Multiple successional pathways of boreal forest stands in central Canada Dr. Han Chen Office: BB-1009F Phone: 343-8342 Email: han.chen@lakeheadu.ca Taylor, A.R., and H.Y.H. Chen. 2010. Multiple successional pathways of boreal forest stands in central Canada. Ecography DOI: 10.1111/j.1600-0587.2010.06455.x Chen, H.Y.H., and Taylor, A.R. 2011. 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)

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

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)

Ageing succession study stands

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

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

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**

Results : Transition probability for shade-intolerant species

Results : Transition probability for shade-tolerant species

MD and MC succession

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

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

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

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

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

Jack pine

Trembling aspen

White birch

Spruce and balsam fire

Cedar

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

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

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

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