Awarded second place in Biological Sciences for GPSC Student Showcase

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Many studies have reported deficient or intermittent patterns of natural regeneration in temperate deciduous woodland. The present study aimed to assess the relative impact of herbivore-mediated plant mortality on the natural regeneration dynamics of representative tree species (Acer pseudoplatanus, Betula pendula, Fraxinus excelsior, Ilex aquifolium, Sorbus aucuparia, Taxus baccata and Ulmus glabra) in representative temperate deciduous woodland (Shipley Wood and Derwent Gorge, County Durham). Sapling density and the density, spatial association and size distribution of adult tree populations varied significantly between tree species and study sites, reflecting contrasting patterns of current and historical regeneration. Rates of post-dispersal seed predation and seedling herbivory were quantified using field-based 'cafeteria' trials. Rodents were the principal agents responsible for seed predation, whereas seedling herbivory was attributable to a mixed suite of herbivores including invertebrates, rodents and larger mammals. Rates of seed predation and seedling herbivory varied significantly between tree species, most likely reflecting individualistic, trade-off responses to chemical and physical attributes. Fine-scale spatial variation most likely reflected the preferential foraging of rodents beneath protective vegetation cover. There was no consistent evidence to support the hypothesis that dispersed seeds may escape disproportionately high offspring mortality beneath parent plants resulting from increased herbivore activity. Natural seedling density varied significantly between years and between tree species, according to the abundance of viable seeds produced by conspecific adult trees. Although seedling survivorship varied significantly between species, the survivorship of each species was similar between years and between cohorts of the same year. Canopy cover, field layer cover or correlated factors were significant determinants of seed germination and seedling emergence, establishment and survival, according to age- and species-specific tolerances. In relative terms, patterns of natural regeneration were primarily herbivore- limited (Acer and Taxus), microsite-limited (Betula, Fraxinus, Ilex and Ulmus) or limited by herbivores and microsites (Sorbus). The availability of viable seeds may have also limited the recruitment of Ilex, Sorbus, Taxus and Ulmus. Vegetative expansion, mast seeding, seed bank regeneration and repeated, prolonged reproduction may have reduced the actual impact of herbivory on natural regeneration, such that long-lived iteroparous tree species were unlikely to have been critically dependent on current recruitment.

Wildfires play a significant role in many different elements of Mediterranean forest ecosystems. In recent years, prescribed fires have started being used more often as a fuel reduction tool, and also as silvicultural treatment to help the regeneration and health improvement of stands. Apart from the fact that fire may alter microsite conditions, very little is known about the impact of prescribed burning on natural regeneration or plant species renewal in Mediterranean pine forests. Likewise, knowledge about the influence of seedling predators on post-fire regeneration is still scarce. In this study, we aimed to compare the effects of seedling predation on recruitment in earlier stages after prescribed burnings in three pine stands in Central Spain: a pure stand of Pinus nigra; a mixed stand of Pinus halepensis and Pinus pinaster and a mixed stand P. nigra with P. pinaster. In situ we superficially sowed seeds from two different species. In the sowing experiment, we tested two different seed provenances (drier and more humid spanish regions) for each species. In all, 60 plots (30 burned, 30 unburned) per site, with 10 seeding units per plot and more than 20,000 seeds, were used in the whole study. Seedling predation was evaluated by replicating the seeding units inside and outside a wire cage as protection for rodents and birds. Our results showed that prescribed fires alter initial seedling predation intensity: predation was significantly higher in the seedlings grown in the plots affected by prescribed fire. The individuals sown before the fire passed showed slightly more predation than those sown after fire passage. Provenances did not appear as an important predation drive. Understanding the role of the predation associated with these treatments can help improve Mediterranean pine forest management.

