Do herbivores facilitate seed germination and seedling recruitment of woody plants?.

Abstract

Woody plant encroachment is a phenomenon whereby trees and shrubs invade grasslands or increase in an already wooded area, resulting in lower yields of herbaceous plants and a reduction in the carrying capacity of rangelands. It is not only the extent of woody plant encroachment, but also the rate at which it occurs, that is a major concern for livestock ranchers interested in herbaceous production. The question of what causes woody plant encroachment still remains unanswered. Animals consume a considerable amount of woody plant seeds during the dry season and could be responsible for spreading undesirable species to new areas or they could enhance the dispersal of species already established. However, under natural conditions, seed dispersal does not guarantee seed germination, which depends on appropriate environmental and seed survival conditions. To better understand the mechanisms involved in woody plant encroachment, we studied the effects of livestock (goats, sheep and cattle), diet quality (high-quality (Medicago sativa hay) vs. low-quality (Digtaria eriantha hay)), seed characteristics (size, shape and hardness), grass competition and fire on germination and seedling recruitment of Dichrostachys cinerea and Acacia nilotica seeds. These objectives were achieved by conducting the following trials: a) recovery and germination of D. cinerea seeds fed to goats, b) diet quality on germination of Dichrostachys cinerea and Acacia nilotica seeds fed to ruminants, c) the effects of gut passage and dung fertilization on seedling establishment of Dichrostachys cinerea and Acacia nilotica seeds and d) the effects of gut passage, dung fertilization, trampling, grass competition and fire on seedling recruitment of the two woody plant species. During the first trial, seed recovery rate of D. cinerea seeds consumed by goats, either voluntarily after mixing them with feed pellets (mixed), or by force-feeding (gavaged) and germination percentage were measured. Seed recovery for the gavaged treatment (32.7%) was significantly higher than for the mixed treatments (9.9%; P < 0.001). Seeds that passed through the digestive tract (mixed (35.5%)) and gavaged (31.2%) treatments) had a significantly higher germination percentage than untreated seeds (19.0%). A non-negligible proportion of D. cinerea seeds remained intact after chewing and passage through the digestive system, and their germination percentage increased. In the second trial, I tested the effects of animal species (goats and sheep, goats and cattle), diet quality (Medicago sativa hay and Digitaria eriantha hay), and seed characteristics (size, shape and hardness) on the effectiveness of animal seed recovery and germination of D. cinerea and A. nilotica seeds. The trial was divided into two experiments. In the first experiment, a significant interaction effect of animal species (goats, sheep), diet (high-quality hay, low-quality hay) and seed species (A. nilotica seeds, D. cinerea seeds) was found on percentage germination (P < 0.0001). There was also a higher percentage seed recovery (P < 0.009) when animals were offered high-quality hay (47.4% + 4.65) compared to low-quality hay (30.2% + 3.24). In goats fed D. eriantha hay, A. nilotica seed germination (9.38% + 3.66) was higher (P < 0.05) than goats fed D. eriantha hay and D. cinerea seeds (6.78% + 1.13). A greater germination percentage was observed in goats fed M. sativa hay with D. cinerea seeds (6.71% + 1.53) than goats fed M. sativa hay with A. nilotica seeds (2.50% + 0.97) (P < 0.05). In the second experiment, animal species had a positive impact, both on seed recovery (P < 0.0325; goats 32.0% + 6.44; cattle 50.3% + 4.27) and germination percentage (P < 0.055; goats 14.1% + 1.48; cattle 9.3% + 0.94; control: D. cinerea 0.64 + 0.06; A. nilotica 0.59 + 0.07). Animal species in experiment one (goats and sheep) and two (goats and cattle) was most important for seed recovery and germination. However, diet (M. sativa hay and D. eriantha hay) and seed species (D. cinerea, A. nilotica seeds) also had important effects on germination of seeds retrieved from experiment one. The interaction of animal species and size, diet quality, and seed characteristics (size, hardness) all played a major role in recovery of viable and scarified seeds either alone or in combination. I also studied the effects of seedling emergence, seedling establishment and recruitment of dispersed A. nilotica and D. cinerea seeds by goats and cattle under natural conditions. The interaction effect of animal species, seed recovery day and seed germination treatment/planting method was significant on seedling recruitment. Seeds retrieved from goats in the last four days and planted 2 cm in the soil with dung (25.85% ± 0.46) and seeds planted 2 cm in the soil with no dung (24.77% ± 0.35), recruited significantly better than seeds planted on top of the soil (16.98% ± 0.46). The results also indicated significant differences in percentage seedling recruitment among goats, cattle and control, with goats and cattle having the highest percentage recruitment than controls or untreated seeds. Overall, seeds can potentially germinate and recruit following passage through the gut, thereby facilitating woody plant encroachment. The results of the last trial showed that seed passage through the digestive tract of goats and cattle compared to untreated seeds (i.e. not ingested) played an important role in improving germination through seed scarification. However, seed recovery by livestock does not guarantee seedling establishment survival, survival and recruitment. Fire and grass mowing treatments affected seedling emergence, seedling survival and recruitment, most probably because of reduced grass competition, thereby reducing competition for resources (light, water and nutrients) between grasses and seedlings. In conclusion, this study indicated that animal species (goats, sheep and cattle), associated diet (low-quality vs. high-quality), seed species (D. cinerea and A. nilotica) and seed characteristics (size, shape, hardness) all played an important role in seed germination. The interactions of animal species, fire, dung, and season either directly or indirectly were pivotal in the emergence, survival and recruitment of D. cinerea and A. nilotica seedlings. Thus, acid scarification in the gut of herbivores in combination with their indirect effects (dung fertilization) and removal of grass competition (either by fire or mowing) can facilitate seedling emergence, seedling survival and recruitment of woody plant species, which may lead to woody plant encroachment.