4.1. Water Fluxes
 The observed daily E and Ec values declined in response to cloudy periods, and also showed a gradual decline over the course of the growing season in response to shorter day-length and reduced incoming solar radiation. On a daily basis, Ec consistently lagged behind E. This lag continued until the end of the day when Ec often exceeded E, indicating that the tree tissues were replenished after transpiration ceased. This has been readily observed in forest transpiration studies and implies that the water storage capacity of living tissues is also important in this forest [Unsworth et al., 2004]. The highest fluxes (both E and Ec) were recorded on hot summer days immediately following precipitation events, and the lowest rates were recorded on cool, cloudy, wet days in the spring. The maximum daily E rate of 5.8 mm d−1 at the Turkey Point white pine forest site was somewhat larger than many other mature temperate conifer forests located in wet environments (4.4 mm d−1 [Barbour et al., 2005]; 4.0 mm d−1 [Grelle et al., 1997]; 3.7 mm d−1 [Humphreys et al., 2003] and 3.6 mm d−1 [Unsworth et al., 2004]). Two extremely high daily water losses were recorded during the study. Without these two values, the maximum observed E rates would have been approximately 4.0 mm d−1. Both extremes (5.7 mm on 15 July and 5.8 mm on 4 August) corresponded with high mean daily VPDs (greater than 1.3 kPa) and occurred less than two days after rain storms. It is likely that these extreme estimates were the result of a large evaporation contribution from the wet canopy, forest floor, and soil.
 The estimated S for the forest was similar to other high-LAI conifer forests [Link et al., 2004]. The S of a Douglas-fir stand with an LAI of 8.6 was 2.7 mm [Link et al., 2004] and was 2.4 mm for another Douglas-fir stand with an LAI of 9–13 [Klaassen et al., 1998]; both similar to the S of 2.4 mm and LAI of 8.0 found at this site. Growing season EI accounted for 18% of PG, well within the range of observed net interception losses in temperate conifer forests [Link et al., 2004]. Hormann et al.  published values of interception loss from a number of interception studies, showing temperate conifer forests having interception losses ranging from 9–48% of gross precipitation, depending on the size of the precipitation event. Despite large fluctuations in monthly PG (62 mm to 221 mm), the relatively large contribution of monthly EI to E and narrow range of observed monthly EI (21 mm to 28 mm) illustrated the influence of canopy storage capacity on forest water loss and interception under varying storm size. As storm size increases, proportional interception loss decreases [Gash and Morton, 1978; Link et al., 2004], resulting in EI accounting for 33% of PG in June, the month with the least amount of rain, and EI accounting 13% of PG in September, the month with the greatest amount of rain (see Table 3).
 The water status of a plant is related to both soil water supply and atmospheric demand. Interception is an important control on the amount of available soil water. Interestingly, during the drought of late June and early July, 50 mm of rain fell. However, due to the low intensity of the rainfall events (never exceeding 7 mm hh−1), a greater proportion of rain was intercepted. It is likely that the relatively high water holding capacity of the canopy (2.4 mm) is exacerbating periodic droughts in the forest growing on well drained sandy soils. The high LAI and S of this forest make the water balance at the site particularly sensitive to reductions in precipitation characteristics, such as frequency, duration or intensity, because the resulting reduction in net precipitation would be amplified by the canopy storage capacity.
 On a daily basis, the remainder of evaporation (after subtracting Ec from E) cannot always be accounted for by modeled EI, due to the different temporal frameworks used to make the estimates. Our estimates of EI on the event scale must be compared to fluxes on a daily scale. The reconciliation of these two scales is not possible because rain events and the evaporation of water afterward are not always confined to a single calendar day. For this reason EI usually exceeds E on wet days, but is under-estimated on the days immediately following rainfall events, when E usually greatly exceeds Ec. This occurs because modeled estimates of EI are given on a per rain event basis rather than on a daily basis, with reported EI representing the sum of the evaporation of water intercepted during and after the event, but can only be reported as the sum for the day the rain event began. It should be noted that for any significant rain event, a large proportion of the reported EI would have likely evaporated within hours following rainfall.
