Allocation strategies of savanna and forest tree seedlings in response to fire and shading: outcomes of a field experiment

Abstract

The forest-savanna ecotone may be very sharp in fire-prone areas. Fire and competition for light play key roles in its maintenance, as forest and savanna tree seedlings are quickly excluded from the other ecosystem. We hypothesized a tradeoff between seedling traits linked to fire resistance and to competition for light to explain these exclusions. We compared growth- and survival-related traits of two savanna and two forest species in response to shading and fire in a field experiment. To interpret the results, we decomposed our broad hypothesis into elementary tradeoffs linked to three constraints, biomass allocation, plant architecture, and shade tolerance, that characterize both savanna and adjacent forest ecosystems. All seedlings reached similar biomasses, but forest seedlings grew taller. Savanna seedlings better survived fire after topkill and required ten times less biomass than forest seedlings to survive. Finally, only savanna seedlings responded to shading. Although results were consistent with the classification of our species as mostly adapted to shade tolerance, competition for light in the open, and fire tolerance, they raised new questions: how could savanna seedlings survive better with a 10-times lower biomass than forest seedlings? Is their shade intolerance sufficient to exclude them from forest understory?

Steal the light: shade vs fire adapted vegetation in forest–savanna mosaics

“…Our results indicate that the deep shade found in forest could play an important role in tree–tree competition and could constitute an environmental barrier for trees from the savanna and thicket biomes. It is well‐established that under low light, trees express different morphological and physiological traits (Kitajima, ; Brenes‐Arguedas et al ., ; Gignoux et al ., ) or plant architectures (Küppers, ; Millet et al ., , ). The absence of savanna seedlings in deep shade (LAI > 1.5) suggests that shade suppression of their establishment consistent with the restriction of adult trees to an almost completely open environment but could also be due to dispersal limitation.…”

“…Our results indicate that the costs related to the display of photosynthetic surface, as represented by branch mass per area (BMA), of thicket and savanna species seem to be too high to survive in the deep shade of forest, supporting the hypothesis of a trade‐off between ‘competitive ability for light’ and ‘fire resistance’ (Gignoux et al ., , ). At LAI > 1.5, the mean community value of BMA drops below 0.1 g cm −2 , a threshold value that separates forest trees from savanna and thicket trees that appear to be intolerant of deep shade.…”

“…With regard to savanna/forest mosaics, these can be considered alternative stable states with savannas maintained by surface fires fuelled primarily by grasses (and not by leaf litter). The boundary separating savannas from forest is thought to be organized along a single trade‐off axis between closed (forest) vegetation adapted to cast and survive deep shade, but intolerant of fire, vs flammable open grassy vegetation (with highly flammable C 4 grasses) able to survive fire, but intolerant of shade (Scholes & Archer, ; Gillson & Ekblom, ; Warman & Moles, ; Hirota et al ., ; Mayer & Khalyani, ; Ratnam et al ., ; Staver et al ., ,b; Higgins & Scheiter, ; Hoffmann et al ., ,b; Parr et al ., ; Gignoux et al ., ). These two distinct ecosystems (forest and savanna) are often separated by a narrow strip of forest margin specialists (which could equally be considered as savanna margin specialists), for whom very limited ecological behavioural information exists.…”

Large‐Scale Geographical Variations and Climatic Controls on Crown Architecture Traits

“…Therefore, they tend to decrease primary branch insertion angles and increase chord length ratios (Figures 8c and 8d) to create tree shapes like the rightmost tree in Figure 2c to maximize light capture (Davies & Ashton, 1999;Sterck et al, 2001;Wright et al, 2005). With the increase of MAT, Q. mongolica trees tend to decrease primary branch insertion angles (Figures 8c and 8d) and create crown shapes like the four trees on the left of Figure 2c to alleviate the water loss (Gignoux et al, 2016;Kuuluvainen & Pukkala, 1987). Meanwhile, the specific leaf area also changed correspondingly to increase the photosynthesis or reduce transpiration (Wright et al, 2017).…”

Factors Affecting to Allocation in Plants

“…Işıklanma süresi ve şiddetindeki değişimler, bitkinin fotosentez metabolizmasını doğrudan etkilediği için, oluşan karbonhidratların bitkideki taşınma modelini ve fidelerinin gelişmerini de etkilenmektedir (Poorter and Nagel, 2000;Gignoux et al, 2016). Çünkü düşük ışık şiddetinde gelişen bitkiler, yüksek ışık isteğindeki bitkilere göre daha az besine ihtiyaç duyarlar ve fotosentezde daha az miktarda C tutulurken, yavaş büyüme ve stoma iletkenliğin az olması nedeniyle daha az su kullanırlar.…”