T. Minckley

ABSTRACT Concerns about the impact of predicted future water deficits on mountain ecosystems can be assessed though analyses of past ecosystem responses to hydrologic variability. Paleoecological records indicate that the composition of subalpine forests in western North America have been resilient to multiple influences over millennia, including severe droughts, insect outbreaks, and widely varying fire regimes. We evaluate the hypothesis that early-succession conifer forests with broad climatic tolerances, such as those dominated by lodgepole pine (Pinus contorta var. latifolia Engelm. ex Wats.) persist because forest dynamics are disrupted by frequent disturbance and climate variations. To assess this prolonged resilience, we use independently reconstructed vegetation, fire, and drought history for a small, forested watershed in southeastern, Wyoming, based on sedimentary pollen and charcoal counts in conjunction with sedimentary lake-level indicators. Our data indicate that prominent vegetation shifts (from sagebrush steppe to spruce-fir parkland at ca. 10.7 ka and spruce-fir parkland to pine-dominated forest at ca. 8.5 ka) coincided with changes in effective moisture. However, once the modern subalpine, lodgepole pine forests establish at ca. 8.5 ka, similar hydroclimatic changes did not produce detectable changes in forest composition. Fire history data show that other aspects of the ecosystem were responsive to changes in effective moisture at multi-centennial-to-millennial timescales with prolonged fire-free episodes coinciding with periods of low effective moisture at >7.2-5.6 and 3.7-1.6 ka. Our results suggest that although current climate changes favor widespread disturbance in Rocky Mountain forests, the composition of these ecosystems could recover through succession dynamics over the next few decades to centuries.

It is widely accepted, based on data from the last few decades and on model simulations, that anthropogenic climate change will cause increased fire activity. However, less attention has been paid to the relationship between abrupt climate changes and heightened fire activity in the paleorecord. We use 35 charcoal and pollen records to assess how fire regimes in North America changed during the last glacial–interglacial transition (15 to 10 ka), a time of large and rapid climate changes. We also test the hypothesis that a comet impact initiated continental-scale wildfires at 12.9 ka; the data do not support this idea, nor are continent-wide fires indicated at any time during deglaciation. There are, however, clear links between large climate changes and fire activity. Biomass burning gradually increased from the glacial period to the beginning of the Younger Dryas. Although there are changes in biomass burning during the Younger Dryas, there is no systematic trend. There is a furthe...

ABSTRACT Two paleohydrological proxies from bog sediments, water level based on testate amoebae assemblages and peat humification, indicate both prolonged and transient drought events over the past 3500 years. High- resolution pollen data for the periods 3000-2200 and 1400-500 cal yr BP from Minden Bog (46.6106N, 82.8347W), Michigan were examined to assess how regional vegetation composition was affected by drought events. Between 3000-2200 cal yr BP, four intervals of multi-decadal drought and one prolonged drought episode were identified. Four of the five dominant arboreal pollen taxa (Betula, Fagus, Quercus and Pinus) do not appear to response to these particular events. In contrast, Tsuga percentages rise at the onset of a prolonged drought ca. 2850 cal yr BP, but decrease rapidly for the duration of this event. Between 1400-500 cal yr BP, five transient droughts and one prolonged dry period were identified. Betula, Pinus, and Tsuga generally increased and Fagus decreased during this period. These trends are punctuated by stepwise changes in pollen percentages associated with the onset and persistence of drought, particularly with the drought starting ca. 1000 cal yr BP. These data suggest that pollen-assemblage responses to climate variation occur across a broad range of scales, involving processes at landscape, community, population, and individual levels. At centennial timescales regional forest composition changes in response to climate variation via demographic processes and altered disturbance regimes. These responses can be rapid if the forcing is large enough. Transient drought events of lower magnitude (e.g., decadal/multidecadal) are accompanied by pollen responses, but these may be linked to changes in pollen productivity at the scale of individual trees across the landscape rather than demographic changes. Tree species differ in their physiological and reproductive responses to drought stress, with some reducing allocations to pollen productivity and others increasing it. We hypothesize that there is a short-term physiological response of Tsuga to drought onset, with increased pollen productivity followed by decline if the drought persists. Further studies of paired high-resolution pollen and paleoclimate records can be used to determine the relative contributions of physiological and demographic responses to the pollen record at different timescales. As timescales become increasingly fine, pollen sequences may represent physiological rather than compositional responses to climate.

