[Show abstract][Hide abstract] ABSTRACT: Several forest types co-exist at low elevations in the North Coast Range of California, forming a mosaic at the landscape scale. The major types are: coast redwood forest (dominated by Sequoia sempervirens), Pacific northwest Douglas-fir forest (dominated by Pseudotsuga menziesii), and a broadleaved forest (dominated by any or all of the following: Arbutus menziesii, Chrysolepis chrysophylla, Lithocarpus densiflora, Quercus agrifolia, Q. chrysolepis, and Umbellularia californica; conifers not dominant. Our hypothesis was that the driving forces for this patchy mosaic pattern had to do with a gradient of micro- and macro-attributes from the coast, inland. We focused on the first 60 km of the gradient and we limited any latitudinal effects (i.e., the flora was kept homogeneous) by locating 17 old-growth stands in only seven counties within the boundaries of the Bay Area, sensu lato. The dominance of conifers relative to broad-leaved evergreen trees and of Pseudotsuga relative to Sequoia were analyzed by non-metric multidimensional scaling (NMS), regression, and Canonical Analysis of Principal Coordinates (CAP). The abiotic traits tested for degree of correlation with vegetation were: latitude, distance inland from the sea, slope aspect and steepness, annual precipitation, annual solar radiation, mean January minimum and mean July maximum temperatures, site elevation, and the elevation of an intervening ridge between the site and the coast (relative to site elevation). Some of these traits were significantly correlated with vegetation, the strongest being distance inland, from site to sea. The most parsimonious regression model that explained conifer dominance was a cubic function based only on distance from site to sea, which had an adjusted R 2 of 68 %.
[Show abstract][Hide abstract] ABSTRACT: Canary Islands pine (Pinus canariensis Chr. Sm. ex DC) is often described as serotinous, even though most serotinous attributes are absent or weakly developed and the trees do not seem to experience a natural fire regime that would favor and sustain serotiny. We studied the age structure of 22 old-growth stands on the slopes of Mt. Teide on Tenerife, Canary Islands. Statistically robust relationships between trunk diameter at breast height and tree age allowed us to use diameter-age regressions to summarize population age structures and to reconstruct disturbance history. The age structure of only one stand statistically fit the null hypothesis's expectation of a smoothly declining L-shaped negative exponential, non-disturbance population model. Departures from the model commonly featured high densities of seedlings and saplings in the absence of recent fires (indicating that regeneration is independent of fire) and age structures that exhibited one or more peaks of establishment, the average number of years between peaks being 78 yr. Onset of sexual reproduction averaged 46 yr, and the age of mature overstory individuals often exceeded 200 yr. Anomalously, variation in most vegetation attributes, including stem diameter growth, failed to significantly correlate with major abiotic gradients-elevation, temperature, precipitation, exposure to trade wind clouds, slope aspect and steepness, and geological substrate in habitats where P. canariensis dominated.
[Show abstract][Hide abstract] ABSTRACT: Background/Question/Methods Californian natural low elevation grassland is an extinct vegetation type. Remaining grasslands, which were not converted to agriculture or residential areas, are heavily dominated by introduced species that displaced natives partially or completely. Lack of knowledge about pristine grasslands makes it difficult to define management and restoration objectives and measure success of restoration efforts. The objective of this study was to reconstruct floristic composition of California grasslands, to understand former diversity grassland types, and to provide guidelines for their management and restoration. We sampled >300 plots (100 m2 each) in 10 locations throughout the Central Valley. Using TURBOVEG, JUICE, TWINSPAN, and PC-ORD software for data analysis followed by Braun-Blanquet method of vegetation classification, we distinguished 14 grassland types defined in the presence of its own set of diagnostic native species. These types were used to define targets for ecological restoration.
Results/Conclusions Our results have been brought to the attention of restoration and conservation ecologists to recognize the broad range of plant communities that once constituted an extensive California prairie. These results shifted common focus on restoration of native bunchgrasses toward restoration of diversity of native forbs. We will discuss: (1) application of vegetation classification for ecological reconstruction and introduce a method of floristic analysis that has not yet been used for these purposes; (2) report results of the reconstruction of floristic composition of native species in grasslands in California Central Valley and discuss their relationship to oak woodlands and riparian forests; (3) explain how classification was used to predict which species should be reintroduced into existing floristically poor grasslands; (4) and demonstrate examples of reintroduction that enhanced the native species component in modern grasslands.
