What drives grassland-forest boundaries? Assessing fire and frost
effects on tree seedling survival and architecture
Abstract
1. 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. 2. We assess the response of
young tree seedlings, which represent a vulnerable stage in tree
recruitment, to traumatic fire and frost disturbances. 3. 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. 4. 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. 5. Synthesis.
Fire caused more severe aboveground damage compared to frost, suggesting
that trees in these open grassland systems may be subjected to a
seedling release bottleneck maintained by fire. However, the woody
species composition will almost certainly 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.