Xeric Slopes Research Articles

Historical patterns of oak population expansion in the Chautauqua Hills, Kansas

AbstractAimRates of tree population expansion have increased in many North American landscapes that were mosaics of grasslands, savannas and woodlands historically. Consequences of woodland expansion include reduced economic return from grazing and changes in native biodiversity, but causes of woodland expansion are poorly understood. We address historical timing of blackjack oak (Quercus marilandica) and post oak (Quercus stellata) population expansion in tree–grass ecosystems, roles of climate and land use in driving this expansion, and future stability of these woodlands.LocationThe Cross Timbers ecosystem in Kansas, USA.MethodsUsing increment cores, we quantified blackjack oak and post oak age structures in four woodlands on sites that were not wooded in the 1860s. We compared timing of oak regeneration with climate fluctuations (using the Palmer drought severity index) and land‐use history. We quantified tree species composition within 5‐m radii of sampled oaks.ResultsRecruitment of both oak species increased between 1925 and 1945. Modal age classes recruited from 1935 to 1960. Recruitment was associated with dry intervals at the two sites with north‐ or east‐facing aspects. This association was driven by blackjack oak recruitment in dry intervals. Woodlands on the sites with south‐ and west‐facing aspects contained only oaks, whereas those on the sites with north‐ and east‐facing aspects contained saplings of fire‐intolerant, shade‐tolerant tree species.Main conclusionsOur results contribute to growing evidence for woodland expansion in the region during dry climate intervals. The association between drought and recruitment was influenced by slope aspect and was more pronounced in the less fire‐tolerant oak species. Although woodland expansion coincided with regional increases in fire frequency, drought and greater use of prescribed burning are likely to have reduced fire intensities by reducing fuel loads. These oak woodlands, which have developed during the 20th century, appear to form stable communities on xeric slopes but to be undergoing succession towards a mesophytic tree community on mesic slopes.

Morphology as Clue to Developmental Regulation: Stomata

Incipient stomata might have appeared in thalloid alga-like land plants as sporophytic structures homologous to gametangial conceptacles of their isomorphic gametophytes and developed in association with vascular tissue and cuticle. Historically, stomatal evolution is correlated with growth habits and synecological events like the early terrestrial plant expansion from wetland to dryland habitats or the appearance of arboreal and grass canopies. The general tendency of elaboration and aggregation of stomatal complexes was reversed with reduction of vegetative growth at such phylum initiating events as the origin of angiosperms. Stomata links ambient environment with leaf surface air layer and the air of intercellular spaces, balancing the reduction and oxidation reactions involved in photosynthesis. Adaptive responses to environmental change require adjustments of transpiration rates through stomatal density regulation, such as revealed in Ricotia lunaria ecotypes of mesic and xeric slopes of a canyon, with the differences in Stomatal Index comparable to those obtained by experimental exposure to mildly elevated CO2 levels. Long term responses include morphological elaboration of stomatal complexes and their aggregates providing for developmental stability, scarcely impaired by occasional aberrations. Distribution of stomatal complexes correlates with vascularizaton and wax biosynthesis controlled by auxin and ABA hormones respectively at the cell differentiation level. Stomatal transcription factors respond to the interfering auxin/brassinosteroid/ABA signaling mediated by kinaze receptors and their ligands. Stomatal developmental feedbacks are probably conferred by the pH activated protease and oxidative stress activated (’mitogen activated’) protein kinase cascades. This way the hormonal response to environmental pressure is converted into the chemical free energy potential of phosphorylation contributing to internal energy of structural innovation. Universality of this scheme may explain correlation of stomatal development with the whole plant growth habit, ecology and evolution.

Slope aspect influences plant association patterns in the Mediterranean matorral of central Chile

The most important processes shaping communities are likely to leave observable imprints on their spatial structure. The net outcome of positive and negative interactions among plants has been proposed to vary inversely with environmental harshness. In the Mediterranean-type matorral of central Chile, slope aspect strongly influences mesoclimatic conditions: equatorial-facing slope are xeric and are dominated by spiny shrubs and cacti, while evergreen sclerophyllous trees dominate polar-facing slopes due to their more mesic conditions. Here we use spatial pattern analyses to infer on the interactions that shape community structure in this matorral. We sampled the vegetation on two polar- and equatorial-facing slopes and assessed the dominant interaction at each slope through: (1) a species co-occurrence index ( C-score) for the entire community; (2) the frequencies of negative and positive associations between species pairs; and (3) the species richness at each slope. Our results indicated that negative spatial associations dominated plant assemblages on all slopes, but the importance of negative associations decreased from mesic (∼30%) to xeric slopes (∼16%). Species richness was higher on xeric than on mesic slopes. Although competition seems to be the main biotic force structuring communities on both mesic and xeric slopes, we suggest that the decrease in importance of negative interactions from mesic to xeric habitats could produce an increase in diversity.

