Introduction
Water availability drives the distribution and dynamics of plant
species, even within relatively wet biomes (Engelbrecht et al.2007, Comita & Engelbrecht 2009, Esquivel-Muelbert et al. 2019,
Krishnadas et al. 2021). Quantifying plant response to water
stress is imperative given that global environmental change will alter
patterns of precipitation and seasonal drought. For closed canopy
forests, much attention has focused on the response of mature trees to
drought (Suresh et al. 2010, Anderegg et al. 2015, Rowlandet al. 2015, Hartmann et al. 2018), but less is known of
seedling responses. Seedlings can be highly vulnerable to drought
because their shallow roots do not allow access to deeper water layers
during dry periods. Moreover, the light-limited forest understory may
impose carbon starvation during drought (Slot & Poorter 2007, Amissahet al. 2015) such that even short dry spells can result in
substantial seedling mortality in tropical wet forests (Engelbrechtet al. 2007). Seedling responses to water availability can
therefore regulate regeneration and persistence of species in a changing
climate and shape community structure of later life-stages (Comita &
Engelbrecht 2009, 2014).
Interspecific variation in drought response can be mediated by species’
traits—anatomical, morphological and physiological characteristics
associated with differences in strategies of resource use (Slot &
Poorter 2007, Poorter & Markesteijn 2008). For example, wood density
and leaf mass per area can correlate positively with drought tolerance
of mature trees (Muscarella et al. 2016, O’Brien et al.2017), but the extent to which these relationships hold true for
seedlings is not known. Notably, the role of root traits in mediating
species’ drought responses remains poorly explored, despite increasing
calls to include below ground traits in assessing plant functional
strategies and whole-plant response to stressors (Freschet et al.2021, Laughlin et al. 2021, Weemstra et al. 2023). In
addition, understanding variation in anatomical features of stomata and
xylem, which help regulate water balance and response to dry conditions,
may serve as stronger proxies for hydraulic function than commonly used
traits such as specific leaf area (Bartlett et al. 2016).
Species of evergreen habit may respond to seasonal droughts either by
avoiding or tolerating desiccation, with functionally distinct
strategies arising from different trait combinations. Desiccation
avoiders try to maintain water status through a drought. Due to higher
metabolic demands, these species may be resource-acquisitive with their
higher rates of photosynthesis and growth selecting for thinner, cheaper
leaves, large stomata and xylem to move adequate water, and deep roots
to maintain access to water (Chitra‐Tarak et al. 2021, Reich
2014). Alternatively, species may tolerate desiccation by decreasing
photosynthesis to reduce water loss. This strategy impedes growth but
reduces the chance of embolism or hydraulic failure (Anderegg et
al. 2015, 2016, Adams et al. 2017, Skelton et al. 2017).
To prevent carbon starvation, desiccation tolerant species should be
resource-conservative, reflected in traits such as low specific leaf
area (SLA), high leaf dry matter content (LDMC), smaller xylem and
stomata, and relatively lower investment in below-ground tissue to
forage for water compared to desiccation avoiders. Alternatively, traits
may not be coordinated and different trait combinations may offer
equivalent responses to drought.
Easily measurable so-called ‘soft’ traits may help categorize species as
avoiding or tolerating desiccation based on an integrated whole-plant
evaluation of photosynthesis, growth and survival, and offer a helpful
adjunct to hydraulic traits (Bartlett et al. 2012, Sun et
al. 2020). With that in mind, we assessed the relationship between
traits and species’ performance in dry vs. well-watered conditions for
seedlings of 16 tree species from a wet tropical forest region in a
global biodiversity hotspot in southern India, the Western Ghats. Across
the region, most seedlings establish during the monsoon, but then have
to withstand dry periods varying from 3-9 months to persist. The species
were chosen to represent a gradient in their affinity to seasonal
climatic water deficit during the dry season (henceforth seasonality)
based on a previous analysis of tree species distributions in the
Western Ghats (Krishnadas et al. 2021). We asked:
Do species from less vs. more seasonal forests vary in their drought
response?
We expected that species associated with greater seasonality would
tolerate drought better than species from less seasonal forests, i.e.,
show smaller decline in survival and/or growth in drought vs.
well-watered conditions.
How well do individual traits explain interspecific variation in
drought response?
We expected that species with higher SLA, lower LDMC, higher root mass
fraction (RMF) or deeper roots, larger stomata and wider xylem would
maintain better function (photosynthesis and growth) early in drought,
but survive less than species with lower SLA, higher LDMC, and lower RMF
or shallow roots, smaller stomata, and narrower vessels.
Do distinct trait combinations characterize species that avoid or
tolerate desiccation?
We expected that traits associated with conservative water-use or safe
hydraulic strategy, e.g., thicker leaves, denser stem, smaller stomata
and xylem, would correspond with responses characteristic of desiccation
tolerance – decreased photosynthesis and growth. Species with thinner
leaves and larger stomata or xylem would avoid desiccation by
functioning at similar levels in drought compared to watered conditions.
We also expected that species with higher SLA, lower LDMC, deeper roots
and higher RMF would avoid desiccation. Species with lower SLA, higher
LDMC, shallow roots and lower RMF were expected to tolerate desiccation.