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\begin{document}

\title{Photosynthetic quantum efficiency in South-Eastern Amazonian trees may
be already affected by climate change}



\author[1]{Rakesh Tiwari}%
\author[1]{Emanuel Gloor}%
\author[2]{Wesley Jonatar Cruz}%
\author[2]{Beatriz Schwantes Marimon }%
\author[2]{Ben Hur Marimon Junior }%
\author[2]{Simone Reis}%
\author[2]{Igor de Souza}%
\author[3]{G. Heinrich Krause}%
\author[3]{Martijn Slot}%
\author[3]{Klaus Winter}%
\author[1]{David Ashley}%
\author[2]{Raiane Béu}%
\author[2]{Camila Borges}%
\author[4]{Maura Da Cunha}%
\author[5]{Sophie Fauset}%
\author[2]{Laura Ferreira}%
\author[2]{Maélly Dállet Gonçalves}%
\author[2]{Thaynara Lopes}%
\author[2]{Eduardo Marques}%
\author[2]{Natalia Mendonça}%
\author[2]{Natana Mendonça}%
\author[2]{Pedro Noleto}%
\author[2]{Carla Heloísa  de Oliveira}%
\author[2]{Milene Oliveira}%
\author[4]{Saulo Pireda}%
\author[2]{Nayane dos Santos Prestes }%
\author[2]{Denilson Santos}%
\author[2]{Eduarda Santos}%
\author[2]{Ediméia Laura da Silva}%
\author[2]{Izabel de Souza}%
\author[2]{Luciana de Souza}%
\author[4]{Angela Vitória}%
\author[1]{Christine Foyer}%
\author[1]{David Galbraith}%
\affil[1]{University of Leeds}%
\affil[2]{Universidade do Estado de Mato Grosso}%
\affil[3]{Smithsonian Tropical Research Institute}%
\affil[4]{Universidade Estadual do Norte Fluminense Darcy Ribeiro}%
\affil[5]{University of Plymouth}%


\vspace{-1em}



  
  \date{June 17, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Tropical forests are experiencing unprecedented high temperature
conditions due to climate change that could limit their photosynthetic
functions. We studied the high temperature sensitivity of photosynthesis
in a rainforest site in southern Amazonia, where some of the highest
temperatures and most rapid warming in the Tropics have been recorded.
The quantum yield (\emph{F\textsubscript{v}}/\emph{F\textsubscript{m}})
of photosystem II was measured in seven dominant tree species using leaf
discs exposed to varying levels of heat stress.
\emph{T\textsubscript{50}} was calculated as the temperature at which
\emph{F\textsubscript{v}}/\emph{F\textsubscript{m}}) was half the
maximum value. \emph{T\textsubscript{5}} is defined as the breakpoint
temperature, at which
\emph{F\textsubscript{v}}/\emph{F\textsubscript{m}}) decline was
initiated. Leaf thermotolerance in the rapidly warming Southern Amazonia
was the highest recorded for forest tree species globally.
\emph{T\textsubscript{50}} and \emph{T\textsubscript{5}} varied between
species, with one mid storey species, Amaioua guianensis, exhibiting
particularly high \emph{T\textsubscript{50}} and
\emph{T\textsubscript{5}} values. While the \emph{T\textsubscript{50}}
values of the species sampled were several degrees above the maximum air
temperatures experienced in southern Amazonia, the
\emph{T\textsubscript{5}} values of several species are now exceeded
under present-day maximum air temperatures.%
\end{abstract}%



\sloppy


\section*{Introduction}

{\label{introduction}}

Temperatures have increased substantially in the Tropics in recent
decades, by as much as 0.5 \selectlanguage{ngerman}°C per decade in some regions (Jiménez-Muñoz,
Sobrino, Mattar, \& Malhi, 2013). In many areas, these increases in mean
temperatures have been accompanied by increasingly frequent heatwaves
and extreme temperature days (Coumou \& Robinson, 2013). Although many
tropical forest tree taxa have been exposed to warmer temperatures in
the past -- e.g. during the Pliocene-Eocene thermal maximum, the rapid
warming rates currently experienced are unprecedented (Dick, Lewis,
Maslin, \& Bermingham, 2013). Forests in the southern Amazon experience
the highest temperatures in Amazonia, with monthly maximum air
temperatures during dry periods frequently reaching \textgreater{}40°C.
They are also amongst the planets most rapidly-warming tropical forests
(Gloor et al., 2018; Jiménez-Muñoz et al., 2013) making them a natural
laboratory for studying the effects of global warming on tropical
forests. However, despite their increasing exposure to heat stress,
there is currently no empirical data on the sensitivity of southern
Amazonian forests to high temperatures.

Photosynthesis, which underpins the substantial productivity and biomass
storage of tropical forests, is heavily temperature-dependent (Berry \&
Björkman, 1980). Most investigations to date have focused on the
temperature sensitivity of net CO\textsubscript{2} exchange, at the leaf
(e.g., Doughty and Goulden (2008), Lloyd and Farquhar (2008) and Slot,
Garcia, and Winter (2016)) and canopy (e.g., Tan et al. (2017)) scales,
leading some authors to suggest that tropical forests currently exceed
their photosynthetic temperature optima (Doughty \& Goulden, 2008).
However, studies focusing on gas exchange generally only span the
typical range of leaf temperatures to which leaves are exposed under
ambient climatic conditions and thus focus only on warming impacts that
are reversible in nature. Irreversible changes to the photosynthetic
apparatus can ensue in leaves exposed to very high temperatures (G. H.
Krause, Cheesman, Winter, Krause, \& Virgo, 2013) but little is known of
the temperature thresholds associated with photosynthetic impairment in
Amazonian tree taxa. In the rapidly-warming southern Amazon, there is a
concern that maximum leaf temperatures may be approaching critical
thresholds, particularly in outer canopy leaves that receive maximum
irradiance, but we currently do not know the point at which irreversible
thermal changes will occur. Furthermore, regions of Amazon rainforest
vary in the length of dry periods as well (Fu et al., 2013) as drought
intensity (Marengo, Nobre, Tomasella, Cardoso, \& Oyama, 2008). In our
study region, the peak of the dry season coincides with the peak of
maximum air temperatures. Seasonal acclimation potential of thermal
traits in trees, particularly the ones exposed to long dry/warm periods
will influence the potential for carbon gain in the forest and may be
very important for maintaining plant photosynthetic function under a
rapidly warming climate. Variation in thermal traits across seasons has
not been measured previously in Amazonia.

