Raf Aerts ,

1. Critical phases in the natural regeneration of African wild olive in Northern Ethiopia(Olea europaea subsp. cuspidata) Raf Aerts, Eva November, Wouter Maes, Ives Van der Borght, Mintesinot Behailu, Martin Hermy and Bart Muys Mekelle University (Ethiopia) K.U. Leuven (Belgium)

2. Olea regeneration occurs under early successional shrubs • Earlier research by Aerts et al. (2003) and Van der Borght (2003) indicates that: • in forest rehabilitation areas (grazing exclosures) and grazing land, Olea recruitment only occurs under early successional shrubs, not in open field; • certain shrub structures are better recruitment foci than others; and • Euclea racemosa supports the majority of recent Olea recruits although Acacia etbaica is the dominant shrub in the research area.

3. The majority of Olea recruits is found under Euclea racemosa [TGY: k’uleaw] • Recruit position is highly dependent on shrub type (P < 0,01), irrespective of land use, or: Olea recruits are not randomly distributed over the shrub pool: Euclea (70%) > Acacia (25%) > Aloe (5%) (a = 0,05) • Euclea shrubs (overall relative density: 11%) are 13 times more attractive than the dominant Acacia shrubs (o.r.d.: 54 %).

4. Hypotheses:why more Olea under Euclea? • Euclea offers better protection and a ‘fruit reward’ and therefore attracts more avian seed dispersers than other shrubs (directed dispersal hypothesis); • germination is more successful in the mulch and microclimate under Euclea shrubs; and • despite expected stronger root competition and post-dispersal seed predation, survival rates of Olea seedlings are higher under Euclea because it offers better protection against grazing pressure and desiccation (nurse plant hypothesis). Acacia Euclea

5. Experimental sites • Experiments designed to test the hypotheses formulated above have taken place in the following sites: • Core area: central Geba river catchment • (Doga Tembien & Enderta districts) • 2 closed areas in Mheni and Sesemat • 10 forest remnants (Van Overtveld, 2003) Additional area: Bubu Hills (Mekelle) - 1 closed area Ex-situ area: Endayesus tree nursery (Mekelle University)

6. Conceptual framework • The following processes have an impact on Olea recruitment; downward arrows are drains or losses (detailed diagram: see excursion guide, page 10): Seed production in the source Seed dispersal Seed influx Germination Seedling growth Sapling Young tree Disease Fruit predation Lack of dispersal Post-dispersal seed predation Seed outflux Unsuitable site (no germination) Seedling predation (grazing/browsing) Competition Sapling termination (browsing/cutting) Regrowth Formation of dwarf shrubs (persistent rootstocks)

7. 1. Seed input • Seed influx/loss (secondary dispersal) • Apart from a higher directed dispersal by birds (primary dispersal), higher seed influx and/or less seed losses by run-off water can result in higher recruitment under Euclea. Shrub structure is believed to play an important role in this process. • M&M: deposition of marked seeds in 5 twin 3*3 m² plots around representative Euclea and Acacia perches; assessment of seed positions after the rainy season.

8. Example: Influx/Loss twinplot Mhe2 (Set 20030822) Mhe2.1 Mhe2.2 • Mhe2.1 • Euclea • = 0,139 • Mhe2.2 • Acacia •  = 0,094 Seed influx 29 marked seeds dropped each 10 cm at the top of each twinplot A ditch at the bottom of each plot allows for recovery and count of seeds that ‘left’ the plot. A pluviometer, measurement of slope and aspect and surface roughness estimation by chain method (Saleh, 1993) allow for comparing different plots. Seed loss 10 marked seeds dropped from random perching sites in the crown of each perch  (rho) = (L-l)·L-1 0    1 0 = perfectly smooth 1 = infinitely rough

9. Mheni: large rain event on 29/08/2003 • Mhe1 31 mm, Mhe2 26 mm in 1 hour • Significant seed movement

10. 1. Seed influx/loss: results • Seed outside shrubs can move over fairly large distances. • Water is not flowing through shrubs: it is diverted by the shrub (bridgehead effect), hence influx effect is very limited. • Two types of rain in terms of seed movement: • Non-erosive rains: embedding rains (seeds are embedded in the gravel matrix in the open field) • Erosive rains: translocating rains (seeds are displaced by run-off, but not into shrubs)

