E. A. Johnson

Compounded Perturbations Yield Ecological Surprises

ABSTRACT All species have evolved in the presence of disturbance, and thus are in a sense matched to the recurrence pattern of the perturbations. Consequently, disturbances within the typical range, even at the extreme of that range as defined by large, infrequent disturbances (LIDs), usually result in little long-term change to the systems fundamental character. We argue that more serious ecological consequences result from compounded perturbations within the normative recovery time of the community in question. We consider both physically based disturbance (for example, storm, volcanic eruption, and forest fire) and biologically based disturbance of populations, such as overharvesting, invasion, and disease, and their interactions. Dispersal capability and measures of generation time or age to first reproduction of the species of interest seem to be the important metrics for scaling the size and frequency of disturbances among different types of ecosystems. We develop six scenarios that describe communities that have been subjected to multiple perturbations, either simultaneously or at a rate faster than the rate of recovery, and appear to have entered new domains or “ecological surprises.” In some cases, three or more disturbances seem to have been required to initiate the changed state. We argue that in a world of ever-more-pervasive anthropogenic impacts on natural communities coupled with the increasing certainty of global change, compounded perturbations and ecological surprises will become more common. Understanding these ecological synergisms will be basic to environmental management decisions of the 21st century.

THE RELATIVE IMPORTANCE OF FUELS AND WEATHER ON FIRE BEHAVIOR IN SUBALPINE FORESTS

Surface fire intensity (kilowatts per metre) and crown fire initiation were predicted using Rothermels 1972 and Van Wagners 1977 fire models with fuel data from 47 upland subalpine conifer stands varying in age from 22-258 yr and 35 yr of daily weather data (fuel moisture and wind speeds). Rothermels intensity model was divided into a fuel component variable and weather component variable, which were then used to examine the relative roles of fuel and weather on surface fire intensity (kilowatts per metre). Similar variables were defined in the crown fire initiation model of Van Wagner. Both surface fire intensity and crown fire initiation were strongly related to the weather com- ponents and weakly related to the fuel components, due to much greater variability in weather than fuel, and stronger relationship to the fire behavior mechanisms for weather than for fuel. Fire intensity was correlated to annual area burned; large area burned years had higher fire intensity predictions than smaller area burned years. The reason for this difference was attributed directly to the weather variable frequency distribution, which was shifted towards more extreme values in years in which large areas burned. During extreme weather conditions, the relative importance of fuels diminishes since all stands achieve the threshold required to permit crown fire development. This is important since most of the area burned in subalpine forests has historically occurred during very extreme weather (i.e., drought coupled to high winds). The fire behavior relationships predicted in the models support the concept that forest fire behavior is determined primarily by weather variation among years rather than fuel variation associated with stand age.

Testing the assumptions of chronosequences in succession

Many introductory ecology textbooks illustrate succession, at least in part, by using certain classic studies (e.g. sand dunes, ponds/bogs, glacial till, and old fields) that substituted space for time (chronosequence) in determining the sequences of the succession. Despite past criticisms of this method, there is continued, often uncritical, use of chronosequences in current research on topics besides succession, including temporal changes in biodiversity, productivity, nutrient cycling, etc. To show the problem with chronosequence-based studies in general, we review evidence from studies that used non-chronosequence methods (such as long-term study of permanent plots, palynology, and stand reconstruction) to test the space-for-time substitution in four classic succession studies. In several cases, the tests have used the same locations and, in one case, the same plots as those in the original studies. We show that empirical evidence invalidates the chronosequence-based sequences inferred in these classic studies.

A Model of Wind Dispersal of Winged or Plumed Seeds

We derived a micrometeorological model for the dispersal of winged or plumed seeds from a point source. The model is based on six measurable parameters: mean release height, mean and standard deviation of the terminal velocities of seeds, standard deviation of vertical wind velocities, and the mean and standard deviation of the natural logarithms of horizontal wind velocities. Predictions of the model include (1) the distri- bution of the dispersal curve (defined as numbers vs. distance from source) is right skewed; (2) the median and long-range dispersal distances need not be well correlated; and (3) increased variance in flight trajectories for a seed population will place the mode of the dispersal curve closer to the source. Empirical tests of the model showed that it adequately characterizes the observed dispersal curves for experimental releases. It is shown that a simple ballistic equation provides a good estimate of the median dispersal distance (and mean distance if the skew of the dispersal curve is slight) for experimental releases. Tests of the model using natural releases from isolated trees indicated that winged seeds do not detach from the parent randomly with respect to horizontal wind velocity. The need to understand the relationship between the probability of detachment and the frequency distribution of horizontal wind velocities is stressed.

Climatically Induced Change in Fire Frequency in the Southern Canadian Rockies

The purpose of this study was to partition the components of a mixed fire frequency, and empirically relate these components to temporal and spatial differences in fire frequency. The method of reconstructing the fire frequency was to build a stand-origin map, from this estimate the mixed time-since-fire distribution, and then use a graphic technique to partition the mixed distribution into two homogeneous fire-frequency distri- butions. The fire frequency for the last 380 yr in the 495-km2 Kananaskis Watershed showed a temporal change in fire frequency at - 1730. This change in fire frequency is related to a change from a warmer and drier climate before 1730 to a cooler and moister climate since then. When the fire-frequency data were partitioned by a graphic technique, the two resulting fire frequencies fit negative exponential distributions. For the period 1730-1980 the fire cycle (the time required to burn an area equal to the area of study, or one divided by the scale parameter of the exponential distribution) was 90 yr. For the period before 1730 the fire cycle was 50 yr. The Watershed could not be subdivided into smaller, spatially ho- mogeneous fire-frequency units. The negative exponential distribution of fire frequencies suggests a constant hazard function (mortality force). This constant hazard was corroborated by a spatial correlation (Morans I) test which found no spatial pattern other than ones expected by chance between forest ages on either side of all fire boundaries. Thus, there was no tendency for young stands to be associated either with only younger- or only older- aged stands. Other studies on fire behavior in the Watershed corroborate the short fire cycle, constant hazard, and lack of spatial fire-frequency differences, and suggest thai the regional climate control of the temporal fire frequency is related to a characteristic synoptic weather pattern and the resulting high intensity and high rate-of-spread of fires.