The measurement of leaf thermotolerance typically focuses on the
response of one of two diagnostics of chlorophyll \emph{a} fluorescence
quenching, measured under dark-adapted conditions (G. H. Krause \& Weis,
1991). This is usually analysed via heat shock treatment of leaf discs
(G. H. Krause et al., 2013; G. Heinrich Krause et al., 2010) where
changes in the `minimal' fluorescence (\emph{F} \textsubscript{0}) the
ratio of variable to maximum fluorescence yield (\emph{F}
\textsubscript{v}/\emph{F} \textsubscript{m}), a parameter that is often
referred to as the maximum quantum yield (QY) of photosystem II (PSII)
are measured (Kitajima \& Butler, 1975). The first measurement defines
\emph{T\textsubscript{crit}} as the temperature at
which\emph{F\textsubscript{0}} rises sharply, while the second
uses\emph{F} \textsubscript{v}/\emph{F} \textsubscript{m} to
calculate\emph{T\textsubscript{50}} as the temperature associated with a
50\% decline in \emph{F} \textsubscript{v}/\emph{F} \textsubscript{m}.
Although the two metrics are indicators of the functional integrity of
photosystem II, they are reporters of different underlying physiological
mechanisms. A temperature-induced increase in \emph{F}
\textsubscript{0}indicates disruption of the light-harvesting antenna,
which is attributed to increased thylakoid membrane fluidity (Figueroa,
2003). The decline in \emph{F} \textsubscript{v}/\emph{F}
\textsubscript{m}, on the other hand, signifies a loss of PSII function
that has been attributed to disassembly of the light-harvesting complex
from the core of PSII (Kou\selectlanguage{polish}ř\selectlanguage{english}il et al., 2004; L\selectlanguage{ngerman}ípová, Krch\selectlanguage{polish}ň\selectlanguage{ngerman}ák, Komenda, \&
Ilík, 2010; Zhang, Liu, \& Yang, 2011). Hence, heat-induced changes
in\emph{F} \textsubscript{v}/\emph{F} \textsubscript{m} indicate damage
to PSII and are less prone to measurement artefacts (G. Heinrich Krause
et al., 2010; Slot, Krause, Krause, Hernández, \& Winter, 2018).

In this study, heat-induced changes in\emph{F}
\textsubscript{v}/\emph{F} \textsubscript{m} were used to characterise
thermal tolerance for seven dominant tree species in a rapidly-warming
southern Amazonian rainforest over two different seasons i.e., the end
of the wet season and end of the dry season. The following questions
were addressed: a) does the high-temperature tolerance of sun-exposed
evergreen trees in the hottest Amazonian site vary across species? b)
what is the extent of seasonal plasticity in photosynthetic thermal
tolerance over dry and wet seasons? In addition, c) how does
thermotolerance of southern Amazonian species compare with published
data for trees from other tropical regions where maximum temperatures
are typically lower? In addition to \emph{T} \textsubscript{50}, as a
proxy for the first heat effects, we assessed two further parameters:
1)\emph{T\textsubscript{5}} , the temperature associated with the onset
of the temperature-induced decline in\emph{F\textsubscript{v}} /\emph{F}
\textsubscript{m} and 2)\emph{T\textsubscript{95,}} the temperature at
which\emph{F} \textsubscript{v}/\emph{F} \textsubscript{m} decreased
below 95\% of the maximum level, taken to be the limit, where PSII
functions are effectively lost.

\section*{Materials and methods}

{\label{materials-and-methods}}

\subsection*{Study site}

{\label{343605}}

The study site is located in a large Amazonian rainforest fragment in
Vera Cruz farm (14.833° S 52.168° W; plot code VCR-02 in the RAINFOR
forest inventory network), Nova Xavantina, Mato Grosso, Brazil. The plot
is located 0.8 km from the nearest forest edge. The vegetation of the
site located at the forest-savanna ecotone, is a transitional Amazonian
rainforest (Marimon et al., 2014) that is mostly (over 80\%)
characterised by typical Amazonian tree species but also containing some
species more commonly found in the \emph{Cerrado} biome. (see Morandi et
al. (2016) for floristic description). Mean canopy height is 13.6 m, but
the height of some trees can exceed 25 m. The soil is dystrophic, acidic
and shallow plinthosol.

The site is at the southernmost dry limit of the Amazon rainforest, is
highly seasonal and is characterised by an average annual rainfall of
1369 mm/year (recent 20-year average). Most of the rain falls during a
six--month wet period (mid--October until April), which also experiences
lower mean temperatures compared to the six--month long dry season
(May--October). The peak of the dry season (August-October) coincides
with the hottest time of the year, where maximum air temperatures
(\emph{T\textsubscript{max}} ) frequently exceed 40°C. The daily maximum
temperatures have risen considerably over the last two decades (Figure
1a). The recent decade recorded 19 heatwave events (consecutive 3-day
spells where maximum daily air temperatures exceed the
90\textsuperscript{th} percentile level for the entire 1999-2008 period)
compared to 13 events during the previous decade (Figure 1b); maximum
air temperatures have exceeded 40°C 95 times during 2009--2018 compared
to 29 times during the 1999--2008 period. The absolute
highest~\emph{T\textsubscript{max}} recorded was 43.9°C (15 October
2017, coinciding with the hot-dry sampling period reported in this
study)

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-011/Figure-011}
\caption{{This is a caption
{\label{874179}}%
}}
\end{center}
\end{figure}

Figure 1: A) Histograms of daily maximum air temperature,
\emph{T\textsubscript{max}}\textsubscript{,} for two time periods and B)
\emph{T\textsubscript{max}} time series indicating
90\textsuperscript{th} percentile 3-day heatwave event peaks for Nova
Xavantina, Brazil. Blue lines represent \emph{T\textsubscript{max}}, the
grey horizontal dashed line represents 90\textsuperscript{th}
percentile\emph{T\textsubscript{max}} temperature of 37.7 \selectlanguage{ngerman}°C for the
period 1999--2018. Data source: Instituto Nacional de Meteorologia,
Brazil ( http://www.inmet.gov.br/)

