Stand structure and vegetation dynamics of a subalpine wooded fen in Rocky Mountain National Park,Colorado

Abstract. The tree population size structure and relationship between tree diameter and age were examined in a subalpine fen and surrounding Picea-Abies forest in northern Colorado. The fen grades from a sedge fen, through an ecotone, to a treed fen. Tree growth rate varies across the vegetational gradient, with the sedge fen having the slowest growth, and the upland forest having the fastest growth. Differences in growth rate are related to the average size of peat hummocks, with areas containing tall hummocks exhibiting the highest tree growth rates. Size structures display the characteristic reverse-J distribution generally indicative of stable populations, but forest vegetation is expanding into the open regions of the fen, and within the treed fen an increase in Abies lasiocarpa is occurring. These changes are primarily attributed to a positive feedback situation wherein the fen's surface is built up by peat accumulation. Distinct hummocks form first on the open fen but then coalesce to form raised peat islands in the treed fen. This new substrate provides habitat with a comparatively low water table and allows the growth of mesophytic forest vegetation. A pathway for this vegetational development is proposed.     

Ecological effects of changes in fire regimes in Pinus ponderosa ecosystems in the Colorado Front Range

Question: What is the relative importance of low‐ and high‐severity fires in shaping forest structure across the range of Pinus ponderosa in northern Colorado? Location: Colorado Front Range, USA. Methods: To assess severities of historic fires, 24 sites were sampled across an elevation range of 1800 to 2800 m for fire scars, tree establishment dates, tree mortality, and changes in tree‐ring growth. Results: Below 1950 m, the high number of fire scars, scarcity of large post‐fire cohorts, and lack of synchronous tree mortality or growth releases, indicate that historic fires were of low severity. In contrast, above 2200 m, fire severity was greater but frequency of widespread fires was substantially less. At 18 sites above 1950 m, 34 to 80% of the live trees date from establishment associated with the last moderate‐ to high‐severity fire. In these 18 sites, only 2 to 52% of the living trees pre‐date these fires suggesting that fire severities prior to any effects of fire suppression were sufficient to kill many trees. Conclusions: These findings for the P. ponderosa zone above ca. 2200 m (i.e. most of the zone) contradict the widespread perception that fire exclusion, at least at the stand scale of tens to hundreds of hectares, has resulted in unnaturally high stand densities or in an atypical abundance of shade‐tolerant species. At relatively mesic sites (e.g. higher elevation, north‐facing), the historic fire regime consisted of a variable‐severity regime, but forest structure was shaped primarily by severe fires rather than by surface fires.     

