Gerald E. Rehfeldt

GENETIC RESPONSES TO CLIMATE IN PINUS CONTORTA: NICHE BREADTH, CLIMATE CHANGE, AND REFORESTATION

Fundamental plant–environment relationships were revealed by analyses of 20-yr height and survival of 118 populations representing two subspecies of Pinus contorta growing in common gardens at 60 environmentally disparate test sites in British Columbia. The approach involved (1) preparing models that described the general climate of British Columbia, (2) developing population-specific response functions driven by predicted climate variables, (3) developing general transfer functions that predict performance from the climatic distances over which populations were transferred, and (4) interpreting the results in terms of niche breadth, effects of climate change on adaptedness of populations, and reforestation in a changing environment. Polynomial regression models used physiographic descriptors to predict seven climate variables from normalized records of 513 weather stations. Values of R2 ranged over 0.80–0.97 for thermal variables and 0.54–0.61 for precipitation variables. Validations with independent data from 45 stations were strong and suggested that the models were generally free of bias within the limits of the original data. Response functions describing the height or survival of each population were developed from quadratic regressions using predicted climate variables for each test site. Mean annual temperature and mean temperature in the coldest month were the most effective variables for predicting population height, while the ratio of summer temperature to summer moisture was the best predictor of survival. Validation of the response functions with independent data from two additional test sites produced values of R2 between actual and predicted values that were as high as 0.93 for height and 0.73 for survival. The results demonstrated that natural populations have different climatic optima but tend to occupy suboptimal environments. Nevertheless, the general transfer functions showed that optimal growth and survival of the species as a whole is associated with the null transfer distance. These seemingly anomalous results suggest that the same processes thought to determine the distribution of species control the distribution of genotypes within species: (1) environmental selection to produce a broad fundamental niche, and (2) density-dependent selection to produce a relatively narrow realized niche within which most populations are relegated to suboptimal environments. Consequently, the steep geographic clines typical of P. contorta seem to be driven more by density-dependent selection than by environmental selection. Asymmetric gene flow from the center of distribution toward the periphery is viewed as a primary regulator that provides the fuel for both environmental and density-dependent selection and thereby indirectly perpetuates suboptimality. The response functions predict that small changes in climate will greatly affect growth and survival of forest tree populations and, therefore, that maintaining contemporary forest productivities during global warming will require a wholesale redistribution of genotypes across the landscape. The response functions also provide the climatic bases to current reforestation guidelines and quantify the adjustments necessary for maintaining adaptedness in planted trees during periods of small (∼1°C) temporal temperature shifts.

Empirical analyses of plant-climate relationships for the western United States

The Random Forests multiple-regression tree was used to model climate profiles of 25 biotic communities of the western United States and nine of their constituent species. Analyses of the communities were based on a gridded sample of ca. 140,000 points, while those for the species used presence‐absence data from ca. 120,000 locations. Independent variables included 35 simple expressions of temperature and precipitation and their interactions. Classification errors for community models averaged 19%, but the errors were reduced by half when adjusted for misalignment between geographic data sets. Errors of omission for species‐specific models approached 0, while errors of commission were less than 9%. Mapped climate profiles of the species were in solid agreement with range maps. Climate variables of most importance for segregating the communities were those that generally differentiate maritime, continental, and monsoonal climates, while those of importance for predicting the occurrence of species varied among species but consistently implicated the periodicity of precipitation and temperature‐precipitation interactions. Projections showed that unmitigated global warming should increase the abundance primarily of the montane forest and grassland community profiles at the expense largely of those of the subalpine, alpine, and tundra communities but also that of the arid woodlands. However, the climate of 47% of the future landscape may be extramural to contemporary community profiles. Effects projected on the spatial distribution of species‐specific profiles were varied, but shifts in space and altitude would be extensive. Species‐specific projections were not necessarily consistent with those of their communities.

North American vegetation model for land-use planning in a changing climate: a solution to large classification problems.

Data points intensively sampling 46 North American biomes were used to predict the geographic distribution of biomes from climate variables using the Random Forests classification tree. Techniques were incorporated to accommodate a large number of classes and to predict the future occurrence of climates beyond the contemporary climatic range of the biomes. Errors of prediction from the statistical model averaged 3.7%, but for individual biomes, ranged from 0% to 21.5%. In validating the ability of the model to identify climates without analogs, 78% of 1528 locations outside North America and 81% of land area of the Caribbean Islands were predicted to have no analogs among the 46 biomes. Biome climates were projected into the future according to low and high greenhouse gas emission scenarios of three General Circulation Models for three periods, the decades surrounding 2030, 2060, and 2090. Prominent in the projections were (1) expansion of climates suitable for the tropical dry deciduous forests of Mexico, (2) expansion of climates typifying desertscrub biomes of western USA and northern Mexico, (3) stability of climates typifying the evergreen-deciduous forests of eastern USA, and (4) northward expansion of climates suited to temperate forests, Great Plains grasslands, and montane forests to the detriment of taiga and tundra climates. Maps indicating either poor agreement among projections or climates without contemporary analogs identify geographic areas where land management programs would be most equivocal. Concentrating efforts and resources where projections are more certain can assure land managers a greater likelihood of success.