\subsection*{Plant material}

{\label{123313}}

The most dominant canopy and subcanopy evergreen tree species were
chosen for the study. For each species, individual trees were randomly
selected from within the VCR2 plot. Branches from the same set of
individual trees were sampled during both the seasons. Leaf samples were
collected during two seasons: 1) the hottest and driest period of the
year during October 2017, referred to as `end of dry' period and 2) the
end of the wet season during March--April 2018, referred to as the `wet'
period (Supporting Information Figure S1). Plant material was collected
in both seasons from three individuals from each of five commonly
occurring evergreen tree species: \emph{Hymenaea courbaril} L.
(Fabaceae), \emph{Brosimum rubescens} Taub. (Moraceae)\emph{, Amaioua
guianensis} Aubl. (Rubiaceae), \emph{Cheiloclinium cognatum} (Miers)
A.C.Sm. (Celastraceae) and \emph{Mouriri apiranga} Spruce ex Triana.
(Melastomataceae). Plant material from three individuals of two further
species, \emph{Chaetocarpus echinocarpus} (Baill.) Ducke (Peraceae)
and\emph{Tetragastris altissima} (Aubl.) Swart (Burseraceae), was only
collected in the wet period; and one tree was sampled during the end of
dry period for \emph{C. echinocarpus} (Supporting Information Table S1).
Among the species studied, \emph{H. courbaril} is the most dominant
emergent tree with an average height of 29.9±0.9 m. \emph{C.
echinocarpus} (12.7±0.5 m), \emph{T. altissima} (11.6±0.5 m), \emph{B.
rubescens} (10.4±0.6 m) and \emph{A. guianensis} (9.5±0.3 m) are mid
storey trees while \emph{C. cognatum} (7.1±0.2 m) and \emph{M.
apiranga}(6.8±0.4 m) being the understorey trees.

Trained climbers harvested sunlit canopy branches of the trees between
07:00 to 08:00 AM. The excised branches were separated into small twigs
and transported to the lab (\textasciitilde{}35km) in closed Styrofoam
boxes with ice packs. Heat treatment assays were done typically within
12h of branch harvest. Fully expanded mature leaves were sampled in all
cases, but during the end of the dry period campaign, \emph{H.
courbaril} trees had a new flush of leaves and hence, unavoidably for
this species younger leaves were also sampled.

\subsection*{Heat tolerance assay}

{\label{199402}}

Following the thermal tolerance measurement protocols adapted from G.
Heinrich Krause et al. (2010), heat treatment assay was conducted on
fully mature healthy leaves were transferred into a water trough in
which leaf discs of \selectlanguage{english}[?] 20.5 mm were cut underwater using a cork borer,
consistently choosing the central part of the lamina, and avoiding the
midrib area. One disc was cut from a single leaf except for a small
number of cases where leaf material for analysis was limited when two
discs per leaf were excised. Leaf discs were then randomly selected and
wrapped in two layers of moist tissue paper on both sides covering the
cut edge of the leaf discs. Care was taken not to touch the leaves, and
the leaf discs always remained in water or covered in damp tissue paper
to avoid anaerobiosis (Harris \& Heber, 1993). The discs were
transferred into separate plastic bags (30 x 100 mm) keeping them flat
and covered in a thin film of water. The bags were subjected to heat
treatment in sets of at least three discs per temperature point. Heat
treatment was conducted in temperature-controlled thermos flasks with
pre-heated water. The water temperature was recorded in the central area
of the flask using an HI 9063 K-type thermocouple probe. Separate sets
of leaf discs were treated to one of the following temperatures points:
30, 35, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, and 70 degC and a set
of untreated discs were used as controls (about 25degC). Hence, one assay
run, in most cases, includes measurements from at least 45 leaves. After
the heat treatment, the leaf discs were dark adapted for at least 30
minutes before measuring \emph{F\textsubscript{v}/F\textsubscript{m}}
with a FluorPen FP100 (Photon System Instruments, Czech Republic). FP100
uses a 455nm light source at 1 kHz modulation frequency to irradiate a \selectlanguage{english}[?]
5mm surface aperture. \emph{F} \textsubscript{0} is read after 40 \selectlanguage{greek}µ\selectlanguage{english}s
exposure to a 0.09 \selectlanguage{greek}µ \selectlanguage{english}mol.m\textsuperscript{-2}s\textsuperscript{-1}
light per pulse and \emph{F} \textsubscript{m}after a 1s exposure to
3,000 \selectlanguage{greek}µ \selectlanguage{english}mol.m\textsuperscript{-2}s\textsuperscript{-1} light pulse. The
response was measured on a PIN photodiode with a 667 to 750 nm bandpass
filter for 10 \selectlanguage{greek}µ\selectlanguage{english}s. The leaf discs were incubated in lab conditions (25\selectlanguage{ngerman}°C,
\textasciitilde{} 20 \selectlanguage{greek}µ\selectlanguage{english}mol.m\textsuperscript{-2}.s\textsuperscript{-1}
light) in Petri plates with a thin film of water.\emph{F}
\textsubscript{v}\emph{/F} \textsubscript{m} recovery after 24h was
measured following a 30-minute period of dark adaptation.

\subsection*{Determination of thermal tolerance
parameters}

{\label{189365}}

Four-parameter logistic curves were fitted for each\emph{F}
\textsubscript{v}/\emph{F} \textsubscript{m} -- temperature response
series (one curve pooling all the leaves per individual run) as follows:\selectlanguage{english}
\begin{longtable}[]{@{}ll@{}}
\toprule
\(f=\ \frac{c+(d-c)}{1+\exp^{\left(b*\left(log(T\right)-log(e\right))}}\) & (Equation 1)\tabularnewline
\bottomrule
\end{longtable}

where\selectlanguage{ngerman} T = treatment temperature, \(b\) =
steepness of the curve, \(c\) = lower asymptote,
\(d\) = upper asymptote or\emph{QY\textsubscript{max}} and
\(e\) is the inflection point. We extracted three measures
from the fitted\emph{F\textsubscript{v}/F\textsubscript{m}} temperature
response curves: 1) a breakpoint temperature referred to
as\emph{T\textsubscript{5}} calculated as the temperature at
which\emph{F\textsubscript{v}/F\textsubscript{m}} starts to decline
below 95\% of the \emph{QY\textsubscript{max}} level,
2)\emph{T\textsubscript{50}} , the temperature
where\emph{F\textsubscript{v}/F\textsubscript{m}} is half
of\emph{QY\textsubscript{max}} level and finally (not same as
parameter\emph{e} from Equation 1) 3) \emph{T\textsubscript{95}} the
temperature at which \emph{F\textsubscript{v}/F\textsubscript{m}} is at
5\% of the\emph{QY\textsubscript{max}} level. The difference
between\emph{T\textsubscript{5}} and \emph{T\textsubscript{95}} is
referred to as decline width (DW= \emph{T\textsubscript{95}}
--\emph{T\textsubscript{5}} ), the window
where\emph{F\textsubscript{v}/F\textsubscript{m}} declines from 95\% to
5\% of the \emph{QY\textsubscript{max}} level (Figure 2). Response
curves were fitted using the `drc' function of the `drm' package (Xia,
Ritz, Baty, Streibig, \& Gerhard, 2015) in R, version 3.5.0 (R Core
Team, 2018).

