François Lebourgeois

Life strategies in intra-annual dynamics of wood formation: example of three conifer species in a temperate forest in north-east France.

We investigated whether timing and rate of growth are related to the life strategies and fitness of three conifer species. Intra-annual dynamics of wood formation, shoot elongation and needle phenology were monitored over 3 years in five Norway spruces (Picea abies (L.) Karst.), five Scots pines (Pinus sylvestris L.) and five silver firs (Abies alba Mill.) grown intermixed. For the three species, the growing season (delimited by cambial activity onset and cessation) lasted about 4 months, while the whole process of wood formation lasted 5-6 months. Needle unfolding and shoot elongation followed the onset of cambial activity and lasted only one-third of the season. Pines exhibited an extensive strategy of cambial activity, with long durations but low growth rates, while firs and spruces adopted an intensive strategy with shorter durations but higher growth rates. We estimated that about 75% of the annual radial increment variability was attributable to the rate of cell production, and only 25% to its duration. Cambial activity rates culminated at the same time for the three species, whereas shoot elongation reached its maximal rate earlier in pines. Results show that species-specific life strategies are recognizable through functional traits of intra-annual growth dynamics. The opposition between Scots pine extensive strategy and silver fir and Norway spruce intensive strategy supports the theory that pioneer species are greater resource expenders and develop riskier life strategies to capture resources, while shade-tolerant species utilize resources more efficiently and develop safer life strategies. Despite different strategies, synchronicity of the maximal rates of cambial activity suggests a strong functional convergence between co-existing conifer species, resulting in head-on competition for resources.

Spatial variation and temporal instability in climate-growth relationships of sessile oak (Quercus petraea [Matt.] Liebl.) under temperate conditions

Temporal instability of forest climate-growth relationships has been evidenced at high elevations and latitudes, and in Mediterranean contexts. Investigations under temperate conditions, where growth is under the control of both winter frost and summer water stress, are scarce and could provide valuable information about the ability of forest to cope with climate change. To highlight the main climatic factors driving the radial growth of Quercus petraea forests and to detect their possible evolutions over the last century, dendroecological analyses were performed along a longitudinal gradient of both decreasing summer water stress and increasing winter frost in northern France (from oceanic to semi-continental conditions). The climate-growth relationships were evaluated from 31 tree-ring chronologies (720 trees) through the calculation of moving correlation functions. Q. petraea displayed a rather low sensitivity to climate. High temperature in March and water stress from May to July appeared to be the main growth limiting factors. The sensitivity to winter precipitation and summer water stress decreased from oceanic to semi-continental conditions, whilst the correlation to winter frost tended to increase. Moving correlations revealed a general instability of climate-growth relationships, with a moderate synchronicity with climatic fluctuations. The main changes occurred during previous autumn for both temperature and precipitation whilst climatic trends were rather low or non-significant. The most coherent trends were pointed out (i) in April with a cooling (−0.9°C) leading to positive correlation to temperature at the end of the century, and (ii) in July with a decreasing inter-annual variability of precipitation resulting in a loss of correlation. On the contrary, the decreasing temperature and increasing precipitation in May and June led to few significant changes climate-growth relationships.

Stand density, tree social status and water stress influence allocation in height and diameter growth of Quercus petraea (Liebl.)

Even-aged forest stands are competitive communities where competition for light gives advantages to tall individuals, thereby inducing a race for height. These same individuals must however balance this competitive advantage with height-related mechanical and hydraulic risks. These phenomena may induce variations in height-diameter growth relationships, with primary dependences on stand density and tree social status as proxies for competition pressure and access to light, and on availability of local environmental resources, including water. We aimed to investigate the effects of stand density, tree social status and water stress on the individual height-circumference growth allocation (Δh-Δc), in even-aged stands of Quercus petraea Liebl. (sessile oak). Within-stand Δc was used as surrogate for tree social status. We used an original long-term experimental plot network, set up in the species production area in France, and designed to explore stand dynamics on a maximum density gradient. Growth allocation was modelled statistically by relating the shape of the Δh-Δc relationship to stand density, stand age and water deficit. The shape of the Δh-Δc relationship shifted from linear with a moderate slope in open-grown stands to concave saturating with an initial steep slope in closed stands. Maximum height growth was found to follow a typical mono-modal response to stand age. In open-grown stands, increasing summer soil water deficit was found to decrease height growth relative to radial growth, suggesting hydraulic constraints on height growth. A similar pattern was found in closed stands, the magnitude of the effect however lowering from suppressed to dominant trees. We highlight the high phenotypic plasticity of growth in sessile oak trees that further adapt their allocation scheme to their environment. Stand density and tree social status were major drivers of growth allocation variations, while water stress had a detrimental effect on height in the Δh-Δc allocation.

Radial growth resilience of sessile oak after drought is affected by site water status, stand density, and social status

Key messageTree resilience to drought was higher in drier sites and lower for suppressed trees grown in higher density stands, highlighting the role of acclimation and selection in tree responses to drought.AbstractOngoing climate change will drive more frequent drought events in the future, with potential impacts on tree community structure and functioning. Growth responses of tree communities may depend on their past water status and on competition pressure. We investigated the effects of site water status, population density, and tree social status on tree growth resistance and resilience following the severe drought of 1976 in even-aged stands of sessile oak (Quercus petraea). We used retrospective growth data collected in permanent plots experiencing contrasted climatic and stand density conditions. We used boosted regression trees to calibrate a tree growth model over 1960–1975, which was then used to provide a baseline of expected tree growth following 1976. Growth dynamics during and after 1976 was examined using the ratio between observed and expected growths over 1976–1983. Tree radial growth was on average 0.6 times its expected values in 1976 and was still 0.63 times its expected value in 1977. Despite experiencing higher summer soil water deficit in 1976, trees growing in drier sites exhibited remarkably faster growth recovery than those in moister sites. Suppressed trees grown in higher density stands recovered their normal growth rate slower than dominant trees. Forest growth is evidenced to be more vulnerable to drought in moister than in drier sites. Competitive pressures also alter tree capacity to recover from a severe drought, accelerating suppression of smaller trees in high-density stands. These results highlight the role of acclimation and selection processes in tree community responses to present and future climates.

Chapter 9 - Tree Phenology

The chapter describes methodologies for harmonized phenological assessments based on a limited set of development phases: flushing, flowering, secondary flushing, color change, and leaf/needle fall. Manual phenological observations are based on a brief examination in the forest stands. More recently, the use of terrestrial digital image photography for forest phenology monitoring has been adopted. Vegetation indices, such as the normalized difference vegetation index (NDVI) have been used for many years to quantify the phenology of different ecosystems. For satellite-based remote sensing of vegetation phenology, phenological metrics are derived from time series of optical data and represent the only possible assessment of phenology over large and inaccessible regions. All indirect methods using optical vegetation indices from digital camera or NDVI sensors need to be validated against ground observations, for which manual tree phenological observations from the forest monitoring plots are often used. Examples from phenological monitoring in Slovenia, France, United Kingdom, and Finland are presented.