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 subsampled for fresh and dry weights. Tissues were then dried at 60C 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 understory 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, grow raspberries in a pot 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.
Compared with mature tundra, the disturbed ground associated with bear digs generally has lower vascular plant species richness. In contrast, Shannon-Weaver diversity levels are indistinguishable between mature tundra and digs, suggesting that the lower overall plant cover in bear digs allows a more even distribution of the abundances of plant species that are present. The results of this direct comparison are intuitive, but grizzlies’ cumulative effect on community richness and structure is more appropriately thought of as marginal addition to the mature tundra community than as a contrast to it. At any given time, recognizable bear digs make up only a small fraction of the ground cover in mature tundra areas. In the northern portion of our study area, Christian found between 7 and 23 bear digs within sight of each kilometer-long transect through ground squirrel habitat, and grizzlies digging for glacier lily bulbs excavate no more than 6% of suitable meadow habitat in a given year in Glacier National Park . The results of our pairwise comparison show that overall, mature tundra alone has fewer plant species than mature tundra with bear digs. The difference in our plots was small but statistically significant. So, although mature tundra harbors more plant species at the landscape level than bear digs, a potentially important effect emerges at the smaller scales relevant to most plants and many animals.
Square meter by square meter, repeated samples from a landscape of mature tundra dotted with bear digs will exhibit greater species richness than samples from a landscape of pure bear digs but also greater richness than samples from purely undisturbed tundra. Although our data cannot define the optimum level of disturbance, they suggest strongly that moderate levels of bear digging will modestly enhance local species richness. In alpine tundra, grizzly bear digs undergo transient increases in plant species richness and diversity that develop over time as recolonization of the dig occurs. This result is consistent with other studies of mammalian disturbance. In desert , coastal prairie , and serpentine annual grasslands , digging mammals have significant impacts on species richness and diversity, with species numbers usually rising after a disturbance event. In some cases, richness and diversity appear to decline as digs undergo succession back to mature tundra. This pattern fits that predicted by the intermediate disturbance hypothesis , with highest diversity or richness at an intermediate stage in succession following the bear digging. We note this result with caution, however, because our data indicate relative richness and diversity of bear digs and because our proxy for dig age is inherently imprecise. As recolonization occurs, in any case, we have shown that species composition of bear digs remains very similar to that of the immediately surrounding tundra, implying that throughout the successional recovery of bear digs most colonization comes from nearby plants.In addition to changing community diversity, bear digs have differential impacts on plants with different life histories. In particular, these disturbances appear to favor plants with certain clonal growth strategies. The biological significance of these effects differs between the two plant communities we studied. In snowbed areas, all clonal types except the most dominant one were favored by digging, suggesting that the statistically significant reduction in above ground guerrilla species may release species of other clonal types from competition, leading to the observed increases in species richness and diversity. In mesic meadows, by contrast, species of the dominant below ground phalanx clonal type were actually favored by digging, while two of the less common clonal types, above ground guerrilla and phalanx species, were suppressed. In this case, it is possible that bear digs are frequent enough to prevent the dominance of above ground species, even in what appears to be mature tundra. Our observations suggest that grizzly bear digs are very concentrated in some areas, but we cannot speculate on the recurrence interval for bear digs in any given location. Ultimately, several of the changes in group dominance by plant species of different clonal types were statistically insignificant. Given the broad range of plant morphologies and strategies grouped within each of our coarsely classified clonal types, we expected to find more significant differences at the scale of individual species. In a study of short-grass prairie communities, for example, Platt showed that a small suite of ‘‘fugitive’’ plant species depended entirely upon badger disturbances for recruitment and reproduction. Unlike Platt, but in common with other studies of digging mammals , we did not find individual species that appeared to absolutely require bear digs for establishment or reproduction. In retrospect, this result is not surprising, given the large number of abiotic disturbances that affect alpine communities; avalanches, frost heaving, solifluction, best grow pots and freeze-thaw ground tears are all significant forces in shaping tundra communities . Indeed, although the species most dependent on bear digs in this study, Epilobium anagallidifolium, also occurred in mature tundra plots, it did so almost exclusively in the cracks caused by frost heaving and ground tears.
