BACKGROUND
the increasing density in the woody component of savannas has been widely reported  <cit>  with special mention being made of acacia karroo hayne  <cit>  and a. nilotica  willd.
ex del.
subsp.
kraussiana  brenan,  <cit> .
in some areas, as major contributors to the phenomenon.
in hluhluwe-imfolozi park dichrostachys cinerea  wight & arn.
and a. luederitzii engl.
var.
retinens  ross & brenan are also thought to contribute to this phenomenon.
in hard seeded legumes dormancy is broken by rupturing part of the seed coat.
the rupturing of the seed coat may be induced by heat from fire  <cit>  enabling water to enter the seed and start the process of germination.
many studies have confirmed a release of legume seeds from dormancy after fire  <cit> .
fire temperature or intensity also has an effect on the germination of seeds  <cit>  and low intensity fires may not be enough to break dormancy of hard-seeded legumes  <cit> .
in other cases lower fire temperatures are preferable for germination with an increase in fire temperature causing seed mortality  <cit> .
while some studies report that a decrease in grass cover favours the establishment of woody seedlings due to reduced competition  <cit> , others  <cit>  challenge these findings.
these differences may however, be a result of species reacting differently to fire or competition.
some acacia species are shade intolerant resulting in decreased seedling establishment in shady areas  <cit> .
other acacia species have been found to be tolerant of low light conditions and may even experience increased seedling survival  <cit> .
the frequency of fires may affect the direction of change in woody plant density  <cit> .
while it has been suggested that fire may increase acacia densities  <cit> , it is also used to clear acacias from grassland  <cit> .
this contradictory situation in the literature concerning the effect of fire necessitates further research, as it is clear that continuous use of incorrect burning practices may have disastrous consequences.
this study investigated the direct  and indirect  effects of fire on seed germination and seedling establishment of a. nilotica, a. karroo, a. luederitzii and dichrostachys cinerea in hluhluwe-imfolozi park , where an increase in woody plant density over the past  <dig> years has been reported  <cit> .
it has also been reported that a. karroo is apparently replacing a. nilotica as the dominant microphyllous element  <cit> .
this study reports on the effects of burning, fire intensity and burning of sites on germination; burning, fire intensity, burning of sites and grass length  on seedling establishment and specific species responses to treatments .
RESULTS
germination
none of the seeds of d. cinerea germinated in the field and it was therefore excluded from the model for the field experiment.
testing for differences among treatments was based on the maximum number of seedlings for each species at each location over the 31-week period .
a description of the factors used in both the germination and establishment models is given in table  <dig>
the ratio of the model deviance to the degrees of freedom was small  indicating that the model was a good fit.
location and species were the only main effects significantly affecting germination .
acacia karroo had the highest germination of all species .
interaction terms that had a significant effect on germination were, location × grass length, burn status × site burn status and grass length × site burn status .
germination of burnt seeds in burnt sites  was significantly higher than that of burnt seeds in unburnt sites .
similarly, unburnt seeds in unburnt sites had a higher germination percentage  than unburnt seeds in burnt sites .
the estimated odds of germination and their associated probabilities for the factors and their interactions are given in additional file  <dig>  the odds ratios for significant effects were calculated.
thus a comparison between a. karroo and a. nilotica with regards to seeds germinating was made, where
thus the odds of germinating are four times more for a. karroo than for a. nilotica.
similarly a. nilotica was four times less likely to germinate than a. luederitzii while a. karroo and a. luederitzii had the same odds of germinating.
differences in germination among species for the various treatments are given in table  <dig>
there was  <dig>  times less germination at le dube than at nombali and  <dig>  times less at le dube than at seme.
germinations were  <dig>  times more likely at seme than at nombali.
seedling establishment
the ratio of the model deviance to the degrees of freedom was small  indicating that the model fitted the data well.
location and species were the only main effects significantly affecting establishment in the field .
