Savanna Grasses Research Articles

Variation in root system traits among African semi‐arid savanna grasses: Implications for drought tolerance

AbstractIn arid to semi‐arid grasslands and savannas, plant growth, population dynamics, and productivity are consistently and strongly limited by soil water and nutrient availability. Adaptive traits of the root systems of grasses in these ecosystems are crucial to their ability to cope with strong water and/or nutrient limitation and the increasing drought stress associated with ecosystem degradation or projected climate change. We studied 18 grass species in semi‐arid savanna of the Kalahari region of Botswana to quantify interspecific variation in three important root system traits including root system architecture, rhizosheath thickness and mycorrhizal colonization. Drought‐tolerant species and shorter‐lived species showed greater rhizosheath thickness and fine root development but lower mycorrhizal colonization compared to later successional climax grasses and those characteristic of wetter sites. In addition, there was a significant positive correlation between root fibrousness index and rhizosheath thickness among species and a weak negative correlation between root fibrousness index and mycorrhizal colonization. These patterns suggest that an extensive fine root system and rhizosheath development may be important complementary traits of grasses coping with drought conditions, the former aiding in the acquisition of water by the grass plant and the latter aiding in water uptake and retention, and reducing water loss in the rhizosphere. Within species, both rhizosheath development and mycorrhizal colonization were significantly greater in a wet year than in a year with below‐average precipitation. The observed patterns suggest that the primary benefit of rhizosheath development in African savanna grasses is improved drought tolerance and that it is a plastic trait that can be adjusted annually to changing environmental conditions. The functioning of mycorrhizal symbiosis is likely to be relatively more important in infertile savannas where nutrient limitation is higher relative to water limitation.

Overlap in soil water sources of savanna woody seedlings and grasses

ABSTRACTSeasonal availability of water is a key controlling factor in semi‐arid savanna vegetation structure, function and interactions. Understanding of woody plant interactions with grasses in savannas has long been underpinned by Walter's two‐layered niche differentiation hypothesis that postulates that grasses and trees source water from different depths. The Walter hypothesis persists in the literature, despite contrary evidence and a lack of quantitative empirical tests of the theory. We conducted a greenhouse experiment to determine the following: (1) whether tree seedlings and grasses obtain water from different depths on rocky and sandy soils; (2) whether interspecific competition affected tissue water content of clipped grasses; and (3) the influence of repeated grass clipping on soil moisture. Grass competition significantly reduced tree seedling rooting depth on both rocky and sandy substrates. Trees had significantly longer roots on rocky substrates than on sandy substrates for all combinations (trees only, trees with unclipped grasses and trees with clipped grasses). Results indicated a three‐tier soil moisture depletion pattern, with a top layer (15 cm) exclusively exploited by grasses, an intermediate zone (25–35 cm) utilised by both grass and tree seedling roots and deeper subsoil exclusively tapped by tree seedling roots. Our results are consistent with Walter's hypothesis, but we distinguished between three rather than two layers of tree and grass root interactions in acquiring soil moisture. Copyright © 2012 John Wiley & Sons, Ltd.