SEED SIZE, NUMBER, AND HABITAT OF A FLESHY-FRUITED PLANT: CONSEQUENCES FOR SEEDLING ESTABLISHMENT

I examined the consequences of local environmental variation on the seedling establishment of Prunus virginiana, common chokecherry. In western Montana, USA, this species produces large crops of small seeds in mesic riparian habitats and small crops of large seeds in xeric slope habitats. From 1996 to 1999, I manipulated seed and seedling distributions. I altered seed size, density, distance from adult canopies, habitat, degree of protection from seedling predators, and the amount of soil moisture. In riparian habitats, seed dispersers tended to deposit seeds away from fruit tree canopies, and seedling establishment was greater away from canopies than under them. In both riparian and slope habitats, large seeds were more likely to be depredated than small seeds. However, seed and seedling depredation were more intense in riparian habitats. In slope habitats, seedling establishment was greatest under fruit tree canopies, where seed deposition by frugivores was most likely. In this habitat, large seeds produced seedlings with higher growth rates, and desiccation, rather than depredation, was the predominant cause of seedling mortality. In slope habitats, seedlings from small seeds experienced reduced survival when I did not provide supplemental water. On slopes, the importance of large seed size for seedling establishment may override the negative effects of size on dispersal likelihood. Because the canopy of slope plants may have overall positive effects on establishment, dispersal may be relatively unimportant or even undesirable. In contrast, in riparian habitats, the production of small, highly dispersible seeds may be critical because it reduces the negative effects of parental canopies.

Variation in vegetation and seed bank in a Chilean semi‐arid community affected by ENSO 1997

Abstract. We followed plant cover and soil seed density of shrubs and herbs in two markedly contrasting years with regard to annual rainfall in two opposite‐facing slopes (mesic vs xeric) and an intervening, relatively wet, ravine of a typical creek in semi‐arid Chile. During the ENSO year 1997 cover of ephemerals increased in all three sites; 43% vs 8% on the xeric slope 75% vs 26% on the mesic slope and 57% vs 32% in the ravine. The number of species was almost three times higher on the xeric slope (27 vs 10 species), increased by 47% on the mesic slope (28 vs 19) and by 14% in the ravine (24 vs 21). Cover of shrubs plus perennial grasses increased from 52% in 1996 to 59% in 1997 on the xeric slope, but no significant cover changes were found on the mesic slope or the ravine. On the xeric slope peak density of seeds was 4500/m2 in 1996, while during the ENSO year it was 24000/m2. On the mesic slope equivalent values were 3000 and 17000 seeds/m2 while in the ravine figures were 8500 and 27000 seeds/m2. The strong responses of ephemerals and seed bank to the ENSO‐driven wet year of 1997 demonstrate the importance of this phenomenon in replenishing, recovering and probably maintaining ephemeral vegetation in this region.

The relationships between vegetation type and topography in Lassen Volcanic National Park

The relationships between topographic features and the relative abundances of vegetation types were analyzed for the mid-montane conifer forests, the upper montane conifer forests and the alpine areas of Lassen Volcanic National Park in the Cascade Range of northern California using Landsat Thematic Mapper data. Vegetation types were identified from Landsat Thematic Mapper data and included pine, fir, montane chaparral, willow/alder and herbaceous communities. Topographic features including aspect, slope steepness, slope configuration (i.e., convex, straight or concave) and slope position (i.e., ridge top to valley bottom) were computed from USGS Digital Elevation Models. The distribution of montane chaparral was consistent with its dependency on fire with chaparral communities being more abundant on more xeric aspects and on steeper and more xeric slope configurations. Slope position was important in determining the distribution of willow/alder communities which were more abundant in valley bottom locations. Both slope steepness and slope position affected the abundance of herbaceous communities. Herbaceous communities were: (1) more prevalent on steeper slopes due to the distribution of dicotyledonous forb species; and (2) more abundant in valley bottoms due to the distribution of monocotyledonous species in wet meadows. Pines and firs were separated along a gradient of slope steepness with the firs being more abundant on steep slopes and the pines being more abundant on shallow slopes. In the alpine areas above 2350 m, the abundance of most vegetation types declined with increasingly mesic aspects. This decline may be related to the prolonged persistence of snow in areas of less intense solar radiation.

Spatial Variation in Diameter Structures of Forests in Lassen Volcanic National Park, California∗

Variation in seedling/sapling densities and stand diameter forms for six coniferous tree species is related to stand structural development and to elevation and topography in Lassen Volcanic National Park, California. Understory density patterns reflect differences in species tolerance; densities decrease with stand development for shade intolerant pines, but increase for shade tolerant firs and mountain hemlock. Pine species exhibit reverse-J diamter structures on recently disturbed sites, and decreaser and random forms elsewhere. More tolerant fir species show topographically mediated patterns of diameter structure, with reverse-J form common on northerly exposures and upland sites, but with decreaser and random forms on more xeric slopes. Interaction among species tolerance, environmental setting, and disturbance history yields a complex mosaic of stand diameter structures in the Lassen landscape.

