Dry season watering alters the significance of climate factors influencing phenology and growth of saplings of savanna woody species in central Zambia,southern Africa

Although tree growth in southern African savannas is correlated with rainfall in the wet season, some studies have shown that tree growth is controlled more by rainfall in the dry season. If more rainfall occurred in the dry season in future climates, it would affect the growth of savanna trees, especially saplings that have shallower roots which limit access to subsoil water during the dry season when leaf flush and shoot extension occur. Recent paleobotanical evidence has revealed that there was relatively more precipitation in the dry season in eastern Africa in the Eocene than under the current climate. Saplings therefore can be expected to respond more to water addition during the dry season than mature trees that have more stored water and deeper roots that access subsoil water. Accordingly, I hypothesized that irrigation in the dry season should (i) advance the onset of the growing season, (ii) increase growth rates and (iii) alter the growth responses of saplings to climate factors. To test these hypotheses saplings of five savanna woody species were irrigated during the hot‐dry season at a site in central Zambia and their monthly and annual growth rates compared to those of conspecifics growing under control conditions. Although the responses among the species were variable, all irrigated saplings had significantly higher monthly and annual growth rates than control plants. In addition, dry season watering significantly altered the climatic determinants of sapling growth by either strengthening the role of the same climatic factors that were important under control conditions or displacing them altogether. In conclusion, more precipitation during the hot‐dry season is likely to have significant positive effects on sapling growth and consequently reduce the sapling‐tree transition periods and promote future tree population recruitment in some southern African savanna tree species.     

Fitness implications of late bud break and time of burning in Lannea edulis (Sond.) Engl. (Anacardiaceae)

Many woody plants of savannas have massive underground parts (“lignotubers”) and diminutive aboveground parts with phenologies that are triggered by fire. Lannea edulis (Anacardiaceae) represents this life form and is widely distributed in south central Africa. The phenology of L. edulis was monitored on 42 permanently marked shoots over a 3-year period, from 2002 to 2004, and under three fire treatments (early and middle dry season burning, and fire protection) to determine whether (i) fire affected time of bud break and (ii) time of fruit ripening affected seed germination. Statistical analyses revealed that shoot reproductive status and fire treatments significantly affected time of bud break and the lag between bud break and leaf production. Shoot reproductive status explained 27% (P<0.001) while the interaction between reproductive status and fire treatments explained 34% (P<0.0001) of the variation in time at bud break. The appearance of the first leaf was delayed by 4 weeks in reproductive shoots compared with vegetative shoots. Shoot reproductive status explained 28% (P<0.0001) of the variation in the lag between bud break and appearance of the first leaf suggesting the existence of resource competition between reproduction and vegetative growth. However, reproductive status and fire treatments did not affect shoot size and leaves produced because replacement shoots after dieback due to fire were as long as shoots surviving from previous years due to insignificant annual shoot extension among the latter. There was high autocorrelation in the germination rate of seed-lots with seeds from fruits ripening early having a significantly higher germination rate (47% in 2002 and 89% in 2004) than those from fruits ripening later (3% in 2002 and 53% in 2004). Since time at bud break and fruit ripening were linked, it is proposed that late bud break results in reduced fitness in L. edulis because of reduced seed germination rate. Early and late dry season fires that delay bud break and destroy reproductive structures, respectively, also reduce this measure of fitness.     