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-021/Figure-021}
\caption{{This is a caption
{\label{662766}}%
}}
\end{center}
\end{figure}

Figure 2: Schematic illustration of a typical four-point logistic curve
fitted to\emph{F\textsubscript{v}} /\emph{F\textsubscript{m}}
temperature response measurement indicating the thermal tolerance
variables extracted. Indicators extracted from the fit are temperatures
at which F\textsubscript{v} /F\textsubscript{m} declines to 5\%, 50\%
and 95\% of the maximum F\textsubscript{v}/F\textsubscript{m} level
expressed as \emph{T\textsubscript{5}} ,\emph{T\textsubscript{50}} and
\emph{T\textsubscript{95}} , respectively. Decline width, \emph{DW}
expressed as \emph{T\textsubscript{95}} --\emph{T\textsubscript{5}} .

\subsection*{Analysis of field data}

{\label{381598}}

The analysis of field data from Nova Xavantina focused on testing for
differences in, \emph{T\textsubscript{50}} , \emph{T\textsubscript{5}}
,\emph{T\textsubscript{95}} and the decline width, across species and
seasons. Two-way repeated measure linear mixed-effect analysis of
variance (ANOVA) models were fitted using the `lme' function in the
`nlme' package (Pinheiro, Bates, DebRoy, and, \& R Core Team, 2018).
Season and species were included as fixed effects and individuals
(trees) were included as a random effect in the models. Tukey post-hoc
tests were used to test for differences between specific pairs of
species in the significant models. Data analysis was performed using the
R program, version 3.5.0 (R Core Team, 2018); all means are presented
with \selectlanguage{ngerman}± standard errors and significance results are presented for 95\%
confidence interval (CI = 95\%, \selectlanguage{greek}α \selectlanguage{english}= 0.05).

\subsection*{Comparison with data from other tropical
sites}

{\label{727083}}

The thermotolerance data gathered in the study presented here was
compared with published data for adult tropical evergreen trees from
other sites, i.e. excluding seedling/greenhouse studies. The final
compilation includes data from 106 species occurring in nine tropical
sites (Table 1). Some of these studies determined thermotolerance using
the \emph{T\textsubscript{crit}} parameter derived from
the\emph{F\textsubscript{0}} rise approach and others
used\emph{T\textsubscript{50}} derived from the
\emph{F\textsubscript{v}/F\textsubscript{m}} approach used in this
study. Data for each of these two metrics were treated separately in our
analyses. Based on climatological records from CRU (Osborn \& Jones,
2014), we classified the season during the sample collection period as
`hot' (three hottest months of the year) and `cool' (three coldest
months of the year). Similarly, from the precipitation data, we treated
the sampling period that took place during the months with rainfall
\textless{} 100 mm as `dry' and periods above 100 mm as `wet'. Hence,
the classification of `dry' is only a relative description of the site's
precipitation status and does not necessarily indicate no precipitation.
Data points for aseasonal sites such as Iquitos were classified as
`wet'. Details of the forest regions and study sites can be found in
Supporting Information Table S2. The MuMIn package (Barto\selectlanguage{polish}ń, \selectlanguage{english}2018) was
used to extract conditional pseudo R\textsuperscript{2} for the complete
model fits.

\section*{Results}

{\label{results}}

\subsection*{Photosynthetic thermal
tolerance}

{\label{534250}}

Measured \emph{F\textsubscript{v} /F\textsubscript{m}} ratios (average
of 0.69 \selectlanguage{ngerman}± 0.02 arbitrary unit, \emph{n} = 35) were stable with
increasing temperatures up to a maximal temperature point and declined
thereafter to near zero levels. The breakpoint
temperature\emph{T\textsubscript{5}} averaged at 43.5 ± 1.7 °C across
species during the end of the dry period and 41.9 ± 0.9 °C during the
wet period (Figure 3A). The paired difference of 1.06 °C over the
seasons was not statistically significant. \emph{A. guianensis} recorded
the highest\emph{T\textsubscript{5}} at 53.9 ± 1.3 °C
(\(P\) \textless{}.0001,\(t_{3}\) = 10.7) during
the end of dry period while the rest of the species recorded
\textasciitilde{} 13.4°C lower\emph{T\textsubscript{5}} at 40.6 ± 3.3°C.
However, during the wet period \emph{T\textsubscript{5}} for \emph{A.
guianensis} was statistically indistinguishable from that of the other
species. Furthermore, \emph{T\textsubscript{5}} for \emph{A. guianensis}
was higher during the dry period compared to wet period -- while on the
contrary, the remaining species showed lower or
indifferent\emph{T\textsubscript{5}} during the dry period. The mixed
effects model to explain \emph{T\textsubscript{5}} variation showed a
weak species effect (\emph{F} = 3.48, \emph{P} =0.01) but no influence
of season was found. The species difference was mainly due to the
response of \emph{A. guianensis} which was significantly different from
\emph{C. cognatum}(difference of 11.6 °C, \emph{P} = 0.005) and \emph{M.
apiranga}(difference of 8.7 °C, \emph{P} = 0.029).

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-031/Figure-031}
\caption{{This is a caption
{\label{491185}}%
}}
\end{center}
\end{figure}

Figure 3: Seasonal variation in photosynthetic thermal tolerance for
sunlit evergreen tree species from the forest of Nova Xavantina, Brazil.
Species average and pooled seasonal averages of
\emph{T}\textsubscript{\emph{5}},~\emph{T}\textsubscript{\emph{50}} and
\emph{T}\textsubscript{\emph{95}} in Panels A, B and C respectively. The
dotted grey line indicates~\emph{T}\textsubscript{\emph{max}} long-term
mean maximum air temperature(34.8\selectlanguage{ngerman}°C) and dashed grey line represents
absolute highest~\emph{T}\textsubscript{\emph{max}} recorded (43.9°C) in
the region. Error bars denote one standard error. Species codes: amgu
=\emph{Amaioua guianensis} , brru = \emph{Brosimum rubescens} , chco
=\emph{Cheiloclinium cognatum} , hyco = \emph{Hymenaea courbaril} , moap
=\emph{Mouriri apiranga} and teal = \emph{Tetragastris altissima}

