Supplementary data: What drives grassland-forest boundaries? Assessing fire and frost effects on tree seedling survival and architecture

Fire and frost represent two major hurdles for the persistence of trees in
open grassy biomes and have both been proposed as drivers of
grassland-forest boundaries in Africa. We assess the response of young
tree seedlings, which represent a vulnerable stage in tree recruitment, to
traumatic fire and frost disturbances. In a greenhouse experiment, we
investigated how seedling traits predicted survival and resprouting
ability in response to fire vs frost; we characterised survival strategies
of seedlings in response to the two disturbances, and we documented how
the architecture of surviving seedlings is affected by fire vs frost
injury. Survival rates were similar under both treatments.
 However, different species displayed different levels of
sensitivity to fire and frost. Seedling survival was higher for older
seedlings and seedlings with more basal leaves. Survivors of a fire event
lost more biomass than the survivors of a frost event. However, the
architecture of recovered fire and frost treated seedlings were mostly
similar. Seedlings that recovered from fire and frost treatments were
often shorter than those that had not been exposed to any disturbance,
with multiple thin branches, which may increase vulnerability to the next
frost or fire event. Synthesis. Fire caused more severe aboveground damage
compared to a single frost event, suggesting that fire is an important
driver of tree distribution in these open grassland systems. However, the
impact of repeated frost events may be equally severe, and needs to be
investigated. Also, woody species composition may be influenced by
phenomena that affect the timing and frequency of seedling exposure to
damage, as mortality was found to be dependent on seedling age. Therefore,
changes in fire regime and climate (esp. changes that bring about less
frost and reduced fire intensity and frequency) are likely to result in
changes in the composition and the structure of the woody components of
these systems.