Seedling ecology of two miombo woodland trees

The development of seedlings of two miombo trees, Brachystegia spiciformis Benth. and Julbernardia paniculata (Benth.) Troupin, was studied during two growing seasons (December 1989–April 1991) at a Zambian grassland site. Seed germination rates under laboratory and field conditions were not significantly different although germination in the field was delayed by 1–2 weeks due to insufficient rainfall. After one year of storage J. paniculata seed germination had declined from 67% to 17% while germination of B. spiciformis seeds remained at about 83%.Leaf production was confined to the rainy season. Leaf fall occurred during the dry season and in J. paniculata this was followed by shoot die-back during the hot dry period (August–November). Two-thirds of B. spiciformis seedlings experienced shoot die-back but shoot die-back did not necessarily result in seedling mortality. Seedling deaths occurred during the germination period (6–10 weeks after planting) and in the hot dry period (40–50 weeks after planting) during September–November. Survivorship of B. spiciformis seedlings was 74% at the end of the second growing season while this was 46% for J. paniculata.Shoot growth was negligible during the second growing season. In fact mean maximum leaf area of B. spiciformis decreased significantly from 19.7 cm² (SD=5.7) per plant at the end of the first growing season to 13.3 cm² (SD=5.8) at the end of the second growing season (t=3.31, P<0.01). However, root biomass of B. spiciformis seedlings increased 2.8 times during the second growing season.These results suggest that shoot die-back in seedlings of miombo trees is caused by drought and that the slow shoot growth is the result of allocating most of the biomass to root growth during seedling development.     

Distribution and abundance of a keystone tree, <Emphasis Type="Italic">Schinziophyton rautanenii</Emphasis>, and factors affecting its structure in Zambia,southern Africa

Schinziophyton rautanenii is a keystone tree species whose fruits are eaten by wildlife and livestock, nuts are used to extract oil for human use and its wood is used for making curios that are sold to tourists. The species occurs in southern African countries of Angola, Namibia, Botswana, Mozambique, Zambia, Zimbabwe and the Democratic Republic of Congo but little is known about its population and conservation status. The objectives of this study were to (i) assess factors determining the distribution and abundance of the species in Zambia, (ii) evaluate its regeneration potential and population status and (iii) assess whether the main determinants of forest and species stand structures also favour S. rautanenii. The study is based on three forest surveys conducted in 2004, 2005–2008 and 2015. In Zambia S. rautanenii abundance and population structure are controlled by soil, climate and human factors. The species is most abundant in southwest Zambia where population recruitment is good but is rare and recruitment is poor elsewhere in the country. Fire appears to be a major cause of tree damage and possibly death. The elephant is a major disperser of S. rautanenii nuts and the low recruitment levels in the country may partly be attributed to the decline in the elephant population from about 250,000 in 1960 to 28,000 in 2008. Different conservation strategies will be required for areas where the species is rare and for those where the species is abundant. Whatever the strategies, it will be important to address issues of fire management, fruit harvesting and the role of wildlife in the conservation of S. rautanenii in Zambia.     

Effects of seed burial and fire on seedling and sapling recruitment, survival and growth of African savanna woody plant species

Seed predation and seedling mortality can act as strong demographic “bottlenecks” to sapling recruitment in African savanna woodlands. Fire also limits tree recruitment from saplings by suppressing their growth. I conducted field experiments with 13 woody plant species to assess the effects of seed burial on seedling emergence rates and effects of fire on seedling and sapling survival and growth rates over a period of 8 years at a savanna plot in central Zambia, southern Africa. Seed removal rates by small rodents varied among years and buried seeds had significantly higher emergence rates than seeds exposed to predators in most but not all the species. Annual burning reduced sapling growth in some species but in other species saplings experienced successive shoot die back even in the absence of fire. The findings show that for some woody species, seed predation is an important constraint to seedling recruitment but not for others and annual fires are important hindrances to demography and growth for some species but not others. Thus, demographic “bottlenecks” occur at different life history stages in different savanna woody species and these have the potential to alter woody tree competitive relationships and ultimately savanna structure.     