11. 2. Post-dispersal seed predation • The spatial pattern of Olea recruits depends on the pattern of seed deposition by avian dispersers. However, this pattern can be altered by post-dispersal seed predation by rodents and gallinaceous birds. • Rodents tend to avoid open areas to prevent being attacked by their natural enemies (e.g. birds of prey). • Therefore, the possibility of seed encounter and subsequent predation is likely to be higher under shrubs as compared to seeds in the open field. • M&M: seed depots with 10 seeds and 3 dummies placed in a regular grid in grazing exclosures. Seed counts after 1, 3, 6, 9 days and so on.

12. 2. Seed predation:results • Post-dispersal Olea seed predation is highly variable, but in general rather low. • Vicinity of stone-bunds rather than the shrub type affects predation rates (most predation at bottom of stone bund B) E A O T B

13. 3. Germination • Two major factors are thought to have significant effect on germination of Olea seeds: • Pulp removal and alteration of the structure of the endocarp by avian seed dispersers • Microclimate of the germination site (e.g. moisture content, temperature and microbial activity)

14. 3. Germination (continued) • M&M: three germination trials • 1. In-situ germination trial: shrub effect. Direct sowing of seeds in the open field and under Euclea and Acacia shrubs. 2 sites (Mheni, Bubu), 2 replications per site, 15 units per replication (5 Euclea, 5 Acacia and 5 open areas), 10 seeds per unit. Total number of seeds: 600 • 2. Ex-situ germination trial: mulch effect. Collection of mulch from random Euclea and Acacia shrubs, from the open field and from Chenferes church forest (natural regeneration of Olea under closed canopy). Reconstruction of the mulch in seedbeds in the Endayesus nursery. Direct sowing of seeds in the reconstructed mulch layers. 5 replications per mulch type, 20 seeds per replication. Total number of seeds: 400. All seeds were regurgitated or defecated by birds.

15. 3. Ex-situ germination trial: bird handling effect. Two-factor experiment to test the effect of birds and the effect of mechanical treatments: I. pulp removal (by birds, manual, intact pulp); II. mechanical treatment (cracking, scarification, control). 7 treatments, 3 replications per treatment, 50 seeds per replication. Total number of seeds: 1050 (2002); 1050 (2003) Ex-situ germination Mulch reconstruction Clockwise from top left: Euclea mulch, Acacia mulch, church forest mulch, open field mulch.

16. 3. Germination: results Cumulative germination rates of O. europaea seeds subjected to different treatments prior to direct sowing (2002-2003): pulp naturally removed by birds, combined with mechanical scarification (), endocarp splitting () and control (); pulp manually removed, combined with mechanical scarification (), endocarp splitting () and control (); intact ripe drupe (overall control) () Results show that around 15 % of regurgitated or defaecated seeds germinate during the same rainy season. The other fraction stays dormant until the start of the next rainy season. It is clear that under natural conditions, pulp removal by birds significantly enhances germination. Bird handling effect

17. 3. Germination: results (continued) Shrub effect Mulch effect In-situ low levels of germination; better germination of Olea under Euclea shrubs Ex-situ no effect of mulch type on germination levels Soil moisture critical for Olea regeneration

18. 4. Seedling survival • Microclimate, protection, soil conditions and competition can affect seedling survival rates. All these factors are highly dependent on shrub type. Since more Olea recruits are found under Euclea, growth and survival conditions are believed to be more suitable under this shrub than under Acacia or in the open field. • 2 experiments: • 4.1 Enrichment planting • 4.2 Controlled grazing

19. 4.1. Enrichment planting • Seedlings of Olea raised under nursery conditions were planted under Euclea, Acacia and in the open field (5 replications per treatment in each site) in 6 sites in Mheni, Bubu and Sesemat. Seedlings were planted twice in Mheni and Bubu (April/May 2003 and July 2003), once in Sesemat (August 2003). Total number of seedlings: 150. Survival rates and overall condition of the seedlings were checked on a weekly basis.

