Randy Calcote

Patch Formation and Maintenance in an Old‐Growth Hemlock‐Hardwood Forest

Cause of patch formation was investigated on a 7.2 ha study area in Sylvania Wilderness Area, a primary forest remnant in Upper Michigan comprising a mosaic of hemlock, sugar maple, and mixed-forest patches. Spatial autocorrelation analysis of the stem map indicates that, although most species pairs have a neutral association between canopy trees and understory trees of other species, hemlock and sugar maple canopy trees both have strong positive self association and negative reciprocal association with each other. No species pairs have a positive reciprocal association on regeneration with each other. MOSAIC, a Markov simulation model in which transition probabilities depend on neighborhood species composition, shows that the negative reciprocal association between hemlock and sugar maple of the intensity observed in this study, could lead to spatial separation into monodominant patches over long time periods (3000 yr). The mixed-forest patches are along spatial continua of varying steepness between sugar maple and hemlock patches. Interactions between sugar maple and hemlock overstory and understory trees, along with the pattern of invasion of hemlock, provide a reasonable explanation for the patch structure. Pedological, topographical, and disturbance history differences do not coincide with the location of patches within upland forests on the study area.

PATCHY INVASION AND THE ORIGIN OF A HEMLOCK-HARDWOODS FOREST MOSAIC

The record of forest invasion by eastern hemlock (Tsuga canadensis) during the course of Holocene migration provides useful information about invasion processes in temperate forest, a system that has been invaded by few exotic species. We used fossil pollen preserved in small forest hollows, which record forest composition on the scale of 1-3 ha, to study hemlock invasion of forests in the Sylvania Wilderness in the upper peninsula of Michigan, where there is now a mosaic of 3-30 ha stands dominated either by hemlock or by sugar maple (Acer saccharum) and basswood (Tilia americana). Fossil pollen was interpreted by comparison with 66 surface samples from small hollows in Michigan and Wisconsin, using three different statistical methods: pollen ratios, a dissim- ilarity index, and canonical variates analysis (CVA) ordination. We found that four hemlock stands along a 10-km transect across Sylvania originated as patches of white pine (Pinus strobus) forest that were invaded by hemlock -3000 yr ago (calibrated 14C dating) when hemlock expanded its range in northern Michigan. Invasion occurred at about the same time (within 800 yr) at all sites but was not associated with disturbance at any site. Over the next several thousand years hemlock coexisted with white pine, but eventually, at a time that differs from site to site, hemlock became dominant, and white pine disappeared from all but one of the four stands. These changes were apparently driven by climate changes over the last 4000 yr that caused the water table to rise (Brugam and Johnson 1997). The history of four nearby maple stands is more variable and less well understood. Unlike the hemlock stands, three of the four maple patches were not dominated by pine at the time of hemlock invasion, but instead had abundant oak (Quercus) and/or maple. Two of these stands were not invaded by hemlock, and the third, if invaded at all, was invaded for a few centuries by low densities of hemlock trees. Thus invasion by hemlock was sensitive to the species composition of the resident forest. Sugar maple and basswood increased in these stands, and by 2000 to 800 yr ago, depending on site, they resembled modern maple stands. The fourth patch was invaded by hemlock, but it was converted to a maple stand 1000-500 yr ago. A wood layer in the sediment is evidence that a catastrophic windstorm may have been responsible.

Identifying forest stand types using pollen from forest hollows

Methods of interpreting pollen assemblages in sediment were examined using surface samples from 66 small forest hollows in Michigan and Wisconsin. All canopy trees in the surrounding 50 m were measured to provide detailed information about the source vegetation of each surface pollen assemblage. Basal area of trees in each forest sample was used to classify them into six stand types: hemlock-dominated, sugar maple/hemlock mixed, sugar maple-dominated, and ash-, oak-, and pine-dominated stands. Various statistical procedures were tested to learn which was most successful in sorting the pollen assemblages into appropriate vegetation groups. Two ordination techniques – detrended correspondence analysis (DCA) and canonical variate analysis (CVA) – give similar overall results, although CVA more successfully separated assemblages from hemlock stands from those of sugar maple-dominated stands. Squared chord distance,0.05 also successfully identified samples from the same forest type. After stand dominants have been identified from pollen assem blages using multivariate methods, a further determination of stand composition is sometimes possible using ratios of pollen counts of individual taxa. Ratios can be calibrated by comparison with species abundances around surface samples. For instance, ratios of pine to hemlock pollen can indicate the abundance of pine within a stand dominated by hemlock, whereas pine pollen percentages alone are affected by variable abundance of other species.

