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Fragmentation disrupts the seasonality of Amazonian evergreen forests

29 Jul 2021-

TL;DR: The plant-climate interactions controlling the seasonality of wet Amazonian forests are demonstrated and it is shown that forest fragmentation will aggravate forest loss under a hotter and drier future scenario.
Abstract: Predictions of the magnitude and timing of leaf phenology in Amazonian forests remain highly controversial, which limits our understanding of future ecosystem function with a changing environment. Here, we use biweekly terrestrial LiDAR surveys spanning wet and dry seasons in Central Amazonia to show that plant phenology of old-growth forests varies strongly across strata but that this seasonality is sensitive to disturbances arising from forest fragmentation. In combination with continuous microclimate measurements, we found that when maximum daily temperatures reached 35 °C in the latter part of the dry season, the upper canopy of large trees in undisturbed forests shed their leaves and branches. By contrast, the understory greens-up with increased light availability driven by the upper canopy loss alongside more sunlight radiation, even during periods of drier soil and atmospheric conditions. However, persistently high temperatures on forest edges exacerbated the upper canopy losses of large trees throughout the dry season, and the understory seasonality in these light-rich environments was disrupted as a result of the altered canopy structure. These findings demonstrate the plant-climate interactions controlling the seasonality of wet Amazonian forests and show that forest fragmentation will aggravate forest loss under a hotter and drier future scenario.

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Fragmentation disrupts the seasonality of
Amazonian evergreen forests
Matheus Nunes ( matheus.nunes@helsinki. )
University of Helsinki
José Luís Camargo
Biological Dynamics of Forest Fragment Project (INPA & STRI) https://orcid.org/0000-0003-0370-9878
Grégoire Vincent
CIRAD
Kim Calders
Ghent University https://orcid.org/0000-0002-4562-2538
Rafael Oliveira
University of Campinas
Alfredo Huete
School of Life Sciences, University of Technology Sydney, NSW 2007
Yhasmin Moura
Karlsruhe Institute of Technology
Bruce Nelson
Brazil's National Institute for Amazon Research (INPA)
Marielle Smith
Michigan State University
Scott Stark
Michigan State University
Eduardo Maeda
University of Helsinki https://orcid.org/0000-0001-7932-1824
Biological Sciences - Article
Keywords: leaf phenology, forests, Amazonian forests, forest fragmentation and loss
Posted Date: July 29th, 2021
DOI: https://doi.org/10.21203/rs.3.rs-722038/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License. 
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Version of Record: A version of this preprint was published at Nature Communications on February 17th,
2022. See the published version at
https://doi.org/10.1038/s41467-022-28490-7.

Fragmentation disrupts the seasonality of Amazonian evergreen forests 1
2
Authors 3
Matheus H. Nunes
1, 11
, José Luis C. Camargo
2
, Grégoire Vincent
3
, Kim Calders
4
, Rafael S. Oliveira 4
5
, Alfredo Huete
6
, Yhasmin Mendes de Moura
7, 8
, Bruce Nelson
9
, Marielle N. Smith
10
, Scott C. 5
Stark
10
, Eduardo E. Maeda
1
6
7
1
Department of Geosciences and Geography, University of Helsinki, Helsinki, 00014, Finland 8
2
Biological Dynamics of Forest Fragment Project, National Institute for Amazonian Research, 9
Manaus, AM, 69067-375 Brazil 10
3
AMAP, Univ Montpellier, IRD, CIRAD, CNRS, INRAE, Montpellier, France 11
4
CAVElab Computational and Applied Vegetation Ecology, Department of Environment, Faculty 12
of Bioscience Engineering, Ghent University, Ghent, Belgium 13
5
Department of Plant Biology, Institute of Biology, University of Campinas, Campinas, Brazil 14
6
School of Life Sciences, Faculty of Science, University of Technology Sydney, Sydney, NSW 15
2007, Australia 16
7
Institute of Geography and Geoecology, Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 17
76131, Karlsruhe, Germany 18
8
Centre for Landscape and Climate Research, School of Geography, Geology and the Environment, 19
University of Leicester, Leicester, LE17RH, United Kingdom 20
9
National Institute of Amazonian Research, Manaus, Brazil 21
10
Department of Forestry, Michigan State University, East Lansing, MI, USA 22
11
Corresponding author (matheus.nunes@helsinki.fi) 23
24

