Collaborative Research: Thresholds and mechanisms of net ecosystem production (NEP) resilience following moderate disturbance: Why does one ecosystem recover and another one crash?

美国的森林是食物、纤维和能源的主要来源。 它们在地球的气候系统中起着重要作用，因为它们可以在植物生物量中螯合碳，否则这些碳可能会形成大气温室气体（如二氧化碳）的分子骨架。森林捕获大气二氧化碳和积累生物量的能力可能会随着年龄和干扰而发生很大变化。 科学家们长期以来一直认为，随着森林的老化，生长和碳吸收会下降。然而，新的观察结果表明，在老化的森林中，低水平的干扰，例如来自虫害、真菌病原体和极端天气的干扰，可能会与直觉相反，维持甚至增加森林的碳固存和增长。森林固碳率高于预期的机制尚不清楚。本研究旨在确定森林生长对干扰恢复力的机制及其阈值。研究人员还将评估不同的计算机模拟是否、如何以及为什么对预测未来森林碳储存、生长和产量至关重要，却无法复制这种弹性。此外，他们将确定美国西部的万年青林和东部的落叶林是否具有不同的干扰机制和气候，遵循独特的年龄森林生长轨迹。该项目对社会、森林和土地管理者、小学教育工作者、大学生和森林科学家的好处是深远的。通过将生物学上知情的实地和模拟实验与北美森林的综合相结合，这项研究将显着推进我们对森林干扰的生态思考，同时产生与生态系统和地球系统模拟直接相关的结果，并为森林管理者提供工作，以最大限度地提高碳储存，增长和木材生产在日益受到干扰的森林景观中。该项目将为小学教师编制公开提供的教学材料，培训若干研究生和本科生，向科学家和土地管理人员提供公开和透明的数据来源和计算机代码，并在美国能源部实验室和一个学术机构之间建立学生培训伙伴关系。 未来的陆地碳汇是不确定的，因为美国上中西部和东部的森林广泛推进从早期到中期的森林演替。随着这一转变，早期演替冠层的优势正在衰老，并让位于更生物和结构复杂的森林，越来越多地受到中度严重干扰。最近的研究表明，净初级生产可能会在这些森林中以高于预期的速度持续，但这种功能恢复力的限制和机制无法从目前的知识中预测，这些知识几乎完全来自于对最近受到干扰的森林中严重的林分更换干扰动态的研究。从同样的知识基础发展起来的生态系统和全球模型也难以再现适度扰动的影响。这项工作的三个核心研究目标是：1）确定机制支持净初级生产力恢复干扰，和他们的阈值;了解如果，如何，以及为什么不同的森林模型无法复制这种NPP恢复力;和3）阐明是否温带落叶和针叶林，不同的干扰制度和气候，遵循独特的年龄生产轨迹。该项目采用实地实验，模型测试和大规模数据合成的三管齐下的方法，以改变对碳循环在老龄化森林中对一系列中等干扰强度的适应能力的理解，阐明确定净初级生产力恢复力和下降之间阈值的基本机制。场分量使用完全复制的干扰严重度梯度，从0到85%落叶，以系统地确定碳循环如何以及为什么响应于干扰水平的上升而发生变化。PI将采用一套碳和氮循环测量，重点是冠层结构，叶片生理学和冠层氮再分配，以确定导致快速净初级生产力恢复或干扰后下降的机制。该项目的建模组件使用数据同化实验，在一个开源的NSF支持的生态信息学工具箱中运行两个非常不同的生态生理模型，以确定模型假设未能模拟净初级生产对干扰的恢复力的过程，并迭代地通知下一个田间季节的采样优先级。最后，一个数据合成组件使用新的观测，以描述干扰对北美温带森林的年龄净生态系统生产轨迹的影响。

Forests of the United States are primary sources of food, fiber and energy. They play a fundamental role in the earth's climate system by sequestering in plant biomass carbon that might otherwise form the molecular backbone of atmospheric greenhouse gases such as carbon dioxide. Forests' capacity to capture atmospheric carbon dioxide and build biomass may change substantially with age and disturbance. Scientists have long theorized a decline in growth and carbon uptake as forests age. New observations, however, suggest that low levels of disturbance, such as those originating from insect pests, fungal pathogens, and extreme weather, in aging forests may, counter-intuitively, sustain or even increase forest carbon sequestration and growth. The mechanisms underlying these higher-than-expected rates of forest carbon sequestration are unknown. This study seeks to identify the mechanisms underpinning forest growth resilience to disturbance, and their thresholds. The researchers will also evaluate if, how, and why different computer simulations, critical to predicting future forest carbon storage and growth and yield, fail to replicate this resilience. Furthermore, they will determine whether evergreen forests in the western United States and deciduous forests in the East, with different prevailing disturbance regimes and climates, follow unique age-forest growth trajectories. The benefits of this project to society, forest and land managers, grade school educators, university students, and forest scientists are far-reaching. By combining biologically-informed field and simulation experiments with a synthesis of North American forests, this study will significantly advance our ecological thinking about forest disturbance, while producing results immediately relevant and accessible to ecosystem and earth system simulations, and to forest managers working to maximize carbon storage, growth, and timber production in increasingly disturbed forest landscapes. The project will produce openly available instructional materials for grade school teachers, train several graduate and undergraduate students, provide open and transparent sources of data and computer code to scientists and land managers, and form a student training partnership between a United States Department of Energy laboratory and an academic institution.  The future terrestrial carbon sink is uncertain as forests of the United States upper Midwest and east broadly advance from early to middle forest succession. With this transition, early successional canopy dominants are senescing and giving way to more biologically and structurally complex forests that are increasingly subject to moderate severity disturbance. Recent studies suggest that net primary production may be sustained in such forests at higher-than-expected rates, but the limits of and mechanisms behind such functional resilience cannot be predicted from present knowledge, which is derived almost entirely from studies of severe, stand-replacing disturbance dynamics in recently disturbed forests. Ecosystem and global models, developed from the same intellectual foundations, also have trouble reproducing the effects of moderate disturbances. The three core research objectives of this work are to: 1) identify mechanisms supporting net primary production resilience to disturbance, and their thresholds; understand if, how, and why different forest models fail to replicate this NPP resilience; and 3) elucidate whether temperate deciduous and coniferous forests, with different disturbance regimes and climates, follow unique age-production trajectories. The project uses a 3-pronged approach of field experiments, model testing, and large-scale data synthesis to transform understanding of how resilient the carbon cycle will be to a range of moderate disturbance intensities in aging forests, elucidating the underlying mechanisms that determine the threshold between net primary production resilience and decline. The field component uses a fully replicated gradient of disturbance severity, from 0 to 85 % defoliation, to systematically determine how and why the carbon cycle shifts in response to rising disturbance levels. The PIs will employ a suite of carbon and nitrogen cycling measurements, focusing on canopy structure, leaf physiology, and canopy nitrogen reallocation, to identify the mechanisms that cause rapid net primary production resilience or decline following disturbance. The modeling component of the project uses data assimilation experiments, running two very different ecophysiological models within an open source, NSF-supported ecoinformatics toolbox, to identify the processes most responsible for the models' hypothesized failure to simulate net primary production resilience to disturbance, and iteratively inform the next field season's sampling priorities. Finally, a data synthesis component uses newly available observations to characterize disturbance effects on age-net ecosystem production trajectories for North American's temperate forests.