Timescales of Climate-Ecosystem Interactions. Scientific Tools

The coupling of ecosystems and climate occurs over a continuum of timescales from minutes to seasons to millennia (Table 1.2). At short timescales, the seasonal emergence and senescence of leaves alters the absorption of radiation, the dissipation of energy into latent and sensible heat, and CO₂ uptake. The effect of these changes can be seen in air temperature, humidity, and the seasonal drawdown of CO₂ in the atmosphere. At seasonal to interannual timescales, photosynthesis, respiration, evapotrans- piration, and reactive gas fluxes influence the physical and chemical state of the atmosphere. Interannual variability in temperature and precipitation alter ecosystem metabolism, which is again evident in the concentration of CO₂ in the atmosphere.

Over several decades, people shape the landscape through clearing of land for agriculture, reforestation of abandoned farmland, and through urbanization. These land uses alter surface energy fluxes, biogeochemical cycles, and the hydrological cycle, and they produce a discernible signal in temperature, precipitation, the concentrations of CO₂, CH₄, and N₂O in the atmosphere, and the deposition of atmospheric pollutants such as reactive nitrogen, ozone, and black carbon aerosols onto land. At longer timescales of decades to centuries, slower suc- cessional changes in response to disturbances control community composition and ecosystem structure and so alter surface energy fluxes, carbon storage, and trace gas emissions. Coupled climate-ecosystem dynamics are particularly evident over periods of centuries to millennia.

Temperature, precipitation, and atmospheric CO₂ are the chief determinants of the geographic distribution of vegetation across the planet. In turn, this biogeography affects climate and atmospheric CO ₂ concentration. The outcome of climate-vegetation interactions can also be seen in the evolutionary record. There is a close relationship between leaf shape and climate. Vascular plants introduced numerous biotic feedbacks on climate, primarily related to plant responses to CO₂ that affected stomatal conductance and leaf form.

Scientific Tools. The influence of plants and terrestrial ecosystems on the atmosphere can be discerned through environmental monitoring, experimental manipulation, or with the use of numerical models of weather and climate. Intensive field campaigns with ground-based measurements of biosphere-atmosphere flux exchanges and aircraft measurements of atmospheric composition provide datasets to analyze biosphere-atmosphere coupling over short time periods (up to several weeks). Environmental monitoring techniques include eddy covariance flux towers that provide continuous measurements of biosphere-atmosphere exchanges of energy, moisture, and trace gases at fast timescales (subhourly).

The longest such sites have been continually operating for over two decades. Ecosystem and watershed studies provide monitoring of carbon and elemental stocks and fluxes and the hydrologic cycle, typically at longer timescales (e.g., annual). Such studies extend over several decades. Satellite observations of leaf area, surface albedo, surface temperature, and other properties provide global coverage at a high spatial resolution for a period extending now for almost three decades. Atmospheric CO₂ observations at numerous locations throughout the world provide information about the seasonal dynamics of land-atmosphere carbon exchange and continental-scale fluxes on timescales of years to decades. The longest such record, at Mauna Loa, Hawaii, dates back to 1958. Ice core measurements reveal the history of CO₂, CH₄, N₂O, and dust in the atmosphere over the past several hundred thousand years and variations with glacial-interglacial cycles.

Whole-ecosystem experimental manipulations provide insight to ecosystem responses to environmental change. Such experiments warm ecosystems or exclude rainfall to study responses to climate change, enrich the air with CO₂ to study responses to elevated atmospheric CO₂ concentrations, and fertilize the soil with nutrients (e.g., nitrogen and phosphorus) to examine responses to perturbed biogeochemical cycles. Watershed manipulation studies that remove vegetation show the biotic control of the hydrologic cycle.

The influence of plants and ecosystems on large-scale climate is difficult to establish directly through observations. Careful examination of climatic data can sometimes reveal an ecological influence, such as the effect of leaf emergence on springtime evapotranspiration and air temperature. Eddy covariance flux towers and field experiments provide local-scale insight to ecosystem-atmosphere interactions, and advances in remote sensing science aid extrapolation to larger spatial scales. More often, however, our understanding of how plants and ecosystems affect climate comes from atmospheric models and their numerical parameterizations of Earth’s biosphere. Paired climate simulations, one serving as a control to compare against another simulation with altered vegetation, demonstrate an ecological influence on climate.

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