For \emph{T} \textsubscript{50}, we found significant species (\emph{F}
= 7.98, \emph{P} = 0.0007) and season (\emph{F} = 6.69, \emph{p} =0.022)
effects (Supporting Information Table S3). \emph{A. guianensis} was
significantly different from the rest of the species except \emph{T.
altissima} . End of dry period \emph{T\textsubscript{50}} for all
species combined was \textasciitilde{} 1.6°C higher at 51.6±0.8 °C
compared to the wet period that averaged at 49.4±0.6°C. \emph{A.
guianensis} showed the highest seasonal plasticity in
\emph{T\textsubscript{50}} while\emph{B. rubescens} the least. \emph{A.
guianensis} recorded the highest\emph{T\textsubscript{50}} during both
at the end of dry (56.4±0.5 °C) and wet (53.2±0.6 °C) periods (Figure
3B). Compared to the rest of the species studied,
\emph{T\textsubscript{50}} for \emph{A. guianensis} was 6.4°C (\emph{P}
= 0.001, \(t_{14}\) =5.08) and 4.1°C (\emph{P} =
0.016,\(t_{21}\) = 2.61) higher during the end of dry and wet
seasons respectively. Excluding \emph{A. guianensis,} there was no
significant difference across species in \emph{T\textsubscript{50}}
during both seasons. It is important to note that all species had
mature, fully expanded leaves except for \emph{H. courbaril,} which had
a young flush of leaves during the end of the dry period.
However,\emph{T\textsubscript{50}} for \emph{H. courbaril} was not
different from the pool of species excluding \emph{A. guianensis} .

\emph{T\textsubscript{95}} averaged 60.1±0.2 °C across species during
the end of the dry period and 57.3±0.3 °C during the wet period.
Although the ranges differed, the difference in means across the season
of 2.9 °C was not statistically significant (Figure 3C). The recovery
of\emph{F\textsubscript{v} /F\textsubscript{m}} , measured 24h after
heat treatment following a 30-minute duration dark period and found no
significant recovery upon incubation under the irradiance conditions
used (\textless{} 20 \selectlanguage{greek}µ\selectlanguage{english}mol m\textsuperscript{-2} s\textsuperscript{-1}and
\textasciitilde{}25\selectlanguage{ngerman}°C room temperature) in\emph{T\textsubscript{50}}
(\emph{t\textsubscript{50}} = 1.23, \emph{P}= 0.22),
\emph{T\textsubscript{5}} (\emph{t\textsubscript{50}} = --0.81,\emph{P}
= 0.42) or \emph{T\textsubscript{95}} (\emph{t\textsubscript{48}} =
1.97, \emph{P} = 0.059).

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-041/Figure-041}
\caption{{This is a caption
{\label{909256}}%
}}
\end{center}
\end{figure}

Figure 4:Relationships between PS II maximum quantum yield
(\emph{F\textsubscript{v}} /\emph{F\textsubscript{m}} ) breakpoint
temperature, \emph{T\textsubscript{5}} , and decline width, the
temperature window of~\emph{F\textsubscript{v}}
/\emph{F\textsubscript{m}} declines from 95\% to 5\% of the maximum
\emph{F\textsubscript{v}/F\textsubscript{m}} level
(\emph{T\textsubscript{95}} -- \emph{T\textsubscript{95}} ). A)
Evergreen trees from Nova Xavantina measured during end of dry and wet
seasons and B) Extended analysis using data from an unpublished study
from a Cerrad\selectlanguage{ngerman}ão (savannah forest) site in Nova Xavantina (wet season)
combined with data from Panama sampled during wet season reanalyzed from
data published by Slot et al. (2018) {[}Error bars in panel A: seasonal
variation whereas panel B, variation across individual tree
replicates{]}

\subsection*{Gradual vs rapid losses in PSII maximum quantum
yield}

{\label{933611}}

A negative relationship between
the~\emph{F\textsubscript{v}/F\textsubscript{m}} breakpoint
temperature,~\emph{T\textsubscript{5}} and decline width was observed
(R\textsuperscript{2} = 0.68 \emph{p} =0.02) (Figure 4A). Moreover,
species that sustain \emph{QY\textsubscript{max}} levels to a high
breakpoint temperature show sudden and steeper \emph{QY} decline with a
narrow \emph{DW} . Conversely, in cases where the \emph{QY} decline
started at lower temperatures, the decline was gradual (wider\emph{DW}
). \emph{A. guianensis} and \emph{H. courbaril} showed the highest
seasonal variation in \emph{DW} compared to other species and showed
similar response over both seasons. \emph{C. echinocarpus} showed the
lowest \emph{T\textsubscript{5}} and widest \emph{DW} . To further
expand this analysis, we also included an as yet unpublished dataset for
four evergreen species from a Cerradão (savannah forest) about 50 km
from our study site and data from three Panamanian tropical forest
species (Slot et al., 2018). The results from this additional analysis
(Figure 4B) showed a similar negative relationship
between\emph{T\textsubscript{5}} and decline width (R\textsuperscript{2}
= 0.96\emph{p} \textless{}0.001) consistent with results from our
dataset.\emph{Calophyllum inophyllum} and \emph{Inga spectabilis} from
Panama, showed similar response to \emph{A. guianensis} in our
measurement with much narrower DW; \emph{A. guianensis} however,
remained the species with highest \emph{T\textsubscript{5}} in our
combined analysis.

\subsection*{Pan-tropical variation in leaf thermal
tolerance}

{\label{343871}}\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-051/Figure-051}
\caption{{This is a caption
{\label{638653}}%
}}
\end{center}
\end{figure}

Figure 5: Photosynthetic thermal tolerance of adult evergreen trees
measured across tropical forest biomes. Except for Indian data for the
Western Ghats, all other studies including current study did the heat
treatment for 15-minutes

Only five studies in the literature reported data for adult evergreen
trees in tropical forests. O'Sullivan et al. (2017) and Zhu et al.
(2018) report the \emph{F\textsubscript{0}} rise inflexion point
metric~\emph{T\textsubscript{crit}} whereas the remaining tropical
studies viz., G. Heinrich Krause et al. (2010), Sastry and Barua (2017),
Slot et al. (2018) and the present study report
\emph{T\textsubscript{50}} based on
\emph{F\textsubscript{v}/F\textsubscript{m}} decline. Although the
number of studies is limited, this dataset includes rainforests in
Australia, India and the NeoTropics (Table 1).
Both\emph{T\textsubscript{50}} (Figure 5) and
\emph{T\textsubscript{crit}}showed very closely located modes around
47--48\selectlanguage{ngerman}°C. While most of the\emph{T\textsubscript{50}} was \textgreater{}
45°C and the mode was around 47°C; the range of
\emph{T\textsubscript{50}} (45--52°C) was much narrower than
\emph{T\textsubscript{crit}} , which showed very high interspecific
variability within sites. Mixed effect models showed that the choice of
metric (\emph{F} = 24.32, \emph{P} \textless{} 0.001) with season
(\emph{F} = 52.89, \emph{P} \textless{} 0.001) and species (\emph{F} =
13.04, \emph{P} \textless{} 0.001) explained significant variation in
the data indicating that both metrics are different given different
underlying mechanisms. The two metrics were therefore compared
separately.

