Manitou Experimental Forest Research Articles

Water‐rock interactions drive chemostasis

AbstractThe western U.S. is experiencing shifts in recharge due to climate change, and it is currently unclear how hydrologic shifts will impact geochemical weathering and stream concentration–discharge (C–Q) patterns. Hydrologists often use C–Q analyses to assess feedbacks between stream discharge and geochemistry, given abundant stream discharge and chemistry data. Chemostasis is commonly observed, indicating that geochemical controls, rather than changes in discharge, are shaping stream C–Q patterns. However, few C–Q studies investigate how geochemical reactions evolve along groundwater flowpaths before groundwater contributes to streamflow, resulting in potential omission of important C–Q controls such as coupled mineral dissolution and clay precipitation and subsequent cation exchange. Here, we use field observations—including groundwater age, stream discharge, and stream and groundwater chemistry—to analyse C–Q relations in the Manitou Experimental Forest in the Colorado Front Range, USA, a site where chemostasis is observed. We combine field data with laboratory analyses of whole rock and clay x‐ray diffraction and soil cation‐extraction experiments to investigate the role that clays play in influencing stream chemistry. We use Geochemist's Workbench to identify geochemical reactions driving stream chemistry and subsequently suggest how climate change will impact stream C–Q trends. We show that as groundwater age increases, C–Q slope and stream solute response are not impacted. Instead, primary mineral dissolution and subsequent clay precipitation drive strong chemostasis for silica and aluminium and enable cation exchange that buffers calcium and magnesium concentrations, leading to weak chemostatic behaviour for divalent cations. The influence of clays on stream C–Q highlights the importance of delineating geochemical controls along flowpaths, as upgradient mineral dissolution and clay precipitation enable downgradient cation exchange. Our results suggest that geochemical reactions will not be impacted by future decreasing flows, and thus where chemostasis currently exists, it will continue to persist despite changes in recharge.

Local Topography and Streambed Hydraulic Conductivity Influence Riparian Groundwater Age and Groundwater‐Surface Water Connection

AbstractThe western U.S. is experiencing increasing rain to snow ratios due to climate change, and scientists are uncertain how changing recharge patterns will affect future groundwater‐surface water connection. We examined how watershed topography and streambed hydraulic conductivity impact groundwater age and stream discharge at eight sites along a headwater stream within the Manitou Experimental Forest, CO USA. To do so, we measured: (a) continuous stream and groundwater discharge/level and specific conductivity from April to November 2021; (b) biweekly stream and groundwater chemistry; (c) groundwater chlorofluorocarbons and tritium in spring and fall; (d) streambed hydraulic conductivity; and (e) local slope. We used the chemistry data to calculate fluorite saturation states that were used to inform end‐member mixing analysis of streamflow source. We then combined chlorofluorocarbon and tritium data to estimate the age composition of riparian groundwater. Our data suggest that future stream drying is more probable where local slope is steep and streambed hydraulic conductivity is high. In these areas, groundwater source shifted seasonally, as indicated by age increases, and we observed a high fraction of groundwater in streamflow, primarily interflow from adjacent hillslopes. In contrast, where local slope is flat and streambed hydraulic conductivity is low, streamflow is more likely to persist as groundwater age was seasonally constant and buffered by storage in alluvial sediments. Groundwater age and streamflow paired with characterization of watershed topography and subsurface characteristics enabled identification of likely controls on future stream drying patterns.

Summertime Soil-Atmosphere Ammonia Exchange in the Colorado Rocky Mountain Front Range Pine Forest

Understanding the NH3 exchange between forest ecosystems and the atmosphere is important due to its role in the nitrogen cycle. However, NH3 exchange is dynamic and difficult to measure. The goal of this study was to characterize this exchange by measuring the atmosphere, soil, and vegetation. Compensation point modeling was used to evaluate the direction and magnitude of surface-atmosphere exchange. Measurements were performed at the Manitou Experimental Forest Observatory (MEFO) site in the Colorado Front Range by continuous online monitoring of gas and particle phase NH3-NH4+ with an ambient ion monitoring system coupled with ion chromatographs (AIM-IC), direct measurements of [NH4+] and pH in soil extracts to determine ground emission potential (Γg), and measurements of [NH4+]bulk in pine needles to derive leaf emission potential (Γst). Two different soil types were measured multiple times throughout the study, in which Γg ranged from 5 to 2122. Γst values ranged from 29 to 54. Inferred fluxes (Fg) from each soil type predicted intervals of emission and deposition. By accounting for the total [NH4+] pool in each compartment, the lifetime of NH3 with respect to the surface-atmosphere exchange in the soil is on the order of years compared to much faster naturally occurring processes, i.e., mineralization and nitrification.