Forest vegetation of the Colorado Front Range

The forest vegetation of the northern Colorado Front Range was studied using a combination of gradient analysis and classification methods. A graphical model of forest composition based on elevation and topographic-moisture gradients was constructed using 305 0.1 ha samples. To derive the topographic moisture gradient, stands were stratified into eight 200 m elevation belts, and then ordinated by correspondence analysis using understory (<1 m) data. Each of the resultant gradients was scaled against a standard site moisture scalar derived from incident solar radiation and topographic position. Except for krummholz sites, the vegetation defined gradients fit the moisture scalar closely. Once scaled, these gradients were stacked vertically, sandwich-style, to create the graphical representation shown in Figure 5.Gradient analysis and ordination (direct and indirect gradient analysis of Whittaker, 1967) are frequently viewed as alternative approaches for analysis of vegetation. With gradient analysis the axes are readily interpretable, but stand placement is often difficult and at times questionable. Ordination defines an optimal arrangement for species and/or stands, but axis interpretation is often impossible. With the present combination of methods, the interpretability of gradient analysis complements the precision of placement obtained with ordination.Forest vegetation was classified by dividing the gradient model into eight series and 29 types on the basis of similar successional trends in canopy dominants. On dry, low-elevation sites above 1 700 m Pinus ponderosa woodlands dominate. With increasing elevation or site moisture, tree density increases and Pinus ponderosa, Pseudotsuga forests prevail. At middle elevations on mesic sites forests of mixed composition occur. Pinus contorta forests dominate at middle elevations over much of the central position of the moisture gradient, though these are primarily post-fire forests. With protection from fire only a small percentage of sites retain dominance by Pinus contorta. Over the lower portion of its range Pinus contorta is succeeded by Pseudotsuga, while at higher elevations Abies lasiocarpa and Picea engelmannii can eventually achieve dominance. At high elevations on all except the driest sites Picea engelmannii and Abies lasiocarpa are exclusive dominants, both after disturbance and in climax forests. Pinus flexilis dominates on the driest high-elevation sites. Above 3 500 m forests are replaced by alpine tundra, often with a transitional krummholz zone.Structure and post-fire development were examined in the context of the gradient-based classification scheme. Three generalized types of forest development were recognized as reference points in a continuum of developmental patterns varying with both elevation and soil moisture.On favorable, middle-elevation sites, trees become established rapidly after disturbance. Rapid growth results in severe overcrowding and competitive elimination of reproduction. As a consequence bell-shaped diameter distributions develop. Diversity and productivity appear to drop while biomass remains roughly constant. Following decades or even centuries of stagnation, the forests eventually breakup through mortality of the canopy trees, thereby allowing regeneration to resume. During this period of renewed regeneration, biomass, diversity, and productivity all show dramatic changes in response to the changing population structure (Fig. 9). This type of forest development can be found in forests dominated by Picea engelmannii and Abies lasiocarpa, Pinus contorta, Pseudotsuga menzeisii, Pinus flexilis or Populus tremuloides. On highest elevation forest sites or at middle elevations on the very driest sites reestablishment rates are greatly reduced. These forests dominated by Picea and Abies or Pinus flexilis gradually approach predisturbance levels of biomass, diversity and productivity, while regeneration remains at a roughly constant level. At lower elevations in the Pinus ponderosa woodlands, regeneration appears episodic, reflecting variation in seed rain and favorable conditions for seedling growth. Here, inter-tree competition is relatively unimportant and diameter distributions show irregular humps resulting from periodic recruitment.A few species pairs presented consistent problems and their treatment as single species was necessary. Garex rossii and C. brevipes were lumped as Carex rossii. Rosa woodsii and R. acicularis were lumped as Rosa sp. Cirsium scopulorum and C. coloradense were lumped as Cirsium coloradense. Extreme forms of Arnica cordifolia and A. latifolia are easily distinguishable, but as these species intergrade and hybridize extensively, they have been lumped as Arnica cordifolia. The native bluegrass, Poa agassizensis, was lumped with Poa paratensis. Solidago missouriensis includes some S. canadensis.Nomenclature follows Weber (1972) for most species. In some cases where Weber's narrow generic concept deviates from the main thrust of present-day North American systematic botany, names were changed to conform with Harrington (1954) and Hitchcock & Cronquist (1973). Voucher specimens have been deposited in the herbarium of Rocky Mountain National Park, with a few unusual species being deposited in the herbarium of the University of Colorado, Boulder.Numerous individuals have contributed generously to this project. Among those to whom I am particularly indebted are B. Chabot, R. T. Clausen, C. V. Cogbill, J. Douglas, H. G. Gauch. Jr., D. C. Glenn-Lewin, D. Hamilton, K. H. Hildebrandt, D. Mueller-Dombois, R. L. Peet, D. Stevens, E. L. Stone, J. Vleck, W. A. Weber, T. R. Wentworth, and P. L. Whittaker. I especially thank R. H. Whittaker for advice and encouragement. Financial support was provided by grants from the National Science Foundation, the DuPont Foundation, Cornell University and the University of North Carolina Research Council. The cooperation and support of the National Park Service is gratefully acknowledged.     

Responses of exotic plant species to fires in Pinus ponderosa forests in northern Arizona

Abstract. Changes in disturbance due to fire regime in southwestern Pinus ponderosa forests over the last century have led to dense forests that are threatened by widespread fire. It has been shown in other studies that a pulse of native, early‐seral opportunistic species typically follow such disturbance events. With the growing importance of exotic plants in local flora, however, these exotics often fill this opportunistic role in recovery. We report the effects of fire severity on exotic plant species following three widespread fires of 1996 in northern Arizona P. ponderosa forests. Species richness and abundance of all vascular plant species, including exotics, were higher in burned than nearby unburned areas. Exotic species were far more important, in terms of cover, where fire severity was highest. Species present after wildfires include those of the pre‐disturbed forest and new species that could not be predicted from above‐ground flora of nearby unburned forests.     

