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Resistance and recolonization of bryophyte assemblages following disturbances: - detecting patterns and exploring mechanisms
Stockholm University, Faculty of Science, Department of Botany.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Disturbances are ubiquitous features of most northern forest ecosystems. The subsequent response of plant assemblages on both short (resistance or not) and long term (recolonization or not) will depend on a number of factors operating at several spatial scales. In boreal forest ecosystems, bryophyte assemblages are a conspicuous and species rich group of plants for which these processes are poorly understood. Using a combination of experimental and observational approaches this thesis explores these questions for closed-canopy bryophyte assemblages in relation to a) microtopography (both for the initial and long-term response), b) environmental constrains during post-logging succession and c) disturbance type. My results clearly show that the shade and shelter provided by microtopographic surface structures can increase survival rates of bryophytes following clear-cut logging by decreasing mortality from microclimatic stress and mechanical disturbance. Following clear-cutting, the recovery of forest floor and dead wood living bryophytes seems to be a relatively steady and progressive process without any major bottleneck episodes in the young or semi-mature forest stages with much of the pre-disturbance composition recovered after 50 years. Although boulders were found to increase the initial survival on clear-cuts and hence increase disturbance resistance, we found no evidence that boulders influenced the subsequent recolonization process. Lastly, strong compositional dissimilarities were found in young forests (40 years) following clear-cut logging, wildfire and insect outbreak, indicating divergent trajectories to occur following different disturbances. Hence, early seral stages of forest ecosystems regenerating after natural disturbances seem to compliment young managed forests in maintaining landscape level diversity.

Place, publisher, year, edition, pages
Stockholm: Department of Botany, Stockholm University , 2010. , 30 p.
Keyword [en]
Bryophytes, boreal forest, boulders, clear-cutting, disturbance, establishment, insect outbreaks, microtopography, recolonization, resistance, spatial heterogeneity, succession, wildfires
National Category
Biological Sciences
Research subject
Plant Ecology
Identifiers
URN: urn:nbn:se:su:diva-38883ISBN: 978-91-7447-053-6 (print)OAI: oai:DiVA.org:su-38883DiVA: diva2:317261
Public defence
2010-06-04, Föreläsningssalen, Botanicum, Lilla Frescativägen 5, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Manuscript. Paper 4: Manuscript.Available from: 2010-05-11 Created: 2010-05-03 Last updated: 2010-05-03Bibliographically approved
List of papers
1. Succession of bryophyte assemblages following clear-cut logging in boreal spruce-dominated forests in south-central Sweden - Does retrogressive succession occur?
Open this publication in new window or tab >>Succession of bryophyte assemblages following clear-cut logging in boreal spruce-dominated forests in south-central Sweden - Does retrogressive succession occur?
2009 (English)In: Canadian Journal of Forest Research, ISSN 0045-5067, E-ISSN 1208-6037, Vol. 39, no 10, 1871-1880 p.Article in journal (Refereed) Published
Abstract [en]

The recovery process of boreal bryophyte communities after clear-cutting was studied in a chronosequence in south-central Sweden. We hypothesized that high initial grass cover on clearcuts, high litter cover and low light levels during canopy closure, and shortage of coarse woody substrates would constrain recovery in different ways. Instead, both epigeic and epixylic guilds (i.e., species growing on forest floor and deadwood) displayed a gradual increase in similarity over time from the clear-cut phase, perhaps because of the absence of distinct peaks in needle litter and canopy cover. Epixylic species started to recover long before the accumulation of deadwood, indicating that microclimate rather than substrate availability was the most constraining factor during the first 50 years. Since we did not find any other bottlenecks during the succession after clear-cutting, conservation measures aiming at decreasing local extinction rates during clearcutting may also increase long-term persistence. On the other hand, as the results from the epixylic guild suggest, other factors during the forest succession, such as the development of a suitable microclimate, might be more important for some organisms, thus possibly mitigating such long-term positive effects of adjusted management during the clear-cutting operation.

Identifiers
urn:nbn:se:su:diva-34466 (URN)10.1139/X09-113 (DOI)000271076100008 ()
Available from: 2010-01-08 Created: 2010-01-08 Last updated: 2017-12-12Bibliographically approved
2. Microtopography creates small-scale refugia for boreal bryophytes during clear-cut logging
Open this publication in new window or tab >>Microtopography creates small-scale refugia for boreal bryophytes during clear-cut logging
(English)In: Ecography, ISSN 0906-7590, E-ISSN 1600-0587Article in journal (Other academic) Submitted
Abstract [en]

The biotic response of ecological systems to disturbances has traditionally been explained by attributes of the disturbance event itself, such as its intensity, the distribution of traits within a community related to resistance (e.g., physiological, morphological or life-history) or their interaction. Another less investigated mechanism explaining variation in response to disturbance is microtopographic heterogeneity, which might modify survival rates unevenly. We tested the hypothesis that forest floor microtopography creates small-scale refugia for bryophytes following conventional clear-cut logging by comparing a) survival of transplanted bryophytes and b) compositional changes of forest floor bryophytes among three different positions: on the northern side of boulders and stumps and on unsheltered forest floor. The investigation was carried out as a before-and-after study in 12 Swedish boreal forests (eight stands subjected to clear-cutting and four reference stands). Significantly more bryophyte transplants survived where they were shelter by boulders and stumps (30 % and 29 % respectively) compared to on level forest floor (10 %) and and less compositional changes occurred in sheltered microtopographic positions compared to on level forest floor. Shelter from boulders and stumps increased survival from both microclimatic stress and mechanical disturbance but not from burying by logging residues. Our findings provide evidence that microtopography can modify initial responses to disturbances by creating small-scale refugia. Further studies are needed to determine whether this phenomenon is commonly occurring among other organisms and in other ecosystems and to what extent scattered in-situ survivors can increase the recovery rate following disturbances.

