The quantification of cambial growth over short time periods has been hampered by problems to discern between growth and the swelling and shrinking of a tree stem. This paper presents a model, which ...separates cambial growth and reversible water‐potential induced diurnal changes from simultaneously measured whole stem and xylem radial variations, from field‐measured Scots pine trees in Finland. The modelled growth, which includes osmotic concentration changes, was compared with (direct) dendrometer measurements and microcore samples. In addition, the relationship of modelled growth and dendrometer measurements to environmental factors was analysed. The results showed that the water‐potential induced changes of tree radius were successfully separated from stem growth. Daily growth predicted by the model exhibited a high correlation with the modelled daily changes of osmotic concentration in phloem, and a temperature dependency in early summer. Late‐summer growth saw higher dependency on water availability and temperature. Evaluation of the model against dendrometer measurements showed that the latter masked a true environmental signal in stem growth due to water‐potential induced changes. The model provides better understanding of radial growth physiology and offers potential to examine growth dynamics and changes due to osmotic concentration, and how the environment affects growth.
•Environment-induced volume increment increase was 8.98million m3a−1.•The increase equals to 37% of the total observed volume increment increase.•The increase was larger in the northern regions (up ...to 45% of increment change).•A larger increase 63% was attributed to growing stock volume and forest structure.•Increase in temperature sum coincides with environment-induced increment increase.
The annual growth of the forests of Finland has more than doubled in less than a century. While the increased growing stock and more efficient silviculture have contributed to the observed growth increase, there are also solid grounds for assuming that environmental changes have played a role. The aim of our study was to analyze and quantify the magnitude of changes of volume, basal area and height increment not attributable to changes in growing stock, forest structure and silvicultural practices.
We used the extensive data from National Forest Inventories during 1971–2010 to develop models for volume, basal area and height increment of individual trees on mineral soils without ditching or paludification with tree and stand characteristics as predictor variables. Differences between the measured and predicted increment were used to detect environment-induced increment changes. Then, we estimated the average changes of volume increment per hectare and totals in millions of m3. Using this approach it was also possible to approximate the enhancing effects of volume increment change in growing stock volume.
From 1971–1975 to 2006–2010, the environment-induced volume increment increase was estimated to be 8.98 million m3a−1 (0.69m3ha−1a−1), which equals to 37% of the total observed volume increment increase. In relative terms the environment-induced increment increase was larger in the northern regions (up to 45% of volume increment change). During 1971–1990, the difference between the observed and predicted change was small. A large shift was observed after the mid-1990s in all regions. While the environment-induced increment change was substantial, a considerably larger increase representing 63% of the change was attributed to growing stock volume and forest structure, which both changed due to differences in forest management.
A comparison between the environment-induced increment changes and growing season temperature sums revealed similarities. In the southern Finland, April-May seemed to be influential, while in the north the temperature sum of May-September showed similar variation. As climate change is predicted to increase growing season temperatures, the trend can be expected to continue in the boreal conditions of Finland.
•Tracheid production initiated later in the north than at more southern sites.•There were no differences in temperature sum by the onset of tracheid production.•Tracheid production ceased earlier and ...at a lower temperature sum in the north.•Trees adjusted their tracheid production according to the local conditions.•Temperature mainly regulates the timing of tracheid production in boreal forests.
Considerable changes in tree growth are projected due to the expected climate change. The expected changes of climate call for a better insight into the growth responses of trees to varying environmental conditions over large geographical regions. The aim of this study was to analyse the intra-annual tracheid production of Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) across a latitudinal gradient in Finland (60–68°N). The number of tracheids and the day of the year for the onset, fastest rate, and cessation of tracheid production were determined from microcores repeatedly collected in nine stands during growing seasons of 2001–2009. The onset of tracheid production varied from late May in southern Finland to mid-June in northern Finland. On all stands, tracheid production initiated earlier and ceased later for Scots pine than for Norway spruce. On average, the fastest tracheid production rate occurred slightly after the summer solstice, but variation between sites and years was high. In the northernmost Scots pine stand, the length of the growing season was less than two months and the onset of tracheid production required clearly lower TS than elsewhere. The results imply that within Finland, year-to-year weather variation has a marked impact on the timing of tracheid production. However, the results indicate that the Norway spruce and Scots pine have adapted and are able to adjust their tracheid production according to the local conditions.
Key message
Onset and cessation of radial and height increment of Norway spruce
(
Picea abies
(L.) Karst.) and Scots pine
(
Pinus sylvestris L.
)
in southern Finland were independent phenomena. They ...both contributed to the increment period duration, which was a more crucial factor defining the magnitude of annual radial and height increment.
Context
Phenology of diameter and height increment is a critical component of growth, also contributing to damage and survival of trees.
Aims
We quantified annual variation in intra-annual tracheid production and height increment of Norway spruce (
Picea abies
(L.) Karst.) and Scots pine (
Pinus sylvestris L.
).