 The range of Ec rates in our forest was similar to those reported from other studies conducted in mature wet temperate conifer forests. Unsworth et al.  reported mean summer Ec rates of 1.5 mm d−1 and 1.4 mm d−1 (June and July). Barbour et al.  reported a range of Ec rates of between 0 and 1.8 mm d−1 in their study on a temperate conifer rainforest and Irvine et al.  report the same range for a temperate Scots pine (Pinus sylvestris L.). Apart from the responses to storm events, there was a distinct lack of seasonality and variability in Ec rates at our forest site compared to the variability exhibited in other studies. This was also evident in the daily ensemble Ec averages for each month of the study, which showed little change until the beginning of the dormant season. However, small but noticeable declines of Ec rates in late June and early July did show a broad response to reductions in soil water content. This relative stability was an indication of the importance of the physiological and climatic limitations on canopy transpiration. It also lends support to the idea that for a single species the range of variation in transpiration is likely to be dampened by an overall coupling with climate and a strong negative feedback between stomatal conductance and VPD [Roberts, 1983].
 The proportion of Ec to E was quite low (47%) in this forest when compared to other conifer forests. Oren et al.  reported that Ec accounted for 69% of E in a temperate Loblolly pine (Pinus taeda) plantation, Unsworth et al.  reported 65% in a temperate Douglas fir–Western hemlock (Pseudotsuga menziesii–Tsuga heterophylla) old growth forest, and Grelle et al.  reported 75% in a boreal mixed conifer (Picea abies–Pinus sylvestris L.) forest. Two studies however, did find similar partitioning in conifer forests: one in a Ponderosa pine (Pinus ponderosa) plantation in California, USA, found that 53% of E came from Ec [Kurpius et al., 2003]; the other in a temperate old growth coniferous (Dacrydium cupressinum) rainforest, in New Zealand, reported that 39% of E was derived from Ec over the growing season and 51% on dry days [Barbour et al., 2005]. Both of these forests had much smaller LAIs than our forest, 2.2 and 2.9 respectively.
 The low Ec proportion and high LAI of our forest imply that we under-estimated stand Ec, because the unmeasured components of soil evaporation (Es) and understorey transpiration (Eu) are unlikely to entirely account for the 49% of forest water lost on dry days from this site. The likely reason for the possible underestimation is the difference between the spatial scales involved in scaling-up of sapflow measurements from the 20 × 20 m study plot to the stand level for estimating canopy transpiration. The eddy covariance technique measures water fluxes over a large variable “footprint” source area, but due to practical and economic restrictions sapflow had to be measured at a much smaller fixed local scale and thus can only estimate the Ec of the dominant species. Our initial assumption that the sapflow from the dominant white pine would represent the response of all species in the forest adequately, and therefore suffice to represent total canopy transpiration, was inadequate. The confounding factor was the unmeasured contribution from the young emergent deciduous trees (or gap species, which make up at least 8% of the species composition) and patches of heavy understorey growth distributed unevenly throughout the forest. Though we have no way of estimating their individual contributions, all three sources (gap species transpiration, understorey transpiration and soil evaporation) do contribute and can plausibly account for the remaining 49% of forest water lost on dry days from this site. Further support for this reasoning may be found in the seasonality of the divergence between E and Ec estimates and its relationship to the approximated deciduous phenology. The difference between the two estimates was relatively small during the earliest part of the growing season (Ec was 73% of E from 1 May to 26 May). This difference grew in late May, corresponding with the timing of gap deciduous species full leaf date, and then shrank again in October, corresponding with the onset of senescence. Hogg et al.  similarly reported that eddy covariance measurements showed a rapid increase in water flux from a deciduous aspen canopy in late May, due to rapid leaf expansion. This reasoning is also supported by the good agreement of our Ec rates with the typical transpiration rates from wet temperate conifer forests dominated by a single species reported by previous studies [Barbour et al., 2005; Unsworth et al., 2004; Irvine et al., 1998]. It is likely that the unmeasured gap species at our site contributed significantly to the total water flux measured at the tower from June to September, though presently we have no way of estimating their contribution due, in part, to their uneven spatial distribution. We assumed that errors associated with the eddy covariance evaporative flux measurements did not contribute significantly to the underestimation. Another possible reason for the seemingly low transpiration proportion of this forest maybe shoot morphology. Chen et al.  observed that both within-shoot and beyond-shoot clumping was particularly high in this forest. The reported needle-to-shoot area ratio of 1.91 is considerably larger than the needle-to-shoot area ratio values reported for several conifer forests across Canada [Chen et al., 2006]. Beyond-shoot clumping value (0.98) for this site was also high when compared with the other sites investigated by Chen et al. . Therefore, despite high LAI (8.0), the understorey and soil of this forest may have been exposed to higher levels of radiation compared to other forest ecosystems with high LAI, resulting in relatively higher evaporative losses. However, this hypothesis needs to be confirmed and should be considered in future studies at this site.