Perspective: Learning new tricks from old trees: revisiting the savanna question

perspectives ISSN 1948-6596 Learning new tricks from old trees: revisiting the savanna question Aristides Moustakas 1,* , Kerstin Wiegand 2 , Katrin M. Meyer 2 , David Ward 3 & Mahesh Sankaran 1,4 Institute of Integrative & Comparative Biology, Faculty of Biological Sci- ences, University of Leeds, Leeds LS2 9JT, UK Ecosystem Modelling, University of Gottingen, Gottingen, Germany School of Biological and Conservation Sciences, University of KwaZulu- Natal, Scottsville 3209, South Africa National Centre for Biological Sciences, Tata Institute of Fundamental Research, GKVK, Bellary Road, Bangalore 560065, India *Corresponding author. e-mail: arismoustakas@gmail.com; http://www.fbs.leeds.ac.uk/staff/ profile.php?tag=Moustakas_A Introduction Savannas are ecosystems comprising of a mixture of woody species (trees and bushes), grasses and forbs. They cover about a fifth of the global land surface and about half of the area of Africa, Aus- tralia and South America (Scholes and Archer 1997, Sankaran et al. 2004). Savannas are charac- terized by a continuous grass understorey and a discontinuous tree layer. A savanna, where trees and grasses co-dominate (often referred to as co- existence), may be viewed as an intermediate eco- system between grassland (grass dominance) and forest (tree dominance). Tree-grass ratios vary widely in savannas, with higher precipitation usu- ally leading to a more continuous tree layer (Sankaran et al. 2005). However, tree canopies in mesic savannas are still discontinuous enough, with significant understorey grass biomass for the system to be characterised as a savanna and not forest. There are several different savanna eco- regions worldwide, each containing different sub- sets of species and displaying substantial variation in physical and structural attributes (Scholes and Archer 1997, House et al. 2003). In this paper, we focus primarily on tropical and subtropical savan- nas, although some of the conclusions might be valid for a broader extent of savannas. Until the early 1990s, it was generally be- lieved that trees and grasses coexist because of a separation of rooting niches (Scholes and Archer 1997). This idea was based on Walter’s (1939) two-layer hypothesis, a version of niche separa- tion theory (Walker et al. 1981). According to this theory, water is the limiting factor for woody spe- cies as well as grasses. It was assumed that grasses were the better competitors for topsoil moisture (usually 30 cm). Topsoil is usually defined by its fast reaction to climatic re- gimes, getting wet during light rain, drying out with a few days of sunshine (Knoop and Walker 1985). The two-layer theory has been verified by field experiments which reported that in the Nylsvlei savanna (South Africa) grasses reduced the supply of water to the trees (Knoop and Walker 1985). Furthermore, subsoil competition between the trees, probably for water but possi- bly also for nutrients, was reported (Smith and Goodman 1986). Several studies have concluded that savanna stability is based on the two-layer theory (e.g. Knoop and Walker 1985, Skarpe There is a growing body of literature that questions the validity of the two-layer theory. A number of field studies have reported facilitative effects of trees on grass biomass, where grass bio- mass is greater below tree canopies rather than away from trees (e.g. Belsky et al. 1989). Among others, Ludwig et al. (2004) reported a field ex- periment where the two-layer theory is inappro- priate. More specifically, according to Ludwig et al. (2004) “prevention of tree–grass interactions frontiers of biogeography 2.2, 2010 — © 2010 the authors; journal compilation © 2010 The International Biogeography Society

Root dynamics influence tree–grass coexistence in an Australian savanna

AbstractBoth resource and disturbance controls have been invoked to explain tree persistence among grasses in savannas. Here we determine the extent to which competition for available resources restricts the rooting depth of both grasses and trees, and how this may influence nutrient cycling under an infrequently burned savanna near Darwin, Australia. We sampled fine roots <2 mm in diameter from 24 soil pits under perennial as well as annual grasses and three levels of canopy cover. The relative proportion of C3 (trees) and C4 (grasses) derived carbon in a sample was determined using mass balance calculations. Our results show that regardless of the type of grass both tree and grass roots are concentrated in the top 20 cm of the soil. While trees have greater root production and contribute more fine root biomass grass roots contribute a disproportional amount of nitrogen and carbon to the soil relative to total root biomass. We postulate that grasses maintain soil nutrient pools and provide biomass for regular fires that prevent forest trees from establishing while savanna trees, are important for increasing soil N content, cycling and mineralization rates. We put forward our ideas as a hypothesis of resource‐regulated tree–grass coexistence in tropical savannas.

Can the shrub Chromolaena odorata (Asteraceae) be considered as improving soil biology and plant nutrient availability?

