Introduction
Environment plays a key role in shaping community assembly (Kraft et
al., 2015). Across an environmental gradient, species performance
relates to their functional traits that mediate the ability to withstand
different environmental conditions (Adler et al., 2013; Paine et al.,
2011; Poorter et al., 2008; Sonnier et al., 2010). Trait-environment
linkages influence compositional change across space and also shape
community responses to anthropogenic change (Bernard-Verdier et al.,
2012; Cornwell and Ackerly, 2009; Harrison and LaForgia, 2019;
Méndez-Toribio et al., 2020; Poorter et al., 2019; Venail et al., 2015).
In human-modified landscapes, remnant habitat exist as fragments
embedded within a matrix of other land-uses (Barlow et al., 2007; Melo
et al., 2013). Fragments experience edge effects that alter the
microclimate (De Frenne et al., 2019, 2015; Zellweger et al., 2020) and
forest-climate feedbacks that can enhance local climatic stress
(Arroyo-Rodríguez et al., 2017; Laurance, 2004). Both processes can
decouple the how different traits mediate community composition across
macro-scale environmental gradients in fragments compared to contiguous
forest (Fernandes Neto et al., 2019; Krishnadas et al., 2018b;
Lebrija-Trejos et al., 2010; Poorter et al., 2019; Zirbel and Brudvig,
2020).
Trait-environment linkages can vary between fragments and contiguous
forest because of changes in the relative importance of different
stressors. For trees, consider wood density and specific leaf area
(SLA). Denser wood and lower SLA correlate with greater shade-tolerance
(Poorter et al., 2008; Wright et al., 2010). In contiguous forest, tree
species with denser wood or lower SLA increase in wetter sites where
shade limits species performance. However, greater light availability in
fragments could increase abundances of species with lighter wood and/or
higher SLA in wetter sites, which would decouple the macro-scale
linkages between traits and water availability. Alternatively, because
denser wood and lower SLA also correlate with dry-tolerance, they may be
favored where climatic water deficit increases across the contiguous
forest (Muscarella et al., 2016; O’Brien et al., 2017; Sterck et al.,
2006). Warmer, drier local conditions in fragments (Asbjornsen et al.,
2004) may favor dry-tolerant traits even in wetter sites. However, if
dry microclimates are more deleterious in drier climates, fragments may
show tighter the macro-scale linkage between water deficit and lower SLA
and/or higher wood density. Instead of individual traits, environmental
filters may select for multi-trait phenotypes (Umaña and Swenson, 2018),
or assembly may be a response to a composite suite of environmental
variables (Shen et al., 2019).
Environmental gradients also shape community assembly via constraints on
trait combinations, reflected in trait covariance (Dwyer and Laughlin,
2017). Environmental stress can curtail viable trait combinations or
select for specific phenotypes among locally co-occurring species,
resulting in stronger covariance at more stressful sites (Dwyer and
Laughlin, 2017; Umaña and Swenson, 2018). The sign of covariance also
matters to recognize the phenotypes being favored. Positive covariance
indicates selection for more extreme trait combinations (e.g., large
seeded and dense wooded or large seeded and tall) as may happen if
multiple stressors impose coordination across a suite of traits.
Negative covariance may imply opposing selective pressures on traits,
e.g., higher light availability supports smaller-seeded species but
concomitant effects of dry conditions select for species shorter stature
or thicker leaves (Muscarella et al., 2016; Tyree, 2003).
In fragments, limited arrival of larger-seeded species and warmer, drier
microenvironments can constrain viable combinations of seed size and
SLA, height or wood density. Trait covariance could therefore be tighter
in fragments compared to contiguous forests, especially where
macroclimatic stress also increases. Alternatively, greater
heterogeneity of microenvironments within fragments could support more
phenotypes than contiguous forest, resulting in a weaker influence of
the macroenvironment on trait covariance. For instance, heterogeneous
light levels within fragments may allow a wider range of viable SLA or
wood density values for the same seed size, whereas more uniformly
shaded conditions in contiguous forests restrict trait combinations.
Finally, trait covariance could reflect the diversity of the trait space
shaped by environmental gradients or local biotic interactions. An
increase in the functional diversity of individual traits, i.e., wider
occupancy of available niche space, could increase the number of
possible phenotypes arising from trait combinations.
Here, I examined how trait-environment linkages and constraints on trait
combinations varied between fragments and contiguous forests across
macroenvironmental gradients in a human-modified wet tropical landscape
in peninsular India. Specifically, I asked:
Do fragmented and contiguous forests differ in trait-mediated species
distributions across macroenvironmental gradients?
In contiguous forest, I expected that traits associated with
shade-tolerance (denser wood and lower SLA) would increase in wetter
sites where there is strong competition for light. Greater light
availability in fragments would favor smaller-seeded, lighter-wooded
species on average, but larger seeded and denser wooded species would
increase in fragments located in warmer, drier conditions. Drier climate
in fragments would favor shorter species (Méndez-Toribio et al., 2020).
Across contiguous forest however, uniformly shaded conditions may buffer
the effects of drier climates allowing similar height profiles in
relation to climatic dryness (Arroyo-Rodríguez et al., 2017; Davis et
al., 2019; De Frenne et al., 2019).
Do contiguous forests and fragments differ in whether the environment
selects for individual traits or multi-trait phenotypes (i.e., trait
syndromes) and does this influence composition along environmental
gradients?
If selection along environmental gradients operates via individual
traits, trait-environment correlations will be stronger for individual
traits than multi-trait phenotypes in both contiguous forest and
fragments (Butterfield and Suding, 2013; Krishnadas et al., 2018b).
Alternatively, successful phenotypes may be determined by a combination
of traits. Species with trait values associated with
resource-conservative strategies would be expected to increase in more
stressful conditions such as warmer, drier climates. If small-scale
abiotic conditions in fragments modulate or override the effects of
broad-scale environmental gradients, then landscape-level
trait-environment linkages seen in contiguous forests will change for
fragments.
Does the environment constrain trait combinations in fragments vs.
contiguous forest? Or, does trait covariance correlate with the
overall trait space occupied by constituent species?
In contiguous forest, I expected stronger trait covariance as sites
became drier and warmer. I expected fragments to show stronger
correlations between trait covariance and climate gradients as forest
loss would reduce the buffering of climatic stress and allow less leeway
for different phenotypes (Zellweger et al., 2020), i.e., stronger
selection for specific phenotypes leading to tighter constraints on
trait combinations. I also expected trait covariance to decrease with
higher species richness, functional diversity, or variance of individual
traits.