Effects of climate on the growth of exotic and indigenous trees in central Zambia

Aim Climate change has far‐reaching effects on species and ecosystems. The aims of this study were to determine how climate factors affect the growth pattern of indigenous and exotic trees in Zambia and to predict tree growth responses to a warmer climate with the use of mathematical models. Location Two savanna sites in central Zambia. Methods Diameter at breast height (1.3 m above ground, d.b.h.) of 91 permanently marked trees belonging to three indigenous and four exotic species was measured fortnightly for periods of 1–2 years from 1998 to 2003. Correlation and regression analysis was used to determine the effect of climate factors (minimum, maximum and average temperature and rainfall) on monthly daily d.b.h. increment of each species. Regression models were used to predict the growth behaviour of trees under a 0.5 °C warmer climate. Results Interactions between temperature and rainfall explained 60–98% of the variation in d.b.h. increment in all the tree species, except the exotic Eucalyptus grandis. For deciduous species, stem expansion was delayed by 2–12 weeks following leaf‐flush and d.b.h. increment peaked during the rainy season. Evergreen and deciduous species could not be separated on the basis of annual d.b.h. increment because the higher growth rates of deciduous species compensated for the shorter growing period. Mathematical models predicted slight changes in d.b.h. growth pattern under a 0.5 °C warmer climate in five of the seven species. Significant changes in d.b.h. growth patterns were predicted in the indigenous Bridelia micrantha and exotic Gmelina arborea under a warmer climate. However, models failed to adequately represent potential soil water stress that might result from changes in tree growth patterns and a warmer climate. Main conclusions Climate factors explained a large proportion of the variation in diameter growth of both indigenous and exotic trees, rendering it possible to model tree growth patterns from climate data. Tree growth models suggest that a rise in temperature of 0.5 °C is unlikely to induce significant changes in the growth behaviour of the majority of the studied species. However, because the growth behaviour of some species may be substantially affected by climate change, it is recommended that strategies for the future production of such climate‐sensitive trees should incorporate aspects of climate change.     

Seedling development of the miombo woodland tree Julbernardia globiflora

Abstract. The development of seedlings of the miombo tree (Julbernardia globiflora) was studied for 28 months (December 1987 - April 1990) at a Zambian grassland site from which fire has been excluded for many years. Germination occurred 10–25 days after seed planting and germination rates under laboratory and field conditions did not differ significantly. Seed germinability after one year of storage declined to 38 %. Leaf production occurred between December and April with peaks in January each year. Leaf fall occurred steadily throughout the dry season but leafless seedlings were first observed in August of each year. All seedlings had shed all their leaves by the end of November during the first year while 10% of the seedlings remained leafy throughout the second year. Annual seedling mortality was about 60 % during the first year and declined to 3 6 % during the second year. Survivorship appeared to stabilise at 3 8 % when seedlings were 5 5 - 5 9 weeks old. Every seedling experienced shoot die-back during the first hot dry season (September - November) when open-pan evaporation rates averaged 5–10 mm / day. However, shoot mortality did not necessarily result in plant mortality. Because of recurrent shoot die-back there was little increase in shoot biomass and height at the end of the second and third growing seasons. Seedlings that survived shoot die-back sprouted from roots which showed a steady annual accumulation of biomass. Early shoot die-back in seedlings of Julbernardia globiflora as documented in this study was not causedby fire but most probably by drought and / or nutritional stress.     

Fruit Production and Seed Predation in Two Miombo Woodland Trees in Zambia

During a six year period (1990–1995) fruiting frequency among marked trees and seed production and predispersal predation by beetle larvae of lsoberlinia angolensis and Julbernardia globiflora of the Leguminosae family and Caesalpinoideae subfamily were studied at two dry miombo woodland sites in central Zambia. Fruit production varied from year to year and was not related to annual rainfall. On average I. angolensis trees fruited once every two years while J. globiflora trees fruited once every two and one-half years. Number of fruits per tree was correlated with tree size for both species. In a peak year fruit biomass represented two to four percent of total above ground biomass but for J. globiflora about 5.5 and 4.5 percent of nitrogen and potassium, respectively, were in fruits compared to 0.6 and 1.8 percent for I. angolensis. The lower fruiting frequency in J. globifora was probably related to the depletion of N reserves during a fruiting year which required a longer replenishment period than in I. angolensis. Predispersal seed predation was low for J. globiflora (10%) and very high in I. angolensis (65–85%) and for the latter species predation rate increased from August to November 1991 and significant variations were observed among years and pod types. Seed predation rate was lowest in a peak fruiting year and this observation provides support for the satiation hypothesis. The results of the study also suggest that I. angolensis and J. globiflora may have different regeneration strategies although reproductive allocation in both species was within the range reported for other iteroparous plants.     