The \emph{T\textsubscript{50}} values for the Central America and
Southern Amazon species in the wet period were 50.4±0.36°C and
49.4±0.73°C respectively whereas for the Western Ghats it was
significantly (\emph{t\textsubscript{31}} =2.40, \emph{p} =0.02) lower
at 46.9±0.31°C. \emph{T\textsubscript{crit}} was found to vary much more
across species and seasons than \emph{T\textsubscript{50}} .
While\emph{T\textsubscript{crit}} ranged from 37.2--66.7 °C
(n=96),\emph{T\textsubscript{50}} ranged from 45.5--56.42°C (n=54). A
linear mixed effect model fitted to explain \emph{T\textsubscript{50}}
showed season (\emph{F} =12.96, \emph{P} \textless{}.0001) and
biogeographical region (\emph{F} =15.26, \emph{P} \textless{}.0001) as
significant variables (pseudo \emph{R\textsuperscript{2}} = 0.885). In
contrast,\emph{T\textsubscript{crit}} showed no effect of season,
however the variability and limited number of sites represented, limits
any meaningful comparison.

\section*{Discussion}

{\label{discussion}}

\subsection*{Thermal tolerance at the hottest Amazonian forest
site}

{\label{494882}}

Tree species measured in Nova Xavantina showed a very high
thermotolerance, with one species, \emph{Amaioua guianensis,} recorded
the highest \emph{T\textsubscript{50}} documented for any tropical
evergreen tree thus far (52.7 ± 1.05 °C). The data reported here
represent the first \emph{F} \textsubscript{v}/\emph{F}
\textsubscript{m~}measurements made on adult Amazon trees. Thus, it is
not possible to directly compare these results with published data from
other Amazonian sites. However, the \emph{T\textsubscript{50}} values
reported here are similar to values reported for four Panamanian species
(Slot et al., 2018) and also to \emph{T} \textsubscript{crit} values
reported for two Amazonian sites(O'Sullivan et al., 2017). However,
although~\emph{F} \textsubscript{v}/\emph{F} \textsubscript{m}
and~\emph{F} \textsubscript{0} are related, they are not equivalent.
Hence, care must be taken when making comparisons across metrics.
However, the species in Nova Xavantina (Table 1) were considerably more
thermotolerant than the Western Ghats forests in India, where the most
extensive datasets by far are available for leaf thermotolerance (Sastry
\& Barua, 2017).

The differences in the long-term \emph{T\textsubscript{max}} mean values
for the site across the two seasons was \textasciitilde{}2.5°C
(\emph{T\textsubscript{max}} = 35.7±0.37°C for October and 33.19 ±0.23°C
for the months March-April). Consistent with the literature\emph{,
T\textsubscript{50}} measurements across seasons showed significant
differences, with \emph{T\textsubscript{50}} values for the hot/dry
season being \textasciitilde{}1.6 °C greater than in the cooler/wetter
season. \emph{A. guianensis} , a characteristic mid-storey species found
across a wide range of ecosystems in the region (Morandi et al., 2016),
had the highest \emph{T\textsubscript{50}} and the highest seasonal
plasticity in \emph{T\textsubscript{50}} . \emph{A. guianensis} is the
slowest growing species among the species sampled (Mews, Marimon, Pinto,
\& Silvério, 2011). These results are consistent with studies that
indicate that slow-growing species have greater capacity for flexible
heat dissipation and thermal protection mechanisms than fast-growing
plants (Adams \& Demmig-Adams, 1994). Maintaining high thermal tolerance
during dry periods could be energetically expensive (Wahid, Gelani,
Ashraf, \& Foolad, 2007), thus requiring down-regulation during wet
periods. Stomatal regulation varies across seasons and species.
Moreover, lower water availability limits the transpiration-dependent
cooling capabilities in dry periods.

The observed leaf level differences could be due to small variations in
the prehistory of the leaves within the canopy for example in terms of
light or temperature exposure (Colombo \& Timmer, 1992) during the peak
dry period, prior to sampling. For example, the acquired tolerance
induced by a pre-exposure to heat is associated with the synthesis of
heat shock proteins (HSPs) and with other low molecular weight proteins
that protect PSII (Gifford \& Taleisnik, 1994). The variations
in\emph{T\textsubscript{95}} values were higher than those observed
in\emph{T\textsubscript{5}} or \emph{T\textsubscript{50}} . This may
indicate that there is greater variation in the high temperature
threshold for loss of photosystem integrity (\emph{T\textsubscript{95}}
) between species than in the initiation temperature of sensitivity
(\emph{T\textsubscript{5}} ) (Figure 6).

The mechanisms that underpin the thermal stability of photosynthesis in
tropical trees have not been fully characterised. The data presented
here clearly show that species such as \emph{A. guianensis} are able to
maintain photosystem II functions up to high temperatures (53.9±0.75°C
during the end of dry period i.e., the highest reported in literature
for C3 plants). These trees could have mechanisms to not only protect
the photosystem II from irreversible thermal inactivation, but also have
a high degree of plasticity in relation to temperature fluctuations. In
such trees, loss of photosynthetic functions may be prevented by a more
rapid repair cycle than that occurring in other species. However, the
precise nature of the specific mechanisms involved remains unclear.
Further molecular and metabolic studies on tropical tree species are
required to understand how photosynthesis can withstand high
temperatures. Some studies indicate that redundancy and diversity of
light harvesting complexes (Tang et al., 2007) could play a role in
buffering the high light, high temperature-induced changes in
photosystem functions. Hence, heat sensitive systems could be replaced
with more thermally stable forms. Similarly, large variations in heat
shock factors/proteins, oxidative stress/signalling and thermal energy
dissipation could exist across taxa, hinting at differential thermal
sensitivity of tropical evergreen trees to high-temperature stress.

