3.3 Effect of girdling on whole-tree H2O and CO2 fluxes
Girdling reduced whole-tree fluxes of H2O and CO2 (Fig. 3) and increased dark respiration of woody aboveground biomass (Table 2). Prior to girdling, mean hourly CO2 fluxes showed a moderately strong positive and relationship with PAR (R2 = 0.68) with a typical curvilinear trajectory that indicates saturation of CO2uptake with increasing PAR as consequence of limited photosynthetic capacity (Fig. 3). Average daytime CO2 fluxes were 0.76 g m-2 h-1 on a leaf-area basis. After girdling, this saturating relationship remained, albeit slightly weaker (R2 = 0.56), and average daytime CO2 fluxes noticeably declined (p = 0.009) to 0.59 g m-2 h-1. Under the same light intensity of 1000 µmol m-2 s-1 the CO2 flux dropped from 1.15 g m-2h-1 pre-girdling to 0.73 g m-2h-1 post-girdling (Fig. 3a). The same effect was observed for H2O fluxes, where girdling significantly (p <0.001) reduced whole-tree transpiration at similar PAR intensities (Fig. 3b).
There was no effect of Tair (and VPD) on whole-tree C uptake (Fig. 3c, e). In contrast, transpiration was significantly (p <0.001) and positively dependent on Tair (and VPD). However, despite warmer mean Tair and higher VPD in the days following girdling, whole-tree transpiration declined in response to girdling (p <0.001) (Fig. 3d, f). Average daily whole-tree transpiration declined from 92.4 ml m-2h-1 pre-girdling to 68.7 ml m-2h-1 post-girdling. At VPD of 1 kPa, average H2O fluxes were around 120 ml m-2h-1 prior to girdling and 70 ml m-2h-1 after girdling.
Dark respiration of leaves was not affected by girdling and remained around 3.40 nmol CO2 g-1s-1 (Table 2). In contrast, dark respiration of branch tissues, following girdling, increased by 42% (p <0.001) from 1.33 nmol CO2 g-1s-1 (± 0.25) to 1.89 nmol CO2g-1 s-1 (± 0.29). Girdling also increased dark respiration of the whole aboveground biomass by 21%, but very large variation among trees (2.20-3.69 nmol CO2g-1 s-1) meant that this increase was statistically not significant p = 0.10).