| 1. |
- D'Orangeville, Loic, et al.
(författare)
-
Drought timing and local climate determine the sensitivity of eastern temperate forests to drought
- 2018
-
Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 24:6, s. 2339-2351
-
Tidskriftsartikel (refereegranskat)abstract
- Projected changes in temperature and drought regime are likely to reduce carbon (C) storage in forests, thereby amplifying rates of climate change. While such reductions are often presumed to be greatest in semi-arid forests that experience widespread tree mortality, the consequences of drought may also be important in temperate mesic forests of Eastern North America (ENA) if tree growth is significantly curtailed by drought. Investigations of the environmental conditions that determine drought sensitivity are critically needed to accurately predict ecosystem feedbacks to climate change. We matched site factors with the growth responses to drought of 10,753 trees across mesic forests of ENA, representing 24 species and 346 stands, to determine the broad-scale drivers of drought sensitivity for the dominant trees in ENA. Here we show that two factors-the timing of drought, and the atmospheric demand for water (i.e., local potential evapotranspiration; PET)-are stronger drivers of drought sensitivity than soil and stand characteristics. Droughtinduced reductions in tree growth were greatest when the droughts occurred during early-season peaks in radial growth, especially for trees growing in the warmest, driest regions (i.e., highest PET). Further, mean species trait values (rooting depth and psi(50)) were poor predictors of drought sensitivity, as intraspecific variation in sensitivity was equal to or greater than interspecific variation in 17 of 24 species. From a general circulation model ensemble, we find that future increases in earlyseason PET may exacerbate these effects, and potentially offset gains in C uptake and storage in ENA owing to other global change factors.
|
|
| 2. |
- Fraver, Shawn, et al.
(författare)
-
Demographics and disturbance history of a boreal old-growth Picea abies forest
- 2008
-
Ingår i: Journal of Vegetation Science. - : Wiley. - 1100-9233 .- 1654-1103. ; 19:6, s. 789-798
-
Tidskriftsartikel (refereegranskat)abstract
- Question: To what extent do tree growth, mortality, and long-term disturbance patterns affect stand structure and composition of an old-growth Picea abies forest?Location: Boreal Sweden.Methods: We linked data from three 50 m × 50 m permanent plots established in 1986 with dendrochronology data to evaluate tree growth and mortality over an 18-year period and to describe a several-hundred-year disturbance history for this forest type.Results: Averaged over all diameters, P. abies trees had an annual mortality rate of 0.60%; however, diameter had a striking effect on both growth and mortality, with trees of intermediate diameters (ca. 20–30 cm) showing faster growth and lower mortality. Their increased vigor gave rise to a diameter distribution resembling the ‘rotated sigmoid’ (not reverse-J) proposed for such conditions, and it led to a deficit of snags of intermediate diameters. Slow-growing trees had an increased likelihood of dying. Although recruitment occurred in most decades over the past 400 years, two prominent recruitment peaks occurred (mid 1700s and 1800s), neither of which appeared to cause a shift in tree species composition. The lack of fire evidence suggests that fire was not responsible for these recruitment peaks.Conclusions: Taken together, these results depict a rather impassive system, where canopy trees die slowly over decades. Field observations suggest that fungal infections, mediated by wind, account for much of the mortality during these periods of relative quiescence. However, these periods are at times punctuated by moderate-severity disturbances that foster abundant recruitment.