FIRE FREQUENCY AND THE SPATIAL AGE MOSAIC OF THE MIXED‐WOOD BOREAL FOREST IN WESTERN CANADA

One approach to ecosystem management is to emulate the effects of natural disturbance in producing landscape patterns; this approach requires a spatial analysis of the pattern and an understanding of the processes producing the pattern. Forested landscapes exhibit mosaic patterns of both stand types and ages. This study investigates the spatial mosaic of stand ages produced by high-intensity stand-replacing fires in the mixed-wood boreal forest of western Canada. A high-resolution, accurately dated, time-since-fire map for a large (3461 km2) contiguous area is used to produce the landscape survivorship distribution in which both spatial and temporal changes in fire cycle are statistically tested. Spatial multivariate analysis of the time-since-fire map is also used to investigate the spatial assembly of the age mosaic. Significant changes in fire cycle can be explained by climatic change as well as land use change in the surrounding area. The shift from a short (15 yr) fire cycle to a longer (75 yr) cycle after 1890 in the northern half of the study area coincides with climatic change at the end of the Little Ice Age. In the southern half of the study area, the short fire cycle continues after 1890 due to the spread of human-caused fires from the adjacent area which was settled and cleared for agriculture during the first half of the 20th century. Upon completion of settlement in 1945, the fire cycle becomes significantly longer due to the fragmentation of the once continuous forest that surrounded the study area and from which the majority of large fires propagated in the past. The different fire cycle histories of the two parts of the study area also explain the spatial mosaic pattern of stand ages, sizes, and shapes. The extended period of the short fire cycle through the first half of this century in the southern region results in it being dominated by younger, larger, oblong-shaped polygons with irregular edges: characteristics that describe the shapes of large burns. The northern region has generally older and smaller, more circular, compact polygons that are the remnants of larger much earlier burns that have since been overburned. The polygons in the northern region are more similar in size and shape but less similar in age to adjacent polygons than are those in the southern region. Thus, this study shows how spatial heterogeneity in the landscape mosaic pattern can be characterized and related to the disturbance history of an area. Furthermore, it provides evidence of the impacts on the age mosaic due to forest fragmentation in surrounding areas.

Wind Dispersal of Seeds from a Forest Into a Clearing

Although there are many studies of wind dispersal of seeds from a forest into an adjacent clearing, no physical model has yet been advanced. The model constructed here calculates the trajectories of seeds from individual trees in the area source to a line of seed traps (in the clearing) oriented perpendicular to the forest edge. The model uses a log-normal distribution of horizontal wind velocities at a nearby reference station to evaluate wind velocities at any point in the forest and clearing as a function of both height above the ground and distance from the forest edge. The model predicts that (1) the slope of the area source dispersal curve (seed density vs. leeward distance) approximates a negative exponential ; and (2) the great majority of seed deposition in the clearing is contributed by source trees lying within a few tree heights of the forest edge. An evaluation of previously published empirical data shows that the area source model adequately characterizes the decline in seed density with leeward distance.

Estimating the Mean Annual Seed Production of Trees

Intraspecifically, plant seed production is a function of both seed size (mean mass per seed) and plant size. In this paper we examined the interspecific relationship between the size of seeds and plants and the mean long-term annual seed production per tree. For canopy trees, we show that seed production is highly (inversely) correlated with the mean seed mass as a power law argument. Tree size (basal area or leaf mass) is directly proportional to seed production over a limited range. Analysis of seed production for herbaceous plants indicates a relationship similar to that for trees. As the exponent relating seed size to seed production is >-1.0, it follows that large-seeded plants produce more total annual crop mass than do small-seeded species. However, this is balanced by the greater investment in ancillary reproductive tissue by smaller seeded species. The results obtained here ought to be of theoretical and applied interest in, for example, stand-level simulations of population dynamics or for planning the optimal size and shape of clear- cuts intended to be regenerated naturally.

Testing a basic assumption of shrubland fire management: how important is fuel age?

This years catastrophic wildfires in southern California highlight the need for effective planning and management for fire-prone landscapes. Fire frequency analysis of several hundred wildfires over a broad expanse of California shrublands reveals that there is generally not, as is commonly assumed, a strong relationship between fuel age and fire probabilities. Instead, the hazard of burning in most locations increases only moderately with time since the last fire, and a marked age effect of fuels is observed only in limited areas. Results indicate a serious need for a re-evaluation of current fire management and policy, which is based largely on eliminating older stands of shrubland vegetation. In many shrubland ecosystems exposed to extreme fire weather, large and intense wildfires may need to be factored in as inevitable events.

Seed mass and dispersal capacity in wind-dispersed diaspores

Is there necessarily a trade-off between seed size (mass) and dispersal capacity for wind-dispersed diaspores? Within three families (Pinaceae, Aceraceae, and Leguminosae) with asymmetric samaras, shape is maintained (isometry) despite size change. Consequently, within these three families, equilibrium descent velocity is proportional to samara mass raised to the 1/6 power and, necessarily, larger samaras are more poorly dispersed