Native tree seedlings grow best near existing forest and beneath shade in highland Madagascar

A team of MBG scientists describes a recent experiment to grow native trees in a degraded part of Madagascar’s central highlands.

Tampo_Raw

The dissected landscape of the Tampoketsa de Ankazobe in central Madagascar. Imagery: Google Earth (2018).

Seen from space, parts of Madagascar’s high plateau look like a wizened, grayish-pink brain drying in the sun. Thin, dark lines demarcate nooks and crannies – nearly the only places where bits of forest remain.

Formerly, the forests here covered more territory. Just how much territory is debated; ancient grasslands are also present in highland Madagascar. But in this area, about three hours northwest of the capital, many forests have been cleared, burned, and converted to new grassland within living memory.

To restore forests to their recent extent would benefit a range of species, including Schizolaena tampoketsana (a threatened, micro-endemic tree) and an undescribed species of fat-tailed dwarf lemur. However, restoration has been easier said than done so far. Natural forest regeneration is slow to non-existent, even near remnant forests where fire is excluded. Planted tree seedlings grow only millimeters each year, if they survive at all. Adding fertilizer seems to inhibit seedling growth. Inoculating seedlings with mycorrhizal fungi seems promising, but we are not yet sure if this will make a difference in the field.

Following a field trip in November 2016, we decided to test a couple of other tactics for growing native trees on this weathered plateau. First, we tested planting trees near existing forest. Being near the forest could help young seedlings by shading them from the hot sun or by sharing beneficial microorganisms. Second, we put shade structures over some of tree seedlings to test how much the bright, hot sunlight prevented tree growth.

We tested four tree species: Baronia taratana (Anacardiaceae), Nuxia capitata (Stilbaceae), Uapaca densifolia (Phyllanthaceae), and Eugenia pluricymosa (Myrtaceae).

ExpDesign

An experiment with native tree seedlings at Ankafobe, a small forest fragment on the highlands northwest of Madgascar’s capital, Antananarivo. Photo: Chris Birkinshaw.

Three of the tree species survived and grew more when we planted them next to the forest. The fourth, Nuxia capitata, was a super species and grew relatively well wherever it was planted.

Two of the four species also survived more often beneath shade structures. But interestingly, this shade effect did not completely account for the effect of proximity to forest. That suggests that shade is important for protecting young seedlings from the hot sun, but something else is going on too. Perhaps trees growing next to the forest get a boost of water, since remnant forests sit at the valley bottom where water collects. If this is true, then tree seedlings might do well in any valley bottom, not just ones with remnant forest in them.

Our study site, called Ankafobe, is only a small area, so it would be a stretch to generalize our observations to the entire region. However, we are not the only ones to have done such a test in this ecosystem. In 2000, Ingar Pareliussen led a study with the same basic elements as ours at a site ten kilometers away, at Ambohitantely. Like us, Pareliussen’s team found that seedlings planted near the edge of a remnant forest grew better than those planted further away. In contrast, shade structures did not improve seedling survival. In fact, one species grew worse in the shade.

Fig_TampoEdges

One vision for landscape-scale forest restoration on the Tampoketsa de Ankazobe. We used an edge-detection algorithm in Inkscape to highlight forest edges and valley bottoms, the places where trees grew best in our study. Imagery: Google Earth (2018).

Together, our two studies begin to suggest the outlines of a vision for landscape-scale forest restoration on the high Tampoketsa de Ankazobe. If native tree seedlings perform better along forest edges, it follows that a cost-effective strategy would be to focus on planting those areas first, leaving the higher, drier areas alone. Planting along edges would also be a conservative strategy given our hazy understanding of past landscapes. Some grasslands in highland Madagascar seem to be very old, and planting trees in such places could destroy habitat for grassland species, which are threatened in their own right.

For more information about this experiment, you can read our open access paper in Plant Diversity. We have also published several other blog posts about Ankafobe.

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Homemade mycorrhizal inoculum improves seedling growth for some native Malagasy trees

MBG Madagascar’s Chris Birkinshaw and Dinasoa Tahirinirainy describe exciting, preliminary results from a forest restoration experiment in highland Madagascar.

