Current Research

Current Research Initiatives

Long-term Ecosystem Studies

Climate Change and Treeline in the Pikes Peak Region, Colorado Rockies

Ecological effects of mechanical fuel treatments at the Manitou Experimental Forest: A joint venture between the USDA Forest Service Rocky Mountain Research Station and CCGSR

**Contribution of the tree-disease organism Phellinus tremula to the maintenance of avian biodiversity in montane forest ecosystems**

Long-term Ecosystem Studies

There is a growing consensus among forest researchers today that present forest stocking, primarily the result of diligent and long-standing fire suppression practice, is both unnatural and undesirable. Accordingly, plans are in the offing to conduct large-scale commercial thinning operations on forested lands throughout the west. Our knowledge, however, of forest community dynamics is incomplete at best, and the many landscape changes that have occurred in the west over the last century, including the spread of aggressive, non-native plant and animal species, present new ecological challenges of largely unknown dimension. With this in mind, we have launched a research program at CCGSR’s primary field site in Woodland Park, CO to assess the present condition of our montane forest ecosystems and project the long-term effects of different management strategies on the health of forest communities throughout the Front Range.

Hypothesis

Changes in the structure of a forest canopy, whether initiated by natural processes or purposeful manipulation, set into motion a series of reactions that may profoundly affect the entire forest community. Opening the forest canopy admits more light to the forest floor, resulting in warmer soil temperatures. Warmer soil accelerates microbial activity and subsequent mineralization of organic matter. Increased nutrient availability (plus sunlight) then stimulates more understory growth, raising the level of resources available to consumers (herbivores and their predators) at all trophic levels. Energy flow is increased and nutrient cycling is accelerated. But there are potential complications with this simple scenario: Nutrients may be leached from soils and lost before new ground cover is established, or, alternatively, tree regeneration, stimulated by release from overhead competition, may quickly sequester nutrients into long-lived tissues. The effects of opening the understory to more light are further confounded by the potential negative feedback (particularly on nutrient cycles) of increasing evapotranspiration and reducing soil moisture. Acting in concert, these changes may shift competitive balances, possibly in favor of alien species that could have a lasting impact on native plant and animal communities.

While a return of our forests to more natural conditions may be desirable, the simplified scheme presented above highlights just a few of the unknowns that can plague well-intentioned management plans. Given our present understanding of complex montane forest ecosystems, we cannot be sure that the outcome of wide-scale forest thinning will be exactly as we anticipate.

A Natural Experiment

At CCGSR’s primary field site we have a natural experiment already in place for testing some of these unknowns. Extensive logging activity in the 1980's has left a legacy of thinned stands in both spruce-fir and limber pine forests. Several of these stands are situated adjacent to unlogged areas of similar slope and aspect, providing us with controls against which the results of 20 years of ecosystem response can be measured.

Toward that end, 10 permanent study plots have been established to date, with several more planned for the future. Extensive sampling is underway to reconstruct stand histories and quantify differences in plant and animal communities in each. The plots presently being evaluated encompass both logged and unlogged areas on moderate to steep northeast and northwest facing slopes. Each is permanently gridded to facilitate precise and replicable sampling over a period of many years. In addition to characterizing differences in forest overstory within control and thinned plots, we are measuring factors such as the number of standing dead trees, herbaceous and shrub groundcover, and the amount of woody debris on the forest floor, as these affect the faunal community. We will also be studying nutrient cycling processes, particularly nitrogen mineralization and its subsequent effect on tree growth and stand development. In addition, we will be evaluating the specific impact of overstory thinning on bird, mammal, and insect populations, measuring, for example, the effects of different stand densities on tree-roosting bats or on the incidence of songbird nest parasitism by cowbirds.

It is our hope that by fully assessing the impact of past management practices on the entire forest ecosystem, we can better anticipate the consequences of management plans currently being developed. Our best glimpse into the future of our forests may come from looking back.

Nearly two decades after thinning, legacies of forest management remain in the structure and chemical composition of these forest communities. Thinned plots support stands with lower basal area, stem density, and canopy cover, and increased ground cover of herbaceous and shrub species compared to paired controls. Selective logging in these high-elevation ecosystems, however, has had little impact on regeneration of early successional species such as aspen (Populus tremuloides). Heterogeneity of forest structure after harvest increased small mammal abundance but decreased predation of artificial songbird nests located in the understory. In contrast, thinning had no impact on Lepidopteran abundance or diversity but significantly altered species composition.