Effect of pollen from regional vegetation on stand-scale forest reconstruction

To investigate the influence of regional pollen inputs on reconstructing local vegetation, we compared modem pollen assemblages deposited in forest hollow sediments from two study areas, Michigan and Wisconsin. Local forest-stand composition (within 50 m) at all sites is dominated by hemlock and northern hardwood trees, but the regional abundance of tree taxa in the two study areas is not the same. Modern pollen assemblages differ between the two study areas, corresponding with differences in regional vegetation. Oak and pine pollen are more abundant in Wisconsin samples, whereas sugar maple, birch and hemlock pollen are more abundant in Michigan samples. Pollen assemblages differed most between study areas for hardwood stands, reflecting lower pollen production of sugar maple and basswood, which exaggerates regional pollen inputs. However, within each study area, surface pollen assemblages are sufficiently different to permit differentiation of hemlock and hardwood stand types, suggesting that regional pollen inputs are similar on the scale of tens of kilometres. Therefore, stand-scale forest histories can be derived from forest-hollow sediments using modern analogues, but our results emphasize the importance of understanding the regional vegetation context and inferring how regional vegetation has changed in the past.

Interpretation of charcoal morphotypes in sediments from Ferry Lake, Wisconsin, USA: Do different plant fuel sources produce distinctive charcoal morphotypes?

We describe five common charcoal morphotypes observed in late-Holocene lake sediments from northern Wisconsin and compare them with charcoal produced by burning modern plant material. Our experiments show that grass cuticle, conifer wood and leaves of some broadleaved taxa all produce recognizable charcoal types that are preserved in sediments. We use the identification of charcoal morphotypes to enhance our interpretation of a previously published charcoal record from Ferry Lake, Wisconsin. The occurrence of the different charcoal morphotypes changed as the vegetation and fire regimes changed over the past 2300 yr. Charred grass cuticle was more common before 1300 cal. yr BP when small charcoal peaks were frequent and the pollen assemblage suggests that an open oak savanna surrounded the lake. Charcoal with bordered pits produced from burned conifer wood was more common after 1300 cal. yr BP, when red/jack pine pollen increased and the frequency of charcoal peaks decreased, suggesting a switch from a surface fire regime to one with less frequent crown fires. Our results suggest that stratigraphic changes in the occurrence of charcoal morphotypes can improve our understanding of past vegetation and fire regimes.

The response of a jack pine forest to late-Holocene climate variability in northwestern Wisconsin

Terrestrial plant communities have the potential to respond to climate change rapidly, if dominant species are killed by a series of extreme events, or slowly, if the cumulative effects of shorter-term climate fluctuations result in long-term compositional change. We used pollen and charcoal records from a lake and a testate amoebae-derived history of water-table depth in a nearby peatland to assess the response of the jack pine-dominated forests of northwestern Wisconsin to the climate variability of the last ~2000 years. The hydrology record and the charcoal record indicate that the climate near Warner Lake over the last ~2000 years was characterized by multidecadal variation in moisture availability with no apparent multicentennial-long trends in moisture balance or fire frequency. However, the pollen record suggests that there were multicentennial-scale changes in the vegetation composition around Warner Lake. Direct comparison of the three proxy records is challenging, because of their differing temporal resolutions and the complexity of potential ecological responses to climate variability. Therefore, we developed an interpretive model to compare multiple simulated proxies under two scenarios of environmental variability in order to determine under what conditions apparently contradictory records are likely to be found. The interpretive model reveals that a record of multicentennial-long change in vegetation is possible if multidecadal climate variability interacts with ecological processes influencing the direction and magnitude of succession. Compositional changes in the Warner Lake pollen record could reflect long-term variation in temperature, seasonality or other climate factors independent of moisture balance; however it is also possible that multidecadal moisture variability interacted with ecological processes affecting recruitment and mortality of species following fires of varying size and severity. Decadal-scale climatic variability can lead to altered successional pathways and to changes in forest composition that last for centuries.

Detecting differences in vegetation among paired sites using pollen records

The ‘Qualitative Assessment of Difference’ method (QAD) is proposed to objectively detect differences in the relative abundance of vegetation between paired sites using pollen percentages. This method corrects for intertaxonomic differences in pollen productivity and neutralizes influences of background pollen on pollen representation of vegetation, using an inverse form of the Extended R-value model. We test the method using modern pollen-vegetation data from small hollows in northern Michigan (6 taxa; 45 sites) and from northwestern Wisconsin (7 taxa; 43 sites) in the USA. Compared with pollen percentages, the one-tailed Fisher Exact test shows that the QAD method significantly improves the accuracy of the results for all taxa. The rank order of sites based on QAD is significantly correlated to the rank order of sites based on a survey of vegetation composition surrounding the hollow for each taxon (Spearmans rank-order correlation coefficients; p<0.001). We apply QAD to 9000-yr pollen records from two forest hollows c. 6 km apart in northern Michigan. Among seven taxa compared, Pinus, a taxon with well-dispersed pollen and a high pollen producer, often displays discrepancies in the direction of difference between QAD and the percentage-based method, demonstrating that pollen percentages alone do not always reflect differences in vegetation composition accurately. When several similarly sized sites are available in the same vegetation zone, QAD can objectively rank-order sites for individual taxa in relation to, for example, differences in soils, topography, patterns of species invasion, natural and anthropogenic disturbances, and other factors.