This paper attempts to provide a new perception of the weed Chromolaena odorata (Asteraceae) which is considered as a plague in agriculture or as a soil fertility indicator. The study was conducted in the forest-savanna transition zone of Cote d’Ivoire and aimed to compare soil biological activity and plant nutrient availability under three well-represented vegetation features, including C. odorata lands (ChrO), shrub savannas (ShrS), and grass savannas (GraS). Each of these vegetation features included five plot replicates (50 m × 50 m size) distributed in the landscape. Soil chemical parameters such as pH, organic matter, soluble phosphorus, exchangeable bases, and biological parameters such as abundance and diversity of earthworms, and soil enzymatic activities were investigated. Composite soil samples were collected and analyzed for chemical and microbial parameters while earthworms were sampled using the Tropical Soil Biology and Fertility 25 × 25 × 30 cm soil monolith method. A principal component analysis showed a clear demarcation between C. odorata plots and the savanna ones. Soluble P and exchangeable bases were significantly higher under ChrO than under both savannas. Earthworm density was twice higher under ChrO (433.3 ± 90.8 ind m−2) than under ShrS (173.9 ± 61.5 ind m−2) and GraS (176.0 ± 40.6 ind m−2) and this was accounted for by the abundance of detritivores and polyhumics. Acid and alkaline phosphatase activities under ChrO (2.9 ± 0.2 and 2.5 ± 0.3 μmol pNP g−1 soil h−1, respectively) were twice higher than under both savannas. Based on the fact that ChrO and ShrS were located on similar soil types and had the same topographic position, we concluded that the establishment of C. odorata in a savanna land and its subsequent high biomass and quality-litter production were mostly the reasons of the improvement in soil biology and plant nutrient availability. Between-savanna comparison showed that ShrS, with higher CEC and exchangeable bases, was somewhat more fertile than GraS, probably because of a better soil physical status. In view of the agronomic potentials of the shrub C. odorata, it may be taken as a basis for improved fallow in Africa.

Tracing the history of South-American neotropical savannas and seasonally dry forests evidences from comparative phylogeography

Background The Neotropical Seasonally Dry Forests (SDF) are treedominated ecosystems that occur in disjunct areas of fertile soils throughout the Neotropics. The hypothesis that the vicariance of a formerly continuous seasonal woodland formation, which may have reached its maximum extension during a dry-cool period 18,000–12,000 bp (the LGM), the Pleistocenic Arc Hypothesis, has been raised to explain the disjunct distribution [1]. Alternatively, based on the distribution of contemporary SDF species in Amazonia Forest and palynological data, Pennington [2] also proposed that SDF may have expanded into Amazonia Basin during the Pleistoce, with rain forest and montane taxa largely confined to gallery forest. In addition, a number of studies based on the fossil pollen record now available show that during the early Holocene period (until ca. 6000-5000 14C B. P.), the climate was drier in most of the South American savannas and distribution of savanna-like vegetation in Central and Southeast Brazil was more extensive in early compared with the late Holocene [3-5]. In southeastern Brazil, the current vegetation exist in the region only in the latest Holocene period (since 970 or 600 B. P. for some regions) under the current wet climatic conditions, with an annual dry season of about 4 months. Hence, the fossil record shows that savanna expansion in the Quaternary, especially in southeastern Brazil was characterized mainly by herbaceous and grass savanna which were favored by the drier and highly seasonal climate. It is possible that arboreal savanna taxa became restricted to sites with moist climatic conditions, which served as refugias. We are interested in test these hypotheses using the genus Tabebuia as model group. We have chosen five species based on the pattern of geographical distribution: T. aurea and T. ochracea, from savanna vegetation (cerrado sensu stricto), T. impetiginosa, T. roseo-alba from SDF, T. serratifolia, widely distributed in Mata Atlantica, SDF, riparian forests and Amazonia. Here, we present the results based on the phylogeography of T. impetiginosa.