Methods
The number of tracheids and the day of the year for the onset and cessation of tracheid production were monitored from microcores collected repeatedly during growing seasons 2001–2012 in southern Finland. Weekly height increment was also measured in an adjacent sapling stand in 2008–2012.
Results
The first tracheids in pine were found around mid-May and in spruce a week later. The cessation of the tracheid production occurred during the last week of August for both tree species. Increment onset and cessation were independent phenomena, both contributing to the magnitude of tracheid production via increment period duration, which appeared to be a more crucial factor defining the number of tracheids. Duration of the height increment period was also related to shoot length but the connection was less tight than the link between the duration of tracheid production and the number of tracheids. A thermal threshold around 100 d.d. (degree days) was found for the onset of radial increment. No single environmental factor triggered the cessation of tracheid production, but in some years, soil water availability appeared to play a role.
Conclusion
The results indicate that extending growing seasons due to the climatic warming may increase growth in the Finnish forests.
•Long-term increase in forest growth in Finland mainly due to improved forest structure.•Environmental factors induced a significant share (20 – 31 %) of the growth increase.•The recent growth ...reduction mainly caused by environmental factors.
After a rising trend for 1971 – 2013, during which the annual volume growth of the forests of Finland increased by more than 70 %, a recent reduction has been observed. We analyzed the development of annual growth in the forest of Finland, focusing on the component not explainable by changes in growing stock. The data originate from nine consecutive Finnish National Forest Inventories. In the data, diameter increments were measured from increment cores and tree height increments from standing sample trees in the field. We developed models predicting periodic (5 years) annual volume increment per hectare with properties of the trees and stands as predictor variables. Deviations from model-predicted values were interpreted to be induced by environmental variation. The development was analyzed for all tree species combined and separately for three species groups: Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.) and broadleaves. We observed a rising growth trend not solely explainable by increased growing stock. The species groups produced rather a similar pattern in different parts of Finland: from the 1960s to the mid-1990s, the observed volume increment was mainly below the model-predicted level, thereafter above it. During the current century, the difference between observed and predicted annual volume increment has shown a downward trend for Scots pine. For Norway spruce, the difference has continued to increase in southern Finland, but shows little change in the north. For broadleaved species, the difference between measured and predicted increment shows a recent increase as well, though not as large as for Norway spruce. The geographical pattern of the environment-induced increment component was described in more detail via maps using a 75 km × 75 km grid. The changing environment has enhanced forest productivity in Finland over a period of nearly six decades, but recent years have not been favorable for Scots pine, which represents 50 % of the growing stock volume of the forests of Finland.
Nitrogen is the nutrient mainly limiting forest growth on mineral soil sites in the boreal regions. The objective of this study was to find out the response of stem wood N to repeated fertilizations ...and to find out their long-lasting effects on soil organic matter composition, focusing on C and N cycling processes and concentrations of condensed tannins. The site was located in a relatively unfertile Scots pine (
Pinus sylvestris
L.) stand in eastern Finland. The treatments were three levels of N fertilization (0, 150, 300 kg/ha) applied four times at 5-year intervals with the last addition 29 years ago. The N additions had not changed the pH of the humus layer but resulted in higher availability of N. The C-to-N ratio of organic matter decreased with increasing N addition. The treatment of 300 kg/ha increased the net N mineralization rate and the ratio of net N mineralization/microbial biomass N and decreased the amount of C in the microbial biomass and its C-to-N ratio and the concentration of condensed tannins. Net nitrification and extractable nitrate were negligible in all soils. In soil diffusive fluxes, NH
4
-, NO
3
- and amino acid-N were all detected by in situ microdialysis sampling; the results showed large variation but supported higher N availability in N fertilized soil. The N fertilization increased tree-ring widths and the effect lasted for about 10 years after the last fertilization event. Nitrogen content and the N isotopic ratio
15
N/
14
N (
δ
15
N) in tree-rings increased both after the first N addition in the treatment of 300 kg/ha. In conclusion, soil properties still indicated higher N availability in the N fertilized soil after three decades since the latest fertilization, but the response of tree diameter growth had faded out after a much shorter period.
•N deposition in 1951–2020 totaled 120–470 kg/ha, i.e., annual average 1.8–6.7 kg/ha.•Soils in pine stands were similar, except decreasing N status with the lowest deposition.•Tree ring δ15N was ...different between the lowest and highest N deposition sites.•δ15N in 1951–2020 correlated negatively with total-, ammonium- and nitrate-N depositions.•Temporal development of δ15N and N deposition coincided only on some sites.
