Radial growth response of four dominant boreal tree species to climate along a latitudinal gradient in the eastern Canadian boreal forest

To address the central question of how climate change influences tree growth within the context of global warming, we used dendroclimatological analysis to understand the reactions of four major boreal tree species –Populus tremuloides, Betula papyrifera, Picea mariana, and Pinus banksiana– to climatic variations along a broad latitudinal gradient from 46 to 54°N in the eastern Canadian boreal forest. Tree‐ring chronologies from 34 forested stands distributed at a 1° interval were built, transformed into principal components (PCs), and analyzed through bootstrapped correlation analysis over the period 1950–2003 to identify climate factors limiting the radial growth and the detailed radial growth–climate association along the gradient. All species taken together, previous summer temperature (negative influences), and current January and March–April temperatures (positive influences) showed the most consistent relationships with radial growth across the gradient. Combined with the identified species/site‐specific climate factors, our study suggested that moisture conditions during the year before radial growth played a dominant role in positively regulating P. tremuloides growth, whereas January temperature and growing season moisture conditions positively impacted growth of B. papyrifera. Both P. mariana and P. banksiana were positively affected by the current‐year winter and spring or whole growing season temperatures over the entire range of our corridor. Owing to the impacts of different climate factors on growth, these boreal species showed inconsistent responsiveness to recent warming at the transition zone, where B. papyrifera, P. mariana, and P. banksiana would be the most responsive species, whereas P. tremuloides might be the least. Under continued warming, B. papyrifera stands located north of 49°N, P. tremuloides at northern latitudes, and P. mariana and P. banksiana stands located north of 47°N might benefit from warming winter and spring temperatures to enhance their radial growth in the coming decades, whereas other southern stands might be decreasing in radial growth.     

Tree species dynamics and disturbance in three Swedish boreal forest stands during the last two thousand years

Abstract. Data from three forest stands for the past 2000 yr show how the shade-intolerant species Pinus sylvestris and Betula pubescens maintain significant populations in the Swedish boreal landscape. Age structure data from a northern stand close to the range limits of Picea abies and Pinus complement a local pollen diagram, and reveal cyclic population fluctuations which can be related to periods of climatic stress and fire. Pollen data from two southern stands show that high fire frequencies in the past prevented the expansion of Picea populations. Reduction of the fire frequency during the last 200 yr has favoured Picea. A long time perspective reveals the population dynamics of long-lived species and indicates the controlling factors. Such knowledge permits assessment of the current status and likely future of forest stands.     

Dynamics of Salix caprea L. populations during forest regeneration after strong herbivore pressure

Abstract. Broadleaved forest communities degenerated through strong pressure from large herbivores. Relief of this pressure led to regeneration, in particular of Salix caprea and other light-seeded pioneer trees: Populus tremula, Betula pendula and B. pubescens. This regeneration proceeded following conservation protection of degenerate stands in a nature reserve and later in Biaowiéza National Park. The emergence and development of the Salix caprea population proceeded following the expansion of Picea abies, which coincided with the period of enhanced animal pressure on the broadleaved forest. Salix caprea filled all the gaps in the tree stand after the destruction of trees and undergrowth by herbivores (in the years 1892–1915). The species also appeared abundantly in old, at the time unforested, clearings and felled areas. Here, S. caprea developed large populations with certain trees in good condition, with a growth form typical of forest trees and attaining considerable heights. The majority of trees were 50 - 60 yr old at the time of death, although some individuals reached 74 yr of age. The process of extinction of the Salix population — observed over 19 yr on permanent plots with marked trees — proceeded very quickly, especially in the first decade of observation. It led to the almost complete disappearance of S. caprea from the forest communities of Biaowiéza National Park. The death of individual trees is preceded by impairment of their health and reduced annual increments in the last 4–9 years of their life. The development of populations of permanent constituents of the forest, notably Carpinus betulus, Tilia cordata, Acer platanoides and Ulmus glabra, under the canopy of light-seeded trees, and the absence of a new generation of pioneer trees points to the end of the process of regeneration in the forest communities of BiaHwiéza National Park.     

Patch colonisation in a savanna grassland

Patches (1 m diameter) were cleared in a heavily grazed and a lightly grazed savanna in South Africa; half of them were covered once with grass litter. The greatest colonisation over three years occurred on heavily grazed patches with litter, the least on open, lightly grazed patches. Annual rainfall affected colonisation rate. Basal cover was lower on patches than in the surrounding vegetation. Patch composition was weakly affected by treatment, and was partly related to the composition of adjacent vegetation, but the proportional representation of species on patches differed from the surrounding vegetation. The stoloniferous Digitaria eriantha and the obligate seed reproducers Aristida bipartita and Heteropogon contortus were major components of patch communities but were uncommon in the surrounding vegetation and in the seed bank. Setaria incrassata and Themeda triandra were the predominant components of the surrounding vegetation and of the seed bank, but Setaria established very poorly and Themeda established well only on heavily grazed patches with litter. The pattern of seedling establishment was the same as that of colonisation. Most seedlings emerged atthe beginning of the wet season, with ca. 50% mortality soon thereafter. The colonising species can be partly predicted from the availability of propagules (vegetative or seed), emergence and establishment success, and subsequent growth.     

Vegetation and soil seed bank of successional stages in tropical lowland deciduous forest

Abstract. We studied vegetation structure and soil seed bank composition in different successional stages of secondary lowland tropical deciduous forest in Yucatán, Mexico. The series of study sites includes: slashed (S), slashed-and-burned (SB), and regenerating for 1, 6, 10, 15, 30, 40 and 100 yr. Species richness (S = 42 - 65), evenness (E = 0.32 - 0.38), and diversity (H' = 1.2 - 1.6) do not vary much as the forest grows older. 20 species of shrubs and trees were present in at least six of the seven regrowth years studied; 10 of these account for more than 50% of the total density values per regrowth year. These species dominate the vegetation due to their capacity to withstand repeated fire and felling. One third of the individuals sampled had regenerated from coppiced shoots. Species composition little resembles that in earlier accounts. The area is now largely covered by young regrowth stages (1 - 20 yr). Species constituting the original woody structure of the mature forest are rare or absent due to the lack of seed sources and failure of dispersal (which is due to limited dispersal capacities), lack of dispersal agents, or long distances. Herbs were the most important life form in the soil seed banks; only one tree species was found. The number of viable seeds varied between sampled areas: 70/m² in the 40 yr-old, to 1 815/m² in the slashed-and-burned (SB). The vegetation of S- and SB-areas was the same, but the number of viable seeds germinating in SB was twice the number in S; the number of species in the seed bank is the same for both areas. We speculate that fire modifies species dominance early in succession, allowing seeds of some species to germinate in great numbers.