[Show abstract][Hide abstract] ABSTRACT: Background/Question/Methods Vernal pool vegetation has only recently been sampled adequately enough to floristically define associations, alliances, and orders. Prior to this, agencies involved in the protection, conservation, and management of vernal pools had only a statewide laundry list of vernal pool endemics, preferents, and federally listed plant taxa with which to judge the health of natural and created vernal pools. There was no understanding of how many community types might exist nor of which might be rare and unprotected. Based on a statewide sample of > 2000 releves in 700 pools we defined 26 associations, 6 alliances, 3 orders, and 1 class and attributed them with diagnostic taxa, associated listed taxa, habitat traits, and relative degree of rarity and protection. Results/Conclusions Our research was subsidized by several agencies that had very pragmatic objectives (California Department of Fish and Game, US Fish and Wildlife Service, and US EPA. We recognized that an emphasis on basic research and publications in peer-reviewed journals would notin and of itselfcause these sponsors to use our results in rewriting protocols for monitoring and evaluating candidates for take and for assessing compensatory mitigation projects. We had to find a way to get the attention of staff at the right agency levels, and then to make our approach so obviously useful and an improvement of past approaches that staff were motivated to beginning a process of evaluation and adoption. At first, our attempts were met with cautious coolness. For example, when we developed a training course to teach staff scientists and private consultants how to sample vernal pool vegetation and how to key it out to appropriate classification levels, it was done pro bono because no agency would subsidize us or even promote the class. A year later, however, US EPA had agreed to help fund a second offering and a few months later, EPA hosted a multi-agency meeting on vernal pool management attended by 140 persons and a new edition of Manual of California Vegetation was published that adopted our classification. There is now talk in several agencies and the release of RFPs about writing new protocols for vernal pool vegetation assessment, conservation, and restoration that would be more ecologically relevant and regionally appropriate than protocols that have been in place for some time.
[Show abstract][Hide abstract] ABSTRACT: Questions: 1. Does resource use efficiency increase with increased species richness in conifer forests? 2. Do patterns found in resource use support niche differentiation/complementarity between species, or is any increase indicative of a selection effect?Location: All data were collected from upper montane (2200–2600 m a.s.l) conifer forests of the Desolation Wilderness in the central California Sierra Nevada, USA.Methods: We established 281 plots of varying levels of conifer richness throughout the wilderness area. Within each plot we used hemispherical photos to measure canopy closure and LAI, total soil carbon and nitrogen from the A-horizon, and stand basal area. We used linear regression and ANOVA to analyse the relationship between stand species richness and resource availability.Results: We found no correlation of either soil nitrogen or carbon with stand biomass. Nor did soil nitrogen and carbon levels change with species richness. Canopy closure increased with species richness but also varied significantly between pure stands of different species. Pure Pinus monticola stands had the lowest canopy closure, Tsuga mertensiana stands the highest. Composition explained more canopy cover variation than did species richness. We found evidence supporting both the sampling effect and niche differentiation models at different stages of stand development.Conclusions: During initial stages of stand development, the interaction between the shade-intolerant Pinus species and shade-tolerant Abies magnifica and T. mertensiana followed the niche differentiation model, but switched to the sampling effect model during the competitive-exclusion stage. In contrast, interaction between A. magnifica and T. mertensiana followed the niche differentiation model.
[Show abstract][Hide abstract] ABSTRACT: The currently prevailing view is that saplings require gaps or larger disturbances in order to grow into the canopy. This study documents an exception. In California's Pseudotsuga-mixed hardwood forests, crowns of Pseudotsuga menziesii (Douglas fir) are within those of angiosperm trees (Arbutus menziesii and Quercus species). In the forests we examined, every Pseudotsuga was younger and all but one were growing more rapidly in girth than the Arbutus or Quercus whose crown it had penetrated. Furthermore, as saplings, the Pseudotsuga had grown at rates between those of suppressed saplings and canopy dominants. The recruitment of emergent Pseudotsuga substantially alters these canopies because of the large size Pseudotsuga attains. Given the density of Pseudotsuga growing in canopy crowns, such recruitment is likely. As a mechanism of recruitment, this through-growth differs from gap recruitment in that the turnover of canopy trees is determined by an understory species' growth rate rather than the overstory species' longevity, and community attributes may change rapidly by replacement of canopy dominants with a dissimilar species. Pseudotsuga could grow through the canopy because of its greater potential height (> 60m vs. 20–40m for the angiosperms), narrower crown and its branches suffering less mechanical damage than those of the angiosperms. In general, resource levels in the understory, canopy height, and interspecific differences in maximum height and crown architecture all influence the likelihood of through-growth. Therefore, for vegetation types whose dominants differ substantially in growth form, through-growth may be a mechanism for rapid ecosystem change.
