|Canopy leaching after disturbance:|
We compared net canopy leaching by rainfall for the principal solutes between the mature forest adjacent to W5 (but outside any of the gauged watersheds) and the young, recovering forest on W5 (Figure 1). Net throughfall fluxes (NTF - the difference between atmospheric input and element flux beneath the canopy) of nitrogen were negative in both watersheds, indicating canopy retention. The NTF values for nitrogen in W5 were more negative than for the mature forest (MH), suggesting relatively high nitrogen demand in the young pin cherry-dominated stands (PC) on W5. Differences in NTF between the forests for the other major solutes were minor. We plan to repeat this study as the forest on W5 matures in coming decades to examine NTF patterns during the transition from dominance by pin cherry to maple-beech-birch forest.
Figure 1 (Figure 2 from Lovett et al. 1996. Can. J. For. Res. 26:2134-2144.)
|Root decomposition and nutrient release:|
We conducted detailed studies of decomposition and nutrient release for tree root systems following the whole-tree harvest of W5. Flux from decaying roots was the largest source of base cation supply to the leaching pathway from the watershed during the first several years after harvest. In contrast, nitrogen was strongly retained in decaying roots, including even the N-rich very fine roots, during the initial stages of decay (Figure 2 and Figure 3). Thus, the increase in nitrogen loss from W5 was not primarily associated with fine root decay; and the timing of nitrogen release from the decaying root systems apparently coincided with the beginning of the interval of rapid nitrogen accumulation in recovering vegetation in the second year after harvest.
Figure 2 (Figure 5 from Fahey et al. 1988. Forest Science 34:744-768.)
Figure 3 (Figure 1 from Fahey and Arthur. 1994. Forest Science 40:618-629.)
|Soil CO2 evolution:|
We have been measuring CO2 evolution from the soil surface on W5 and the adjacent mature hardwood forest (outside of the gauged watersheds) on a monthly basis since 1992. During the first three years of measurements (years 8-10 after the forest harvest), CO2 evolution was consistently and significantly higher for W5 than for the mature forest (Figure 4). In contrast, by year 14 the reverse was true, with the cross-over occurring in years 11-13. Although the cause of this changing pattern remains uncertain, budgetary calculations suggest that the decomposition of woody roots of the pre-harvest forest probably was the principal contributor to the earlier difference. Direct studies of woody root decay using a chronosequence approach illustrated that by year 14 the total CO2 flux from the residual roots was probably only about one-fourth as high as during the peak interval around year 8 (Fahey et. al. 1988).
Figure 4 (Tim Fahey, unpublished data)
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