Edmund C. February

Determinants of woody cover in African savannas

Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than ∼650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered ‘stable’ systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of ∼650 mm, savannas are ‘unstable’ systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation may considerably affect their distribution and dynamics.

EFFECTS OF FOUR DECADES OF FIRE MANIPULATION ON WOODY VEGETATION STRUCTURE IN SAVANNA

The amount of carbon stored in savannas represents a significant uncertainty in global carbon budgets, primarily because fire causes actual biomass to differ from potential biomass. We analyzed the structural response of woody plants to long-term experimental burning in savannas. The experiment uses a randomized block design to examine fire exclusion and the season and frequency of burn in 192 7-ha experimental plots located in four different savanna ecosystems. Although previous studies would lead us to expect tree density to respond to the fire regime, our results, obtained from four different savanna ecosystems, suggest that the density of woody individuals was unresponsive to fire. The relative dominance of small trees was, however, highly responsive to fire regime. The observed shift in the structure of tree populations has potentially large impacts on the carbon balance. However, the response of tree biomass to fire of the different savannas studied were different, making it difficult to generalize about the extent to which fire can be used to manipulate carbon sequestration in savannas. This study provides evidence that savannas are demographically resilient to fire, but structurally responsive.

Diverse functional responses to drought in a Mediterranean-type shrubland in South Africa

• Mediterranean-type ecosystems contain 20% of all vascular plant diversity on Earth and have been identified as being particularly threatened by future increases in drought. Of particular concern is the Cape Floral Region of South Africa, a global biodiversity hotspot, yet there are limited experimental data to validate predicted impacts on the flora. In a field rainout experiment, we tested whether rooting depth and degree of isohydry or anisohydry could aid in the functional classification of drought responses across diverse growth forms. • We imposed a 6-month summer drought, for 2 yr, in a mountain fynbos shrubland. We monitored a suite of parameters, from physiological traits to morphological outcomes, in seven species comprising the three dominant growth forms (deep-rooted proteoid shrubs, shallow-rooted ericoid shrubs and graminoid restioids). • There was considerable variation in drought response both between and within the growth forms. The shallow-rooted, anisohydric ericoid shrubs all suffered considerable reductions in growth and flowering and increased mortality. By contrast, the shallow-rooted, isohydric restioids and deep-rooted, isohydric proteoid shrubs were largely unaffected by the drought. • Rooting depth and degree of iso/anisohydry allow a first-order functional classification of drought response pathways in this flora. Consideration of additional traits would further refine this approach.

A depth-controlled tracer technique measures vertical, horizontal and temporal patterns of water use by trees and grasses in a subtropical savanna

• As described in the two-layer hypothesis, woody plants are often assumed to use deep soils to avoid competition with grasses. Yet the direct measurements of root activity needed to test this hypothesis are rare. • Here, we injected deuterated water into four soil depths, at four times of year, to measure the vertical and horizontal location of water uptake by trees and grasses in a mesic savanna in Kruger National Park, South Africa. • Trees absorbed 24, 59, 14 and 4% of tracer from the 5, 20, 50, and 120  cm depths, respectively, while grasses absorbed 61, 29, 9 and 0.3% of tracer from the same depths. Only 44% of root mass was in the top 20 cm. Trees absorbed tracer under and beyond their crowns, while 98% of tracer absorbed by grasses came from directly under the stem. • Trees and grasses partitioned soil resources (20 vs 5  cm), but this partitioning did not reflect, as suggested by the two-layer hypothesis, the ability of trees to access deep soil water that was unavailable to grasses. Because root mass was a poor indicator of root activity, our results highlight the importance of precise root activity measurements.

Influence of competition and rainfall manipulation on the growth responses of savanna trees and grasses

In this study, we explored how rainfall manipulation influenced competitive interactions between grasses and juvenile trees (small nonreproductive trees capable of resprouting) in savanna. To do this, we manipulated rainfall amount in the field using an incomplete factorial experiment that determined the effects of rainfall reduction, no manipulation, rainfall addition, and competition between grasses and trees on grass and tree growth. As response variables, we focused on several measures of tree growth and Disc Pasture Meter settling height as an estimate of grass aboveground biomass. We conducted the study over four years, at two sites in the Kruger National Park, South Africa. Our results show that rainfall manipulation did not have substantial effects on any of the measures of tree growth we considered. However, trees at plots where grasses had been removed grew on average 15 cm more in height and 1.3-1.7 times more in basal area per year than those in plots with grasses. Grass biomass was not influenced by the presence of trees but was significantly and positively influenced by rainfall addition. These findings were not fundamentally influenced by soil type or by prevailing precipitation, suggesting applicability of our results to a wide range of savannas. Our results suggest that, in savannas, increasing rainfall serves to increase the competitive pressure exerted by grasses on trees. The implication is that recruitment into the adult tree stage from the juvenile stage is most likely in drought years when there is little competition from grass for resources and grass fuel loads are low.

