4.5 Forest Dynamics

4.5.1 Introduction

Although forests appear to be permanent features of the mountain landscape, forest cover changes with the passage of time and occurrence of events. Changes in forest structure include the age, size, and density of individual forest stands. Changes in forest composition include tree species changing in dominance or even presence on a given site. Forest change results from several factors, including changes induced by the trees themselves by altering the habitat during the life cycle of the trees, and from external environmental factors. External factors include climate fluctuations such as prolonged drought and disturbances such as fire, forest pathogens, insect infestations, and human induced alterations.

It is unlikely that any of these changes would result in the disappearance of the common forest communities in the Planning Area. However, it should be expected that these communities will shift in occurrence with the passing of time, with respect to both where these forest communities occur and in the structure of the communities. The effect of these changes, if an observer could fast-forward several centuries of change in a sufficiently large area, would be a shifting mosaic of vegetational patterns, where all community and structural types are represented but their location and extent of coverage is constantly changing.

4.5.2 Disturbance Regimes

Disturbance regimes are used to characterize the spatial scale and temporal patterns of specific disturbances, and to describe the response and recovery of the affected forest type. Frequency of disturbance regimes refers to the average number of years before the recurrence of a given disturbance type. As disturbance regimes work upon the landscape, the resulting mosaic is governed, in part, upon the types of disturbances at work in the landscape, the frequency and magnitude of the disturbance events, and the intrinsic landscape elements of topography, soils, and moisture.

Different forest communities have different responses and abilities to recover quickly from disturbances. This concept is expressed as ecosystem resiliency. Ecosystems that are disturbed frequently are considered less stable than those disturbed infrequently. However, because species in the less stable ecosystems have generally adapted to more frequent disturbance regimes, they may be more resilient in the wake of disturbance events. In many cases, these species may have evolved to be dependent upon the fairly regular occurrence of the disturbance regimes.

The primary natural disturbance agents in forests in the Planning Area include drought, fire, pathogens and wind. These agents may act singly, but more often they work in concert to create change in the forests. For example, insect outbreaks are often associated with drought, and together, these agents greatly increase the chance of fire.

Forest pathogens (organisms that cause disease) are naturally occurring components in forest ecosystems, and play a significant role in forest dynamics. Forest pathogens include insects, parasites such as dwarf mistletoe, and fungi that decay the roots of live trees. All conifers are host to a number of insects, many of which infest specific tree species. Among the most significant are bark beetles (Dendroctonus species), which infest all Rocky Mountain conifers, and the western spruce budworm (Choristoneura occidentalis), which feeds on true firs, Douglas-fir, and spruce. Dwarf mistletoes are highly specialized plant parasites on all of the local conifers. The most significant local damage occurs to ponderosa and lodgepole pines and Douglas-fir. Root diseases are caused by decay fungi, and injure or even kill trees by decaying and killing roots. Although these pathogens are typically cast in a negative context, they provide habitat and forage for wildlife, introduce structural diversity into stagnant or homogeneous forests, and are an intrinsic part of biological diversity. The fossil record indicates, for example, that dwarf mistletoe has been co-evolving with conifer forests for at least 2 million years, and probably far longer.

4.5.3 Forest Dynamics of Local Conifer Communities

How and when changes in the forest occurs is dependent in part upon the characteristics of the specific forest communities. The three most common conifer communities in the Planning Area are ponderosa pine, Douglas-fir, and lodgepole pine. Ponderosa pine and Douglas-fir are most common in the eastern half of the Planning Area, lodgepole pine is most common in the western half. Each of these forest communities has different characteristics, which are summarized in Table 4.3.

4.5.3.1 Ponderosa Pine

Under the natural range of variability, ponderosa pine and Douglas-fir (in association with ponderosa pine or otherwise found on dry, lower elevation sites) were characterized by open stands, in which trees covered less than 50% of the ground (i.e. a woodland), that were subject to relatively frequent, low intensity fires. Many of the trees were old, snags were abundant, and vegetation on the forest floor, primarily grasses, was plentiful. Disturbance agents were present, but in most cases did not produce catastrophic, stand-destroying events. Fires, carried mainly by grass fuels, were unable to develop into crown fires due to low tree density and lack of lower limbs on the trees. Ponderosa pine forests are fire-adapted and fire-dependent, meaning that they naturally require fire to maintain their optimum structure and relationships with other biotic systems and communities.

Ponderosa pine has probably been altered the most by human activities including logging, residential and recreational development, and especially fire suppression. Old-growth ponderosa pine has been reduced to the point of being absent. Fire suppression has resulted in dense stands with multiple canopy layers (ladder fuels). Because ponderosa pine does not reproduce well in its own shade, Douglas-fir, which is shade tolerant, has greatly increased its presence in ponderosa pine woodlands. Vegetation on the forest floor is less abundant, since many of these plants are also shade intolerant.

