In particular, we highlight the combination of independent and particularly joint effects of climate and soil on trait variation, an interaction that has to date been neglected because few studies include both in a single analysis, at the global scale as we have done here. In doing so, we identify an important gap in knowledge: what is the nature of climate–soil interactions that drive whole-plant trait variation and what distinguishes the majority of climate and soil factors having joint effects on plant traits from those with independent effects? These are the sorts of questions that require answers to increase our capacity to predict plant functional diversity in a changing environment. Such predictive power would contribute to a sound basis for assessing long-term feedbacks between global environmental change and the terrestrial biosphere, helping to constrain parameters of global coupled climate–vegetation models. Humans are currently modifying both climatic and edaphic conditions at the global scale. Climate envelope models used to predict vegetation shifts must be complemented by drivers related to large-scale anthropogenic alterations of soil conditions resulting, for example, from land-use change, atmospheric nitrogen deposition, fertilization, black plastic nursery pots liming and salinization. Our global analysis provides an essential context for finer-scale studies to directly tackle questions of biological processes and mechanisms at landscape and community scales.
Fire is part of the natural disturbance regime of many boreal regions, although recent evidence suggests that anthropogenically induced climate change may be increasing the burned area in North American and Eurasian forests . Because high-latitude ecosystems store approximately 40% of global carbon stocks in biomass and soils , an amount equal to the atmospheric C pool, there has been considerable inTherest in understanding how these systems will respond to climate warming. Combustion of vegetation and forest floor transfers C directly from terrestrial ecosystems to the atmosphere, so increased burned area or fire intensity in the boreal biome could be a strong, positive feedback to atmospheric CO2 concentrations , at least in the early years after fire . At a regional scale, the effect of fire on species composition, soil drainage, and stand age distribution will ultimately regulate whether the CO2 feedback is positive or negative. The response of this long-term signal to the combination of climate change and alThered fire regime is largely unknown for the boreal biome. Patterns of plant species composition, biomass accumulation, and productivity across post-fire succession are important determinants of the amount , structure , residence time , and decomposability of C inputs to these systems. In Interior Alaska, black spruce BSP stands cover approximately 70% of the forested area and occupy landscape positions that range from permafrost-free well-drained soils to permafrost dominated and poorly drained soils . These forests are highly flammable due to their architecture and resin production as well as the thick moss layer on the forest floor , and fire return intervals range from 70 to 100 years . Wildfires tend to be large and high-intensity and while few boreal overstory species survive fire, many understory species re-sprout after fire from buried or protected meristems .
Black spruce are semi-seritonous and release seed after fire, with the majority of trees recruiting in the first 5 years after fire . Stands may or may not go through a deciduous phase where trembling aspen and tall shrubs , which may resprout after fire , dominate over the 15–50 years prior to closure of the black spruce canopy . Moss and lichen expansion across the forest floor follows similar timing, with moss cover reaching its maximum between 30 and 50 years , concurrent with canopy closure by black spruce and a reduction in deciduous litter. The deciduous phase appears to be related to interactive effects of fire severity and site drainage as evidenced by the fact that sites that burn severely , or at a high frequency have the highest abundance of aspen and willow species. At the landscape level, both the severity and frequency of fire appear to be related to soil drainage . Although patterns of species dominance over post-fire succession have been described for Alaskan black spruce stands, there are few published measurements of productivity and biomass after fire. In conjunction with a chronosequence study of soil C dynamics, O‘Neill and others used a mass balance model to conclude that C inputs balanced C losses 7–15 years after fire. Yarie and Billings used forest inventory data from stands across Alaska to estimate generalized biomass accumulation curves for black spruce green timber. They show that biomass accumulation peaked between 75 and 150 years. Simulation modeling of ecosystem C dynamics over post-fire succession suggests that C balance is most sen-sitive to N fixation, moss accumulation, organic layer depth, soil drainage, and fire severity.