 

Ankafobe Forest on Malagasy highlands - experiment located on grassy ridges

This sliver of riparian forest is one of the last vestiges of Madagascar’s highland forests. Decades of Missouri Botanical Garden research in Madagascar have shown that more than 80% of all plant species on the island exist nowhere else. Many are threatened with extinction due to habitat loss. Several previous posts have described forest restoration efforts at this site, home to the largest population of the endemic sohisika tree (Schizolaena tampoketsana) – a species that belongs to a family (Sarcolanaceae) that only exists on Madagascar.

A small number of forest restoration projects in Madagascar routinely inoculate the tree seedlings in their nurseries with a homemade mycorrhizal inoculum. While the nurserymen are convinced that this technique promotes growth and survival of tree seedlings, there seems to be no published data objectively demonstrating these positive outcomes. In an effort to provide the evidence to justify investment in this technique, we designed a simple experiment that will compare the survival and growth under four treatments of young plants of six native trees planted in grassland adjacent to the Ankafobe Forest on the central Malagasy highlands.

Table – Four experimental treatments to test the effects of mulch and mycorrhizal inoculum on native tree seedling growth in highland Madagascar

  Inoculated Not inoculated
Mulched Treatment 1 Treatment 2
Not mulched Treatment 3 Treatment 4 (control)

In our experiment, fifteen seedlings of each of six native tree species will be grown under each of the four treatments listed above. The mycorrhizal inoculum was made by filling a pit (150 cm long × 50 cm wide × 30 cm deep) lined with sacks with topsoil collected from around the roots of three native tree species, then growing maize and beans in this soil for three months before cutting these plants down and letting the substrate dry out for two weeks. The substrate remaining in the pit is the inoculum and was used by adding one tablespoon to each seedling container.

Making the inoculum

Beans and maize are grown in topsoil collected from a remnant forest to amplify local mycorrhizae populations. This enriched soil (i.e., inoculum) is then added to seedling containers.

The tree seedlings that received mycorrhizal enrichment were inoculated in November 2017, and all of the seedlings were otherwise grown under the same conditions in the nursery until January 2018 when they were planted out into an experimental plot at Ankafobe. Half of the tree seedlings were surrounded by a thick layer of grass-based mulch (~30-cm deep). The comparison of seedling performance with and without the addition of mulch is interesting because of the possibility that mulch helps to maintain a relatively cool and moist environment in which the mycorrhizae can flourish.

Table – Mean difference in tree seedling height (cm) between seedlings inoculated versus not inoculated with homemade mycorrhizae, after two months in the nursery (N = 30 seedlings per species).

Species Inoculated seedling height Non-inoculated seedling height t p1
Aphloia theiformis 29.5 ± 10.2 33.4 ± 6.5 -1.75 1.0000
Baronia tarantana 18.1 ± 6.1 11.4 ± 3.5 5.21 <0.0001
Brachylaena ramiflora 27.2 ± 6.0 31.8 ± 6.5 -2.87 1.0000
Craspidospermum verticillatum 43.0 ± 5.9 42.6 ± 3.7 0.37 1.0000
Macaranga alnifolia 34.8 ± 8.6 39.2 ± 5.2 -2.43 1.0000
Uapaca densifolia 23.0 ± 7.9 11.5 ± 2.6 7.58 <0.0001

1 t and p values are from a one-tailed student’s t-test asking whether inoculated seedling height was greater than non-inoculated seedling height. P values are adjusted for multiple comparisons with Bonferroni correction.

Although we plan to measure seedling survival and growth 12 months from the time when they were planted (i.e., in January 2019), we were interested to see that for two of the species the height of inoculated seedlings was significantly greater than the height of non-inoculated seedlings after a mere two months in the nursery. On average, inoculated seedlings of Baronia tarantana are 1.6× taller than non-inoculated seedlings; while the seedlings of Uapaca densifolia are a full 2× taller. For the other species there was no significant difference between the height of the inoculated and non-inoculated plants.

Experiment showing line of seedlings some with and some without mulch (1)

Tree seedlings are planted out in a field experiment at Ankafobe in January 2018. These seedlings are planted adjacent to a line of “green manure” (i.e., nitrogen-fixing Tephrosia shrubs planted to improve the degraded highland soil prior to planting native tree seedlings).

 

Hill of Honey: Forest Recovery on Madagascar’s Central Highlands

This post was co-written by Leighton Reid and Chris Birkinshaw after a three-day field trip in the tampoketsa with Cyprien Mandriamanana and Jeannie Raharimampionona.