Litter Decomposition, Nitrogen Dynamics and Litter Microarthropods in a Southern Appalachian Hardwood Forest 8 Years Following Clearcutting

.' SUMMARY (1) Litter decomposition rates, nitrogen dynamics and litter microarthropods on the xeric slopes of a watershed 8 years after clearcutting (WS 7) and on an adjacent reference watershed (WS 2) at the Coweeta Hydrologic Laboratory were measured using litterbags. Litter of Comusflorida L., Acer rubrum L. and Quercusprinus L. was used as experimental substrates in three plots on adjacent areas of both watersheds. Results from this study were compared with those obtained before and after cutting to assess the longer-term changes induced by canopy removal. (2) Reduced litter decomposition rates were associated with clearcutting. Litter nitrogen dynamics were also affected. Net immobilization of nitrogen in litter substrates was lower on WS 7. Lower net nitrogen immobilization was related to slower increases in nitrogen concentration per unit mass lost in litter on WS 7. (3) Mean annual densities of total litter microarthropods remained 28% lower on WS 7 than on WS 2, 8 years after cutting. Previous studies indicated clearcutting initially reduced mean annual densities of litter microarthropods by>50%. Mesostigmata and Oribatei densities averaged 50 and 54% lower, respectively, than on WS 2. Prostigmata and Collembola densities averaged 20 and 24% lower, respectively, than on WS 2. This differential response changed the relative abundances of major groups. Changes in litter decomposition rates and nitrogen dynamics were consistent with effects associated with lower microarthropod densities and suggest that reduced microarthropod densities may be important. (4) Results of this study differ from those at northern hardwood forest sites where clearcutting caused increased decomposition rates. This suggests that generalizations from northern hardwood forests may not apply to other regions. Instead the effects of canopy

Forest Dimensions and Production in the Great Smoky Mountains

Samples based on stand measurements and borings of trees, and clippings of undergrowth, were taken from 24 forests and forest heaths in the Great Smoky Mountains. Samples were taken also from heath balds and grassy balds and a California coastal redwood forest. Available information on ratios and regressions of forest tree and shrub dimensions were used to estimate stand biomass and net annual production above ground. Mature climax forests of mesic environments below 1,400 m are characterized by: wood basal areas of 50—64 m2/ha and basal area increments of 0.3—0.6 m2/ha/yr, stem wood volumes (parabolic estimate) of 750—900 m3/ha and estimated wood volume increments of 530—590 cm3/m2/yr, aboveground biomasses of 500—610 t/ha and aboveground net annual productions of 1,000—1,200 g/m2, and biomass accumulation ratios of 40—50. These and other stand dimensions decrease along the moisture gradient to xeric sites and decrease toward higher elevations. Above—ground net annual productions of forest heaths of xeric slopes and forests of highest elevations are 420—650 g/m2. Evergreen spruce—fir forests are more productive than deciduous forests above 1,400 m. Among mesic high—elevation beech and fir forests, production is higher on south slopes than on north slopes. Production and biomass of steady—state forests were estimated from multiple correlations using elevation and weighted—average index of site moisture as independent variables. Apart from some high—elevation stands, production is not significantly correlated with evergreen vs. deciduous foliage or with direction of exposure affecting incident sunlight. When unstable stands are excluded, production and biomass are strongly correlated with each other and with mean tree height and other stand dimensions. A wide range of temperate—zone climax forests of relatively favorable environments have net annual productions above and below ground of 1,200—1,500 g/m2. Productions of stable closed heath balds are lower, 700—1,200 g/m2, but productions of heath balds and less productive forests overlap broadly. Production of unstable forests, both successional stands and those opened by removal of large trees, exceeds that of steady—state forests of similar environments. Canopy coverage and light penetration are not strongly correlated with forest production. Light penetration to the herb stratum ranges from 1.4—7.0% of incident sunlight in more open forests and forest heaths to 0.3—0.9% in cove forests. Foliage live/dry ratios decrease along the moisture gradient from mesic to xeric stands–from about 5.0 to 2.8 in shrub clippings, 7.6 to 2.8 in herb clippings. Undergrowth production and biomass are trivial compared with the tree stratum in many forests. Shrub production is generally higher in xeric environments and is 20—145% of tree production in forest heaths. Herb production is higher at the extremes of the moisture gradient (exceeding 3% of total aboveground production in mesic and in open xeric stands) than in intermediate stands (below 1%). Apart from such differences, her and thallophyte biomass and production increase with elevation to maximum values in fir forests of the highest summits.