The role of time in ecology has a long history of investigation, but ecologists have largely restricted their attention to the influence of concurrent abiotic conditions on rates and magnitudes of important ecological processes. Recently, however, ecologists have improved their understanding of ecological processes by explicitly considering the effects of antecedent conditions. To broadly help in studying the role of time, we evaluate the length, temporal pattern, and strength of memory with respect to the influence of antecedent conditions on current ecological dynamics. We developed the stochastic antecedent modelling (SAM) framework as a flexible analytic approach for evaluating exogenous and endogenous process components of memory in a system of interest. We designed SAM to be useful in revealing novel insights promoting further study, illustrated in four examples with different degrees of complexity and varying time scales: stomatal conductance, soil respiration, ecosystem productivity, and tree growth. Models with antecedent effects explained an additional 18-28% of response variation compared to models without antecedent effects. Moreover, SAM also enabled identification of potential mechanisms that underlie components of memory, thus revealing temporal properties that are not apparent from traditional treatments of ecological time-series data and facilitating new hypothesis generation and additional research.

We explore the hypothesis that a passing satellite or dark matter subhalo has excited coherent oscillations of the Milky Way's stellar disk in the direction perpendicular to the Galactic midplane. This work is motivated by recent observations of spatially dependent bulk vertical motions within ∼ 2 kpc of the Sun. A satellite can transfer a fraction of its orbital energy to the disk stars as it plunges through the Galactic midplane thereby heating and thickening the disk. Bulk motions arise during the early stages of such an event when the disk is still in an unrelaxed state. We present simple toy-model calculations and simulations of disk-satellite interactions, which show that the response of the disk depends on the relative velocity of the satellite. When the component of the satellite's velocity perpendicular to the disk is small compared with that of the stars, the perturbation is predominantly a bending mode. Conversely, breathing and higher order modes are excited when the vertical velocity of the satellite is larger than that of the stars. We argue that the compression and rarefaction motions seen in three different surveys are in fact breathing mode perturbations of the Galactic disk.

Emissions of CO 2 from soils make up one of the largest fluxes in the global C cycle, thus small changes in soil respiration may have large impacts on global C cycling. Anthropogenic additions of CO 2 to the atmosphere are expected to alter soil carbon cycling, an important component of the global carbon budget. As part of the Duke Forest Free-Air CO 2 Enrichment (FACE) experiment, we examined how forest growth at elevated (+200 ppmv) atmospheric CO 2 concentration affects soil CO 2 dynamics over 7 years of continuous enrichment. Soil respiration, soil CO 2 concentrations, and the isotopic signature of soil CO 2 were measured monthly throughout the 7 years of treatment. Estimated annual rates of soil CO 2 efflux have been significantly higher in the elevated plots in every year of the study, but over the last 5 years the magnitude of the CO 2 enrichment effect on soil CO 2 efflux has declined. Gas well samples indicate that over 7 years fumigation has led to sustained increases in soil CO 2 concentrations and depletion in the d 13 C of soil CO 2 at all but the shallowest soil depths.

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1. Variation in antecedent (past) climate conditions is likely to govern tree growth over long periods of time. Antecedent conditions are rarely considered in models of tree growth, representing a weakness in quantitative understanding of forest responses to climate variations.2. We applied the stochastic antecedent modelling (SAM) framework to 367International Tree Ring Data Bank chronologies in the southwestern US ("Southwest") representing eight conifer species. To better understand climatic and physiologic controls on tree growth, we quantify the effects of antecedent precipitation, temperature and Palmer Drought Severity Index (PDSI) over 60 months preceding and including the year of ring formation.3. In Pinus edulis, Pinus ponderosa and Pseudotsuga menziesii, growth responded primarily to recent precipitation and temperature conditions (43%-49% of the response was driven by conditions during the year of ring formation), but to less recent PDSI conditions (>50% of response driven by conditions 13-48 months prior to the year of ring formation), though PDSI significantly affected growth at only 21% of sites. Combining extensive tree-ring data with monthly resolution climate data also reveals key climatic events, such as the effect of monsoon arrival date on growth, especially in P. menziesii, highlighting the ability of the SAM framework to identify climate effects at multiple time-scales. 4. Sensitivity to antecedent climate, baseline growth at average climate conditions and the strength of first order autocorrelation varied spatially, suggesting variation in mean growing conditions, non-structural carbohydrate storage, and/or seasonal precipitation contribution of the North American Monsoon may drive differences in growth sensitivities across species' ranges. 5.Synthesis. Our findings provide further evidence for multi-year legacy effects of climate conditions, particularly drought metrics, on tree growth. Antecedent climate and especially drought are key drivers of growth in these species, and associated climatic sensitivities and growth indices vary spatially. We argue such factors should be considered in modelling efforts. The spatial variability in antecedent climate sensitivities points to key differences in how different populations within a species range may respond to climate change, particularly if timing of weather events, such as monsoon arrival date, or annual precipitation amounts undergoes significant changes. |