acacia karroo showed significantly higher percentage establishment than any of the other species .
interaction terms, location × burn status, location × grass length, location × site burn status, burn status × site burn status and grass length × site burn status had a significant effect on establishment  .
additional file  <dig> gives the estimated odds of non-establishment and their associated probabilities for the factors and their interactions.
the odds ratios for significant effects were calculated and are given .
acacia karroo was  <dig>  times more likely to establish than a. nilotica.
similarly a. luederitzii was  <dig>  times more likely to establish than a. nilotica while a. karroo had  <dig>  times more chance of establishing than a. luederitzii.
species differences in establishment for the various treatments are given in table  <dig>
the odds of establishment were  <dig>  times less at le dube than at nombali and  <dig>  times less at le dube than at seme.
<dig>  times more seedlings were likely to establish at nombali than at seme.
discussion
the lack of germination of d. cinerea in the field suggests that some disturbance other than fire is needed to cause a release from dormancy and commence germination.
germination of all species in the field was low.
as the seeds relied on cotyledons for food, soil moisture may have been a limiting factor.
as rainfall was not recorded, this should be kept in mind when interpreting the results.
five point one percent of a. karroo seeds germinated, which was higher than the other two species.
story  <cit>  found similar levels of germination for a. karroo, with  <dig> % of seeds germinating under natural conditions in the field.
he also found that a. karroo germination was erratic, with germinations still being recorded after  <dig> days.
this was similar to what was found in this study, with the number of a. karroo seedlings still increasing until the end of the experiment.
acacia nilotica also showed dormancy with sporadic germination events over the 31-week period.
acacia luederitzii did not show dormancy with most germinations taking place in the first  <dig> weeks of the experiment.
acacia nilotica has a thick seed coat, which could account for it's poor level of germination.
one would predict increased germination of burnt seeds due to a breaking of dormancy  <cit> , but this was not the case.
a possible explanation is that the temperature of the fires in this study, though not measured, might not have been sufficient to break dormancy in this species.
some acacia species are temperature specific, suggesting a temperature threshold for germination  <cit> .
this is unlikely in this case as radford et al.
<cit>  found a. nilotica seeds to be highly vulnerable to fire with a 80% mortality of seeds on the soil surface.
the current study, however, found no difference in germination between burnt and unburnt seed or seeds burnt at different temperatures.
this finding is inconsistent with the recent study by kanz  <cit>  who found increased seed germination in low fires compared to the control as well as that of okello and young  <cit>  who found increased germination of unburnt seeds.
auld & o'connell  <cit>  had similar results to that of kanz  <cit>  with strong germination responses to heat.
location had a significant effect on germination with le dube having very low germination overall and seme having the most germinations.
germination at nombali and seme were similar.
site-specific effects may be attributed to various factors such as microclimate or soil type.
sites may also have different water infiltration rates and runoff, which may result in differences in germination levels.
okello and young  <cit> , however, found that soil type did not affect germination or establishment of acacia drepanolobium in kenya.
the current study did not find a difference in the number of seedlings in burnt and unburnt patches.
while neither burning of seeds nor burning of sites had any effect on germination, the interaction factor proved significant with unburnt seeds showing increased germination in unburnt sites as did burnt seeds in burnt sites.
kanz  <cit>  also found greater seedling emergence of unburnt seeds in unburnt areas.
this might be a result of burnt seeds imbibing faster than unburnt seeds, possibly making them more susceptible to rot.
burnt seeds would therefore show poorer germination in unburnt areas due to increased moisture retention.
similarly, unburnt seeds would require more moisture to imbibe, resulting in decreased germination in burnt areas due to decreased moisture in these open areas.
whilst more seeds germinated in short grass at both le dube and nombali, those at the short-grass site  had higher levels of germination in tall grass sites.
the short grass site at seme is a white rhinoceros  grazing lawn with very short grass, which may lead to seeds losing moisture through more direct sunlight.
