David E. Hibbs

Forty Years of Forest Succession in Central New England

The first 40 yr of forest succession on permanent plots at the Harvard Forest in central New England followed the initial floristic composition model of forest succession. After the 1938 hurricane removed the previous white pine (Pinus strobus) canopy, species regenerated within 4-6 yr by sprouts, buried seed, and wind-blown seed, with no method of regeneration uniformly contrib- uting more to species success than another. Hemlock (Tsuga canadensis) was the only species suc- cessfully regenerating after 1948. Pin cherry (Prunus pennsylvanica) was the early dominant in size and numbers (5000 stems/ha). At year 10, pin cherry, red maple (Acer rubrum), white ash (Fraxinus americana), and red oak (Quercus rubra) were dominant. Species diversity had reached a maximum. By year 40, red oak and paper birch (Betula papyrifera) showed strong canopy dominance, making up only 7.5 and 4.9W, respectively, of total density but 37.5 and 12.5%, respectively, of the size- dominant stems. Red maple and white pine were also codominant in 1978. Some evidence for an intermediate stage dominated by red maple and gray birch (B. populifolia) was found. On one pre- viously hardwood plot, the same species were present, and similar trends in species composition and dominance were followed, but there was more surviving hemlock advance regeneration and a lower density of some shade-intolerant early dominant species. The canopy structure was loosely multilay- ered, and at any given point in succession, species tended to be found in characteristic layers, although these relative positions could change with time.

Mixed‐severity fire regimes: lessons and hypotheses from the Klamath‐Siskiyou Ecoregion

Although mixed-severity fires are among the most widespread disturbances influencing western North American forests, they remain the least understood. A major question is the degree to which mixed-severity fire regimes are simply an ecological intermediate between low- and high-severity fire regimes, versus a unique disturbance regime with distinct properties. The Klamath-Siskiyou Mountains of southwestern Oregon and northwestern California provide an excellent laboratory for studies of mixed-severity fire effects, as structurally diverse vegetation types in the region foster, and partly arise from, fires of variable severity. In addition, many mixed-severity fires have occurred in the region in the last several decades, including the nationally significant 200,000-ha Biscuit Fire. Since 2002, we have engaged in studies of early ecosystem response to 15 of these fires, ranging from determinants of fire effects to responses of vegetation, wildlife, and biogeochemistry. We present here some of our important early findings regarding mixed-severity fire, thereby updating the state of the science on mixed-severity fire regimes and highlighting questions and hypotheses to be tested in future studies on mixed-severity fire regimes. Our studies in the Klamath-Siskiyou Ecoregion suggest that forests with mixed-severity fire regimes are characterized primarily by their intimately mixed patches of vegetation of varied age, resulting from complex variations in both fire frequency and severity and species responses to this variation. Based on our findings, we hypothesize that the proximity of living and dead forest after mixed-severity fire, and the close mingling of early- and late-seral communities, results in unique vegetation and wildlife responses compared to predominantly low- or high-severity fires. These factors also appear to contribute to high resilience of plant and wildlife species to mixed-severity fire in the Klamath-Siskiyou Ecoregion. More informed management of ecosystems with mixed-severity regimes requires understanding of their wide variability in space and time, and the particular ecological responses that this variability elicits.

A new method for modeling the heterogeneity of forest structure

Abstract Despite the critical ecological roles of structural complexity, ecologically relevant quantitative measures of structural complexity that allow comparisons among forest stands are still lacking. The objective of this study was to develop a method that allows comparisons of structural heterogeneity among stands. To encompass a broad range of potential structural complexities, we simulated three spatial point patterns each for tree-size distributions from five inventoried natural stands. Forest structure was modeled and analyzed by simulating point patterns of trees and constructing triangular networks to connect neighboring tree tops to one another. This method is based on the concept of spatial tessellation of tree positions, where point patterns are converted into 2-dimensional nearest neighbor triangles. A structural complexity index (SCI) was defined as the sum of the areas of 3-dimensional triangles (with x , y , and z coordinates) divided by the sum of the areas of 2-dimensional triangles. Vertical gradients were defined as the maximum size difference among the trees forming a triangle, that is, the greater the difference, the greater the structure and the larger the SCI value. Patch-types were defined as classes of structural gradients at different positions within the canopy. More patch-types are also indicative of more structurally heterogeneous stands. Both the SCI and the number of patch-types and patch-size heterogeneity related closely to conventional descriptors of forest structure. While the number of patch-types and patch-type heterogeneity related more to the vertical component of structural complexity, SCI integrated well with both the vertical and horizontal structure. SCI and the concept of patch-types complement one another and can be used to quantitatively compare the structure of different stands. The applicability of this modeling approach in characterizing the structural heterogeneity of forests across spatial scales is discussed.

