## 4 Discussion  
We have shown that the timing of aspen and oak tissue NSC accrual and depletion was often coupled with the periodicity of NPP (forest growth) and GPP (gross C assimilation), phenologies that are commonly constrained by temperature in temperate forests [19-23]. Temporal shifts in tissue NSC have been broadly shown to correspond with tree phenological changes and with climatic events [8-17]. We are among the first to demonstrate the potential coupling of temperature regulated C cycling processes with temporal changes in tissue NSC concentrations [1-3], a result that suggests future warming may disrupt current cycles of NSC accumulation and depletion in our forest. Seasonal cycles of tissue NSC accumulation and depletion at our site are consistent with those reported for other temperate, deciduous forests. We observed a general decline in NSC concentrations of major storage tissues (coarse roots and stems) from dormancy until GPP initiation and, subsequently, an accrual of tissue NSC after leaf-out and during the latter half of summer when NPPhad ceased and GPP was sustained. Seasonal trends that were especially pronounced in oak were more attenuated in aspen. Our findings reinforce prior results that tissue NSC of temperate deciduous trees accumulates when the supply of C assimilation exceeds current demands for NPP and, conversely, NSC may become depleted when current GPP is not sufficient to meet NPP demands [2,4,6,7]. Thermal regulation of NPP and GPP at our site suggests that seasonal NSC dynamics are partly constrained by temperature controls on the phenologies of C assimilation and allocation to growth. In our forest there was strong correspondence between cumulative heating degree days and the initiation of NPP and GPP, and between cumulative chilling degree days and the cessation of GPP in the autumn. Indirect thermal regulation of tissue NSC dynamics at our site may be caused by changes in the temporal relationship between the supply of recently assimilated C, and C demand for growth as temperatures rise. Numerous studies support a strong relationship between air temperatures and the duration of the growing season and canopy C assimilation or gross primary production [19-23], but none have related temperature to shifts in tissue NSC allocation. Our results provide evidence that future warming could increase NSC accumulation in aspen and oak tissues. We predict an approximate 1% increase in tissue NSC concentrations for every 1 \({}^{\circ}\)C rise in mean annual temperature; the cause is the lengthening of the period between growth and C assimilation termination, a stage in which photosynthetic allocation is redirected from growth to storage in NSC pools. If growth terminates during mid-summer independent of temperature as we observed, then extending C assimilation later into autumn might augment NSC accumulation by increasing the period in which assimilated C is allocated to storage NSC rather than to growth. We did not adjust our predictions of future late season NSC accumulation for a concurrent rise in plant respiration that might occur as temperatures increase [41-43]; however, we suggest that lengthening of the C assimilation period and, consequently, higher gross NSC production under warmer conditions may provide an important mechanism for offsetting rising metabolic costs, which have been  
Figure 5: Projected increases in tissue NSC concentrations for oak and aspen bole and roots, +1 to 7 \({}^{\circ}\)C mean annual temperature. Grey shading and dotted lines are 95% C.I. for bole and root NSC concentration projections, respectively. demonstrated to increase with temperature [5, 44, 45]. Interestingly, our findings suggest that rising air temperature may compound forecasted increases in tissue NSC caused by rising atmospheric CO\({}_{2}\)[32].