The structure and dynamics of Abies magnifica forests in the southern Cascade Range,USA

Abies magnifica (Red fir) forests in the Cascade Range and Sierra Nevada of California are composed of groups, or patches, of even-sized individuals that form structurally complex stands. Patches may be even-aged, resulting from synchronous post-disturbance establishment, or multi-aged, reflecting continuous recruitment of seedlings moderately tolerant of shade. We analyze the population structure (i.e. age, size, and spatial patterning) of A magnifica, and associated A concolor, White fir, and reconstruct the disturbance history of two mature to old-growth A. magnifica forests in order to determine the relationship between disturbance and forest structure. Within both stands examined, the distributions of A. magnifica seedlings, saplings, and small understory trees were clumped, with clump sizes corresponding to the area of canopy gaps. Gaps were created by frequent wildfire (mean fire return interval of 41 yr) and by windstorms. Severe fire initiated mass establishment of Abies magnifica, whereas gaps created by windthrowreleasedalreadyestablishedindividuals. Lowinten-sity fire stimulated little recruitment, but Red fir established continuously during fire-free intervals. Thus, the complex age and structures of Red fir forests reflect both episodic and continuous recruitment, as determined by the type and severity of natural disturbance.     

Forest vegetation of the Colorado Front Range: Patterns of species diversity

Summary Plant species diversity patterns of the Rocky Mountain forests were found to be at variance with patterns reported from other regions. The most centrally located forests in terms of elevation, site moisture and successional status were found to have the lowest diversity. In contrast, the peripheral and environmentally more severe sites were found to have relatively high diversity. In particular, the forest-grassland transition and the low elevation riparian forests have species diversity values as high as any yet reported from western North America.When diversity was examined in terms of variation across elevation or moisture gradients, varying results were obtained due to the interaction of these factors. The failure of previous studies to converge on generalizations about plant diversity reflects, in part, the failure of most investigators to view diversity in a regional context of variation across several interacting gradients.Diversity was seen to vary inversely with the degree of development of the forest canopy. The interaction of different components of the forest community is one reason for the failure of general patterns of plant species diversity to emerge from previous studies. A potentially rich herb community can be greatly suppressed by a single species tree stratum.Among the most successful work to date on species diversity is that on birds, a distinct albeit large and functional group. It is unlikely that similar success could have been achieved through work on all animal species simultaneously. This suggests the need to examine plant species diversity, not in terms of total diversity, but in terms of component functional groups, perhaps guilds, growing under similar microclimatic conditions and subject to similar competitive pressures.Nomenclature follows Weber (1976).     

Spatial and temporal variation in historic fire regimes in subalpine forests across the Colorado Front Range in Rocky Mountain National Park, Colorado, USA

Aim The historical variability of fire regimes must be understood in the context of drivers of the occurrence of fire operating at a range of spatial scales from local site conditions to broad‐scale climatic variation. In the present study we examine fire history and variations in the fire regime at multiple spatial and temporal scales for subalpine forests of Engelmann spruce–subalpine fir (Picea engelmannii, Abies lasiocarpa) and lodgepole pine (Pinus contorta) of the southern Rocky Mountains. Location The study area is the subalpine zone of spruce–fir and lodgepole pine forests in the southern sector of Rocky Mountain National Park (ROMO), Colorado, USA, which straddles the continental divide of the northern Colorado Front Range (40°20′ N and 105°40′ W). Methods We used a combination of dendroecological and Geographic Information System methods to reconstruct fire history, including fire year, severity and extent at the forest patch level, for c. 30,000 ha of subalpine forest. We aggregated fire history information at appropriate spatial scales to test for drivers of the fire regime at local, meso, and regional scales. Results The fire histories covered c. 30,000 ha of forest and were based on a total of 676 partial cross‐sections of fire‐scarred trees and 6152 tree‐core age samples. The subalpine forest fire regime of ROMO is dominated by infrequent, extensive, stand‐replacing fire events, whereas surface fires affected only 1–3% of the forested area. Main conclusions Local‐scale influences on fire regimes are reflected by differences in the relative proportions of stands of different ages between the lodgepole pine and spruce–fir forest types. Lodgepole pine stands all originated following fires in the last 400 years; in contrast, large areas of spruce–fir forests consisted of stands not affected by fire in the past 400 years. Meso‐scale influences on fire regimes are reflected by fewer but larger fires on the west vs. east side of the continental divide. These differences appear to be explained by less frequent and severe drought on the west side, and by the spread of fires from lower‐elevation mixed‐conifer montane forests on the east side. Regional‐scale climatic variation is the primary driver of infrequent, large fire events, but its effects are modulated by local‐ and meso‐scale abiotic and biotic factors. The low incidence of fire during the period of fire‐suppression policy in the twentieth century is not unique in comparison with the previous 300 years of fire history. There is no evidence that fire suppression has resulted in either the fire regime or current forest conditions being outside their historic ranges of variability during the past 400 years. Furthermore, in the context of fuel treatments to reduce fire hazard, regardless of restoration goals, the association of extremely large and severe fires with infrequent and exceptional drought calls into question the future effectiveness of tree thinning to mitigate fire hazard in the subalpine zone.     

