Recruitment of Quercus agrifolia in central California: the importance of shrub-dominated patches

Abstract. Many perennial plants strongly enhance the survival of seedlings of other species. We studied patterns of long-term recruitment of Quercus agrifolia (Coastal live oak) associated with shrub-dominated communities by counting Q. agrifolia recruits on a time sequence of historical aerial photographs and comparing recruitment among mapped patches of coastal sage scrub, chaparral, and grassland in an 1120-ha landscape. Because we could not identify new recruits in existing woodlands with aerial photographs, we studied the recruitment of Q. agrifolia in this vegetation type indirectly by comparing population size structures and the spatial relationships between shrubs and recruits among woodlands that varied in understory community type. At the landscape scale, recruitment was higher in coastal sage scrub vegetation than predicted by the extent of its coverage, commensurate with the spatial coverage of chaparral, and very low in grassland. Recruitment within woodland communities also varied considerably. In woodland communities on sheltered, north-oriented topography with understories dominated by shrubs, there were large numbers of small Q. agrifolia, and recruits were not significantly spatially associated with shrubs within plots. In woodlands with herbaceous understories there were few individuals in the small size classes, and recruits were strongly spatially associated with shrubs within plots. Woodlands with shrub-dominated understories have population structures that appear to be stable, but woodlands with herbaceous understories exhibit size structures associated with declining populations. Quercus recruitment into shrub-dominated patches corresponds with previous documentation of facilitative relationships between shrubs and oak seedlings, and suggests the occurrence of an unusual form of patch dynamics in these landscapes.     

An Analysis of Vegetation Restoration on Opencast Oil Shale Mines in Estonia

We compared four types of 30‐year‐old forest stands growing on spoil of opencast oil shale mines in Estonia. The stand types were: (1) natural stands formed by spontaneous succession, and plantations of (2) Pinus sylvestris (Scots pine), (3) Betula pendula (silver birch), and (4) Alnus glutinosa (European black alder). In all stands we measured properties of the tree layer (species richness, stand density, and volume of growing stock), understory (density and species richness of shrubs and tree saplings), and ground vegetation (aboveground biomass, species richness, and species diversity). The tree layer was most diverse though sparse in the natural stands. Understory species richness per 100‐m² plot was highest in the natural stand, but total stand richness was equal in the natural and alder stands, which were higher than the birch and pine stands. The understory sapling density was lower than 50 saplings/100 m² in the plantations, while it varied between 50 and 180 saplings/100 m² in the natural stands. Growing stock volume was the least in natural stands and greatest in birch stands. The aboveground biomass of ground vegetation was highest in alder stands and lowest in the pine stands. We can conclude that spontaneous succession promotes establishment of diverse vegetation. In plantations the establishment of diverse ground vegetation depends on planted tree species.     

Species richness and soil properties in Pinus ponderosa forests: A structural equation modeling analysis

Question: How are the effects of mineral soil properties on understory plant species richness propagated through a network of processes involving the forest overstory, soil organic matter, soil nitrogen, and understory plant abundance? Location: North‐central Arizona, USA. Methods: We sampled 75 0.05‐ha plots across a broad soil gradient in a Pinus ponderosa (ponderosa pine) forest ecosystem. We evaluated multivariate models of plant species richness using structural equation modeling. Results: Richness was highest at intermediate levels of understory plant cover, suggesting that both colonization success and competitive exclusion can limit richness in this system. We did not detect a reciprocal positive effect of richness on plant cover. Richness was strongly related to soil nitrogen in the model, with evidence for both a direct negative effect and an indirect non‐linear relationship mediated through understory plant cover. Soil organic matter appeared to have a positive influence on understory richness that was independent of soil nitrogen. Richness was lowest where the forest overstory was densest, which can be explained through indirect effects on soil organic matter, soil nitrogen and understory cover. Finally, model results suggest a variety of direct and indirect processes whereby mineral soil properties can influence richness. Conclusions: Understory plant species richness and plant cover in P. ponderosa forests appear to be significantly influenced by soil organic matter and nitrogen, which are, in turn, related to overstory density and composition and mineral soil properties. Thus, soil properties can impose direct and indirect constraints on local species diversity in ponderosa pine forests.     