Table 4.3. Characteristics of Three Conifer Forest Communities in the Magnolia Planning Area.

4.5.3.2 Douglas-fir

Douglas-fir dominates on moister, north-facing slopes below 8,200’. Ponderosa pine, aspen, and lodgepole pine are often present. Forest structure is more closed than in ponderosa pine woodland, but usually not as dense as in lodgepole pine stands. Fires were more likely to become crown fires than in ponderosa pine woodland, but were very patchy in occurrence due to the variability of site characteristics and fuel conditions.

Douglas-fir has been subject to the same human impacts as ponderosa pine. In the absence of fire, Douglas-fir has expanded its presence in drier ponderosa pine woodlands. This forest type now has multi-storied, dense stands that are susceptible to budworm epidemics and wildfires. There have been three western spruce budworm epidemics in this forest type in the Front Range in this century.

4.5.3.3 Lodgepole Pine

Lodgepole pine is a fire dependent species, subject to infrequent, high intensity fires. Lodgepole pine generally replaced itself after these fires, in part because of cones that open and drop seed under extreme heat. These regenerated stands tend to be single-storied and even-aged. In some locations, aspen may have been the first tree to establish after a stand replacement fire, and in the absence of further disturbance, will be replaced by lodgepole pine.

Over 80% of this forest type now exceeds the basal area per acre threshold for medium to high susceptibility for infestation by the pine bark beetle. Stand structure is widely variable, with only 30% to 60% of the stands in late structural stages. Patch sizes (stands of one age class) generally exceed 1,000 acres.

4.5.3.4 Conclusions

In the ponderosa pine and Douglas-fir stands, the forest conditions are conducive to larger and more intense wildfires than was typical under the range of natural variability. These wildfires are capable of destroying the stands, and substantially impacting the vegetation cover and soils on the forest floor.

In all conifer forest types, forest structure conditions are conducive to insect and disease disturbances that are potentially larger, more frequent and more intense than was the case in the past century. This situation is exacerbated by the absence of fire.

4.5.4 Ecosystem Management

Historic forest management practices have typically varied between two polarities. At the time of settlement in the 1800s, the landscape was viewed as a resource to be exploited. Some of the excesses of this attitude were curbed in the early 1900s with the development of concepts such as stewardship, multiple use, and sustained yield, which were first implemented by the US Forest Service. Nevertheless, the dominant attitude continued to be one of resource extraction. In the mid to late 1900’s, an opposing view became common, that of preservation of the resource - to let “nature have its way.” This attitude was most visible with the public movement to officially designate wilderness areas on federal lands, where resource extraction was prohibited.

Ecosystem management represents a new management paradigm that has slowly been evolving over the past decade, and it is re-shaping the polarized management practices of the past. The following guiding principles for ecosystem management are paraphrased from Kaufmann et al. (1994).

Humans are an integral part of today’s ecosystems and depend on natural ecosystems for survival; ecosystems must be sustained for the long-term well being of humans.

All components of ecosystems should potentially be present in adequate numbers.

Ecosystem processes, such as disturbances and shifting patterns of vegetation, that characterize the natural range of variability should be present and functioning across adequately large areas.

Human intervention should not impact ecosystem sustainability by destroying or significantly degrading components that affect ecosystem capability.

The cumulative effects of human influences, including the production of commodities and services, should maintain resilient ecosystems capable of returning to the natural range of variability if left alone.

Management activities should conserve or restore natural ecosystem patterns.

Other key ecosystem management concepts include:

• conservative management that gives the benefit of doubt to the resource rather than to extraction or development.
• management actions must be within the physical and biological capabilities of the land, based upon an understanding of ecosystem function.
• management actions must maintain viable populations of all native species.

There are several problems and limiting factors at this time with ecosystem management. Much is not known about ecosystems such as the minimum viable populations of species, or even many fundamental ecosystem processes and functions. The principles of ecosystem management have been carefully developed, but have not yet been widely applied in real world situations. Specific results of ecosystem management will be difficult to test, and difficult to verify. A political limitation is that ecosystem management requires agreement on guiding principles and extensive cooperation among stakeholders. This is good for democratic representation, but problematic for efficiency.

Despite these limitations, ecosystem management is an official management direction of the Forest Service, and is being applied to varying degrees by other local public land management agencies. Ecosystem management has important implications for forest management in the Planning Area since it recognizes and plans for the shifting patterns of forest cover and disturbance regimes as described earlier in this section. Most importantly for PUMA, ecosystem management is the management paradigm of the cooperative Winiger Ridge Forest Health Restoration Project described below.