Disturbance history and tree establishment in old-growth Pinus koraiensis-hardwood forests in the Russian Far East

Abstract. Dendro-ecological studies were undertaken to document the disturbance history in two old-growth mixed Pinus koraiensis-hardwood forests in the southern part of the Sikhote-Alin mountains in the Russian Far East. Establishment of four common canopy conifers, Abies holophylla, A. nephrolepis, P. koraiensis and Picea ajanensis, and three common canopy hardwoods, Acer mono, Betula costata and Tilia amurensis, were also inferred from population age structures and spatial dispersion patterns. Growth releases on increment cores suggested that peak periods of growth releases indicating partial canopy disturbances have repeatedly occurred over the past 230 yr at intervals from ca. 35 to 100 yr. Slight releases and suppressions other than the peak releases occurred in many years of both histories, suggesting the formation of smaller-scale canopy gaps. Despite the predominance of anthropogenic fires in Primorskiy Kray at present, destructive fires had not affected either forest. Wind disturbances and low intensity fires are likely factors controlling the dynamics of the forests. Under the disturbance regime, P. koraiensis has maintained its populations through its dependence on canopy gaps for establishment. Age distribution and gap dependence of P. ajanensis, A. nephrolepis and A. mono suggested continuous establishment of these species under a closed canopy, whereas occasional establishment of T. amurensis was derived largely from vegetative reproduction. Restricted establishment of A. holophylla and B. costata suggested a variety in kinds of disturbance throughout the histories. Continuous habitation of the study area by P. koraiensis is likely under the disturbance regime without destructive fires.     

Regeneration in subalpine coniferous forests

The regeneration process of a subalpine coniferous forest, a mixed forest ofTsuga diversifolia (dominant species),Abies veitchii, Abies mariessi, andPicea jezoensis var.hondoensis, was studied on the basis of annual ring data. The age class distribution was discontinuous and four age groups occurred in the study plot (30m×30m). The canopy layer was a mosaic of patches (83.8–133.7 m² patch area), which had different mean ages. The recruitment of canopy trees was carried out only by advance regeneration in the plot. The diameter growth ofAbies andPicea exceeded diameter growth ofTsuga in the gap.Abies lived for 200–300 years and their trunks were susceptible to heart rot.Picea lived for 300–400 years andTsuga for more than 400 years. The regeneration process derived from the analysis of the plot consisted of three phases leading to the development of a even-aged patch; (1) the establishment of saplings before a gap opening, (2) the opening of a gap in the canopy and repair of the canopy by advance regenerated saplings dominated by rapid growth species,Abies andPicea, and (3) the dying off of canopy trees as each species reached the end of its life-span, resulting in pure patches of long-livedTsuga.     

