NOTES ON POST-FIRE STAND DEVELOPMENT IN THE SUBALPINE ZONE OF NORTHERN COLORADO

1. Inference of stand development pattern requires age data. Inference from size data is easily confounded.

2. Typical pattern at mesic sites is joint colonization by spruce and lodgepole with fir establishing several decades later (but at some sites fir also is an initial colonist). Availability of seed after the fire is a critical determinant of the initial pattern of stand development. Aspen (mostly from root suckers) may also be an initial dominant.

3. Initial dominance by lodgepole reflects both seed availability and more rapid growth.

4. Aspen due to its short life span and shade intolerance is the first seral species to be replaced. Shade-intolerance of lodgepole results in its eventual replacement by spruce and fir.

5. Slight differences in site (e.g. aspect, topographic position) can result in major differences in patterns of stand development (e.g. initial dominance by limber on the most xeric sites versus lodgepole on less xeric sites).

6. The general pattern of stand development fits Oliver's "whole stand replacement model." Single cohorts are evident in the post-fire populations of lodgepole, spruce and fir. The lodgepole cohort does not regenerate; the fir cohort regenerates and becomes all-aged. There is uncertainty over the general pattern for the spruce cohort. According to one view, the spruce cohort does not begin to regenerate until the initial colonists begin to die at an age of c. 200-350 yrs and a second cohort establishes. However, in many old-growth stands this alleged two-cohort age structure is not evident (i.e. spruce age structures are not bimodal and instead are all-aged or sporadic).

7. Spruce and fir codominate apparently steady-state stands.

8. Coexistence of spruce and fir occurs despite the much greater abundance of young (small) fir. This coexistence may be explained by three non-mutually exclusive hypotheses:

a. Regeneration niche differentiation.

Spruce and fir seedlings have different niches in relation to log sites, litter sites, and tolerance to shade and drought.

b. Non-equilibrium coexistence.

Because coarse-scale disturbance occurs fairly frequently (i.e. large disturbances tend to occur in less than 600 years), there is not enough time for the slightly more shade-tolerant fir to replace the spruce.

c. Life history differentiation.

Spruce lives longer and therefore needs to establish less frequently than fir which has a higher mortality rate.

9. Limber pine may play a facilitative role as suggested by spatial pattern evidence.

10. On sites too xeric for spruce and fir, lodgepole or limber pine may form steady-state stands.

IMPACTS OF BLOWDOWNS IN THE SUBALPINE ZONE

1. Spruce, fir and lodgepole are shallow rooted and easily susceptible to blowdown.

2. Over extensive areas (e.g. 15 kilometers or more) years of exceptionally strong winds are reflected by synchronous blowdowns in different stands.

3. Major differences between blowdown and fire are:

a. the relatively small area of exposure of bare mineral soil created by blowdown.

b. the survival of small trees in a blowdown versus their destruction by fire.

4. In late seral lodgepole-spruce-fir stands, the post-disturbance vegetation response is dominated by a "reorganization response" (i.e. release of advance regeneration) rather than by a "new establishment" response. In such stands, the increased dominance of the stand by fir and spruce following blowdown is an example of "disturbance-accelerated succession" and of the importance of Connell and Slatyer's inhibition mechanism.

5. Given the long-time (>300 yrs) required for a post-fire site to succeed to a steady-state spruce-fir stand, there is a high probability that the stand will be affected by a significant blowdown that will affect the course of stand development. Thus, the influences of more than a single type of coarse-scale disturbance must be taken into account to understand patterns of stand development.

6. Blowdown increases the hazard of a spruce beetle outbreak.

7. Aspen is more resistant to blowdown than are the conifers.

SPRUCE BEETLE IN THE COLORADO ROCKIES

General Background on the spruce beetle (Dendroctonus rufipennis)

1. Range– boreal and subalpine spruce forests across North America.

2. Host species-- principal host is Engelmann spruce, but under outbreak conditions lodgepole pine is also attacked.

3. Endemic vs. epidemic beetle populations– under endemic conditions spruce beetles mainly attack dead trees; attack live trees under epidemic conditions.

