Relationship between leaf traits
In the present study, there was a positive correlation between
Emass, gswmass, PNUE, and PPUE with
Amass; Earea, gswareawith Aarea; and Earea with
gswarea. There was a negative correlation between
WUEi and WUE with Aarea; gwsarea with WUEi and Earea with WUE in all three tree age stages (Fig.4).
Intrinsic water use efficiency (WUEi) and water use
efficiency were significantly negatively correlated with PNUE in all
three tree age stages. There was a positive correlation between midday
water potential (Ψmd) with Aarea,
Earea and gswarea (Fig.5). The specific
leaf area (SLA) was positively correlated with Amass,
Emass, PNUE, and PPUE in all three tree age stages but
negatively related to N concentration per unit leaf mass across all
three tree age stages. In all three tree stages, the PNUE and PPUE were
negatively correlated with N and P concentration per unit of leaf mass.
The total chlorophyll concentration was positively correlated with
Aarea (Fig.6).
DiscussionVariation in leaf ecophysiological traits along with the leaf
age and tree age
Our first objective was to understand how key ecophysiological traits
changing from resourse acquisition to resource conservation with plant
age and leaf age. As expected, we noticed a shift toword
resource-conservatite traits with tree age. Thus, our findings reinforce
the idea that older tree have more conservative traits (Manson et al.
2013; Damián et al. 2018; Dayrell et al. 2018; Funk et al. 2021). Older
tree had higher SLA, lower leaf Nmass and reduced
photosynthetic traits compared with young trees. The modifications in
photosynthetic rate were strongly dependent on E, gsw, PNUE, WUE, and
SLA (Fig.4). Our results demonstrated that leaf age and tree age had a
significant effect on gsw, which may be due the tree age-related changes
in hydraulic conductance and differences in Ψleaf. The
lower gsw of old tree than young and mature trees would be constant with
the hydraulic limitation hypothesis and the influence of the
gravitational hydrostatic gradient (Ryan and Yoder 1997). It is quite
well recognised that stomatal closure is linked to decreased soil to
leaf hydraulic conductance and variations in Ψleaf(Hubbard et al. 1999; Kolb and Stone 2000). As a result, plant
strategies gradually shifted from more resource-acquisitive to more
resource-conservative (Guariguata and Ostertag, 2001). This was also
reflected in higher leaf chlorophyll concentrations and greater PNUE and
PPUE in the young tree, which triggered a shift of leaf traits towards
faster growth strategies involving greater photosynthesis and light
capture to enhance the growth rate.
At the leaf scale, leaf age-driven differences in chemical and
morphological traits have contributed significantly to the change in
physiological traits along with the leaf age. The total chlorophyll,
Nmass, and Pmass was higher in young
tree which supports the higher growth rate, and productivity. This is
because the combination of optimum morphological and chemical traits
with more favorable environmental factors (light and temperature) of
fully expanded leaf posed a positive impact on nutrient and water
cycling. This supported optimum productivity during the summer season
when the leaf was fully expanded than the leaf bud burst (spring) and
leaf senescence stage (autumn). Previous studies also suggested the
similar leaf age trends of the physiological traits related to leaf age;
they showed peak values during fully expanded leaf (Wilson et al. 2000,
2001; Bauerle et al. 2004; Grassi et al. 2005). In the summer, day
length was longer, highest photosynthetically active radiation, optimum
air temperature and higher leaf Nmass, and
Pmass combined to highest physiological traits (Grassi
et al. 2005, Hikosaka et al. 2007, Wright et al. 2006). Leaf nitrogen
and chlorophyll dropped to their lowest level in the leaf senescence
stage due to the nutrient remobilization process after leaf senescence,
associated with lowest physiological traits. Such leaf age and tree
age-dependent decline in leaf morphological and chemical traits are
significantly associated with changes in leaf physiological traits over
time (i.e., photosynthetic rate) (Reich et al. 1991). In addition, the
nutrient resorption process at the time of leaf senescence negatively
affected the overall productivity during the leaf senescence stage
(Crous et al. 2019). Photosynthetic rate in fully expanded leaf was
significantly associated with other physiological traits including
transpiration rate, stomatal conductance, water potential, and
photosynthetic nutrient-use efficiency (Reich et al. 1998; Wright et al.
2005b). This indicated that, at a given photosynthesis rate,
transpiration rate, stomatal conductance, and photosynthesis nutrient
use efficiency were higher in the young stage than mature and old
stages. The tight connection between the levels of photosynthesis,
chlorophyll, and Nmass reflected contribution of
nitrogen to the Calvin-Benson cycle enzyme (in particular,
ribulose-1,5-biphosphate carboxylase/oxygenase—Rubisco) and
chlorophyll for better light harvesting. The high chlorophyll
concentrations in the young stage indicated supported the high
light-harvesting and higher carbon assimilation per unit leaf area.
