Exotic tree species displace indigenous ones on farms at intermediate altitudes around Mount Kenya

Abstract

Agroforestry systems are potentially suitable for conservation of tree genetic resources. Farmers around Mt. Kenya usually integrate trees into their farm. Large parts of these trees seem to be of exotic origin, whereas indigenous species have priority for conservation. This study aimed at determining on-farm richness, composition and frequency of indigenous and exotic woody species around Mount Kenya to assess the suitability of farms for the conservation of indigenous tree species. 265 onfarm plots of 0.5 ha size each were selected in 18 different agro-ecological zones by using a stratified sampling scheme. All woody species within the plot were recorded with their local and scientific names. Total species richness was 424 (including 306 indigenous ones), mean richness per plot 16.5 species (including 8.8 indigenous ones). Eight out of the 10 most frequent species were exotic ones with Grevillea robusta from Australia ranking first (found on almost 76% of the surveyed farms). The proportion of indigenous species increased with increasing aridity and temperature. Dominance of exotic species was found at farms of humid mid-and highlands. Ordination analysis revealed that mostly exotic species contributed to separation of farms in the highlands and upper midlands, whereas indigenous species in the lower midlands and lowlands. As the frequencies of most indigenous trees were low, only parts of the surveyed farms can contribute to conservation of tree genetic resources, particularly the less intensively managed farms of the more arid lands. Farmers' access to knowledge on valuable indigenous tree species and to quality seedlings of these trees need to be improved to increase indigenous species' frequencies on farms and possibly to replace some of the exotic species in the future.

Evaluating factors influencing heterogeneity in agroforestry adoption and practices within smallholder farms in Rift Valley, Kenya

“…Only Sesbania spp., M. lutea and C. macrostachyus are ranked among the 10 most abundant tree species in the farms studied. This observation is consistent to results obtained from western and central Kenya (Kindt et al, 2006;Kehlenbeck et al, 2011;Lengkeek et al, 2005) whereby exotic species had tree count compared to indigenous species but lower species count. The existence of high abundance of exotics demonstrates deliberate planting and choice (Simons and Leakey, 2004).…”

“…For example, Carsan (2012) recorded 165 tree species on smallholder coffee farms in Central Kenya with mean species richness of 17 species per farm. Kehlenbeck et al (2011) found a mean tree species richness of 17 per farm and Lengkeek et al (2005) observed a mean of 54 species per farm. The species richness of 44 we found in our study area and a mean of 8 tree species per farm are therefore relatively low compared to other studies in agricultural landscapes of Kenya.…”

Why do farmers plant more exotic than native trees? A case study from the Western Ghats, India

“…Thus, in southern Bahia, Brazil, farmers often plant non-native rubber (Hevea brasiliensis) and jackfruit (Artocarpus heterophyllus) trees rather than native timber and fruit trees for shade in their cocoa farms (Schroth et al, 2011); in Costa Rica, Eucalyptus species have become popular as coffee shade (Tavares et al, 1999;Schaller et al, 2003); and in Ghana, cocoa and coffee farmers wishing to diversify into timber production often prefer South American Cedrela odorata or Asian Tectona grandis to native species (Ruf, 2011). Similar preferences for planting exotic tree species on farm land have also been observed in other tropical regions (Dewees, 1995;Elouard et al, 2000;Takaoka, 2008a,b;Ambinakudige and Sathish, 2009;Nath et al, 2011;Kehlenbeck et al, 2011;Tefera et al, 2014;Nyaga et al, 2015;Valencia et al, 2015). In addition to the threat of declining environmental quality and ecosystem services caused by exotic species monocultures, diversity and continuity of the tree canopy may be compromised, thus preventing wildlife migration across agroforests and between nearby forest fragments (Perfecto et al, 1996;Vandermeer and Perfecto, 2007;Schroth et al, 2011).…”

“…Farmers worldwide have contributed substantially towards this diversity by planting trees that provide economic value, food security and environmental improvement (Dewees, 1995;Scherr, 1995;Akinnifesi et al, 2006;Takaoka, 2008a;Anglaaere et al, 2011;Kehlenbeck et al, 2011;Goodall et al, 2015;Nyaga et al, 2015). However, a recent globally observed threat to farmland biodiversity is the ongoing transformation of traditional complex agroforests into simpler land use forms dominated by exotic species, which may eventually culminate in unshaded crop monocultures (Siebert, 2002;Peeters et al, 2003;Ruf, 2011;Jha et al, 2014).…”

Environmental filtering of agroforestry systems reduces the risk of biological invasion

“…Agroforestry systems contribute significantly to the conservation of biodiversity and its genetic resources in fragmented landscapes (Kehlenbeck et al 2011;Nath et al 2011;Jose 2012;Souza et al 2012). Such systems contain up to 50 % of the diversity found in natural forests (Nobel and Dirzo 1997).…”