The shape is more important than we ever thought: plant to plant interactions in a high mountain community

Abstract

Working with fully mapped communities provides the ecologist the opportunity to estimate precisely who interacts with who, how frequently and which of those interactions are important for the assembly of the community. Most analyses of fully mapped communities have employed point pattern analysis techniques, as these extract the maximum information available in the dataset. However, when dealing with high mountain communities, an important part of the spatial structure of the community is lost by approximating plant individuals as points when most chamaephyte and hemicryptophyte individuals have non-symetric, irregular sizes that are best represented by planar shapes.
Based on this, we have developed a new method to test for the existence of interactions between individuals. Shapes of the individuals are mapped in the field and intersections (i.e., interactions) between them are quantified and tested against the expectations of a null model of spatial configuration of shapes (e.g., random rotation).
We applied our new method to study community assembly in a two-phase pasture-like alpine Mediterranean community in central Spain analysing pair-wise interactions between the 11 most abundant species and compared the results to those obtained by a traditional point pattern analysis (ppa).
We found that our new method increased the number of significant interactions from 21 % (ppa) to 55 % and changed the sign of around half the interactions detected with ppa. Most significant interactions were negative. Therefore, our method provides a new efficient tool to study community assembly in alpine communities.

Full text

The shape is more important than we
hh l l
ever t
h
oug
h
t: p
l
ant to p
l
ant interactions
in a high mountain community
in
a
high
mountain
community
De la Cruz, M., Pescador, D. S. & Escudero, A.

Working with fully-mapped communities

Study of interactions with point pattern analysis

positiveassociations
(focalspecies)
g(r)
focaldistance (m)

¿Points or shapes?

TYPICAL SPANISH t
wo
-
phase
TYPICAL
SPANISH
t
wo
phase
,
pasture-like, alpine Mediterranean
community

Festuc
a
Ati
dli tl
Caespitose
hemicryptophytes
curvifolia
A
gros
tis
d
e
li
ca
t
u
la
hemicryptophytes
Rosette
hemicryptophytes
Jurinea humilis Hieracium vahlii
Prostrate chamaephytes
Jasione crispa
Silene ciliata Armeria caespitosa Minuartia recurva

Mapping shapes in the field
frame of
methacrylate
frame
of
methacrylate
s
heets of PVC
scanned shapes
s
heets
of
PVC

9 x 6 m plot
22 111
individuals of 17 perennial plant species
22
,
111
individuals
of
17
perennial
plant
species

Study of interactions with shape intersections
OBSERVED
OBSERVED
overlapping area

Study of interactions with shape intersections
OBSERVED
NULL MODEL: SHAPE ROTATION
OBSERVED
SIMULATED
overlapping area

Study of interactions with shape intersections
NEGATIVE
ASSOCIATION
NULL MODEL: SHAPE ROTATION
POSITIVE
ASSOCIATION
ASSOCIATION
SIMULATED
overlapping area

RESULTS: POINT PATTERN ANALYSES (pcf)
G.o.F.
(
0
.
0 - 0
.
2
m)
G.o.F.
(.
.m)
Focal S
p
ecies
Agr Ar
m
Ery Fes Hie Jas Jur Min Sed S.c Sil
Agr
‐++‐
p
+
Positive association
Arm
++ ++++ ‐
Ery
‐+‐
+
Positive

association
‐
Negativeassociation
Fes
‐
Hie
+
Jas
+
+
Jas
+
+
Jur
+
Min
+
+
+
+
+
+
+
+
Sed
+
S.c
++
Sil