Astragalus lentiginosus, which consists of 36 varieties, is one of the principal locoweeds of western North America. The objectives of this study included identification of some of the major factors affecting germination of the seed and survival of the newly established seedlings for several of these varieties. The seeds of the varieties studied exhibited two mechanisms for dormancy. One is a seed coat that is impermeable to water, and the other is a water-leachable inhibitor. The presence or absence of low-intensity incandescent light (in 14-hour day lengths) had no effect on germination. Of three temperature regimes studied, the regime 7/13 C proved optimal for both total and rapid rate of germination. However, though germinating at lower rates, weeds at -1/4 C and 21/27 C both ultimately reached high percentages of germination. Germination decreased linearly with increasing salinity to almost zero in NaCl solutions of -8.52 bars water potential. Variety araenosus, collected in a salt desert shrub vegetation type showed the highest germination at low temperatures and the greatest salinity tolerance. Seedling survival work was done with both pre-established and established seedlings at the cotyledon to five leaflet stages of development. Seedlings were considered pre-established from the time of germination until the cotyledons were completely unfolded. Afterwards, the seedlings were considered established. Germination and survival of pre-established seedlings were inhibited by day temperatures of 21 C under laboratory conditions. Maximum emergence and survival occurred at the low temperature regime of 4/13 C. However, seedlings of the three to five leaflet stages grew best at day temperatures of 30 C; growth was substantially lower at lower temperatures. Seedlings of var. araenosus displayed the most rapid rate of root growth at 7/12 C while roots of var. vitreus grew at a rate nearly 50 percent lower.

A trade-off between seed size and seed number is central in seed ecology, and has been suggested to be related to a trade-off between competition and colonization, as well as to a trade-off between stress tolerance and fecundity. Large seeds endure hazards during establishment, such as shading, drought, litter coverage and competition from other plants, better than do small seeds, due to a larger amount of stored resources in the seed. Small seeds, however, are numerous and small-seeded species are therefore more fecund. Moreover, a pattern with small-seeded species being associated with open habitats and large-seeded species being associated with closed habitats has been reported in the literature. In this thesis I assess effects of seed size on recruitment, and how relationships between seed size and recruitment may relate to habitat conditions. Seed sowing experiments were performed in the field to assess inter- and intra-specific relationships between seed size and recruitment in open and closed habitats (Paper I and II). Seed removal experiments were performed in the field to assess what effects seed predation may have on a relationship between seed size and recruitment (Paper III). A garden experiment was performed based on contests between larger-seeded and smaller-seeded species, in order to examine different models on co-existence of multiple seed size strategies. The results showed that there was a weak positive relationship between seed size and recruitment in the field, and that this relationship was only weakly and inconclusively related to habitat (Paper I and II). Seed removal was negatively related to seed size in closed habitats and unrelated to seed size in open habitats (Paper III). This indicates that any positive relationship between seed size and recruitment may be an effect of higher seed removal in small-seeded species. However, when grown under controlled conditions in a garden experiment, there was a clear advantage of larger-seeded species over smaller-seeded species (Paper IV). This advantage was unaffected by seed density, indicating that there was no competitive advantage of the larger-seeded species. Instead, indirect evidence suggests that larger-seeded species exhibit higher tolerance to stress. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Accepted.</p><p> </p>

Abstract The restoration of degraded landscapes has become one of our most valuable tools for conservation, however there are many factors which can restrict natural regeneration and impede active restoration attempts. The purpose of this study was to investigate three key processes which commonly limit the establishment of forest tree species into abandoned pasture in tropical forests: i) dispersal limitation, ii) seed predation, and iii) competition from the grass sward. Seed dispersal I identified 59 species of birds that were using the grassland habitat. Through 216 hours of focal tree observations I established that isolated trees in the grassland that had larger canopies, and those that were providing a food source (i.e. flowers or fruit) had significantly higher bird visitation rates and average stay lengths. I found evidence of the “perch effect” as patches of remnant trees encouraged more birds into areas of grassland, and the density of seedlings under tree canopies was significantly positively correlated with bird visitations. 95% of the seedlings found beneath tree canopies in grassland were of a different species to that of the tree canopy above them, demonstrating the dispersal of seeds from elsewhere into these microhabitats. 98% of these seedlings are grassland or forest edge species showing forest core species are still dispersal or microsite limited despite the effect of these trees. Seed predation Removal rates of seeds from experimentally laid out seed piles varied among seed species, the habitat the pile was in, and the predator guild able to access the piles. Preliminary results indicate that these trends are driven by the ecology of the seed predator. Removal of seeds by vertebrates was highest in the core forest, while ant predation was constant across all habitats. Vertebrates removed the larger seeds (Entandrophragma angolense and Sterculia tragacantha) while ants preferred the smaller Celtis gomphophylla and Croton macrostachyus. Overall predation rates in grassland were lower than those in the forest, and the presence of remnant trees did not influence predation rates, a positive sign for regeneration and the survival of seeds dispersed into these areas. Competition from the grass sward While the grass sward provides shade for seedlings of forest tree species it is also a harsh environment for them, as the grass competes with seedlings for water. Removing the grass and covering planted seedlings with artificial shading structures significantly increased both the survival and growth of these seedlings. Recommendations From this study I was able to make recommendations for a low input restoration program at Ngel Nyaki. Planting seedlings in small „islands‟ takes advantage of the natural increase in dispersal of seeds under isolated trees, while low seed predation rates increase the chance of survival of these seeds to germination. Planting these seedlings under shade will lead to increases both their growth rates and their survival. Once the secondary forest develops, under-planting seedlings of core forest trees will introduce them to the system, as the natural establishment of these seeds appears to be limited in the current environment. This study has also served to remind us how little we know about this particular forest-grassland system, and has led to the development of ideas for further investigations into several more aspects of regeneration.

