DOI: 10.5061/dryad.ncjsxksrx
title: Supplementary data: What drives grassland-forest boundaries? Assessing fire and frost effects on tree seedling survival and architecture

description: 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.
description: Nineteen woody species representing a range of habitat
 preferences were selected. These species are all native to one geographic
 area, namely Buffelskloof Private Nature Reserve (BKNR)
 (25°17'53.6"S 30°30'31.7"E) in Mpumalanga, South
 Africa. Seedlings were grown from seed, which were sown in plastic
 seedling trays filled with standard nursery seedling soil mix.
 Germinations were recorded every second day over seven months. Within a
 week of germinating, newly germinated seedlings were transferred to 3.57 L
 soil bags filled with a growth medium composed of a 1:1:1 ratio of
 topsoil, compost and river sand. <i> </i>Up to 15 individuals
 were assigned to either a fire, frost or control treatment. Assignment was
 done to ensure similar aged trees were assigned to each treatment. On 3
 May 2018 an experimental fire was applied to seedlings assigned to the
 fire treatment and a frost treatment was applied to assigned seedlings
 from 16 to 17 July 2018. The control (no treatment) trees remained
 undisturbed for the duration of the experiment.
 Baseline trait measurements of all the seedlings were taken one
 day before each treatment was applied. (Baseline measures for frost and
 fire treated individuals took place on different days because these two
 treatments happened on different days). Additionally, these, and some
 additional traits, were measured on the same seedlings two months after
 treatment, and again on 1 November 2018 at the end of the
 experiment. Survival status (dead or alive) was
 recorded at the end of the experiment to provide information on survival
 rates for different species under the different treatments. Live plant
 material present two weeks after the treatments was recorded as an
 indication of which seedlings retained aboveground live foliage after the
 disturbance (resprouting only started to appear approximately one month
 after treatment). ‘Live foliage’ was defined as any visibly live material
 such as leaves, stems and active buds. A number of
 traits were measured prior to disturbance to assess their contribution to
 seedling survival. These were plant height (height of the highest green
 material), diameter of the main stem at 1 cm above the soil surface), main
 stem length, and the number of leaves below 1 cm. Plant
 height, stem length, lateral branching, and stem tip counts were also
 measured post-disturbance.  Plant height: stem diameter ratio was
 calculated by taking the height of the highest green material divided by
 the diameter of the main stem. Also, plant height:stem tip ratio was
 calculated by taking the height of the highest green material divided by
 the number of stem tips. These measures were used to assess changes in
 architecture due to fire and frost exposure. Main stem length was also
 used at the end of the experiment to determine the degree of recovery
 (regrowth).