 These findings highlight the difficulties in adequately measuring canopy transpiration in a mature planted conifer forest, where the natural succession toward a mixed-wood forest has begun. The results also provide strong support for the simultaneous use of sapflow and eddy covariance measurements to monitor forest water fluxes. Since the errors of the two techniques are different and generally independent, their simultaneous measurements helped to identify possible problems and methodological weaknesses [Hogg et al., 1997]. Despite these limitations, we were still able to account for the majority (81%) of the total growing season evapotranspiration from the forest by estimating the two largest components (transpiration from the dominant white pine and interception evaporation).
 During the driest periods in which θ0−25cm would approach the seasonal minimum and the water table depth was presumably at its deepest, the shallow soil layers began to gain more noticeable amounts of water at night without any apparent atmospheric input. A possible reason is the presence of a strong water potential gradient between non-adjacent soil layers during these periods, leading to the passive nocturnal redistribution of soil from deeper depths to the root-zone, sometimes referred to as Hydraulic Redistribution [Brooks et al., 2002]. This recharge may explain why soil water contents were maintained above those associated with the approximate wilting point through dry periods at the site (see Figure 2). We hypothesize that night-time increases during extremely dry conditions, which were large enough to change net water use (calculated as the difference between the maximum soil water storage (mm) of a single day and the maximum soil water storage of the following day), were likely due to hydraulic redistribution. Hydraulic redistribution has been shown to occur in a number of different soil types, climates and species [Brooks et al., 2002; Burgess et al., 1998; Caldwell et al., 1998; Dawson, 1993; Oliveira et al., 2005; Unsworth et al., 2004]. Numerous possible benefits have been proposed as a consequence of hydraulic redistribution, including; providing water to shallow rooted seedlings and understorey plants, enhancing mineral nutrient availability, enhancing microbial processes and heightening the acquisition of nutrients by roots, by keeping the fine root zone hydrated [Caldwell et al., 1998].
4.2. Controls and Limitations on Canopy Transpiration
 Shifts in the timing and magnitude of Ec rates, caused by increased VPDin, masked an important relationship between Ec rate and θ0−25cm. Several studies have indicated that the pattern of the limitation responses of Ec to high VPD and to soil water deficit appeared similar and were often concurrent, sometimes making interpretations difficult [Kurpius et al., 2003; Oren and Pataki, 2001]. Our results suggest that VPDin could be used as an analytical tool to investigate the combined effects of soil water content and atmospheric demand on transpiration.
 It has been postulated that Ec rates are linked to VPD through stomata, which work to maintain leaf-needle water potential above a critical minimum value, thereby, limiting maximum Ec rates [Hogg and Hurdle, 1997]. Our results seem to support this hypothesis, though we did not test it using a direct measurement of leaf water potential. Since our Ec rates reflect water uptake into the stem and not actual transpiration rates at the needles, it is likely that leaf water potentials are reaching critical minimums earlier in the day, and at lower water uptake rates, when early morning VPD is high. This is because when VPD is high at the beginning of the diurnal transpiration cycle, the initial rates of water uptake are inherently lower regardless of VPD and can be exceeded by lower rates of leaf water loss than usual. Thus the critical minimum leaf water potentials that signal a stomatal response (and hence the daily maximum Ec rates) are met earlier in the day. There is a growing body of evidence supporting the notion of isohydric regulation (the homeostasis of minimum leaf water potential) of plant water status [Bucci et al., 2005; Buckley, 2005; Brodribb and Holbrook, 2006; O'Grady et al., 2008]. A recent study by O'Grady et al.  found that as pre-dawn leaf water potentials in a Tasmanian Eucalyptus plantation declined, in response to soil desiccation and increasing VPD, the difference between pre-dawn leaf water potentials and midday leaf water potentials declined, in turn causing declines in both transpiration and canopy conductance. Their observations lead to a conclusion that there was a strong case for the existence of a critical minimum leaf water potential as soil water deficits increase. Similarly, Bucci et al. found minimum leaf water potentials in a Brazilian savanna (Cerrado) were isohydric, but that pre-dawn leaf water potentials decreased as soils dried.
