Delpierre, N., Guillemot, J., Dufrêne, E., Cecchini, S., and Nicolas, M., 2017. Tree phenological ranks repeat from year to year and correlate with growth in temperate forests. Agric. For. Meteorol. 234-235, 1-10. https://doi.org/10.1016/j.agrformet.2016.12.008
Leaves are arguably the most important anatomical component of a tree. They allow for atmospheric exchange of carbon and water and energy capture from sunlight. In fact almost every metric of tree growth and productivity can be traced back to leaf area through the tight relationship between leaf area and photosynthetic capacity. In my previous post I discussed some potential changes in forest productivity under various climate change scenarios. Increases in productivity were in large part due to the potential for longer growing seasons for temperate deciduous forests (i.e. forests that inhabit mid latitudes and go through a dormant period in the winter). Longer growing seasons equate to increases in the amount of time that trees are photosynthetically active and thus producing biomass. Growing season length in temperate deciduous forests is a function of both the timing of spring leaf flush (when the leaves appear on the trees) and fall leaf senescence (when the leaves change colors and drop). But what exactly are trees responding to when deciding it is time to grow or drop their leaves?
Large Scale Trends
The general trend of deciduous tree phenology (a fancy term for the study of cyclic and seasonal natural phenomena) is thought to be controlled by temperature and photoperiod or the amount of daylight that the tree experiences. If you look at satellite imagery of North America during the spring and fall you will notice the slow progression of seasonal change as it migrates north in the spring and south in the fall (Fig. 2). At this spatial scale it is evident that temperature and photoperiod are the main controls on seasonal change. However questions still remain about the drivers of seasonal change on a more local scale. In mixed species hardwood forest stands, there are substantial differences in the timing of leaf flush and senescence between individuals both within the same species (intraspecific) and between different species (interspecific). When summed up across large geographic areas such as the eastern United States, this small scale variability in growing season length adds up to a large amount of uncertainty in predicting carbon and water cycling in trees through computer simulation. Thus understanding the influences of this small scale variability helps us to predict how trees will manage their future carbon, water, and, nutrient budgets. The main question is, what might be causing this small scale variability?
Small Scale Variability
Within a forest stand there can exist small scale differences in environmental variables. For instance, there is evidence that temperatures accumulate more rapidly (i.e. it gets warmer quicker in the spring) in the understory as compared to the canopy. This may lead to juvenile trees having earlier leaf out times as compared to adult trees within the same forest (Augspurger and Bartlett 2003). A new study, Delpierre et al. (2017), found that differences in soil moisture availability on a fine scale (~10 m) may play a role in influencing spring leaf on and fall senescence variability within a forest. This is one of few studies that have concluded that soil moisture may play a role in influencing forest phenological cycles. By making the assumption that phenological ranking (the order in which individual trees leaf on and leaf off) within a stand is largely genetically determined and thus significantly consistent from year to year, Delpierre et al. (2017) asked the question whether interannual variability in phenological ranks was correlated with soil moisture. In other words, within a given stand of trees, does the ordering of tree leaf on and leaf off across comparable individuals (similar height and health status) change when soil moisture conditions are unfavorable for that species?
The role of water
This study used data collected from 52 deciduous forests across France from 2009 to 2013 on the timing of leaf unfolding and leaf senescence for three dominant tree species: European Beech (Fagus sylvatica), Sessile Oak (Quercus petraea), and Common Oak (Quercus robur). To evaluate the effect of soil moisture on phenological rank, the study used a mathematical model (CASTANEA) to simulate soil water content for the top 30 cm of soil. The study found that there was a positive correlation between rank repetition and soil moisture availability for Common Oak and European Beech whereas there was a negative correlation between the two variables for Sessile Oak. In other words, during a year of average precipitation, phenological rankings were consistent but during abnormal years (either less soil moisture than average or more soil moisture than average) there was a higher amount of deviation in phenological ranks. Common Oak and European Beech had larger deviations in phenological rank during dry years whereas Sessile Oak had larger deviations during wet years. This may be explained by the fact that Common Oak and European Beech are more sensitive to water limitation whereas Sessile Oak is more sensitive to water logging.
In addition to Delpierre et al. (2017), other studies have also found that moisture may play a role in driving phenophases or the timing of leaf on and leaf off. Fu et al. (2014) found that more precipitation in the winter increases the amount of heat required to break dormancy in the spring. Other studies have found conflicting conclusions about the role of moisture in driving fall senescence timing. Hwang et al. (2014) found that a drier late growing season is associated with earlier leaf senescence at low elevation sites while Xie et al. (2015) found that drought stress during the growing season delayed dormancy. It is clear that water status may play a larger role in determining when trees decide to grow/drop their leaves. Although we currently lack a robust modeling framework to predict fall senescence, most predictive models of spring leaf on include only the degree of winter cooling, the amount of heating in late winter and spring, and photoperiod parameters. Including some type of metric regarding water status such as soil moisture content may improve these models predictive power especially at smaller spatial scales. Water may be more important than we think but now we need to determine its exact role in controlling phenology.
Augspurger, C.K., Bartlett, E.A., 2003. Differences in leaf phenology between juvenile and adult trees in a temperate deciduous forest. Tree Physiol. 23, 517–525.
Fu, Y.H., Piao, S., Zhao, H., Jeong, S.J., Wang, X., Vitasse, Y., Ciais, P., Janssens, I.A., 2014a. Unexpected role of winter precipitation in determining heat requirement for spring vegetation green-up at northern middle and high latitudes. Glob. Change Biol. 20, 3743–3755,
Hwang, T., Band, L.E., Miniat, C.F., Song, C., Bolstad, P.V., Vose, J.M., Love, J.P., 2014.Divergent phenological response to hydroclimate variability in forested mountain watersheds. Glob. Change Biol. 20, 2580–2595.
Xie, Y., Wang, X., Silander, J.A., 2015. Deciduous forest responses to temperature, precipitation, and drought imply complex climate change impacts. Proc. Natl. Acad. Sci. U. S. A. 112, 13585–13590.