1 Introduction
Global climate change has led to and is continuously resulting in
increases not only
in
drought intensity but also in the frequency and duration of drought
events globally (IPCC, 2013). Two main hypotheses have been proposed and
are currently debated to explain the mechanisms for the widespread
forest dieback caused by increased drought events (Hartmann et
al. , 2018; Spinoni et al. , 2018): trees would die (1) due to
hydraulic failure or (2) as a result of carbon starvation (McDowell &
Allen, 2015; Gessler et al. , 2018). The hydraulic failure
hypothesis proposes that the tree mortality results from the embolism of
the xylem vessels under high evaporative demand and restricted soil
water availability (Nardini et al. , 2013) whereas the carbon
starvation hypothesis suggests that tree mortality would be caused by a
carbon supply limitation due to stomatal closure and thus reduced
photosynthesis that cannot cover the carbon and energy demand for
maintenance processes (McDowell et al. , 2008; Rowland et
al. , 2015). Concerning carbon starvation, several studies have shown
that the contents of starch, a compound that serves as carbon storage
and is build up when assimilation exceeds plant’s demand for carbon,
were strongly reduced by severe soil water deficit (McDowell, 2011),
implying carbon limitation (Reinhardt et al. , 2015). Previous
studies also suggested that hydraulic perturbation could prevent phloem
transport (Sevanto, 2014) and consequently constrains carbon
accessibility (Sala et al. , 2012; Hartmann & Trumbore, 2016),
even though the carbon availability in the tree crown area might not be
restricted. Thus, hydraulic failure and carbon starvation are often seen
to be associated with each other in the cascade of drought events
leading to tree mortality (Adams et al. , 2017).
Droughts, however, are very tougher to be defined, and thus a
universally accepted drought definition considering both water deficit
intensity and duration is still lack (Buitink et al ., 2021).
Previous drought-related forest and
agricultural studies, especially
manipulation experiments, mainly focused on the effects of drought
intensity (e.g. various levels of watering such as very limited, limited
and optimum watering) on trees or plants (Allen et al. , 2010; Yuet al. , 2021), while less is known about the effects of drought
duration on trees’ physiology, growth and mortality. A long-lasting
drought event, may cause irreversible changes in plant physiology which
are different from those found in relatively short-term severe droughts.
For example, recurrent or short-term lasting drought events may allow
trees to recover and even acclimate to water restriction (Gessleret al. , 2020), and thus, permit them to survive in the long-term.
However, longer term lasting drought events may strongly affect the
recovery ability of trees. Currently, unexpected whole-season drought
occurs more frequently in many regions around the world. For instance,
since the beginning of the 21st century Europe already experienced
severe drought summer of 2003, 2010, 2013, 2015, and 2018 (Hanelet al. , 2018; Brunner et al. , 2019). Therefore,
mechanistic understanding of tree and forest responses to various
drought duration is particularly critical for sustainably forest
management under future climate change.
Theoretically, trees’ resilience and resistance to drought stress and
the recovery and thus survival ability should be associated with the
resource storage and availability. Non-structural carbohydrates (NSC =
soluble sugars + starch) among other resources (e.g. nutrients, see
Gessler et.al 2017) are known to contribute to tree resilience after
stress (Li et al. , 2002; Li et al. , 2008b). For many
years, reserve storage was considered as a passive process resulting
from an accumulation of resources when uptake and assimilation exceeded
growth demand (Sala et al. , 2012; Wiley & Helliker, 2012). An
alternative hypothesis was proposed in which reserves storage would be
rather an active process during the growing season and would act as a
sink competing with other sinks (e.g. growth and reproduction) for
available resources (Wiley & Helliker,
2012).
Several studies investigated the NSC levels of trees during and at the
end of the growing season following a growing-season-long drought (Liet al. , 2013; Schönbeck et al. , 2018; Schönbeck et
al. , 2020a; Schönbeck et al. , 2020b). They found a
drought-induced growth reduction but did not observe a drought-induced
NSC decrease of trees, suggesting an active NSC storage under drought at
the expense of growth (Wiley & Helliker, 2012; Li, M-H et al. ,
2018). Recent evidence indicates that stress actively induces NSC
transfer from aboveground tissues to roots stored (Kannenberg et
al. , 2018; Li, W et al. , 2018). In contrast, Li W. et al.(2018) analyzed 27 case studies and found that drought decreased NSC
concentration by 17.3% in roots, while it did not change NSC in
aboveground tissues in the current season. To our knowledge, even less
is known about the changes of NSC over winter (post- vs. pre-winter) in
trees previously stressed by drought.
Recently, it was proposed that nutrient addition (i.e. fertilization)
will affect the fitness of trees under dry conditions, showing
intensifying or mitigating effects on trees’ tolerance to drought
(Kreuzwieser & Gessler, 2010; Gessler et al. , 2017; Schönbecket al. , 2020b). Nitrogen
(N) deficiency can increase the sensitivity of stomata to low leaf water
potential (Radin & Ackerson, 1981; Ghashghaie & Saugier, 1989), which
further increases the risk of drought-induced carbon starvation
(McDowell, 2011). N itself is a main growth limiting nutrient in
temperate terrestrial ecosystems and is also a major component of
Rubisco and other photosynthetic enzymes and structures which regulate
the photosynthetic activity and thus carbon gain and the NSC level of
trees in response to environmental factors such as drought (Bondet al. , 1999; Meng et al. , 2016). Schönbecket al. (2020a) found that
negative effects of moderate drought intensity (but not of severe
drought) could be compensated by increased nutrient availability in Scot
pine saplings. In contrast, Jacobs et al. (2004) reported that
fertilization with blended fertilizer impaired the root system
development and drought avoidance ability of drought-stressed
Douglas-fir seedlings. Similarly, Dziedek et al. (2016) found
that nitrogen addition increased the drought sensitivity of saplings of
several deciduous tree species (Dziedek et al. , 2016). Despite
these studies, there is a strong knowledge gap about drought and
nutrient interaction and especially about whether and to what extent
nutrient addition affects winter NSC consumption and thus growth
recovery in the season following drought.
The species, Quercus petraea (Matt.) Liebl. (oak ) andFagus sylvatica L. (beech), are two coexisting species in
European forests. According to (Ellenberg, 2009), oak will become more
competitive than beech at sites where as July-temperatures increase to
>18°C and precipitation decreases to <600 mm/year
as a result of climate change. We carried out a greenhouse experiment to
explore the effects of different drought duration in combination with
fertilization on the physiology, mortality and growth of these two
species. We mainly focused on the following research questions: 1) How
does drought duration influence trees’ performance; 2) How does various
drought duration in previous growing season affect tree’s winter NSC
consumption and thus the over-winter NSC change? 3) Does post-winter NSC
level (at the early beginning of next growing season) rather than
pre-winter NSC level determine growth recovery of previously
drought-stressed trees? 4) Can fertilization mitigate the negative
drought effects on trees as proposed by Gessler et al. (2017)?