A narrow, paved road winds north from Antananarivo through a high, windswept plain. It is the wet season, and the hills are green and close-cropped, but in the long dry season the landscape burns black. Orange rivers wind through the valleys, muddied by massive erosion. Here and there are thin strips of riparian forest, chock full of endemic species.

The biggest chunk of remaining forest is Ambohitantely, Malagasy for “hill of honey”. Ambohitantely encompasses 1,800 ha of humid forest (about three times the size of Saint Louis’s Forest Park). We visited the reserve to observe natural forest recovery in one of the few places it can still be seen on Madagascar’s Central Highlands.

Ambohitantely map

Ambohitantely: the last large tract of forest on Madagascar’s Central Highlands. Ten kilometers to the northwest is Ankafobe, a much smaller forest fragment managed by a local community with assistance from Missouri Botanical Garden.

Forest transition feedback

Why is forest recovery so rare in the Malagasy highlands? Madagascar’s Central Highlands are currently undergoing a complete ecological transition, from forest and wooded savanna to grassland. The degradation cycle often starts when people cut forest trees to extract wood for timber and charcoal production. Small-scale cutting opens the canopy, dries the forest floor, and creates debris, all of which increase forest vulnerability to annual wildfires that sweep across the grasslands during the eight month dry season. C4 grasses quickly colonize the burned land, inhibiting forest recovery, and creating ideal conditions for future fires. The reason that Ambohitantely has natural forest recovery for us to observe is because reserve staff maintain a wide fire break for more than thirty kilometers around the reserve.

Our guide at Ambohitantely took us on a hike through several areas where forest had once been cleared and burned but where fire had been excluded for 15 or 25 years, allowing the forest to begin to recover. Frankly, the vegetation was uninspiring. Low shrubs and forbs were scattered through a matrix of C4 grass (mostly Aristida), but trees and tree seedlings were nowhere to be seen outside of the forest (where they were abundant). By tropical forest standards, this looked slow. But for natural forest recovery on the Central Highlands, it’s hard to imagine a better situation than being protected from fire and immediately adjacent to the largest remaining tract of forest.

Ambohitantely forest edge

After 25 years of recovery, forest edges were sharp. Tree seedlings were almost totally restricted to forest, and grasses dominated the ground layer just outside.

Our main interest in Ambohitantely was to compare natural forest recovery there to our observations at another forest fragment, Ankafobe, 10 km northwest. For the last decade, MBG has partnered with a local community to preserve a thin, riparian forest containing several critically endangered plant species. Community members constructed a fire break and have begun to restore the surrounding hillsides by turning over the orange clay and planting fast-growing legumes to develop the soil.

Ankafobe hillside with Schizolaena

A degraded hillside at Ankafobe; the remnant forest is down the hill to the right, near the edge of the photo. The tree at the top of the hill is Schizolaena tampoketsana, a critically endangered microendemic in an endemic Malagasy plant family (Sarcolaenaceae). It is a remnant forest tree that likely escaped repeated fires by being nestled in a deep, protective gully. The multi-stemmed shrub in the foreground is actually a large tree species, Brexia montana, which has likely resprouted many times from a well-developed root system.

Some of our comparative observations were promising. We were happy to find examples at Ambohitantely where recovering land dominated by heather and blueberry seemed to have continued developing into a more diverse thicket, including Nuxia capitata, Psiadia altissima, and Razafimandimbisonia minor. At Ankafobe, we had worried that the heather growing in some areas signified poor soil conditions and possibly arrested development.

Overall our visit left us with more questions than answers. We hope to answer at least a couple of them over the coming years.

  • Why is natural forest recovery so slow on the Central Highlands?
  • Has it always been this slow?
  • Are Malagasy tree species poor pioneers because of their long, relatively stable evolutionary ecological history?
  • Is there any way to make Malagasy trees grow any faster on the degraded grasslands?1
  • Are the fire-stoking C4 grasses introduced from east Africa rather than being native species?
  • If so, when?
  • Is the soil too far gone to ever recover?
  • How important were now-extinct seed dispersers and grazers, like 200-kg lemurs and elephant birds?
Ambohitantely cloud cover

Although Ambohitantely is the only remnant forest fragment of any size, we learned recently that it may not be a perfect reference system for Ankafobe. For one thing, Ambohitantely is slightly higher and farther east, which results in considerably greater cloud cover during the dry season. This probably ameliorates the harsh conditions outside of the forest, at least a little. Shown here is cloud frequency from May – October, taken from NOAA MODIS satellite imagery. Thanks, Michael Douglas!