The dynamics of old-growth Pseudotsuga forests in the western Cascade Range,Oregon, USA

Size and age structure, spatial analysis, and disturbance history were used to analyse the population structures and regeneration patterns of 8 conifer stands in the central western Cascade Range, Oregon, USA. Variation in forest structure reflected the effects of frequent (20–50 yr) low-intensity fires and treefalls, infrequent (100–200 yr) localised, intense fires, and extensive fires that resulted in stand replacement (every ca 400 yr?).The amount of canopy removed and the size of openings formed by fires and treefalls were important determinants of subsequent forest establishment. Single or several species stands of Pseudotsuga and/or Abies procera, or mixed species stands of Pseudotsuga, Abies procera, Tsuga heterophylla and Abies amabilis established in openings where intense fires had removed most of the canopy trees over several ha. Multi-tiered and multi-aged stands, often containing 400–500 yr-old Pseudotsuga and variously-sized more or less even-aged patches of younger shade tolerant Tsuga heterophylla and/or Abies amabilis, occurred where lower-intensity fires did not kill all overstorey trees or where treefalls occurred after the initial fire.Current regeneration processes are influenced by overstorey composition, the availability and size of canopy openings, and the availability of substrates suitable for regeneration. Tsuga heterophylla and Abies amabilis established under Pseudotsuga menziesii and Abies procera canopies and in small canopy openings (<400 m²) created by windfalls, but rarely under Tsuga. Down logs and stumps were favoured establishment sites for Tsuga.The disturbance regime of fires of low-to moderate-intensity, windfalls, and occasional fires that result in extensive stand replacement contrasts with the pattern of infrequent, catastrophic disturbances proposed for other areas of the Pacific Northwest. Although fires at stand establishment commonly determine much of the composition, structure, and subsequent stand development, canopy replacement by shade tolerant species occurs as the different life histories of the species are expressed in response to various disturbances differing in intensity and frequency. Such a non-equilibrium view of vegetation change is consistent with many other fire-dominated forests of the western United States.     

Relationships of subalpine forest fires in the Colorado Front Range with interannual and multidecadal‐scale climatic variation

Aim An understanding of past relationships between fire occurrence and climate variability will help to elucidate the implications of climate‐change scenarios for future patterns of wildfire. In the present study we investigate the relationships between subalpine‐zone fire occurrence and climate variability and broad‐scale climate patterns in the Pacific and Atlantic Oceans at both interannual and multidecadal time‐scales. Location The study area is the subalpine zone of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa), and lodgepole pine (Pinus contorta) in the southern sector of the Rocky Mountain National Park, which straddles the continental divide of the northern Colorado Front Range. Methods We compared years of widespread fire from AD 1650 to 1978 for the subalpine zone of southern Rocky Mountain National Park, with climate variables such as measures of drought, and indices such as the El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO). Results Years of extensive subalpine‐zone fires are significantly related to climate variability, phases of ENSO, the PDO, and the AMO, as well as to phase combinations of ENSO, the PDO, and the AMO at both interannual and centennial time‐scales. Main conclusions Years of extensive fires are related to extreme drought conditions and are significantly related to the La Niña phase of ENSO, the negative (cool) phase of the PDO, and the positive (warm) phase of the AMO. The co‐occurrence of the phase combination of La Niña‐negative PDO‐positive AMO is more important to fire occurrence than the individual influences of the climate patterns. Low‐frequency trends in the occurrence of this combination of climate‐pattern phases, resulting from trends in the AMO, are the primary climate pattern associated with periods of high fire occurrence (1700–89 and 1851–1919) and a fire‐free period (1790–1850). The apparent controlling influence of the AMO on drought and years of large fires in the subalpine forests of the Colorado Front Range probably applies to an extensive area of western North America.     

Vegetation structure and dynamics of Pinus gerardiana forests in Balouchistan,Pakistan

Abstract. 44 forest stands, including 42 stands with Pinus gerardiana Wall, ex Lamb dominant and two stands with broad-leaved trees, were sampled in the Suleiman Range in Balouchistan. Density oi Pinus gerardiana trees ranged from 24 to 930 trees / ha with a mean of 266 individuals / ha; the average basal area was 25.5 m² ha^-1. Adequate recruitment of Pinus seedlings was observed; higher seedling density is recorded from east-facing slopes, while tree density was higher on west-facing slopes. The average growth rate was estimated as 0.08 cm / yr radial growth. However, trees on high elevations and cooler slopes grow faster. Soil variables showed no correlation with density, basal area or importance values. It is suggested that the present degraded stage of the forests in the study area is of anthropogenic origin.     