Since nitrogen (N) is the main forest-growth limiting nutrient in the boreal region, atmospheric N deposition may be an important source of available soil N. The objective of the study was to determine whether the variation in N deposition is reflected in the stem wood N and in its isotopic signatures (variation of 15N/14N ratios relative to atmospheric N2, reported as δ15N values), as well as in current soil properties with a special focus on N cycling. The study material consisted of twelve Scots pine (Pinus sylvestris L.) stands located along the N deposition gradient from south to north in Finland, representing similar site types with relatively unfertile and N-limited soils. Tree-ring N from 5-year segments, spanning a period of 70 years (1951–2020), were examined. Based on the modelled open-place deposition amounts, annual N deposition increased until around 1990 up to 9 kg/ha/year at the southernmost site and 1 kg/ha/year in the north but decreased substantially thereafter. The deposition of N totaled 472 kg/ha at the southernmost and 123 kg/ha at the northernmost site during the 70-year period. δ15N values in tree rings varied between −5.8 and + 1.3 and were higher at the northern than at the southern sites. Tree-ring δ15N ratio correlated negatively with total N, nitrate-N and ammonium-N depositions. The negative correlation still existed when stand age was used as a controlling factor. The correlation also remained negative when the dataset was divided into periods of generally increasing (1951–1990) and decreasing (1991–2020) deposition, or over 30-year age classes, except for the oldest age class (>90-year-old stands). Humus layer pH did not vary much between the sites, but slight signs of decreasing N status existed from south to north. The temporal development of tree-ring δ15N ratio and the amount of N deposition coincided only on some sites after the effect of stand age was controlled. In conclusion, although climate effects cannot be totally excluded, given the same tree species, mycorrhizal type (ectomycorrhiza), site and soil factors, the N isotopic ratios in tree rings distinguished the highest and the lowest N deposition levels. Within small differences in N deposition other factors affecting the ratio may dominate and tracing of the N input from the past is questionable.
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•Large trees have increased in Finland during 1921–2013.•In southern boreal subzone, the densities of large trees are today more than twice as high as in the other subzones.•Old trees ...have not increased during 1971–2013.•Old trees are far more common in the northern boreal subzone.•Densities of trees that are both large and old have increased.
Large and old trees have a vital role in preserving biodiversity in forest ecosystems. We used National Forest Inventory data from 1921 to 2013 for studying changes in densities (stems per ha) of large trees (diameter ≥40 cm) in Finland. In addition, densities of old trees (age ≥150 years) are reported from 1971 to 2013. We present results separately for the three subzones of the boreal biogeographical zone. Large trees have increased as much as 325%. The change has occurred mainly since the 1970s. On country level, old trees have become slightly less common (−4%) since the 1970s, although a decrease was actually observed only in the northern boreal subzone. The large majority of old trees in Finland are quite small in diameter, however. Trees that are both large and old show a notable increase from 1971 to 2013. During the 2010s, densities of large trees were higher in the southern boreal subzone than in the northern boreal subzone, but in the 1920s the opposite was true. Densities of old trees have been much higher in the northern boreal subzone. The observed densities of large trees are still considerably smaller than those observed in unmanaged old-growth forests in Scandinavia. High densities of large and/or old trees were observed in areas with restrictions on wood production emphasizing their role in maintaining biodiversity. The results reflect the destructive effects of former land use and the transition from dimensional cuttings to clear cuts and thinning from below after the 1940s. Proportionally larger changes were observed for southern Finland, where a higher human population density and the resulting more intensive land use had more severe detrimental effects on forests. As the densities of large trees and old trees have developed in a completely different manner in Finland, our results suggest that monitoring only the size distribution of trees will not sufficiently describe the role of old trees as constituents of biodiversity. Thereagainst, densities of large trees and large old trees developed in a similar manner.
•We studied provenance differences in growth responses to climate in Norway spruce.•We used tree-ring data from old provenance tests established already in the 1930s.•Main differences were found ...between the central European and Finnish provenances.•Provenances differed in their growth responses to winter and spring temperatures.•Responses to growing season weather did not show significant provenance differences.
Changing climate is expected to cause range shifts and reduced growth in Norway spruce (Picea abies (L.) Karst). In order to mitigate these changes, genetic variation between populations can be utilized in selecting alternative tree origins that are better suited to the new conditions. The aim of this study was to examine the intraspecific differences in the climatic drivers of radial growth in Norway spruce. We used tree-ring data from seven Norway spruce provenance experiments in Finland, located in different climatic conditions and including a large variety of provenances. The annual ring-width indices were studied with hierarchical clustering, correlation analysis with climate variables, pointer year analysis and linear models to identify the provenance differences in growth variation and its climatic control, and compare them on a latitudinal gradient. The cluster analysis revealed patterns of provenance differences in growth variation: north European and central European provenances were grouped in separate clusters within sites, although with some exceptions. Largest provenance differences in climate-growth responses were found in relation to winter and spring temperatures. In the southern provenances warm winters were typically associated with faster growth whereas for the northern provenances the correlations varied from non-significant to negative. In addition, the pointer year analysis showed negative growth anomalies only in the southern provenances for years with exceptionally cold winters. These patterns may reflect the physiological differences between the provenances relating to, for example, cold tolerance and the timing of spring phenology. As the climate warming in Europe is predicted to be strongest during the winter months, acknowledging the intraspecific growth responses to climate in Norway spruce becomes increasingly important.
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