this suggests a similar pattern to the seed burn × site burn interaction.
the tall grass site at seme had higher germination than any of the other tall or short grass sites.
this may be due to possible site-specific effects mentioned earlier.
there was also an interaction between grass length and site burn with seeds in burnt, short grass showing higher germination than those in burnt, tall grass and unburnt sites showing higher germination in tall grass.
as half of the seeds on a burnt or unburnt site were burnt themselves, it is possible that this interaction is due to temperature sensitivity in seeds.
burning in tall grass  may be detrimental to the germination of seeds  <cit>  while cooler fires may be sufficient to break dormancy and cause germination.
higher germinations in unburnt tall grass areas suggest a shade effect.
this is not certain, as the effects of shade and grass competition were not separated in this study.
acacia karroo has however been reported as having an increased ability to survive in shade with recruitment of seedlings being dependent on moisture availability  <cit> .
tall grass species may retain more moisture than short grass species, affording seeds a better opportunity for germination.
no species factor interactions were observed suggesting that though species had different germination levels, they did not respond differently to the treatments.
the same factors and interactions found to be significant influences on germination were found to influence establishment.
this was expected as increased germination for these treatments would result in better establishment.
the interaction patterns for most of the treatments, however, were different to those of the germination model.
owing to the low levels of germination, interspecific and intraspecific competition was thought to play a minor role in seedling establishment.
le dube again had the least seedlings at  <dig> weeks while nombali had the best establishment.
seme, which had the highest level of germination, had establishment levels somewhere between that of the other two sites.
it is again suggested that this may be due to soil or rainfall factors.
forty-five out of forty-eight seedlings established at nombali and thirty-three out of thirty-five at seme were a. karroo seedlings.
this species is known to be dependent on moisture availability for survival  <cit>  and these two sites might have better water retaining ability than le dube.
at week  <dig>   <dig> a. karroo seedlings had established as opposed to six of a. nilotica and three of a. luederitzii.
the high germination, but poor survival of a. luederitzii suggests that the absence of this species in the hluhluwe section of hip is not due to seed limitation or germinability, but possibly due to environmental factors decreasing its ability to establish.
the differences in seedling survival between species are consistent with those reported by kanz  <cit>  who found higher seedling survival for a. karroo than a. nilotica.
the location × grass length interaction revealed the same patterns as for germination with regards to nombali and seme with seme showing better establishment in tall grass and nombali showing better establishment in short grass.
there was no difference between establishment on tall and short grass at le dube.
the short grass site at nombali had the highest number of seedlings surviving at week  <dig>
the grass length × site burn interaction displayed the same patterns as for the germination model, but this was not the case for the seed burn status × site burn status interaction.
while unburnt seeds still did well on unburnt sites, burnt and unburnt seeds showed decreased establishment on burnt sites suggesting that, as a result of increased irradiance, burnt  sites may not hold sufficient moisture for seedlings to survive.
the interaction effects found to be significant for establishment only, both suggest the importance of fire temperature.
location × seed burn status and location × site burn status could both relate to the different grass lengths, and thus specific fire temperatures, at the three sites.
temperature sensitivity in acacia species have been reported elsewhere  <cit> .
kanz  <cit>  found increased survival and growth in burnt areas.
in this study, nombali was the only location to have higher establishment on burnt sites, while seme had increased establishment on unburnt sites and le dube very little establishment overall.
in general, however, this study found no difference in establishment in burnt and unburnt areas.
chirara, frost & gwarazimba  <cit>  found that intensity of grass defoliation does not affect seedling establishment of a. karroo during the first year.
similarly, there was no difference in establishment of a. karroo in burnt or unburnt and tall or short grass sites.
smith & goodman  <cit>  reported that a. nilotica seedlings, however, almost exclusively occurred away from canopy cover, suggesting an inability to establish in shaded environments.
acacia tortilis also showed a greater proportion of established seedlings in open than shaded areas  <cit> .
we did not find a difference in establishment of a. nilotica in tall and short grass, but its establishment was so low that no real prediction can be made.