Plant functional types in Brazilian savannas: The niche partitioning between herbaceous and woody species

An essential characteristic of savannas is the presence of two main plant functional types: grasses and trees. The tree–grass coexistence in savannas is expected to be stabilised either deterministically by water availability or by occurrences of fire and herbivory. However, under a neutral macroevolutionary perspective, functional types could arise purely as a consequence of branching processes in plant lineages. We sought plant functional types in cerrado by assessing 360 species and 11 functional traits that correspond to important dimensions of the niche of plants. Then, we tested whether random branching processes in plant lineages could generate the observed plant functional types. We sought functional types with the k -means partitioning analysis. We constructed a phylogenetic tree for all species sampled and tested whether the phylogeny could be generated by one of the branching processes that can randomly produce functional types: (1) pure birth, (2) rate-constant birth–death, and (3) rate-variable birth–death models. Then, we compared the functional types we observed with a multivariate analysis of variance in a phylogenetic context. Two major functional types characterised the cerrado plant community – one type dominated by grasses and herbaceous shrubs and other by woody shrubs and trees. These functional types were different from the functional types simulated under Brownian motion of evolution of traits. Thus, underlying adaptive processes rather than random evolutionary processes accounted for the plant functional types. The herbaceous and woody functional types seem to have appeared as a consequence of adaptive processes of plants to cope mainly with water availability, fire, and herbivory. However, the niche partition between herbaceous and woody species supports that the reduction in the competitive interactions may be also important to the stable coexistence between trees and grass in savannas.

Is there a temporal niche separation in the leaf phenology of savanna trees and grasses?

Aim It has been proposed that, in tropical savannas, trees deploy their leaves earlier in the growing season and grasses deploy their leaves later. This hypothesis implies a mechanism that facilitates the coexistence of trees and grasses in savannas. If true, this hypothesis would also allow algorithms to use differences in the phenological timing of grass and tree leaves to partition the relative contribution of grasses and trees to net primary production. In this study we examine whether a temporal niche separation between grasses and trees exists in savanna. Location A semi-arid, subtropical savanna, Kruger National Park, South Africa. Methods We use a multi-spectral camera to track through an entire growing season the normalized difference vegetation index (NDVI) of individual canopies of grasses and trees at eight sites arranged along a precipitation and temperature gradient. Results Among trees, we identified two distinct phenological syndromes: an early flushing syndrome and a late-flushing syndrome. Leaf flush in the tree strategies appears to pre-empt rainfall, whereas grass leaf flush follows the rain. The growing season of trees is 20 (late-flushing trees) to 27 (early flushing trees) days longer than that of the grasses. Main conclusions We show that grasses and trees have different leaf deployment strategies. Trees deployed leaves at lower temperatures than grasses and retained them for longer at the end of the growing season. The timing of the increase in NDVI is, however, similar between grasses and late-flushing trees and this complicates the separation of grass and tree signals from multi-spectral satellite imagery.

Water-removed spectra increase the retrieval accuracy when estimating savanna grass nitrogen and phosphorus concentrations

Information about the distribution of grass foliar nitrogen (N) and phosphorus (P) is important for understanding rangeland vitality and for facilitating the effective management of wildlife and livestock. Water absorption effects in the near-infrared (NIR) and shortwave-infrared (SWIR) regions pose a challenge for nutrient estimation using remote sensing. The aim of this study was to test the utility of water-removed (WR) spectra in combination with partial least-squares regression (PLSR) and stepwise multiple linear regression (SMLR) to estimate foliar N and P, compared to spectral transformation techniques such as first derivative, continuum removal and log-transformed (Log( 1 / R )) spectra. The study was based on a greenhouse experiment with a savanna grass species ( Digitaria eriantha). Spectral measurements were made using a spectrometer. The D. eriantha was cut, dried and chemically analyzed for foliar N and P concentrations. WR spectra were determined by calculating the residual from the modelled leaf water spectra using a nonlinear spectral matching technique and observed leaf spectra. Results indicated that the WR spectra yielded a higher N retrieval accuracy than a traditional first derivative transformation ( R 2 = 0.84 , RMSE = 0.28) compared to R 2 = 0.59 , RMSE = 0.45 for PLSR. Similar trends were observed for SMLR. The highest P retrieval accuracy was derived from WR spectra using SMLR ( R 2 = 0.64 , RMSE = 0.067), while the traditional first derivative and continuum removal resulted in lower accuracy. Only when using PLSR did the first derivative result in a higher P retrieval accuracy ( R 2 = 0.47 , RMSE = 0.07) than the WR spectra ( R 2 = 0.43 , RMSE = 0.070). It was concluded that the water removal technique is a promising technique to minimize the perturbing effect of foliar water content when estimating grass nutrient concentrations.

