(1) Understanding tree seedling responses to water, nutrient, and light availability is crucial to precisely predict potential shifts in composition and structure of forest communities under future climatic conditions. (2) We exposed seedlings of widespread Central European tree species with contrasting leaf habit, deciduous broadleaves (DB) and evergreen conifers (EC), to factorial combinations of manipulated precipitation (100% and 50% of ambient), shade (40% and 60% of full sunlight), and nutrient availability (low and high NPK), and measured specific leaf area, C/N ratio, soluble sugars, starch and non-structural carbohydrate concentration, and δ¹³C of the leaves. (3) We found contrasting effects of water and nutrient availability on foliar traits of the two species groups: EC exhibited higher tolerance to low resource availability but also less plasticity in foliar traits, which is congruent with a “slow” resource strategy. In contrast, foliage of DB reacted particularly to altered nutrient availability, corresponding to a “fast” resource strategy with high foliar plasticity and rapid adjustments to resource fluctuations, commonly adopted by species with high growth rates. (4) We conclude that DB will respond to environmental change with foliar acclimation, while EC will either tolerate, to some extent, or shift their distribution range in response to environmental change.

Summary: In most tree species, roots serve as major carbon (C) sinks, where C is depleted first when C assimilation is limited. Recent methodological advancements in sugar infusion allow for a better understanding of physiological processes alleviating root C limitation. We conducted a glasshouse experiment with maple ( Acer pseudoplatanus L.) and pine ( Pinus sylvestris L.) saplings that underwent defoliation followed by either slow, fast, or no ¹³ C‐labeled glucose infusion. We measured photosynthetic parameters, nonstructural carbohydrate (NSC) concentrations, and δ ¹³ C in cellulose of leaves, twigs, and fine roots, as well as the isotopic composition of dark‐respired CO ₂ . Sugar infusion induced photosynthetic downregulation and leaf senescence in maple but not in pine. Leaf photosynthesis was negatively correlated with leaf NSC concentration in maple. These responses exacerbated root C limitation in maple. Conversely, pine maintained stable photosynthetic rates and needle NSC concentrations across treatments, showing the potential of sugar infusion to mitigate root C limitation. Our study suggests that exogenous sugar supply reduces the root C availability when it impairs a plant's photosynthetic performance. Species‐specific differences influence infused sugar transport and overall source–sink responses. Alleviating C limitation in roots via exogenous sugar addition is feasible only if photosynthesis is not impeded.

Plants’ non structural carbohydrates (NSCs) serve as their capital for growth, reproduction, defense and survival. To increase the NSC availability of carbon-limited trees, a recent study revealed the possibility of adding exogenous soluble sugars to carbon-starved trees. This provides an opportunity to investigate carbon allocation between source and sink, as well as the growth and physiological responses to external sugars. Using this method, we infused 13C-labeled glucose solution into the stem xylem of sycamore maple (Acer pseudoplatanus L.) trees (Experiment 1) and immersed branch cuttings of various tree species in a 13C-labeled glucose solution (Experiment 2). Our aim was to study whether infused sugars contribute to structural growth and how they influence photosynthesis. Specifically, we focused on whether trees can transport and utilize exogenous sugars for growth, and if sugar addition might trigger negative feedback mechanisms on carbon gain. We then traced the 13C label in bulk tissue and cellulose, and measured tissue NSC concentrations and leaf photosynthesis. Glucose addition consistently increased leaf NSC concentrations (Experiments 1 and 2), and exogenous sugars added were transported and incorporated into biomass formation in Experiment 1. However, excessive sugar addition triggered a negative feedback response, leading to leaf senescence (Experiments 1 and 2) and reduced photosynthesis (Experiment 2). Our findings validate the recently developed sugar addition method but emphasize the importance of carefully controlling the amount and rate of sugar addition to avoid negative feedback responses. This study will contribute to carbon physiological research, particularly in understanding carbon balance and source –sink relationships at the whole-plant level.