Finally, there are several comprehensive studies of post-fire succession in Central and Eastern Canada , but the trees in these sites inhabit different soil drainage and temperature regimes than their Alaskan relatives, potentially resulting in different rates of ecosystem C dynamics . The goals of this study were to describe the changes in community structure and above ground net primary productivity and biomass that occur over post-fire succession in the upland black spruce forests of Interior Alaska. We present measurements that span two different time scales: recovery 1–4 years after fire and recovery over the entire successional cycle. For the former, we followed vegetation recovery for 4 years after the 1999 Donnelly Flats fire near Delta Junction, Alaska. We used a chronosequence approach for the latter by selecting two sequences of sites in the region that varied primarily in time since fire: a mesic sequence on moderately well-drained soil with permafrost and a dry sequence located on well-drained soils without permafrost . These sequences represent transitions in environmental factors that might occur with climate warming, including loss of permafrost and subsequent increases in soil drainage .This study was conducted in the Donnelly Flats area located near Delta Junction in Interior Alaska, in seven upland sites that were previously dominated by black spruce . All sites were located within a 100-km2 area on gently sloped alluvial flats that range from moderately well-drained soils dominated by permafrost to well-drained soils where permafrost was largely absent. Our study include three sites on well-drained soils that burned in stand-killing wildfires in 1999, 1987, and approximately 1921 , hereafter the dry chronosequence, and four sites on moderately well-drained soils that burned in 1999, 1994, 1956, and approximately 1886 , hereafter the mesic chronosequence. Time since last fire was determined by historical record in the younger sites and by tree ring analyses in the older sites. Some or all of these sites have been used to assess the effects of fire on soil C storage and emissions , soil chemistry , hydrogen fluxes , fungal community composition and dynamics , seasonal CO2 and 18O–CO2 fluxes and energy exchange . Within each chronosequence, sites were chosen to have similar state factors other than time . Climate: Micrometerological data collected in the 1999, 1987, and 1921 dry sites and the 1994 , 1999 and 1886 mesic sites support the idea that sites in both chronosequences experienced a similar climatic regime. The regional climate is cold and dry with an annual mean surface air temperature of 2.1C during the 1970–2000 period . Over this same period, mean temperatures in January and July were 20 C and 16.0C, respectively, and mean annual precipitation was 290 mm. Approximately 65% of precipitation fell during June, July, and August. Potential biota: Although all stands were currently or historically dominated by black spruce and were in a close enough proximity that they belong to the same regional pool of potentially colonizing organisms, 30 plant pot the understory vegetation and ground cover varied with soil drainage and stand age .
The oldest dry stand was a lichen woodland , with ground cover dominance split between feather moss and lichens. Vaccinium uliginosum and V. vitis-idaea were the most abundant understory species, with deciduous shrubs and trees, forbs and graminoids present but at low abundance. Many of the same species resprouted or recruited after fire in the 1999 dry site and dominated the understory in the 1987 dry site. Species characteristic of well-drained ecosystems that were present in all dry chronosequence sites and absent from the mesic sites were the grass Festuca altaica and the evergreen shrub Arctostaphylos uva-ursi. These species were present, however, on trails and roadsides around the mesic sites. The oldest mesic stand had continuous feathermoss ground cover and a high abundance of Vaccinium spp. Feathermoss occupied almost the entire ground surface in the 1956 and 1886 mesic sites. In the 1994 mesic site it persisted in patches that appeared to have escaped burning. Vascular nomenclature follows Hulte´n and non-vascular nomenclature follows Vitt and others . Relief: Sites in both chronosequences were within a 100- km2 area with little variation in slope or topography . Parent material: Soils along both chronosequences were mainly derived from the Donnelly moraine and wind blown loess and have been described in detail elsewhere . Differences in drainage between the chronosequences are thought to be related to differences in water table depth and texture . Although great care was taken to control state factors within and between chronosequences, it was difficult to fully constrain the effects of past fires on productivity or biomass pools. In the 1999, 1994, and 1987 sites, fires were stand replacing . In the 1957 mesic site, the relatively small range of tree sizes suggests a single cohort of black spruce. In the mature 1886 mesic and 1921 dry sites where tree sizes are quite variable, however, the number of trees sampled for age was not large enough to determine whether stands are comprised of a single cohort . At the landscape-scale, the severity and frequency of fire are likely to be related to soil drainage . At the site level, however, stochastic factors such as weather conditions, time since last fire, and neighboring vegetation can also affect fire severity. Post-fire vegetation recovery is similarly affected by stochastic processes such as timing of fire in relation to both vegetative and reproductive phenology, proximity of seed source, and/or the effects of past and present climate conditions on demographic processes. Finally, we caution the reader to keep in mind at all times that this is an observational study; we depend on the assumptions of the chronosequence approach to make inferences about time.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. Organic depth was measured at the four corners of each quadrat and averaged. In the mesic chronosequence sites and the 1987 dry site, 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 .