Ecological Adaptations in Douglas-Fir (Pseudotsuga menziesii var. glauca): a Synthesis

Abstract Measurements of 3rd-year height of 228 seedling populations, grown in four separate studies in two of the same common gardens, were used to summarize patterss of genetic variation for Douglas-fir across 250 000 km2 of forested lands in Idaho and Montana, U.S.A. Because each study was conducted in different years with a different set of populations, measurements were transformed to standard deviates and then were scaled according to the performance of populations common between studies. Genetic variation in 3rd-year height was related to the elevation and geographic location of the seed source by a regression model that accounted for 87% of the variance among populations. In addition, 3rd-year height of 169 of the populations was strongly correlated (r = 0.80) to freezing injury observed in previous studies. Both variables showed hat populations from elevationally or geographically mild sites were tall but had low freezing tolerance. Populations from harsh sites were short and cold-hardy. In Douglas-fir, adaptation to heterogeneous environments can be viewed as physiological specialization for a relatively small portion of the environmental gradient; populations separated by a relatively short distance along the environmental gradient (e.g., 20 frost-free days) tend to be different genetically.

A spline model of climate for the Western United States

Monthly climate data of average, minimum, and maximum temperature and precipitation normalized for the period 1961 through 1990 were accumulated from approximately 3,000 weather stations in the Western United States and Southwestern Canada. About two-thirds of these observations were available from the weather services of the two countries while the remaining third were added to the normalized base from daily weather records of stations of short duration. Tests of the procedures used to normalize these supplemental data showed that estimates on average were within 0.2 oC for temperature variables and 2.7 mm for precipitation.Weather data for the 48 monthlies were fit to geographic surfaces with thin plate splines. Relationships between predicted values and observed monthlies for about 245 records withheld from the modeling process produced values of R2 that averaged about 0.95 and ranged from 0.87 to 0.99. The slope of the regression line for these relationships was essentially 1.0 for all 48 comparisons. Predictions from the climate model can then be converted to variables of demonstrated importance in plant geography, ecology, or physiology. As an illustration, algorithms are presented and justified for estimating 18 variables derived from predicted values. These derived variables range from the straightforward such as mean annual temperature or mean temperature in the coldest month to those of degree-days >5 oC or freezing dates. Applications of the model in plant biology are illustrated for (1) generating climate estimates for locations specified by latitude, longitude, and elevation, (2) mapping climate variables, (3) separating species distributions in climatic space, and (4) relating genetic variation among populations to climatic gradients.

Height-growth response to climatic changes differs among populations of Douglas-fir: a novel analysis of historic data

Projected climate change will affect existing forests, as substantial changes are predicted to occur during their life spans. Species that have ample intraspecific genetic differentiation, such as Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), are expected to display population-specific growth responses to climate change. Using a mixed-effects modeling approach, we describe three-year height (HT) growth response to changes in climate of interior Douglas-fir populations. We incorporate climate information at the population level, yielding a model that is specific to both species and population. We use data from provenance tests from previous studies that comprised 236 populations from Idaho, Montana, and eastern Washington, USA. The most sensitive indicator of climate was the mean temperature of the coldest month. Population maximum HT and HT growth response to changes in climate were dependent on seed source climate. All populations had optimum HT growth when transferred to climates with warmer winters; those originating in sites with the warmest winters were taller across sites and had highest HT growth at transfer distances closest to zero; those from colder climates were shortest and had optimum HT growth when transferred the farthest. Although this differential response damped the height growth differences among populations, cold-climate populations still achieved their maximum growth at lower temperatures than warm-climate populations. The results highlight the relevance of understanding climate change impacts at the population level, particularly in a species with ample genetic variation among populations.