Fire Season Effects on Flowering Characteristics and Germination of Longleaf Pine (Pinus palustris) Savanna Grasses

Pinus palustris (longleaf pine) savannas depend on contiguous grass cover to facilitate frequent surface fires. Enhanced flowering, seed production, and germination may be linked to season of fires. We assessed the effect of month of prescribed fire (February, April, May, and July) on percentage of plants with flowering culms (FCs%), FC/area, FC/plant, seed production, and germination for five warm seasons, fall‐flowering grasses. Multivariate analysis indicated the response of flowering and fruiting to burn month varied among the grass species. The dominant species, Sporobolus junceus and Schizachyrium scoparium var. stoloniferum, had greater numbers of most flowering characteristics when burnt during April, May, and July. Aristida purpurascens had increased FC/plant after May and July burns. In contrast, Ar. mohrii had the fewest FC/plant and seeds/FC when burnt in July. Germination was greatest (26–60%) for Ar. purpurascens. Seeds collected following July burns for Ar. purpurascens and Ar. ternarius and after May burns for S. junceus were within the highest germination values recorded. Germination of Sc. scoparium var. stoloniferum was very low after February and July burns (≤5%). With Ar. mohrii, only seed collected following February (2%) and April burns (3%) germinated. April, May, and July fires increased seed production of dominant matrix grasses, thus facilitating the potential for recruitment of these species and facilitating seed collection from potential donor sites for ground‐layer restoration projects. Varying prescribed fire burn month captured variation in flowering characteristics among these grasses.

Experimental evidence that fire causes a tree recruitment bottleneck in an Australian tropical savanna

Abstract:A fire-mediated recruitment bottleneck provides a possible explanation for the coexistence of trees and grasses in mesic savannas. The key element of this hypothesis is that saplings are particularly vulnerable to fire because they are small enough to be top-killed by grass fires, but unlike juveniles, they take several years to recover their original size. This limits the number of recruits into the adult size classes. Thus savanna vegetation may be maintained by a feedback whereby fire restricts the density of adult trees and allows a grass layer to develop, which provides fuel for subsequent fires. Here, we use results from a landscape-scale fire experiment in tropical Australia, to explore the possible existence of a recruitment bottleneck. This experiment compared tree recruitment and survival over 4 y under regimes of no fire, annual early and annual late dry-season fire. Stem mortality decreased with increasing stem height in the fire treatments but not in the unburnt treatment. Tree recruitment was 76–84% lower in the fire treatments than the unburnt treatment. Such fire-induced stem loss of saplings and reduced recruitment to the canopy layer in this eucalypt savanna are consistent with the predictions of the fire-mediated recruitment bottleneck hypothesis.