ABSTRACT Background/Question/Methods Disturbance regimes, and their sensitivity to climate, may alter the spatial and temporal responses of vegetation to climate change because disturbances can shape total ecosystem biomass, species assemblages, and biogeochemical processes. Consequently, as climate changes in the future, ecosystem responses may depend on local disturbances and the responses of the disturbances to climate. Here, we evaluate the hypothesis that disturbance-prone ecosystems, such as fire-dominated lodgepole pine forest of the Rocky Mountains, prairie-woodland ecotones of the Great Plains, and human-occupied forests of New England, have been less responsive to long-term climate changes over the past 15,000 years than other settings. To do so, we use dissimilarity metrics to measure past vegetation changes based on fossil pollen assemblages from several North American settings, and compare these metrics with past magnitudes of climate change and past fire regimes. Fire history is reconstructed using sedimentary charcoal records. Results/Conclusions Dissimilarity metric from a wide network of Rocky Mountain sites show dramatically less vegetation change in these fire-dominated ecosystems than at eastern North American sites where fire has been less common. Eastern pollen records commonly reveal that climate changes produced ecosystem responses of the same magnitude as the ecosystem changes associated with European land-clearance; such large changes are uncommon in the Rocky Mountains. However, exceptions in the east, such as Crooked Pond in Plymouth, Massachusetts, and Minnetoga Pond in Minneapolis, Minnesota, reveal that frequent fire in human occupied landscapes of the east also dramatically reduced responsiveness to past climate change. Complex vegetation-fire-climate interactions on the prairie-forest ecotones both in the Rocky Mountains and in Minnesota point to the possibilities of multiple steady states in disturbed settings. Taken together, the results reveal the importance of disturbance as a mediator of climate-vegetation interactions.

Forest dynamics at centennial to millennial timescales can be identified using paleoecological records with high spatial, temporal, and taxonomic resolution. These dynamics are linked to climate changes by comparing the paleoecological records with independent paleoclimate records of complementary sensitivity and temporal resolution. We analyzed plant macrofossils at contiguous 1cm intervals (representing 5 to 35 yr/cm) from late Holocene sediments of

ABSTRACT Future climate projections predict warming at high elevations that will impact treeline species, but complex topographic relief in mountains complicates ecologic response, and we have a limited number of long-term studies examining vegetation change related to climate. In this study, pollen and conifer stomata were analyzed from a 2.3 m sediment core extending to 15,330 cal. yr BP recovered from a treeline lake in the Rocky Mountains of Wyoming. Both pollen and stomata record a sequence of vegetation and climate change similar in most respects to other regional studies, with sagebrush steppe and lowered treeline during the Late Pleistocene, rapid upward movement of treeline beginning about 11,500 cal. yr BP, treeline above modern between ~9000 and 6000 cal. yr BP, and then moving downslope ~5000 cal. yr BP, reaching modern limits by ~3000 cal. yr BP. Between 6000 and 5000 cal. yr BP sediments become increasingly organic and sedimentation rates increase. We interpret this as evidence for lower lake levels during an extended dry period with warmer summer temperatures and treeline advance. The complex topography of the Rocky Mountains makes it challenging to identify regional patterns associated with short term climatic variability, but our results contribute to gaining a better understanding of past ecologic responses at high elevation sites.