Genetic variation, climate models and the ecological genetics of Larix occidentalis

Abstract Provenance tests of 143 populations of Larix occidentalis revealed genetic differentiation for eight variables describing growth, phenology, tolerance to spring frosts, effects of Meria laricis needle cast, and survival. Geographic variables accounted for as much as 34% of the variance among Rocky Mountain populations. Patterns of genetic variation were dominated by the effects of latitude and elevation, with populations from the north and from high elevations having the lowest growth potential, the least tolerance to the needle cast, and the lowest survival. However, the slope of the geographic clines was relatively flat. Populations in the same geographic area, for instance, need to be separated by about 500 m in elevation before genetic differentiation can be expected. Regression models using geographic predictors were developed to describe variation in ten climatic variables from 192 weather stations that best represented the Rocky Mountain distribution of L. occidentalis . Values of R 2 ranged from 0.42 to 0.92 and were higher for temperature than for precipitation variables. Using climatic effects predicted for each provenance to describe genetic variation produced values of R 2 between 0.15 and 0.27 which accounted for nearly as much (68–100%) of the genetic variance as had geographic variables. The analyses suggested that genetic differentiation was controlled primarily by the relative mildness of the climate. Secondary effects of precipitation were implicated for variables measuring the impact of Meria needle cast and survival.

Ecological adaptations in Douglas-fir (Pseudotsuga menziesii var. glauca) populations: I. North Idaho and North-East Washington

SummaryGrowth, phenology and frost tolerance of seedlings from 50 populations of Douglas-fir (Pseudotsuga menziesii var. glauca) were compared in 12 environments. Statistical analyses of six variables (bud burst, bud set, 3-year height, spring and fall frost injuries, and deviation from regression of 3-year height on 2-year height) showed that populations not only differed in mean performance, but also reacted differently to the environmental gradient. Most of the population-environment interaction was attributable to heterogeneous regressions of population means on environmental means. For all variables except growth rate, the variance of heterogeneous regression coefficients was explained by convergence of regression lines to a common point on the environmental gradient. Consequently, mean values for populations were significantly correlated with regression coefficients. Thus, main effects of populations in those single environments that induced the greatest mean differences reflected the interaction.Multiple regression analyses associated adaptive differentiation of populations with geographic and ecologic characteristics of the seed source. Differentiation was controlled primarily by elevation and secondarily by latitude. Whereas longitude was a minor factor, habitat types accounted for no differentiation beyond that associated with elevation, a factor closely correlated with habitat types. From these results it is recommended that seed for reforestation should not be moved more than 140 m elevation, 1·6 degrees latitude, or 2·7 degrees longitude in northern Idaho and eastern Washington.

Quantifying the abundance of co-occurring conifers along Inland Northwest (USA) climate gradients

The occurrence and abundance of conifers along climate gradients in the Inland Northwest (USA) was assessed using data from 5082 field plots, 81% of which were forested. Analyses using the Random Forests classification tree revealed that the sequential distribution of species along an altitudinal gradient could be predicted with reasonable accuracy from a single climate variable, a growing-season dryness index, calculated from the ratio of degree-days >5 degrees C that accumulate in the frost-free season to the summer precipitation. While the appearance and departure of species in an ascending altitudinal sequence were closely related to the dryness index, the departure was most easily visualized in relation to negative degree-days (degree-days < 0 degrees C). The results were in close agreement with the works of descriptive ecologists. A Weibull response function was used to predict from climate variables the abundance and occurrence probabilities of each species, using binned data. The fit of the models was excellent, generally accounting for >90% of the variance among 100 classes.

Projections of suitable habitat for rare species under global warming scenarios

UNLABELLED PREMISE OF THE STUDY Modeling the contemporary and future climate niche for rare plants is a major hurdle in conservation, yet such projections are necessary to prevent extinctions that may result from climate change. • METHODS We used recently developed spline climatic models and modified Random Forests statistical procedures to predict suitable habitats of three rare, endangered spruces of Mexico and a spruce of the southwestern USA. We used three general circulation models and two sets of carbon emission scenarios (optimistic and pessimistic) for future climates. • KEY RESULTS Our procedures predicted present occurrence perfectly. For the decades 2030, 2060, and 2090, the ranges of all taxa progressively decreased, to the point of transient disappearance for one species in the decade 2060 but reappearance in 2090. Contrary to intuition, habitat did not develop to the north for any of the Mexican taxa; rather, climate niches for two taxa re-materialized several hundred kilometers southward in the Trans-Mexican Volcanic Belt. The climate niche for a third Mexican taxon shrank drastically, and its two mitotypes responded differently, one of the first demonstrations of the importance of intraspecific genetic variation in climate niches. The climate niche of the U.S. species shrank northward and upward in elevation. • CONCLUSION The results are important for conservation of these species and are of general significance for conservation by assisted colonization. We conclude that our procedures for producing models and projecting the climate niches of Mexican spruces provide a way for handling other rare plants, which constitute the great bulk of the worlds endangered and most vulnerable flora.