1. Multiple successional pathways of boreal forest stands in central Canada
Dr. Han Chen
Office: BB-1009F
Phone:
Taylor, A.R., and H.Y.H. Chen Multiple successional pathways of boreal forest stands in central Canada. Ecography DOI: /j x
Chen, H.Y.H., and Taylor, A.R A test of ecological succession hypotheses using 55-year time-series data for 361 boreal forest stands. Global Ecology and Biogeograpgy (accepted on March 2, 2011)

2. Boreal succession “under fire”
Fire cycle is too short for succession to occur
Chronosequence approach assumes pathways, therefore not suitable for studying pathways
Dendrochronological reconstruction method is too expensive to study many stands needed to determine pathways

3. Repeated measurements
Each stand was measured 4-6 times between 1946 and 2003.
Ground and aerial photo plots
Re-construct stand history, including composition, edaphic conditions, disturbance history, and stand age (i.e., TSF)

4. Ageing succession study stands

5. Ageing succession of boreal forest
Multiple successional pathways hypothesis
Time
Disturbance
Genetic makeup (stand type)
Physical environment: climate, soil
Probability of stand type change =

6. Analysis-Multinomial Logistic regression
Response variable
Response variable was stand “Cover Type”
Single species types: dominated by ≥ 50% individual species BA
Mixed deciduous or coniferous types: dominated by either deciduous or coniferous BA
Independent variable
Time since fire (TSF)
Land Class
LC 1 = fine to medium texture, wet to fresh moisture conditions
LC 2 = medium to coarse texture, fresh to dry moisture conditions
Intermediate Disturbance
Presence or absence of windthrow, FTC, and SBW

7. Significance test and predictor variables retained for each multinomial regression model.k G
ΔAIC
Variable a 1. Pb 4
34.7**
26.7
TSF**, LC** 2. Pt 3
18**
11.9
TSF** 3. Bp
20.2**
14.3 4. Pm
21.8**
13.8 5. Ab
32.6**
26.6
DIS** 6. To 1
4.5*
2.5
LC* 7. MD 2
6.4*
2.4
TSF* 8. MC 12
36.9**
12.9
TSF*, LCns, DIS**

8. Results : Transition probability for shade-intolerant species

9. Results : Transition probability for shade-tolerant species

10. MD and MC succession

11. Conclusions
First time to use repeatedly measured data to show in boreal forest
Evidence of a directional change in species composition related to TSF
Multiple pathways affected by neighborhood, site condition and intermediate disturbance
Convergence/semi-climax, i.e., in the prolonged absence of stand replacing fire, stands converge to shade-tolerant dominance

12. A test of ecological succession hypotheses using 55-year time-series data for 361 boreal forest stands
Chen, HYH and AR Taylor, Global Ecology and Biogeography

13. Four hypotheses
Neighbourhood effect hypothesis: Tree species abundance is dependent on time since disturbance (Frelich & Reich, 1995, 1999)
2) Negative density dependence hypothesis: Rare or minor species is more favoured over time due to a negative intraspecific effect (Connell, 1970) or their reproduction advantage with temporal environmental variability (Chesson & Warner, 1981)
3) Resource ratio hypothesis: species that can grow at the lowest resource level tend to dominate resource limited sites throughout succession (Tilman, 1985; Tilman, 1994)
4) Intermediate disturbance hypothesis: Intermediate disturbances act as mechanisms to increase abundance of rare species (Connell & Slatyer, 1977; Connell, 1978), promoting species coexistence and leading to higher local species diversity

14. Data
Same as multiple successional pathways, but this paper focuses on individual tree population dynamics
Measured by, for each species, change in basal area, Δba (m2·ha-1·yr-1)
And change in relative abundance or composition, Δ%ba (%ba·ha-1·yr-1)
Use only initial data and last data to avoid many zeros between short measurement intervals

15. Explanatory variables
Table 1. Summary statistics for each linear statistical model for each study species.
Species
Model
Response variable R2
Explanatory variables
Source df F η2
Pinus banksiana
1.1
Δba
0.51
TSFmean 1
118.29***
0.20
Stand Type: 2
44.36***
0.15
con a, msi b, mst b
Land Class
4.15*
0.01
TSFmean · Stand Type:
47.40***
0.16

16. Jack pine

17. Trembling aspen

18. White birch

19. Spruce and balsam fire

20. Cedar

21. Conclusions
Using repeated measurements over a ~ 55 year period and covering a large geographical area, we found limited support for species-replacement of shade-intolerant pioneers by late-seral shade-tolerant species,
However, TSF alone is an insufficient predictor for species dynamics in the spatially structured boreal landscape with varying initial species abundance, site resources, and disturbance regimes

22. Conclusions
Species either persisted longer or increased its abundance more in non-conspecific stands over time, supporting the density-dependence hypothesis.
Shade-intolerant species persisted longer in mixed stands of shade-tolerant species, whereas shade-tolerant species had higher overall increases in abundance when associated with shade-intolerant species, indicating that species coexistence is enhanced by complementary shade tolerance niches

23. Conclusions
For species with large edaphic amplitude, varying edaphic conditions also influenced species abundance over time. Very dry, coarse textured soils or water saturated, nutrient poor soils favoured those species most capable of tolerating limited resources. Rich soils permitted invasion and promoted species mixtures during succession, as predicted by the resource-ratio hypothesis of plant succession.
Intermediate disturbances such as SBW and FTC on Betula papyrifera, wind on Picea spp., and SBW on Abies balsamea had the strongest effects in conspecific stands, suggesting that these disturbances have served as the key mechanism to promote coexistence of boreal tree species

24. Conclusions
Our study is the first to demonstrate that, contrary to past findings that have relied on space-for-time substitution, time since fire alone only partially explains species dynamics. Rather, density dependence, site resource, and intermediate disturbances are key mechanisms for species dynamics and coexistence over time in boreal forests