BIODIVERSITY RESEARCH: Turning up the heat: the impacts of Andropogon gayanus (gamba grass) invasion on fire behaviour in northern Australian savannas

AbstractAim This study aimed to quantify changes in fire severity resulting from the invasion of Australia’s tropical savannas by the African grass Andropogon gayanus Kunth. (gamba grass).Location Mesic savannas of the Northern Territory, Australia.Method Byram’s fire‐line intensity (If), fuel load and architecture, and two post‐fire indicators of fire intensity – scorch height (SH) and char height (CH) of woody vegetation – were determined for fires in native grass savanna and A. gayanus invaded savanna. Leaf scorch is the height at which the fire’s radiant heat browns leaf tissue, and leaf char is the height that radiant heat blackens or consumes leaf tissue and provides an indirect measure of flame height. These data, and 5 years of similar data collected from the Kapalga Fire Project in Kakadu National Park, were used to develop empirical relationships between If and the post‐fire indices of fire intensity.Results A relationship between A. gayanus If and SH could not be developed because complete canopy scorch occurred in most A. gayanus fires, even at low If. In contrast, A. gayanus If was strongly correlated with CH. This empirical relationship was substantially different from that for native grass fires. For a given If, there was a significantly greater CH in invaded sites. This increase in radiant heat is attributable to the increased biomass (mean 3.6 t ha−1 in native grasses compared to 11.6 t ha−1 in A. gayanus) and height (approximately 0.5 m in native grasses compared to 4 m in A. gayanus) of the standing fine fuel.Main conclusion Andropogon gayanus invasion resulted in substantial changes in fire behaviour. This has important regional implications owing to the current (10,000–15,000 km2) and predicted (380,000 km2) area of invasion and the negative consequences for the native savanna biota that has evolved with frequent but relatively low‐intensity fire.

Tree–grass co-existence in savanna: Interactions of rain and fire

The mechanisms permitting the co-existence of tree and grass in savannas have been a source of contention for many years. The two main classes of explanations involve either competition for resources, or differential sensitivity to disturbances. Published models focus principally on one or the other of these mechanisms. Here we introduce a simple ecohydrologic model of savanna vegetation involving both competition for water, and differential sensitivity of trees and grasses to fire disturbances. We show how the co-existence of trees and grasses in savannas can be simultaneously controlled by rainfall and fire, and how the relative importance of the two factors distinguishes between dry and moist savannas. The stability map allows to predict the changes in vegetation structure along gradients of rainfall and fire disturbances realistically, and to clarify the distinction between climate- and disturbance-dependent ecosystems.

Influence of herbicides and felling, fertilization, and prescribed fire on longleaf pine growth and understory vegetation through ten growing seasons and the outcome of an ensuing wildfire

Restoring longleaf pine (Pinus palustris Mill.) over much of its original range requires artificial regeneration. In central Louisiana, USA, two fertilization levels—No (NF) or Yes (F-36 kg/ha N and 40 kg/ha P) in combination with three vegetation treatments—Check, four prescribed fires (PF), or multi-year vegetation control by herbicidal and mechanical means (IVM) were applied to container-grown longleaf pine plantings in a grass savanna. After 10 years, P concentration in longleaf pine foliage was less on NF plots than F plots, but fertilization did not significantly affect tree stature. Survival was greater on NF plots than F plots, and so, NF plots were more productive (NF—63 m3/ha and F—45 m3/ha). Pine trees on IVM plots (37 dm3/tree) were significantly larger than on Check and PF plots, which averaged 17 dm3/tree. Survival was better on IVM plots (88%) than PF plots (78%), which was better than Checks (58%). Consequently, IVM plots were most productive (99 m3/ha), followed by PF plots (44 m3/ha), and lastly Checks (28 m3/ha). PF plots had greater grass cover (38%) than Check and IVM plots, which averaged 5%, whereas PF and IVM plots had less understory arborescent cover (an average of 25%) than Checks (91%). A wildfire in March 2007 reduced pine survival by 22, 14, and 1 percentage points on IVM, Check, and PF plots, respectively. Seven months later, longleaf pine production had decreased to 92 m3/ha on IVM plots while increasing to 55 m3/ha on PF plots and 30 m3/ha on Checks. Overall, the wildfire rejuvenated the herbaceous plant community. Grass cover on Check and IVM plots averaged 20% while grass cover on PF plots was 36%. Forb cover increased on all treatments from 2% before the wildfire to 13% seven months after the wildfire. Understory arborescent cover decreased on Checks to 62% but increased slightly on PF and IVM plots and averaged 30%.

