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Fine root growth and mortality in different-aged ponderosa pine stands

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Abstract:

Root minirhizotron tubes were installed at two sites around three different age classes of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) to follow patterns of fine root (≤2mm diameter) dynamics during a 4year study. Both sites were old-growth forests until 1978, when one site was clear-cut and allowed to regenerate naturally. The other site had both intermediate-aged trees (50–60years) and old-growth trees (>250years old). Estimates of fine root standing crop were greatest around young trees and least around intermediate-aged trees. Root production was highly synchronized in all age classes, showing a single peak in late May–early June each year. Root production and mortality were proportional to standing root crop (biomass), suggesting that allocation to new root growth was proportional to root density regardless of tree age. The turnover index (mortality/maximum standing crop) varied from 0.62 to 0.89·year–1, indicating root life spans in excess of 1 year. It appears that young ponderosa pine stands have greater rates of fine root production than older stands but lose more fine roots each year through mortality. The results indicate that soil carbon may accumulate faster in younger than in older stands.

Des minirhizotrons tubulaires ont été installés dans deux stations autour de pins ponderosa (Pinus ponderosa Dougl. ex Laws.) appartenant à trois classes d’âge différentes pour étudier la dynamique des racines fines (diamètre ≤ 2 mm) pendant quatre ans. Les deux stations étaient situées dans des vieilles forêts jusqu’en 1978 lorsque la forêt dans une des stations a été coupée à blanc et régénérée naturellement. Dans l’autre station, on trouvait des arbres d’âge intermédiaire (50–60 ans) et de vieux arbres (plus de 250 ans). La population de racines fines vivantes était la plus abondante autour des jeunes arbres et la moins abondante autour des arbres d’âge intermédiaire. La production de racines fines était étroitement synchronisée dans toutes les classes d’âge et elle culminait une seule fois entre la fin mai et le début juin chaque année. La production et la mortalité des racines étaient proportionnelles à la population de racines vivantes (biomasse), ce qui indique que l’allocation à la croissance de nouvelles racines est proportionnelle à la densité des racines peu importe l’âge de l’arbre. L’indice de renouvellement (mortalité/population vivante maximale) variait de 0,62 à 0,89·an–1, indiquant que la longévité des racines dépassait 1 an. Il semble que les jeunes peuplements de pin ponderosa ont un taux de production de racines fines plus élevé que les peuplements plus vieux, mais également qu’ils perdent plus de racines fines chaque année à cause de la mortalité. Ces résultats indiquent que le carbone peut s’accumuler dans le sol plus rapidement dans les jeunes peuplements que dans les vieux peuplements.

Document Type: Research Article

Publication date: July 1, 2008

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  • Published since 1971, this monthly journal features articles, reviews, notes and commentaries on all aspects of forest science, including biometrics and mensuration, conservation, disturbance, ecology, economics, entomology, fire, genetics, management, operations, pathology, physiology, policy, remote sensing, social science, soil, silviculture, wildlife and wood science, contributed by internationally respected scientists. It also publishes special issues dedicated to a topic of current interest.
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