Post-fire seedling establishment in Florida sand pine scrub

Abstract. This study deals with a quantification of pre- and post-fire seedling establishment and microsite characteristics in two Florida sand pine scrub sites burned in May 1993. In addition, life history characteristics related to seedling establishment are described for five perennial species –Calamintha ashei, Chapmannia floridana, Eriogonum floridanum, Garberia heterophylla and Palafoxia feayi. Post-fire seedling establishment in sand pine scrub was sparse (median = 1, 12 seedling/m²), with 17 of 35 species establishing seedlings. Chapmannia, Eriogonum, Garberia and Palafoxia resprouted and flowered after fire; Eriogonum and Garberia had strong post-fire seedling establishment responses within 19 months post-fire. Calamintha individuals were killed by fire, but this species had a strong post-fire seedling establishment response, presumably from seeds in a soil seed bank. Eriogonum and Calamintha seedlings established preferentially in plots centered on conspecific adults. For these species with poor seed dispersal, spatial patterns of seedling establishment may be influenced more by pre-fire adult plant location than by post-fire microsite conditions. Post-fire seedling density in sand pine scrub was much lower than in California chaparral and South African sand plain lowland fynbos.     

Relationship between plant species richness and biomass in a coastal Maine Quercus-Pinus forest

Abstract. Several researchers have hypothesized that plant species richness has a unimodal relationship with biomass, while others have argued for a linear relationship. Data from various types of herbaceous communities show some support for the unimodal hypothesis, but this has not been tested extensively for forests and questions remain concerning its generality. We used linear and quadratic regression models to examine the relationship between overstory biomass and richness in a coastal Maine Quercus-Pinus forest across and within cover types using data from two plot sizes (2500-m² quadrats and 625-m² sub-quadrats). Understory data from 1-m² plots were also analysed. Richness was quadratically related to biomass at both plot sizes for all cover types combined, but the amount of variation explained by the models was very low (R²s < 0.09). Richness and biomass were not significantly related at either plot size for the mixed mesic cover type, the most common type in the forest. The best fit (R²= 0.43) was obtained with a quadratic model for the conifer cover type at the sub-quadrat level, with the quadratic model for the 1-m² data having the second highest R² (0.24). Across all six data sets, the quadratic model was the only one with a significant fit in two cases, and had considerably higher R²s (1.3–1.9 ×) in two others. The remaining two data sets could not be fit with a significant model of either type. For this forest, these results suggest little support for a linear relationship between plant species richness and biomass and variable, often weak, support for a unimodal relationship. Density, a potentially confounding variable in this type of analysis, was only weakly correlated with richness and was not found to alter the relationship between biomass and richness.     

Environmental controls of NDVI dynamics in Patagonia based on NOAA-AVHRR satellite data

Abstract. Variation in vegetation in extra-Andean Patagonia (Argentina) was analyzed using spectral data derived from AVHRR/NOAA satellite. The study of seasonal dynamics of the Normalized Difference Vegetation Index (NDVI, i.e. a combined index of the reflection in the red and infrared bands) highlighted similarities in functional aspects between regional vegetation units which are dissimilar in a geographical, physiognomical and/or floristical way, and also suggested that gross primary production is correlated with mean annual rainfall. The first axis in a Principal Component Analysis of NDVI data was correlated (r² = 0.90) with NDVI as integrated for the study period. The second axis was correlated (r² = 0.50) with the differences in NDVI during the growing season, reflecting seasonality. Mean annual rainfall accounted for 60% of integrated NDVI variability among vegetation units. Much of the residual variance (62%) was accounted for by the inverse of the distance to the Atlantic Ocean, which is interpreted as an ocean effect on vegetation functioning in the extra-Andean Patagonia.     

