Power analysis from a larger set of black spruce tree ring increments from three stands in Interior Alaska , indicated that as few as four tree ring widths could be used to estimate site ANPP within a 95% confidence interval. Such a low sampling intensity for tree rings may be a reflection of the even-aged, structurally simple nature of black spruce forests. The mean of the average annual ring width for the last 10 years was used with the stem allometry equation to calculate secondary growth for each tree measured in the inventory. Stand biomass was also calculated with our inventory but regional Alaskan or Canadian equations , and these values were compared to estimates derived from the local equations.Above ground biomass of vascular plants, mosses and lichens was measured across all sites by destructive harvest in July 2001 at approximately peak biomass. To more closely examine the dynamics of regrowth in the first several years after fire, biomass was also measured in the 1999 dry site 2 months after the fire as well as mid-summer in 2000–2002; it was also measured in 2000, 2001, and 2002 in the mature dry site for comparison. Trees less than 1.37 m in height that were excluded from the inventory described above were included in these harvests. In harvests of the 1999 dry site, we determined whether each species was a re-sprouter by assessing the presence of charred stems or large rhizomes. We also monitored species or generic richness on an annual basis in these sites by recording the presence of all species within a 144-m2 plot surrounding the 1 m2 harvest blocks. We did not survey species richness in the other sites. In each site, above ground biomass was clipped from either 6 or 12 randomly located 1 m2 quadrats. In the mesic chronosequence sites and the 1987 dry site,blueberry container size above ground biomass of vascular species was clipped from six 1 m2 quadrats randomly located along two 100 m long permanent transects .
Mosses and lichens were collected from a 400-cm2 organic soil plug sawed from a randomly selected corner of the 1 m2 quadrat following vascular plant clipping. In the 1987 dry site and the 1994 mesic site, we also harvested tall shrubs in a 4-m2 quadrat surrounding the 1-m2 quadrat to account for their larger stature. Vegetation was harvested similarly in the 1999 and 1921 dry sites, except that 12 quadrats were harvested. Samples were returned to the lab and sorted into species and tissues within 1 day of harvest. Each vascular species was separated into several tissue categories including current year and previous year leaves, current year and previous year stems, and fruits or inflorescences following methods modified from Shaver and Chapin and Chapin and others . Mosses were separated to species and lichens to genera. We included all structurally intact moss and lichen tissue in the live biomass category. This was determined by tugging gently on the brown part of the moss or lichen ramet; the part that broke off was determined to be litter. Large samples were chopped into small pieces, mixed, and then sub-sampled for fresh and dry weights. Tissues were then dried at 60 C for 48 h or more before weighing. Above ground vascular net primary production was estimated as the sum of the current year‘s apical growth, including leaves and stems. We did not measure secondary growth for under story plants and thus our ANPP values represent an underestimate for shrubs where secondary growth is likely important, mainly Salix spp. and trees less than 1.37 m tall. Apical growth was defined as that produced from apical or intercalary meristems during the current growing season; it was calculated by summing the masses of all current year‘s leaf, stem, and reproductive tissues in the quadrat harvested. Harden and others measured moss production in these sites by measuring the apical growth of individual species and then scaling growth to the plot level with digital mapping. At each site, an average of ten 60 · 60 cm2 moss plots were arranged along greater than 100 m transects with plots spaced every 20–40 m. Percent cover values for up to five dominant moss species within each plot were recorded in fall 2001 via digital photos, extensive field notation, and digitization with Arcview 8.0 software .
Apical growth for each species within each plot was based on growth between June and September of 2001. Within each plot, 10 cm2 dense, generally single-species patches of moss were dyed with a fluorescent brightener in early June. Sprayed moss samples were harvested in late September using a coring device of known area and refrigerated until measurement. Apical growth of each ramet was measured individually under a black light using calipers and new growth was harvested, dried, and weighed to estimate per ramet production. The density of stems per m2 was determined from the % cover plots described above. Moss NPP per species was then estimated on a per plot basis as the mass of apical growth per ramet times the ramet density per unit area times the areal coverage. To validate this method of estimating moss NPP, Harden and others compared estimates of H. splendens productivity to estimates based on a morphological growth marker . They found that the fluorescent dye method underestimated H. splendens production relative to the morphological method, possibly due to an offset in the timing of harvest of the two methods. We have chosen to report the fluorescent dye methods here because these estimates likely represent the most conservative estimate of moss NPP.In the 1999 dry site where we followed growth for 4 years, ANPP was surprisingly resilient to fire disturbance and returned to the level of the mature 1921 dry stand by year four despite radical changes in species composition. Treseder and others observed that root length production was not different between these two sites in 2002, suggesting that below ground production was similarly resilient. The re-sprouters that dominated post-fire productivity must have had roots and/or rhizomes buried in deep organic or mineral soil because over 70% of organic soil depth was consumed in the fire . Rhizomes and roots may have been lingering in mineral soil since the last burn, which is a pattern observed for grasses in black spruce/feather moss sites , as well as trembling aspen which may re-sprout from root suckers after fire . Species known to root in the organic layer dominated pre-fire under story biomass, namely blueberries and cranberries ; these species recovered slowly after fire in the 1999 dry site.