Taking the long view: Integrating recorded, archeological, paleoecological, and evolutionary data into ecological restoration

Historical information spanning different temporal scales (from tens to millions of years) can influence restoration practice by providing ecological context for better understanding of contemporary ecosystems. Ecological history provides clues about the assembly, structure, and dynamic nature of ecosystems, and this information can improve forecasting of how restored systems will respond to changes in climate, disturbance regimes, and other factors. History recorded by humans can be used to generate baselines for assessing changes in ecosystems, communities, and populations over time. Paleoecology pushes these baselines back hundreds, thousands, or even millions of years, offering insights into how past species assemblages have responded to changing disturbance regimes and climate. Furthermore, archeology can be used to reconstruct interactions between humans and their environment for which no documentary records exist. Going back further, phylogenies reveal patterns that emerged from coupled evolutionary-ecological processes over very long timescales. Increasingly, this information can be used to predict the stability, resilience, and functioning of assemblages into the future. We review examples in which recorded, archeological, paleoecological, and evolutionary information has been or could be used to inform goal setting, management, and monitoring for restoration. While we argue that long-view historical ecology has much to offer restoration, there are few examples of restoration projects explicitly incorporating such information or of research that has evaluated the utility of such perspectives in applied management contexts. For these ideas to move from theory into practice, tests performed through research-management partnerships are needed to determine to what degree taking the long view can support achievement of restoration objectives.

Response of temperate lakes to drought: a paleolimnological perspective on the landscape position concept using diatom-based reconstructions

The hydrological position of a lake within the landscape can affect a number of lake chemical, physical, and biological features, as well as how lakes respond to environmental change. We present a paleolimnological test of the model for landscape position and lake response to climate change proposed by Webster et al. (2000). To investigate how diatom communities have responded to drought relative to landscape position, we examined sedimentary diatom profiles extending through the twentieth century from an upland site (Crystal Lake) and a lowland site (Allequash Lake) in the Northern Highlands region of north-central Wisconsin (USA). To explore changes in diatom communities at each site, we developed a calibration set and transfer functions from 48 lakes in Wisconsin’s Northern Highland Lake District. We further determined planktic:benthic ratios in the two target lakes, developed lake level models, and investigated the sensitivity of planktic:benthic diatom ratios to climatic variability over the past century. In the upland lake, diatom communities responded indirectly to climate via drought-induced changes in lake level, which resulted in shifts in planktic versus benthic habitat availability. This response of diatoms to changes in habitat availability provides an alternative approach for tracking climate change in upland lakes, though careful consideration must be given to the effect of the bathymetry and its relationship to lake level change and habitat zonation at individual sites. In the lowland lake, changes in diatom communities were related to temperature (and possibly lakewater chemistry) and physical changes secondarily. These results are consistent with the model by Webster et al. (2000), with chemical changes occurring in the lowland system and little chemical response in the upland system. However, the biological changes in sediment records presented here provide additional insight into how lake response to climatic change is shaped by landscape position, contributing to a clearer understanding of potential changes in ecosystem structure and function during drought conditions.

Geophysical features influence the climate change sensitivity of northern Wisconsin pine and oak forests

Landscape-scale vulnerability assessment from multiple sources, including paleoecological site histories, can inform climate change adaptation. We used an array of lake sediment pollen and charcoal records to determine how soils and landscape factors influenced the variability of forest composition change over the past 2000 years. The forests in this study are located in northwestern Wisconsin on a sandy glacial outwash plain. Soils and local climate vary across the study area. We used the Natural Resource Conservation Services Soil Survey Geographic soil database and published fire histories to characterize differences in soils and fire history around each lake site. Individual site histories differed in two metrics of past vegetation dynamics: the extent to which white pine (Pinus strobus) increased during the Little Ice Age (LIA) climate period and the volatility in the rate of change between samples at 50-120 yr intervals. Greater increases of white pine during the LIA occurred on sites with less sandy soils (R² = 0.45, P < 0.0163) and on sites with relatively warmer and drier local climate (R² = 0.55, P < 0.0056). Volatility in the rate of change between samples was positively associated with LIA fire frequency (R² = 0.41, P < 0.0256). Over multi-decadal to centennial timescales, forest compositional change and rate-of-change volatility were associated with higher fire frequency. Over longer (multi-centennial) time frames, forest composition change, especially increased white pine, shifted most in sites with more soil moisture. Our results show that responsiveness of forest composition to climate change was influenced by soils, local climate, and fire. The anticipated climatic changes in the next century will not produce the same community dynamics on the same soil types as in the past, but understanding past dynamics and relationships can help us assess how novel factors and combinations of factors in the future may influence various site types. Our results support climate change adaptation efforts to monitor and conserve the landscapes full range of geophysical features.