Experimental and Modeling Study of Forest Fire Effect on Soil Thermal Conductivity

An understanding of soil thermal conductivity after a wildfire or controlled burn is important to land management and post-fire recovery efforts. Although soil thermal conductivity has been well studied for non-fire heated soils, comprehensive data that evaluate the long-term effect of extreme heating from a fire on the soil thermal conductivity are limited. The purpose of this study was to evaluate the long-term impact of fire on the effective thermal conductivity of soils by directly comparing fire-heated and no-fire control soils through a series of laboratory studies. The thermal conductivity was measured for ten soil samples from two sites within the Manitou Experimental Forest, Colorado, USA, for a range of water contents from saturation to the residual degree of saturation. The thermal conductivity measured was compared with independent estimates made using three empirical models from literature, including the Campbell et al. (1994), Côté and Konrad (2005), and Massman et al. (2008) models. Results demonstrate that for the test soils studied, the thermal conductivity of the fire-heated soils was slightly lower than that of the control soils for all observed water contents. Modeling results show that the Campbell et al. (1994) model gave the best agreement over the full range of water contents when proper fitting parameters were employed. Further studies are needed to evaluate the significance of including the influence of fire burn on the thermal properties of soils in modeling studies.

Contribution from biogenic organic compounds to particle growth during the 2010 BEACHON-ROCS campaign in a Colorado temperate needleleaf forest

Abstract. New particle formation (NPF) is an important atmospheric phenomenon. During an NPF event, particles first form by nucleation and then grow further in size. The growth step is crucial because it controls the number of particles that can become cloud condensation nuclei. Among various physical and chemical processes contributing to particle growth, condensation by organic vapors has been suggested as important. In order to better understand the influence of biogenic emissions on particle growth, we carried out modeling studies of NPF events during the BEACHON-ROCS (Bio–hydro–atmosphere interactions of Energy, Aerosol, Carbon, H2O, Organics & Nitrogen – Rocky Mountain Organic Carbon Study) campaign at Manitou Experimental Forest Observatory in Colorado, USA. The site is representative of the semi-arid western USA. With the latest Criegee intermediate reaction rates implemented in the chemistry scheme, the model underestimates sulfuric acid concentration by 50 %, suggesting either missing sources of atmospheric sulfuric acid or an overestimated sink term. The results emphasize the contribution from biogenic volatile organic compound emissions to particle growth by demonstrating the effects of the oxidation products of monoterpenes and 2-Methyl-3-buten-2-ol (MBO). Monoterpene oxidation products are shown to influence the nighttime particle loadings significantly, while their concentrations are insufficient to grow the particles during the day. The growth of ultrafine particles in the daytime appears to be closely related to the OH oxidation products of MBO.

Overview of the Manitou Experimental Forest Observatory: site description and selected science results from 2008 to 2013

Abstract. The Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics & Nitrogen (BEACHON) project seeks to understand the feedbacks and inter-relationships between hydrology, biogenic emissions, carbon assimilation, aerosol properties, clouds and associated feedbacks within water-limited ecosystems. The Manitou Experimental Forest Observatory (MEFO) was established in 2008 by the National Center for Atmospheric Research to address many of the BEACHON research objectives, and it now provides a fixed field site with significant infrastructure. MEFO is a mountainous, semi-arid ponderosa pine-dominated forest site that is normally dominated by clean continental air but is periodically influenced by anthropogenic sources from Colorado Front Range cities. This article summarizes the past and ongoing research activities at the site, and highlights some of the significant findings that have resulted from these measurements. These activities include - soil property measurements; - hydrological studies; - measurements of high-frequency turbulence parameters; - eddy covariance flux measurements of water, energy, aerosols and carbon dioxide through the canopy; - determination of biogenic and anthropogenic volatile organic compound emissions and their influence on regional atmospheric chemistry; - aerosol number and mass distributions; - chemical speciation of aerosol particles; - characterization of ice and cloud condensation nuclei; - trace gas measurements; and - model simulations using coupled chemistry and meteorology. In addition to various long-term continuous measurements, three focused measurement campaigns with state-of-the-art instrumentation have taken place since the site was established, and two of these studies are the subjects of this special issue: BEACHON-ROCS (Rocky Mountain Organic Carbon Study, 2010) and BEACHON-RoMBAS (Rocky Mountain Biogenic Aerosol Study, 2011).