RESULTS: SHAPE INTERSECTIONS
1000 R
O
TATI
ONS
p.s.
Focal S
p
ecies
OONS
p.s.
Agr Ar
m
Ery Fes Hie Jas Jur Min Sed S.c Sil
Agr 0.00 0.10 0.01 0.05 0.11 0.22 0.05 0.16 0. 17 0.80 0.21
p
+
Positive association
Arm 0.15 0.00 0.26 0.25 0. 17 0.36 0.12 0.17 0. 00 0.00 0.01
Ery 0.00 0.04 0.00 0.10 0.02 0. 03 0.03 0.00 0.00 0.00 0.00
+
Positive

association
‐
Negativeassociation
Fes 3.64 11.82 31.45 0.00 13.67 29.86 20.69 17.29 14.66 5.67 15.33
Hie 0.36 0.38 0.24 0.63 0.14 0.83 0. 47 0.70 0.04 0.06 0. 43
Jas
095
107
064
186
112
000
113
163
006
043
141
Jas
0
.
95
1
.
07
0
.
64
1
.
86
1
.
12
0
.
00
1
.
13
1
.
63
0
.
06
0
.
43
1
.
41
Jur 0.14 0.22 0.37 0. 76 0.37 0.67 0.52 0.81 0. 00 0.23 0.43
Min 0.16 0.12 0.00 0.26 0.22 0.39 0.32 0.00 0.00 0.00 0.18
Sed 0.02 0.00 0. 00 0.03 0.00 0.00 0. 00 0.00 0.00 0.00 0.00
S.c 0.02 0.00 0.00 0.00 0.00 0. 00 0.00 0.00 0.00 0.00 0.00
Sil 0.34 0.01 0.00 0.35 0.22 0.52 0.27 0. 28 0.00 0.00 0.00

RESULTS: SHAPE INTERSECTION vs. POINT PATTERN
ANALYSIS
Focal S
p
ecies
"Apparent"
facilitation
masks
Agr Ar
m
Ery Fes Hie Jas Jur Min Sed S.c Sil
Agr
NDNNCN NDD
p
facilitation
masks
real small-scale
inhibition
Arm
CD CCCCN
Ery
DDNN D
41
N
ew interaction
Fes
NNN NNNNNN
Hie
NNNC NN N
Jas
C
N
D
N
N
N
N
N
41
N
ew

interaction
12
Changedsign
12
Disa
pp
eared
Jas
C
N
D
N
N
N
N
N
Jur
NN DNN N N
Min
DC DCN N
pp
Sed
NNC
S.c
CD
Sil
NN

Pescador et al. 2014 New Phytol.

WHERE DOES ALL THIS COULD LEAD TO?
•Architectural patterns in dwarf shrubs and grasses
Ph i l i i h h
•
Ph
enotyp
i
c p
l
ast
i
c
i
ty on t
h
ose c
h
aracters
•Shape as a functional trait
•Variance in overlapping interactions
sky
'
sthelimit!
•…
sky s
the
limit!

THANK YOU!

The shape is more important than weever thought: plant to plant interactions in a high mountain
community.
De la Cruz, M., Pescador, D. S. & Escudero, A.
XIV MEDECOS & XIII AEET Meeting. Seville, 31st January - 4th February 2017
1. TITLE: Good morning.
2. Working with fully mapped communities provides the ecologist the opportunity to estimate precisely
who interacts with who, how frequently and which of those interactions are important for the assembly of
the community.
Different methods could provide the coordinates of each individual plant in a community.
3. Most analyses of fully mapped communities have employed point pattern analysis techniques, as these
extract the maximum information available in the dataset.
They measure the observed number of neighbours of other species at successive distances around the
individuals of the focal species [e.g. the cyan one] and compare it with the expected number under a
null model of independence [[(each species is constrained by its dispersal parameters and
environmental heterogeneity preference, independently from the others)]].
4. Repeating this for each species in the community as a focal species, give us the matrix of interactions
in the community.
Positive or negative deviations from the simulated counts at each spatial scale show the extent of positive
or negative interactions
5. However, when dealing with high mountain communities, an important part of the spatial structure of
the community is lost by approximating plant individuals as points when most chamaephyte and
hemicryptophyte individuals have non-symetric, irregular sizes that are best represented by planar shapes.
6. To test this idea, we have developed a new method to test for the existence of interactions between
individuals based on the disposition of the shapes of individual plants.
We applied our new method to study community assembly in a two-phase pasture-like alpine
Mediterranean community in central Spain
Most of the vegetation biomass is formed by the grass Festuca curvifolia
7. …accompanied by at least another sixteen perennial species, mostly hemicriptophytes and dwarf
chamaephytes
8. To record the shapes of each individual,
- we extended a frame of methacrylates over the plot (5cm),
- we delineated the outline of each plant over A3 PVC sheeeets (different color for each species) and

