## 1 Introduction  
Plant non-structural carbohydrates (NSC), comprised of starch and sugars, are primary intermediate products of carbon assimilation (C) that can be stored and used to meet future demands for growth and metabolism [1-3]. Stored NSC is fundamental to sustaining plant growth and metabolism during periods of high physiological stress and through dormancy, when photosynthetic C supply is low [2-7]. Because of a shifting balance between plant metabolic C demands and photosynthetic C supply, NSC concentrations in plant tissues can change on hourly to seasonal timescales [8-17].  
Seasonal cycles of plant tissue NSC accumulation and depletion are strongly tied to the timing and magnitude of C assimilation and growth [18], indicators of C source and sink demand, respectively. In temperate deciduous trees, tissue NSC concentrations may decline during winter dormancy and early leaf expansion, when current C assimilation is insufficient to meet metabolic and early growth demands [1,2,12,16]. Non-structural carbohydrate concentrations in tissues may be replenished once leaf production has peaked and structural growth demands for current C assimilate decline [2,4-7]. The timing and magnitude of tissue NSC accrual and depletion is thus a function of two coupled, but sometimes temporally offset C cycling processes of growth and C assimilation.  
Climate change may exert a substantial, indirect influence on seasonal cycles of tissue NSC by modifying the temporal relationship between growth and C assimilation. In particular, thermal regulation of growing season length and duration of the photosynthetic period may have important consequences for the periodicity of seasonal tissue NSC cycles in temperate deciduous forests. Numerous studies support a strong relationship between temperature and growing season duration, with warmer air or soil temperatures accelerating the initiation of spring net primary production (NPP) and delaying autumn senescence [19-23]. Warmer spring temperatures generally prompt an earlier onset of gross primary production (GPP), but temperature effects on the termination of GPP are less clear. If the temporal relationship between NPP and GPP is altered as temperatures rise, then shifts in C source-sink relationships may, in turn, affect tissue NSC concentrations, an outcome that could have broad ecological consequences for the resilience of forest growth following disturbance or climatic stress [3,24-31]. Forecasts of future plant tissue NSC have focused on the effects of rising atmospheric CO\({}_{2}\)[32]. Understanding how climate change will alter tissue NSC concentrations additionally requires investigation into how temperature constrains the timing of key regulatory ecosystem C supply and demand processes of C assimilation (_i.e._, GPP) and growth (_i.e._, NPP).  
We examined how seasonal fluctuations in tissue NSC of _Populus grandidentata_ and _Quercus rubra_ relate to stand-scale C cycling processes of NPP and GPP and their climatic drivers, and we consider how future temperature rise may affect seasonal NSC cycles of depletion and accrual in a north temperate mixed deciduous forest of Michigan, USA. We focus our analysis on the phenological and environmental regulation of C cycling processes that constrain tissue NSC concentrations, and which may inform predictions of future forest C allocation to NSC. Using standard assays of tissue NSC concentration together with stand-scale assessments of GPP and NPP [18], we demonstrate how these coupled ecosystem C cycling processes interact to regulate highly dynamic, but generally predictable seasonal fluctuations in tissue NSC concentrations, and we consider how these labile C pools are likely to change as temperature increases.