Eddy covariance measurements of isoprene and 232-MBO based on NO+ time-of-flight mass spectrometry

Isoprene and 2-methyl-3-buten-2-ol (232-MBO) are the dominant biogenic VOCs released throughout the US, thus requiring simultaneous measurements. Recent measurements suggest the presence of isoprene in 232-MBO dominated ecosystems, however analytical difficulties make it problematic to detect both species independently. Based on a new chemical ionization scheme we use eddy covariance measurements to selectively measure fluxes of both species without analytical interference at the Manitou Experimental Forest (MEF) in Colorado. Our measurements show a concentration ratio between isoprene and 232 MBO of 0.24 (ppbv/ppbv), and a flux ratio of 0.10 ([ppbvm/s]/[ppbvm/s]). Daytime average emission factors of isoprene and 232 MBO were 1.4±0.3 and 16.0±3.0mg/m2/h respectively. Both compounds exhibit light and temperature dependent fluxes. These findings suggest that isoprene is both locally produced and transported to the measurement site dominated by ponderosa pine. This makes it difficult to use MVK and MAC as tracers for estimating the photochemical age of isoprene chemistry at this site. Further our measurements suggest that isoprene can contribute to about 15% of the OH reactivity relative to 232 MBO.

Seasonal cycles of fluorescent biological aerosol particles in boreal and semi-arid forests of Finland and Colorado

Abstract. Biological aerosol particles have become increasingly important for atmospheric study, but continuous measurements at high time and size resolution have not been available until recently. Here we report seasonal cycles of fluorescent biological aerosol particles (FBAP) from the boreal forest in Hyytiälä, Finland (18 months) and the semi-arid Manitou Experimental Forest, Colorado (10 months). FBAP at both locations were observed to be highest in summer and lowest in winter, increasing by factors of 12 and 5 between these seasons, respectively. In addition to the low temperatures and reduced sunlight during winter, we suggest that snow cover inhibited FBAP release from local terrestrial surfaces and that more extensive snow cover at the Finland site contributed to lower winter FBAP concentrations. Average size distributions at each site exhibited peaks between 1.5 and 6 μm in aerodynamic diameter. The Finland site consistently showed a dominant, narrow FBAP peak at ~ 3 μm in addition to discreet modes at ~ 1.5 and ~ 5 μm, whereas the Colorado site showed broader peaks at 1.5 and 5 μm, suggesting different modes of biological particles at the two sites. FBAP concentrations in both locations were shown to correlate with daily patterns of relative humidity (RH) during each season. Also during summer at each site, average FBAP concentration scaled with RH, but at the Finland site RH values above ~ 82% led to a significant decrease in FBAP concentration. We hypothesize that this is due to dew formation that inhibits bioparticle release. Lastly we show that rain during summer at each location led to pronounced increases in both fluorescent and total particle concentrations with FBAP peak particle size at ~ 2 μm and concentration scaling with rain intensity. We suggest that these particles are primarily fungal spores and other bioparticles lofted from splashing of rain droplets hitting soil and leaf surfaces. During the summer at the Colorado site we consistently observed a mode of ~ 4 μm particles appearing several hours after rain events that we suggest are fungal spores actively emitted when ambient conditions are most advantageous for spread and germination. The pronounced patterns of fluorescent bioparticles observed here suggest that parameterizations of both daily and seasonal cycles will be important to accurately reflect bioparticle emissions in future studies of atmospheric bioaerosols and their potential effects on clouds and precipitation.

Total OH reactivity measurements in ambient air in a southern Rocky mountain ponderosa pine forest during BEACHON-SRM08 summer campaign

Total OH reactivity was measured during the Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen-Southern Rocky Mountain 2008 field campaign (BEACHON-SRM08) held at Manitou Experimental Forest (MEF) in Colorado USA in August, 2008. The averaged total OH reactivity was 6.7 s−1, smaller than that measured in urban (33.4 s−1, Yoshino et al., 2012) and suburban (27.7 s−1, Yoshino et al., 2006) areas in Tokyo in the same season, while sporadically high OH reactivity was also observed during some evenings. The total OH reactivity measurements were accompanied by observations of traces species such as CO, NO, NOy, O3 and SO2 and Volatile Organic Compounds (VOCs). From the calculation of OH reactivity based on the analysis of these trace species, 46.3% of OH reactivity for VOCs came from biogenic species that are dominated by 2-methyl-3-buten-2-ol (MBO), and monoterpenes. MBO was the most prominent contribution to OH reactivity of all trace species. A comparison of observed and calculated OH reactivity shows that the calculated OH reactivity is 29.5% less than the observed value, implying the existence of missing OH sinks. One of the candidates of missing OH is thought to be the oxidation products of biogenic species.

