SummaryDuring the Last Glacial Maximum (LGM; 18 000-20 000 yr ago) and previous glacial periods, atmospheric [CO 2 ] dropped to 180-190 ppm, which is among the lowest concentrations that occurred during the evolution of land plants. Modern atmospheric CO 2 concentrations ([CO 2 ]) are more than twice those of the LGM and 45% higher than pre-industrial concentrations. Since CO 2 is the carbon source for photosynthesis, lower carbon availability during glacial periods likely had a major impact on plant productivity and evolution. From the studies highlighted here, it is clear that the influence of low [CO 2 ] transcends several scales, ranging from physiological effects on individual plants to changes in ecosystem functioning, and may have even influenced the development of early human cultures (via the timing of agriculture). Through low-[CO 2 ] studies, we have determined a baseline for plant response to minimal [CO 2 ] that occurred during the evolution of land plants. Moreover, an increased understanding of plant responses to low [CO 2 ] contributes to our knowledge of how natural global change factors in the past may continue to influence plant responses to future anthropogenic changes. Future work, however, should focus more on the evolutionary responses of plants to changing [CO 2 ] in order to account for the potentially large effects of genetic change.
Forests cover 30% of the terrestrial Earth surface and are a major component of the global carbon (C) cycle. Humans have doubled the amount of global reactive nitrogen (N), increasing deposition of N onto forests worldwide. However, other global changes—especially climate change and elevated atmospheric carbon dioxide concentrations—are increasing demand for N, the element limiting primary productivity in temperate forests, which could be reducing N availability. To determine the long-term, integrated effects of global changes on forest N cycling, we measured stable N isotopes in wood, a proxy for N supply relative to demand, on large spatial and temporal scales across the continental U.S.A. Here, we show that forest N availability has generally declined across much of the U.S. since at least 1850 C.E. with cool, wet forests demonstrating the greatest declines. Across sites, recent trajectories of N availability were independent of recent atmospheric N deposition rates, implying a minor role for modern N deposition on the trajectory of N status of North American forests. Our results demonstrate that current trends of global changes are likely to be consistent with forest oligotrophication into the foreseeable future, further constraining forest C fixation and potentially storage.
Summary• While studies of modern plants indicate negative responses to low [CO 2 ] that occurred during the last glacial period, studies with glacial plant material that incorporate evolutionary responses are rare. In this study, physiological responses to changing [CO 2 ] were compared between glacial (La Brea tar pits) and modern Juniperus trees from southern California.• Carbon isotopes were measured on annual rings of glacial and modern Juniperus. The intercellular : atmospheric [CO 2 ] ratio (c i ⁄ c a ) and intercellular [CO 2 ] (c i ) were then calculated on an annual basis and compared through geologic time.• Juniperus showed constant mean c i ⁄ c a between the last glacial period and modern times, spanning 50 000 yr. Interannual variation in physiology was greatly dampened during the last glacial period relative to the present, indicating constraints of low [CO 2 ] that reduced responses to other climatic factors. Furthermore, glacial Juniperus exhibited low c i that rarely occurs in modern trees, further suggesting limiting [CO 2 ] in glacial plants.• This study provides some of the first direct evidence that glacial plants remained near their lower carbon limit until the beginning of the glacial-interglacial transition. Our results also suggest that environmental factors that dominate carbon-uptake physiology vary across geologic time, resulting in major alterations in physiological response patterns through time.
1Disturbances affect almost all terrestrial ecosystems, but it has been difficult to 2 identify general principles regarding these influences. To improve our understanding of 3 the long-term consequences of disturbance on terrestrial ecosystems, we present a 4 conceptual framework that analyzes disturbances by their biogeochemical impacts. We 5 posit that the ratio of soil and plant nutrient stocks in mature ecosystems represents a 6 characteristic site property. Focusing on nitrogen (N), we hypothesize that this 7 partitioning ratio (soil N: plant N) will undergo a predictable trajectory after disturbance. 8We investigate the nature of this partitioning ratio with three approaches: (1) nutrient 9 stock data from forested ecosystems in North America, (2) a process-based ecosystem 10 model, and (3) conceptual shifts in site nutrient availability with altered disturbance 11 frequency. Partitioning ratios could be applied to a variety of ecosystems and 12 successional states, allowing for improved temporal scaling of disturbance events. The 13 generally short-term empirical evidence for recovery trajectories of nutrient stocks and 14 partitioning ratios suggests two areas for future research. First, we need to recognize and 15 quantify how disturbance effects can be accreting or depleting, depending on whether 16 their net effect is to increase or decrease ecosystem nutrient stocks. Second, we need to 17 test how altered disturbance frequencies from the present state may be constructive or 18 destructive in their effects on biogeochemical cycling and nutrient availability. Long-19 term studies, with repeated sampling of soils and vegetation, will be essential in further 20 developing this framework of biogeochemical response to disturbance. 21 22 4
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