Several species of overstory and understory plants in thinned plots were chemically distinct from individuals in plots that had not been logged. Foliar phenolic concentrations in three of six species surveyed were significantly and negatively related to basal area of overstory trees. Furthermore, thinning increased the ratio of total foliar phenolic/nitrogen content in some species, suggesting that gap formation though management may drive long-term changes in litter quality and rates of herbivory.

Despite significant changes in forest structure, thinning has not left a strong legacy in surface soil properties or microbial processes associated with nitrogen transformations, likely due to the integrity of soil organic matter reserves. Selective logging has left a long-term impact on plant and animal communities in these high-elevation, mixed coniferous forests but has only subtly altered soil nutrient cycling, possibly due to trade offs between litter decomposability and microclimate associated with reductions in canopy cover.

In the past two decades, numerous published reports have cited evidence of advancing treelines in the northern hemisphere. Commonly documented are instances of abundant seedling establishment above existing treelines and sustained increases in radial growth of mature trees in forest-tundra ecotones, both at high latitudes and high elevations. Explanations often point to the positive effects of CO₂(and in some places N) fertilization as well as climate warming, but each of these is problematic and definitive causal relations have yet to be discerned.

The potential effects of increasing CO₂ and rising temperature on tree growth at the forest limit are not difficult to understand. With wood production a low priority for trees under stress, and with growth at the upper forest limit thought to be inhibited by low respiration rates, any increase in respiration and carbon availability in excess of maintenance requirements for roots and foliage could be expected to go into cambial growth. In such case we might see an increase in wood production efficiency at treeline, but not necessarily at lower elevations, particularly if some critical temperature threshold may be involved.

Whether CO₂ fertilization alone can affect growth rates at high elevation, has been vigorously questioned. Arguments favoring a CO₂ fertilization explanation for the trends seen above are based largely on the fact that increased radial growth at treeline in the western U.S. exactly mimics the results of long-term CO₂ fertilization experiments with sour orange trees at Arizona State University. Arguments against the CO₂ hypothesis generally center on the frequent observation that trees at high elevation often contain adequate carbohydrate reserves (i.e. carbon acquisition is not limiting), but that low temperatures heretofore limited the rate at which assimilated carbon could be converted to biosynthetic products. Because rising atmospheric CO₂, N-pollution, and climate warming co-vary, sorting these effects from the tree-ring record has been difficult.

Our present study was designed to explore further the possibility of increased carbon sequestration at treeline in the Pikes Peak region by comparing volumetric wood production, as a function of total foliage area, in Engelmann spruce growing at treeline and in subalpine forests. Our working hypothesis is simply that if respiratory efficiency is increasing at treeline in the southern Rockies – whether due to CO₂ and/or N fertilization or to temperature effects – the amount of wood produced per unit of foliage area would be expected to increase. This is testable by using the ratio of annual wood increment (volume or mass) to projected foliage area (linearly related to sapwood conducting area) as an index of assimilation rate. The allometric relationship between cross-sectional area of sapwood and leaf area for Engelmann spruce has been determined and appears consistent for all but the most suppressed trees. Thus, all that would be needed to compare wood production efficiency between treeline and subalpine sites would be measurements of sapwood conducting area and annual volume increment, easily obtainable from increment cores and tree height measurements.

To test this hypothesis we selected 4 high-elevation study sites in the Pikes Peak area, all similarly exposed on west-facing slopes. Two sites were located at Sentinel Point on the western shoulder of Pikes Peak, a third at Almagre Mountain on the south slope of Pikes Peak, and the fourth at Greenhorn Peak in the Wet Mountain range. Treeline sites were chosen within the forest-tundra transition zone (mean elevation 3620m) and then paired with a subalpine site directly down the fall-line, 120 m lower in elevation. At each site, 8-13 trees were selected for measurement and coring, the number depending on suitability of trees (principally size and accessibility with increment borer at treeline) and the condition of cores (principally degree of heartrot). Tree diameters were measured just above the root swell and tree heights were determined using a hand-held clinometer. In order to accurately determine sapwood cross-sectional areas, 4 cores were obtained at cardinal directions for each tree.