A Spatially-Explicit Reconstruction of Historical Fire Occurrence in the Ponderosa Pine Zone of the Colorado Front Range

A key issue in ecosystem management in the western U.S. is the determination of the historic range of variability of fire and its ecological significance prior to major land-use changes associated with Euro-American settlement. The present study relates spatial variation in historical fire occurrence to variation in abiotic and biotic predictors of fire frequency and severity across the elevational range of ponderosa pine in northern Colorado. Logistic regression was used to relate fire frequency to environmental predictors and to derive a probability surface for mapping purposes. These results indicate that less than 20% of the ponderosa pine zone had an historic fire regime (pre-1915) of relatively frequent fires (mean fire intervals, MFI, <30 years). More than 80% is reconstructed to have had a lower frequency (MFI ≥ 30 years), more variable severity fire regime. High fire frequency is clearly associated with low elevations. Lower and more variable fire frequencies, associated with high and moderate severities, occur across a broad range of elevation and are related to variations in other environmental variables. Only a small part of the ponderosa pine zone fits the widespread view that the historic fire regime was characterized mainly by frequent, low-severity that maintained open conditions. Management attempts to restore historic forest structures and/or fire conditions must recognize that infrequent severe fires were an important component of the historic fire regime in this cover type in northern Colorado.     

Tree species dynamics and disturbance in three Swedish boreal forest stands during the last two thousand years

Abstract. Data from three forest stands for the past 2000 yr show how the shade-intolerant species Pinus sylvestris and Betula pubescens maintain significant populations in the Swedish boreal landscape. Age structure data from a northern stand close to the range limits of Picea abies and Pinus complement a local pollen diagram, and reveal cyclic population fluctuations which can be related to periods of climatic stress and fire. Pollen data from two southern stands show that high fire frequencies in the past prevented the expansion of Picea populations. Reduction of the fire frequency during the last 200 yr has favoured Picea. A long time perspective reveals the population dynamics of long-lived species and indicates the controlling factors. Such knowledge permits assessment of the current status and likely future of forest stands.     

Critical zone properties control the fate of nitrogen during experimental rainfall in montane forests of the Colorado Front Range

Several decades of research in alpine ecosystems have demonstrated links among the critical zone, hydrologic response, and the fate of elevated atmospheric nitrogen (N) deposition. Less research has occurred in mid-elevation forests, which may be important for retaining atmospheric N deposition. To explore the fate of N in the montane zone, we conducted plot-scale experimental rainfall events across a north–south transect within a catchment of the Boulder Creek Critical Zone Observatory. Rainfall events mimicked relatively common storms (20–50% annual exceedance probability) and were labeled with ¹⁵N-nitrate (\( {\text{NO}}_{3}^{ - } \)) and lithium bromide tracers. For 4 weeks, we measured soil–water and leachate concentrations of Br^?, \( {}^{15}{\text{NO}}_{3}^{ - } , \) and \( {\text{NO}}_{3}^{ - } \) daily, followed by recoveries of ¹⁵N species in bulk soils and microbial biomass. Tracers moved immediately into the subsurface of north-facing slope plots, exhibiting breakthrough at 10 and 30 cm over 22 days. Conversely, little transport of Br^? or \( {}^{15}{\text{NO}}_{3}^{ - } \) occurred in south-facing slope plots; tracers remained in soil or were lost via pathways not measured. Hillslope position was a significant determinant of soil ¹⁵N-\( {\text{NO}}_{3}^{ - } \) recoveries, while soil depth and time were significant determinants of ¹⁵N recovery in microbial biomass. Overall, ¹⁵N recovery in microbial biomass and leachate was greater in upper north-facing slope plots than lower north-facing (toeslope) and both south-facing slope plots in August; by October, ¹⁵N recovery in microbial N biomass within south-facing slope plots had increased substantially. Our results point to the importance of soil properties in controlling the fate of N in mid-elevation forests during the summer season.     