Keyword
boreal forest, boulders, bryophytes, mechanical disturbance, liverworts, mosses, stumps
National Category
Biological Sciences
Research subject
Plant Ecology
Identifiers
urn:nbn:se:su:diva-38897 (URN)
Available from: 2010-05-03 Created: 2010-05-03 Last updated: 2017-12-12Bibliographically approved
3. Microtopographic influence on recolonization patterns of forest floor bryophytes following clear-cut logging
Open this publication in new window or tab >>Microtopographic influence on recolonization patterns of forest floor bryophytes following clear-cut logging
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The extent to which a plant assemblage might recolonize a disturbed system is in general related to the availability of propagule sources and sites with appropriate conditions for establishment. Both of these factors might be sensitive to aspects of spatial heterogeneity. Microtopographic variation may enhance initial resistance by reducing the impact of the disturbance and facilitate establishment of incoming propagules by providing shaded “safe-sites” for establishment. This study explores the influence of microtopographic heterogeneity (caused by variation in surface boulder cover) on the recolonization of closed-canopy forest floor bryophytes using a chronosequence of 75 spruce-dominated forests in south-central Sweden (2-163 years after clear-cutting). We found that high boulder cover did increase survival and subsequent persistence in young forests at the scale of 1000 m2, but that this effect disappeared on a smaller spatial scale (100 m2). At the smaller scale there was a steady accumulation of species over time in both sites with few and many boulders. Furthermore, species accumulation in boulder-poor subplots was not different when surrounded by boulder-rich compared with boulder-poor subplots suggesting short-distance recolonization from boulder-created refugia to be of little importance during the recolonization process. Apparently, boulders function as creators of refugia, but such events seem to be relatively infrequent, which can explain why we were only able to detect such a pattern at larger scales. To conclude it seems like boulders (and probably microtopographic surface structures in general) increase initial resistance to clear-cutting for this bryophyte guild, but that the subsequent recolonization process is more likely to depend on external propagule sources and factors affecting establishment such as microclimate.

Keyword
boreal forest, boulders, bryophytes, clear-cutting, microtopography, refugia, resistance, scale
National Category
Biological Sciences
Research subject
Plant Ecology
Identifiers
urn:nbn:se:su:diva-38898 (URN)
Available from: 2010-05-03 Created: 2010-05-03 Last updated: 2010-05-03Bibliographically approved
4. Bryophyte species density and composition in young forests regenerating after clear-cut logging, wildfire and spruce budworm outbreak
Open this publication in new window or tab >>Bryophyte species density and composition in young forests regenerating after clear-cut logging, wildfire and spruce budworm outbreak
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The disturbance regime in the interface of the boreal forests and the north-temperate deciduous forests in eastern Canada is characterized by both natural disturbances such as wildfires and insect outbreaks as well as anthropogenic disturbances such as production forestry. The current understanding of how understorey plant assemblages respond to different disturbances is mostly based on short-term wildfire-logging comparisons and has traditionally emphasized vascular plants. In this study we explore patterns of species density and composition of four bryophyte guilds in young forests (approximately 40 years old) regenerating after clear-cut logging, wildfire, and spruce budworm outbreak in the Acadian forest region of New Brunswick, eastern Canada. Although being similar in overall species density at the scale of 1000 m2 all three young forest types had fewer species than mature reference forests. All groups were found to be compositionally distinct. Stands developed after spruce budworm outbreaks had the highest canopy closure and the highest amount of coarse woody debris. These stands had similar number of woody debris species as mature forests and an overall species composition that was most similar to mature forests among the three groups. Wildfire-disturbed sites were characterized by a high litter cover, perhaps due to the larger deciduous component of the canopy. A high number of treebase species was typical of these sites. Finally, young managed forest had the highest number of forest floor bryophytes at the scale of 100 m2 among the three young forest types, but was compositionally very far from mature forests in their woody debris flora. In conclusion, young seral stages of forest succession following different disturbances seem to have complementary roles in maintaining landscape level diversity, but if natural disturbances are eliminated certain species (e.g., among the epixylics and treebase species) might become more restricted to older stands in the landscape.

Keyword
boreal forest, bryophyte, Canada, clear-cut logging, disturbance, environmental constraint, recolonization, spruce budworm, succession, wildfire
National Category
Biological Sciences
Research subject
Plant Ecology
Identifiers
urn:nbn:se:su:diva-38899 (URN)
Available from: 2010-05-03 Created: 2010-05-03 Last updated: 2010-05-03Bibliographically approved

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