CONCLUSIONS
seedling establishment of a. karroo is strongly moisture dependent  <cit>  and one would expect that a. karroo is more likely to invade moist rather than semi-arid grassland.
this suggests that hluhluwe game reserve, being an area with moist grassland, would be more prone to invasion by a. karroo.
it has also been reported that a. karroo has the ability to withstand fire  <cit> .
a combination of these factors may contribute to the success of a. karroo in the field and may be the reason for a. karroo's success over a. nilotica as the most important encroaching acacia species in hip at present.
the literature does, however, suggest that high intensity fires may result in seed mortality  <cit> .
it has, however, been reported that a. karroo seedlings survive fires from as little as  <dig> months of age  <cit> .
therefore, if fires are not hot enough to kill the seeds allowing them to germinate and seedlings to establish, management burns in the following year may not be useful in its attempt to control the establishment of this species.
back fires have higher fire intensities than head fires  <cit> .
we therefore suggest that backfires be used during management burns and that fire frequency be increased in suitable areas in an attempt to slow down the rate of encroachment by a. karroo.
it has been reported that spring burns are the most effective  and this should be taken into account.

 the increasing density in the woody component of savannas has been widely reported with special mention being made of acacia karroo hayne and a. nilotica willd. ex del. subsp. kraussiana brenan,. in some areas, as major contributors to the phenomenon. in hluhluwe-imfolozi park dichrostachys cinerea wight & arn. and a. luederitzii engl. var. retinens ross & brenan are also thought to contribute to this phenomenon. in hard seeded legumes dormancy is broken by rupturing part of the seed coat. the rupturing of the seed coat may be induced by heat from fire enabling water to enter the seed and start the process of germination. many studies have confirmed a release of legume seeds from dormancy after fire. fire temperature or intensity also has an effect on the germination of seeds and low intensity fires may not be enough to break dormancy of hard-seeded legumes. in other cases lower fire temperatures are preferable for germination with an increase in fire temperature causing seed mortality. while some studies report that a decrease in grass cover favours the establishment of woody seedlings due to reduced competition, others challenge these findings. these differences may however, be a result of species reacting differently to fire or competition. some acacia species are shade intolerant resulting in decreased seedling establishment in shady areas. other acacia species have been found to be tolerant of low light conditions and may even experience increased seedling survival. the frequency of fires may affect the direction of change in woody plant density. while it has been suggested that fire may increase acacia densities, it is also used to clear acacias from grassland. this contradictory situation in the literature concerning the effect of fire necessitates further research, as it is clear that continuous use of incorrect burning practices may have disastrous consequences. this study investigated the direct and indirect effects of fire on seed germination and seedling establishment of a. nilotica, a. karroo, a. luederitzii and dichrostachys cinerea in hluhluwe-imfolozi park, where an increase in woody plant density over the past years has been reported. it has also been reported that a. karroo is apparently replacing a. nilotica as the dominant microphyllous element. this study reports on the effects of burning, fire intensity and burning of sites on germination; burning, fire intensity, burning of sites and grass length on seedling establishment and specific species responses to treatments.  germination none of the seeds of d. cinerea germinated in the field and it was therefore excluded from the model for the field experiment. testing for differences among treatments was based on the maximum number of seedlings for each species at each location over the 31-week period. a description of the factors used in both the germination and establishment models is given in table the ratio of the model deviance to the degrees of freedom was small indicating that the model was a good fit. location and species were the only main effects significantly affecting germination. acacia karroo had the highest germination of all species. interaction terms that had a significant effect on germination were, location × grass length, burn status × site burn status and grass length × site burn status. germination of burnt seeds in burnt sites was significantly higher than that of burnt seeds in unburnt sites. similarly, unburnt seeds in unburnt sites had a higher germination percentage than unburnt seeds in burnt sites. the estimated odds of germination and their associated probabilities for the factors and their interactions are given in additional file the odds ratios for significant effects