ABSTRACT 25
Predictions of the magnitude and timing of leaf phenology in Amazonian forests remain highly 26
controversial, which limits our understanding of future ecosystem function with a changing 27
environment. Here, we use biweekly terrestrial LiDAR surveys spanning wet and dry seasons in 28
Central Amazonia to show that plant phenology of old-growth forests varies strongly across strata 29
but that this seasonality is sensitive to disturbances arising from forest fragmentation. In combination 30
with continuous microclimate measurements, we found that when maximum daily temperatures 31
reached 35 °C in the latter part of the dry season, the upper canopy of large trees in undisturbed forests 32
shed their leaves and branches. By contrast, the understory greens-up with increased light availability 33
driven by the upper canopy loss alongside more sunlight radiation, even during periods of drier soil 34
and atmospheric conditions. However, persistently high temperatures on forest edges exacerbated the 35
upper canopy losses of large trees throughout the dry season, and the understory seasonality in these 36
light-rich environments was disrupted as a result of the altered canopy structure. These findings 37
demonstrate the plant-climate interactions controlling the seasonality of wet Amazonian forests and 38
show that forest fragmentation will aggravate forest loss under a hotter and drier future scenario. 39
40
INTRODUCTION 41
Leaf phenology of Amazonian forests is a key component controlling the exchange of energy and 42
trace gases water vapour, carbon dioxide and volatile organic compounds - with influences on 43
vegetation feedbacks on the regional and global climates
15
. In the past decade, several studies have
44
demonstrated from field data and remote sensing products that a majority of Amazonian forests 45
respond to climatic variations
2,6
. There is also mounting evidence that evergreen canopies have a
46
seasonal variation
711
with changes in leaf demography and canopy structure
12
. Long-term studies
47
have shown that 60 - 70% of species of humid Amazonian forests flush new leaves in the dry months 48
12,13
linked to higher solar radiation
4,14
, which leads to increases in gross primary productivity as a
49
result of new young leaves with higher photosynthetic capacity and water-use efficiency
4,15,16
.
50
However, when some Amazonian forests are impacted by water stress, leaf development is reduced 51
17
, and trees shed their leaves, increasing litterfall
10,18
, which interact to alter leaf area dynamics
19
.
52
To complicate matters further, leaf phenology also responds to different gene expressions that have 53
evolved to maximize photosynthetic and water use efficiency during the dry season, reduce plant 54
competition for light and water, and minimise herbivore pressure
7,16,2022
.
55

The effects of climatic variations on leaf phenology can also be amplified by forest fragmentation
23
. 56
Forest edges contain a large abundance of early successional species with rapid acquisition of 57
resources that maximise new leaf production and growth
24,25
, but may be more vulnerable to droughts
58
26
. Forest fragmentation can increase the evaporative demand due to higher temperatures and wind
59
exposure, and soil moisture can be lower at fragment edges
27
, which may cause leaves to drop and
60
lead to higher branch turnover
12,23
. However, ground observations of litterfall in Amazonian forests
61
have shown only a mild seasonality near edges
28
. Indeed, large uncertainty remains regarding the 62
responses of fragmented forests to climatic seasonality, particularly because some species can benefit 63
from higher solar radiation
29,30
, drought resistance varies among species
3133
and surviving trees may 64
acclimate or be adapted to the drier, hotter conditions near edges
34
. As the number of contiguously
65
forested areas are significantly decreasing in the Amazon
35
, understanding the effects of forest 66
fragmentation on phenology is vital for predicting the benefits of protecting non-fragmented 67
Amazonian forest landscapes. 68
Seasonal variations in leaf quantity and leaf area across evergreen Amazonian forests have frequently 69
been considered negligible or small
4,12,21,36
. However, spaceborne remote sensing approaches tend to 70
detect only trees that dominate the upper canopy, thereby obtaining more information from those 71
species that are adapted to more stressful conditions such as high solar radiation and temperatures 72
and low air humidity
37
. LiDAR-based observations may provide fresh insights into the interacting 73
factors controlling vegetation dynamics and have more recently shown that leaf phenology in 74
Amazonian forests is stratified over canopy positions and conditions
19,38
. Here, we investigate the
75
phenology of forests in Central Amazonia with terrestrial laser scanning (TLS, also terrestrial 76
LiDAR) surveys collected every 15 days spanning the wet and dry seasons. We use TLS 77
measurements to investigate how forest fragmentation and microclimatic seasonality interact to affect 78
plant area of the understory and the upper canopy. Repeated TLS measurements can monitor subtle 79
changes in forest structure in specific horizontal layers
42
(Figure 1). Furthermore, the detailed and
80
precise structural measurements offered by this system can help answer fundamental questions about 81
the three-dimensional (3D) ecology of trees
43
without suffering from potentially confounding 82
artefacts present in passive optical satellite images
11,36
. Using a combination of 11 repeat TLS
83
surveys, as well as continuous air temperature and soil moisture measurements in old-growth, 84
undisturbed forests and fragmented forests under edge effects, we test: (1) whether vertically stratified 85
plant phenology in undisturbed forests varies with microclimatic conditions, and (2) whether plant 86
phenology is sensitive to disturbances arising from forest fragmentation. We predict that the hotter 87
and drier conditions of edges exacerbate leaf loss during the dry season. To our knowledge, the work 88

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