Climate and nitrogen deposition constrain the maximum size-density boundary for mature and old-growth stands
Forest stocking guidelines traditionally reference self-thinning lines representing the tradeoff between maximum trees per unit area vs. maximum mean tree size for even-aged stands. While self-thinning lines are roughly linear on logarithmic scales, certain forest types display a curvilinear “mature...
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Frontiers Media S.A.
2025-03-01
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| Series: | Frontiers in Forests and Global Change |
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| Online Access: | https://www.frontiersin.org/articles/10.3389/ffgc.2025.1566459/full |
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| author | Christopher E. Looney John D. Shaw |
| author_facet | Christopher E. Looney John D. Shaw |
| author_sort | Christopher E. Looney |
| collection | DOAJ |
| description | Forest stocking guidelines traditionally reference self-thinning lines representing the tradeoff between maximum trees per unit area vs. maximum mean tree size for even-aged stands. While self-thinning lines are roughly linear on logarithmic scales, certain forest types display a curvilinear “mature stand boundary” (MSB). The existence of the MSB suggests that beyond self-thinning, processes such as recruitment limitation, density-independent mortality, and their interactions with site quality may also contribute to a more universal maximum size-density boundary (MSDB). To advance forest modeling and the management of mature stands under global change, we investigated: (1) how the MSDB may differ as stands biologically mature in response to climate and N deposition, (2) whether mortality and recruitment contribute to the curvilinearity of the MSDB. To accomplish this, we compiled forest inventory, climate, and total N deposition data for four western U.S. forest types (California mixed-conifer, ponderosa pine, Douglas-fir, and pinyon-juniper). We examined three aspects of climate: thermal loading, aridity, and seasonality of precipitation. We used 0.95 quantile regression to model the MSDB and generalized linear modeling for mortality and recruitment. Unlike studies of even-aged stands that found abrupt MSBs, we found evidence for curvilinear MSDBs in all four forest types, with climate and/or N deposition modulating the degree of curvilinearity. Aridity constrained maximum stocking in medium-large diameter stands of California mixed-conifer and Douglas-fir, while higher growing-season precipitation constrained maximum stocking in large-diameter ponderosa pine. Heavier N deposition lowered maximum stocking in large-diameter stands of California mixed-conifer and pinyon-juniper. In California mixed-conifer and Douglas-fir, N deposition steepened the slope of the MSDB in small-diameter stands. Mortality was consistent along the MSDB for ponderosa pine, concentrated in large-diameter California mixed-conifer and Douglas-fir stands, and small-diameter pinyon-juniper stands. Recruitment was elevated in small-diameter stands of all four forest types. Our results support roles for both mortality and recruitment in driving curvilinear MSDBs. Our findings caution against assuming that self-thinning consistently defines the MSDB throughout stand development, while having important implications for the management of mature and old-growth stands under global change, especially at extremes of resource availability where the limitations of traditional tools may be most acute. |
| format | Article |
| id | doaj-art-745892a241e14816ab2b020cd4ce3a97 |
| institution | Kabale University |
| issn | 2624-893X |
| language | English |
| publishDate | 2025-03-01 |
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| series | Frontiers in Forests and Global Change |
| spelling | doaj-art-745892a241e14816ab2b020cd4ce3a972025-08-20T03:40:57ZengFrontiers Media S.A.Frontiers in Forests and Global Change2624-893X2025-03-01810.3389/ffgc.2025.15664591566459Climate and nitrogen deposition constrain the maximum size-density boundary for mature and old-growth standsChristopher E. Looney0John D. Shaw1U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station, Vallejo, CA, United StatesU.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Ogden, UT, United StatesForest stocking guidelines traditionally reference self-thinning lines representing the tradeoff between maximum trees per unit area vs. maximum mean tree size for even-aged stands. While self-thinning lines are roughly linear on logarithmic scales, certain forest types display a curvilinear “mature stand boundary” (MSB). The existence of the MSB suggests that beyond self-thinning, processes such as recruitment limitation, density-independent mortality, and their interactions with site quality may also contribute to a more universal maximum size-density boundary (MSDB). To advance forest modeling and the management of mature stands under global change, we investigated: (1) how the MSDB may differ as stands biologically mature in response to climate and N deposition, (2) whether mortality and recruitment contribute to the curvilinearity of the MSDB. To accomplish this, we compiled forest inventory, climate, and total N deposition data for four western U.S. forest types (California mixed-conifer, ponderosa pine, Douglas-fir, and pinyon-juniper). We examined three aspects of climate: thermal loading, aridity, and seasonality of precipitation. We used 0.95 quantile regression to model the MSDB and generalized linear modeling for mortality and recruitment. Unlike studies of even-aged stands that found abrupt MSBs, we found evidence for curvilinear MSDBs in all four forest types, with climate and/or N deposition modulating the degree of curvilinearity. Aridity constrained maximum stocking in medium-large diameter stands of California mixed-conifer and Douglas-fir, while higher growing-season precipitation constrained maximum stocking in large-diameter ponderosa pine. Heavier N deposition lowered maximum stocking in large-diameter stands of California mixed-conifer and pinyon-juniper. In California mixed-conifer and Douglas-fir, N deposition steepened the slope of the MSDB in small-diameter stands. Mortality was consistent along the MSDB for ponderosa pine, concentrated in large-diameter California mixed-conifer and Douglas-fir stands, and small-diameter pinyon-juniper stands. Recruitment was elevated in small-diameter stands of all four forest types. Our results support roles for both mortality and recruitment in driving curvilinear MSDBs. Our findings caution against assuming that self-thinning consistently defines the MSDB throughout stand development, while having important implications for the management of mature and old-growth stands under global change, especially at extremes of resource availability where the limitations of traditional tools may be most acute.https://www.frontiersin.org/articles/10.3389/ffgc.2025.1566459/fullself-thinning boundary linestand density indexmature and old-growth forestmortality and recruitmentnitrogen depositionquantile regression |
| spellingShingle | Christopher E. Looney John D. Shaw Climate and nitrogen deposition constrain the maximum size-density boundary for mature and old-growth stands Frontiers in Forests and Global Change self-thinning boundary line stand density index mature and old-growth forest mortality and recruitment nitrogen deposition quantile regression |
| title | Climate and nitrogen deposition constrain the maximum size-density boundary for mature and old-growth stands |
| title_full | Climate and nitrogen deposition constrain the maximum size-density boundary for mature and old-growth stands |
| title_fullStr | Climate and nitrogen deposition constrain the maximum size-density boundary for mature and old-growth stands |
| title_full_unstemmed | Climate and nitrogen deposition constrain the maximum size-density boundary for mature and old-growth stands |
| title_short | Climate and nitrogen deposition constrain the maximum size-density boundary for mature and old-growth stands |
| title_sort | climate and nitrogen deposition constrain the maximum size density boundary for mature and old growth stands |
| topic | self-thinning boundary line stand density index mature and old-growth forest mortality and recruitment nitrogen deposition quantile regression |
| url | https://www.frontiersin.org/articles/10.3389/ffgc.2025.1566459/full |
| work_keys_str_mv | AT christopherelooney climateandnitrogendepositionconstrainthemaximumsizedensityboundaryformatureandoldgrowthstands AT johndshaw climateandnitrogendepositionconstrainthemaximumsizedensityboundaryformatureandoldgrowthstands |