Matthew F. Bekker

Positive Feedback Between Tree Establishment and Patterns of Subalpine Forest Advancement, Glacier National Park, Montana, U.S.A

Abstract The development and maintenance of several types of visually striking vegetation patterns are controlled by positive feedback between pattern and process. These patterns are particularly common at ecotones, where the influence of positive feedback may affect the position and dynamics of the boundary between the adjacent biotic communities. In this study, I use dendrochronology to examine the role of feedback between existing trees and the establishment and survival of seedlings in the advancement of linear, finger-like strips of subalpine forest in Glacier National Park, Montana. A general upslope, windward to leeward pattern of older trees followed by progressively younger trees was evident in all sample transects, although in some cases this pattern repeated several times along the length of a transect, with each repetition originating leeward of boulders. Overall advancement rates varied from 0.28 to 0.62 m yr−1. The oldest trees established in the early to mid-1700s, but establishment and advancement increased rapidly after 1850, and peaked in the early 1900s. In addition, almost all seedlings established within 5 m downwind of existing trees between 1700 and 1850, while establishment beyond this distance was common after 1850. These patterns suggest that existing trees facilitate leeward seedling establishment and survival, by depositing wind-blown snow. These seedlings in turn modify their leeward environment, thus allowing forest advancement in a linear pattern. Feedback was critical for the survival of seedlings before 1800, and strongly controlled advancement between about 1800 and 1850, but appears to have had little effect on establishment patterns since that time. The importance of feedback between pattern and process may change over time and space as a result of changes in climatic conditions or biotic surroundings.

Gradient Analysis of Fire Regimes in Montane Forests of the Southern Cascade Range, Thousand Lakes Wilderness, California, USA

Species distribution and abundance patterns in the southern Cascades are influenced by both environmental gradients and fire regimes. Little is known about fire regimes and variation in fire regimes may not be independent of environmental gradients or vegetation patterns. In this study, we analyze variation in fire regime parameters (i.e., return interval, season, size, severity, and rotation period) with respect to forest composition, elevation, and potential soil moisture in a 2042 ha area of montane forest in the southern Cascades in the Thousand Lakes Wilderness (TLW). Fire regime parameters varied with forest composition, elevation, and potential soil moisture. Median composite and point fire return intervals were shorter (4-9 yr, 14-24 yr) in low elevation and more xeric white fir (Abies concolor)-sugar pine (Pinus lambertiana) and white fir-Jeffrey pine (P. jeffreyi) and longest (20-37 yr, 20-47 yr) in mesic high elevation lodgepole pine (Pinus contorta) and red fir (Abies magnifica)-mountain hemlock (Tsuga mertensiana) forests. Values for mid-elevation red fir-white fir forests were intermediate. The pattern for fire rotation lengths across gradients was the same as for fire return intervals. The percentage of fires that occurred during the growing season was inversely related to elevation and potential soil moisture. Mean fire sizes were larger in lodgepole pine forests (405 ha) than in other forest groups (103-151 ha). In contrast to other parameters, fire severity did not vary across environmental and compositional gradients and >50% of all forests burned at high severity with most of the remainder burning at moderate severity. Since 1905, fire regimes have become similar at all gradient positions because of a policy of suppressing fire and fire regime modification will lead to shifts in landscape scale vegetation patterns.

Lithologic, structural, and geomorphic controls on ribbon forest patterns in a glaciated mountain environment

Abstract So-called “ribbon forests” have been attributed to snowdrift patterns and fire history without reference to geomorphology [Vegetatio 19 (1969) 192.]. This paper illustrates how site conditions of geomorphology and geology explain the origin of ribbon forests. In Glacier National Park, MT (USA), regional tectonic uplift associated with the Laramide Orogeny produced structural features that amplify lithologic differences. Pleistocene glaciation scoured deeply along the strike of bedding planes, highlighting this pattern and in some cases producing fine-scale parallel finger lakes between forested ribbon strips. Twelve ribbon forest sites on both sides of the Continental Divide were closely studied on stereoscopic aerial photographs, and several of these sites were examined in the field or from helicopter overflights. In all cases, geologic and geomorphic conditions explain the location and distribution of the ribbon forests. Change-detection of the distribution of trees versus nontree-covered surfaces in an area of ribbon forest on Flattop Mountain, a complex uplifted synclinal structure, was undertaken using panchromatic, low-altitude aerial photographs from 1966 to 1991. Areas changed from forest to meadow and from meadow to forest in roughly equal amounts in a generally random spatial pattern. No evidence was seen to suggest that the creation of one ribbon eventually created another downwind, as suggested by Billings. Aerial photograph interpretation, field examination and soils analyses of forest ribbons and adjacent unforested meadows clearly illustrated that trees occupy higher, parallel to subparallel, well-drained sites where the spatial pattern is in turn a distinct reflection of the spatial pattern of structure and stratigraphy. Meadows occupy topographically lower positions between ridges where erosion along bedding plane strike was concentrated. Topography sets conditions that allow tree growth in certain locations while precluding it in immediately adjacent areas. Ribbon forests there are thus a spatial manifestation of the interaction between structure, lithology, and topography.