[Show abstract][Hide abstract] ABSTRACT: Background/Question/Methods When vegetation consists of a fine-scale mosaic of many communities, individual communities often cannot be mapped because of their small size. Instead, the minimal mapping units can be complexes that re-occur in particular habitats. Finding and defining these stable complexes is an additional process necessary for linking classification and mapping. Our objective in studying Californian vernal pools was to classify individual community types and then to define their complexes that can be mapped. We sampled >2000 plots in >700 pools throughout California. One finding was that a typical pool consists of several plant communities that differ in duration of inundation. Each community within the pool was sampled with 10 m2 plots. Using TURBOVEG, JUICE, and TWINSPAN software, and following the Braun-Blanquet technique, we defined one class (Downingio-Lasthenietea), three orders (Downingio-Lasthenietalia, Frankenio-Lasthenietalia, and Lasthenietalia glaberrimae), five alliances, and >30 associations. The class includes all types of hardpan, claypan, volcanic, terrace, freshwater, and saline pools in California, Oregon, Washington, and Baja California (Barbour et al. 2007).
Results/Conclusions The associations were not mappable because the pools themselves are generally <500 m2 and dominated by ephemerals that change seasonally and annually. We applied the same technique for defining mappable complexes as we used for defining associations. Mappable complexes consist of 2-3 associations with high constancy and additional associations with lower constancy, and they are local -- restricted to one vernal pool regions. We found two principal types of complexes, one with hardpan and one with claypan pools, each of which was subdivided to multiple locally restricted complexes. Each complex was defined based on its own set of associations. At a fine scale, diversity of complexes increases with pool depth. At a coarser scale, diversity of complexes depends upon the uniqueness of the set of locally restricted associations. Most associations are restricted to only one complex, while a few associations, such as Downingio bicornutae-Lasthenietum glaberrimae, occur in many complexes. Transformation of ground-based vegetation classification to mapping units permits us to address fine-scale patchiness and to incorporate valuable ecological information into a vegetation map. We suggest using this process as an additional step of on-the-ground data analysis for mapping other vegetation types that consist of fine-scale mosaic of plant communities.
[Show abstract][Hide abstract] ABSTRACT: To identify the influence of interannual and interdecadal climate variation on the occurrence and extent of fires in montane conifer forests of north-western Mexico. This study was conducted in Jeffrey pine (Pinus jeffreyi Grev. & Balf.)-dominated mixed-conifer forests in the central and northern plateau of the Sierra San Pedro Mártir, Baja California, Mexico. Fire occurrence was reconstructed for 12 dispersed sites for a 290-year period (1700-1990) from cross-dated fire-scarred samples extracted from live trees, snags and logs. Superposed epoch analysis was used to examine the relationships of tree-ring reconstructions of drought, the El Niño/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) with fire occurrence and extent. Years with no recorded fire scars were wetter than average. In contrast, years of widespread fires were dry and associated with phase changes of the PDO, usually from positive (warm) to negative (cold). The influence of the PDO was most evident during the La Niña phase of the ENSO. Widespread fires were also associated with warm/wet conditions 5 years before the fire. We hypothesize that the 5-year lag between warm/wet conditions and widespread fires may be associated with the time necessary to build up sufficient quantity and continuity of needle litter to support widespread fires. Two periods of unusually high fire activity (1770-1800 and 1920-1950) were each followed by several decades of unusually low fire activity. The switch in each case was associated with strong phase changes in both PDO and ENSO. Climate strongly influences fire regimes in the mountains of north-western Mexico. Wet/warm years are associated with little fire activity. However, these years may contribute to subsequent fire years by encouraging the production of sufficient needle litter to support more widespread fires that occur in dry/cool years.