Plant communities and landscape diversity along a Canadian Arctic river

Abstract. We analysed the structure and diversity of the vegetation along an Arctic river to determine the relationship between species richness and plant community structure. We examined whether variation in species richness along the corridor is structured as (1) an increase in the number of communities due to increasing landscape heterogeneity, (2) an increase in the floristic distinctiveness (β-diversity) of communities, or (3) an increase in within-community richness (α-diversity) as species-poor communities are replaced by species-rich communities. We described 24 community types and analysed the relationship between site vascular species richness (γ-diversity) and β-diversity, α-diversity, site environmental heterogeneity, and the number of distinct plant communities. We also measured diversity patterns of vascular, bryophyte, and lichen species within communities and examined their relationship to community-level estimates of environmental factors. We found that an increase in site species richness correlated with an increase in the number of communities (r²= 0.323, P= 0.0173) and β-diversity (r²= 0.388, P= 0.0075), rather than an increase in the α-diversity of individual communities. Moisture and pH controlled most of the differences in composition between communities. Measures of species richness and correlations with moisture and pH within communities differed among vascular, bryophyte, and lichen species. Bryophyte richness was positively correlated with moisture (r²= 0.862, P= 0.0010) and lichen richness was negatively correlated with moisture (r²= 0.809, P= 0.0031). Vascular plants had a peak in richness at pH 6.5 (r²= 0.214, P < 0.0001). We conclude that site variation in vascular richness in this region is controlled by landscape heterogeneity, and structured as variation in the number and distinctiveness of recognizable plant communities.     

Environmental factors influencing spatial patterns of shrub diversity in chaparral,Santa Ynez Mountains,California

We examined patterns of shrub species diversity relative to landscape‐scale variability in environmental factors within two watersheds on the coastal flank of the Santa Ynez Mountains, California. Shrub species richness and dominance was sampled at a hierarchy of spatial units using a high‐powered telescope from remote vantage points. Explanatory variables included field estimates of total canopy cover and percentage rock cover, and modeled distributions of slope, elevation, photosynthetically active radiation, topographic moisture index, and local topographic variability. Correlation, multiple regression, and regression tree analyses showed consistent relationships between field‐based measurements of species richness and dominance, and topographically‐mediated environmental variables. In general, higher richness and lower dominance occurred where environmental conditions indicated greater levels of resource limitation with respect to soil moisture and substrate availability. Maximum richness in shrub species occurred on high elevation sites with low topographic moisture index, rocky substrate, and steep slopes. Maximum dominance occurred at low elevation sites with low topographic variability, high potential solar insolation, and high total shrub canopy cover. The observed patterns are evaluated with respect to studies on species‐environment relations, resource use, and regeneration of shrubs in chaparral and coastal sage scrub. The results are discussed in the context of existing species‐diversity hypotheses that hinge on reduced competitive dominance and increased resource heterogeneity under conditions of resource limitation.     

Plot shape effects on plant species diversity measurements

Abstract. Question: Do rectangular sample plots record more plant species than square plots as suggested by both empirical and theoretical studies? Location: Grasslands, shrublands and forests in the Mediterranean‐climate region of California, USA. Methods: We compared three 0.1‐ha sampling designs that differed in the shape and dispersion of 1‐m² and 100‐m² nested subplots. We duplicated an earlier study that compared the Whittaker sample design, which had square clustered subplots, with the modified Whittaker design, which had dispersed rectangular subplots. To sort out effects of dispersion from shape we used a third design that overlaid square subplots on the modified Whittaker design. Also, using data from published studies we extracted species richness values for 400‐m² subplots that were either square or 1:4 rectangles partially overlaid on each other from desert scrub in high and low rainfall years, chaparral, sage scrub, oak savanna and coniferous forests with and without fire. Results: We found that earlier empirical reports of more than 30% greater richness with rectangles were due to the confusion of shape effects with spatial effects, coupled with the use of cumulative number of species as the metric for comparison. Average species richness was not significantly different between square and 1:4 rectangular sample plots at either 1‐ or 100‐m². Pairwise comparisons showed no significant difference between square and rectangular samples in all but one vegetation type, and that one exhibited significantly greater richness with squares. Our three intensive study sites appear to exhibit some level of self‐similarity at the scale of 400 m², but, contrary to theoretical expectations, we could not detect plot shape effects on species richness at this scale. Conclusions: At the 0.1‐ha scale or lower there is no evidence that plot shape has predictable effects on number of species recorded from sample plots. We hypothesize that for the mediterranean‐climate vegetation types studied here, the primary reason that 1:4 rectangles do not sample greater species richness than squares is because species turnover varies along complex environmental gradients that are both parallel and perpendicular to the long axis of rectangular plots. Reports in the literature of much greater species richness recorded for highly elongated rectangular strips than for squares of the same area are not likely to be fair comparisons because of the dramatically different periphery/area ratio, which includes a much greater proportion of species that are using both above and below‐ground niche space outside the sample area.     

