Ole R. Vetaas

Micro-site effects of trees and shrubs in dry savannas

. The physiognomy of dry savannas is described as a combination of discontinuous woody perennials and a continuous grassland matrix. Interactions between these two components are of vital importance for the persistence of a savanna landscape. Earlier savanna models have emphasized competitive interactions for water between the two components. Recent studies have argued that small-scale facilitating interactions between woody perennials and the herbaceous understorey are also important. This phenomenon has been given little theoretical consideration in the savanna literature, but it has been an important topic in agroforestry and arid-grassland ecology. This paper reviews some of the evidence for micro-site effects of trees and shrubs, and attempts to integrate their interactions with the surrounding open grassland. Woody perennials modify the microclimate by interception of solar radiation and rainfall. Their root systems extract nutrients horizontally and vertically, which are concentrated in the sub-canopy soil from litter decomposition and root turnover. Legumes are abundant in dry savannas, and may have symbiotic relationships with Rhizobium bacteria. This symbosis increases the availability of nitrogen in the soil. Isolated trees and shrubs initiate feedback mechanisms in their interactions with other organisms, and contribute to an uneven distribution of water and nutrients in dry savanna. This influences the species composition, and community diversity. Small-scale facilitating interaction between the woody and herbaceous components and competitive interaction on larger scales, are complementary processes which together explain a dynamic coexistence.

Species Richness and Altitude: A Comparison between Null Models and Interpolated Plant Species Richness along the Himalayan Altitudinal Gradient, Nepal

We compare different null models for species richness patterns in the Nepalese Himalayas, the largest altitudinal gradient in the world. Species richness is estimated by interpolation of presences between the extreme recorded altitudinal ranges. The number of species in 100‐m altitudinal bands increases steeply with altitude until 1,500 m above sea level. Between 1,500 and 2,500 m, little change in the number of species is observed, but above this altitude, a decrease in species richness is evident. We simulate different null models to investigate the effect of hard boundaries and an assumed linear relationship between species richness and altitude. We also stimulate the effect of interpolation when incomplete sampling is assumed. Some modifications on earlier simulations are presented. We demonstrate that all three factors in combination may explain the observed pattern in species richness. Estimating species richness by interpolating species presence between maximum and minimum altitudes creates an artificially steep decrease in species richness toward the ends of the gradient. The addition of hard boundaries and an underlying linear trend in species richness is needed to simulate the observed broad pattern in species richness along altitude in the Nepalese Himalayas.

THE EFFECT OF CANOPY DISTURBANCE ON SPECIES RICHNESS IN A CENTRAL HIMALAYAN OAK FOREST

Non-epiphytic species richness was studied in different disturbance classes within a Quercus semecarpifolia forest. Nine disturbance classes were defined according to the degree of biomass removal (lopping) and their spatial mixture. Six of these were observed in the study area. The species were divided into three functional groups: climbers, phanerophytes, and field-layer plants. The primary aim was to test if there is an elevated species richness under an intermediate disturbed canopy for (i) all vascular plants, (ii) lianas, (iii) phanerophytes and (iv) field-layer species. The richness of the different plant groups and all species were fitted against the disturbance gradient by means of Generalized Linear Models (GLM). Other environmental variables such as altitude, potential solar radiation, light intensity, canopy cover and soil parameters were also evaluated as predictors. Disturbance classes, canopy cover and light intensity were combined into a new variable, disturbance-complex, using Principal Component Analyses.Phanerophytes did not respond to any variable. Climbers were mostly related to pH and canopy cover, and were the only group related to altitude, nitrogen and loss-on-ignition. Herbaceous plants and total species richness showed a unimodal response to disturbance classes and the complex disturbance gradient, which supports the intermediate disturbance hypothesis. Relative radiation and slope also supported a unimodal response in herbaceous plants, but disturbance had a significant additional contribution to this pattern. The most significant predictor for these two groups was pH. The responses to organic carbon and phosphorus were not significant for any of the subsets.The results indicate that a small-scale lopping regime will enhance species richness of vascular plants; only a few species in the intermediate disturbed forest are weedy ruderals. In such a situation, the conservation policy may accept small-scale human impact as part of the forest landscape.