References

Goodman, S.M. & Jungers, W.L. 2014. Extinct Madagascar: Picturing the Island’s Past. University of Chicago Press, Chicago, IL.

Pareliussen, I., Olsson, E.G.A. & Armbruster, W.S. 2006. Factors Limiting the Survival of Native Tree Seedlings Used in Conservation Efforts at the Edges of Forest Fragments in Upland Madagascar. Restoration Ecology 14: 196-203.

1Two datasets (one from our team and one from Pareliussen et al’s [2006]) suggest that NPK fertilizer, even in relatively small doses, reduces native tree seedling performance. It is unclear whether this is because of toxicity (a direct effect) or because other plants, like shrubs, are better able to utilize the nutrient pulse and then compete more strongly against the native tree seedlings (an indirect effect).

Can fungus help grow trees in Madagascar?

Thomas Timberlake and Cyprien Miandrimanana write from Madagascar about a field experiment using fungus to help tree seedlings survive.

One of the problems that has long bedeviled ecological restoration efforts in Madagascar is persuading young seedlings to grow at a pace of more than just a few centimetres per year. The site of Ankafobe in the central highlands is a prime example, with many five year old individuals, planted in the anthropogenic grassland surrounding the remaining forest fragments, still no taller than waist height. Clearly, the environment into which the seedlings are planted is in some way inhospitable.  One hypothesis to explain seedling underperformance  is that they are not managing to establish their normal symbiotic relationships with vesicular arbuscular mycorrhizae (VAM) fungi on which most higher plants depend.

Scaled visual comparison of VAM and VAM-less seedlings at Mitsinjo in Andasibe, Madagascar.

Scaled visual comparison of VAM and VAM-less seedlings at Mitsinjo in Andasibe, Madagascar.

In a VAM symbiosis, plants exchange a significant carbohydrate donation to the fungus in return for important nutrients, particularly phosphorus, and often increased drought tolerance. So if mycorrhizae propagules are absent in the savanna soil, this could well explain the slow growth rates and high mortality observed among planted tree seedlings at sites like Ankafobe.

In response to concern about poor seedling performance, various restoration projects in Madagascar have begun inoculating their nursery seedlings with VAM using a simple protocol pioneered by Mitsinjo, a restoration project in the eastern rain forest of Andasibe. Soil (presumed to contain mycorrhizal fungus) is gathered from underneath forest trees, mixed with sand in a sack-lined pit and then sown with rice and beans to act as hosts for the developing VAM. After three months of maturation, you have a sack-full of VAM inoculum, ready to be applied to the young germinating seedlings – one teaspoon per plant.

Many groups in Madagascar swear by the VAM protocol and the visual results can be compelling, but as yet there have been no experiments in the country to rigorously test whether this method is actually effective. This lack of clear evidence is what prompted us to work on a series of experiments testing and perhaps refining the VAM protocol.

We planted 480 native tree seedlings with and without VAM inoculation to test whether this method increases seedling survival and growth in the degraded savanna around Ankafobe. Digging into the solid laterite and planting the experimental seedlings was hard work but our efforts were rewarded one day with the sighting of a family of 10 young Tenrecs (Tenrec ecaudatus) who ventured bravely out of the security of the forest to observe the progress.

Planting complete, we took our “Time Zero” measurements and then a small sample of roots from both VAM and control seedlings to return to Antananarivo and check for the presence of mycorrhizae vesicles. The process of staining involved cooking up some rather nasty chemicals in our improvised laboratory – the kitchen – back in Tana.

Our next project will be to replicate our VAM study in Ananalava, a humid site on the east coast that contrasts with the drier climate of the Malagasy Highlands. Repeating our study in different environments will help generalize our results and recommendations for people working across this heterogeneous island.

Cyprien in our kitchen laboratory preparing an improvised stain to look for VAM vesicles.

Cyprien in our kitchen laboratory preparing an improvised stain to look for VAM vesicles.