The Electronically Steerable Flash Lidar: A Full Waveform Scanning System for Topographic and Ecosystem Structure Applications

The electronically steerable flash lidar (ESFL) is a waveform lidar sensor that incorporates two advances relevant to the design of future spaceborne lidar sensors. The first is a nonmechanical scanner that splits a single incoming beam into a variable number of output beams that can be aligned independently across track; the transmitted beam pattern can be changed up to 60 Hz. The second is a flash focal plane array (FFPA) capable of recording waveforms simultaneously from a 128 t 128 pixel grid with individual footprints spread over multiple pixels. In this paper, the incoming beam was used to illuminate eight 8.4-m footprints which were imaged simultaneously on 12 t 12 pixel subsets of the FFPA. The FFPA digitizes waveforms at a vertical resolution of 75 cm over 41 vertical bins to create waveforms of 30.75-m depth. Multiple waveforms obtained using range-gating were combined for these analyses. ESFL data were collected at Manitou Experimental Forest (MEF), located in the Pikes Peak National Forest, Colorado, USA and the Stephen F. Austin Experimental Forest (AEF), located in the Angelina Forest, Nacogdoches, TX. We evaluated the use of individual pixel-level and aggregated footprint-level waveforms and alternate approaches to define the extent of each footprint in the focal plane array. Using discrete return lidar data as a reference, we evaluated the ability of ESFL lidar to estimate canopy height and compared the two sensors' rates of penetration to the terrain surface. We found the footprint-level waveforms were better suited for use with existing waveform processing techniques, although techniques for processing at the pixel-level appear feasible. Relationships between height estimates from each lidar data set were most closely related when footprint-level ESFL waveforms were calculated after removing pixels that had less than 50% of the maximum energy within that footprint. Regressions between ESFL and reference lidar data estimates of height explained 84% (AEF) and 85% (MEF) of variance; this study could not say definitively which method yielded the more accurate estimate of height.

An annual cycle of size‐resolved aerosol hygroscopicity at a forested site in Colorado

The ability of particles composed wholly or partially of biogenic secondary organic compounds to serve as cloud condensation nuclei (CCN) is a key characteristic that helps to define their roles in linking biogeochemical and water cycles. In this paper, we describe size‐resolved (14–350 nm) CCN measurements from the Manitou Experimental Forest in Colorado, where particle compositions were expected to have a large biogenic component. These measurements were conducted for 1 year as part of the Bio‐hydro‐atmosphere Interactions of Energy, Aerosols, Carbon, H2O, Organics, and Nitrogen program and determined the aerosol hygroscopicity parameter, κ, at five water supersaturations between ∼0.14% and ∼0.97%. The average κ value over the entire study and all supersaturations was κavg = 0.16 ± 0.08. Kappa values decreased slightly with increasing supersaturation, suggesting a change in aerosol composition with dry diameter. Furthermore, some seasonal variability was observed with increased CCN concentrations and activated particle number fraction, but slightly decreased hygroscopicity, during the summer. Small particle events, which may indicate new particle formation, were observed throughout the study period, especially in the summer, leading to increases in CCN concentration, followed by a gradual increase in the aerosol mode size. The condensing material appeared to be predominantly composed of organic compounds and led to a small decrease in κ at the larger activation diameters during and immediately after those events.