Primary Succession of Plant Assemblages on a Glacier Foreland-Bodalsbreen, Southern Norway

Primary successions on glacier forelands have been widely described as changes of plant assemblages, although many studies show that the assemblages may be more apparent than real. Some doubt has also been cast on the occurrence of successional convergence on deglaciated terrain. Vegetation data from five dated terminal moraines in southern Norway, created between 1750 and 1930, were classified by TWINSPAN. The distinctness of the TWINSPAN groups and their relation to the spatial and temporal gradients were analysed by Canonical Correspondence Analysis. The successional sequence can be described as a physiognomic development; from cryptogam-mats, to a heath-phase, which may then be over-grown by Betula shrubs that develop into a sub-alpine Betula pubescens forest. Most species (79%) in the Racomitrium canescens and Stereocaulon assemblage on the youngest moraine are also found in the Betula pubescens forest on the oldest moraine. However, the establishment of some species seems to be facilitated by other species. The floristic difference between the TWINSPAN groups decreases with time suggesting that successional convergence has occurred. The spatial micro-variation is reduced once a closed canopy of Betula is established on the oldest moraine, but there are clear variations related to moraine positions on the other moraines.

RELATIONSHIP BETWEEN PLANT SPECIES RICHNESS AND BIOMASS IN AN ARID SUB-ALPINE GRASSLAND OF THE CENTRAL HIMALAYAS, NEPAL

The hump-shaped relationship between plant species richness and biomass is commonly observed at fine scale for herbaceous vegetation in temperate climates. This relationship predicts that herbaceous species richness is highest at an intermediate level of biomass that corresponds to moderate competition or disturbance. However, this relationship has not previously been investigated in high arid sub-alpine mountain grasslands. We tested the humped-back prediction in the arid Trans-Himalayan mountain grassland with a seasonal grazing system. The study area is located in the bottom of a U-shaped valley, in the Manang district (3500 m a.s.l.). We sampled two hundred plots (1m × 1m) in two different types of pastures: common pasture and old field, which both have similar grazing practices. There was a significant unimodal relationship between species richness and biomass only in the common pasture, and when the two sites were analyzed together. The species turnover is estimated by DCA in standard deviation unit. The turnover was lower in the old field than in the common pasture. The unimodal relationship between plant species richness and biomass did not disappear after accounting for unknown environmental gradients expressed as DCA (detrended correspondence analysis) axes and spatial variables. The species richness is highest at 120 ± 40 g/m2. The results indicate that a hump-shaped relationship is also found in arid Trans-Himalayan grasslands.

Relationships between floristic gradients in a primary succession

. Correlations between five floristic gradients at small spatial extents (10 - 20 m) and one successional gradient over a larger spatial extent (1.5 km) are analysed. Floristic data (62 taxa) were sampled on five terminal moraines of known age deposited after the ‘Little Ice Age’ (1750–1930). The floristic gradients on the moraine ridges were sampled by two or three transects on each moraine, and the successional gradient by 114 plots on all moraines. The sequential orders of species were determined by constrained and unconstrained Correspondence Analysis. The similarities between species order on the successional gradient and the exposure gradients at the same age were tested against the null hypothesis of no correlation. The null hypothesis was rejected using both Monte Carlo permutation tests and Spearmans rank correlations, except on the oldest moraine. Here a closed canopy has developed, which eliminates the environmental variability associated with ridge morphology. The similarity between the successional gradient and the moraine-ridge gradients is attributed to variation in environmental severity, mainly caused by glacier wind and related factors such as temperature and moisture. Similarity was highest on the moraines in the middle of the glacier foreland, which have many successional stages present and have a relatively exposed relief. The distribution of growth/life-forms along gradients of small spatial extent are comparable to the successional gradient, but lichens, herbs and graminoids differ in their behaviour. The resemblance between species gradients at a small spatial extent and species gradients on a larger spatial extent is interpreted as an ecological self-similar pattern, where young and old substrate are linked to exposed-xeric and protected-mesic habitats, respectively.

Gradients in field-layer vegetation on an arid misty mountain plateau in the Sudan

. Earlier studies have described how moist, on-shore winds cause meso-scale vegetation patterns on arid mountains near the sea. However, all protruding objects such as trees, micro-relief, and hill slopes influence the distribution of sea-mist. The influence of the tree-canopy, aspect, and distance to the sea on the field-layer vegetation in montane savanna was investigated on 16 hills in the Red Sea Hills, at 34 - 38 km from the sea. At 32 sites, total field-layer cover, species cover, and species number were estimated in a sub-canopy plot and in a nearby open plot on seaward and leeward slopes. Cover and species number in the understorey are significantly higher than in the open. The difference is highest on seaward slopes. Detrended correspondence analysis reveals short species-axes of ca. 2 SD-units. Differences between plots are mainly in species cover. This fits a principal components ordination model. PCA and its constrained version RDA give concordant results. The explanatory variables, Tree-cover and Relative Radiation Index (aspect), have similar indirect influences on plants, and are significantly correlated with axis 1, which is interpreted as a moisture and temperature gradient. The moist seaward plots show an independent trend in species composition along axis 2, which correlates with distance to the sea. On a presence basis the variables, all representing different spatial separation, correlate on the first axis. Presumably, the species composition, at all spatial scales, is directly or indirectly related to the variation in temperature and moisture.