-we georeferenced the acetates and digitalized the shapes.
9. We obtained a 9 x 6 m map with the position and shape of more than 22-thosand individual plants.
10. To analyze the existence of interactions, we calculated the sum of overlapping areas of the
individuals of a focal species with every individual of the other species
11. And to test the significance of those interactions, we compared the observed overlapping area with the
distribution of overlapping areas obtained with a null model of random rotation.
Each shape of the focal species is rotated around its centroid and the new, simulated, overlapping areas
are calculated.
12. Observed overlapping areas smaller and larger than the simulated ones show respectively negative
and positive interactions.
13. We tested the existence of interactions between the eleven (11) most abundant species in the
community.
For comparison purposes, we show first the RESULTS OF a classical Point Pattern ANALYSES (PPA)
with the crossed-pair correlation function.
We tabulate when a GoF test shows evidence of positive or negative associations up to 20 cm of distance
form the focal species. [[Note that these are not necessarily symmetrical]].
[Positive (and negative) association in PPA means that the centroid of individuals of other species are
neighbours (0-20 cm) of the centroid the focal species more (and less) frequently than expected by a null
model of independent spatial distribution] [[(IPP, PC, IPC)]].
Most non-neutral associations are positive as would be expected from classical ecological theory
(facilitation in stressful mountain environments). But note that most species-pairs appear as
"independent".
14.[ RESULTS OF SHAPE INTERSECTION and ROTATION ]
On the contrary, with shape intersection, most significant associations were negative.
(i.e., with shape rotation, "negative association" means that observed shape-overlap is significantly less
than expected by random rotation, and the contrary for "positive association").
Figures on the table show observed "% of overlapping area" between the focal species and the others.
Even although most of them show high % overlap with Festuca (the engineer), it is considerably less than
what would be expected by random rotation.
Half of "neutral associations" (26) happen when species do not coexist close enough to overlap
(figures = 0.0). In this case, the "shape rotation" null model can't explain whether this is caused by
different micro-environmental affinities or because other inhibitory processes, acting at a large
spatial scale have prevented individuals of these species to coexist (although this last hypothesis
doesn't seem probable with the results of PPA, where most significant associations were positive)

15. We found that our new method increased the number of significant interactions from 2 % (ppa) to
55% and changed the sign of around half the interactions detected with ppa. Most significant interactions
were negative.
No one of associations (i.,e., "interaction") detected with point pattern remained with shape intersection.
All changes of sign are from positive to negative: this means that although there is some tendency of
centroids to be closer than expected, species refuse to intimate.
The changes of sign, and the new detected associations mean that "apparent" facilitative interactions
and even those which may be considered neutral form a "point-centroid" centred perspective, in
reality mask *real* small scale inhibition. This shows that even in stressful mountain environments,
where facilitation is supposed to happen, there are strong signals of competition.
16. Using a sophisticated point pattern analysis we previously showed that this small-scale inhibition
occurred between the community engineer (Festuca) and most other species but with our new method we
can confirm that it also occurs among many species pairs and that the shape of the individuals (i.e., the
grow-form strategies of plants) should be blamed for it.
17. Our method provides a new efficient tool to study community assembly in alpine communities AND
in any OTHER where individual canopies could be precisely outlined
The implications of this are several.
• Architectural patterns in dwarf shrubs and grasses
• Phenotypic plasticity on those characters
• Shape as a functional trait
• Variance in overlapping interactions:
(large overlaps for a few individuals vs. average overlaps for most individuals: evenness of overlapping)