were calculated. thus a comparison between a. karroo and a. nilotica with regards to seeds germinating was made, where thus the odds of germinating are four times more for a. karroo than for a. nilotica. similarly a. nilotica was four times less likely to germinate than a. luederitzii while a. karroo and a. luederitzii had the same odds of germinating. differences in germination among species for the various treatments are given in table there was times less germination at le dube than at nombali and times less at le dube than at seme. germinations were times more likely at seme than at nombali. seedling establishment the ratio of the model deviance to the degrees of freedom was small indicating that the model fitted the data well. location and species were the only main effects significantly affecting establishment in the field. acacia karroo showed significantly higher percentage establishment than any of the other species. interaction terms, location × burn status, location × grass length, location × site burn status, burn status × site burn status and grass length × site burn status had a significant effect on establishment. additional file gives the estimated odds of non-establishment and their associated probabilities for the factors and their interactions. the odds ratios for significant effects were calculated and are given. acacia karroo was times more likely to establish than a. nilotica. similarly a. luederitzii was times more likely to establish than a. nilotica while a. karroo had times more chance of establishing than a. luederitzii. species differences in establishment for the various treatments are given in table the odds of establishment were times less at le dube than at nombali and times less at le dube than at seme. times more seedlings were likely to establish at nombali than at seme. discussion the lack of germination of d. cinerea in the field suggests that some disturbance other than fire is needed to cause a release from dormancy and commence germination. germination of all species in the field was low. as the seeds relied on cotyledons for food, soil moisture may have been a limiting factor. as rainfall was not recorded, this should be kept in mind when interpreting the results. five point one percent of a. karroo seeds germinated, which was higher than the other two species. story found similar levels of germination for a. karroo, with % of seeds germinating under natural conditions in the field. he also found that a. karroo germination was erratic, with germinations still being recorded after days. this was similar to what was found in this study, with the number of a. karroo seedlings still increasing until the end of the experiment. acacia nilotica also showed dormancy with sporadic germination events over the 31-week period. acacia luederitzii did not show dormancy with most germinations taking place in the first weeks of the experiment. acacia nilotica has a thick seed coat, which could account for it's poor level of germination. one would predict increased germination of burnt seeds due to a breaking of dormancy, but this was not the case. a possible explanation is that the temperature of the fires in this study, though not measured, might not have been sufficient to break dormancy in this species. some acacia species are temperature specific, suggesting a temperature threshold for germination. this is unlikely in this case as radford et al. found a. nilotica seeds to be highly vulnerable to fire with a 80% mortality of seeds on the soil surface. the current study, however, found no difference in germination between burnt and unburnt seed or seeds burnt at different temperatures. this finding is inconsistent with the recent study by kanz who found increased seed germination in low fires compared to the control as well as that of okello and young who found increased germination of unburnt seeds. auld & o'connell had similar results to that of kanz with strong germination responses to heat. location had a significant effect on germination with le dube having very low germination overall and seme having the most germinations. germination at nombali and seme were similar. site-specific effects may be attributed to various factors such as microclimate or soil type. sites may also have different water infiltration rates and runoff, which may result in differences in germination levels. okello and young, however, found that soil type did not affect germination or establishment of acacia drepanolobium in kenya. the current study did not find a difference in the number of seedlings in burnt and unburnt patches. while neither burning of seeds nor burning of sites had any effect on germination, the interaction factor proved significant with unburnt seeds showing increased germination in unburnt sites as did burnt seeds in burnt sites. kanz also found greater seedling emergence of unburnt seeds in unburnt areas. this might be a result of burnt seeds imbibing faster than unburnt seeds, possibly making them more susceptible to rot. burnt seeds would therefore show poorer germination in unburnt areas due to increased moisture