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

Citation

Botha, Monique; Archibald, Sally; Greve, Michelle (2021), Supplementary data: What drives grassland-forest boundaries? Assessing fire and frost effects on tree seedling survival and architecture, Dryad, Dataset, https://doi.org/10.5061/dryad.ncjsxksrx

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 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.

Methods

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.  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).

Funding

National Research Foundation, Award: 98889

National Research Foundation, Award: 116333

University of the Witwatersrand, Johannesburg