Environmental factors influencing the establishment, height and fecundity of the annual grass Sorghum intrans in an Australian tropical savanna

Abstract:Environmental factors influencing grass establishment and performance in tropical savannas are poorly understood, particularly in relation to disturbance. We describe a seed sowing experiment that examined the effects of fire regime, canopy cover and litter cover on the emergence, establishment, height and fecundity (seed production) of the regionally dominant annual grass Sorghum intrans in northern Australia. Establishment was significantly lower under the woody canopy compared with canopy gaps, and where seeds were sown on a layer of litter compared with bare soil. However, variation in fire regime had no significant effect on establishment or seed production. Additionally, a shade-house experiment was conducted to test the effects of litter on seedling emergence of S. intrans and six other grass species representative of the local flora (Pseudopogonatherum contortum, Sorghum plumosum, Chrysopogon latifolius, Eriachne triseta, Heteropogon triticeus and Alloteropsis semialata). All species showed reduced emergence when sown either above or below litter, compared with bare soil. Our results demonstrate the importance of the overstorey as a determinant of S. intrans abundance and savanna grass composition more generally, through its effect on establishment. The aversion of savanna grasses to litter (and S. intrans to canopy shading) supports the notion of savanna species being highly adapted to disturbance.

A resource ratio model of the effects of changes in CO2 on woody plant invasion

It is expected that elevated CO2 levels may have an important positive effect on the dominance of woody plants over grasses in savannas and grasslands. I propose that these changes in the relative abundance of trees and shrubs over grasses may be explained by Tilman’s resource ratio models. This change will occur because C3 trees will have higher net photosynthetic rates than C4 grasses which predominate in savannas. This will cause trees to have higher growth rates than grasses. An additional factor in trees and shrubs with carbon-based defences (such as tannins or other polyphenols) is that they may be better defended and, thus, lose less material to herbivory. Consequently, trees and shrubs should invade savannas and grasslands because their R* values will be lower. I compare this model to another less parsimonious model based on fire and carbon storage and allocation. Although these models are not necessarily mutually exclusive, the resource ratio model may be differentiated from the fire-carbon model on the basis of the presence of fire and/or the sensitivity to elevations in global CO2 levels.

Pleistocene environmental evolution in the Nihewan Basin and implication for early human colonization of North China

The fluvio-lacustrine sequence in the Nihewan Basin is an important archive of Pleistocene climate and environmental changes in northern China, which provides excellent sources of early human settlements in high-latitude East Asia. Here a long-term environmental record from the Pleistocene fluvio-lacustrine sequence in this basin is revealed, based on selected major element pairs that are sensitive to paleoenvironmental changes. Results reveal a long-term increasing trend in aridification/cooling in the Nihewan Basin since the early Pleistocene. This provides a general setting for early human colonization in high-latitude northern China (around 40° N). The intensified aridification/cooling is envisaged to reduce tree cover and increase open savanna grass area. This, in turn, would promote prominent flourishing population and migration of savanna-adapted early humans in the Old World. Therefore, the cold/dry climate may have contributed to the evolution and expansion of early humans in the high-latitude northern China, rather than bringing hardship, as is often thought.

Biomass and leaf‐level gas exchange characteristics of three African savanna C4 grass species under optimum growth conditions

