As the leaves fall this October and the canopies bare their skeletal limbs, there’s suddenly more light filtering across the riverside trails in Maine and I’m wearing sunglasses on runs where I used to be totally engulfed in the shade. It’s hot toddy season, pumpkin spice season, submit-your-GRFP season. When the weather finally chills we’ll get into ugly sweater season, rush-to-take-family-photos-for-a-holiday-cards season, and grading-endless-finals season. Culturally, we humans divide the year into more than just autumn-winter-spring-fall. A recent PLOS ONE paper makes the case that understory plants probably do this too.
Janice Hudson and her coauthors explored the seasonal dynamics of sunlight in a temperate deciduous forest and the ecology of the common shade-tolerant shrub, spicebush. They were inspired, in part, by a relatively obscure 1977 Ecological Monographs paper* with the unassuming title “The Distribution of Solar Radiation within a Deciduous Forest,” in which the authors, Boyd A. Hutchison and Detlef R. Matt, outline the concept of phenoseasons.
Get ready to update your calendars — the seven phenoseasons for life under a forest canopy are: winter leafless, spring leafless, spring leafing, summer leafing, summer fully-leafed, autumnal fully-leafed, autumnal partially-leafed. I only wish that Hutchison and Matt had dined with Tolkien. Imagine the invitation: “Let’s meet for second breakfast to celebrate the end of spring leafless.” [Insert ent joke here.] Hudson was interested in how changes in light availability affected understory plants like spicebush. As Hudson explains “broadly, this study was an attempt to better understand the pre-existing conditions of the forest…[are] light conditions...a controlling factor in the distribution and presence of plant species?” The phenoseason construct hasn’t taken off in ecology and the annual cycle of subcanopy light exposure is not well understood. Hudson and her coauthors stumbled on Hutchinson and Matt while working on a literature review, but the idea of phenoseasons — now update-able with a high-tech piece of equipment called line quantum sensors — seemed ecologically intriguing. Hudson’s background is in eco-hydrology and the link between seasonal changes in light and phenology had immediate implications for her. She wanted to know “how understory plants acclimate…[and] plant contributions to nutrient and water cycling during individual phenoseasons, and yet, the literature on the subject of phenoseasons is scant.”
Hudson’s team combined a year of intense field measurements with experimentally manipulated light conditions in growth chambers to explore light intensity through the phenoseasons. At Fair Hill Natural Resource Management Area in Maryland, Hudson and her team carried a light sensor through the forest of American beech and yellow poplar trees to measure light conditions above, within, and under the spicebush canopy, compiling over 4,500 measurements in a year across 26 sites (25 in the forest, one open area just outside the forest for comparison).
When Hudson talks about light, she talks about photosynthetically active radiation (PAR) and, for this study, subcanopy photosynthetic photon flux density (PPFD), which is a measure of PAR. Unsurprisingly, the highest PPFD values under the beech and poplar canopy occur in spring leafing — the days are growing longer, the northern hemisphere is tilted toward the Sun, the trees are still mostly bare. During summer leafing, the subcanopy PPFD values drop, and continue to decrease into summer and autumnal fully-leafed, before a slight bump for the autumnal partially-leafed phenoseason. In a nod to Hutchison and Matt, Hudson recreates their 1977 figure mapping the contours of PPFD through the year at different canopy levels with her own data. It’s the scientific equivalent of siblings re-staging family photos as adults.
But what does it mean to be a spicebush living in the light environment depicted in these figures? In general, Hudson found that there’s almost 10 times more subcanopy light available during the leafless seasons than the leafing and leafed seasons. During the leafing and leafed seasons there are high-energy sun flecks and hot spots — think of a sun-dappled forest floor — which contribute to the variability of light measurements throughout the phenoseasons. But, mostly the understory species must make proverbial hay (read: Germinate! Flower! Leaf out! Photosynthesize like crazy!) while the sun shines in the short leafing seasons. Even in the leafless seasons, the open site received much more PPFD than the subcanopy: the woody surfaces of the trees were intercepting plenty of winter and early spring light.
The spicebush plants in the field and in the growth chambers grew best under the highest PPFD conditions found in the Maryland woods. This is the light niche. In the growth chambers, plants that received higher PPFD conditions were actually less healthy, produced fewer leaves and less biomass. Hudson wrote a beautiful explanation of this when we emailed and I have to let this paragraph speak for itself:
We know that all organisms have an ecological sweet spot, but very rarely are all conditions ideal. Canopy species are "less limited" in the sense that they may experience some shading by neighbors but are primarily subject to changes in light due to latitude, season, and sky conditions. This "light intensity niche" is especially important for shade-adapted and shade-loving plant species when you consider spectral filtration (one way that plants "communicate" with each other and adapt growth direction and strategy) and temporal sequences of incident radiation at both long and short time scales (the timing and amount of light availability is crucial for physiological and biochemical processes for these species). It puts a sort of "ceiling" on the amount of light that is useful for the understory plant, whereas for canopy species there really isn't such a thing as too much light – their growth is primarily limited by the lower boundaries of light availability.
Finally, this study’s implications for climate change research are quite interesting. In the decades while the ‘phenoseasons’ concept was languishing, research in phenology has taken off: the timing of seasonal events like leaf out and flowering are almost universally creeping earlier in response to warming temperatures. This advancing spring phenology has been definitively tracked in temperate deciduous forests like Hudson’s study site. As the climate changes, leafing phenoseasons may bite into the leafless phenoseasons. The density of the canopy may change as the species composition, size, and height of canopy trees changes. As Hudson wrote, these are the pre-existing conditions in the forest from the perspective of an understory species. We often think about species migrations and no-analog communities when we talk about the ecological effects of climate change: now I think I’ll imagine the reshuffling of the pre-existing conditions, and the interactions between biotic and abiotic factors that create the “ecological sweet spots” that we study. And now, as we enter the autumn leafless, I’ll soak up the sun on my unseasonably warm October runs.
*This paper’s obscurity is not helped by the fact that the google scholar pdf link takes you to a 627-page annual report hot off the mimeograph with old-timey typer-writer kerning; Hutchison and Matt’s paper is buried in this report (just scroll to page 327), though much easier to find via JSTOR.