Demographic implications of life-history stage characteristics in two African acacias at a Makeni savanna plot in Zambia

Aims In spite of the importance of African acacias in vegetation succession and provision of goods and services, little is known about life-history variations within and among species. Much of the work done on African acacias has focused on seed predation and germination and seedling establishment, especially of Acacia tortilis, Acacia nilotica and Acacia karroo. The primary aim of the present work is to investigate differences in the demography of Acacia polyacantha and Acacia sieberiana and the relationship between life-history characteristics and population size. A secondary objective is to assess how fire, an important ecological factor in savanna vegetation, might modify the growth and demographic dynamics of the two acacias.Methods The study was conducted at the Makeni savanna plot in central Zambia, southern Africa. Seedling emergence from both non-scarified and scarified seeds sown at different times in the wet season and the fate and growth of seedlings and saplings were monitored over a period of 4 years. Annual growth of permanently marked sample trees in annually burnt and fire-protected blocks was recorded over a 6-year period (2002–08) in order to assess inter-specific differences and how fire modifies tree growth patterns. Censuses of natural saplings and trees were conducted periodically in sample blocks to determine recruitment into these life-history stages.Important findings Seedling emergence and sapling survival rates were much higher in A. sieberiana than in A. polyacantha. However, both seedling and sapling growth rates were higher in A. polyacantha than in A. sieberiana but tree growth rates were similar in the two species. Under fire protection tree growth was significantly influenced by tree size and year while under annual burning only tree size significantly affected tree growth. The dominance of A. sieberiana over A. polyacantha at the study site was attributed to higher seedling emergence rate, higher sapling survival rate and a large sapling bank that forms a reliable source of tree recruitment. The life-history stage characteristics of A. polyacantha suggest that this is an early successional species.     

Fitness Implications of Clonal Integration and Leaf Dynamics in a Stoloniferous Herb, Nelsonia canescens (Lam.) Spreng (Nelsoniaceae)

Very little has been published on the life-history significance of clonal plants inhabiting southern African savanna environments. This study investigated the fitness implications of clonal integration, resprouting behaviour and growth phenology in a stoloniferous herb, Nelsonia canescens (blue pussyleaf) at a savanna site in Zambia, central Africa. Census data on growth and survival were obtained regularly on permanently marked ramets over a 4-year period, from 2001 to 2005, and analyzed to assess how physiological integration and module demography contribute to fitness in Nelsonia. Above ground and below ground growth occurred during the dry and rainy seasons, respectively. Dry season growth was characterized by resprouting and production of stolons that bore small pubescent leaves with high mortality (30–80% month⁻¹). Deep roots and high leaf turnover appear to contribute to sustained growth during the dry season when topsoil moisture and nutrient availability are low. The interaction between maximum temperature and precipitation explained a significant proportion (59%, p<0.01) of the monthly variation in leaf size and increasing evapo-transpiration levels appeared to trigger the shift in leaf size from a large wet season type to a small dry season one. During the dry season Nelsonia resprouted from dormant buds buried at the time of root development in daughter ramets in the rainy season. Temporal integration significantly (p<0.05) enhanced survival of daughter ramets. However, daughter ramets with severed mother–daughter ramet inter-connectors experienced high initial mortality that was caused by both early stolon severing and drought stress during the root development phase. The majority of ramets lived for 5–10 months and 25% were still alive at the age of 3.5 years. The study showed that although the growth phenology of Nelsonia has serious ecological implications for accessing scarce resources during the dry season, the species utilizes a number of strategies to overcome resource limitations in a seasonally heterogenous environment. Co-ordinating editor: G. P. Cheplick