Does the Type of Disturbance Matter When Restoring Disturbance‐Dependent Grasslands?

The reintroduction of burning is usually viewed as critical for grassland restoration; but its ecological necessity is often untested. On the one hand, fire may be irreplaceable because it suppresses dominant competitors, eliminates litter, and modifies resource availability. On the other hand, its impacts could be mimicked by other disturbances such as mowing or weeding that suppress dominants but without the risks sometimes associated with burning. Using a 5‐year field experiment in a degraded oak savanna, we tested the impacts of fire, cutting and raking, and weeding on two factors critical for restoration: controlling dominant invasive grasses and increasing subordinate native flora. We manipulated the season of treatment application and used sites with different soil depths because both factors influence fire behavior. We found no significant difference among the treatments—all were similarly effective at suppressing exotics and increasing native plant growth. This occurred because light is the primary limiting resource for many native species and each treatment increased its availability. The effectiveness of disturbance for restoration depended more on the timing of application and site factors than on the type of treatment used. Summer disturbances occurred near their reproductive peak of the exotics, so their mortality approached 100%. Positive responses by native species were significantly greater on shallow soils because these areas had higher native diversity prior to treatment. Although likely not applicable to all disturbance‐dependent ecosystems, these results emphasize the importance of testing the effectiveness of alternative restoration treatments prior to their application.     

Influence of soil fertility on the physiognomy of the African savanna tree Colophospermum mopane

This study investigated the influence of soil fertility on the physiognomy of Colophospermum mopane, one of the dominant tree or shrub species in savannas of central and southern Africa. The study was conducted in Gonarezhou National Park (GNP) in the south‐east of Zimbabwe. Mopane stands at the study site were divided into three physiognomic forms or mopane ‘types’: short mopane (SM, 1–2 m in height); medium mopane (MM, 8–12 m) and tall mopane (TM, >15 m). In each mopane type, soil samples were collected up to a depth of 15 cm to investigate the level of soil nutrient concentrations. SM stands were associated with low phosphorus and high levels of calcium and sodium relative to MM and TM. The low levels of phosphorus in SM relative to other mopane types suggest that phosphorus may be an important nutrient limiting growth in dwarf mopane. Alternatively, plant nutrients may be limiting because of interaction with factors such as the high soil sodicity present in the dwarf mopane habitats that may affect nutrient uptake or plant nutrient demand.     

Causes of increased nutrient concentrations in post-fire regrowth in an East African savanna

The aim of the present study was to investigate the causes of increased macronutrient concentrations in above-ground post-fire regrowth in an East African savanna (Northern Tanzania). Experiments were set up to discriminate between the following possible causes: (1) increased soil nutrient supply after fire, (2) relocation of nutrients from the roots to the new shoots, (3) rejuvenation and related changes in plant tissue composition and (4) changes in nutrient uptake in relation to above-ground carbon gains. N, P, K, Ca and Mg concentrations in post-burn graminoid vegetation were compared with clipped and with unburned, control vegetation during the post-burn growth season. One month after burning and clipping, nutrient concentrations in live grass shoots in the burned and clipped treatments were significantly higher than in the control. This effect, however, declined in the course of the season and, except for Ca, disappeared three months after onset of the treatments. There were no significant differences in live grass shoot nutrient concentrations between burned and clipped treatments which suggests that the increased nutrient concentration in post-fire regrowth is not due to increased soil nutrient supply via ash deposition. The relatively low input of nutrients through ash deposition, compared to the amount of nutrients released through mineralisation during the first month after burning and to the total nutrient pools, supports this suggestion. There was no difference between burned and unburned vegetation in total root biomass and root nutrient concentrations. Relocation of nutrients from the roots to the new shoots did not, therefore, appear to be a cause of higher post-fire shoot nutrient concentrations. The present study shows that in this relatively nutrient-rich savanna, the increased nutrient concentration in above-ground post-fire regrowth is primarily due to increased leaf:stem ratios, rejuvenation of plant material and the distribution of a similar amount of nutrients over less above-ground biomass. This revised version was published online in June 2006 with corrections to the Cover Date.     

Burning the Seasonal Mosaic: Preventative Burning Strategies in the Wooded Savanna of Southern Mali

Data are presented indicating a seasonal mosaic pattern of burning in the savanna of southern Mali. A seasonal mosaic is a landscape that is annually re-created by people, and which contains patches of unburned, early burned, and recently burned vegetation. A survey of over 100 farmers and in-depth interviews demonstrates that rural inhabitants of southern Mali begin an annual burning regime early in the dry season in order to fragment the landscape, with the goal of preventing later fires that can damage natural resources. The process of gradually burning off the driest vegetation creates a seasonal mosaic of habitat patches that increases the potential of the landscape for a variety of dry season land uses, including hunting, gathering of savanna products, and grazing. An analysis of a series of Landsat images shows that the practice of mosaic burning is widespread in the wooded savanna, in which burning usually begins early and large fires are rare. On the basis of recent developments in ecological theory and empirical evidence from similar burning regimes in parts of Australia, it is suggested that seasonal mosaic burning in Mali not only prevents damaging late-season fires but increases biodiversity. It is concluded that discourse on African savanna burning overemphasizes the ecologically detrimental aspects of fire, while neglecting the beneficial ones resulting in misguided policies that pose a threat to human livelihoods and savanna ecosystems.     

Germination responses of native and invasive Cerrado grasses to simulated fire temperatures

Background: Fire is an important ecological factor in the Cerrado (Brazilian savanna). However, comparative studies on the effect of high temperatures experienced during fires on seed germination of native and invasive grass species are few.

Aims: To assess germination responses to simulated fire temperatures by seeds of invasive and native Cerrado grasses.

Methods: Heat-shock treatments (50 °C, 70 °C, 90 °C, 110 °C, 130 °C or 150 °C) were applied to seeds of 10 species of native and invasive grasses. For each temperature, the seeds were heated in a dry-air flow for 2 or 5 min. This combination of temperatures and exposure times simulated the soil conditions during typical Cerrado fires.

Results: Temperature treatment was significantly related to germination, and the effect varied according to species. Heat shock did not increase germination in either the native or the invasive species. Exposure time was important for only two species, and four species showed a significant increase in mean germination time.

Conclusions: Species showed different tolerances to high temperatures. It was not possible to differentiate the native and invasive grasses only by their tolerance to high temperatures, suggesting that fire alone may not be an efficient management tool to control the invasive species studied here.