AbstractC4 savanna grass species, Digitaria eriantha, Eragrostis lehmanniana and Panicum repens, were grown under optimum growth conditions with the aim of characterizing their above‐ and below‐ground biomass allocation and the response of their gas exchange to changes in light intensity, CO2 concentration and leaf‐to‐air vapour pressure deficit gradient (Dl). Digitaria eriantha showed the largest above‐ and below‐ground biomass, high efficiency in carbon gain under light‐limiting conditions, high water use efficiency (WUE) and strong stomatal sensitivity to Dl (P = 0.002; r2 = 0.5). Panicum repens had a high aboveground biomass and attained high light saturated photosynthetic rates (Asat, 47 μmol m−2 s−1), stomatal conductance, (gsat, 0.25 mol m−2 s−1) at relatively high WUE. Eragrostis lehmanniana had almost half the biomass of other species, and had similar Asat and gsat but were attained at lower WUE than the other species. This species also showed the weakest stomatal response to Dl (P = 0.19, r2 = 0. 1). The potential ecological significance of the contrasting patterns of biomass allocation and variations in gas exchange parameters among the species are discussed.

Local versus landscape‐scale effects of savanna trees on grasses

Summary1. Savanna ecosystems – defined by the coexistence of trees and grasses – cover more than one‐fifth the world’s land surface and harbour most of the world’s rangelands, livestock and large mammal diversity. Savanna trees can have a variety of effects on grasses, with consequences for the wild and domestic herbivores that depend on them.2. Studies of these effects have focused on two different spatial scales. At the scale of individual trees, many studies have shown net positive effects of trees on sub‐canopy grass nutrient concentrations and biomass. At the landscape scale, other studies have shown negative effects of high tree densities on grass productivity. These disparate results have led to different conclusions about the effects of trees on forage quality and ungulate nutrition in savannas.3. We integrate these approaches by examining the effects of trees on grasses at both spatial scales and across a range of landscape‐scale tree densities. We quantified grass biomass, species composition and nutrient concentrations in these different contexts in an Acacia drepanolobium savanna in Laikipia, Kenya.4. Individual trees had positive effects on grass biomass, most likely because trees enrich soil nitrogen. Grass leaf phosphorus in sub‐canopy areas, however, was depressed. The effects of individual trees could explain the effects of increasing landscape‐scale tree cover for the biomass of only two of the four dominant grass species.5. The negative effects of trees on grass and soil phosphorus, combined with depressed grass productivity in areas of high tree cover, suggest that ungulate nutrition may be compromised in areas with many trees.6. Synthesis. We conclude that few, isolated trees may have positive local effects on savanna grasses and forage, but in areas of high tree density the negative landscape‐scale effects of trees are likely to outweigh these positive effects. In savannas and other patchy landscapes, attempts to predict the consequences of changes in patch abundances for ecosystem services (e.g. rangeland productivity and carbon sequestration) will depend on our understanding of the extent to which local, patch‐scale dynamics do or do not predict landscape‐scale dynamics.

Browsing and fire interact to suppress tree density in an African savanna

Disturbances from fire and herbivory strongly affect savanna vegetation dynamics. In some savannas, fire especially may be instrumental in preserving the coexistence of trees and grasses. The role of herbivory by large mammals is less clear; herbivory has been shown variously to promote and to suppress tree establishment. Here we ask how interactions between herbivory and fire act to shape savanna vegetation dynamics via their effects on tree populations in Hluhluwe iMfolozi Park in KwaZulu Natal, South Africa, a savanna with a full complement of native large mammals. We examined the effects of herbivore exclusion on tree growth, mortality, and seedling establishment from 2000 to 2007 at 10 sites located in areas of low and high herbivore pressure throughout the park. Results were analyzed statistically and using Leslie matrix models of population dynamics. Herbivory and fire acted primarily to suppress sapling growth rather than on sapling mortality or seedling establishment. This indicates that browsing, like fire, suppresses tree density by imposing a demographic bottleneck on the maturation of saplings to adults. Model results suggest that, while browsing and fire each alone impacted growth, a combination of browsing and fire had much greater effects on tree density. Only fire and browsing together were able to prevent increases in tree density. These results suggest that, while soil resources, including nutrients and moisture, are probably instrumental in determining tree growth rates, disturbances from fire and herbivory may be instrumental in limiting tree cover and facilitating the coexistence of trees and grasses in savannas.