20. Enrichment planting Open field Acacia Euclea

21. Example: Survived seedlings Site: Bubu Date of planting: 17/04 and 09/05 Left: under dense Acacia Right: under Euclea • Although enrichment planting was carried out in grazing exclosures, many seedlings in the Mheni site were destroyed by human activity (deliberately). • Few seedlings from the first planting (small rainy season) still survive. • Seedlings were watered with only one liter just after planting. • Drought stress caused the seedlings of the first planting action to shed their leaves soon after planting. Those seedlings that survived formed smal twigs and small, thicker leaves at the original leaf bases (reduced leaf area).

22. Euclea In the drier sites (e.g. Sesemat), seedling survival is better under Euclea. In Bubu (more moist), seedling survival is better under shrubs (both under Acacia and Euclea). Seedlings in the open field have low chance for survival. Euclea and Acacia

23. 4.2. Controlled grazing • Van der Borght (2003) showed that structural traits of shrubs such as number of stems, diameter at ground level, crown density and thorniness determine presence or absence of Olea recruits. • These results suggest that protection against grazing and browsing is an essential factor for Olea recruitment. • To test the effect of shrub type on protection against grazing and browsing, a controlled grazing experiment was designed: • Olea seedlings were simulated with fresh cuttings from saplings • For each replication, a demarcated area of 30*30 m² was enriched with 30 simulated seedlings – 3 cuttings under each of 10 randomly selected Euclea shrubs, Acacia shrubs and in 10 open areas. • Controlled grazing pressure was applied to the plot and seedling survival checked after a predetermined time interval.

24. Example: controlled grazing • Under high browsing pressure, seedling survival approaches 0%. • Seedling survival is highly dependent on movement of browsers. • Euclea shrubs even attract animals due to high herbaceous cover and shade underneath.

25. Example: controlled grazing 16 LSU.ha-1.h (13 goats, 20’)

26. 4.2. Controlled grazing (continued) • Grazing pressure in ‘pressure-hours’: • Lifestock-units per ha • hours • GP = fLSU • nanimals • t/60 • 10000m² • ha-1/900m² • fLSU = 1 (cows); fLSU = 0.20 (goats) • This experiment is designed to test the effect of different species (browsers vs. grazers) on seedling predation at comparable levels of grazing pressure. Results can be used to indicate GP limits with minimal impact on Olea regeneration.

27. Survival of (simulated) olive seedlings in relation to uncorrected (top) and corrected (bottom) grazing pressure. Seedlings in the open field are detected and predated sooner at low grazing pressures.

28. 4.2. Controlled grazing: empirical model Initial mortality under low grazing pressure: seedlings in open areas very vulnerable Critical grazing pressure Rapid decline of survival under overgrazing Seedling survival remains relatively stable under increasing grazing pressure

29. Conclusions • The only process with a significant difference between Euclea and Acacia shrubs is germination. • The differences found in this process can not explain the total difference in O. europaea seedling density between the two shrub species. Higher seedling densities under Euclea are probably caused by a higher number of seeds dispersed directly under Euclea shrubs by birds. • These results suggest that O. europaea regeneration is subject to passive (directed dispersal) and active (moisture, shade) facilitation in the germination stage, leading to the conclusion that initial recruitment patterns are defined by disperser-mediated facilitation rather than herbivore-mediated recruitment limitation. • Efforts to restore African wild olive woodlands should therefore concentrate on increasing facilitation for seedlings and reducing competition for saplings.

30. Recommendations • Direct sowing of regurgitated or defecated O. europaea seeds under dense Euclea shrubs (or similar shrub types, such as Dodonea) is an inexpensive technique to increase O. europaea regeneration in grazing exclosures (increase facilitation). • A second technique is enrichment planting of seedlings raised in nurseries. Seedlings planted under the protective crown of Euclea are more likely to survive than seedlings planted in the open field. • Once seedlings are established, pruning of the nurse shrubs and removal of competitive grasses will enhance seedling growth (reduction of competition).

31. Acknowledgements The people of Mheni and Sesemat, the woreda councils of Doga Tembien and Enderta, Mekelle City Agriculture office, Mekelle University, K.U. Leuven, VL.I.R. EI and UOS, the Endayesus tree nursery staff members, Aklilu Negussie and all our M.U. PAP and B.Sc. students: thank you very much.