Influence of Climate on Oak Savanna Tree Species in the Midwestern United States

Aims: The previously abundant high quality and open canopy oak savanna communities in the Midwest have been reduced by more than 98% of their pre-settlement (pre-1840) area because of changing land use and represent some of the most threatened ecosystems in North America. Prior knowledge of oak savanna communities’ climatic resilience to potential impact of climate change and competition is critical to restoration success. This study examined sensitivity to climatic stress, and effects of competition, which are important considerations during oak savanna restoration. Methodology: Dendrochronological methods were used to sample oak savanna communities located in MacCready Reserve (MR) situated in southern Michigan, U.S.A. The influence of climate (mainly temperature and precipitation) on white oak (Quercus alba L.), red maple (Acer rubrum L), and black cherry (Prunus serotina Ehrh) were correlated using dendroclimatic techniques. The effect of competitor species (A. rubrum and P. serotina) on Q. alba were examined using competitor ratio chronologies and examining correlations with climatic variables. Results: Findings indicate that precipitation in winter, spring, and summer is beneficial for radial growth of white oak. White oak is more resilient to drought stress than red maple and black cherry due to its ecophysiological adaptations but tends to grow rather slower when in competition with shade tolerant and fire sensitive competitor species. Conclusion: Overall, this study has shown that temperature and precipitation play key roles in tree productivity and thus climatic sensitivity should be incorporated in the restoration of oak savanna ecosystems.

Quantifying the woody component of savanna vegetation along a density gradient in the Kalahari Bushveld: a comparison of two adapted point-centered quarter methods

Bush encroachment is a serious problem in savanna rangelands of southern Africa. There is a strong interest in practical and reliable assessment methods to quantify related vegetation changes in the woody layer such as the widely applied point-centred quarter (PCQ) methods. Several variations of these distance methods exist but their results differ due to differences in sampling effort and methodological accuracy. The aim of this study was to compare the performance of two recently developed adapted PCQ methods. These methods were used to estimate density, productivity and diversity of the woody layer of a semiarid savanna along a degradation gradient in the Kalahari rangelands. It was found that both adapted PCQ methods (APCQ10 and APCQ20, with the APCQ20 method using less recording points but a larger sampling area and higher sampling intensity per recording point) provided similar results for density, phytomass, available browse and browsing capacity in open, dense and encroached savanna types. Significant differences between the methods were obtained in differentiating height classes, which were, however, largely restricted to the woody layer above 2 m in open savanna types. There, applying the APCQ20 method avoided an under-sampling of larger shrubs and trees and increased precision in data assessment. This was confirmed by a better representation of species frequency distributions, as well as the density, phytomass and diversity status of the woody layer. These differences disappeared as the woody vegetation became denser with the APCQ10 method providing similar results to that of the APCQ20 method in densely vegetated and encroached savanna types. From a practical point of view, the APCQ10 method has a range of advantages in dense vegetation, where restricted movement impedes effective data collection. It is concluded that the APCQ20 method should be used to quantify open savanna communities, whereas the APCQ10 method is more suitable in dense stands of >1200 tree equivalents ha–1. Overall, the two APCQ methods were effective for assessing and monitoring woody savanna layers for management purposes but, for research, their accuracy still needs to be investigated in comparison to other assessment methods.

Viewpoint: social and economic dimensions of involving savanna communities in carbon management systems

In this paper we discuss the social, economic and institutional aspects of the development of carbon management systems within Australia’s tropical savannas. Land-use values in savanna landscapes are changing as a result of changing economic markets, greater recognition of native title, and growing social demands and expectations for tourism, recreation and conservation. In addition, there is increasing interest in developing markets and policy arrangements for greenhouse gas abatement, carbon sequestration and carbon trade in savannas. We argue that for carbon management to lead to national greenhouse outcomes, attention must be paid to social, economic and institutional issues in environmental planning and policy arrangements. From an economic perspective, the financial impact of carbon management on savanna enterprises will depend on appropriate and available policy mechanisms, unit price for carbon, landscape condition, existing management strategies and abatement measurements used. Local social and cultural features of communities and regions may enhance or constrain the implementation of carbon abatement strategies, depending on how they are perceived. In terms of institutional arrangements, policies and plans must support and enable carbon management. We identify three areas that require priority investigation and adjustment: regional planning arrangements, property rights, and rules for accounting at enterprise and regional scales. We conclude that the best potential for managing for carbon will be achieved while managing for range of other natural resource management outcomes, especially where managing for carbon delivers collateral benefits to enterprises.