Comparing species temperature response curves: population density versus second‐hand data

. Descriptions of individual species responses to temperature are required in order to assess the impact of future global warming. The response of Rhododendron arboreum to estimated mean annual temperature was investigated in the Himalayas using General Additive Models (GAM) and Generalized Linear Models (GLM). The aim was to evaluate the consistency between the response in population density along elevation gradients versus response curves based on elevation data from herbarium specimens and vegetation surveys. The comparison was made with respect to (1) estimated temperature at the point of maximum response and (2) the shape of the response curves i.e. symmetric vs skewed. All data indicate a single optimum between 12.3 and 10.8°C. The difference is only 0.4°C between the optimum estimated from localities of herbarium specimens (frequency) and the population density data. The difference is larger (0.7°C) when the vegetation survey data are combined with the data from the herbarium specimens. However, the differences are small when the uncertainties in temperature estimation are taken into consideration. The response curves based on herbarium specimens and vegetation survey data (frequencies) are symmetric. A sigmoid response curve was estimated from herbarium specimens (binomial data). The population density along the elevation gradients was, to some extent, asymmetric. This may reflect the underlying biological structure, but sampling bias and the numerical analyses may also influence the results.

Establishment of Pinus Wallichiana on a Himalayan Glacier Foreland: Stochastic Distribution or Safe Sites?

ABSTRACT The establishment of tree seedlings in primary succession is thought to occur only after an adequate reserve of nutrients has accumulated in the soil. Individuals of Pinaceae are sometimes reported to grow on very recently deglaciated substrates. This study analyzed the colonization of a glacier foreland by Pinus wallichiana. Physical, chemical, and biotic aspects of potential and observed seedling microsites were analyzed with regression methods and tests for proportions. Microsites with intermediate to high moisture levels and alkaline nutrient-poor soils were found to be conducive to seedling establishment. The most recently deglaciated parts of the foreland have soils with little nutrients and high pH. There is a linear change in soil variables from low nutrient content and high pH at the most recently deglaciated parts to more nutrient-rich and neutral toward the pre-neoglacial moraines. Surrounding old-growth forests of Pinus wallichiana shed an abundance of seeds onto the foreland, are able to germinate and grow, and are predominant among the early pioneers, which makes this species an unusual pioneer of primary succession. Colonization by P. wallichiana is not restricted to particular safe sites. Even though individuals look chlorotic and stunted, they grow at near normal rates. Leaf discoloration of seedlings occurs in soils with high pH and low nitrogen content. P. wallichiana is also a canopy dominant on some of the oldest terrains and outside the foreland.

Does Regulated Land Use Allow Regeneration of Keystone Forest Species in the Annapurna Conservation Area, Central Himalaya

Abstract This quantitative study assesses the ecological impacts of varying degrees of forest use, describing community structure, population structure, and regenerative capacity of the understudied but much utilized Quercus semecarpifolia Sm. in a mixed oak and Rhododendron arboreum Sm. forest. The Middle Hills of the Himalayas have long traditions of mixed farming, with animal husbandry and agriculture as interdependent components. Major biomass demands come from grazing and collection of fodder and fuelwood. In forested areas lopped fodder is one of the main components, and as elevation increases, so does its importance. Human impact has been used to explain low regeneration of these evergreen oaks. The anthropogenic disturbance gradient spanned from highly disturbed savanna-like sites to minimally disturbed shaded sites dominated by rhododendron. The population structure showed a bell-shaped distribution with a pronounced under-representation of saplings throughout the forest. Data indicated the best regeneration in the least disturbed sites. The degree of lopping was the best explanatory variable for the distribution of recruits. Factors preventing trees from surviving the sapling phase may inhibit long-term regeneration more than factors causing high seedling mortality. Results indicate that regeneration of Q. semecarpifolia may suffer to the benefit of rhododendron, which escapes biotic stress due to its poor fodder and fuelwood qualities.