Journal of Biogeography 01/2008; · 4.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Our objective was to examine the age structure of successionally young and mature old-growth stands dominated by Quercus pyrenaica, widely distributed on silicious substrates at 400-1800 m elevation in the Iberian Peninsula, France, and Morocco. Many hectares of young forest are recovering from intensive grazing and wood cutting as a result of land abandonment during the past several decades. Eight early-successional undisturbed stands, one intermediate-aged stand, and four old-growth stands were sampled. Age was determined by coring a subset of trees of various diameters and applying a regression formula to the dbh of 1316 trees among all 13 stands, so that every diameter could be expressed in years of age. Linear regression formulas accounted for 69-81 % of all diameter-age variation. For young stands, almost 2/3 of all trees were 30-39 yr of age, a dip in regeneration characterized the most recent 30 yr, and the oldest tree encountered was 90 yr old. For old stands, more than half the trees were 56-125 yr of age, a dip in regeneration characterized the most recent 75 yr, and the oldest tree encountered was 358 yr old. Thus, both young and mature stands showed depressed regeneration for at least the past three decades, a phenomenon shared by ecologically related oaks in California, where such age structures are interpreted as evidence of long-term population imbalance and instability.
[Show abstract][Hide abstract] ABSTRACT: Theoretical and empirical studies have long suggested that stability and complexity are intimately related, but evidence from long-lived systems at large scales is lacking. Stability can either be driven by complex species interactions, or it can be driven by the presence/absence and abundance of a species best able to perform a specific ecosystem function. We use 64 years of stand productivity measures in forest systems composed of four dominant conifer tree species to contrast the effect of species richness and abundance on three stability measures. To perform this contrast, we measured the annual growth increments of > 900 trees in mixed and pure forest stands to test three hypotheses: increased species richness will (1) decrease stand variance, (2) increase stand resistance to drought events, and (3) increase stand resilience to drought events. In each case, the alternate hypothesis was that species richness had no effect, but that species composition and abundance within a stand drove variance, resistance, and resilience. In pure stands, the four species demonstrated significant differences in productivity, and in their resistance and resilience to drought events. The two pine species were the most drought resistant and resilient, whereas mountain hemlock was the least resistant and resilient, and red fir was intermediate. For community measures we found a moderately significant (P = 0.08) increase in the community coefficient of variation and a significant (P = 0.03) increase in resilience with increased species richness, but no significant relationship between species richness and community resistance, though the variance in community resistance to drought decreased with species richness. Community resistance to drought was significantly (P = 0.001) correlated to the relative abundance of lodgepole pine, the most resistant species. We propose that resistance is driven by competition for a single limiting resource, with negative diversity effects. In contrast resilience measures the capacity of communities to partition resources in the absence of a single limiting resource, demonstrating positive diversity effects.
[Show abstract][Hide abstract] ABSTRACT: Vernal pools contain ephemeral wetland vegetation in the class Downingio bicornutae-Lasthenietea fremontii. In this article, those communities with the longest period of inundation were classified into a new order Lasthenietalia glaberrimae, the alliance Lasthenion glaberrimae, four associations, seven variants, and one unranked community. The analysis was based on vernal pools in northern California. The Downingio bicornutae-Lasthenietum glaberrimae characterizes duripan vernal pools on foothill terraces. It is the most widely distributed association of the four, and it contains seven variants. The Pogogyno douglasii-Lasthenietum glaberrimae is more restricted to higher elevation basaltic table mountains. The Trifolio variegati-Lasthenietum glaberrimae is characteristic of relatively shallow pool bottoms on foothill terraces. Finally, the Pleuropogono californici-Lasthenietum glaberrimae occurs in slightly saline claypan pools located in the valley floor. The Eleocharis macrostachya [Lasthenion glaberrimae] community represents floristically depauperate stands. The syntaxa tend to be locally restricted and their diagnostic species are typically related to the period of inundation.