Predicting mountain plant richness and rarity from space using satellite-derived vegetation indices

Can species richness and rarity be predicted from space? If satellite‐derived vegetation indices can provide us with accurate predictions of richness and rarity in an area, they can serve as an excellent tool in diversity and conservation research, especially in inaccessible areas. The increasing availability of high‐resolution satellite images is enabling us to study this question more carefully. We sampled plant richness and rarity in 34 quadrats (1000 m²) along an elevation gradient between 300 and 2200 m focusing on Mount Hermon as a case study. We then used 10 Landsat, Aster, and QuickBird satellite images ranging over several seasons, going up to very high resolutions, to examine the relationship between plant richness, rarity, and vegetation indices calculated from the images. We used the normalized difference vegetation index (NDVI), one of the most commonly used vegetation indexes, which is strongly correlated to primary production both globally and locally (in more seasonal and in drier and/or colder environments that have wide ranges of NDVI values). All images showed a positive significant correlation between NDVI and both plant species richness and percentage tree cover (with R² as high as 0.87 between NDVI and total plant richness and 0.89 for annual plant richness). The high resolution images enabled us to examine spatial heterogeneity in NDVI within our quadrats. Plant richness was significantly correlated with the standard deviation of NDVI values (but not with their coefficient of variation) within quadrats and between images. Contrary to richness, relative range size rarity was negatively correlated with NDVI in all images, this result being significant in most cases. Thus, given that they are validated by fieldwork, satellite‐derived indices can shed light on richness and even rarity patterns in mountains, many of which are important biodiversity centres.     

An improved approach for remotely sensing water stress impacts on forest C uptake

Given that forests represent the primary terrestrial sink for atmospheric CO₂, projections of future carbon (C) storage hinge on forest responses to climate variation. Models of gross primary production (GPP) responses to water stress are commonly based on remotely sensed changes in canopy ‘greenness’ (e.g., normalized difference vegetation index; NDVI). However, many forests have low spectral sensitivity to water stress (SSWS) – defined here as drought‐induced decline in GPP without a change in greenness. Current satellite‐derived estimates of GPP use a vapor pressure deficit (VPD) scalar to account for the low SWSS of forests, but fail to capture their responses to water stress. Our objectives were to characterize differences in SSWS among forested and nonforested ecosystems, and to develop an improved framework for predicting the impacts of water stress on GPP in forests with low SSWS. First, we paired two independent drought indices with NDVI data for the conterminous US from 2000 to 2011, and examined the relationship between water stress and NDVI. We found that forests had lower SSWS than nonforests regardless of drought index or duration. We then compared satellite‐derived estimates of GPP with eddy‐covariance observations of GPP in two deciduous broadleaf forests with low SSWS: the Missouri Ozark (MO) and Morgan Monroe State Forest (MMSF) AmeriFlux sites. Model estimates of GPP that used VPD scalars were poorly correlated with observations of GPP at MO (r² = 0.09) and MMSF (r² = 0.38). When we included the NDVI responses to water stress of adjacent ecosystems with high SSWS into a model based solely on temperature and greenness, we substantially improved predictions of GPP at MO (r² = 0.83) and for a severe drought year at the MMSF (r² = 0.82). Collectively, our results suggest that large‐scale estimates of GPP that capture variation in SSWS among ecosystems could improve predictions of C uptake by forests under drought.     

Why Pine Barrens Restoration Should Favor Barrens Over Pine

Pine barrens include an assortment of pyrogenic plant communities occurring on glacial outwash or rocky outcrops scattered along the Atlantic coastal plain from New Jersey to Maine, and inward across New England, New York, Pennsylvania, and the northern Great Lakes region. At least historically, pine barrens provided some of the highest quality terrestrial shrublands and young forests in the eastern North American sub‐boreal and northern temperate region. However, the mosaic open‐canopy, sparse‐shrub, and grassland early successional state is generally lacking in contemporary pine barrens. Many sites in the northeastern United States have converted to overgrown scrub oak (Quercus ilicifolia, Quercus prinoides) thickets and closed canopied pitch pine (Pinus rigida)‐dominated forests. Thinning pitch pine is a contentious issue for the imperiled pitch pine‐scrub oak barrens community type (G2 Global Rarity Rank, 6–20 occurrences). Here we provide a historical, ecological, and resource management rationale for thinning pitch pine forest to restore savanna‐like open barrens with a mosaic of scrub oaks, heath shrubs, and prairie‐like vegetation. We postulate that the contemporary dominance of pitch pine forest is largely of recent anthropogenic origin, limits habitat opportunities for at‐risk shrubland fauna, and poses a serious wildfire hazard. We suggest maintaining pitch pine‐scrub oak barrens at 10–30% average pitch pine cover to simultaneously promote shrubland biodiversity and minimize fire danger.     

