PNW 98 Session - Full AbstractsPNW 98 Session - Full AbstractsThe management plan was based upon two working hypotheses. First, that _Phalaris_ can be controlled by
maintaining a high water level for a relatively long duration in the spring, and by maintaining inundation
through the intermediate growing season, at least July 15 (Naglich, 1994). Second, the _Sagittaria_
population will be maintained by this hydrological regime, and cover will increase relative to that of
_Phalaris_.
Dalton Lake is primarily a deep emergent marsh system, dominated by _Sagittaria_, _Phalaris_, and other
native emergent and floating aquatic species, with a forested component dominated by _Salix lucida_ ssp.
_lasiandra_ and _Fraxinus latifolia_. Freshwater input is mainly from two intermittent streams. Overflow
from the adjacent Columbia River may influence the water level during spring freshets. At estimated
Ordinary High Water, mean water depth is approximately 76 cm, which normally decreases steadily after
early May upon cessation of input. In dryer years, standing water disappears by August or September.
We observed percent cover and height of the two plants, and water depth, in late summer along 100m
transects from 1993 to 1997. We also recorded monthly rainfall totals for this period. We found a variety
of late summer water depths, from none in 1994 to a mean of 74.9 cm in 1997. During the period, overall
_Phalaris_ cover decreased significantly to a low of 4.9%, while that of _Sagittaria_ increased at first, then
declined dramatically in 1997. The highest water depths were recorded in 1996 and 1997, corresponding
with low cover of _Phalaris_ and (in 1997) a crash in the _Sagittaria_ population. This summary time-series
data suggests that, while _Phalaris_ can be controlled by maintaining deeper inundation, _Sagittaria_ can be
adversely affected by the same high water levels. This adverse effect is supported by experimental data
from Garbisch (1993), who investigated _Sagittaria_ establishment from tubers (its primary means of
reproduction) at different water depths, and found a range of water depths (46-56 cm) beyond which the
experimental populations could not reproduce asexually.
A summary of point-data over this period showed a low level of correlation between water depth and
_Sagittaria cover_ (r^2 = -0.02) and _Phalaris_ cover (r^2 = -0.02). Annual average data for the period showed
a moderate negative correlation between water depth and _Sagittaria_ cover (r^2 = -0.55) and a very low
correlation with _Phalaris_ (r^2 = -0.08). We also plotted spring/summer rainfall against percent cover, and
found generally low levels of correlation (r^2 = -0.30 for _Sagittaria_, and r^2 = -0.12 for _Phalaris_).
We explain the results as follows: while both species are sensitive to water depths greater than a certain
threshold, both are found at a wide variety of water depths in the system, creating a scatter in the data when
cover is plotted against water depth. _Phalaris_ is able to tolerate deeper water to an extent by forming
floating mats. We were measuring water depth in late summer, but based on the research by Garbisch, we
believe that water depth at the time of emergence (approximately June 1) has a stronger influence on
_Sagittaria_ cover and vigor.
Consequently, an “adaptive management” strategy will need to be adopted to avoid impacting the
_Sagittaria_ population, which was severely reduced by naturally high water in 1997. The approach may
need to optimize for one species or the other in a given year. For 1998, the strategy will involve a spring
draw down to aid _Sagittaria_ emergence. This may temporarily compromise the goal of _Phalaris_ control.
Higher water levels can be maintained to control _Phalaris_, as originally planned, once _Sagittaria_ recovers.