retention. similarly, unburnt seeds would require more moisture to imbibe, resulting in decreased germination in burnt areas due to decreased moisture in these open areas. whilst more seeds germinated in short grass at both le dube and nombali, those at the short-grass site had higher levels of germination in tall grass sites. the short grass site at seme is a white rhinoceros grazing lawn with very short grass, which may lead to seeds losing moisture through more direct sunlight. this suggests a similar pattern to the seed burn × site burn interaction. the tall grass site at seme had higher germination than any of the other tall or short grass sites. this may be due to possible site-specific effects mentioned earlier. there was also an interaction between grass length and site burn with seeds in burnt, short grass showing higher germination than those in burnt, tall grass and unburnt sites showing higher germination in tall grass. as half of the seeds on a burnt or unburnt site were burnt themselves, it is possible that this interaction is due to temperature sensitivity in seeds. burning in tall grass may be detrimental to the germination of seeds while cooler fires may be sufficient to break dormancy and cause germination. higher germinations in unburnt tall grass areas suggest a shade effect. this is not certain, as the effects of shade and grass competition were not separated in this study. acacia karroo has however been reported as having an increased ability to survive in shade with recruitment of seedlings being dependent on moisture availability. tall grass species may retain more moisture than short grass species, affording seeds a better opportunity for germination. no species factor interactions were observed suggesting that though species had different germination levels, they did not respond differently to the treatments. the same factors and interactions found to be significant influences on germination were found to influence establishment. this was expected as increased germination for these treatments would result in better establishment. the interaction patterns for most of the treatments, however, were different to those of the germination model. owing to the low levels of germination, interspecific and intraspecific competition was thought to play a minor role in seedling establishment. le dube again had the least seedlings at weeks while nombali had the best establishment. seme, which had the highest level of germination, had establishment levels somewhere between that of the other two sites. it is again suggested that this may be due to soil or rainfall factors. forty-five out of forty-eight seedlings established at nombali and thirty-three out of thirty-five at seme were a. karroo seedlings. this species is known to be dependent on moisture availability for survival and these two sites might have better water retaining ability than le dube. at week a. karroo seedlings had established as opposed to six of a. nilotica and three of a. luederitzii. the high germination, but poor survival of a. luederitzii suggests that the absence of this species in the hluhluwe section of hip is not due to seed limitation or germinability, but possibly due to environmental factors decreasing its ability to establish. the differences in seedling survival between species are consistent with those reported by kanz who found higher seedling survival for a. karroo than a. nilotica. the location × grass length interaction revealed the same patterns as for germination with regards to nombali and seme with seme showing better establishment in tall grass and nombali showing better establishment in short grass. there was no difference between establishment on tall and short grass at le dube. the short grass site at nombali had the highest number of seedlings surviving at week the grass length × site burn interaction displayed the same patterns as for the germination model, but this was not the case for the seed burn status × site burn status interaction. while unburnt seeds still did well on unburnt sites, burnt and unburnt seeds showed decreased establishment on burnt sites suggesting that, as a result of increased irradiance, burnt sites may not hold sufficient moisture for seedlings to survive. the interaction effects found to be significant for establishment only, both suggest the importance of fire temperature. location × seed burn status and location × site burn status could both relate to the different grass lengths, and thus specific fire temperatures, at the three sites. temperature sensitivity in acacia species have been reported elsewhere. kanz found increased survival and growth in burnt areas. in this study, nombali was the only location to have higher establishment on burnt sites, while seme had increased establishment on unburnt sites and le dube very little establishment overall. in general, however, this study found no difference in establishment in burnt and unburnt areas. chirara, frost & gwarazimba found that intensity of grass defoliation does not affect seedling establishment of a. karroo during the first year. similarly, there was no difference in