\subsection*{Thermal sensitivity thresholds for photosystem
II}

{\label{846225}}

A key question concerns how close tree species are to their thermal
sensitivity thresholds in the warmest region of the Amazon? The data
presented here show that \emph{T\textsubscript{50}} for seven focal
species is at least 4 °C above the highest air temperatures ever
experienced at the study site (43.9 °C). However, for most
species,~\emph{T\textsubscript{5}} , the breakpoint
temperature\emph{\textsubscript{,}} is already surpassed in the peak of
the warm/dry season (Figure 6). Thus, the site temperatures are already
reaching levels where incipient irreversible loss of PSII activity
occurs during the dry/warm periods. Such periods are predicted to become
more frequent with future warming (Yao, Luo, Huang, \& Zhao, 2013).
However, these inferences are based on air temperatures, while leaf
temperatures are arguably a more meaningful measure of proximity to
thermal sensitivity thresholds. During the dry/warm season, limited
access to soil moisture restricts stomatal opening. This is likely to
lead to leaf temperatures that exceed air temperatures (G. Heinrich
Krause et al., 2010; Slot \& Winter, 2016), at least for part of the
day. Under these circumstances, plants in our study area, especially
those exposed to full sunlight, may operate closer to thermal
sensitivity thresholds than the simple examination of air temperatures
suggests.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-061/Figure-061}
\caption{{This is a caption
{\label{537963}}%
}}
\end{center}
\end{figure}

Figure 6: Leaf thermal tolerance for six dominant evergreen tree species
during the end of the dry season from the hottest Amazonian forest site,
Nova Xavantina measured with \emph{F\textsubscript{v}
/F\textsubscript{m}} temperature response to 15-minute duration heat
treatment. Inset histogram shows warming trends in daily maximum air
temperature for the site for the dry period of July-October (primary
ordinate). Four-point logistic curves indicate combined fit for all the
species measured (secondary ordinate). Box plots are overall measures of
thermal tolerance with temperature on the abscissa. Box plot ordinate
positioning indicative and not absolute. {[}See Supporting Information
Figure S2 for wet season status{]}

\subsection*{Variations in the responses of PSII to
heat}

{\label{520669}}

The negative relationship between \emph{T\textsubscript{5}} and decline
width (Figure 4) indicates a range of response strategies in trees. The
two extremes of PSII thermal sensitivity could be described as
`sensitive' and `tolerant' (See Figure 7). Tolerators were found to
sustain their PSII QY values up to a remarkably high temperature point
(high \emph{T\textsubscript{5}} ). They were characterised by a rapid
decline to near zero levels at higher temperatures
beyond\emph{T\textsubscript{5}} . Conversely, heat `sensitive' responses
are characterised by a sensitivity of PSII to much lower
(low\emph{T\textsubscript{5}} ) temperatures. The decline in PSII QY to
near zero levels in these trees, however, occurs gradually over a very
wide temperature range. Hence, in heat sensitive trees, when the leaf
temperatures are at this `decline width' range, PSII is already
negatively affected by temperature stress. The slow decline could
potentially indicate PSII protection mechanisms that ensue mediated by
heat shock proteins and/or xanthophyll mediated thermal protection
mechanisms. Given the change in PSII activity, it is reasonable to
hypothesise that electron flow could switch from the non-cyclic pathway
to cyclic electron transport around PSI, a mechanism that is known to
protect PSII against thermal inactivation (Essemine, Xiao, Qu, Mi, \&
Zhu, 2017; Sun, Geng, Du, Yang, \& Zhai, 2017).

High temperature tolerant trees, on the other hand have better protected
PSII which can continue to function up to a very high temperature point.
By sustaining PSII activity under extremely high leaf temperature
conditions, tolerators could maintain better photosynthetic electron
flow rates than thermosensitive species. The effective thermal
sensitivity of heat tolerant trees could be linked to stomatal controls
and transpiration cooling. However, evaporative cooling can reduce the
need for investment in thermal tolerance mechanisms. If transpiration
cooling is strong, the mechanisms that facilitate high PSII thermal
tolerance might not be required. Hence, thermal tolerance of leaf
metabolism must consider the molecular physiology of the leaf.

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-071/Figure-071}
\caption{{This is a caption
{\label{121911}}%
}}
\end{center}
\end{figure}

Figure 7: Schematic illustration of two variants
of\emph{F\textsubscript{v}/F\textsubscript{m}} sensitivity to
temperature. Tolerant runs indicate that PSII activity and potentially
linear electron transport are unaffected at very high temperatures
whereas sensitive response is characterized by initiation
of\emph{F\textsubscript{v}/F\textsubscript{m}} activity decrease at low
temperatures followed by slow and gradual decline
of\emph{F\textsubscript{v}/F\textsubscript{m}} at higher temperatures.
This gradual decline window indicates PSII activity decline. The slow
decline could be due to possible protection mechanisms and possibly the
temperatures where cyclic electron transport ensues.

\subsection*{Ecophysiological relevance of PSII QY temperature
sensitivity}

{\label{518903}}

During the dry and hot period of the seasonal cycle, leaf to air
temperature differentials (\emph{T\textsubscript{leaf}}
--\emph{T\textsubscript{air}} ), in evergreen trees in some tropical
sites have been measured to be as high as 18\selectlanguage{ngerman}°C (Fauset et al., 2018;
Pau, Detto, Kim, \& Still, 2018; Tribuzy, 2005). Furthermore, during the
parts of the day when transpirational cooling capacity is limited by
stomatal closure (Slot et al., 2018), leaf temperatures reach as high as
48°C in some tropical trees (Slot \& Winter, 2016). Comparing these leaf
temperatures with the range of \emph{T\textsubscript{5}} measured,
consistent with the ideas presented in Mau, Reed, Wood, and Cavaleri
(2018) and Doughty and Goulden (2008), it is likely that some sensitive
species are already experiencing temperatures that are affecting PSII
functioning. Given the species level variability and largely unexplored
mechanisms of thermal protection strategies of trees, it is probable
that there will be varied range of response to high temperature
conditions across species. Early leaf senescence is triggered by
exposure to high temperatures (De la Haba, De la Mata, Molina, \&
Agüera, 2014; Way, 2013). Strategies such as increased leaf turnover
rates could therefore be triggered by high temperatures with
implications for carbon acquisition by the trees. Transpirational
cooling mechanisms under high temperatures could be limited and vary
between species, making species with less water available for cooling
more susceptible to thermal stress. However, some species continue to
transpire even at extremely high temperatures (Drake et al., 2018).

