Introduction
Climate stress and competition
influence plant growth through different physiological pathways and vary
inversely in importance along gradients of productivity and climatic
harshness (e.g., Coomes and Allen 2007; Ettinger et al. 2011; Rollinson
et al. 2016). Adapting for the unique challenges posed by competition or
climate is thereby considered a fundamental tradeoff differentiating
primary plant life-history strategies (i.e., strong competitors vs.
stress tolerators; Grime 1977; Reich et al. 2003). In turn, the
stress-tradeoff hypothesis explains compositional patterns along
climatic gradients by arguing that the biogeographic range limits of
these primary strategies reflect the poor performance of i) stress
tolerant species under competitive suppression, and ii) competitive
species under climatic stress (Louthan et al. 2015). However, despite
broad acceptance of this foundational theory, observations confirming a
role for the stress-tradeoff hypothesis in dictating species range
limits – especially those of tree species – are surprisingly sparse
(see Anderegg and HilleRisLambers 2019 and references therein). This
shortage likely persists because additional ecological dimensions (e.g.,
population structure, community composition, natural disturbances) alter
the interplay between climate sensitivity and competition in forests
(Babst et al. 2018) and highlights outstanding complications in the
reliable scaling of tree-level mechanisms to explain forest biogeography
(Mori 2019).
Climate and competition impart unique spatiotemporal patterns on tree
growth, which could aid in disentangling their respective influences
throughout biogeographic regions. Climatic stress is relatively coherent
across larger regions and varies interannually. This spatiotemporal
character of climatic variability synchronizes the growth of trees
co-occurring within macroclimatic envelopes—growth series from trees
sharing common climatic drivers will demonstrate heightened correlation
(Fig. 1; e.g., Shestakova et al. 2016; Schurman et al. 2019).
Mechanistically, climatic stress acts as a direct constraint on wood
synthesis by affecting the rate and duration of wood cell division,
expansion and cell wall formation (Wardlaw 1990; Körner 2003). Growth
under such constraints is commonly referred to as sink limited ,
(a term deriving from the role of woody tissues as a carbon sink).
In contrast to climate, competition is a localized constraint that
operates within tree communities, and is considerably more stable and
predictable over time (Fritts 1976; McDowell et al. 2020). Trees
occurring in close proximity will compete for finite resources and
larger trees will garner disproportionately more resources (Weiner
1990). The growth rates of trees with differential access to resources
will diverge (unlike climate, which synchronizes tree growth) (Clark
2011). If resource acquisition fails to sustain the rates of wood
synthesis permissible under ambient climatic conditions, then growth
will be resource limited and likely decouple from climatic
variability. For example, understory trees are generally less
climate-sensitive than overstory cohorts (e.g., Teets et al. 2018;
Saulnier et al. 2020), and less responsive to the alleviation of climate
stress (e.g., boreal warming, Luo et al. 2020). The size-dependent
impact of competition further suggests that the growth synchrony between
overstory and understory tree cohorts will decline towards more benign
growing conditions, as the growth of understory trees increasingly
decouples from climate (see Fig 1.g-i).
The functional differences between deciduous angiosperms and evergreen
conifers, especially regarding wood anatomy, provide a particularly
clear example of tradeoffs between competitive and stress-tolerant life
histories (e.g., Bond 1989; Brodrib et al. 2012). Whereas conifer xylem
consists of a relatively homogeneous matrix of tracheids (slim
water-conducting cells several millimeters in length), angiosperm wood
contains more recently evolved vessels (wider water-conducting conduits
potentially reaching meters in length). Vessels are substantially more
efficient in water transport, allowing angiosperms to support high leaf
area canopies (Tyree & Ewers 1991) capable of intercepting large
proportions of incoming light and bolstering the competitive proclivity
of late-successional angiosperms (Bazzaz 1979). However, large diameter
vessels are vulnerable to climatic hazards, such as frost- or
drought-induced embolism, to which the more compartmentalized matrix of
conifer wood is substantially more resistant (Sperry, Hacke, &
Pitterman 2006). This phylogenetic contrast highlights the appeal of the
stress-tradeoff hypothesis as a concise explanation for emergent
biogeographic patterns, where conifers persist under cold or arid
climates and angiosperms dominate under benign temperate conditions
(Bond 1989). However, despite this strong theoretical foundation, the
role of angiosperm competition as a constraint on conifer distributions
is not well grounded in actual field observations (Augusto et al. 2014).
Dendroecological studies have emerged as a robust tool to understand how
competition (e.g., Teets et al. 2018; Saulnier et al. 2020), climate
gradients (e.g., Primicia et al. 2015; Schurman et al. 2019; del Rio et
al. 2020) and their interactions (Rollinson et al. 2016; Buechling et
al. 2017) alter tree growth. Yet, the higher density of tree-ring
sampling that is required for such spatially explicit studies has rarely
been achieved at sufficiently large scales to empirically assess
range-limit mechanisms (but see Anderegg and HilleResLambers 2019). In
this respect, the present study constitutes a more than ten-fold
increase in the number of tree-ring samples collected compared to
conventional dendroecological studies of growth and range dynamics. This
allowed us to quantify how the interplay between competition and climate
sensitivity changes with variation in local forest structure and
community composition, and along macroclimatic gradients (i.e., altitude
and latitude) throughout a biogeographic region (Fig. 2). After
confirming tree-growth synchrony as a reliable indicator of sink vs.
resource-limited growth, we examined spatial patterns in synchrony and
absolute growth rates to determine the importance of climate and
interspecific competition as range constraints for a shade-tolerant
angiosperm (Fagus sylvatica ) and a frost-hardy conifer
(Picea abies ) throughout a mid-latitude montane ecoregion (the
Carpathian Mountains). We further demonstrate how competition and
climate sensitivity interact with structural, compositional and
historical factors likely to obfuscate climate and competition as
range-limit mechanisms and scale to produce regional patterns in climate
sensitivity and forest productivity.