establishment of a. karroo in burnt or unburnt and tall or short grass sites. smith & goodman reported that a. nilotica seedlings, however, almost exclusively occurred away from canopy cover, suggesting an inability to establish in shaded environments. acacia tortilis also showed a greater proportion of established seedlings in open than shaded areas. we did not find a difference in establishment of a. nilotica in tall and short grass, but its establishment was so low that no real prediction can be made.  seedling establishment of a. karroo is strongly moisture dependent and one would expect that a. karroo is more likely to invade moist rather than semi-arid grassland. this suggests that hluhluwe game reserve, being an area with moist grassland, would be more prone to invasion by a. karroo. it has also been reported that a. karroo has the ability to withstand fire. a combination of these factors may contribute to the success of a. karroo in the field and may be the reason for a. karroo's success over a. nilotica as the most important encroaching acacia species in hip at present. the literature does, however, suggest that high intensity fires may result in seed mortality. it has, however, been reported that a. karroo seedlings survive fires from as little as months of age. therefore, if fires are not hot enough to kill the seeds allowing them to germinate and seedlings to establish, management burns in the following year may not be useful in its attempt to control the establishment of this species. back fires have higher fire intensities than head fires. we therefore suggest that backfires be used during management burns and that fire frequency be increased in suitable areas in an attempt to slow down the rate of encroachment by a. karroo. it has been reported that spring burns are the most effective and this should be taken into account. 
this study investigated germination of acacia karroo, a. luederitzii and dichrostachys cinerea, thought to be increasing in density, as well as the historically successful encroaching woody species, a. nilotica, in savanna grassland, hluhluwe-imfolozi park, south africa. a. karroo is thought to be replacing a. nilotica as the dominant microphyllous species in the park. on average, burning of a. karroo, a. nilotica and a. luederitzii seeds did not affect germination. although germination and establishment of a. karroo was higher than for a. nilotica a competitive advantage after fire could not be shown. there was a significant difference in the germination of burnt seeds on burnt sites and burnt seeds on unburnt plots. similarly, unburnt seeds on unburnt sites germinated better than unburnt seeds on burnt sites. seeds of the shrub dichrostachys cinerea did not germinate in the field after fire and it is thought that some other germination cue is needed. we tested the hypothesis that observed increases in certain woody plants in a savanna were related to seed germination and seedling establishment. germination is compared among species for burnt and unburnt seeds on burnt and unburnt plots at three different locations for both hot and cool fires. acacia karroo showed higher germination levels and better establishment. we conclude that a combination of factors may be responsible for the success of a. karroo and that fires may not be hot enough or may occur at the wrong time of year to control a. karroo establishment in hip.
 while fire has been used in some instances to control the increase of woody plants, it has also been reported that fire may cause an increase in certain fire-tolerant acacia tree species. this study investigated germination of acacia karroo, a. luederitzii and dichrostachys cinerea, thought to be increasing in density, as well as the historically successful encroaching woody species, a. nilotica, in savanna grassland, hluhluwe-imfolozi park, south africa. a. karroo is thought to be replacing a. nilotica as the dominant microphyllous species in the park. we tested the hypothesis that observed increases in certain woody plants in a savanna were related to seed germination and seedling establishment. germination is compared among species for burnt and unburnt seeds on burnt and unburnt plots at three different locations for both hot and cool fires.  acacia karroo showed higher germination levels and better establishment. seeds of the shrub dichrostachys cinerea did not germinate in the field after fire and it is thought that some other germination cue is needed. on average, burning of a. karroo, a. nilotica and a. luederitzii seeds did not affect germination. there was a significant difference in the germination of burnt seeds on burnt sites and burnt seeds on unburnt plots. similarly, unburnt seeds on unburnt sites germinated better than unburnt seeds on burnt sites.  we conclude that a combination of factors may be responsible for the success of a. karroo and that fires may not be hot enough or may occur at the wrong time of year to control a. karroo establishment in hip. although germination and establishment of a. karroo was higher than for a. nilotica a competitive advantage after fire could not be shown. 