[Show abstract][Hide abstract] ABSTRACT: We described 38 relictual old-growth stands – with data on the mortality, regeneration, floristic richness, fuel load and disease incidence in our study area in the Tahoe Basin of California and Nevada. The stands are within the lower and upper montane zones (1900–2400 m a.s.l.) and they are rare, occupying < 2% of the land in the Basin's watershed. Correlation matrices and ANOVAs of forest types and conifer species with environmental gradients revealed significant relationships with elevation, distance east of the Sierran crest, slope aspect, annual precipitation, date of complete snow melt, litter depth and degree of soil profile development. Pathogens, parasites and wood-boring insects were present on 23% of living trees; 16% of all trees were dead. We compared these stands to a reconstruction of pre-contact Basin forests and to ecologically analogous old-growth forests of Baja California that have never experienced fire suppression management. Currently, overstorey trees (> 180 yr old) in the Basin stands have ca. 33% cover, 54 m2.ha-1 basal area and 107 individuals.ha-1, values very similar to reconstructions of pre-contact Basin forests and to modern Baja California forests. Understorey trees (60–180 yr old), however, are several times more dense than historic levels and species composition is strongly dominated by A. concolor, regardless of the overstorey composition. The ratio of Pinus: Abies has increased – and the age structure of extant stands predicts that it will continue to increase – from approximately 1:1 in pre-contact time to 1:7 within the next century. Disease incidence and mortality in Baja forests were lower. Although we quantitatively defined current Basin old-growth forests – in terms of stand structure – we realize that our definition will differ from that of both past and future old-growth forests unless management protocols are changed.
[Show abstract][Hide abstract] ABSTRACT: Growth and xylem water potential of the lower elevation conifers Pinus jeffreyi and Abies concolor and the higher elevation Pinus monticola and Abies magnifica were monitored in their montane Mediterranean habitat of the southernmost Sierra Nevada mountains of California. Measurements were made across the ecotone between the midmontane and upper montane forests and through light and heavy snowfall years.Radial stem growth, averaging ∼1.5 mm/yr, started 2 wk after snow melt, providing that maximum air temperatures had reached 21°C, and ended when predawn water potentials fell rapidly at the onset of the summer drought. Leader growth started on or after a fixed date, providing that minimum air temperatures were above -4°C for Pinus species or +2.5°C for Abies species. The cue for leader growth was inferred to be photoperiodic. Leader growth ended when either a determinate internode length of ∼1 mm was reached or predawn water potentials fell rapidly. Abies magnifica grew more rapidly than the low-elevation species, but had a shorter growth period; its annual leader growth, as a consequence, was only 35 mm/yr vs. 50 mm/yr for the low-elevation species. Needle growth was similarly determinate in the absence of early drought. This growth phenology contributes to determining species distribution across the ecotone.
American Journal of Botany 06/2001; 88(5):919-32. · 2.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Xylem water potential of the midelevation conifers Pinus jeffreyi, Pinus lambertiana, Abies concolor, and Calocedrus decurrens, the higher elevation Pinus monticola and Abies magnifica, and co-occurring evergreen angiosperm shrubs, together with soil moisture under these plants, were monitored at three sites on the Kern Plateau in the southernmost Sierra Nevada Range of California. Site locations spanned the ecotone between the mid- and upper montane forests at elevations of 2230-2820 m. Measurements were made through a low-snowfall year and a heavy-snowfall year.In the Mediterranean climate of the Sierra Nevada, the heavy winter snowpack persists into late spring, after precipitation has effectively stopped. We found the subsequent depletion of soil moisture due to plant water uptake to result in predawn xylem water potentials for conifers more negative by 0.6-1.4 MPa than those for shrubs or inferred soil potentials. Shrubs generally depleted soil moisture more rapidly and ultimately extracted a greater fraction of the available soil moisture than did the conifers. This depletion of soil moisture by shrubs, particularly Arctostaphylos patula, may limit conifer growth and regeneration by prematurely terminating growth on the shallow soils studied. The conifers all generally showed similar patterns of soil moisture use, except that A. magnifica extracted moisture more rapidly early in the season.
American Journal of Botany 06/2001; 88(5):911-8. · 2.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A first classification for serpentine annual grasslands distributed throughout northern and central California is proposed. This study has followed the Braun-Blanquet phyto-sociological system based on floristical, biogeographical and bioclimatic features of the sampled areas. Numerical analyses of classification and ordination were applied to the floristic relevés. Minimum Variance Clustering grouped relevés into basic classification units that allowed us to define low-hierarchical syntaxonomical units (associations) and ‘communities’. A Principal Coordinate Analysis was used to extract those ecological parameters related to the axes that separate those classification units from the previous dendrogram. The results showed that differences in species composition was mainly due to a continentality gradient and the shady effect of an overstory vegetation. On the basis of both analyses we propose a first syntaxonomic scheme on ultramafic (mainly serpentine) annual plant communities of the biogeographical Californian Region that comprises four associations, two subassociations and some provisional communities.