Fire cycle of the Canada's boreal region and its potential response to global change

Interactions of fire cycle and plant species＇ reproductive characteristics could determine vegetation distribution pattern of a landscape. In Canada＇s boreal region, fire cycles before the Little Ice Age （c. 1850s） ranged from 30-130 years and 25-234 years afterwards until the settlement period （c. 1930s） when longer fire cycles occurred in response to climatic change and human interference. Analysis indicated that fire cycles were correlated with growing season （April-October） temperature and precipitation departure from the 1961-1990 normal, varying by regions. Assuming that wildfires will respond to future warming similar to the manner during the past century, an assessment using climatic change scenarios CGCMI, CGCM2 and HadCM2 indicates fire cycles would divert to a range of 80-140 years in the west taiga shield, more than 700 years for the east boreal shield and east taiga shield, and 300-400 years for the boreal plains in 2050.

Evaluation of deriving fire cycle of forested landscape based on time-since-fire distribution

Estimation of fire cycle has been conducted by using the negative exponential function as an approximation of time-since-fire distribution of a landscape assumed to be homogeneous with respect to fire spread processes. The authors imposed predefined fire cycles on a virtual landscape of 100 cell×100 cell, and obtained a mosaic composing of patches with different stand ages （i.e. time since fire）. Graphical and statistical methods （Van Wagner 1978; Reed et al. 1998） were employed to derive fire cycle from the virtual landscape. By comparing the predefined and the derived fire cycles, the two methods and tested the effects of sample size and hazard of burning （i.e., stand＇s susceptibility to fire in relation to its stand age） were evaluated on fire cycle deviation. The simulation results indicated a minimum sample size of l0 times of the annual burnt area would be required for.partitioning time-since-fire distribution into homogeneous epochs indicating temporal change in fire cycle. Statistically, there was significant difference among the imposed and the derived fire cycle, regardless of sample sizes with or without consideration of hazard of burning. Both methods underestimated the more recent fire cycle without significant difference between them. The results imply that deviation of fire cycle based on time-since-fire distribution warrants cautious interpretation, especially when a landscape is spatially partitioned into small units and temporal changes in fire cycle are involved.

FIRE CYCLE AND FIRE ADAPTATION OF MAIN TREE SPECIES IN DAXING'ANLING FOREST REGION, CHINA

Based on the fire statistics, the Daxing'anling forest area were classified into three fire cycle regions: northern coniferous virgin forest region with a fire cycle of 110-120 years, middle conifer--broad-leaved mixed forest region with a fire cycle of 30-40 years, and southem broad-leaved secontw forest region with a fire cycle of 15-20. The percentage of conifers and broad-leaved trees, for est age and natural mature period of main tree species in different fire cycle regions were discussed in concem with fire occurrence. The characteristics of fire adaptation and fire resisboce of main tree species, such as sexual and asexual reproduction, were discussed and evaluation of the synthetical fire adaptation was made.

Have some landscapes in the eastern Canadian boreal forest moved beyond their natural range of variability?

Background: In the contxt of ecosystem management, the present study aims to compare the natural and the present-day forested landscapes of a large territory in Quebec(Canada). Using contemporary and long-term fire cycles, each natural forst landscape is defined according to the variability of its structure and composition, and compared to the present-day landscape. This analysis was conducted to address the question of whether human activities have moved these ecosystems outside the range of natural landscape variability.Methods: The study encompassed a forested area of 175 000 km2 divided into 14 landscapes. Using a framework that integrates fire cycles, age structure and forest dynamics, we characterized the forest composition and age structures that resulted from three historical fire cycles(110,140, and 180 years) representative of the boreal forest of eastern Canada. The modeled natural landscapes were compared with present-day landscapes in regard to the proportion of old-growth forests(landscape level) and the proportion of late-successional forest stands(landscape level and potential vegetation type).Results: Four landscapes(39%) remain within their natural range of variability. In contrast, nine landscapes(54%)show a large gap between natural and present-day landscapes. These nine are located in the southern portion of the study area, and are mainly associated with Abies-Betula vegetation where human activities have contributed to a strong increase in the proportion of Populus tremuloides stands(early-successional stages) and a decrease of oldgrowth forest stands(more than 100 years old). A single landscape(7%), substantially changed from its potential natural state, is a candidate for adaptive-based management.Conclusion: Comparison of corresponding natural(reference conditions) and present-day landscapes showed that ten landscapes reflecting an important shift in forest composition and age structure could be considered beyond the range of their natural variability. The description of a landscape's natural variability at the scale of several millennia can be considered a moving benchmark that can be re-evaluated in the context of climate change.Focusing on regional landscape characteristics and long-term natural variability of vegetation and forest age structure represents a step forward in methodology for defining reference conditions and following shifts in landscape over time.

Fire occurrence in relation to bamboo dominance in the Qinling Mountains of China: Evidence from phytolith and charcoal records in Holocene sediments

he internal genetic clock of semelparous mast-flowering bamboo species creates gregarious regions of flowering and death, thus leading to fuel-load accumulation that potentially promotes fire（Fire-Cycle Hypothesis）. Higher abundance of bamboo could be expected to provide greater fuel loads after flowering. Here, we investigated the possible link between bamboo dominance and fire occurrence. As the main food source for the giant panda in the Qinling Mountains of China, Bashania fargesii（Farges Canebrake） and Fargesia qinlingensis（Arrow Bamboo） dominate lower（1000–1900 m） and higher elevations（1800–2700 m）, respectively. Four soil profiles（elevation ranging from 1240 to 2170 m） in areas of known recent bamboo flowering events（~1980） and adjacent non-flowering sites were assessed for phytoliths and charcoal. Characteristic phytoliths were chosen to describe bamboo dynamics, and charcoal particles were extracted from the soil to reconstruct fire history. Carbon 14 dating of soil charcoal samples was conducted using an accelerator mass spectrometry technique. In addition, we chose two depths（28–52 and 90–100 cm） in one Bashania profile（B. fargesii forest） and one Fargesia profile（F. qinlingensis forest） for phytolith dating. Our findings indicate that bamboo has been a dominant component of these ecosystems almost for the entire profile; some 6400 yrs for Bashania and ca. 10000 yrs for the Fargesia. It is also clear from the charcoal and phytolith data that bamboo abundance and recent mast flowering were significantly related to fire occurrence for lower elevation Bashania while the higher elevation Fargesia was not. One of the four profiles had chronological charcoal records throughout and the oldest charcoal aged was ~1410 yr BP, suggesting an anthropogenic origin. The mixing of phytolith and charcoal made it unrealistic to describe temporal dynamics of bamboo and fire events, and thus our interpretation is cognizant of the dating discrepancies and conservative（less-speculative）. Due to the long history of bamboo and much shorter history of fire within the profile, our data lend little evidence to the Fire-Cycle Hypothesis.