 Though the maximum Ec rates observed were shown to decrease and occur earlier in the day as VPDin increased, mean daily Ec rates remained largely unchanged, indicating that the trees were still able to maintain relatively high levels of daily transpiration, even when VPD's were quite high. The shifting of the diurnal center of Ec into the morning hours has been linked to a specific water use strategy in pine trees. In pines, transpiration rates are maximized in the morning on warm dry days, rather than throughout the day thus maximizing water use efficiency during optimal conditions for stomatal opening (i.e. when light levels (PPFD) are close to maximum, but atmospheric demand (VPD) has not yet peaked) [Kurpius et al., 2003]. Similarly, our findings showed that the timing of peak Ec shifted toward the morning on days when the air was warm and dry. Furthermore, this shifted peak was followed by a steady, but reduced Ec, relative to days when the early morning atmospheric demand was not as high. We found a general correspondence between high morning VPDs and early peaks in Ec, during the driest period of the study (June). Our results suggest that, during dry periods, the dominant white pines at our site moderate the timing of stomatal behavior to maintain a constant rate of Ec with changes in VPD. This interpretation of generalized stomatal function supports the hypothesis that stomatal behavior is shaped by selection, such that the underlying control mechanisms approach a quantifiable goal [Buckley, 2005], where by transpiration is being constrained to a critical rate by the need to both maximize water use efficiency and avoid cavitation at the same time.
 As a result of these findings, we hypothesize that a feedback exists between VPD and Ec at our site, whereby initial daily increases in VPD cause increases in Ec. In turn, increases in Ec cause decreases in leaf water potential. If VPD continues to increase, the decreasing leaf water potentials will eventually meet a critical minimum, prompting a stomatal response, lowering the maximum obtainable Ec rate, which ultimately results in Ec rates leveling-off with subsequent increases in VPD. Further to this point of isohydric regulation, we also hypothesize that either an increase in the magnitude of this negative feedback on Ec with increasing mean early morning VPD, or increasing soil water deficit, would similarly prompt stomata to close progressively sooner and sooner. Likewise, Buckley  made the case that stomata respond similarly to any perturbation in the hydraulic continuum of a tree (i.e. including either soil water supply or atmospheric demand) because the effect on leaf water status is the same. It is thought that plants respond to the cumulative effects of daily weather over extended periods [Schwartz et al., 2006], and it may also be the case that trees can respond to the cumulative effects of daily weather on short time scales in this manner.
 The fact that diurnal Ec rates were significantly affected by atmospheric demand at the onset of the daily course of transpiration is an important finding of our study. The shifts in the timing and magnitude of daily Ec rates, caused by increased early morning VPD, were masking the relationship between transpiration and root zone soil water content. The reason behind the masking was likely that high mean early-morning VPDs and low soil water contents have much the same effect on the relationship between water loss at the leaf surface and water uptake at the root surface. Both can cause a reduction in leaf water potential, signaling a similar stomatal response [Buckley, 2005]. This suggests that the actual degree of VPD is not as critical to stomatal operation as is the balance between the rate of water loss at the crown and the rate of water uptake into the stem. It is likely that critical minimum leaf water potentials can be met at any VPD and could explain the relative insensitivity of Ec to changes in soil water content over short time scales, reported in several past studies (mentioned above). This being said, the results outlined in detailed literature reviews suggest that there doesn't seem to be a single response mechanism that can explain all features of stomatal behavior and it is likely that stomatal operation is controlled by a combination of multiple mechanisms, including root-shoot signaling (via abscisic acid or ABA), leaf water status, atmospheric demand and soil water supply [Jones, 1998].