Nitrogen availability is not affected by frequent fire in a South African savanna

There is a perception that sustained frequent fires cause nitrogen limitation over the long term (50-100 y) by volatilizing the nitrogen in soil, plant biomass and litter. Here we test this perception in a South African savanna located in the Kruger National Park. At our study site we compare the effects of 50 y of fire exclusion, season (August and February) and frequency (triennial and annual August and triennial February) of burn on nitrogen cycling and availability. We do this using three different methods to determine nitrogen mineralization; in situ incubations, laboratory incubations and ion-exchange resin bags. On each treatment we established two parallel transects 100 m apart with 10 sampling points per treatment along these transects. Daily mineralization rates for in situ incubations were determined monthly from August 2004 to June 2005 at each of the sampling points. Ion-exchange resin bags were buried (5 cm) at the same points and left in the field from August 2004 to August 2005. In February 2005 five randomly located soil samples from each of the four treatments were collected for laboratory incubations using a 7-cm-diameter soil auger. Regardless of method used our results show that there are no significant differences in daily nitrogen mineralization rates after 50 y of different burning treatments from annual burning to fire exclusion. In fact, both in situ and laboratory incubations show that nitrogen availability is higher on the annual burn than the fire exclusion (0.16 μg g -1 soil d -1 vs. 0.11 μg g -1 soil d -1 and 0.46 μg g -1 soil d -1 vs. 0.30 μg g -1 soil d -1 respectively). Perceived negative effects of fire on ecosystem functioning has curbed the use of fire as a management tool with fire often actively suppressed in savanna. The results of our study show that fire can be used more vigorously in mesic African savanna to manipulate tree:grass ratios without negatively affecting the nitrogen cycle.

History matters: tree establishment variability and species turnover in an African savanna

Demographic variability in the tree component of savannas, arising from both spatial and temporal variability in fire, herbivory, and climate, is thought to be a key driver maintaining tree-grass co-existence in savannas. Modeling work has shown that this variability can be vital to the persistence of savanna as a biome and that variability in climate, herbivory and fire affect tree establishment. However, empirical evaluations of the idea that savanna tree establishment is variable have so far been limited to tests of the effects of major events (such as the East African rinderpest epidemic) on tree establishment. The degree to which tree establishment varies in savannas under less extreme variability in climate, herbivory and fire remains unknown. We tested whether establishment of adult trees was constant or episodic using tree ring counting of three Acacia species in Hluhluwe iMfolozi Park in South Africa. Patterns of establishment of adult trees were analyzed at three spatial scales for temporal and spatial patchiness. Establishment of adult trees was not constant for any of the three species of Acacia, and all three species showed significant spatial patchiness and larger-scale temporal variability in establishment patterns. Spatial patchiness highlights the importance of landscape processes like fire and herbivory, while larger-scale temporal variability indicates that climatic variability also structures tree demography. These results suggest that current tree cover in savannas is a product of ecological history and is not necessarily determined by current ecological processes. An historical perspective is essential for interpreting patterns of tree cover in savannas, in models of the effects of climate, fire and herbivory (especially elephants), in experimental work, and in analyses of continental patterns of tree cover alike.

The use of pre-dawn leaf water potential and MODIS LAI to explore seasonal trends in the phenology of Australian and southern African woodlands and savannas

Trends in global soil moisture are needed to inform models of soil–plant–atmosphere interactions. Predawn leaf water potential (Ψpd), a surrogate for soil moisture and an index of plant water stress, has been routinely collected in Australian forests, woodlands and savannas, but the associated leaf area index (LAI) has seldom been available to enable the preparation of a Ψpd on LAI relationship. Following an analysis of Ψpd and MODIS LAI data from Australian forests, woodlands and savannas, we identified patterns in Ψpd which provide an understanding of the role of soil-moisture status in controlling LAI. In the savanna of northern Australia, the MODIS LAI product had a basal value of 0.96 during the dry season as compared with a mean value of 2.5 for the wet season. The dry season value is equivalent to the LAI of the tree component and corresponds with ground-truthed LAI. Ψpd is lowest (more negative) during the height of the dry season (late October) at −2.5 MPa, and highest (−0.1 MPa) during the wet season (early March). We present two models which predict Ψpd from the MODIS LAI product. These may be useful surrogates for studying trends in soil moisture in highly seasonal climates and may contribute to climate change research.

Rapid Leaf Deployment Strategies in a Deciduous Savanna.

Deciduous plants avoid the costs of maintaining leaves in the unfavourable season, but carry the costs of constructing new leaves every year. Deciduousness is therefore expected in ecological situations with pronounced seasonality and low costs of leaf construction. In our study system, a seasonally dry tropical savanna, many trees are deciduous, suggesting that leaf construction costs must be low. Previous studies have, however, shown that nitrogen is limiting in this system, suggesting that leaf construction costs are high. Here we examine this conundrum using a time series of soil moisture availability, leaf phenology and nitrogen distribution in the tree canopy to illustrate how trees resorb nitrogen before leaf abscission and use stored reserves of nitrogen and carbon to construct new leaves at the onset of the growing season. Our results show that trees deployed leaves shortly before and in anticipation of the first rains with its associated pulse of nitrogen mineralisation. Our results also show that trees rapidly constructed a full canopy of leaves within two weeks of the first rains. We detected an increase in leaf nitrogen content that corresponded with the first rains and with the movement of nitrogen to more distal branches, suggesting that stored nitrogen reserves are used to construct leaves. Furthermore the stable carbon isotope ratios (δ13C) of these leaves suggest the use of stored carbon for leaf construction. Our findings suggest that the early deployment of leaves using stored nitrogen and carbon reserves is a strategy that is integrally linked with the onset of the first rains. This strategy may confer a competitive advantage over species that deploy leaves at or after the onset of the rains.

New national and regional bryophyte records, 50

1. Andreaea flexuosa R.Br.bisContributors: R. Ochyra and N. J. M. GremmenHeard Island, Subantarctica: Eastern slope of Scarlet Hill, 340 m a.s.l., 53°06ʹ18ʺS, 73°38ʹ23ʺE, on red consolidated scoria...