The interannual and seasonal variability of gross ecosystem photosynthesis (P), ecosystem respiration (R) and evapotranspiration
(E), and their relationships to environmental factors were used to explain changes in net ecosystem productivity (FNEP) at the
onset of, during, and following a 3-year-long (2001–2003) drought in a mature boreal aspen stand in central Saskatchewan, Canada.
The forest was a moderate carbon (C) sink over its entire 11-year data record (FNEP = 153 99 g C m2 year1), including the
peak drought years of 2002 and 2003. In 2001, the depletion of water near the soil surface likely reduced heterotrophic soil
respiration while water remaining deep in the root zone maintained P above the pre-drought mean, resulting in above-average FNEP.
In 2002 and 2003, the forest remained a C sink even though P was below average because R was also below average—a likely
consequence of the influence of low soil water content in deep and shallow soil layers on both autotrophic and heterotrophic
respiration. In 2004, the recharge of soil water in shallow soil layers allowed R to recover to its pre-drought values, whereas low
spring temperatures, the slow recharge of soil water in deep soil layers in spring, late leaf emergence and diminished leaf area index
combined to suppress P and produce the lowest annual FNEP of the 11-year record (4 g C m2 year1). The low FNEP and P were
mirrored in the lowest stem growth and LAI values of the 11-year record. In 2005, a warm wet year, both the annual values and
seasonal variations of FNEP, P and R returned to those of pre-drought years; the partial recovery of LAI to pre-drought values
suggests that aspen P was able to adjust to this restriction on C assimilation. Growing season average dry surface conductance (gsv),
the Priestley–Taylor coefficient (a) and light use efficiency (LUE) reached their lowest values in 2003 and became similar to predrought
years in 2004–2005.Water use efficiency (WUE) was highest in 2003 and remained above average in 2004 and 2005. At the
ecosystem scale, the above-average gains made in C sequestration in the first year of the drought were significantly offset by belowaverage
stand FNEP in the final 2 years of the drought, and in the year following the drought.