The role of soil seed banks in the restoration of dry acidic dune grassland after burning of Ulex europaeus scrub

Question: Can the seed bank play a significant role in the restoration of plant communities of dry acidic dune grassland where fire has destroyed Ulex europaeus scrub? Location: Northern French Atlantic coast. Methods: One year after the fire, the seed bank and vegetation were sampled in 1 m × 1 m plots along three transects from the oldest scrub vegetation towards the grassland. Differences in species richness, seed density and contribution of ecological groups in the seed bank and vegetation along the transects were analysed. Results: Seed density and species richness in the seed bank decreased significantly from the grassland towards the centre of the scrub vegetation; 50% of the seed bank consisted of core species of the target plant community, such as Carex arenaria, Aira praecox, Rumex acetosella and Agrostis capillaris. Seeds of these species were also found in the deeper soil layers beneath the oldest scrub vegetation, indicating that they can be considered to be long‐term persistent. Beneath the youngest scrub vegetation, seeds of rare satellite target species also occurred. However, no target species were established on the burned site after one year, resulting in a large discrepancy between seed bank and vegetation. Conclusions: Although the seeds present in the soil indicate that restoration of the acidic grassland based on the seed bank is possible, additional management actions such as mowing and soil disturbance may be necessary to restrict resprouting of Ulex and to stimulate the germination of seeds of target species in the deeper soil layers.     

Relating rainfall and vegetation greenness to the biology of spur‐throated and Australian plague locusts

• 1 Rainfall and vegetation greenness are widely claimed to influence fat content and egg development in locusts. Body size, abdominal fat and egg length of spur‐throated Austracris guttulosa and Australian plague Chortoicetes terminifera locusts were related to the normalized difference vegetation index (NDVI; a greenness indicator) to test these assumptions and quantify biological responses.
• 2 Cumulative rainfall was highly correlated with NDVI values of an area (25 × 25 km) surrounding the trap to which locusts were attracted. Catches of locusts were greater in hotter periods preceding rain, associated with an increase in NDVI indicative of brown (0.298) versus green (0.465) vegetation.
• 3 Pre‐overwintering descendent A. guttulosa were larger and fattier than post‐overwintering antecedents. Egg maturation of antecedents occurred coincident with increasing NDVI. Higher abdominal fat in pre‐overwintering locusts was positively associated with higher NDVI.
• 4 Male C. terminifera of two descendent generations were larger than their antecedents but only pre‐overwintering locusts were significantly fattier than their predecessors. Only pre‐overwintering females were significantly larger than their predecessors and no generation was significantly fattier than any other. Fat content was negatively correlated with egg maturation and differed significantly with NDVI, although strong and consistent relationships were not obtained.
• 5 The findings suggest that locust–environment interactions are species‐ and habitat‐specific. If A. guttulosa hoppers develop during periods when regional NDVI exceeds 0.4, fatter adults will arise. The elucidation of relationships between greenness, fat and egg development in C. terminifera is unlikely using NDVI values from areas comprising mixtures of monocotyledonous and dicotyledonous vegetation.