Long-term impacts of prescribed burns on soil thermal conductivity and soil heating at a Colorado Rocky Mountain site: a data/model fusion study

Heating any soil during a sufficiently intense wild fire or prescribed burn can alter that soil irreversibly, resulting in many significant, and well studied, long-term biological, chemical, and hydrological effects. On the other hand, much less is known about how fire affects the thermal properties and the long-term thermal regime of soils. Such knowledge is important for understanding the nature of the soil’s post-fire recovery because plant roots and soil microbes will have to adapt to any changes in the day-to-day thermal regime. This study, which was carried out at Manitou Experimental Forest (a semiarid site in the Rocky Mountains of central Colorado, USA), examines three aspects of how fire can affect the long-term (post-fire) thermal energy flow in soils. First, observational evidence is presented that prescribed burns can alter the thermal conductivity of soils to a depth of at least 0.2 m without altering its bulk density. Second, data are presented on the thermal properties of ash. (Such data are necessary for understanding and modeling the impact any remaining post-fire ash layer might have on the daily and seasonal flow of thermal energy through the soil.) Third, observational data are presented on the long-term effects that prescribed burns can have on soil surface temperatures. In an effort to quantify long-term changes in the soil temperatures and heat fluxes resulting from fire this study concludes by developing and using an analytical model of the daily and annual cycles of soil heating and cooling, which incorporates observed (linear variation of) vertical structure of the soil thermal properties and observed changes in the surface temperatures, to synthesise these fire-induced effects. Modeling results suggest that under the dry soil conditions, typical of the experimental forest, the amplitudes of the daily and seasonal cycles of soil heating/cooling in the fire-affected soils will greatly exceed those in the soils unaffected by fire for several months to years following the fire and that these effects propagate to depths exceeding one metre.

Long-Term Seedfall, Establishment, Survival, and Growth of Natural and Planted Ponderosa Pine in the Colorado Front Range

Abstract Seedfall, natural regeneration establishment, and growth of planted seedlings was observed from 1981 to 2001 under shelterwood and seedtree overstories in a replicated study in ponderosa pine in the Manitou Experimental Forest in the Colorado Front Range. Good seed crops were produced only every 4 to 6 years, with almost no viable seed produced in intervening years. With seed predation, only 14% of total seedfall was available for germination. Shelterwood overstories containing between 6 and 14 m2 ha−1 stem basal area over scarified seedbeds provided optimal conditions for natural seedling establishment. Survival and growth of planted seedlings was much better than that of natural seedlings. However, poor survival and slow initial growth may require many years to establish a fully stocked forest of natural seedlings.

Effect of a controlled burn on the thermophysical properties of a dry soil using a new model of soil heat flow and a new high temperature heat flux sensor

Some fires can be beneficial to soils but, if a fire is sufficiently intense, soil can be irreversible altered. We measured soil temperatures and heat fluxes at several soil depths before, during, and after a controlled surface burn at Manitou Experimental Forest (southern Colorado, USA) to evaluate its effects on the soil’s thermophysical properties (thermal conductivity and volumetric specific heat capacity). During the burn the soil was heated to over 400°C at a depth of 0.02 m and to almost 100°C at 0.30 m. Relatively high temperatures persisted for several hours to days even over 1 m deep into the soil. At these intensities and durations significant changes in soil chemistry, structure, and nutrient cycling are likely. However, soil thermophysical properties, estimated before and after the fire with a new model of periodic heat flow in soils, were not significantly changed between the times shortly after sensor installation (October 2001) and 1 month after the fire (February 2002). Estimates of the soil thermophysical properties derived with the new model underestimate laboratory analyses performed on soil samples obtained after the fire. Also presented in this study are some of the first soil heat flux measurements made during a surface fire. Furthermore, data and analyses of the type discussed in this study should aid modeling studies of the soil thermal pulse associated with fire. The ultimate goal of this experiment was to provide tools to assist land managers in the use of prescribed fire to benefit ecosystems and to reduce the potential for harm by examining how the soil’s physical properties and different fuel amounts, geometries, and loading densities influence soil recovery and forest regeneration after fires.

Effect of Grazing on Soil Compaction as Measured by Bulk Density on A High Elevation Cattle Range

ful establishment of Sherman big bluegrass were studied at the Manitou Experimental Forest, Colorado, from 1963 to 1965. Recommendations for seeding areas having similar soil and climatic conditions are as follows: 1. Sherman big bluegrass should be planted into a moist seedbed in July or August. The seedlings can be expected to emerge 7 to 10 days after planting and be well established within 2 to 3 months. Bluegrass emergence is poor if the seed must remain in the ground for a prolonged period of time. Although spring seeding is less favorable, it is a possible alternative providing soil moisture is good. 2. On sandy-loam or sandyclay-loam soils, seed should be drilled as uniformly as possible to a 0.5or 5/%-inch depth. Five pounds per acre at a maximum 12-inch row spacing is a suggested seeding rate. A doubledisc, depth-band or comparable grass drill is recommended for controlling seed placement depth. 3. Leave the soil rough plowed until just prior to seeding to reduce the establishment of weeds and the interval the ground is susceptible to erosion.