Remote sensing of the environment has become an effective and useful research approach applied across a wide range of scientific and professional disciplines. Generally remote sensing is used to evaluate patterns and processes at broad spatio-temporal scales, such as classifying landscape vegetation patterns or for creating digital surface models, however, there are increasing opportunities to expand the use of remotely sensed information to a wider range of applications at variable spatial and temporal scales. In the field of plant seedling and germination research methods are needed to improve plant establishment and restoration monitoring, particularly in areas that have historically low success rates such as in semi-arid and arid rangeland landscapes. The purpose of this research is to assess the efficacy of remote sensing for tracking seedling height, seedling density, and seedling fate, and determine the biotic causes of seedling mortality in a rangeland revegetation site in northwestern Utah. In Chapter 1, we use 28 time-lapse and motion sensing infrared cameras (Reconyx) to measure seedling density and height in fenced and unfenced plots during the initial four months of seedling establishment and growth. We compare imaged-based measurements of seedling height and density with similar measurements collected in the field and at different daylight hours to determine the accuracy and reliability of remotely sensed measurements. We found that the ideal sample periods for capturing the clearest images were at the time the sun passed zenith and shadows were minimized. Average seedling height was 14% lower in image-based versus field estimates. Seedling density was underestimated by approximately 30% when using cameras. Our study establishes that remote sensing of seedlings using time-lapse cameras is a method for seedling research and monitoring in restoration efforts which merits further research and development. In Chapter 2, we track biotic causes of seedling fate using the methods developed in Chapter 1, and compare seedling survival in fenced and unfenced plots. Fencing led to a four-fold increase in the number of seedlings emerged from the soil. Herbivory and damage caused by trampling and burial resulted in the death of 61.4 % of all unfenced seedlings. Fencing plots increased the probability of seedling survival by seven times. Using cameras to track seedling fate at two restoration sites revealed that small herbivores, including Lepus californicus, Thomomys bottae, and Dipodomys sp. drastically reduced seedling survival during the first year after planting. Effects of herbivores on seedling survival should be taken into consideration when planning revegetation operations, and further research can increase knowledge of how herbivory affects restoration efforts. Using cameras can provide meaningful information to managers and researchers about seedling status and fate.

This project evaluates the survival and initial growth response of oak seedlings produced under special nursery protocols (?enhanced? seedlings) compared to nursery-run oak seedlings of the same species, and the influence of various cultural practices on these same nursery stocks. Survival did not differ significantly between nursery stocks. Significant differences in survival among competition control treatments did occur. Intensive competition control regimes using glyphosate can negatively impact seedling survival if seedlings come in contact with herbicide spray drift.Nursery-run seedlings had significantly greater growth and many were larger than enhanced seedlings after two growing seasons. Height growth was typically greater with seedlings receiving less intensive competition control, while groundline diameter (GLD) growth was greatest with seedlings receiving more intensive competition control. Subsoil and auger planting typically resulted in greater growth than normal hand planting. Fertilization improved growth in certain areas, but the effects diminished after the first growing season.

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