A HIERARCHICAL PERSPECTIVE OF PLANT DIVERSITY

Predictive models of plant diversity have typically focused on either a landscape’s capacity for richness (equilibrium models), or on the processes that regulate competitive exclusion, and thus allow species to coexist (nonequilibrium models). Here, we review the concepts and purposes of a hierarchical, multiscale model of the controls of plant diversity that incorporates the equilibrium model of climatic favorability at macroscales, nonequilibrium models of competition at microscales, and a mixed model emphasizing environmental heterogeneity at mesoscales. We evaluate the conceptual model using published data from three spatially nested datasets: (1) a macroscale analysis of ecoregions in the continental and western U.S.; (2) a mesoscale study in California; and (3) a microscale study in the Siskiyou Mountains of Oregon and California. At the macroscale (areas from 3889 km2 to 638,300 km2), climate (actual evaporation) was a strong predictor of tree diversity (R2 = 0.80), as predicted by the conceptual model, but area was a better predictor for vascular plant diversity overall (R2 = 0.38), which suggests different types of plants differ in their sensitivity to climatic controls. At mesoscales (areas from 1111 km2 to 15,833 km2 ), climate was still an important predictor of richness (R2 = 0.52), but, as expected, topographic heterogeneity explained an important share of the variance (R2 = 0.19), showed positive correlations with diversity of trees, shrubs, and annual and perennial herbs, and was the primary predictor of shrub and annual plant species richness. At microscales (0.1 ha plots), spatial patterns of diversity showed a clear unimodal pattern along a climate‐driven productivity gradient and a negative relationship with soil fertility. The strong decline in understory and total diversity at the most productive sites suggests that competitive controls, as predicted, can override climatic controls at this scale. We conclude that this hierarchical, multiscale model provides a sound basis to understand and analyze plant species diversity. Specifically, future research should employ the principles in this paper to explore climatic controls on species richness of different life forms, better quantify environmental heterogeneity in landscapes, and analyze how these large‐scale factors interact with local nonequilibrium dynamics to maintain plant diversity.

A characterization of unmanaged riparian areas in the central Coast Range of western Oregon.

Abstract As an approach to providing baseline information about riparian ecosystems, this study characterized the dominant riparian vegetation along unmanaged streams in central Oregon Coast Range forests. We systematically sampled along various reaches of nine first- to fourth-order streams, all of which were subject to stand-replacing fires ca. 145 year ago. The near-stream communities were divided into different vegetative and/or topographic units called landscape units (LUs); LU1s were closest to the stream, and LU2s were farther from the stream. Fifty-two percent of LU1s had no trees, and among all LUs, red alder was the most frequently found tree species. Although in some cases sample plots simply fell between widely spaced trees, we hypothesize that red alder originally dominated many of the current treeless patches and has since senesced to release understory shrubs. With increased distance from the stream, hardwoods decreased in compositional importance relative to conifers, not because hardwood frequency changed, but because conifer frequency increased. Our results suggest that the competitive advantage of hardwoods and shrubs is the biggest limiting factor of conifer growth in the near-stream micro-environment and that without vigorous competition, conifers have the potential to grow over more of the riparian area than that on which they occurred in unmanaged areas. Calculations of disturbance frequency, based on ages of shade-intolerant stand dominants, indicate that along the stream reaches we sampled, a minimum of 2.6 disturbances per stream km per century occurred since the last stand-resetting fire. Riparian areas are spatially and temporally diverse, and any riparian management model should incorporate this variability.

Stand structure and dynamics of young red alder as affected by planting density

Abstract The response of stand structure and dynamics of young red alder ( Alnus rubra Bong.) was examined over a gradient in planting density. Data from three Nelder plots, representing ages 1–7 years and planting densities from 238 to 101 218 trees ha −1 , were used to develop biomathematical models for dominant height, diameter, and survival. The models accounted for 88–99% of the observed variation in projected height, diameter, and survival, with planting density accounting for 0.6–2.6% of the variation in diameter and height and 7.1% of the variation in survival. Height growth rate exhibited a quadratic relationship with planting density up to 24 670 trees ha −1 and a linear relationship at greater planting densities. The natural logarithm of planting density exhibited a linear relationship with change in relative diameter and quadratic mean diameter growth rates. These functional relationships between growth rates and planting density were consistent with the concepts of competition thresholds: height and diameter growth rates were more affected by planting density at lower than at higher planting density while mortality rate increased linearly with increasing density. At high planting density, annual height and quadratic mean diameter growth were less and reached a maximum at younger ages than at low planting density. The dominant height projection function depicts a temporal ripple in maximum height growth that progresses outward over time from the high planting densities in the center of Nelder plots to the lower densities near the edge of the plots.

HABITAT REQUIREMENTS AND GROWTH OF STRIPED MAPLE (ACER PENSYLVANICUM L.)

Surveys of distribution and habitat characteristics, growth patterns, and growth in recently logged areas of striped maple (Acer pensylvanicum L.) were made in western Massachusetts, USA. Highest densities at a given altitude were found on mesic sites on middle and upper slopes. Density increased with increasing slope and altitude but was not affected by aspect or light conditions. Ninety percent of the striped maples were found in the northern hardwood—hemlock forest type although the type covered only 52% of the study area. Optimum height growth occurred at intermediate light intensities, on higher and more northerly slopes, and on mesic sites. Half of the plants in the nonlogged areas were growing <3 cm/yr in height. In logged areas, 75% of the plants present before overstory removal were released by removal, but most subsequent new seedlings were suppressed. The distribution and growth patterns of striped maple suggest that it is a gap—phase replacement species that utilizes temporary forest openings for growing space.