|
|
| 3. |
|
|
| 4. |
- Fraver, Shawn
(författare)
-
Disturbance dynamics of old-growth Picea rubens forests of northern Maine
- 2005
-
Ingår i: Journal of Vegetation Science. - 1100-9233 .- 1654-1103. ; 16:5, s. 597-610
-
Tidskriftsartikel (refereegranskat)abstract
- Question: How have the spatial and temporal aspects of past disturbance shaped the current structure and composition of old-growth Picea rubens forests?Location: Northern Maine, USA.Methods: We established three 50 m x 50 m plots in old-growth Picea rubens forests and mapped the location of trees and saplings. We extracted increment cores from canopy trees, and recorded growth releases indicating past disturbance. By linking spatial data (tree positions) and temporal data (dated growth releases), we reconstructed the location and size of former canopy gaps back to 1920, and determined a more general disturbance chronology extending as far back as 1740.Results: We found no evidence of stand-replacing disturbances. The disturbance dynamic includes pulses of moderate-severity disturbances caused by wind storms and hostspecific disturbance agents (spruce budworm, spruce bark beetle) interposed upon a background of scattered smaller canopy gaps. Consequently, rates of disturbance fluctuated considerably overtime. Reconstructed canopy gaps were temporally and spatially scattered; during disturbance peaks, they were both larger and more numerous.Conclusions: Despite peaks in disturbance, several of which created relatively large gaps, this system has experienced no significant change in species composition. Instead, the shade-tolerant Picea rubens has maintained canopy dominance. The patch dynamics described here consist of dramatic structural, not compositional, changes to the forest. The persistence of Picea rubens is attributed to a combination of traits: (1) abundance of advance regeneration; (2) ability to endure suppression and respond favourably to release; and (3) longevity relative to ecologically similar species.
|
|
| 5. |
- Fraver, Shawn, et al.
(författare)
-
Identifying growth releases in dendrochronological studies of forest disturbance
- 2005
-
Ingår i: Canadian Journal of Forest Research. - 0045-5067 .- 1208-6037. ; 35:7, s. 1648-1656
-
Tidskriftsartikel (refereegranskat)abstract
- Information on historical disturbances is vital to our understanding of current forest conditions. Dendro chronological methods provide one means of reconstructing disturbance histories in temperate and boreal forests. In particular, the dates of significant growth releases recorded on surviving trees provide strong inferential evidence of past disturbance events. The most common method of detecting releases (the percent-increase method) expresses the postevent growth increase as a percentage of the preevent rate. Despite its widespread use, the method is known to be overly sensitive at low rates of prior growth and overly stringent at high rates. We present an alternative method that directly follows the percent-increase method, but instead of dividing the postevent growth rate by the preevent rate, we simply subtract the two. If the difference exceeds a predetermined species-specific threshold, the event is considered a release. This absolute-increase method has convenient properties that remedy the shortcomings of the percent-increase method. We tested the validity of the absolute-increase thresholds by binary logistic regressions, and we compared the absolute- and percent-increase methods by various methods. We conclude that for the species evaluated in this study, the absolute-increase method represents an improvement over the standard percent-increase method.
|
|
| 6. |
- Fraver, Shawn, et al.
(författare)
-
Refining volume estimates of down woody debris
- 2007
-
Ingår i: Canadian Journal of Forest Research. - 0045-5067 .- 1208-6037. ; 37:13, s. 627-633
-
Tidskriftsartikel (refereegranskat)abstract
- Down woody debris (DWD) plays a vital role in forest ecosystem structure and function. Although volume is likely the most common metric used to characterize DWD, an evaluation of the formulae used for volume estimation on individual DWD pieces has received little attention. We determined actual volume of 155 diverse DWD pieces (types, species, lengths, and diameters) by detailed field measurements. By comparing the actual and calculated volumes from six commonly used formulae, we assessed their bias, precision, and accuracy. Based on observed DWD forms, we developed a new formula, namely the “conic-paraboloid”, which was included in the assessment. Among the formulae that require length and two end diameter measurements, the conic-paraboloid had the lowest bias, highest precision, and hence greatest accuracy. Newton’s and the centroid formulae had higher accuracy yet require more field measurements. Smalian’s, conical frustum, and average-of-ends formulae had poor performance relative to the others. Accuracy of all formulae decreased with increasing piece length. Thus, partitioning pieces into two, three, and four sections for additional measurement improved accuracy. As decay advances, pieces become progressively more elliptical in cross section. Using the cross-sectional area derived from only the long axis of the ellipse leads to substantial volume overestimates for well-decayed DWD. [ABSTRACT FROM AUTHOR]
|
|
| 7. |
- Fraver, Shawn, et al.