ABSTRACT The impact of short-term climate anomalies and disturbances on past plant communities can be understood using high temporal or contiguous sedimentary pollen analysis. The response of pollen assemblages to decadal- to multidecadal-scale moisture variability was analyzed for the time intervals 3000–2200 and 1400–500 cal. yr BP. The hydroclimate and vegetation history for Minden Bog in southeastern Michigan were reconstructed using analyses of sedimentary testate amoebae and pollen. Results indicated that moisture anomalies accounted for 3% to 24% of the variation in arboreal pollen abundance for Fagus, Pinus, Quercus, and Tsuga. Betula pollen percentages did not appear to be affected by moisture variability. Given the longevity of these taxa, rapid arboreal pollen assemblage responses to moisture variability at decadal to multidecadal timescales were likely due to climate-induced changes in pollen productivity. Our data suggest that pollen–climate relationships may be counterintuitive at these fine temporal scales. For example, Tsuga pollen percentages generally increased during short-term dry events, in contrast to expected decreases in abundance with drying at millennial timescales based on empirical pollen–climate relationships. Results suggest caution should be used when inferring subdecadal to multidecadal climate variation from highly resolved pollen records. Rather, high-resolution pollen data may more accurately represent superimposed plant responses that are the composite of reproductive output nested within long-term plant community compositional changes in response to climatic variation.

ABSTRACT A ∼7000-year record of sediment accumulation from the San Bernardino ciénega in southeastern Arizona/northeastern Sonora records changes in effective moisture. Periods of rapid sedimentation between ca. 700 to 1100 cal yr BP and ca. 4100 to 4400 cal yr BP at San Bernardino are associated with a highstand at pluvial Lake Cochise, the presence of aquatic pollen taxa in New Mexican packrat middens and periods of incision in river channels in the San Pedro and Santa Cruz river valleys. These results suggest that ciénega deposits represent records of hydrological change and, as such, are important but under-utilized repositories of paleoclimatic information. These results also inform ciénega restoration efforts by highlighting the importance of subsurface and surface water flow through these environments. Effective restoration requires the development of conditions where groundwater maintains surface vegetation and seasonal floods are allowed to inundate the surface.

ABSTRACT In the practice of pollen sampling from sediment cores, temporal sampling density is usually indexed to depth (e.g., at 5, 10, or 20 cm intervals). Choices concerning temporal sampling density, whether or not made explicitly, can influence the kinds of events represented in a pollen sequence, the ages assigned to those events, and how the events are interpreted. We explored the effects of temporal sampling density by simulating pollen time-series of three types of ecological changes (abrupt, gradual, and hysteresis) at a single site, and then simulating the sampling at different temporal densities. We found that temporal sampling density effects the age determination of ecological events, especially when concerning the hysteresis type of ecological change. We then applied the same analysis to a high-resolution pollen record (contiguous samples 1cm in thickness, representing ~20 years) spanning the mid-Holocene hemlock decline from Tower Lake, Michigan, where the abrupt decline is preceded by high-frequency, hysteretic-like fluctuations in hemlock pollen percentages. Within this record, the estimated age of the steepest portion of the hemlock decline varies among different sampling densities. Using the highest pollen-sampling density (1-cm resolution, contiguous), the age of the event was estimated at ~5200 yr BP. However, under the lower sampling density more typical of pollen studies (i.e. ~3-5 cm sample spacing, representing ~50-100 yrs between samples), the probability of estimating a substantially earlier date of the decline (5600 yr BP) approaches 50%. We verified our result by conducting the same simulation using a high-resolution pollen record from High Lake, Ontario (data obtained from the Neotoma Database). We are applying similar analyses to high-resolution pollen records spanning the Younger Dryas interval. Our findings on effects of temporal density of pollen sampling have implications for assessing and interpreting rapid events in the paleoecological record.

A dilemma has long confronted paleoecologists: pollen data are required to assess past vegetational changes, and at the same time pollen data serve as a primary source of paleoclimate inference. Paleoecologists have had to use the same data sets to infer past climate changes and the ecological responses to those changes. The emergence of independent paleoclimate proxies and archives is providing relief from this dilemma. Peatland archives are especially effective at providing independent paleoclimatic records, ...