This is the presentation given during Diego Sotomayor's PhD defence at the Department of Geography at York University. This is the abstract of the dissertation: In arid environments, dominant woody plants such as shrubs or trees, usually facilitate a high density of species in their understories. This phenomemon is composed by a series of direct and indirect effects from the dominant plant to the understory species, and among understory species. The aim of this project was to determine these direct and indirect consequences of dominant plant-plant facilitation in a collection of field sites along the coastal Atacama Desert. The following objectives and hypotheses were examined in this project: (1) to summarize and contextualize the breadth of research on indirect interactions in terrestrial plant communities; (2) that the positive effects of dominant plants on understory communities are spatiotemporally scale dependent, from micro- to broad-scale spatial effects, and from within-seasonal to among-year temporal effects; (3) that dominant plants via their different traits determine the outcome of plant-plant interactions; (4) that dominant plants determine the outcome of interactions amongst understory species and that their responses are species-specific; and (5) that facilitation by dominant plants generates sufficiently different micro-environmental conditions that lead to consistent differences in seeds traits of understory plants. Overall, we found that multiple factors determine the outcome of plant-plant interactions along the field sites studied in this project. These factors impact both the direct and indirect effects of dominant woody plants on their understory communities and include species-specific traits of both the dominant and understory species, and the spatial and temporal environmental gradients that manifest their effects at different scales. Dominant plants usually facilitate increased species richness and density of plants in their understory, that in turn mediates effects amongst these species. However, these direct effects seem to have a limit given that at extremely stressful environmental conditions they tend to change to neutral and even competitive effects of canopies on their understories. This provides evidence that positive effects of dominant plants collapse under extreme spatiotemporal stress. Although we did not find evidence of evolutionary effects of top-down facilitation, the methodology proposed here represents a contribution to test the conditions under which these results hold. Overall, this project illustrates the importance of understanding the multiple drivers that determine the outcome of biotic interactions.

1. Direct and indirect consequences of dominant
plants in arid environments

2. Canopies in stressful environments nurse a great diversity
of understory plant communities
Strauss 1991, Callaway 2007, Sotomayor & Lortie 2015

3. Dominant plants are centers of interactions in deserts
Lortie, Filazzola & Sotomayor 2016 Funct Ecol

4. tive interactions can be produced by competitive effects
alone, such as when at least one species of lower rank
outcompetes one or more species of higher rank
(Karlson and Jackson 1981, Aarssen 1983, Berlow
1999). Not only do changes in potential chains of
from a schema used to illustrate the factors that lim
geographic ranges, Krebs 2001, and a previously pr
posed general concept of biological filters, Grime 199
Laakso et al. 2001). This schema is just one of ma
possible ways that these different processes might int
Fig. 1. The main processes o
filters that structure a plant
community. The IC concept
proposes that all four proces
can be important in
determining the extant plant
community at a given site bu
that the relative importance
each process will vary in spa
and time. Each process/filter
represented by a pair of
horizontal lines and the
corresponding description is
bold italics adjacent to the
symbol (sub-sets of a process
such as herbivory or
competition are labeled in pl
text). Solid arrows depict the
movement of species through
the filters, and hatched lines
illustrate where each process
might influence the plant
community.
434 OIKOS 107:2 (20
Community assembly
Lortie et al. 2004

5. Plants interact and can have positive effects on
each other
Lortie et al 2004, Callaway 1995, 1997; Bruno et al. 2003

6. Bruno et al. 2003

7. Predicting context-dependence of interactions across
multiple scales is a great challenge
Agrawal et al. 2007, Bruno et al. 2003, Chamberlain et al. 2014

8. Diversity of indirect interactions in plants
Sotomayor & Lortie 2015 Ecosphere

9. Indirect interactions in terrestrial communities
Biases towards Northern Hemisphere and 2-
trophic level studies Sotomayor and Lortie 2015

10. Sotomayor and Lortie 2015

11. Experimental design: dominant plants
effects
Large-scale spatial and temporal patterns
Small-scale factors: scale of influence, nurse identity,
temporal shifts
Removal manipulations
Growth chamber experiments

