Competition and climate dictate range limits of Picea and Fagus, respectively 
The processes regulating tree performance at their respective upper and lower range limits differed for F . sylvatica and P .abies . In the Carpathians, the growth rate of F .sylvatica declines as growth synchrony increases towards its upper elevation limit, which is highly indicative of a species growing at the limit of its climatic tolerance (Jump et al. 2006). Aspects of xylem anatomy related to hydraulic conductivity are closely associated with the upper altitudinal limits of most European tree species, including F . sylvatica , with large vessels amplifying the risks of embolism formation under freezing and thawing cycles (Charrier et al. 2013). The capacity of F . sylvatica to repair cavitation is temperature sensitive (Cochard et al. 2001), similarly reflected in the heightened sensitivity of F . sylvatica to winter temperatures in the Carpathians (Supporting Info 1) and towardsF . sylvatica ’s northern distribution (Weigel et al. 2018), which almost certainly prevents this species from reaching dominant/mature size classes at high altitudes in the Carpathians.
Unlike F . sylvatica , the maximum growth rates of P .abies increased as its abundance declined towards its lower altitudinal limit; this misalignment between abundance and growth optimum strongly suggests that P . abies is outcompeted at sites that offer more favorable growing conditions (Canham et al. 2004). Species composition had fundamentally different impacts on the growth patterns of F . sylvatica and P . abies within the overlapping segments of their altitudinal distributions, especially for understory trees. Understory F . sylvatica grew faster and in greater synchrony with climate under interspecific competition, implying an alleviation of competitive suppression. The opposite was true for P . abies , which exhibited lower growth rates and growth synchrony under mixed-species canopies. These patterns reinforce the notion of shade tolerant angiosperms as species that rise in abundance through monopolization of resources, namely light (Bazzaz 1979), and the competitive advantages afforded to angiosperms by their improved conductivity, higher leaf area, and faster growth (Bond et al. 1987).