In the laboratory, cores were glued into grooved mounting blocks, dried, and surfaced with graded sand-paper to 400-grit. Cores were then cross-dated using several prominent signal years, and annual increments were measured at 40X magnification to an accuracy of 0.025mm.

At 3 of our 4 study sites, spruce growing at treeline tended to carry an equal or slightly greater amount of foliage than trees at lower elevations. This is probably related to differences in stand densities (not quantified) with a greater amount of self-shading and leaf shedding in the subalpine stands. It is noteworthy, however, that at the same 3 treeline sites, wood production efficiency over the past 5 years has been equal to or significantly greater (P=.001) than at corresponding subalpine sites.

While instrumental records for the historical period show only a modest increase in mean annual temperature for Colorado during the last century (1.4°- 2.3º C; EPA 1997), nighttime minima may be increasing faster than daytime maxima (twice as fast at lower elevation sites according to some studies) and the resultant biological effects could be manifold and disproportionate to the climatic stimulus, especially if threshold values for physiological processes are exceeded. A small increase in nighttime minima, for example, may have a disproportionate effect on length of growing season, and this, combined with the slight warming, is likely to be more important near the limits of tree

growth, particularly if low temperature is the dominating constraint at treeline as is often supposed. Early snowmelt could affect length of the growing season as well, through a change in albedo and earlier soil warming.

Could the modest increase in mean annual temperature for the region over the last century have exceeded some critical threshold – enough to significantly influence assimilation of carbon at treeline?

While it is too early yet to answer this question definitively, important landscape-level responses to recent climate change have already been recorded within the tree-ring record in this part of the Rockies. Short-term climate change has brought about shifts in grassland-woodland and woodland-forest ecotones in northern New Mexico, and tree encroachment is occurring in subalpine meadows on the North Rim of the Grand Canyon. The high wood production efficiency observed in our initial sampling at treeline in the Pikes Peak region suggests that we may be witnessing similar changes here. We are currently planning a new round of field studies to test these ideas further.

Ecological effects of mechanical fuel treatments at the Manitou Experimental Forest: A joint venture between the USDA Forest Service Rocky Mountain Research Station and CCGSR

In 2004 we undertook a new project with the USFS to assess the long-term ecological effects of mechanical thinning in a mixed Ponderosa pine-Douglas fir forest following two different treatment strategies: (a) chip-harvesting with all biomass retained on-site, and (b) whole-tree harvesting with all thinned trees removed from the site. Changes in forest community structure under each of the above treatment regimes are being tracked and compared with unthinned control plots over a 10-year period by monitoring soil chemical properties (including ammonia and nitrate-N pools, net mineralization, nitrification, and total C/N ratios), understory plant composition, insect emergence patterns, and small mammal population dynamics. Thinning was completed by mid-July, 2004, and field-work was started a month later.

Net mineralization rates were significantly reduced in thinned treatments one year after harvest, but there were no differences between removal and chipped plots. This suggests to us that the effects of thinning per se, rather than disposition of harvested biomass, dominated below-ground response during the first year after treatment. Contrary to our expectations, the large mass of wood chips on the surface of the thinned/chipped plot did not effect soil C:N ratios in the first year. This may simply reflect the recalcitrant nature of the wood or it may indicate a functional separation between fungi in the wood chips and the soil beneath.

Above-ground response to thinning was generally most pronounced in the chipped treatment. While there were no statistical differences in groundcover between removal plots and controls, herbaceous vegetation in the chipped treatment was significantly lower. Grasses were reduced more than 60% and forbs were nearly eliminated. Juniper, the most common shrub in our study area, was unaffected by treatment differences, but the prostrate bearberry was entirely missing from our chipped plots. Douglas fir seedlings were most abundant in the removal plots where soil disturbance created suitable conditions for germination and seedlings were uninhibited by a layer of wood chips. Two alien species, cheat grass (Bromus tectorum) and common mullein (Verbascum thapsus), appeared in both chipped and removal treatment areas – but not in control plots – late in the year following treatment.