(författare)
-
Tree growth and competition in an old-growth Picea abies forest of boreal Sweden : influence of tree spatial patterning
- 2014
-
Ingår i: Journal of Vegetation Science. - : Wiley. - 1100-9233 .- 1654-1103. ; 25:2, s. 374-385
-
Tidskriftsartikel (refereegranskat)abstract
- Question What factors best characterize tree competitive environments in this structurally diverse old-growth forest, and do these factors vary spatially within and among stands? Location Old-growth Picea abies forest of boreal Sweden. Methods Using long-term, mapped permanent plot data augmented with dendrochronological analyses, we evaluated the effect of neighbourhood competition on focal tree growth by means of standard competition indices, each modified to include various metrics of trees size, neighbour mortality weighting (for neighbours that died during the inventory period), and within-neighbourhood tree clustering. Candidate models were evaluated using mixed-model linear regression analyses, with mean basal area increment as the response variable. We then analysed stand-level spatial patterns of competition indices and growth rates (via kriging) to determine if the relationship between these patterns could further elucidate factors influencing tree growth. Results Inter-tree competition clearly affected growth rates, with crown volume being the size metric most strongly influencing the neighbourhood competitive environment. Including neighbour tree mortality weightings in models only slightly improved descriptions of competitive interactions. Although the within-neighbourhood clustering index did not improve model predictions, competition intensity was influenced by the underlying stand-level tree spatial arrangement: stand-level clustering locally intensified competition and reduced tree growth, whereas in the absence of such clustering, inter-tree competition played a lesser role in constraining tree growth. Conclusions Our findings demonstrate that competition continues to influence forest processes and structures in an old-growth system that has not experienced major disturbances for at least two centuries. The finding that the underlying tree spatial pattern influenced the competitive environment suggests caution in interpreting traditional tree competition studies, in which tree spatial patterning is typically not taken into account. Our findings highlight the importance of forest structure - particularly the spatial arrangement of trees - in regulating inter-tree competition and growth in structurally diverse forests, and they provide insight into the causes and consequences of heterogeneity in this old-growth system.
|
|
| 8. |
- Jönsson, Mari, et al.
(författare)
-
Eighteen years of tree mortality and structural change in an experimentally fragmented Norway spruce forest
- 2007
-
Ingår i: Forest Ecology and Management. - : Elsevier BV. - 0378-1127 .- 1872-7042. ; 242:2-3, s. 306-313
-
Tidskriftsartikel (refereegranskat)abstract
- Long-term experimental forest fragmentation studies remain uncommon, despite their critical role in the advancement of ecological theory and conservation planning. In 1986 five circular forest fragments (1/16-1 ha) were exposed through clearcutting within an old-growth Norway spruce (Picea abies) forest in northern Sweden. Initial responses to fragmentation (1986-1991) showed very high tree mortality and structural degradation of the fragments. In the present study we re-inventoried these fragments to evaluate tree mortality patterns and structural changes occurring over a longer time period (1991-2004). The fragments can readily be viewed as harvest retention patches or 'woodland key habitats' (i.e., set-aside patches of high conservation value), allowing us to make inferences about the effectiveness of these novel conservation tools. Tree mortality rates dropped markedly (to 1.2-3.9%/year) compared to the initial responses, yet remained elevated over those of control plots in the nearby unfragmented forest (0.7%). Mortality increased with tree diameter, resulting in smaller-diameter, more homogenous stands. Mortality also generally increased with decreasing fragment size and was dependent of tree location within fragments. Standing death (45% of dead trees, 1991-2004) replaced uprootings (71%, 1986-1991) as the dominant mode of mortality. Numbers of dying and standing dead trees increased during the second sampling period, further adding to structural change and reduced stand density. Elevated tree mortality resulted in uncharacteristically high volumes of coarse woody debris. Results clearly show that adverse edge-related changes to forest structure and function persist up to two decades after fragmentation. Fragments of this size largely fail as remnants intended to maintain forest interior conditions and late-successional forest structure. However, when embedded within a harvested landscape, they: (1) provide abundant coarse woody debris and snags for deadwood-dependent species that risk extirpation in the surrounding matrix and (2) retain important structures for the developing stands.
|
|
| 9. |
- Jönsson, Mari, et al.
