxes using a combination of relevant plant traits and rainfall data. We will apply our models to 15 plots along the rainfall gradient to obtain estimates of forest greenhouse gas exchange across the landscape. By using tree traits rather than species identities, our approach will offer a way to estimate greenhouse gas uptake and emissions across wide areas of the tropics, which will improve the representation of tropical forests in global greenhouse gas budgets and inform impact assessments of tropical land-use change.