Developmental contributions to phenotypic variation in functional leaf traits within quaking aspen clones

Phenotypic variation in plant traits is strongly influenced by genetic and environmental factors. Over the life span of trees, developmental factors may also strongly influence leaf phenotypes. The objective of this study was to fill gaps in our understanding of developmental influences on patterns of phenotypic trait variation among different-aged ramets within quaking aspen (Populus tremuloides Michx.) clones. We hypothesized that phenotypic variation in leaf functional traits is strongly influenced by developmental cues as trees age. We surveyed eight aspen clones, each with eight distinct age classes ranging from 1 to 160 years in age, and selected three ramets per age class for sample collection. Leaf traits measured included photosynthesis, stomatal conductance, water use efficiency, specific leaf area, and concentrations of N, phosphorus, sucrose, starch, condensed tannins and phenolic glycosides. Using regression analysis, we examined the relationships between ramet age and expression of leaf functional traits. The data showed significant correlations between ramet age and 10 of the 12 phenotypic traits measured. Eight of the phenotypic traits demonstrated a non-linear relationship in which large changes in phenotype occurred in the early stages of ramet development and stabilized thereafter. Water relations, nutrient concentration, leaf gas exchange and phenolic glycosides tended to decrease from early to late development, whereas sucrose, condensed tannin concentrations and water use efficiency increased with ramet age. We hypothesize that ontogenetically derived phenotypic variation leads to fitness differentials among different-aged ramets, which may have important implications for clone fitness. Age-related increases in phenotypic diversity may partially underlie aspens ability to tolerate the large environmental gradients that span its broad geographical range.

Fire Disturbance, Forest Structure, and Stand Dynamics in Montane Forests of the Southern Cascades, Thousand Lakes Wilderness, California, USA

Abstract: We examined tree diameter, age structure, and successional trends in 100 montane forest plots to identify the effects of variation in the return interval, severity, and extent of fires on forest structure and dynamics in the southern Cascade Range, California. We classified 100 forest plots into 8 groups based on stand structural characteristics. Median point fire return intervals were shortest in lower montane mixed conifer and Jeffrey pine—white fir stands (13–25 y) and upper montane red fir—white fir stands (14.5–19.5 y), intermediate in lodgepole pine stands (50–76.5 y), and longest in high-elevation red fir—mountain hemlock stands (100 y). Fire severity was mainly moderate to high in all forest structural groups except red fir—mountain hemlock. In the late 19th century, large, mostly high-severity fires burned through all forests. Fire extent varied among structural groups, burning from 13% to 85% of plots in a group on average. Stands differed in composition, but size and age structures were similar across structural groups, with few trees > 100 y old and peaks of establishment between 1895 and 1955 for all groups except red fir—mountain hemlock. Distinct pulses of tree recruitment followed the most recent (1883, 1885, 1889, 1918) large and mainly high-severity fires. Suppression of fire since 1905 has increased understory density of shade-tolerant, fire-intolerant species and caused forest compositional shifts, particularly in lower-elevation Jeffrey pine—white fir and mixed conifer stands, and lodgepole pine stands on well-drained sites. Structural or compositional change is less pronounced in upper montane red fir—white fir and red fir—mountain hemlock forests. The combination of gently sloping terrain with few fire breaks, extensive, moderate- to high-severity fires in all forest types and gradient positions and fire suppression has promoted homogenization of forest structure that may lead to large and severe fires in the future. Nomenclature: Hickman, 1993.

Tree-ring reconstruction of the level of Great Salt Lake, USA

Utah’s Great Salt Lake (GSL) is a closed-basin remnant of the larger Pleistocene-age Lake Bonneville. The modern instrumental record of the GSL-level (i.e. elevation) change is strongly modulated by Pacific Ocean coupled ocean/atmospheric oscillations at low frequency, and therefore reflects the decadal-scale wet/dry cycles that characterize the region. A within-basin network of seven tree-ring chronologies was developed to reconstruct the GSL water year (September–August) level, based upon the instrumental record of GSL level from 1876 to 2005. The result was a 576-year reconstruction of the GSL level that extends from 1429 to 2005; all calibration-verification tests commonly used in dendroclimatology were passed. The reconstruction explains 48% of the variance in the instrumental GSL level and exhibits significant periodicity at sub-decadal scales over the past six centuries. Meanwhile, predominance of multi-decadal periodicity in the early half of the record shifted to quasi-decadal dominance in the latter half, and this is consistent with that of proxy reconstructions of the Pacific Decadal Oscillation. The GSL-level reconstruction is a crucial component to improving our insight into the possible controls of coupled ocean-atmosphere interactions on precipitation delivery.