12. System and field sites
Atiquipa, Peru (15oS)
Fray Jorge, Chile (31oS)
El Romeral, Chile (29oS)

13. Selection of study sites
Combining common plant
ecological surveys with data
obtained from world climate
models
Bioclimate data from
Hijmans et al. 2002, available
at 1 km resolution (1950-2000
period, average) for 19 env.
variables: rainfall and
temperature
Elevation gradient regionally

14. Methodology
Quadrat sampling in relation to
target species, open and
understory with 3 different sizes
of quadrats
Sampling in multiple sites: species
richness, abundance
Measures were taken along 3
growing seasons
Field and laboratory experiments
designed to disentangle patterns

15. Large-scale spatial patterns
Purpose:
To examine how facilitation
changes within and between
regional stress gradients and their
temporal dynamics using dominant
plants with different traits locally
Observational study over 3 years at
the peak of growing season: 3
regions, 5 sites per region and 8
different nurses across field sites
(one thorny and one non-thorny
per site)

16. Results species richness
Among regions and temporal differences, no nurse or regional differences

17. Plant density
Contrasting responses to increased stress: increase in intensity or collapse

18. Michalet et al. 2006 Ecol Lett
Collapse of facilitation?

19. Small-scale factors
Purpose:
To examine the within and
between seasonal effects of
facilitation, concurrently
with the micro-scale effects
of two dominant species
Micro-scale, seasonal
factors and increased
competition
Randia armata
Caesalpinia spinosa

20. Microclimate differences
Intensification of differences towards the end of growing season

21. Species richness
Temporal
changes: growing
season, and
between years
Facilitation
intensity similar
between
dominants, and
microscales

22. Abundance
Consistent
results, less
pronounced
during 2011
(wetter year)
No small-scale
differences

23. No increased
competition in
understories,
but higher
species
richness
Understory
interactions

24. Community differences

25. Community differences decreased towards the end of the
growing season

26. Canopies determine plant community spatiotemporal
dynamics in arid systems

27. Neighborhood removal experiment
Purpose:
To examine how dominant
plants mediate the outcome
of interactions amongst
understory species and their
species-specific responses
Manipulative experiment on
target understory species

28. Experimental design
Full neighborhood removal of
target understory species in open
and under microsites
Measurements: plan density,
flowering, fruit production, biomass
Neighbors present
Neighbors removed

30. Results

31. Removal
consequences
P. limensis
(rossette-like
annual)
displayed the
greatest
effects after
removals

32. Neighborhood
effects
P. limensis:
stronger
negative effects
of neighbors in
open microsites
This indicates
reduced
competition for
this target

33. Reduced competition in understories could promote
stable coexistence via facilitation

34. Facilitation consequences on seed biology
Purpose:
To examine how facilitation
by dominant plants
generates microsites leading
to consistent differences in
seed traits of understory
plants
5 annual understory species
Growth chamber
experiment simulating
conditions encountered in
the field
Open
(3 chambers)

36. Field observations

37. No seed size or viability differences due to dominant plants
Sotomayor et al. 2014
Austral Ecol.

38. Seeds germinated better in open microenvironments irrespective
of their source

39. Local adaptation and plasticity of beneficiaries should
be further explored with facilitation

40. Synthesis

41. Dominant plants have important indirect effects with
system-wide consequences for arid ecosystems

42. Overall consequences of dominant plants should be
considered when planning their management

43. Acknowledgments and collaborators
Johns Hopkins University, USA
Ben Zaitchik
Universidad de La Serena, Chile
Francisco A. Squeo, Julio Gutiérrez, Danny Carvajal
Universidad Nacional de San Agustín, Peru
Francisco Villasante, Italo Revilla, Briggeth Flores, Jessica
Turpo, Paola Medina
York University, Canada
Supervisory Committee, Department of Geography and
Faculty of Graduate Studies
Ecoblender Lab at York University

44. Thank you