Yearto Year Climate Variability and Plant Phenology Physiology and Ecosystem Exchange

The date of the snowmelt near the LTER greenhouse plots varied by 26 days over the last 7 years; the earliest date was 13 May in 1995 and the latest was 9 June in 2000 (table 5.2). A longer record from nearby Imnavait Creek had a range of 30 ± 10 days (sd) over the past 17 years (Kane et al. 2000).

The timing of snowmelt has a significant effect on soil temperatures and thickness of the active or thawed layer (figure 5.3). On experimental plots where snow was removed two to three weeks before the snow melted on nearby control plots, soil temperatures averaged more than 1°C higher than in control plot soils (Oberbauer et al. 1998). Such differences undoubtedly affect belowground processes such as nutrient mineralization rates and root growth.

Plant photosynthesis begins at snowmelt. Obviously, late snowmelt leads to a shorter growing season, but more subtly, late snowmelt shifts the window of peak leaf performance further toward the declining sun angles and lower irradiances that follow the summer solstice. The weather after snowmelt can also be important; for

Table 5.2 Date of complete snowmelt and snow depth for a site near Toolik LTER

Date of

Snow depth

Year

snowmelt

on 2 May

1995

13 May

52.8

1996

29 May

71.3

1997

3 June

65.4

1998

24 May

48.6

1999

22 May

49.5

2000

9 June

62.2

2001

8 June

65.0

example, very low spring temperatures may slow development of photosynthetic capacity or even damage overwintering leaves. Evergreens are the exception— they very quickly ramp up to full photosynthetic capacity after snowmelt (Oberbauer et al. 1996).

Snowmelt also marks the beginning of the accumulation of forcing temperatures required for bud break of deciduous and evergreen shrubs and for initiation of leaf expansion of graminoids and forbs. The date of bud break is strongly dependent on spring temperatures after snowmelt (Pop et al. 2000). Consequently, the effects of late snowmelt can be partially offset by warm spring temperatures. Variation in the timing of snowmelt in conjunction with spring temperatures has a strong effect on when and how much leaf area develops. In experimentally manipulated plots, Oberbauer et al. (1998) found that plants in the experimental plot with early snow removal developed leaf area earlier and to a greater extent than plants in control plots. These increases in leaf area translate to higher gross ecosystem uptake capacity, though they may be partially offset by higher respiration resulting from higher soil temperatures. Also, for some species, early bud break or leaf initiation leads to early senescence (Oberbauer et al. 1998; Starr et al. 2000).

Variation in the timing of fall freezes and development of snow cover at the end of the season has less impact on ecosystem uptake capacity because shoots of most plant species near Toolik become dormant in response to declining photoperiod. Early freezes unquestionably do accelerate the end-of-season leaf senescence (Mc-Graw et al. 1983). However, perhaps of more importance is the timing of snow cover with respect to hard freezes; late development of snow cover exposes evergreens to potentially damaging hard freezes from which they otherwise would be protected. Die-offs of evergreens noted at the beginning of the growing season in some years are probably a result of such conditions.

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