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.