Range Condition Classification of Bunchgrass Range at the Manitou Experimental Forest in Colorado

Range Condition Classification of Bunchgrass Range at the Manitou Experimental Forest in Colorado

Grazing Intensity Trials on Seeded Ranges in the Ponderosa Pine Zone of Colorado

Seeding ranges for the improvement of forage values is now a recognized practice. Depleted ranges and abandoned farmlands in the ponderosa pine zone of Colorado can be successfully seeded (Hull and Johnson, 1955). The kind of grass to plant, when, where, and how, have been determined through research, but comparable information on how to graze seeded ranges is scarce. Proper grazing use of established stands is essential to good management. The Rocky Mountain Forest and Range Experiment Station conducted a study to determine the proper degrees of grazing use on some of the more important grasses used in seeding. The study was made from 1946 to 1956 at the Manitou Experimental Forest, 28 miles northwest of Colorado Springs, Colorado.

The Effect of Grazing Intensity on Plant Composition, Vigor, and Growth of Pine‐Bunchgrass Ranges in Central Colorado

What happens to individual forage plants when subjected to different degrees of grazing by cattle? In what way does grazinig change growtlh habits and compositioni of the herbaceous vegetation? Answers to these basic questions will help the range operator to properly evaluate his rangemanagement practices. Six experimen-tal pastures on native ponderosa pine-bunchgrass ralnge at the Manitou Experimental Forest near Woodland Park, Colorado, furnished anexcellen-t opportunity to study changes in native vegetation is relation to past grazing use. These ulnits had been grazed at 3 intensities for 9 years. In each unit, exclostures, which had not beenl grazed for 10 years, were available.

Natural Revegetation of Abandoned Crop Land in the Ponderosa Pine Zone of the Pike's Peak Region in Colorado

Cultivation in the ponderosa pine zone of Colorado has, in many instances, been unprofitable because of the uncertainty of crops and eventual loss of soil fertility. Consequently, many areas have been abandoned, and natural revegetation has resulted in the presence of all stages of the secondary succession, from the initial invasion to almost complete return to the original subclimax grassland community. Some of the more recently abandoned fields have been protected from grazing, while others have been grazed at different rates and different seasons of the year. Therefore, areas representing different periods of abandonment and different types of grazing use are available in the Pike's Peak region for the reconstruction of the abandoned field sere in the ponderosa pine-Douglas fir association. Previous investigations of the rate and manner in which abandoned farm lands in the ponderosa pine zone revert to natural cover are not abundant. Hanson ('24) discussed briefly the secondary succession on abandoned fields in the ponderosa pine zone of northeastern Arizona. He described a first weed stage (Verbesina-Amaranthus associes), a second weed stage (Senecio-Eriogonum associes), a first grass stage (Bouteloua-Agropyron associes), and a subclimax grass stage (Festuca-M uhlenbergia-Blepharoneuron associes). In some instances the subclimax grass stage was omitted, and the succession passed directly from the first grass stage to the ponderosa pine climax. Shantz ('11), and Savage and Runyon ('37) discussed various aspects of the succession that occurs on abandoned farm lands in the central and southern Great Plains. Similar discussions for other areas have been presented by Booth ('41) for Kansas and Oklahoma; Whitman, Hanson, and Loder ('43) for western North Dakota, and Costello ('44) for northeastern Colorado. In general, on the areas where these investigators have worked, there are four distinct associes, beginning with an annual weed type and progressing upward through perennial forb, mixed grass and forb, and, usually, a subclimax grass type-all preceding the final or climax association. Other factors affecting the rate of revegetation of abandoned farm lands have been discussed by various workers. Runyon ('36) evaluated the distribution of the seed supply, Booth ('41) the effect of cultural methods and burning, and Weaver and Albertson ('40) the effects of drought. The present study was made to determine, insofar as possible, the rate of revegetation and the stages of secondary succession on abandoned farm lands in the vicinity of the Manitou Experimental Forest 2 near Woodland Park, Colorado. Detailed studies of ten fields in various stages of succession were initiated in 1936 and were concluded in 1944. The development of the vegetation has been traced only from the initial invasion to the subclimax grass community. It is recognized that the ponderosa pine-Douglas fir association is the potential climax of this area and that the subclimax grass community may eventually be replaced by the conifers.