     

Understory vegetation response to thinning disturbance of varying complexity in coniferous stands

Question: Can augmented forest stand complexity increase understory vegetation richness and cover and accelerate the development of late‐successional features? Does within‐stand understory vegetation variability increase after imposing treatments that increase stand structural complexity of the overstory? What is the relative contribution of individual stand structural components (i.e. forest matrix, gaps, and leave island reserves) to changes in understory vegetation richness? Location: Seven study sites in the Coastal Range and Cascades regions of Oregon, USA. Methods: We examined the effects of thinning six years after harvest on understory plant vascular richness and cover in 40‐ to 60‐year‐old forest stands dominated by Douglas‐fir (Pseudotsuga menziesii). At each site, one unthinned control was preserved and three thinning treatments were implemented: low complexity (LC, 300 trees ha^?1), moderate complexity (MC, 200 trees ha^?1), and high complexity (HC, variable densities from 100 to 300 trees ha^?1). Gaps openings and leave island reserves were established in MC and HC. Results: Richness of all herbs, forest herbs, early seral herbs and shrubs, and introduced species increased in all thinning treatments, although early seral herbs and introduced species remained a small component. Only cover of early seral herbs and shrubs increased in all thinning treatments whereas forest shrub cover increased in MC and HC. In the understory, we found 284 vascular plant species. After accounting for site‐level differences, the richness of understory communities in thinned stands differed from those in control stands. Within‐treatment variability of herb and shrub richness was reduced by thinning. Matrix areas and gap openings in thinned treatments appeared to contribute to the recruitment of early seral herbs and shrubs. Conclusions: Understory vegetation richness increased 6 years after imposing treatments, with increasing stand complexity mainly because of the recruitment of early seral and forest herbs, and both low and tall shrubs. Changes in stand density did not likely lead to competitive species exclusion. The abundance of potentially invasive introduced species was much lower compared to other plant groups. Post‐thinning reductions in within‐treatment variability was caused by greater abundance of early seral herbs and shrubs in thinned stands compared with the control. Gaps and low‐density forest matrix areas created as part of spatially variably thinning had greater overall species richness. Increased overstory variability encouraged development of multiple layers of understory vegetation.     

Ectomycorrhizal, vesicular-arbuscular and dark septate fungal colonization of bishop pine (Pinus muricata) seedlings in the first 5 months of growth after wildfire

 We followed the colonization frequency of ectomycorrhizal (EM), vesicular-arbuscular mycorrhizal (VAM), and dark septate (DS) fungi in 1- to 5-month-old bishop pine seedlings reestablishing after a wildfire. Seedlings were collected on a monthly basis at either a VAM-dominated chaparral scrub site or an EM-dominated forest site, both of which were burned. In both vegetation types, fully developed EM were observed from the third month after germination. EM fungi observed on the seedlings from the scrub site were limited to Rhizopogon subcaerulescens, R. ochraceorubens and Suillus pungens. Seedlings from the forest were colonized by a greater variety of EM fungi including Amanita spp., Russula brevipes and a member of the Cantharellaceae. VAM structures (vesicles, arbuscules or hyphal coils) were observed in the seedling root systems beginning 1 month after germination at the scrub site and 3 months after germination at the forest site. Seedlings from the scrub site consistently had more frequent VAM fungal colonization than those from the forest site through the fifth month after germination. DS fungi were observed in most seedlings from both the scrub and forest sites beginning in the first month post-germination. We propose that these fungi survived as a resident inoculum in the soils and did not disperse into the sites after the fire. Accepted: 14 February 1998     

Spatial characteristics of AVHRR‐NDVI along latitudinal transects in northern Alaska

Abstract. Two‐weekly AVHRR images were used to examine spatial patterns of the normalized difference vegetation index (NDVI) and their relationships with environmental variables for moist acidic tundra (MAT) and moist non‐acidic tundra (MNT) along two latitudinal transects in northern Alaska. The NDVI database was derived from a 5‐yr time series (1995–1999) of two‐weekly AVHRR composites for Alaska. A digital climate map, digital elevation map and vegetation map were processed and overlain with the NDVI grid. Homogeneous vegetation patches for both MAT and MNT were defined as sample sites using infrared aerial photos, MSS images and the vegetation map along the transects. Linear and non‐linear regression modeling were performed between NDVI indices and environmental variables, total summer warmth (TSW) and elevation. It was demonstrated that along both western and eastern transects, there were obvious latitudinal trends of peak NDVI (AP‐NDVI), average growing season NDVI (GS‐NDVI), and early June NDVI (EJ‐NDVI). In most cases, MNT had lower NDVI values than MAT throughout the year. There were significant (p<0.01) relations between NDVI (AP‐NDVI, GS‐NDVI and EJ‐NDVI) and total summer warmth (TSW) and elevation in the region. EJ‐NDVI showed the strongest correlation with TSW or elevation, making it the most sensitive NDVI indicator along environmental gradients in northern Alaska. NDVI was likely controlled by TSW and elevation, with the former being dominant.     