The degree to which bear digs’ effects on biotic communities resemble the effects of these other, abiotic disturbances is not well understood, but it seems clear that the ground disturbance associated with bear foraging is not wholly unique in this environment. Still, we found several plant species that showed a statistically significant difference in abundance between bear digs and mature tundra. Furthermore, both species dominant on bear digs were of the phalanx clonal type, while five of the six more common in mature tundra were guerrilla species. These results raise questions about the role of clonal growth form in mediating competitive interactions. Theoretical work has emphasized that differences in clonal form can mediate competitive interactions , and Boeken and Shachak’s work in the Negev desert documented various plant traits favored by disturbances, including seed size and dispersal characteristics. Given the low seedling recruitment rates and long life spans thought to be typical of most tundra species, we suggest that grizzly bear digs may provide critical microsites for reproduction and survival of phalanx-type plants that are at a competitive disadvantage relative to aggressively spreading guerrilla species in densely vegetated mature tundra. Clearly, though, these interactions occur within a diverse plant community where species with widely varying clonal forms interact on small spatial scales. This diversity is probably maintained by the interaction between a diversity of clonal growth forms in the context of numerous microand mesoscale disturbance events. Our observation that the second most common species in a study plot frequently differs in clonal form from the dominant species supports the possibility that the varying clonal forms common in this system may help to allow coexistence of species.The growth of the “critical zone” paradigm has added impetus to closer investigation of soil-plant atmosphere interactions in ecohydrology . This follows from work emphasizing the importance of vegetation in regulating the global terrestrial hydrological cycle, with transpiration being the dominant “green water” flux to the atmosphere compared to evaporation from soils and canopy interception in most environments . More locally, the role vegetation plays in partitioning precipitation into such “green water” fluxes and alternative “blue water” fluxes to groundwater and streamflow has increased inTherest in the feedbacks between vegetation growth and soil development in different geographical environments . The emerging consequences of climatic warming to changes in vegetation characteristics and the implications of land use alterations add further momentum to the need to understand where plants get their water from, and how water is partitioned and recycled in soil-plant systems . Stable isotopes in soil water and plant stem water have been invaluable tools in elucidating ecohydrological interactions over the past decade . Earlier work by Ehleringer and Dawson explained the isotope content of xylem water in trees in terms of potential plant water sources. Building on that, Brooks et al. showed that the isotope characteristics of xylem water did not always correspond to bulk soil water sources as plant xylem water was fractionated and offset relative to the global meteoric water line compared to mobile soil water, groundwater and stream flow signatures. This led to the “Two Water Worlds” hypothesis which speculated that plant water was drawn from a “pool” of water that was “ecohydrologically separated” from the sources of groundwater recharge and stream flow . Research at some sites has found similar patterns of ecohydrologic separation and suggested it may be a ubiquitous characteristic of plant-water systems . Others have found that differences between plant water and mobile water may be limited only to drier periods , or may be less evident in some soil-vegetation systems . Direct hypothesis testing of potential processes that may explain the difference between the isotopic composition of xylem water and that of potential water sources has been advanced by detailed experiments in controlled environments, often involving the use of Bayesian mixing models which assume all potential plant water sources have been sampled . However, as field data become increasingly available from critical zone studies, more exploratory, inferential approaches can be insightful in terms of quantifying the degree to which xylem water isotopes can or cannot be attributed to measured soil water sources . As this research field has progressed, it has become apparent that extraction of soil and plant waters for isotope analysis is beset with a number of methodological issues . Soil waters held under different tensions may have different isotopic characteristics: for example, freely moving water sampled by suction lysimeters often shows a much less marked evaporative fractionation signal than bulk soil waters dominated by less mobile storage extracted by cryogenic or equilibration methods .