Introduction
Tree age may result in variation in leaf morphological, chemical,
physiological traits and the ability to absorb nutrients. Incresing tree
age can cause alterations in leaf morphological traits,
vegetative/reproductive allocation, nutritional absorption, hormonal
control, and environmental adaptability, all of which can lead to
variation in tree structure and function (Thomas et al., 2002; Thomas
2011; Damián et al. 2018). All of these traits may induce the associated
diffrances in the structure and function of the forest (Thomas 2010;
Thomas 2011; Martin and Thomas 2013; Damián et al. 2018).
The leaf ecophysiological traits vary significantly with plant life
history, light and nutrients availabilities (Reich et al., 1992; Ackerly
et al. 2000; Han et al. 2020). Many studies investigated the
physiological traits (i.e. photosynthesis rate) appear to decline in
tall trees, because of the limitation of hydraulic transponrt
(Mencuccini and Grace 1996a). It has been objerved that hydraulic
transport may reduced with tree age as a results of the longer path
lenth from soil to stomata, which derive the reductions in stomatal
conductance and photosynthetic rate that directly influence tree
productivity (Ryan and Yoder 1997). The high values of leaf
physiological traits represent acquisitive strategies (high
productivity) for plants, and low values represent conservative
strategies (low productivity) (Wright et al. 2005a; Gorne et al. 2020).
Several investigations evaluated that younger tree have greater trait
values related to resource acquisition, but mature and older have more
conservative traits (Damián et al. 2018; Dayrell et al. 2018). During
the forest chronosequence, plants shift from acquisitive physiological
strategy to conservative physiological strategy (Han et al. 2020) to
maintain the plant overall productivity at its optimum as per the
metabolic need.
The leaf phenological cycle offers an intriguing system for
investigating the interconnection between leaf gas exchange, water
relations, and leaf functional traits with respect to leaf phenology
(Fajardo, and Siefert 2016). Leaf physiological traits significantly
change with phenological phases. During the growing leaf age, leaf
physiological traits change as a result of the in seasonal environmental
conditions and leaf age (Escudero et al. 2003; Chvaana et al. 2017;
Chvaana et al. 2019). Leaf phenological cycle and leaf economic spectrum
(i.e. leaf initiation, expansion, and senescence) varied among species,
among individual plants, and also among leaves on a plant (Chabot and
Hicks 1982; Reich et al. 1991; Mediavilla et al. 2014; Bai et al. 2015).
Previous research has revealed that with leaf age, leaf mass area (LMA)
rises while leaf nitrogen and phosphorus (N and P) are decline
(Niinemets and Lukjanova 2003; Niinemets et al. 2006; Athokpam and
Garkoti 2015). Previous workers also revealed the leaf and tree
age-related decline in photosynthesis traits which could be associated
with the change in leaf morphology and nutrient over time (Reich et al.
1991). Leaf N is positively correlated with the activity of Rubisco
(ribulose-1,5-biphosphate carboxylase/oxygenase), and its concentration
often decreased with tree and leaf age (Kitajima et al. 2002, Wright et
al. 2006, Fajardo and Siefert 2016, Chavana-Bryant et al. 2019). Leaf P
is involved in various metabolic activities during the photosynthesis
processes and also decreases with leaf and tree age (Wright et al. 2006,
Mediavilla et al. 2011, Chavana-Bryant et al. 2017, 2019). Likewise
stomatal conductance and stomatal control (i.e, stomatal opening and
closing) also decrease with leaf and tree age (Reich and Borchet 1988)
and affect photosynthesis.
Water (H2O), N, and P are essential resources for plant
survival, growth, and photosynthesis. Leaf physiological traits such as
net photosynthetic capacity, leaf diffusive conductance, and
transpiration rate are indicators of CO2 assimilation,
resource use strategies, and water exchange. The water use efficiency
(WUE) and photosynthetic N and P use efficiency (PNUE and PPUE) is an
essential characteristic of different species and determines the leaf
physiology, leaf economics, strategy, and competition expected to change
with plant age and leaf age (Robinson et al. 2001; Wright et al. 2004;
Nabeshima and Hiura 2004; Bai et al. 2015). The WUE, PNUE and PPUE are
excellent ecological indicator of species performance in different
environmental conditions expected to change with palnt age, and leaf age
(Funk and Vitousek 2007). The PNUE and WUE describe the N concentration
per unit leaf area and the amount of water transpired, respectively, for
a given rate of photosynthesis. At the leaf level, WUE is the ratio
between net CO2 assimilation and water loss via
transpiration, and intrinsic water-use efficiency (WUEi) is the quick
ratio between net CO2 assimilation and stomatal
conductance. The PNUE, PPUE, and WUE predict how the photosynthetic
assimilation is optimized per unit of N, P, and water in leaves (e.g.,
Poorter and Evans 1998; Castellanos et al. 2005; Sheng et al. 2011).
Thus, leaf phenological cycle and resource allocation strategies are
physiologically related (Ackerly and Bazzaz 1995; Hikosaka 2005), making
leaf lifespan a crucial characteristic controlling plant carbon and
nutrient economies, which eventually result in adaptive modification in
response to ecological heterogeneity.
Nitrogen-fixing Alnus nepalensis (D. Don) is one of the
fast-growing early successional tree species that often forms pure
stands in areas affected by landslide/ slip stages over 1400 m.a.s.l.
but also occurred in a forest chronosequence in the central Himalaya
(Joshi and Garkoti 2021b). A. nepalensis is an important economic
and reforestation tree species in central Himalaya. Previous research
has demonstrated that A. nepalensis is an extremely important for
soil restoration and degraded forest management (Joshi and Garkoti
2021b). Most studies of A. nepalensis have focused on ecosystem
carbon dynamics (Joshi and Garkoti 2021b), soil physicochemical
properties, below ground biomass and litter dynamics (Joshi and Garkoti
2020; Joshi and Garkoti 2021a). However, it is still not understood how
the leaves of A. nepalensis response (photosynthetically,
chemically and morphologically) along with the leaf age, and tree age.
Therefore, we investigated the effect of leaf age, and tree age on leaf
ecophysiology traits in A. nepalensis . In this study, we proposed
two questions 1) how do the leaf ecophysiological and morphological
traits change with leaf and plant age? We expect a shift from a more
acquisitive physiological strategy in the young stage to a conservative
physiological strategy in the older stage, and 2) A resource
conservation strategy adopted by the older stage may have high WUE,
tough leaf construction, and low leaf N and P concentration, low
photosynthetic N-, and P-use efficiency and low rates of carbon
assimilation when compared to the young stage.