Lifeform distributions of woodland plant species along a moisture availability gradient in Australia's monsoonal tropics

A series of vegetation sites was established in Australia's Northern Territory between Darwin and Tennant Creek, a distance of approximately 1000 km and 7° latitude (12°30'–19°30'S). This region encompasses a strong environmental gradient in mean annual moisture availability (450–1600 mm) whilst remaining within a predominantly summer monsoonal rainfall regime. All sites are within eucalypt-savanna habitats on lighter textured soils (sands–loams). Major changes in family and species representation occur at approximately 16–17° latitude, supporting findings of other workers. Within these eucalypt-savanna communities, the percentage of annual species is consistently around 30% regardless of latitude. However, the distribution of resource allocation strategies used by perennial plants exhibits distinct latitudinal trends. The proportion of deciduous and seasonally perennial species declines with latitude whilst suffrutescent shrub species become increasingly abundant. Species possessing root structures adapted for storage purposes appear to be limited to latitudes north of 15°S.

Factors influencing tree growth in tropical savanna: studies of an abrupt Eucalyptus boundary at Yapilika, Melville Island, northern Australia

ABSTRACTMost of the land surface of Melville Island, Australia's second largest island, is covered in Eucalyptus savanna. One exception is an area at Yapilika where a large tract of savanna is dominated by Acacia shrubs. An ordination analysis of 122 quadrats revealed that the boundary of Eucalyptus dominance did not correspond to a major change in floristic composition. Detailed transect studies at one site on the boundary showed that Eucalyptus trees were abruptly replaced by a band of Grevillea trees which gradually gave way to Acacia shrub dominance. There was a gradual change in the floristic composition of the savanna across the boundary. The distributional limit of Eucalyptus was found to be independent of any hydrological discontinuity. There was a slight decrease (<2.5 m) in altitude from Eucalyptus to Acacia savanna. The Acacia savanna soils were sandier and their surface soil had significantly lower concentrations of Ca and Mg and significantly greater concentration of Al compared with the Eucalyptus savanna soils. Eucalyptus seedlings planted in the three savanna communities were not found to be under drought stress (pre-dawn leaf xylem potentials of > – 0.9 MPa) during the dry season. Over a 12 month period Eucalyptus tetrodonta and E. miniata seedling growth was not significantly different on the Acacia or Eucalyptus savanna, although this result may be due to the counteracting effects of greater soil fertility and tree competition in the Eucalyptus savanna and lower soil fertility in the treeless, and hence competition-free, Acacia savanna. This hypothesis is supported by the significantly greater growth of Eucalyptus seedlings on fertilized Acacia savanna soils. The limited production, dispersal and establishment of Eucalyptus seeds and the greater frequency of fires in the Acacia savanna probably explains the abrupt limit to Eucalyptus dominance along the edaphic gradient.

Responses of lizards to three experiments fires in the savanna forests of Kakadu National Park

A species-rich lizard community responded variably to a range of experimental fire regimes in a tropical savanna. Heteronotia binoei was the only lizard species that showed a short-term response to fire, decreasing in abundance directly after the early- and late-burns. H. binoei and Diporiphora bilineata were significantly more abundant in early-burn treatments. Carlia amax was more abundant in unburnt and early-burn treatments. C. munda was more abundant in unburnt and early-burn sites. Differences in the relative abundance of species between treatments is attributed to site differences in vegetation structure, and fire-induced changes to the structure of habitat. Early-burn sites were significantly richer in lizard species (P< 0.05); however, overall lizard abundance, Shannon-Wiener diversity and Detrended Correspondence Analysis (DCA) 1 and DCA 2 scores were not significantly different, suggesting that habitat partitioning has a stronger influence than the direct effects of fire. Many lizards were associated with a direct gradient of moisture availability, including a seepage assemblage, with Carlia gracilis and Sphenomorphus darwiniensis correlated with increased moisture, a well-developed canopy and abundant leaf litter. An assemblage associated with the drier end of the gradient included Carlia triacantha, Ctenotus kurnbudj, Diporiphora magna and D. bilineata. The lizard composition of most quadrats was intermediate along the moisture gradient and was associated with typical eucalypt savanna communities. Lizard species that largely are restricted to the moist seepage zones may be particularly sensitive to late dry-season fires which alter this habitat type.