Feedback and Pattern in Computer Simulations of the Alpine Treeline Ecotone

The pattern of ecotones relative to gradients in the abiotic environment may be affected by spatial biotic positive feedback, i.e., where trees improve the conditions for other trees in their neighborhood. Computer simulation models were used to investigate the consequences of degrees of positive feedback. A hybrid model combining FOREST-BGC, ATE-BGC, and FORSKA was used. The BGC models created a surface of potential growing conditions at the alpine treeline. FORSKA was then used to compute the leaf area that was produced with varying levels of feedback strength. Feedback must be strong relative to the abiotic environment for growth to occur. Such feedback created an abrupt boundary on a smooth abiotic gradient and was higher than needed to generate more complex patterns. These results have implications for some of the major hypotheses about treeline and for studies combining modeling and field parameterization.

DENDROARCHAEOLOGY OF THE SALT LAKE TABERNACLE, UTAH

We examined tree rings from Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) timbers in the Salt Lake Tabernacle, constructed from 1863–1867 in Salt Lake City, Utah. A seismic upgrade to the Tabernacle initiated in 2005 required the replacement of wooden timbers with steel beams. Our objectives were to 1) determine cutting dates for the timbers to identify logs that may have been salvaged from previous structures, and consequently would have greater historical significance, 2) identify the species and provenance of the timbers, and 3) develop a chronology that could extend or strengthen the existing tree-ring record for environmental and historical applications in northern Utah. We built a 162-year floating chronology from 13 cores and 15 cross-sections, crossdated visually using skeleton plots and verified statistically with COFECHA. Statistically significant (p < 0.0001) comparisons with established chronologies from northern Utah indicated that the Tabernacle chronology extends from 1702–1862. Cutting dates ranged from 1836–1863, with most in 1862 or 1863 and a smaller cluster around 1855. The broad range of cutting dates suggests that some of the timbers were used in previous structures, and that some trees were dead before they were cut. This study provides valuable information for the preservation of historical materials, and increases the sample depth of existing chronologies during the 18th and 19th Centuries.

Tree Rings and Earthquakes

The lithosphere, earth’s rigid outer shell comprising crust and upper mantle rock, is broken into about 14 tectonic plates (Christopherson 2009) that move a few centimeters per year over superheated, pliable rock underneath. Forces within earth’s interior push, pull and twist the plates in different directions, producing three types of plate boundaries: convergent (colliding with one another), divergent (moving away from one another) and transform (sliding past one another). Earthquakes occur when plates become locked together, building strain between and within them that is suddenly released, sending a burst of seismic waves that cause shaking and displacement of the surface. Nearly 95% of earthquakes are due to movement along plate boundaries, particularly convergent boundaries surrounding the Pacific Ocean and a mix of transform and convergent boundaries extending southeast from the Mediterranean region of Europe to Indonesia (Wicander and Monroe 2009) (Fig. 1). However, faults can also develop within plates, and intraplate earthquakes strong enough to affect humans and to be recorded in tree rings have occurred (e.g. Sheppard and White 1995; VanArsdale et al. 1998; Carrara 2002; Bekker 2004).

Dendrochronology of Utah Juniper (Juniperus osteosperma (Torr.) Little)

ABSTRACT Utah juniper was a foundational species for the discipline of dendrochronology, having been used in the early 20th Century investigations of Mesa Verde, but has been largely ignored by dendrochronologists since. Here we present dendrochronological investigations of Utah juniper core and cross-sectional samples from four sites in northern Utah. We demonstrate that, contrary to the general opinion among many dendrochronologists, Utah juniper exhibits excellent crossdating that is reflective of its sensitivity to climate — a desirable characteristic for dendroclimate reconstruction. Across all four sites the dominant signal for annual ring-width increment occurred during the growing season and was positive for precipitation and negative for temperature. This corroborates ecophysiological studies that highlight Utah juniper’s aggressive water-use behavior and desiccation tolerance that together enable survival at extremely negative soil water potentials. This behavior differs from co-occurring Pinus spp. (i.e. P. edulis and P. monophylla) that avoid cavitation at the cost of carbon starvation. We determine that the annual radial increment of Utah juniper rings is particularly responsive to soil moisture availability, and is in fact a useful proxy for hydroclimatic variables such as precipitation, drought, and streamflow. Its geographic distribution spans a large swath of the Interior West, including areas where other more commonly sought-after species for dendrochronology do not occur, and ought to be considered crucial for complementing the rich network of tree-ring chronologies in the western U.S.