The~\emph{F\textsubscript{v}} /\emph{F\textsubscript{m}}
or\emph{F\textsubscript{0}} based fluorescence data only indicate the
responses of PSII (See Supporting Information Figure S3
for~\emph{F\textsubscript{v}} /\emph{F\textsubscript{m}}
and\emph{F\textsubscript{0}} relationship). However, the limits of other
processes such as thermal protection, stomatal and metabolic controls of
CO\textsubscript{2} assimilation are largely unknown except in a few
model plant species. Hence, such integrated definitions of thermal
thresholds could be more meaningful and provide a better understanding
of the thermal sensitivity of trees or plants in general. Thus, concepts
such as thermal safety margins (O'Sullivan et al., 2017; Sunday et al.,
2014) could be expanded further to derive ecophysiologically meaningful
conclusions (See Supporting Information 7 and Table S4).

Overall, the data presented here show that there is significant
diversity in the temperature sensitivity of photosynthesis across
tropical evergreen trees (Cseh et al., 2005; Holm, Várkonyi, Kovács,
Posselt, \& Garab, 2005), much of which remains uncharacterised. It is
probable that with increasing temperatures, especially during the
summer, the leaf temperature differences between species, together with
differences in transpiration cooling capacity, will reveal large
interspecific variations in tree sensitivity to extreme heat conditions.

\section*{Conclusion}

{\label{conclusion}}

The present study quantified the photosynthetic thermal tolerance of
seven dominant evergreen trees from the hottest and most rapidly warming
forest site in the Amazonia. The key findings of this study are firstly,
that evergreen trees from the site exhibit a high level of PSII
thermotolerance. Although variations in thermotolerance was observed
between species, this was largely caused by data from one mid-storey
species, which showed exceptionally high thermotolerance (the highest
recorded in the tropics). A significant but weak seasonal acclimation
of\emph{T\textsubscript{50}} was observed that is consistent with the
literature (Sastry \& Barua, 2017; Zhu et al., 2018).
Although\emph{T\textsubscript{50}} values were much higher than the
absolute maximum air temperature of the region, there is some evidence
of incipient loss of PSII function even under current conditions. Daily
maximum air temperatures during the dry periods at our study site in
recent years has reached levels that overlap with the range
of\emph{T\textsubscript{5}} measured in our study. The trees in the
hottest Amazonian forest site are therefore already sensitive and
experiencing temperatures close to the thresholds of their
high-temperature sensitivity.

\subsection*{Summary of contributions}

{\label{358974}}

The work was envisioned by DG. RT and DG jointly led the research. RT,
CHF and DA developed the thermal tolerance protocols with inputs from
HGK. RT trained the teams led by WJAC and SMR in thermal tolerance
measurements; BSM and BHM oversaw the field campaign. IAS collected
additional data from Cerradão forest, Nova Xavantina. DMS sampled trees
with support from CHLO, EQM and WJAC. MS, KW and HGK provided data for
Panama. Remaining team members assisted with thermal tolerance
measurements. RT wrote the manuscript with primary support from DG and
additional support from EG, CHF and SF. All co-authors commented on the
manuscript.

\subsection*{Acknowledgement}

{\label{744331}}

The field campaign was part of and funded by BIORED (NE/N012542/1) and
ECOFOR (NE/K01644X/1) projects. RT's PhD was funded by University of
Leeds, Leeds International Research Scholarship (2016--2018). RT
acknowledges additional funding support from NERC IOF India grant
(NE/R005079/1), Ecology and Global change cluster funds at School of
Geography, University of Leeds and Climate Bursary Funds from the
University of Leeds. DG acknowledges support from two NERC awards:
TREMOR (NE/N004655/1) and ARBOLES (NE/S011811/1). We acknowledge
CNPq/PELD Project 441244/2016-5 for field work support during the forest
inventories in Nova Xavantina. AV acknowledges the funding support by
the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior - Brasil
(CAPES) - Finance Code 001; Conselho Nacional de Desenvolvimento
Científico e Tecnologico (CNPq); and Fundacao de Amparo a Pesquisa do
Rio de Janeiro (FAPERJ); Centro Multiusuario CME-LBCT the infrastructure
provided. Thanks to Chetan Deva, Amy Bennet, Somashekhar Achar KG, Emma
Docherty and Alexander Chambers-Ostler for helpful discussions.

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{\label{references}}

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Table 1: Mean photosynthetic thermal tolerance for adult evergreen trees
across tropical forest sites.\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
Location / study site & Wet season & Dry season\tabularnewline
\midrule
\endhead
& \emph{T\textsubscript{crit}} in \selectlanguage{ngerman}°C & \emph{T\textsubscript{crit}} in
\selectlanguage{ngerman}°C\tabularnewline
Atherton, Queensland & 46.2\selectlanguage{ngerman}±0.83 (14) &\tabularnewline
Iquitos, Peru & & 50.8\selectlanguage{ngerman}±2.32 (13)\tabularnewline
Paracou, French Guiana & 49.8\selectlanguage{ngerman}±0.91(19) &\tabularnewline
Republic of Panama & 48.0 (1) & 47.9\selectlanguage{ngerman}±0.75 (2)\tabularnewline
& \emph{T\textsubscript{50}} in \selectlanguage{ngerman}°C & \emph{T\textsubscript{50}} in
\selectlanguage{ngerman}°C\tabularnewline
Cape Tribulation, Far North Queensland & & 48.4\selectlanguage{ngerman}±0.58 (12)\tabularnewline
Nova Xavantina, Brazil & 49.5\selectlanguage{ngerman}±0.77(7) & 51.1\selectlanguage{ngerman}±1.23 (6)\tabularnewline
Pune, India & 48.0\selectlanguage{ngerman}±0.27 (18) & 47.0\selectlanguage{ngerman}±0.31(17)\tabularnewline
Republic of Panama & 51.0 (1) & 50.4\selectlanguage{ngerman}±0.28(5)\tabularnewline
Robertson Creek, Far North Queensland & 46.8\selectlanguage{ngerman}±0.53(11) &
49.2\selectlanguage{ngerman}±0.28(11)\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}

Values are mean ± standard error. The number of species is shown in
parenthesis.

\selectlanguage{english}
\FloatBarrier
\end{document}