The 1999 mesic site had substantially thicker organic soil layers both before and after fire then the 1999 dry site . Blueberry biomass was not different but ANPP was twice as large in the 1999 mesic site then in the mature mesic chronosequence site . Cranberries, by contrast,growing raspberries in container had 93% less biomass in the 1999 than in the 1886 mesic site, which may be due to the fact that their rhizomes tend to be only 2–3 cm deep in the moss litter layer , whereas blueberries tend to root in the fibric layer . Most important in the rapid recovery of ANPP after fire appears to be the survival of key species, which may, in part, be related to whether meristems are protected in unburned layers of soil. Other factors that likely contribute to the rapid rate of ANPP recovery include increased resource availability due to release of N and P via combustion , decreased competition , warmer soils stimulating microbial decomposition and mineralization of nutrients from soil organic matter , and more available moisture due to reduced evapotranspiration . Across all sites, ANPP was highest in the 1987 dry site where deciduous trees and shrubs dominated biomass and production. Aspen and willow resprouts were present in the 1999 dry site , and stems of dying or dead aspen and willow were visible in the dry mature site , so it is plausible that these sites are part of a successional trajectory that includes co-dominance of black spruce with deciduous trees during mid-succession . In the mesic chronosequence, deciduous trees and tall shrubs were present but at low abundance in intermediate aged sites and were absent from the mature site. Therefore, in contrast to the dry chronosequence, the mesic sequence may represent self-replacement, or spruce-to-spruce successional trajectory . Fire severity, drainage and soil temperature have been identified as factors driving the abundance of aspen and tall shrubs in Interior Alaska. Aspen density was positively related to fire severity in a nearby mesic site that burned in 1994 and aspen and willow were more abundant in more frequently burned sites in the Yukon presumably due to the better survivorship of species with rhizomes and roots in the mineral soil. Across the landscape, single species stands of aspen are found in relatively warm, well-drained sites . Severe or frequent fires can increase soil temperature though the removal of insulating moss and soil organic matter and increase drainage through thermal erosion of permafrost. Thus, the high abundance of deciduous tree and tall shrub species in the 1987 dry site could be related to thermal effects of severe fire or alternatively, the site could just be an anomalously warm, well drained patch of the landscape.
Finally, stochastic processes such as the proximity to seed sources could play a role in the establishment of deciduous species as well and cannot be ruled out as a factor contributing to compositional differences between the chronosequences. Black spruce density, basal area, biomass and ANPP in the mature sites were within the range reported for Interior Alaska . Biomass in our mesic 1886 stand was 90% of peak black spruce biomass estimated from large-scale forest inventory measurements across the state of Alaska , suggesting that this stand may be at or near peak biomass. The mature dry stand, by contrast, accumulated 78% of biomass predicted for its age class . If the biomass accumulation curve for this chronosequence is projected to 150 years estimate of maximum stand biomass, peak biomass would be 2,831 g m)2 ; still substantially less than peak biomass in the mesic chronosequence. Lower black spruce biomass and ANPP in the mature dry site than in the mature mesic site appears to be driven primarily by higher tree density in the latter site because biomass and ANPP per tree were similar between sites . Yarie and Van Cleve similarly found black spruce production to be constant over variably drained stand ages ranging from 50 to 150 years when productivity was standardized to full stocking rate. Lower density in the mature dry than in the mature mesic site could be related to processes directly attributable to drainage, such as self thinning due to water competition , interactions with abundant deciduous tree species or feedbacks between fire and drainage . Alternatively, differences in density could be caused by processes that are relatively independent of drainage, such as climatic extremes during the sensitive early years of spruce seedling establishment. Our 1956 mesic site had only 25% of the tree density of the 1886 mesic site and contained only 16% of black spruce biomass predicted for its age class by Yarie and Billing‘s accumulation curve. Because upland black spruce stands tend to be comprised of a single cohort , it seems unlikely that density will quadruple in the next 50 years. These differences in density, then, may represent poor site matching in the mesic chronosequence and confound estimates of biomass accumulation and ANPP. To explore the impact of this on our biomass estimates, we multiplied per tree biomass of the 1956 mesic site 1 by density of the mature mesic site , yielding an estimate of 1,135 g m2 , which is within 10% of Yarie and Billing‘s estimate for this age class. When this value was plotted on our chronosequence biomass accumulation curve , the curve is still best-fit by a linear equation 169.4, R2 = 0.97, P = 0.01, suggesting that the functional shape of biomass accumulation would still differ between mesic and dry chronosequences if density was held constant across the mesic chronosequence. Our Alaskan black spruce stands had less biomass and were less productive than comparable well drained stands in Manitoba, Canada, due to both lower tree density and lower per tree biomass and growth.