Lisa J. Graumlich

Interactive Canopies for a Climate Model

Abstract Climate models depend on evapotranspiration from models of plant stomatal resistance and leaf cover, and hence they depend on a description of the response of leaf cover to temperature and soil moisture. Such a description is derived as an addition to the Biosphere–Atmosphere Transfer Scheme and tested by simulations in a climate model. Rules for carbon uptake, allocation between leaves, fine roots, and wood, and loss terms from respiration, leaf, and root turnover and cold and drought stress, are used to infer the seasonal growth of leaf area as needed in a climate model, and to provide carbon fluxes (assuming also a simple soil carbon model) and net primary productivity. The scheme is tested in an 11-yr integration with the NCAR CCM3 climate model. After a spinup period of several years, the model equilibrates to a seasonal cycle plus some interannual variability. Effects of the latter are noticeable for the Amazon. Overall, drought stress has nearly as large an effect on leaf mortality as cold...

Sustainability or Collapse: What Can We Learn from Integrating the History of Humans and the Rest of Nature?

Abstract Understanding the history of how humans have interacted with the rest of nature can help clarify the options for managing our increasingly interconnected global system. Simple, deterministic relationships between environmental stress and social change are inadequate. Extreme drought, for instance, triggered both social collapse and ingenious management of water through irrigation. Human responses to change, in turn, feed into climate and ecological systems, producing a complex web of multidirectional connections in time and space. Integrated records of the co-evolving human-environment system over millennia are needed to provide a basis for a deeper understanding of the present and for forecasting the future. This requires the major task of assembling and integrating regional and global historical, archaeological, and paleoenvironmental records. Humans cannot predict the future. But, if we can adequately understand the past, we can use that understanding to influence our decisions and to create a better, more sustainable and desirable future.

HOLOCENE DYNAMICS OF TREELINE FORESTS IN THE SIERRA NEVADA

We reconstructed a 3500-yr history of fluctuations in treeline elevation and tree abundance in the southern Sierra Nevada. Treeline elevation was higher than at present throughout most of the last 3500 yr. Declines in the abundance of live trees and treeline elevation occurred twice during the last 1000 yr: from 950 to 550 yr BP and from 450 to 50 yr BP The earlier decline coincided with a period of warm temperatures (relative to present) in which at least two severe, multidecadal droughts occurred. This decline was apparently triggered by an increase in the rate of adult mortality in treeline forests. The more recent decline occurred during a period of low temperatures lasting for up to 400 yr and was apparently caused by a sustained failure of regeneration in combination with an increased rate of adult mortality. The apparent past importance of precipitation in controlling the position and structure of the treeline ecotone suggests that climatic controls over treeline may be more complex than previously thought. In the Sierra Nevada, responses of high- elevation forests to future warming may depend strongly on water supply.

Subalpine Tree Growth, Climate, and Increasing CO_2: An Assessment of Recent Growth Trends

Five tree-ring series from foxtail pine (Pinus balfouriana), lodgepole pine (P. murrayana), and western juniper (Juniperus occidentalis) collected in the Sierra Nevada, California, were analyzed to determine if the temporal and spatial patterns of recent growth were consistent with the hypothesized CO{sub 2}-induced growth enhancement. Specifically, the author addresses the following questions: (1) can growth trends be explained solely in terms of climatic variation; (2) are recent growth trends unusual with respect to long-term growth records While the results offer no support for the hypothesized CO{sub 2} fertilization effect, they do provide insights into the response of subalpine conifers to climatic variation. Response surfaces demonstrate that precipitation during previous winter and temperature during the current summer interact in controlling growth and that the response can be nonlinear. Although maximum growth rates occur under conditions of high winter precipitation and warm summers for all three species, substantial species-to-species variation occurs in the response to these two variables.

Climate Variability and Change in High Elevation Regions: Past, Present and Future

This special issue of Climatic Change contains a series of researchand review articles, arising from papers that were presented and discussed at a workshop held in Davos, Switzerland on 25–28 June 2001. The workshop was titled `Climate Change at High Elevation Sites: Emerging Impacts, and was convened to reprise an earlier conference on the same subject that was held in Wengen, Switzerland in 1995 (Diaz et al., 1997). The Davos meeting had as its maingoals, a discussion of the following key issues: (1) reviewing recent climatic trends in high elevation regions of the world, (2) assessing the reliability of various biological indicators as indicators of climatic change, and (3)assessing whether physical impacts of climatic change in high elevation areas are becoming evident, and to discuss a range of monitoring strategies needed to observe and to understand the nature of any changes.