Small mammal numbers were also dramatically lower in the chipped treatment compared to removal and control plots – a likely result of lower resource levels and the relative scarcity of protective cover in chipped plots. In all treatments only two species, the least chipmunk (Tamias minimus) and deer mouse (Peromyscus maniculatus), were captured in a session of 1800 trap nights. The combined population density of P. maniculatus and T. minimus was similar in control and removal plots, but T. minimus numbers in the removal treatment were considerably higher than expected, possibly reflecting displacement of animals from the neighboring chipped treatment.

In contrast to small mammals, insect emergence was significantly greater within the chipped treatment, compared to control plots. A smaller, but still significant increase in emergence occurred within the removal treatment. Fungivorous flies accounted for most of the effect in the chipped treatment, probably in response to a favorable environment for the growth of fungi beneath the wood chips, while an unexplained spike in the emergence of tiny parasitoid wasps accounted in large part for the higher numbers in the removal treatment.

Comparisons of insect community composition based on morphological species showed considerable differences between controls and the two thinning treatments, even though insect richness proved almost identical in all three treatments. These observations indicate that as some species disappeared from plots due to thinning, they were quickly replaced by others. This suggests important alterations in the ecology of the forest floor and raises several questions for future research, including the ecological implications of sudden large numbers of fungivores (that also feed on roots), the unexplained reduction in thinned plots of soil pupating moth species (many of which are important foliar feeders on pines), and the consequences of unusually large numbers of parasitoid wasps in our removal plots. It is our expectation that continued sampling will provide us with sufficient time-series analyses to factor out natural variation and clarify the effects of our treatments on insect community dynamics and the ponderosa ecosystem as a whole.

Contribution of the tree-disease organism Phellinus tremula to the maintenance of Avian Biodiversity in Montane forest ecosystems.

Studies conducted at CCGSR’s primary field site from 2001 to 2004 have revealed that (a) obligate cavity-nesting birds comprise 60% of the avian insectivore species in the upper montane, mixed-conifer forests of the Pikes Peak region, (b) aspen provides 70% of the total nest-cavity resource in the mixed-conifer forest even though this tree species makes up only 11% of the overstory, and (c) 87% of the live aspen cavity-trees in this forest ecosystem are visibly infected with the heart-rot fungus Phellinus tremula.

If successful excavation of aspen by primary cavity nesters follows infection of the tree as the literature suggests (rather than infection following excavation), then attempts to cull the disease organism from forests through biased management efforts could seriously impact cavity-nesting bird populations by eliminating a critical nesting resource. Reducing the numbers of cavity-nesting birds could, in turn, lead to (1) fewer avian insectivores with an accompanying increase in herbivorous insect populations, negatively impacting forest primary production, or alternatively (2) an increase in numbers non-cavity-nesting insectivores through release from foraging competition, offsetting the loss of cavity-nesters.

While hypothesis (1) above would be difficult to prove without investing heavily in labor and equipment, hypothesis (2) is now testable, given our 4 prior years of census data, by excluding cavity-nesting birds from the census area and monitoring subsequent breeding bird activity through our normal survey procedure.

Toward that end we completed in the spring of 2005 a field manipulation whereby we blocked access to 108 aspen cavities in our 5.5 hectare core census area. Four satellite census areas were left un-manipulated for controls. If hypothesis (2) above is correct, we would now expect an increase in numbers of non-cavity insectivores breeding in the core area, compared to previous years, with no change in relative numbers (or a slight increase in cavity-nesters as they seek nest resources in outlying areas) in the satellite census plots. If hypothesis (2) proves incorrect (i.e. overall numbers of insectivorous birds are lower due to the exclusion of obligate cavity nesters), we will have a strong case in support of our primary hypothesis; that the health of the forest may depend on a few diseased trees.

With 3 years of observation behind us since excluding cavity nesters from the core area, we have seen no increase in numbers of non-cavity nesters. While we were successful in reducing the number of cavity nesters in our core census area by 77% on average (85% for 2 out of 3 years), non-cavity nesters have remained relatively constant at pre-treatment levels, as have all breeding birds in our unmanipulated satellite plots. Thus, we have tentatively rejected hypothesis (2), though our next step is to allow cavity nesters back into the core area while continuing our survey for three more years.