THE ORIGIN OF COASTAL SAGE VEGETATION,ALTA AND BAJA CALIFORNIA

Coastal sage is a new pioneer-type vegetation that only spread widely after the Early Quaternary, when species on the dry open borders of forest, woodland and arid tropic scrub vegetation shifted into expanding dry sites there and in adjacent grasslands. These new sites were created by a coincidence of major climatic and tectonic events and by accompanying erosion and mass movement on steep new slopes. Attaining most of its present area during the hot, dry Xerothermic, coastal sage scrub spread further as man's activities disturbed the landscape.     

The Influence of Vegetation Height Heterogeneity on Forest and Woodland Bird Species Richness across the United States

Avian diversity is under increasing pressures. It is thus critical to understand the ecological variables that contribute to large scale spatial distribution of avian species diversity. Traditionally, studies have relied primarily on two-dimensional habitat structure to model broad scale species richness. Vegetation vertical structure is increasingly used at local scales. However, the spatial arrangement of vegetation height has never been taken into consideration. Our goal was to examine the efficacies of three-dimensional forest structure, particularly the spatial heterogeneity of vegetation height in improving avian richness models across forested ecoregions in the U.S. We developed novel habitat metrics to characterize the spatial arrangement of vegetation height using the National Biomass and Carbon Dataset for the year 2000 (NBCD). The height-structured metrics were compared with other habitat metrics for statistical association with richness of three forest breeding bird guilds across Breeding Bird Survey (BBS) routes: a broadly grouped woodland guild, and two forest breeding guilds with preferences for forest edge and for interior forest. Parametric and non-parametric models were built to examine the improvement of predictability. Height-structured metrics had the strongest associations with species richness, yielding improved predictive ability for the woodland guild richness models (r² = ∼0.53 for the parametric models, 0.63 the non-parametric models) and the forest edge guild models (r² = ∼0.34 for the parametric models, 0.47 the non-parametric models). All but one of the linear models incorporating height-structured metrics showed significantly higher adjusted-r² values than their counterparts without additional metrics. The interior forest guild richness showed a consistent low association with height-structured metrics. Our results suggest that height heterogeneity, beyond canopy height alone, supplements habitat characterization and richness models of forest bird species. The metrics and models derived in this study demonstrate practical examples of utilizing three-dimensional vegetation data for improved characterization of spatial patterns in species richness.     

Dynamics of understory vegetation in an old-growth boreal coniferous forest, 1988–1993

Abstract. Understorey vegetation changes in a South Norwegian old-growth coniferous forest were studied between 1988 and 1993 in 200 1-m² vegetation plots. Our aims were to quantify the amount of between-year compositional change, and to elaborate the environmental basis for long-term vegetation change, including the previously identified gradient structure with a major gradient related to topography (and soil nutrient status and soil depth) and a minor gradient reflecting paludification and canopy coverage. Species richness (yearly mean and cumulative species number) and change in species richness differed between vascular plants and cryptogams, and between forest types. The number of vascular plant species decreased in pine forest in dry years; bryophyte species number increased in spruce forest. Statistically significant vegetation change, as tested by constrained ordination (CCA) with time as the constraining variable, is demonstrated for most one-year periods and for the five-year period in most forest types. Vegetation change along identified gradients, measured as plot displacement along DCA ordination axes, also occurred. The magnitude of year-to-year vegetation change was related neither to forest type nor to one-year period; different responses to climatic and environmental change were observed in each forest type. The largest average displacement observed, from medium-rich spruce forest towards poor spruce forest, was interpreted as a long-term trend. Humus-layer pH decreased by ca. 0.25 units from 1988 to 1993, most strongly in medium-rich spruce forest where exchangeable Ca decreased and Al and Mn increased strongly. Our study supports the hypothesis that vascular plants show a long-term and broad-scale response to soil acidification. Change in bryophyte composition is linked to some very long growing-seasons. Detailed analysis of short-term vegetation dynamics enhances the interpretation of long-term changes and stresses the complementarity of univariate and multivariate methods in the analysis of vegetation change.