Ecosystem Responses to Nitrogen Deposition in the Colorado Front Range

We asked whether 3–5 kg N y⁻¹ atmospheric N deposition was sufficient to have influenced natural, otherwise undisturbed, terrestrial and aquatic ecosystems of the Colorado Front Range by comparing ecosystem processes and properties east and west of the Continental Divide. The eastern side receives elevated N deposition from urban, agricultural, and industrial sources, compared with 1–2 kg N y⁻¹ on the western side. Foliage of east side old-growth Englemann spruce forests have significantly lower C:N and lignin:N ratios and greater N:Mg and N:P ratios. Soil % N is higher, and C:N ratios lower in the east side stands, and potential net N mineralization rates are greater. Lake NO₃ concentrations are significantly higher in eastern lakes than western lakes. Two east side lakes studied paleolimnologically revealed rapid changes in diatom community composition and increased biovolumes and cell concentrations. The diatom flora is now representative of increased disturbance or eutrophication. Sediment nitrogen isotopic ratios have become progressively lighter over the past 50 years, coincident with the change in algal flora, possibly from an influx of isotopically light N volatilized from agricultural fields and feedlots. Seventy-five percent of the increased east side soil N pool can be accounted for by increased N deposition commensurate with human settlement. Nitrogen emissions from fixed, mobile, and agricultural sources have increased dramatically since approximately 1950 to the east of the Colorado Front Range, as they have in many parts of the world. Our findings indicate even slight increases in atmospheric deposition lead to measurable changes in ecosystem properties. Received 16 November 1999; accepted 8 February 2000.     

Contrasting long-term alpine and subalpine precipitation trends in a mid-latitude North American mountain system,Colorado Front Range,USA

Background: Long-term climate trends in mountain systems often vary strongly with elevation.

Aims: To evaluate elevation dependence in long-term precipitation trends in subalpine forest and alpine tundra zones of a mid-continental, mid-latitude North American mountain system and to relate such dependence to atmospheric circulation patterns.

Methods: We contrasted 59-year (1952–2010) precipitation records of two high-elevation climate stations on Niwot Ridge, Colorado Front Range, Rocky Mountains, USA. The sites, one in forest (3022 m a.s.l.) and the other in alpine tundra (3739 m), are closely located (within 7 km horizontally, ca. 700 m vertically), but differ with respect to proximity to the mountain-system crest (the Continental Divide).

Results: The sites exhibited significant differences in annual and seasonal precipitation trends, which depended strongly on their elevation and distance from the Continental Divide. Annual precipitation increased by 60 mm (+6%) per decade at the alpine site, with no significant change at the subalpine site. Seasonally, trends at the alpine site were dominated by increases in winter, which we suggest resulted from an increase in orographically generated precipitation over the Divide, driven by upper-air (700 hPa) north-westerly flow. Such a change was not evident at the subalpine site, which is less affected by orographic precipitation on north-westerly flow.

Conclusions: Elevation dependence in precipitation trends appears to have arisen from a change in upper-air flow from predominantly south-westerly to north-westerly. Dependence of precipitation trends on topographic position and season has complex implications for the ecology and hydrology of Niwot Ridge and adjacent watersheds, involving interactions among physical processes (e.g. snowpack dynamics) and biotic responses (e.g. in phenologies and ecosystem productivity).     

Effects of reindeer grazing on understorey vegetation in dry Pinus sylvestris forests

Abstract. Data on floristic composition and environmental variables were collected in floristically homogeneous oligotrophic pine (Pinus sylvestris) forests with heath-like under- storey vegetation in eastern Fennoscandia, and ordinated by non-linear multidimensional scaling (NMDS) in order to study the effect of lichen grazing by reindeer on the understorey vegetation. The study sites included areas with varying grazing pressure, as well as 50-yr old grazing exclosures. Sites rich in respectively bryophytes and lichens were placed at opposite ends of the ordination axes, and heavily grazed sites were placed in between them. Reindeer grazing increased the abundance of bryophytes, especially Dicranum spp. and Pleurozium schreberi. Grazing changed the vegetation to the extent that it resembled more mesotrophic sites, but this did not show any relationship with tree volume or other site productivity indicators. This was observed both in the ordination and, in a more compelling way, when exclosures with adjacent grazed areas were compared. No such signs were evident at ungrazed sites, where especially Cladina spp. spatially replace Cladonia spp. and tiny bryophytes like Barbilophozia spp., Polytrichum spp. and Pohlia nutans during succession. Cladina stellaris had almost disappeared from the most intensively grazed sites. The soil at ungrazed sites was characterized by high Al and Fe concentrations and bryophyte-rich sites by high Mn concentrations. Shannon's diversity index, depth of humus layer and proportion of bare ground also increased in sites getting richer in bryophytes.