4. Stand susceptibility– determined largely by tree size.

5. Triggers of outbreaks– blowdown and logging

6. The life cycle of the spruce beetle– eggs, larva (grubs), pupa, adults.

The life cycle is commonly 2 years, but can be 1 year under warmer conditions or 3 to 4 years under cooler conditions. In the 2-year cycle, eggs hatch during summer to late spring, larvae overwinter, resume development in spring, the larvae pupate during late spring and summer and transform into adults, the second winter is passed as a hibernating adult near the ground, and the following summer they emerge to attack the host.

7. Influences of weather on the beetle’s life cycle– extreme cold kills; warmth accelerates the life cycle.

8. Evidence of attack: a) frass; b) entrance holes; c) pitch tubes; d) bark chips; e) egg galleries and larval mines; f) discolored and dead foliage; and g) blue stain.

9. Cause of tree death– occlusion of xylem by blue stain plus consumption of phloem.

10. Influences on duration of outbreaks– a) availability of food for beetles; b) tree vigor/pitching; c) weather; and d) bird and insect predators and parasites.

GENERAL QUESTIONS ABOUT SPRUCE BEETLE IMPACTS ON SUBALPINE FORESTS IN COLORADO

Stand development:

1. How do outbreaks affect patterns of stand development (i.e. changes in species composition and/or relative dominance)? Does this disturbance impede or accelerate successional development in stands of spruce, subalpine fir, and lodgepole pine?

2. Does a beetle outbreak have a similar influence on the stand as a fire?

Natural vs. anthropogenic origin:

3. Are large-scale beetle outbreaks natural events or are they the result of management activities in these forests during the 20^th century? Have humans altered the fire regime in the subalpine zone in some way (either suppressing fires or perhaps increasing the area burned in the late 19^th century) that has changed the susceptibility of subalpine forests to spruce beetle outbreaks?

Disturbance interactions:

4. How does the prior occurrence of a fire alter the susceptibility of a spruce-fir stand to spruce beetle outbreak?

5. Do beetle outbreaks increase the hazard of subsequent fires by increasing the amount of dead fuels?

SOME ANSWERS ABOUT SPRUCE BEETLE IMPACTS

Stand development:

1. Predominant response is the release of advance regeneration of spruce and fir. Some new establishment may also occur, but in the old stands affected by spruce beetle outbreak the seedlings and saplings of spruce and fir already present in the understory have a major competitive advantage over any new seedling establishment.

This is a major difference from the impact of fire which kills the advance regeneration and creates extensive areas of bare mineral soil favorable for seedling establishment.

2. In seral stands with lodgepole pine, beetle outbreaks can accelerate succession towards spruce and fir. The opening of the canopy due to the death of the large spruce will favor growth of advance regeneration of spruce and fir whereas lodgepole pine is typically not present as advance growth. In addition, in the absence of fire there will not be abundant seed dispersal of lodgepole pine, and under severe outbreak conditions the mature lodgepole pine are killed by spruce beetle.

3. Both small spruce and small fir accelerate their growth following a beetle outbreak. Thus, the shift towards greater dominance by fir (due to the death of main canopy spruce) does not result in a stand of purely fir. Given the longer longevity of spruce, it gradually regains importance in the main canopy relative to fir. Thus, the beetle outbreak results in a major fluctuation in the relative dominance of these species but not in the elimination of spruce.

Natural vs. anthropogenic origin:

Northwestern Colorado was affected by a spruce beetle outbreak during the mid-1800s which was at least as severe and probably more extensive as the one that occurred during the 1940s. Thus, long before Euro-Americans affected fire regimes or otherwise altered these forests through logging, massive beetle outbreaks were an important part of their dynamics.

Disturbance interactions:

We will return to these questions after considering fire history.