MULTISCALE CONTROLS ON WOODY PLANT DIVERSITY IN WESTERN OREGON RIPARIAN FORESTS

Riparian forests are known to be floristically diverse and influenced by multiscale phenomena, yet few studies have explicitly compared how these factors contribute to various aspects of riparian plant diversity. We analyzed woody riparian plant species and environmental data from four western Oregon watersheds distributed across a wide climate- driven productivity gradient at three scales (40-m 2 sample plots (alpha diversity), 1-ha plots, and species pools from 16 1-ha plots in each watershed) to compare three hypotheses of control on species diversity: (1) local control, (2) direct climatic control, and (3) indirect climatic control. We used a process model (3-PG) to model gross primary productivity (GPP) as a functional climate index across our study area. We performed multiple linear regression to determine the best predictors of alpha (sample-plot scale) diversity, compositional change within riparian forests (beta diversity), and hectare-scale diversity and used path analysis to explore hypothesized causal linkages between climate and other factors and species diversity. We also analyzed a companion data set of gap and forest environments from a subset of the same sites to determine the influence of disturbance on species diversity across the gradient. We found evidence for strong spatial patterning in woody plant richness consistent with indirect climatic control on woody plant richness. Climate (GPP) showed negative relationships with alpha diversity and hectare richness of trees, shrubs, and woody plant species and was the most commonly selected explanatory variable in regression analyses. GPP and Rubus spectabilis cover increased from the least to most productive climates while understory light and moisture heterogeneity across the riparian area decreased. These environmental changes coincided with declines in alpha, beta, and hectare-scale diversity. Disturbance gaps yielded higher richness at most sites, but differences in species richness between gap and forest sample plots did not increase at high levels of GPP, as hypothesized. This study points toward an integrated conceptual model whereby regional and landscape scale controls such as climate and watershed position complement and interact with local controls (i.e., vegetation structure, environmental gradients) to jointly govern woody plant diversity in riparian forests.

The self-thinning rule and red alder management

Abstract The self-thinning rule (the —3/2 power rule) has been used frequently to describe the competition-regulated relationship between size and density in plant populations. Because the parameters of the self-thinning rule and principles are easily derived, the application of this rule to tree stands could simplify the development of stand density management guides. This paper briefly examines the structure and development of the few existing stand density management guides which are based on the self-thinning rule. The self-thinning rule is then applied to seven existing data sets to develop a guide for red alder (Alnus rubra Bong.). The self-thinning line for red alder has a slope of -1.5 and a (y-intercept (average stem volume) of 3.3 m3 (1.175 × 102 ft3). The density unit is stems ha−1. The other two lines on the density management diagram, the line of imminent competition mortality and the lower thinning limit, are placed at relative densities of 55 and 30%. The relative-density values for crown closure, mortality and the lower thinning limit correspond to those for other species. It was not possible to use a seedling alder study to determine the intercept of the self-thinning line for trees.

Interactions of tissue and fertilizer nitrogen on decomposition dynamics of lignin-rich conifer litter

High tissue nitrogen (N) accelerates decomposition of high-quality leaf litter in the early phases of mass loss, but the influence of initial tissue N variation on the decomposition of lignin-rich litter is less resolved. Because environmental changes such as atmospheric N deposition and elevated CO2 can alter tissue N levels within species more rapidly than they alter the species composition of ecosystems, it is important to consider how within-species variation in tissue N may shape litter decomposition and associated N dynamics. Douglas-fir (Pseudotsuga menziesii) is a widespread lignin-rich conifer that dominates forests of high carbon (C) storage across western North America, and displays wide variation in tissue and litter N that reflects landscape variation in soil N. We collected eight unique Douglas-fir litter sources that spanned a two-fold range in initial N concentrations (0.67–1.31%) with a narrow range of lignin (29–35%), and examined relationships between initial litter chemistry, decomposition, and N dynamics in both ambient and N fertilized plots at four sites over 3 yr. High initial litter N slowed decomposition rates in both early (0.67 yr) and late (3 yr) stages in unfertilized plots. Applications of N fertilizer to litters accelerated early-stage decomposition, but slowed late-stage decomposition, and most strongly affected low-N litters, which equalized decomposition rates across litters regardless of initial N concentrations. Decomposition of N-fertilized litters correlated positively with initial litter manganese (Mn) concentrations, with litter Mn variation reflecting faster turnover of canopy foliage in high N sites, producing younger litterfall with high N and low Mn. Although both internal and external N inhibited decomposition at 3 yr, most litters exhibited net N immobilization, with strongest immobilization in low-N litter and in N-fertilized plots. Our observation for lignin-rich litter that high initial N can slow decomposition yet accelerate N release differs from findings where litter quality variation across species promotes coupled C and N release during decomposition. We suggest reevaluation of ecosystem models and projected global change effects to account for a potential decoupling of ecosystem C and N feedbacks through litter decomposition in lignin-rich conifer forests.