Modeling the growth rates of potential forestry species in Rwanda: a tree structural traits-based approach

إنجليزي. Forest plantations play a crucial role in the livelihoods of Rwandan people. They provide valuable wood resources, such as timber and biomass energy. The first plantations created in the colonial period from 1920 to 1948 contained only exotic Eucalyptus species, which is thought to out-perform native species in terms of more rapid wood production. Over time, this resulted in the domination of non-native species in forest plantations across Rwanda, primarily Eucalyptus species. However, replacing native species with monocultures of the exotic ones impacts the local environment in multiple negative ways. Therefore, more sustainable forestry practices are needed, such as indigenous species planting. Modeling the performance of potential Rwandan forestry species may serve as a useful tool for forest management strategies. Thus, dynamic vegetation model LPJ-GUESS was used in this study, with the aim to try and adjust the model so that it can represent the growth of five species interesting for the context of Rwandan forestry: Eucalyptus microcorys, Eucalyptus citriodora, Grevillea robusta, Entandrophragma excelsum and Polyscias fulva. Since tree performance essentially depends on their functional traits, with structural traits playing a key role, this study tried to determine the extent up to which structural tree traits can explain the differences in growth rates between potential forestry species in Rwanda. Specifically: wood density, sapwood turnover rate, tree crown area to diameter at breast height (DBH) and tree height to DBH allometric parameters, specific leaf area and leaf area to sapwood area ratio. The impact of several additional functional traits on growth was examined as well, to help explain remaining growth differences not captured by the structural traits alone. Differences between the growth of the modeled species due to the species-specific structural traits were evident. Still, these traits could explain only a fraction of the modeled differences, leaving much unexplained when comparing to the observed differences in growth. Of all the additional functional traits examined, maintenance respiration coefficient had the strongest influence on the growth patterns. The outcomes of this study provide valuable insights in species-level tree growth modeling and could thus potentially help in applying more sustainable forest management practices in Rwanda.

اللغات غير المشفرة. Amashyamba afite uruhare rukomeye mu buzima bw’ Abanyarwanda. Atanga iby’ ingenzi bikomoka ku biti nk’ibiti byo kubaka ndetse n’ ingufu zituruka ku bimera. Amashyamba yatangiye guterwa hagati y’ umwaka w’ 1920 n’ 1948 mu gihe cy abakoloni, yari agizwe n’ amoko y’ ibiti mvamahagana bigizwe ahanini n’ inturusu kuberako zarushaga ibiti gakondo gukura neza. Uko iminsi yagiye yiyongera, gutera amashyamba agizwe n’ ibiti mvamahanga byagiye bigabanya ubuso buteyeho amashyamba gakondo mu Rwanda. Gusa, kwiyongera kw’ amashyamba agizwe n’ ubwoko bumwe bw’ igiti mvamahanga ryagize ingaruka mbi ku bidukikije mu buryo bugaragara, akaba ari yo mpamvu hakwiriye gutekerezwa ku buryo burambye bwo gutera amashyamba hibandwa ku moko y’ ibiti gakondo. Kubaka icyegeranyo ndebakure cy’ imikurire ku bwoko bw’ ibiti bishobora kwifashisha mu hazaza h’ amashyamba mu Rwanda ni igikoresho cya ngombwa mu micungire myiza y’ amashyamba. Ni muri urwo rwego icyegeranyo -ndebakure-Dynamic model LPJ_GUESS cyifashishijwe muri ubu bushakashatsi mu igerageza n’ ikosora ryacyo ku mikurire y’ amoko atanu y’ ibiti akoreshwa mu mu mashyamba y’ u Rwanda ari yo: Inturusu ya kagwegwe, Inturusu ihumura neza, Gerevireya, umuyove n’ umwungo. Gukura neza kw’igiti biterwa n’ imiterere yacyo, imiterere ijyane n’uko igiti cyubakitse igira akamaro kaninini cyane mu mukurire myiza yacyo. Ubu bushakashatsi bugamije kugaragaza uko uruhare rw’ imiterere y’ uko igiti cyubatse igira uruhare mu gusobanura itandukaniro mu mikurire rigaragara hagati y’ amoko y’ ibiti byifashisha mu mashyamba y’ uRwanda. Imiterere y uko igiti cyubakitse yibanzweho harimo: ireme bwite ry’igiti, ingano y’ umugogo w’ igiti gihagaze, isano riri hagati y’ ubugali bw’ umutwe w’ igiti n’ umubyimba wacyo ku buhagarike bureshya no mu gituza, isano riri hagati riri hagati y’ ubuso bw’ umutwe wi‘igiti n’ ubuhagarike bwacyo, ubuso bwa amababi bwa buri bwoko bw’ igiti, n’ isano riri hagati y’ ubuso w’ amababi n’ umutima w’ igiti. Uruhare rw’ iyindi miterere myinshi itandukanye ifitanye isano n’ imikurire y’ igiti narwo rwarebweho kugirango hamenywe byisumbuyeho ibindi bigenga itandukaniro tubona mu mikurire y’ ibiti. Byagaragaye ko itandukaniro ry’ imikurire y’ amoko y’ ibiti byizweho rifite aho rihuriye n’ uko buri bwoko b’ igiti cyubatse. Iyi miterere isobanura igice gito ku bijyanye n’ imikurire y’ibiti ugereranije n’ ikinyuranyo kinini cyaranze imikurire y’ amoko y’ ibiti byifashishijwe. Mu yindi miterere y’ igiti isobanura imikurire yabyo, twasanze uko igiti gikoresha umusaruro w’ ibikigize mu kwiyitaho nabyo bifite uruhare rukomeye cyane mu kugena icyerekezo cy’ imikurire yacyo. Umusaruro w’ ibyavuye muri ubu bushakashatsi n’ ingenzi mu gukora icyegerano ndebakure ku mikurire ya buri bwoko bw’ igiti gishobora kwifashishwa muri gahunda irambye yo gutera amashyamba mu Rwanda.