Long‐Term Trends in Forest Net Primary Productivity: Cascade Mountains, Washington

Estimates of annual net primary productivity since 1880 for four high—elevation forest stands in western Washington indicated that productivity has increased 60% during the 20th century. Because these stands were separated by up to 200 km and differed in species composition, elevation, and time since establishment, the observed trends in productivity imply a response to regionwide changes in environmental factors rather than to site—specific stand dynamics. Annual production is significantly correlated with long—term variation in summer temperature and short—term variation in annual precipitation since 1983, the beginning of continuous local meteorological records. Production is uncorrelated with atmospheric CO2 concentrations, suggesting that direct CO2 fertilization is currently unimportant in these forests. See full-text article at JSTOR

Multimillennial dendroclimatic studies from the western United States

Multimillennial tree-ring chronologies from the mountains of the western United States provide an excellent opportunity to study decade to century scale variability in pre-instrumental times. This results from the lack of ecological interaction between the widely-spaced, climate-sensitive, long-lived trees characteristic of the higher elevations of this region. The availability of climate-sensitive > 1000 year tree-ring records in the western conterminous United States is unique. For example, 80 tree-ring chronologies of at least 1000 years length and 23 of at least 2000 years length, have been developed at our Laboratory for this region. A number of climate reconstructions of high quality have already been published using subsets of these data. The great potential of this resource will be demonstrated by reference to these reconstructions. In particular, we introduce a preliminary reconstruction of precipitation in the Great Basin covering the period 6000 B.C. to A.D. 1979. We will use this to place in context a striking pattern of two multidecade droughts each terminated by intense wet periods in the period between A.D. 900 and 1400.

Long-Duration Drought Variability and Impacts on Ecosystem Services: A Case Study from Glacier National Park, Montana

Instrumental climate records suggest that summer precipita- tion and winter snowpack in Glacier National Park (Glacier NP), Montana, vary significantly over decadal to multidecadal time scales. Because instru- mental records for the region are limited to the twentieth century, knowledge

Holocene Variation in Spatial Scales of Vegetation Pattern in the Upper Great Lakes

While continental-scale patterns of vegetation change during the Holocene clearly record the influence of climatic change, the factors governing change at the landscape scale are less clearly defined. In order to characterize the scales of processes determining vegetation patterns during the Holocene, we analyzed a network of 52 pollen sites in the upper Great Lakes region. Pollen percentage data for three dominant tree genera (pine, Pinus; oak, Quercus; birch, Betula) were interpolated from samples bracketing four target years (500, 2500, 4500, and 6500 yr BP). Smoothed isopoll maps of taxon abundance for each target year show broad trends in pollen abundance that correspond to climatic gradients. Residuals, representing the de- viation of each pollen datum from the smoothed value, indicate the amount of spatial variation in pollen abundance independent of that already modeled as a broad gradient. The three genera differ in the magnitude and pattern of residual variation. Oak residuals are relatively small in magnitude, while pine and birch residuals are relatively large and show greater local variability in sign and magnitude. This indicates local variability in tree abundance, as pollen of all three taxa is readily dispersed by wind. Spatial correlograms, which summarize the strength of spatial autocorrelation as a function of distance between pairs of sites for a given taxon, were calculated separately for each target year and allow the quantification of the dominant scale of variability of each taxon. Oak correlograms corroborate the mapped data in indicating the dominance of region-wide trends. In contrast the birch and pine correlograms indicate that factors operating at scales of 150 to 300 km are as important as region-wide trends in governing pollen abundances. The structure of the correlograms for birch and pine pollen changes through time, with birch showing a more patchy spatial pattern in the mid-Holocene (4500 and 6500 yr BP) as compared to the late-Holocene (500 and 2500 yr BP). Pine, in contrast, shows a more strongly auto- correlated pattern in the mid-Holocene. Our results suggest that substrate, an environmental constraint on vegetation at scales of tens to hundreds of kilometres, has been important in governing the spatial distribution of birch and pine in the upper Great Lakes region. The changing distribution of birch and pine is attributed, in part, to changes in the relative abundance of ecologically dissimilar species within these genera. Further, these observations suggest that spatial scales of tree abundances are dynamic and that constraints imposed by substrate vary in importance in response to long-term climatic variation.

Reflected-light image analysis of conifer tree rings for reconstructing climate

We use reflected-light image analysis to measure brightness of conifer rings, and we use brightness as an alternative to density for reconstructing climate. Using densitometry and image analysis, we measured cores from red spruce (Picea rubens Sarg.) growing at Elephant Mountain, Maine, and then compared statistical characteristics of density and brightness and climate-tree growth models using density or brightness. Auto- correlational and cross-correlational statistics of density and brightness do not differ substantially, and late wood density and brightness both correlate with April-May average temperature, which both tree-ring variables reconstruct equally well. April-May temperature was highly variable during most of the nineteenth century and was below average during the AD 1830s, 1870s and late 1880s. When done carefully, reflected-light image analysis can substitute for X-ray densitometry for measuring tree rings to reconstruct climate of the latest Holo cene.