(författare)
-
Forest history and the development of old-growth characteristics in fragmented boreal forests
- 2009
-
Ingår i: Journal of Vegetation Science. - : Wiley. - 1100-9233 .- 1654-1103. ; 20:1, s. 91-106
-
Tidskriftsartikel (refereegranskat)abstract
- Questions: Can small and isolated high-conservation value forests (e.g. designated woodland key habitats) maintain old-growth forest characteristics and functionality in fragmented landscapes? To what extent have past disturbances (natural and anthropogenic) influenced the development of old-growth characteristics of these forests? How long does it take for selectively cut stands to attain conditions resembling old-growth forests? Location: Southern boreal zone of central Sweden. Methods: We linked multiple lines of evidence from historical records, biological archives, and analyses of current forest structure to reconstruct the forest history of a boreal landscape, with special emphasis on six remaining core localities of high-conservation value forest stands. Results: Our reconstructions revealed that several of these stands experienced wildfires up to the 1890s; all had been selectively harvested in the late 1800s; and all underwent substantial structural and compositional reorganization over the following 100-150 years. This time interval was sufficient to recover considerable amounts of standing and downed dead wood (mean 60.3 m3 ha−1), a range of tree ages and sizes (mean basal area 32.6 m2 ha−1), and dominance of shade-tolerant spruce. It was insufficient to obtain clearly uneven tree age structures and large (>45 cm diameter) living and dead trees. Thus, these forests contain some, but not all, important compositional and structural attributes of old-growth forests, their abundance being dependent on the timing and magnitude of past natural and anthropogenic disturbances. Our landscape-level analysis showed marked compositional and structural differences between the historical forest landscape and the present landscape, with the latter having a greater proportion of young forests, introduction of non-native species, and lack of large trees and dead wood. Conclusions: The remnant high-conservation value stands were not true representatives of the pre-industrial forests, but represent the last vestige of forests that have regenerated naturally and maintained a continuous tree cover. These traits, coupled with their capacity for old-growth recovery, make them valuable focal areas for conservation.
|
|
| 10. |
- Jönsson, Mari T., et al.
(författare)
-
Spatio-Temporal Variation of Coarse Woody Debris Input in Woodland Key Habitats in Central Sweden
- 2011
-
Ingår i: Silva Fennica. - 0037-5330 .- 2242-4075. ; 45:5, s. 957-967
-
Tidskriftsartikel (refereegranskat)abstract
- The persistence of many saproxylic (wood-living) species depends on a readily available supply of coarse woody debris (CWD). Most studies ofCWD inputs address stand-level patterns, despite the fact that many saproxylic species depend on landscape-level supplies of CWD. In the present study we used dated CWD inputs (tree mortality events) at each of 14 Norway spruce (Picea abies) dominated woodland key habitat sites to analyze the spatial and temporal patterns of CWD additions between 1950 and 2002 within a small landscape in central Sweden. We found that inputs were episodic within sites, where local windstorms created pulses in CWD input. Pulses occurred simultaneously in many sites, yielding landscape-level synchrony of CWDinput. These synchronous pulses, and importantly, the breaks between pulses, may have negative implications for saproxylic species that are dependent on large volume inputs of freshly killed Norway spruce. In addition, the inherent small size and relative isolation of these sites may further increase extinction risks due to stochastic events. However, background CWD input rates occurring between pulses varied substantially among sites, presumably the result of the sites' varied histories and structural characteristics. This finding suggests that the different sites have varied abilities to provide habitat for saproxylic species during periods with low landscape-level input of CWD.
|
|