Acquisitive vs. conservative resource-use strategy
In the present study, we found a significant PNUE-WUE trade-off for the
young, mature, and old stage. Young stage achieved higher PNUE and lower
WUE, whereas mature and old trees followed the reverse trend. The
trade-off between WUE and PNUE may explain the greater rates of
physiological traits in young stage. The PNUE for the young stage was 12
% higher than the mature stage and 18 % higher than the old stage.
Young stage exhibited higher water potential suggesting a trade-off
between leaves with greater photosynthetic rates in young stage and
leave are water-stressed (low water potential) in the old stage. The
PNUE and PPUE were higher in the young stage, favoring low investment
and quick return resource strategy, than mature and old stages, which
favored slow resource return strategy (Wright et al. 2004; Reich and
Flores- Moreno 2017). In the present study, the young tree showed
greater N assimilation efficiency and more focused allocation of N to
chlorophyll tissues than other non-photosynthetic tissue to ensure
optimum growth. This was supported by the morphological traits, as SLA
showed comparatively lower values in the young stage than the older one
(Abdul and Mencuccini 2009), to reduce the recourse investment in
non-photosynthetic tissues in leaves of young stage. The trade-off N
partitioning between photosynthetic and non-photosynthetic tissues in
varying tree age has also been reported by previous studies (Hikosaka
and Hirose, 2000; Hikosaka, 2004). In the present study, the majority of
mass based-physiological traits (i.e Amass,
Emass, and gswmass) and photosynthesis
nutrient-use efficiency, showed a positive correlation with SLA, which
is similar to many previous studies (Wright et al. 2005b; Bahar et al.
2017; Crous et al. 2017). Onoda et al. (2017) also revealed that leaves
with greater SLA tend to enhance Amass,
Emass, and PNUE to support fast growth. Our results
indicated that young trees allocate more nutrients to photosynthetic
tissues to support their rapid growth than mature and old trees. This
reveals the differences between leaf size and weight (Shipley et al.
2006). The old trees had higher WUE than young and mature trees, leading
to significantly lower physiological traits. In general, the high WUE
indicates a more conservative resource use pattern (Lambers et al.
1998). The midday water potential (Ψmd) was more
negative (indicating higher potential stress) during the summer in old
tree, because of the enhanced water supply pressure and higher hydraulic
pressure. Consequently, to adapt to the pressure and balance of demand
and supply, the old tree acquires the more conservative approach to
shift toward higher WUE.
Leaf age and tree age leads to adaptive changes in ecophysiological
process, such as variation in water relation, gas exchange, and growth
rate among varying tree age stages (Sala et al., 2010). Physiological
responses, including evolution and adaptation to environmental shifts,
characterize phenotypic plasticity, which is believed to be the dominant
underlying process with implications for ecological processes (Hovenden
and Vander Schoor, 2003; Thomas, 2011). A better comprehension of the
physiological traits of major native trees is required for introducing
resilient forest management strategies to minimize the expected effects
of climatic change on plant development and water stress. The
stage-specific physiological traits of A. nepalensis appear to
have important for the ecosystem process. The WUE, PNUE, and PPUE
apparently play an important regulatory role in the functioning ofA. nepalensis -dominated ecosystem. The PNUE and PPUE was
significantly greater in the young stage, providing additional evidence
that A. nepalensis has developed a mechanism for efficient use of
N and P nutrient and the modifications in physiological traits among theA. nepalensis dominated forest stages accordingly. Ishida et al.
(2005) have also explained the similar ontogenetic morphological,
anatomical, and chemical modifications leading to evolution and
adaptation in leaf physiology along the age gradient for the pioneer
tree species, Macaranga gigantean . All the stage-specific changes
of physiological traits of A. nepalensis had important
implications with respect to nutrient cycling, carbon assimilation, and
feedback interaction between leaf, stand, and ecosystem-level process of
degraded forests in central Himalaya.
In conclusion, our findings indicate that most of the physiological
traits of A. nepalensis decreased with a natural age gradient,
indicating that the ecological strategy of A. nepalensis changed
from a resource acquisitive approach to conservative resource approach
with a change in age gradient with time. The relationship between the
leaf traits and the structural and chemical traits changed during the
age gradients, indicating different trade-off strategies across the age
gradient. High photosynthetic nitrogen- and phosphorus use efficiency in
the young stage could support the rapid growth of A. nepalensis .
Specific leaf area and total chlorophyll concentration strongly
influenced most of the physiological traits, being one of the vital
regulators. Our results contributed to a more dynamic understanding of
the relationship between leaf physiological traits and their interaction
with leaf morphological and chemical traits. We also anticipated
ecophysiological response to varying age gradients through the A.
nepalensis forest stands. Additional studies are needed to understand
the interaction with soil physicochemical properties and soil moisture
concentration along the age gradient in the A. nepalensis forest
stand in central Himalaya.